Geophys. J. R.astr. SOC.(1966) 10, 539-548.
Seismic Studies of the Earth’s Crust in Continents
II: Analysis of Wave Propagation in Continents and Adjacent
Shelf Areas*
Mark Landisman and Stephan Muellert
(Received 1965 May 10)
1. Introduction
New evidence has been presented in Part I (Mueller & Landisman 1965) for
the existence of a low-velocity zone overlying an abrupt increase of velocity in the
uppermost non-sedimentary region of the Earth’s crust, often called the sialic layer.
This distribution of velocities was shown to be capable of accounting for large
second arrivals, termed P,, which closely follow the first refracted arrival, P,, for
seismic refraction observations approximately 50-1 50 km from the shotpoint.
Further, it was shown that the abrupt increase in velocity below the low-velocity
zone also manifests itself in strong normal-incidence crustal reflections.
In this paper, an analysis of wave propagation in continents and adjacent
shelf areas is presented, which demonstrates how the postulated low-velocity zone
and related concepts can bring the results of seismic crustal refraction and reflection
measurements into better agreement. As an illustrative example, the present study
will re-examine the well-known Haslach profile, which runs S.E. from the Black
Forest into the German foreland of the Alps (e.g. Byerly 1956). The adopted
method of solution will be briefly described, and a comparison of refraction and
reflection data will be presented. Future measurements will doubtless modify
some details of the proposed explanation, but most of the features determined seem
to have at least regional significance.
Earthquake focal depths in several moderately stable regions will then be related
to the sialic low-velocity zone, and it is confirmed that the inclusion of this zore in
crustal models can be used to explain the propagation of the guided waves P and
L,. Finally, some implications for future seismological investigations of the Earth’s
crust will be discussed.
2. Haslach refraction profile
Two sets of compressional arrivals through the sialic crustal layer of S.W.
Germany were first analysed by RothC & Peterschmitt (1950) and Foertsch (1951)
in their studies of the 1948 Haslach explosion. In an attempt to explain the two
groups of onsets, these authors deduced the presence of two different materials.
An apparent velocity of 5-63-5.88 km/s was associated with a layer of 2.4-6.0 km
thickness, classified as granitic by Foertsch (1951). The granitic layer was considered
* Southwest Center for Advanced Studies, Geosciences Division, Contribution No. 16.
t On leave from Technische Hochschule Karlsruhe, Germany.
539
540
Mark L.ndisanao and Stephan M u e k
to be separated from a layer of diorite associated with a velocity of approximately
6-0 km/s. The interface between these two layers was subsequently called the
Foertsch (F) discontinuity by Reich (1957), Schulz (1957), Dohr (1957, 1959),
Bederke (1957) and Liebscher (1962). Schulz (1957) first associated deep reflections
at about 4 s with the F discontinuity, corresponding to a depth of approximately
10 km. This constituted a serious, previously unresolved discrepancy, since the
refraction interpretationshad originally placed the F discontinuity at much shallower
depths. The present study confirms that these reflections from depths near 10 km
are important evidence of the abrupt velocity increase at the bottom of the sialic
low-velocity zone, a d that this concept provides a straightforward means for
bringing the two sets of observations into better accord.
0
20
40
60
80
100
120
140
160
180
200
220
A(kd
FIG. la. Reinterpretation of Haslach refraction profile. Reduced times
t-A/6(s) oersus distance A(km). Later arrivals, P,, reflected and refracted
from abrupt velocity increase below the sialic low-velocity zone. For discussion see text.
A reinterpretation of the Haslach profile appears in Fig. la. The results of
theoretical calculations are superimposed upon the same seismogrps which were
first presented by Reich, Schulze & Foertsch in 1948. The records have been
mounted at the appropriate distances (A) from the shotpoint. A reduced time scale,
(t -A/6), has been used in order to magnify the resolution for signals arriving with
velocities near 6.0 km/s.
Two short travel time segments near the origin account for the first arrivals
observed at the few stations closer than 20 km from the shotpoint. The velocities
associated with these lines are meant to correspond to the weathered top of the
outcropping crystalline basement, in agreement with the results of RothC &
Peterschmitt (1950) for the uppermost layer.
At greater distances, the first amvals are correlated as the P, line which can be
traced to about 120km, where they become too small to be observed (see also
e.g. Giese 1963, Pakiser 1963, Prodehl 1964a). This line fixes the velocities down to
depths of about 5 km (Giese 1963). Strong second arrivals first appear at about
60km from the shotpoint and are delayed by approximately 0 7 s relative to P,.
These importht arrivals have never before been properly related to the deep
crustal reflection data. Following the notation introduced in Part I, they are termed
Seismic studies of the Eprth’s crust in contineofs
541
P,. The wide-angle reflections from the bottom of the low-velocity channel form
the retrograde branch of P,, which comes in from a distance of nearly 230 km.
This branch is indicated by a dashed line. The other lines in Fig. l a correspond to
refracted arrivals from deeper layers in the crust. Those retrograde branches which
account for reflections from the intervening horizons of velocity increase, have been
omitted from the figure for the sake of clarity.
3. Velocity-depth relation for the Haslacb profile
The lines in Fig. l a @re the result of theoretical calculations for a best fitting
velocity-depth model, for which, of course, no claims of uniqueness are made.
The merit of the solution presented in Fig. l b is its ability to account for many
more details of the observations than has hitherto been possible.
FIG. lb. Haslach crustal model with sialic low-velocity channel overlying
abrupt velocity increase. Compressional velocity, V@/s) uersus depth,
z(km). Weathered layers, crystalline basement, low-velocity channel and
deeper crustal layers. MohoroviU discontinuity at base of crust. For
discussion see text.
The velocity-depth relation in Fig. l b is primarily the result of the signal
correlations-shown in Fig. la. The measured apparent velocities and intercept
times for each of the correlated lines then determine the velocities and thicknesses
of the corresponding crustal layers. The normal-incidence crustal reflection data,
to be discussed below, in combination with tbese refraction data require the insertion of a low-velocity layer in the upper part of the crust, with an abrupt increase
of velocity at about 10 km for its lower boundary.
The delay between Paand P,, introduced by the sialic low-velocity zone, can be
reproduced by a variety of velocity distributions within the low-velocity region.
It is quite difficult to distinguish between these various possibilities using only
’
542
Mark Landisman and Stephan Muellex
refraction data, since no ray path can bottom within the low-velocity region. The
velocity distribution adopted is a simple approximation to the values likely to be
found, using auxiliary seismic information as a guide.
The PB velocities measured for earthquakes and explosions, 5 5 and 6.0 km/s
respectively (Wiechert 1926, Gutenberg 1950, Steinhart & Meyer 1961), exhibit a
disparity which far exceeds the experimental error for either type of measurement.
As discussed in Part I, the value of 5.5 km/s determined from shallow crustal
shocks is believed to be a reasonable average channel velocity.
An abrupt increase in velocity, from about 5.5 to 6.0 km/s at a depth of about
10 km, produces strong normal-incidence reflections and the outgoing P, refraction
line. The higher velocity, characteristically 0.2 km/s greater than that just above the
channel, combined with the abrupt velocity increase below the channel, is required
if the calculated point of reversal for P, is to come #asclose to the shotpoint as is
observed.
It has been found necessary in the Haslach profile to divide the lower crust into
several layers in order to account for the more important arrivals seen on the
records.
Just below the low-velocity zone there are two first-order velocity increases at
depths of about 15 km and 21 km, associated with mean velocities of about 6.2
and 6-5km/s. Below these layers, and just above the MohoroviEiC discontinuity, is a
zone with a velocity of about 7.2 km/s, corresponding to the deepest crustal layer
found from near earthquakes (Peterschmitt, personal communication). It has also
been detected by explosion studies in the eastern Alps (e.g. Prodehl 1964b, Behnke
1964) and in many other areas.
The calculation used here produces the times, distances and amplitudes
associated with ray propagation through an arbitrary number of spherical shells,
each having its own velocity gradient (Landisman, Sat6 & Usami 1965). Velocity
gradients have been introduced into some of the layers in order to satisfy the
amplitudes recorded on the seismograms. The effects of these gradients can be
seen in the termination points of the refraction lines in Fig. la. In general, a stronger
gradient will cause the corresponding outgoing branch to end sooner and to have
larger amplitudes. More closely spaced observations would have permitted better
control of the resulting velocity distribution.
The velocity distribution above and just below each interface determines the
critical distance and intercept time for each particular refraction line. No corrections
have been made for relief along the various horizons, even though there is evidence
of topography in the case of at least one line, associated with the 6.5 km/s layer below
the Conrad discontinuity. A dotted line forms the extension back from the calculated critical point to shorter distances. This was done to account for some of
the observed larger arrivals which must be near the critical point.
4. Deep reflections
In our re-examination of the Haslach profile, an important piece of evidence
has been the normal-incidence reflection results.for the German foreland of the
Alps, reported by Liebscher (1962, 1964). The data in Fig. 2a were obtained near
Kaufbeuren, not too far from the end of the Haslach profile. An explanation of
these statistical results was given in Part I. The most clearly separated peaks in
Fig. 2a occur at about 3.2, 4.2, 5.8, 7.3, 9.2 and 10.3s. These peaks are related to
the first-order discontinuities below the ‘sediment-basement boundary’ for the
Seismic studies of the Earth’s crust in continentrl
543
FIG.2a. NormaLincidence reflection histogram for Kaufbeuren, not too far
from end of Haslach refraction profile. Number of reflections recorded per
0.1 s Versus echo time. Peaks near 3.2, 4.2, 5.8, 7.3, 9.2 and 10.3 s correspond
to abrupt velocity changes in the Haslach velocity model, Fig. lb. The
echo near 4.2s is from the abrupt velocity increase at the bottom of the
sialic low-velocity channel. (After Liebscher 1962)
Haslach velocity distribution shown in Fig. lb. The earliest two peaks are
associated with the low-velocity channel and the others correspond to velocity
increases in the lower crust. The reflection times are not precisely equal to the
theoretical values for this velocity distribution since the reflection area lies to the
north and not very far from the end of the refraction line. In the foreland of the Alps
S
.--
FIG.2b. Reflection profile in Bavarian Molasse Basin near end of Haslach
refraction profile showing interfaces dipping downward towards the Alps.
Base of Tertiary, top of crystalline basement, top and bottom of low-velocity
channel, top of 6.2km/s layer, top of 6.5 km/s, top of 7.2 km/s layer,
MohoroviEi6 discontinuity at base of crust. (After Liebscher 1962.)
544
Mark Lmdianan nndstephan Msdlcr
it is reasonable to expect the presence of topography on the reflecting horizons in
a N.-S.as well as an E.-W. direction (Liebscher 1964).
A measure 6f this topography is shown in the reflection section in Fig. 2b
(Liebscher 1962). Beginning at the surface, the top two lines correspond to the
base of the Tertiary and the top of the crystalline basement. The reflection section,
which roughly parallels the Haslach refraction profile, shows that these two horizons
dip to the S.E. These sloping interfaces would tend to move the various critical points
to greater distances southeastward along the refraction profile. According to the
reflection section (Fig. 2b), the 65km/s and P
. lines in Fig. la should be most
strongly affected since the associated interfaces have steeper dips than the other
surfaces below the sediment-basement boundary.
2 lkm)
FIG. 3. Histogram: number of earthquakes per kilometre of depth versus
depth, z(km),in S.E. Australia for the interval 1958-62. Note concentration
of hyp0centre.s within sialic low-velocity zone. (Data from Cleary, Doyle &
Moye 1964.)
5. Earthquake focal depths
Careful studies of earthquake epicenters and focal depths for moderately stable
regions, i.e. S.E. Australia (Cleary, Doyle & Moye 1964), S.W. Germany (Schneider
1964) and parts of N. America have shown that focal depths in these relatively
stable areas are commonly less than 10-15km below the surface. A summary plot
is shown in Fig. 3, which depicts the number of hypocentres measured during the
interval 1958-62 within each kilometre of depth for S.E.Australia. The greater
proportion of these shocks almost certainly lies within the region of velocity decrease
in the sialic layer, in agreement with the proposals of Gutenberg (1950, 1951).
Laboratory measurements by Hughes & Maurette (1956, 1957) indicate the
possibility of velocity reversals in the ‘granitic’ layer at depths between 5 and 10km,
caused by the increasing influence of temperature over pressure. By analogy with
similar results for the mantle (Gutenberg& Richter 1954), it is reasonable to suppose
that the elastic properties associated with the low-velocity zone in the sialic layer
permit the occasional release of tectonic stresses at these depths.
6. Guided waves
The concept of the low-velocity channel in the sialic layer provides a basis for
explaining details of the propagation of guided waves associated with the crust.
seismic stpdics of the EUtb’S
clpst in
contknts
545
P arrivals, for instance, have often been observed to have group velocities of about
5.6 km/s (Gutenberg 1950, 1955), and an intercept time as much as 1 s later than
that for P, (e.g. Roller & Healy 1963). P can now be reinterpreted as an RSR (re.fracted, surface-reflected)arrival of nearly critical incidence at the lower boundary
of the crustal low-velocity channel. In this view, it can also be thought to consist
of the various overlapping orders of the P, wave which have repeatedly traversed
the sialic low-velocity zone. At large distances, only the high amplitude portions
near the point of reversal, which joins the retrograde and progressive branches of
P,,will be recorded. Interference between various orders of reflection will characteristically be observed, as ‘the maximum of the amplitudes moves gradually from one
phase of the group to another’ (Gutenberg 1955). Newer data (Ryall & Stuart
1963) indicate that the maximum amplitudes of P can be found at appropriate
distances;the apparent surface velocities correspond to the compressional velocities
below the wave guide (Roller & Healy 1963). P is not observed at distances less
than about 40-50 km, which corresponds to the point of reversal for P,. As shown
by Sat6 et al. (1963) and Ben-Menahem (1964) this ray description is completely
equivalent to a modal interpretation.
L,, another guided wave associated with the continental crust, has been considered to be an RSR arrival (Press & Ewing 1952). Following the reasoning outlined for P, above, L, is thought to consist of various orders of surface reflected
S, waves. Alternatively, since its measured group velocity lies within the range of
crustal shear velocities, it can be classified as a normal-mode arrival. La is thus
the S Y and SH analogue of P and, as such, propagates to the geographical limit
of the sialic layer, since P can be efficiently converted to SV upon reflection (e.g.
Gutenberg 1944, Herrin 1961). Simultaneous recordings of Lg on all three components indicate negligible anisotropy in the sialic layer. Microseisms, whose
dominant periods and regional propagation characteristics closely resemble those
for L,, should propagate in the same manner as L,.
The presence of La propagation has often been used as a test of continental
crustal structure in off-shore areas. The association of L,, P,S, and P, propagation
in off-shore as well as continental areas is strengthened by recent reports (e.g.
Barrett et al. 1964) in which indications of P, and S, arrivals recorded during
refraction experiments on the continental shelf can be found.
The short-period PF phase discussed by Cameron (1961) could possibly be
another example of P, propagation, judging by the fact that it is only excited by
shallow shocks in continental and nearby shelf areas. Its intimate association with
long-period arrivals, classified as PL by Oliver & Major (1960), shows the close
relation of the modal waveguide to the sialic low-velocity zone.
’ 7. ~mplicationsfor future c~118f~1
studies
The recognition of a low-velocity channel in the sialic layer would require
revision of focal depths in the crust and relocation of many shallow epicenters.
Corrections to the magnitude estimates for shallow sources also would be necessary.
Crustal travel times would need revision, and if evidence for the presence of a lowvelocity zone is overlooked, its omission would introduce errors into computed
crustal models. Measurements and calculations of crustal surface wave dispersion
and the inversion problem for the crust will have to take into account the presence
of a low-velocity zone in the sialic layer.
Seismic field measurements will be able to resolve the structural details of the
546
Mark Landisman and Stephan Mueller
Earth’s crust pointed out by this study, only if the planning for future reversed and
overlapping profiles is guided by the following recommendations :
1. More closely spaced observations. A spacing of 5km for reconnaissance
work and 500m for precision measurements is necessary if the velocity gradients
and critical points are to be accurately determined. The use of a number of shots
for filling in the profile can help to achieve this result.
2. A recording speed of about 50mm/s seems to be a reasonable compromise
of the requirements for preservation of arrival character and time resolution.
3. The time elapsed after the shot instant should be known on all records to
a precision of at least 10 ms at all times.
4. The records should be run long enough, 13min, to record deep reflections
and surface wave arrivals.
5. The amplitude and phase characteristics of all instruments should be determined by an electrical calibration pulse or a mechanical tap. This should be done
before or after each shot with the instruments in place at their recording sites.
6. Either auxiliary or primary provision should be made for wide-band tape
recording facilities to permit later frequency and/or velocity filtering.
If these recommendations are followed, many more compressional, shear and
surface wave arrivals will be observed. These short-period surface waves from
explosions, and earthquake surface waves with periods of less than 15s will be
needed for studying the detailed velocity distribution in the crust.
Acknowledgments
The authors appreciate the advice, comments and constructive criticism of
Professors John W. Graham and Anton L. Hales of the Southwest Center for
Advanced Studies.
Computing facilities were generously furnished by the Southern Methodist
University Computing Laboratory, R. G. McAfee, Operations Manager, and by
the NASA Institute for Space Studies, Columbia University, under the direction of
Professor R. Jastrow.
This study was supported in part by the National Aeronautics and Space
Administration, contract NsG-269-62.
Southwest Center for Advanced Studies,
Dallas,
Texas.
1965 May.
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