Bulletin of the Seismological Society of America, Vol. 95, No. 2, pp. 521–539, April 2005, doi: 10.1785/0120040068
The Use of Engineering Seismological Models to Interpret
Archaeoseismological Findings in Tolbiacum, Germany: A Case Study
by Klaus-G. Hinzen
Abstract Archaeological excavations carried out from 2001 to 2003 at the vicus
Tolbiacum site, located in the present day city of Zülpich, Germany, unearthed
heavily damaged and destroyed late Roman fortification works. Damage includes
0.17-m-wide tensile cracks in a 3.1-m-wide wall, tilted walls and fortification towers,
horizontal displacement of wall sections of 0.95 m, and rotation of wall fragments.
Engineering seismological models are used to test the hypothesis that the observed
building damage is of seismogenic nature. The site is located at the western edge of
the Lower Rhine Embayment (LRE), in the northern part of the Rhine rift valley.
More than 20 damaging earthquakes occurred in the area over the past 300 years.
Several paleoseismic events with magnitudes of at least 6.4 occurred during the
Holocene. Site-specific strong motion seismograms are modeled for assumed large
earthquakes at active faults in the LRE using stochastic methods and calibrated Q
models and duration models. Nonlinear site amplification is calculated based on an
S-velocity model from refraction seismic experiments and downhole measurements.
Finite element simulation of the dynamic behavior of a fortification tower shows a
natural period of the soil-building system of about 3.0 sec. Horizontal and vertical
displacements at the top of the 8-m-tall tower reached 0.12 m and 0.06 m, respectively, for the simulated event. No indications of man-made activity exist that could
explain the damage. Running water can also be excluded as a threat to the buildings
due to the orographic situation, and slow-acting gravitational causes can be excluded
due to the damage structure. The destruction and damage is regarded as most probably seismogenic, and site intensity is assessed at IX on the European Macroseismic
Scale (EMS98) (Grünthal, 1998). This result marks the first time an intensity above
VIII is described for a tectonic earthquake in Germany. Preliminary dating results
from three carbon-14 (14C) accelerator mass spectometry (AMS) samples lead to the
second half of the fourth century A.D.
Introduction
Earthquake damage to man-made structures can have
very different appearances. The degree of damage depends
not only on the strength of the vibrations at the construction
site, but also on the type and quality of the construction
itself. Changing construction techniques have forced seismologists to alter macroseismic scales several times since
the early Modified Mercalli scale (MM-31 and MM-56) or
the Mercalli-Cancani-Sieberg scale. Numerous studies in the
past two decades also show how important the influence of
the dynamic behavior of the geological underground is in
terms of damage to man-made structures. An overview is
found in ESG (1998); studies in the Lower Rhine Embayment (LRE) were made by Parolai et al. (2002), Scherbaum
et al. (2003), Röhner and Savidis (2003), and Ohrnberger et
al. (2004). These physical effects are not only important in
contemporary seismic hazard analysis and mitigation; they
also play a crucial role in archaeoseismology. Archaeoseismology is defined as “the detailed study of pre-instrumental
earthquakes that, by affecting locations of human occupation
and their environments, have left their mark in the archaeological record” (Buck and Stewart, 2000).
Most archaeoseismic case studies are from the Mediterranean, where a more than 5000-year building record and
numerous well-explored archaeological sites exist in a region of high seismicity. Four major features are used in archaeoseismology to detect and analyze seismogenic nature
of damage and changes: (1) ancient structures are offset by
seismic faulting (Skuphos, 1984; Stiros, 1989; Stiros and
Rondoyianni, 1985; Stiros and Pirazzoli, 1994; Galadini and
Galli, 1999); (2) changes of shorelines due to sudden coseismic sea level changes proven by on- and off-shore archaeology (Stiros et al., 1992); (3) structure and preferred
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directions of collapsed and damaged constructions, including fracturing and rotation of building stones (Karcz and
Kafri, 1978, 1981; Mazor and Korjenkov, 2001; Korjenkov
and Mazor, 1999a; Guidoboni et al., 2002); and (4) synchronism of damage throughout a significantly large areas
(Stiros, 2001; Guidoboni et al., 2000; Galadini and Galli,
2000).
Relatively few studies in recent years have quantified
engineering seismological parameters based on archaeoseismological observations within the frame of this young
branch of paleoseismology (Sinopoli, 1991; Augusti and
Sinopoli, 1992; Galadini and Galli, 1999; Hinzen and
Schütte, 2003). Archaeoseismology represents a powerful
tool to increase the knowledge of past ancient earthquakes.
Though numerous case studies have been made in the past
(Stiros and Jones, 1996; Maund and Eddleston, 1998;
McGuire et al., 2000), much work must to be done to define
a reliable methodology of investigation. A combination of
archaeological, engineering geological, engineering seismological, and civil engineering techniques are essential in
this context. The work of historians and sociologists also
may be necessary, in addition to dating techniques. Improved knowledge about poorly known or unknown historical earthquakes is not only an interesting academic issue, but
also is important for seismic hazard assessment, especially
in regions of present low seismic activity. Archaeoseismic
events can even become key events for seismic hazard analysis.
During archaeological excavations in the city of Zülpich, Germany, heavily damaged late-Roman fortification
works were discovered. Structure and extent of the damage
indicate an earthquake as a possible cause. This article assesses the seismic loading at this site by calculation of fossil
strong motion seismograms using the stochastic modeling
technique (Beresnev and Atkinson, 1997, 1998), modeling
of nonlinear site effects, and simple 2D finite element modeling of the soil-building system. The site is located on the
western edge of the LRE, the northwest part of the Rhine rift
system (Fig. 1).
was built on a plateau that is raised about 30 m above the
foreland in the northwest, and 10–15 m above the foreland
in the southeast, with a moderate slope toward west and a
smooth slope to the east. The topography of the fortification
works of the city is pronounced in the west-east crosscut of
the digital elevation model (DEM). The profile crosses the
southern tip of the former city limits, where the excavation
site is located. The moat is visible east and west of the site,
which is marked by an arrow in Figure 2. The moat is also
visible in the north-south profile directly south of the site
(marked by an arrow).
History
The Roman name for the city of Zülpich was vicus Tolbiacum. From the middle of the first century A.D., Tolbiacum was an important junction of roads, connecting the
major Roman settlements in present-day Bonn, Neuss, and
Cologne with Reims and Trier (Horn, 1987). Tolbiacum was
located on the Mühlenberg, or Kirchberg, which is the highest elevation in the area (Fig. 2). The exact extension of the
city during Roman times is not known. The Mühlenberg
elevates the city above the slightly undulated Zülpicher
Börde. The major road crossing was located below the city.
The location was first mentioned by Tacitus in 70 A.D. Tolbiacum was probably fortified in the beginning of the fourth
century A.D. The fortification was an approximately 2.5-mwide wall, with jutting towers at roughly a 25-m distance.
A note by Gregor von Tour claims that a fortification still
existed in the sixth century A.D. (Horn, 1987); however, it
is not certain that this was the original late-Roman fortification. Several small fortifications, called burgi, were
erected in the fourth century A.D. in the surroundings of
Tolbiacum. Some were privately owned, and others belonged to the state. The decisive battle between the Franks
under Clovis I and the Alemanni (496/497 A.D.) was probably fought out in the plains close to Tolbiacum. The city
remained an important junction in Franconian times. The
church (Fig. 3) was first mentioned in 848. Construction of
the first medieval fortifications began between 1275 and
1278.
The Tolbiacum Site
Topographic Situation
Figure 2 shows a digital elevation model of the present
topography of the city of Zülpich and its vicinity. The main
fortification of the city is clearly visible in the center of the
map through trenches and ramparts, which still significantly
influence the present day topography. The linear feature running almost diagonally through the map from northeast to
southwest is the surface expression of a road, which follows
the trend of the Roman road between Cologne (Colonia
Claudia Ara Agrippinensium, or CCAA) and Trier (Augusta
Treverorum), which is shown on the main map of Figure 1.
The two features in the west and southeast with flat surfaces
are lakes filling former lignite opencast pits. The city itself
The Roman Thermae
The first traces of Roman construction on top of the
Mühlenberg were found between the late nineteenth and
early twentieth centuries. When a sewer system was built in
1929–1930, unearthed walls were interpreted as parts of a
Roman bath (thermae). Archaeological excavation of this
site started in 1931. The caldarium and parts of the tepidarium of the thermae were excavated (Pesch, 1932). During
excavations for new foundations for the church north of the
bath complex, walls and a channel, obviously belonging to
the Roman thermae, were found. Excavations in 1978–1979
uncovered large parts of the thermae. The bath complex itself was obviously modernized and extended in several
The Use of Engineering Seismological Models to Interpret Archaeoseismological Findings in Tolbiacum, Germany
523
Figure 1. Map A shows the major roads (after Horn, 1987) in the Roman province
Germania Inferior and the eastern part of Belgica. The vicus Tolbiacum (Zülpich) was
located at a major road junction. Stars show the location of known historic and recent
earthquakes (1445–2003) with maximum intensities larger than VII (Hinzen and Oemisch, 2001; Leydecker, 2003). The location of map A is outlined in map B, which
shows the main tectonic features of the Rhine Rift system (after Ziegler, 1987, 1994).
The outline of map B is shown in the overview map of western Europe (C).
phases; three main phases, and four to five minor phases
(Dodt, 2003). Natural layers of Tertiary sand and gravel lie
about 3.5 m below the current walking horizon. Gechter et
al. (1979) discovered during the 1978–1979 excavation
campaign that the hilltop was flattened down to a gravel
layer.
Datable finds for the Roman thermae in Tolbiacum are
rare. Following Dodt (2003), the best terminus ante for the
bath comes from seven cremation burial sites, which date
from the second half of the first century A.D. to the first
quarter of the second century A.D. Painted plaster of the
porticus from the second building phase is probably from
the second half of the second century A.D. The third building
phase was probably during the third quarter of third century.
Dodt (2003) interprets deconstruction of the bath at the time
when the Constantinien fortification of the vicus was built.
Damage to the Late Roman Fortification
The city of Zülpich plans to build a new museum of
Roman bath culture incorporating the Roman thermae. Prior
to the construction, further archaeological excavation of the
site was necessary. These excavations were started in 2001
by the Rheinische Amt für Bodendenkmalpflege. The first
excavation phase (2001–2002) was done in cooperation with
the University Pécs (Wagner, 2003). In addition to the thermae, the excavation also uncovered large parts of the fortification walls of Tolbiacum. Some parts of the fortification
show severe structural damage. The major features of the
excavation were surveyed for this study (Fig. 3). The walls,
which play an important part in the damage scenario, are
labeled with a capital W followed by a number, which increases from the southwest corner of the site counterclockwise. In order to address subsections of walls between
towers, sublabels W1.1, W1.2, and so forth, are used. Fortification towers are labeled with a capital T followed by a
number increasing in the same direction as the walls. The
origin point of the site map in Figure 3 is on top of a 3.1m-wide late-Roman wall (W1.1). The plan also shows the
trend of a seismic refraction line (RS) and two electric tomography profiles (ET1 and ET2), which were measured to
deduce parameters for the local engineering seismological
model. A plan of findings from an earlier excavation (Gechter et al., 1979) is overlaid on the survey in Figure 3. Gray-
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Figure 2. DEM of the current morphology
of the city of Zülpich (vicus Tolbiacum) and
its surroundings. Tick marks on the main map
indicate distances of 500 m. The circled
crosses mark the end points of east–west and
north–south striking crosscuts, which are plotted above and left of the map, respectively (elevation data: Landesvermessungsamt NRW,
Bonn, 1921/2002).
filled parts are an attempt to reconstruct the original position
of the fortification works (Gechter et al., 1979; Horn, 1987).
While the position of the round towers T1 and T2 is clear
from the findings, the existence and exact location of T3 is
not completely clear. T3 was probably a corner tower and
most probably of rectangular shape.
This article concentrates on the severe structural damage to the late Roman city wall and fortification works discovered from 2001 to 2003. The damage can be classified
into four parts, which are described below in the chronology
of their discovery. During the excavation of the southwestern section, south of the current Probstei museum building
(Fig. 3), a hole was found in the ground, which at first was
assumed to indicate an open cellar room. Continued excavation showed that it was not part of a basement, but a wideopen crack in the late Roman city wall (W1.1). The wall is
here approximately 3.1 m wide, roughly corresponding to
10 Roman feet (pes monetalis). The wall shows a very sophisticated building technique. The space between two accurately built wall shells was filled with crushed stones and
bricks mixed with Roman concrete (opus caementitium)
(Lamprecht, 1987). The remaining height of the wall is
roughly 3.5 m. The upper portion juts about 0.15 m with
respect to the foundation. The photo in Figure 4 shows a
view from the north toward the W1.1 section of the fortification. The wide-open crack has a width of 17.5 (Ⳳ 1.5) cm
as measured at several points on the north-facing side of the
wall. East of this crack, a second crack is visible in Figure
4. This second crack also cuts through the entire height of
the remaining part of the wall. While the first crack is subvertical, the second is inclined about 20 toward the west
with respect to the vertical direction. A detail of the large
crack in inset I of Figure 4 shows the current top row of
building stones that is separated horizontally by about 15 cm.
A measurable vertical displacement along the major crack
is not observed, but the eastern part shifted horizontally toward north by roughly 5 cm (Fig. 4, inset II). Insets III and
IV in Figure 4 show details of the second crack in section
W1.1. Both cracks in section W1.1 are clearly of tensile
character due to strong horizontal forces, which acted mainly
parallel to the trend of the wall. Both cracks not only followed seams, but also broke several stones of the outer wall
shells, as can be seen in the inset IV in Figure 4. Following
Korjenkov and Mazor (1999b), this crack continuity is a sign
of seismically induced damage.
East of section W1.1, a roughly 3.4-m-long section of
the city wall (W1.2) was excavated that is inclined at an
angle of 15 towards south. The waste edge of this section
fits with the edge on the upright part to the west (Fig. 5).
After the bottom of this section was excavated, a shear surface on which the remaining block moved southward by
0.95 m became apparent (Fig. 5, inset II).
Next to the inclined section W1.2, part of the foundation
of a round fortification tower was excavated. A pie-slice with
an opening angle of about 35 remained in situ. The contour
of the tower was surveyed along a joint, and a circle was
The Use of Engineering Seismological Models to Interpret Archaeoseismological Findings in Tolbiacum, Germany
525
Figure 3. Plan of the major features discovered during the archaeological campaigns
in 2001–2003 at the Tolbiacum archaeological
site (Zülpich, Germany). Position of geophones of a refraction seismic profile (RS) and
end points of two electrical tomography profiles (ET1 and ET2) are indicated. Major lateRoman walls and fortification towers are labeled W1, W2, . . . and T1, T2 . . . respectively.
Black thin lines connect the points surveyed in
this study. Dashed lines enclosing gray areas
indicate the approximate original position of
walls and towers. Gray lines are from a plan
by Gechter et al. (1979) indicating the outline
of the thermae. Positions of sampling sites for
14
C-dating are labeled TOLC14 A–C. The inset
shows the current elevation (thin line) along
the profile PP⬘ and the estimated extension of
the trench before it was backfilled (heavy line).
Scale of the map is in meters.
fitted to the measured points in a least-squares sense. The
diameter of the best fitting circle is 8.34 m (about 28 Roman
feet). The tower is inclined in a southeast direction by 22.5
(Fig. 5). On the western side of the tower, the wall connection to the city wall (section W1.2) is still obvious (Fig. 5,
inset IV). The tower and wall were built directly connected,
without a joint. The part of the wall still attached to the
tower—the former inner shell of the wall—is about 50 cm
wide and 1.5 m high. With a wall thickness of 1.5–2 m and
a height of 9 m including the foundation, the gross weight
of the tower was roughly 6 ⳯ 105–8 ⳯ 105 kg.
Several younger walls (probably medieval and modern;
see Horn, 1987) were built more or less parallel and orthogonal to the damaged Roman walls. These walls are labeled
with a circled M in Figures 4, 5, and 6. In some parts, the
Roman foundations and/or remaining parts of the upgoing
walls were reused for these walls. A striking example is the
younger wall sitting directly on the inclined section W1.2
(Fig. 5, inset I). The massive block of the foundation of
tower T1 was in the way during the construction of a
younger wall in front (northwest) of it (Fig. 5, inset III).
Instead of demolishing part of the solid tower foundation,
the younger wall was literally built around the bend of the
tower. While all late Roman walls are of very good quality,
and built with Roman concrete, the younger walls are of
poorer quality and constructed with loam-mortar. Some of
the younger walls are also cracked, but this is probably due
to the poor quality of the construction. The younger wall
between W1.2 and T1, which strikes perpendicular to the
trend of the Roman wall W1.2, contains several spolia obviously taken from the late Roman walls (Fig. 5). Location
and building quality of the younger walls indicate that at
least some of them were intended to make quick repairs to
the damaged late Roman walls.
About 29 m (100 Roman feet) northeast of T1, foundations of a second tower (T2) were excavated by Gechter
et al. (1979), and opened again in the recent excavations.
Survey points along the contour of an approximately 30 pieslice that was accessible during the measurement lead to a
diameter of 8.32 m, almost exactly the same as for T1. The
remaining foundation of T2 is about 0.8 m high and slightly
inclined to the south. About 25 m further, the 1977–1978
excavation revealed the rectangular corner of the assumed
corner tower T3 (Fig. 3).
Two massive wall-fragments (W4.1 and W4.2) were
discovered in 2003 north of tower T2 and east of tower T3
(Fig. 3). Due to the massive size of the walls (up to 2.5 m
in width), it can be assumed that they were also part of the
city fortification works, though their exact purpose is not yet
clear. A free face at the northern end of fragment W4.2 in-
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K.-G. Hinzen
Figure 4.
The main photo (left) shows the north facing side of wall W1.1 (Fig. 3).
The height of the remaining part of the 3.1-m-wide wall is roughly 3.5 m. Strike and
dip of wall fragments are indicated. The major crack on the right cuts through the entire
wall. The crack has an opening width of 17.5 cm. While this crack is subvertical, the
crack on the left with a smaller width (5.0 cm) is inclined about 20 in the vertical
direction. The major crack is shown from a different viewing angle in the right photo.
The inset I shows a detail of the major crack photographed from above. While the
stones on the current top layer were separated about 15 cm horizontally, a building
stone in the second layer was pulled out from the left side and is still connected to the
right side wall of the crack. The tip of the shoe gives a relative scale. Inset II shows
the right crack in a side view. The eastern part of the wall W1.1 is dislocated horizontally toward north by roughly 5 cm. Insets III and IV give close views of the crack on
the left side with an opening of 5 cm. Letters R and M indicate Roman walls and
younger (medieval) walls, respectively. Dates of the photos are given on the bottom.
dicates that the wall was probably part of some kind of entrance building belonging to a smaller court in the corner of
the fortified city (M. Gechter and P. Wagner, personal
comm., 2004). Nevertheless, the damage to these walls is at
least as severe as in sections W1.1 and W1.2 The inclination
of the southern part (W4.1) of the wall could not be surveyed
in situ. Fragments of the upward part of this wall were found
positioned almost horizontally behind the foundation. We
assume that a horizontal crack opened within the wall during
dynamic loading; the top part toppled over toward the southwest, and the bottom part came to rest in the position from
which it was excavated.
The second block (W4.2) (Fig. 6) is larger and measures
roughly 2.4 ⳯ 2.5 ⳯ 2.6 m with a weight of approximately
3 ⳯ 104 kg. This block is inclined 40–43 towards east with
respect to the vertical direction and joints are inclined about
10 downward and north. A soil profile north of the wall
fragment clearly shows the traces of a former trench (Fig. 6,
insets I and II). The west dipping slope of the trench is very
clear and has a dip angle of about 53. The border between
the natural ground and the fillings can be followed on the
excavation floor (Fig. 6, inset II). Clearly, the trench was
backfilled with artificial fill and possibly partially reopened,
creating the slope on which the fallen block rests. On the
foot of this newer trench, slope clay deposits that were
washed into the trench indicate that this trench was open for
some time. The inclined wall fragment is leaning on the
southwest dipping trench wall. The archaeological interpretation of the structure and chronology at this section of the
excavation is continuing.
The Use of Engineering Seismological Models to Interpret Archaeoseismological Findings in Tolbiacum, Germany
527
Figure 5. The central photo shows a view of the Tolbiacum excavation in April
2001 from north-northeast toward wall W1.2 and tower T1. Roman and younger walls
are labeled with a circled R and M, respectively; a younger wall containing spolia from
a Roman wall is marked with a heavily circled M. Details of the horizontally-displaced
wall W1.2 are shown in the insets I and II. The horizontal shear surface and location
of dating sample TOLC14 A are indicated. Insets III and IV give details of the inclined
foundations of the round tower T1. Inset III shows a bird’s-eye view of the tower T1
foundation. A recent drainage tube was placed in a channel, which cuts through the
remains of tower T1. In inset IV the torn part of the inner wall shell of wall W1 still
attached to the tower W1 is clearly visible. Dates of the photos are shown on the edges.
Dating
As the excavation is still in progress and the detailed
archaeological interpretation of all recovered material will
take a few years, we attempted to get an initial estimate of
a time window for the damaging event by dating three charcoal samples with accelerator mass spectrometry (AMS)
carbon-14 (14C) radiocarbon dating. The samples TOLC14
A–C were taken from a charcoal-rich layer located below
the upright part of wall section W1.2 (A), a soil-and-debris
layer beneath one of the younger repair walls south of T2
(B), and from an approximately 5–8-cm-thick occupation
horizon within the repair wall behind W4.2 (C). The locations where the samples were taken are indicated in Figure
3. While sample TOLC14 A gives a terminus ante for the
construction of the Roman fortification of Tolbiacum, both
samples TOLC14 B and C probably give a terminus post for
the destructive event. The samples contained a sufficient
amount of carbon, and results are summarized in Figure 7.
While samples TOLC14 B and C show the same age with
respect to the measuring accuracy (difference ⳱ 20Ⳳ35
years; 0.6 r), sample TOLC14 A is approximately 200 years
older. If the samples TOLC14 B and C really indicate a terminus post, the destructive event probably occurred in the
second half of the fourth century (Fig. 7). So far, the database for dating is very poor, and further interpretation of the
archaeological findings from the ongoing excavation will
help to clarify the time sequence of the events.
Simulation of Site-Specific Strong Motion
Seismograms
Modeling Technique
A deterministic approach was chosen to model possible
seismic loading at the excavation site. The procedure can be
structured into six steps: (1) determination of segments of
active faults in the LRE; (2) assignment of magnitudes to the
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K.-G. Hinzen
Figure 6. The central photo shows the inclined section of wall W4 (viewed roughly
toward north). The wall is 2.5 m wide and inclined by roughly 45. A backfilled ditch
is visible to the right of the inclined wall. Inset I enlarges the contact zone between
wall and fillings. Inset II shows a detail of the border between the natural Tertiary
sediments and the filling in the ditch. In inset III the horizontal continuation of the
ditch is obvious. Inset IV shows a detail of the foundation under the inclined wall
section, and inset V shows a view of the wall from above. Dates of the photos are given
on the bottom.
Figure 7. Results of the AMS radiocarbon dating of three samples (C14 A, B, and
C) from the Tolbiacum archaeological excavation. The location where samples were
taken is shown in Figure 3 and described in the text. The first column gives the radiocarbon age before present with one standard deviation (r). The middle column shows
the probability distribution of calibrated ages (Stuiver et al., 1998), with 1r and 2r
ranges indicated by horizontal bars. The standard deviation values are shown in the
right columns. Measurements were made by the Leibniz Laboratory, Kiel University,
Germany.
The Use of Engineering Seismological Models to Interpret Archaeoseismological Findings in Tolbiacum, Germany
fault segments, based on the relations derived by Wells and
Coppersmith (1994) from a worldwide database; (3) simulation of pseudoacceleration response spectra and accelerograms with the stochastic modeling technique suggested by
Beresnev and Atkinson (1997); (4) calculations of nonlinear
site amplification functions with the computer code
SHAKE91 (Schnabel et al., 1972; Idriss and Sun, 1992); and
(5) modeling of the dynamic behavior of the city wall with
a 2D finite-element model.
In addition to the fault geometry, which is assumed planar and rectangular, the method described in detail by Beresnev and Atkinson (1997), and implemented in the FINSIM Beresnev and Atkinson (1998) code, uses a crustal Q
model and a duration model to simulate propagation effects.
The source area is divided into subsources. Subsource time
series are generated by the procedure of Boore (1983, 2003),
assuming an x2 spectrum. The propagation to the observation point (site) is calculated with duration and attenuation
operators (Boore and Atkinson, 1987). The FINSIM program employs a summation procedure, in which the rupture
propagates radially from the hypocenter, triggering subsources as it passes them. A random component is included in
the subsource trigger times (Beresnev and Atkinson, 1997,
1998). The method was applied successfully to large earthquakes of different character, such as in Central America
(Beresnev and Atkinson, 1997), and in the Cascadia region
(Atkinson, 1995). Castro et al. (2001) showed that the
method might be used also for smaller events down to magnitudes of 6.
Seismotectonic Model
The LRE is part of the Rhine-Rhone Rift system. Figure
1 shows the main tectonic features of the rift system north
of the Alps: the Upper Rhine Graben in the south between
the Vosges mountains and the Black Forest, the Hessian Graben, and the LRE with the Roer Graben. The current period
of tectonic movements in the LRE is closely related to the
late Tertiary Graben structures. The current cycle started
with small but widely distributed displacements along faults
in the late Miocene. During the Pliocene, faulting was more
intense, cumulating in the late Pliocene and early Pleistocene. Considerable synsedimentary and intersedimentary
crustal displacements happened in the Quaternary, when the
Older and Younger Main Terraces of the Rhine and Meuse
rivers were accumulated (Ahorner, 1962). The young geological structural activity in the LRE is characterized by regional flexures, tilted blocks, basin-like subsidence, and by
numerous normal faulting movements on mostly verticallyoriented faults. The main faults, with a cumulative length of
roughly 400 km, are generally directed in a northwest–southeast strike. In the Quaternary, some faults show a maximum
vertical movement of 174 m. The structural geological features are expressions of a regional tension of the crust in a
northeast–southwest direction. Stress inversions from fault
plane solutions of 110 earthquakes from the past 25 years
529
confirm this orientation of the stress tensor (Hinzen, 2003).
The amount of total extension in the Quaternary is estimated
to 90–180 m (Ahorner, 1962). Furthermore, structural features of the whole northern Rhine area indicate an uplift and
tilting of the Westdeutsche Großscholle towards the northwest. Uplift in the Eifel mountain region, south of the LRE,
exceeds 1 mm/year in the central part (Meyer and Stets,
2002). The Upper Rhine Graben and the LRE show moderate
seismicity in present and historic times, while the Hessian
Graben is mostly aseismic.
A detailed map of Quaternary active faults in the LRE
was presented by Ahorner (1962). This map was digitized
and some of the faults were extended toward the northwest
beyond the map’s border, based on a map by Geluk et al.
(1994). Linear segments were fitted to the prominent fault
lines and separated into west and east dipping faults. Figure
8 shows the result of the segmentation. The Tolbiacum site
is indicated in the southwestern corner of the LRE. Each
linear segment is regarded in the following as the surface
projection of a possible fault plane.
The segment length determined from the digital map
was assumed to be equal to the surface rupture length (SRL),
and the relation
Mw ⳱ 5.08 Ⳮ 1.16 ⳯ log10(SRL)
(1)
from Wells and Coppersmith (1994) was used to calculate a
yield magnitude for the FINSIM simulation procedure. The
down-dip rupture width of the fault surface (WID) was derived from:
WID ⳱ 10(ⳮ1.01Ⳮ0.32⳯Mw) (Wells and Coppersmith, 1994).
(2)
The seismogenic zone extends from about 5 km to 22 km in
the LRE (Hinzen, 2003). Rupture was assumed to originate
in the deeper part of the seismogenic zone. A total of 12
west-dipping (labeled w1 to w12) and 6 east-dipping (labeled e1 to e6) fault planes were considered for the deterministic determination of the seismic loading functions
(Fig. 8, Table 1). The unlikely event of fracturing of the
whole Peelrand fault (w10) or the Viersen fault (w1), of 55
and 80 km length, respectively, also were considered.
Q and Duration Model
No comprehensive crustal Q model is available for the
complete LRE. El Bouch et al. (2002), and Goutbeek et al.
(2003) recently did studies for Belgium and the southern
Limburg region (The Netherlands), respectively. In the following, the Q model from Goutbeek et al. (2003), with a
frequency dependent Q, is used:
Q ⳱ Q0 • f g with Q0 ⳱ 190; g ⳱ 0.9 .
(3)
The model implemented in the FINSIM code uses a
530
K.-G. Hinzen
Figure 8. Map of active quaternary faults
in the LRE. Fault trend is based on maps by
Ahorner (1962) and Geluk et al. (1994). Heavy
lines indicate linear segments fitted to the trend
of the main fault systems. The Tolbiacum site
is indicated by a star.
Table 1
Parameters of 18 Assumed Earthquakes at Active Faults (Fig. 8) in the LRE
Start Point
Code
Fault Name
Length
(km)
w1
w2
w3
w4
w5
w6
w7
w8
w9
w10
w11
w12
e1
e2
e3
e4
e5
e6
Viersener
Swift Sprung
Erft-Horremer
w2 & w3
Lövenicher
Rurrand (north)
Rurrand (middle 1)
Rurrand (middle 2)
Peelrand (south)
Peelrend
Rurrand
Kirspenicher
Sandgewand
Feldbiss
Herlenheider
Heinsberg-Montfort
Stockheimer
Birgel
55.3
23.1
29.8
47.4
25.4
19.2
23.1
36.7
38.1
79.6
58.0
16.4
21.8
48.6
53.6
32.0
18.4
16.3
Distance
Width
(km)
Strike
NE
Lat. N
Lon. E
18.3
13.2
14.5
17.3
13.7
12.4
13.2
15.7
15.9
21.0
18.6
11.7
12.9
17.4
18.1
14.9
12.2
11.6
142.8
142.1
139.2
144.6
102.4
147.5
141.3
140.0
145.5
146.5
138.3
128.5
329.5
315.7
321.5
296.6
336.2
314.9
51.2
50.6
50.8
50.6
51.0
50.9
50.8
50.8
51.8
51.1
50.7
50.6
50.9
51.1
51.2
51.2
50.9
50.8
6.6
7.0
7.0
7.1
6.6
6.4
6.6
6.6
6.2
6.2
6.8
6.8
6.2
5.7
5.6
5.6
6.5
6.3
Mw
Number of
Subsources
Minimum
(km)
Maximum
(km)
7.1
6.7
6.8
7.0
6.7
6.6
6.7
6.9
6.9
7.3
7.1
6.5
6.6
7.0
7.1
6.8
6.5
6.5
593
120
151
373
137
64
120
275
304
1091
624
57
105
395
560
166
64
57
52.4
18.3
21.7
19.7
35.8
31.3
12.3
13.7
56.0
55.8
8.3
6.5
27.5
38.7
44.8
66.4
8.4
14.3
103.4
31.4
34.9
36.8
52.1
49.6
35.3
49.8
92.6
134.7
52.3
20.0
47.2
85.3
94.7
94.0
25.4
30.3
Distances are maximum and minimum direct distances between the subfaults simulated with the FINSIM code (Beresnev and Atkinson, 1998) and the
Tolbiacum site.
The Use of Engineering Seismological Models to Interpret Archaeoseismological Findings in Tolbiacum, Germany
531
subsource-radiation duration model, incorporating a linear
increase of duration with distance, with slopes depending on
the distance range. A total of 257 digital seismograms from
earthquakes in the northern Rhine area that occurred between 1997 and 2002 (Reamer and Hinzen, 2004) and recorded with the BENS network (Cologne University), were
used to derive a duration model. Magnitudes of these events
range from 2.9 to 4.4. The duration model by Atkinson
(1995) represents the duration of the shear wave by:
T ⳱ T0 Ⳮ Td (R) ,
(4)
where T0 is the source duration and Td (R) describes the
increase of duration with distance due to dispersion and scattering effects. For the relatively small events of this study
(i.e., with magnitudes less than 4.5), the T0 are small and
errors have only minor influence on values of T. Source duration was estimated following the Brune model. The duration T is plotted in Figure 9 versus the hypocentral distance.
The data were binned in distance ranges with a width of 0.1
in logarithmic units, and the median of the duration term
was calculated for each bin (Fig. 9). The median values of
the duration show a linear increase with distance in sections.
The minimum duration introduced by Atkinson (1995) was
0 sec in the duration model for the northern Rhine area. In
the distance range from 10 to 20 km, the slope of the fitted
duration curve is 0.88. For the second section, from 25 to
140 km, the slope is 0.12. At larger distances (up to 250 km)
the data are sparse. The slope of the fitted line increases
slightly to 0.17.
Site-Specific Ground Amplification
The site is located at the western edge of the LRE. The
sedimentary cover of the Jurassic and Triassic bedrock is
mainly made up of Tertiary deposits. The uppermost sedimentary layers are Pliocene silicic oolith layers, fine sand,
and clay. Some clay layers contain lignite seams, which were
mined in the vicinity of the site (Fig. 2). A thin cover of
sandy gravel of the Rhine Main Terrace is deposited on top
of the clay layers. Uppermost layers under the present day
walking horizon are Medieval and Roman fillings. The Roman walking horizon is at about 180 m NN (Normal Null).
Groundwater level in spring 1988, a year of generally high
water table in the LRE, was at about 170 m NN, roughly 10
m below the constructions. A borehole next to the wall segment W1.2 showed some strata water at about ca. 3 m in
depth. Below the thermae, a layer of 1.0–2.8 m of the Rhine
Main Terrace is made up of silt and silty sand and gravel.
These are underlain by fine sands and clay layers. The consolidated fraction is approximately 65%, and water content
is between 15% and 18% (M. Schalkowski, unpublished
manuscript, 2002).
The seismic site effects of the sedimentary layers at the
Tolbiacum site were determined with a 1D model for vertically propagating S waves. According to the geological
Figure 9. Duration model for the calculation of
strong ground motion with the FINSIM code (Beresnev and Atkinson, 1998). The gray circles give the
duration of the S phase as measured from 257 seismograms of earthquakes with magnitudes 2.9–4.4
that occurred between 1995 and 2002 in the Northern
Rhine area. Open circles are the average duration values calculated for bins of 0.1 width in logarithmic
distance units, with error bars indicating one standard
deviation. The lines indicate the slope of the distance
function for three distance ranges, fitted to the binned
values in a least-squares sense.
map of the area (Schröder, 1979), the top of the base rock
is at a depth of 490 m. Shear-wave velocities of the Tertiary
sediments were taken from a downhole measurement that
was made in a borehole about 4 km east-northeast from the
site in the depth range from 15 to 160 m (Budny, 1984).
Velocities between the bottom of the borehole depth and the
base rock (160–490 m) were assumed to increase from 470
to 600 m/sec (Ohrnberger et al., 2004). The velocities in the
uppermost 15 m were determined from a refraction seismic
experiment carried out at the site (Fig. 3). The depth distribution of the shear-wave velocity is shown in Figure 10. Unit
weight was assumed to increase slightly from 18.6 to 20.5
kN/m3 between the surface and the bedrock. Nonlinear material properties of clay and sand were taken from Seed and
Idriss (1970), Seed and Sun (1989), and Idriss (1990), and
assigned to the layers as identified in the borehole. The normalized shear modulus G/Gmax and damping ratio are shown
as a function of shear strain in Figure 10 for sand and clay.
The program SHAKE91 (Idriss and Sun, 1992) was used to
calculate site amplification functions individually for each
simulated strong motion seismogram from the FINSIM calculations. The amplification functions were calculated for
the surface as an outcropping layer, and for the top of the
third layer as an internal amplification function. The latter
532
K.-G. Hinzen
Figure 10. The left diagram shows the depth distribution of the S-wave velocity
derived from refraction seismic experiments on the site and a nearby downhole measurement (Budny, 1984). The two diagrams on the right show the dependency of the
shear modulus, G, and the damping ratio on the shear strain for clay and sand after
Seed and Idriss (1970), Seed and Sun (1989), and Idriss (1990).
was used to calculate the excitation time series for the base
of the finite element model used in the last modeling step.
Maximum amplification reaches a factor of about 3.5 at frequencies around 2 and 5 Hz; above 8 Hz, the amplification
is smaller than 1.0 for most events.
Verification of Model Parameters
In order to verify the FINSIM model parameters, in particular the Q and duration models, recordings of a recent
event (22 July 2002) with a local magnitude of 4.9 were
used. The normal faulting earthquake occurred close to the
city of Alsdorf, 14 km northeast of Aachen (54 km west of
Cologne). The event was modeled with a fault plane of 2 ⳯
2 km and a hypocenter depth of 14 km. fmax was varied
between 10 Hz and infinity (). Figure 11 shows the acceleration seismograms calculated from velocity proportional
recordings at station KLL at an epicentral distance of 26 km.
The station site was modeled as a hard rock location. The
peak ground acceleration (PGA) is 4.1 and 5.7 cm/sec2 in the
north and east components, respectively. The simulated seismogram, with fmax ⳱ 20 Hz, shows a PGA of 5.2 cm/sec2.
While the S/Lg phase is more pronounced in the measured
seismograms, frequency content and duration of the measured traces agree fairly well with the simulated ground acceleration (Fig. 11).
Results of FINSIM Calculations
Some of the modeling parameters for the FINSIM calculations, were used for all deterministically modeled
events. Fault dip was assumed to be 80. Test calculations
with less steep faults (down to 65) did not significantly alter
the engineering seismological parameters of the results. The
stress parameter was assumed to be 50 bars, and a geometric
spreading of 1/R was used. The factor controlling the subsource strength was set to 1, because there seemed no reason
to assume unusually fast or slow earthquakes for the area.
A Saragoni-Hart windowing function for the subsource time
histories was applied; crustal shear-wave velocity and density in the source region was 3.7 km/sec and 2.8 Mg/m3,
respectively. An fmax of 20 Hz was used, as the near surface
site effects were simulated in a separate modeling step. Measures of ground motion dominated by frequencies significantly less than fmax are not sensitive to fmax (Boore, 1983).
As the aim of the modeling was not to match measured seismograms, the slip was assumed to be randomly normally
distributed. Table 1 lists geometric parameters, which
changed for the individual events, and Figure 12 shows examples of seismograms simulated for the 18 fault segments.
The bubble plot in Figure 13 shows the PGA in relation to
magnitude and distance. Distance here is measured between
the site and the center of the closest subfault. Values are
indicated for both the PGA at the surface and the PGA at a
The Use of Engineering Seismological Models to Interpret Archaeoseismological Findings in Tolbiacum, Germany
533
Figure 11.
The top two traces show horizontal acceleration seismograms calculated
from velocity proportional records at station
KLL during the 22 July 2002 Alsdorf earthquake (local magnitude 4.9). The bottom trace
gives the result of a FINSIM (Beresnev and
Atkinson, 1998) simulation of the Alsdorf
event for a hard rock site at 26 km distance.
Values at the end of the traces indicate the
PGA.
15 m depth, which is used as the excitation acceleration time
series in the finite element model. The largest PGAs at the
site result from three events, with magnitudes between 6.5
and 7.1 at distances less than 20 km.
Figure 14 shows the pseudoacceleration response spectra for the 18 earthquakes considered in the deterministic
approach to evaluate the possible ground movement at the
Tolbiacum site. The shape of the response spectra is influenced by the earthquake size, distance, and the amplitudedependent site amplification. In general, the spectra show a
plateau starting at 0.1–0.2 sec to 0.3–0.7 sec. In addition to
the calculated response spectra, a spectrum from the new
proposed German building code is shown (Brüstle et al.,
1999). This spectrum represents the effects of an earthquake
with a 475-year return period, in the highest hazard level
zone in Germany, at a site located in a sedimentary basin
structure. This scenario conforms to conditions at the Tolbiacum site. For comparison with the norm spectrum, the
median of the 18 spectra from the simulations is shown.
The horizontal surface ground motion simulated for
earthquake w12 from Table 1 is shown in detail in Figure
15, together with the velocity and displacement. The corresponding pseudoresponse spectrum, calculated for 5% of the
critical damping, shows peaks at 0.26 sec (3.9 Hz) and approximately 0.6 sec (1.7 Hz), with response amplitudes
above 0.5 g.
Dynamic Behavior of the City Wall
A 2D finite element model (Fig. 16) with 456 elements
was used to examine the behavior of a stiff block, representing the tower of the Roman fortification with dimensions
of 8 ⳯ 9 m, located next to a trench with slopes dipping 45
and a depth of 5 m, as suggested by the north–south trending
elevation profile (PP⬘) in the survey plan (Fig. 3). The
block’s foundation reaches 1 m below the free surface. The
uppermost 15 m of the sedimentary package were assumed
to be homogeneous. Young’s modulus and Poisson’s ratio
of this layer were estimated from the measured P- and Swave velocities, from the on-site measurements of 550 m/
sec and 220 m/sec to 2500 kPa and 0.38, respectively. The
unit weight was assumed to be 18.6 kN/m3, corresponding
to a density of 1.9 Mg/m3. The water table was set at 10 m
below the surface at the site.
The accelerograms of all 18 events simulated with FINSIM were used to calculate the dynamic response of the 2D
model. Vertical acceleration was scaled to 50% of the horizontal movements and assumed to act simultaneously with
the horizontal movements. In addition to the FE model, Figure 16 shows six time snapshots of the displacement during
excitation of the model with the accelerogram from earthquake w2. This event shows a PGA of 0.09 g at the base of
the FE model. Maximum horizontal and vertical displacements of the node at the upper right corner of the tower are
1.5 cm and 1.1 cm, respectively. The maximum displacement at the bottom right corner of the tower is essentially
the same in the vertical direction, due to the stiffness of the
building; the maximum horizontal displacement is smaller
and not in phase with the movement of the top. The resulting
tilt of the tower is obvious in the time snapshots in Figure
16. The soil structure system shows an eigenperiod of about
3.0 sec. The movement pattern with the back-and-forth
swinging tower is similar for all 18 earthquakes. Only amplitude and duration of the tower’s movement vary with the
strength and distance of the events.
Damage Scenario
Erosion and deterioration can be excluded a priori as
possible causes of the damage to the late Roman fortifica-
534
K.-G. Hinzen
Figure 12.
Simulated strong motion seismograms
of the horizontal acceleration at the Tolbiacum site
for 18 earthquakes at active faults in the LRE. Labels
at the beginning of the accelerograms indicate the
events as listed in Table 1. Vertical scale is identical
for all seismograms.
tions in Tolbiacum. These factors would have emerged during the archaeological excavation. Sudden strong exogenous
forces must have cracked and dislocated walls with a width
up to 3.1 m of good building quality. Rapp (1986) and Nikonov (1988) list three main groups of possible damage
sources that should alternatively be considered in a complete
archaeoseismological study: (1) slow-acting natural destructive actions, (2) fast-acting natural processes, and (3) actions
brought about by man. The appearance of the damage in
Tolbiacum excludes slow-acting forces. The large horizontal
displacements and total absence of any vertical movement
of wall W1.1 contradicts static or quasi-static settlements.
The topographic and orographic situation of Tolbiacum
excludes any running water or flooding as a possible threat
to the fortification walls and towers. The elevated position
on top of the Mühlenberg and the lack of streams (Fig. 2)
in the vicinity rules out the influence of running water.
Manmade damage is very unlikely. Digging off the sand
underneath tower T1 during a siege of Tolbiacum would
have been part of a suicide attack and not easy to facilitate.
Specifically, the extent of damage on wall W4, in addition
to W1.1, W1.2, and T1, precludes human activity as a cause.
Several burnt layers are found in different horizons of the
Tolbiacum excavation, but the archaeological finds so far do
not indicate actions of war.
The most probable cause of the described damage in
Tolbiacum is seismic shaking. It is assumed that the damage
west of tower T1 occurred when the shaking produced a
failure of the trench slope at the location of the tower. The
massive tower T1, with a weight of 600–800 metric tons,
leaned toward the south because of the reduced strength of
the subsoil on the trench side. This movement of the tower
induced large tensile forces in the directly connected wall
W1. A section of the wall about 1.6–2.2 m in length directly
west of T1 fell over. This site is where the younger (posibly
repair) wall was built. The following western section, W1.2,
came to a rest at a southward inclination of 22.5, where the
foundation is still found in situ. The section W1.1 was practically pulled apart and broken at two major fractures, displacing the wall fragments by 5–17.5 cm horizontally. The
section W1.1, between the large cracks (Fig. 4), was turned
slightly clockwise, producing the 5-cm lateral offset at the
wide open crack. The sketch in Figure 17 illustrates the
above proposed damage scenario (not intended to be an archaeological reconstruction).
The top part of section W4.1 fell off and came to rest
to the south. Section W4.2 was sheared off from the foundation and slid northward into the trench on top of a clay
and silt layer.
The European Macroseismic Scale (EMS) (Grünthal,
1998) classifies buildings into six vulnerability classes (A to
F) and damage into five grades (1 to 5). The stones of the
outer wall shells of the fortification are manufactured stones,
which are well cemented. The gap in between the shells is
filled with a mixture of crushed stones and opus caementitium. The construction quality is high, and the walls have a
very low level of irregularity. Walls and towers of the fortification are considered as vulnerability class B. “Heavy
structural damage and serious failure of walls” is classified
as grade 4, and grade 5 is “very heavy structural damage,
total or near total collapse.” The observed damage is grade
4–5. As often is the case in archaeoseismological studies,
the percentage of damaged buildings versus undamaged
structures can no longer be accurately assessed. However in
The Use of Engineering Seismological Models to Interpret Archaeoseismological Findings in Tolbiacum, Germany
535
liest earthquake in 813 A.D. (Leydecker, 2003), does not list
any maximum intensities in Germany above VIII for tectonic
events. Locations of earthquakes with maximum intensity
above VII are shown in Figure 1 (Hinzen and Oemisch,
2001; Leydecker, 2003).
Conclusions
Figure 13. Bubble plot of the PGA from simulated
accelerograms at the Tolbiacum site for 18 earthquakes in the LRE (Table 1). The circle radius increases linearly with the PGA; the scale for 0.1 and
0.2 g is indicated. The circles represent the PGA at
the surface, while the gray discs indicate the PGA at
a depth of 15 m.
Figure 14. Pseudoacceleration response spectra
for the Tolbiacum site calculated with the FINSIM
code for 18 model earthquakes in the LRE (Beresnev
and Atkinson, 1998). The dashed line is the median
of the 18 spectra, and the heavy line is a norm spectrum from the German building code (DIN4149 new,
2003) for the site, which is located in a sedimentary
basin.
Tolbiacum, heavy damage is observed at least at two locations separated roughly 60 m; thus we do not have a case of
isolated damage, and the influence of the subsoil should not
be considered in the process of intensity determination for a
specific site. Therefore, the intensity at Tolbiacum is assessed as IX. The German earthquake catalog, with the ear-
Foundations and parts of the upgoing walls of late Roman fortification works in vicus Tolbiacum (in the present
day city of Zülpich, Germany) were recently excavated by
the Rheinische Amt für Bodendenkmalpflege. Walls with
widths up to 3.1 m, and round fortification towers with diameters of 8.34 m, show severe structural damage. Wide
open tensile fractures, absence of vertical static settlement,
rotation of walls, inclination of wall fragments, and foundations of towers tilted more than 20 with respect to the
vertical are strong evidence for a seismogenic origin of the
damage (Korjenkov and Mazor, 1999a,b). Indications of
manmade destruction to the fortification have not been found
(P. Wagner, personal comm., 2003). Due to the orography
of the terrain in the vicinity of the site, running water can
be excluded as cause of the observed damage, and groundwater level is about 10 m below the site.
In order to test the hypothesis of a seismogenic origin
of the damage, deterministic evaluation of ground motion at
the Tolbiacum site from earthquakes along the major active
faults of the LRE with the Roer Graben have been made.
Strong ground motion seismograms were calculated with the
FINSIM code (Beresnev and Atkinson, 1997, 1998). Results
from a refraction seismic experiment and published downhole measurement data were used to quantify the distribution
of S-wave velocities in a 1D model. Nonlinear site effects
were calculated with SHAKE91 (Schnabel et al., 1972; Idriss and Sun, 1992). The vibration behavior of the wall-soil
system was simulated in a 2D-FE model. The tower of the
fortification shows a pronounced fundamental eigenperiod
of roughly 3 sec in the direction orthogonal to the trend of
the wall.
Most of the modeled events cause maximum horizontal
displacements at the top of the tower between 1.0 and
3.0 cm. Significantly larger displacements (in the range of
10 cm) come from earthquakes with code e5, w8, w11, and
w12 (Table 1). Among these earthquakes, c5 and w12 have
the smallest magnitudes of 6.5 (and therefore a higher probability of occurrence), combined with the smallest minimum
distances between subsources on the fault plane and the Tolbiacum site of 8.4 and 6.5 km, respectively. The exact damage threshold of a late-Roman fortification wall or tower is
hard to quantify (Augusti and Sinopoli, 1992). However,
maximum horizontal and vertical displacements of 12 cm
and 6 cm, respectively, as predicted for earthquake w12,
would probably have damaged the walls heavily. These two
earthquakes, e5 and w12, belong to the so-called Stockheimer and the Kirspenicher faults, respectively, alongside the
descent from the Eifel mountains to the Erft-Scholle and the
536
K.-G. Hinzen
Figure 15. Example of the ground motion modeling for the event w12 (Table 1).
The top trace shows the accelerogram of the horizontal ground movement at the surface
of the Tolbiacum site. The diagram on bottom left is the pseudoacceleration response
spectrum calculated for a damping of 5% of the critical value. The two bottom right
seismograms show the velocity and displacement derived from the accelerogram by
one and two times integration, respectively. The additional dashed seismogram in the
bottom right graph is the horizontal displacement of the top south corner of the tower
T1 from the FE model (Fig. 16).
Roer Graben. The Stockenheimer fault shows movements in
the middle and young Pleistocene. The macroseismic epicenter of the ML 6.4 earthquake of 1756, the strongest historical earthquake in Germany, is close to the Stockenheimer
fault (Hinzen and Oemisch, 2001). This earthquake was the
culmination point of a series of 71 events between 1755 and
1756 with macroseismic magnitudes above 3.0 (Meidow,
1995), of which five had macroseismic magnitudes above
5.0—a clear sign for the continuing seismic activity of the
western border faults of the LRE. The Kirspenicher fault has
a vertical Tertiary offset of 100 to 150 m, mainly displacing
layers of old Tertiary and Pliocene lignite formations (Ahorner, 1962). Quaternary dislocation ranges from 10 to 15 m
in the Satzvey area and 8 m at Enzen, to about 3 m southwest
of Zülpich. Earthquakes west of Euskirchen in 1950, 1951,
and 1957 with local magnitude up to 5.1 (Hinzen and Oemisch, 2001) are an indication for ongoing seismic activity of
this fault. For similar situations at the Breé Fault, Belgium,
Ebel et al. (2000) showed that at least part of present-day
seismicity might be an expression of aftershock activity of
past large earthquakes.
Simulations of strong ground motions at the Tolbiacum
site from this study show that an earthquake with moment
magnitude of 6.5 at one of the close active faults would be
sufficient to explain seismogenic damage at the archaeolog-
ical site. Frequency-moment statistics calculated on the basis
of recent and historic seismicity for the whole LRE suggest
an average recurrence interval of a Mw 6.5 earthquake of
1500 years (Ahorner, 2001). However, paleoseismic investigations along the western border of the Roer Graben
(Camelbeeck and Meghraoui, 1998; Vanneste et al., 1999;
Camelbeeck et al., 2000) suggest recurrence intervals for
large surface-rupturing earthquakes at the Breé section of
the Feldbiss fault, of 12 Ⳳ 5 ka during the last 50 ka. This
recurrence interval is a factor 4–5 smaller than that predicted
by the Gutenberg-Richter relation for the whole LRE based
on instrumental and historic earthquakes. Earthquakes with
larger distances to the site (i.e., w11) would have to be significantly larger to produce ground motion of comparable
size (M 7). These events are in the range of the proposed
maximum magnitude assumed for the LRE with a very low
probability (Ahorner, 2001; Schmedes et al., 2004).
Assuming the observed intensity IX is the maximum
intensity of the damaging earthquake, empirical relations
suggested by Johnston (1996) for stable continental regions
can be used to estimate the moment magnitude. Johnston’s
model predicts Mw ⳱ 6.4 Ⳳ 0.53, which is fully compatible
with the modeled events e5 and w12.
With the current status of the Tolbiacum excavation in
Zülpich, Germany, seismic loading from a strong (magni-
The Use of Engineering Seismological Models to Interpret Archaeoseismological Findings in Tolbiacum, Germany
537
Figure 17. The drawing summarizes the proposed
damage scenario for the southern part of the Tolbiacum fortification. The tower (T1) moved and inclined towards the trench, pulling parts of the wall
with it. (The sketch is not intended to be an archaeological reconstruction.)
Acknowledgments
Figure 16.
The top graph (Model) is a 2D finite
element model to study the dynamic behavior of an
8-m-tall tower (dark gray elements) next to a trench
founded in sandy soil (light gray elements). The trend
of the model is roughly parallel to the profile PP⬘
shown in Figure 3. The following six graphs are deformation snapshots during the excitation of the
model, with a strong motion record generated for the
w2 model earthquake (Table 1). Vertical movement
was assumed to be half the size of the horizontal
movement. Model and displacement scales are given
in the upper left corner. The deformation is enlarged
by 20 times with respect to the model dimension.
Numbers on the right give the time of snapshot.
The surveying of the site was done by C. Fleischer, and fieldwork
was assisted by J. Mackedanz, B. Weber, J. Rolshoven, and K. Weber. The
latter also helped with some model calculations and the preparation of figures. Seismic and geoelectric profiles were measured by H. Krummel and
M. Janik. The work presented in this paper benefited greatly from the close
and fruitful cooperation of the Landesamt für Bodendenkmalpflege. In particular, P. Wagner was of great help. Archaeological aspects were also
discussed with M. Gechter, J. Weiner, and S. Schütte. F. Scherbaum, A.
Korjenkov, and P. Wagner gave very helpful comments on the first draft
of the manuscript. I thank R. Castro and F. Galadini for reviewing the
manuscript. Their comments greatly improved the paper. Special thanks
goes to S. K. Reamer for many discussions, important comments, and carefully reading the manuscript.
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Department of Earthquake Geology
Geological Institute, University of Cologue
Vinzenz-Pallotti-Str. 26
D-51429 Bergisch Gladbach, Germany
[email protected]
Manuscript received 8 April 2004.