Tectonophysics 400 (2005) 241 – 254
www.elsevier.com/locate/tecto
Seismic activity along the Central America volcanic arc:
Is it related to subduction of the Cocos plate?
Marco Guzmán-Spezialea,T, Carlos Valdés-Gonzálezb,
Enrique Molinac, Juan Martı́n Gómeza
a
Centro de Geociencias, UNAM, Campus Juriquilla, 76230 Querétaro, México
Instituto de Geofı́sica, UNAM, Cd. Universitaria, 04510 México D.F., México
c
INSIVUMEH, Guatemala
b
Received 16 October 2003; accepted 1 March 2005
Available online 8 April 2005
Abstract
We determine seismic strain rate of tectonic earthquakes along the Central America Volcanic Arc. We then compare this
result to those obtained from earthquakes related to the convergence of the Cocos and Caribbean plates and to earthquakes in
the back-arc region of northern Central America.
The seismic strain-rate tensor for shallow-focus earthquakes along the Central America volcanic arc since 1700, has a
compressive eigenvector with a magnitude of 0.7 108 year1, and oriented in a 3578 azimuth. The extensive eigenvector is
oriented in a 868 azimuth, with a magnitude of 0.82 108 year1. When only Centroid Moment-tensor solutions (CMT) are
considered, the respective eigenvectors are 1.2 108 year1 and 1.0 108 year1.
The compressive eigenvector from the seismic strain-rate tensor for earthquakes along the Cocos-Caribbean convergent
margin is 2.0 108 year1, plunging at 258, and oriented in a 298 azimuth. Its magnitude and direction are similar to those of
the compressive eigenvector for earthquakes along the volcanic arc. The extensive eigenvector along the convergent margin, on
the other hand, has a large vertical component. The compressive and extensive eigevenvectors are 4.9 108 year1 and
4.6 108 year1, using only CMTs as the database.
Earthquakes along the grabens of northern Central America yield a seismic strain-rate tensor whose extensive eigenvector
has a magnitude of 2.4 108 year1, oriented in a 1098 azimuth. Magnitude and direction are similar to those of the extensive
eigenvector for earthquakes along the volcanic arc. The compressive eigenvector along the grabens is practically vertical.
Similarities in magnitudes and directions for compressive and extensive eigenvectors suggest to us that the strain field along
the Central America volcanic arc is the result of compression along the convergent Cocos-Caribbean margin, and extension in the
back-arc region, along the grabens of northern Central America. This field is resolved as strike-slip faulting along the arc.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Central America volcanic arc; Cocos plate; Subduction; Compression; Extension; Seismic strain rate
T Corresponding author.
E-mail address: [email protected] (M. Guzmán-Speziale).
0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2005.03.006
242
M. Guzmán-Speziale et al. / Tectonophysics 400 (2005) 241–254
1. Introduction
Medium-sized, shallow earthquakes occur frequently along the Central America Volcanic Arc.
These earthquakes, which have proved highly
destructive for some of the main cities in Central
America, are of tectonic origin and display strikeslip faulting with one of the nodal planes aligned
parallel to the volcanic arc (Harlow and White,
1985; White, 1991; White and Harlow, 1993).
Several authors (e.g., Fitch, 1972; Harlow and
White, 1985; Guzmán-Speziale, 1995a; DeMets,
2001) have argued that oblique plate convergence
is the driving mechanism responsible for these
shallow-focus earthquakes along the volcanic arc.
Recently, however, evidence has been presented
which suggests that this is not the mechanism that
produces these earthquakes (Guzmán-Speziale and
Gómez, 2002).
-95 °
In this paper we calculate seismic strain rates
along the Central America Volcanic Arc, and also
along the convergent margin of the Cocos and
Caribbean plates, as well as the back-arc region of
northern Central America, to determine whether a
relationship exists between seismic activity along the
volcanic arc and along the plate interface and the
back-arc region.
2. Tectonic setting
Central America is located in the northwestern
corner of the Caribbean Plate, which is overriding the
subducted Cocos Plate along the Middle America
Trench (Fig. 1). Convergence of these two plates takes
place at a rate of 7–8 cm year1 and an azimuth of
about 20–228, (e.g., DeMets et al., 1990; DeMets,
2001). The subducted slab dips at a fairly steep and
-90°
-85°
Polochic Fault
Motagua Fault
Grabens
15°
15°
Middle America Trench
NORTH AMERICA PLATE
10°
10°
COCOS PLATE
-95°
CARIBBEAN PLATE
Volcanic Arc
-90°
-85°
Fig. 1. Tectonic framework of northern Central America. White, thin arrows along the Middle America Trench show direction of CocosCaribbean relative convergence, with length proportional to magnitude. Grey, thick arrows are oriented in a direction normal to the trench.
Arrows in inset show direction of relative plate motion with respect to North America, with length proportional to speed. All figures drawn with
the help of GMT software (Wessel and Smith, 1991).
M. Guzmán-Speziale et al. / Tectonophysics 400 (2005) 241–254
constant angle of about 458 (Bevis and Isacks, 1984;
Burbach et al., 1984).
The North America Plate bounds the Caribbean
Plate to the north along a left-lateral transform
boundary which in Central America is marked principally by the Motagua-Polochic Fault System (e.g.,
Molnar and Sykes, 1969; Malfait and Dinkelman,
1972). Relative motion between the North America and
Caribbean plates is about 2 cm year1 (e.g., Sykes et
al., 1982; Dixon et al., 1998; DeMets, 2001).
The volcanic front consists of 75 basaltic to dacitic
volcanoes with documented Holocene activity, 31 of
which have been active in historic times (Simkin et
al., 1981; Carr and Stoiber, 1990). They lie along a
line which closely parallels the Middle America
Trench, and some 150 km from it (Fig. 1). The
volcanic arc extends from the Motagua-Polochic
system to central Costa Rica, onshore of where the
Middle America Trench looses its surface expression.
Volcanoes are closely-spaced, 12–30 km apart, with
elevations ranging from 100 m to more than 4000 m
(Carr, 1984). In general, the volcanic front is 10–15
km wide (e.g., Carr and Stoiber, 1990). Only a few
Holocene volcanoes do not lie along the volcanic
front, the most notorious being the 10 or so which are
located behind the arc, in an extensional environment
(Burkart and Self, 1985) and whose volcanic products
are petrologically and geochemically distinct from the
basaltic cones along the arc (Walker, 1981).
Just south of the Motagua-Polochic system and
east of the volcanic arc lie a system of grabens
which are oriented N–S (Fig. 1) (e.g., Dengo, 1968;
Dengo and Bohnenberg, 1969; Weyl, 1980; Mann et
al., 1990; Gordon and Muehlberger, 1994). These
grabens are seismically active and recently GuzmánSpeziale (2001a) has calculated a rate of opening of
8 mm year1. Further to the southeast, Donnely et al.
(1990), summarizing the results of earlier workers
(e.g., McBirney and Williams, 1965), identify Neogene alkaline basalt centers located along north–
south alignments in eastern Nicaragua and eastern
Costa Rica, in the back-arc region, which they
suggest are associated with E–W extension. Mann
and Burke (1984) proposed that the N–S trending
Wagwater and Montpelier-Newmarket rifts in
Jamaica, as well as the Southern Nicaragua Rise
Graben, are part of this extensive regime along the
northern Caribbean Plate.
243
3. Seismic activity in Central America
Seismicity is dominated by shallow, thrust-faulting
earthquakes related to subduction of the Cocos Plate
beneath the Caribbean Plate (e.g., Molnar and Sykes,
1969; Dean and Drake, 1978; Burbach et al., 1984;
Dewey and Suárez, 1991; Pacheco and Sykes, 1992;
Ambraseys and Adams, 1996). These earthquakes
have magnitudes sometimes reaching 8.0. There is
also a well-defined Wadati-Benioff zone dipping at an
angle of about 458 and reaching depths to 250 km
(e.g., Burbach et al., 1984; Dewey and Suárez, 1991).
The boundary between the North America and
Caribbean plates is also seismically active. Several
large earthquakes have taken place along the Motagua
and the Polochic faults. White (1984) has catalogued
25 destructive historical earthquakes along the plate
boundary since 1530.
There is upper-crustal seismicity associated to the
N–S-trending grabens, including an M = 6.0 aftershock of the 1976 Motagua fault earthquake along the
Guatemala City Graben (Langer and Bollinger, 1979;
White and Harlow, 1979). These grabens have
experienced large historical earthquakes, with magnitudes sometimes reaching 7.0 or more (e.g., White,
1991).
Shallow-focus earthquakes with 5.7 V Ms V 6.9
occur along the volcanic arc at an average of one
every 2.5 years (Fig. 2). This activity is well
documented, at least since the 16th century (e.g., Carr
and Stoiber, 1977; White and Harlow, 1993; Peraldo
and Montero, 1999). The earthquakes have been
highly destructive, affecting most of the large cities
in Central America; the city of San Salvador, for
example, has been severely damaged in at least 12
occasions since 1594 (Harlow et al., 1993; Peraldo
and Montero, 1999). White and Harlow (1993)
compiled a catalog of destructive upper-crustal earthquakes in Central America since 1900, the vast
majority of which occurred at shallow depth and
within 20 km of the volcanic arc. Peraldo and
Montero (1999), on the other hand, collected historical documents on Central American earthquakes
from the 16th to the 19th century. Additionally, they
constructed, when possible, isoseismal maps, determined a range of probable magnitude (Ms), and made
a tectonic interpretation. Again, most of the events are
related to the activity along the volcanic arc.
244
M. Guzmán-Speziale et al. / Tectonophysics 400 (2005) 241–254
-95˚
-90˚
-85˚
15˚
15˚
10˚
10˚
15˚
15˚
10˚
10˚
-95˚
-90˚
-85˚
Fig. 2. CMTs (Harvard University, 2004) used in this study. Also shown are the areas of the volumes considered: top, volcanic arc; bottom,
Cocos-Caribbean convergence zone and grabens of northern Central America.
M. Guzmán-Speziale et al. / Tectonophysics 400 (2005) 241–254
These events are apparently of tectonic origin,
because they bear no direct temporal relationship with
volcanic eruptions (White and Harlow, 1993) and
because the largest earthquakes due to motion of
magma are no larger than 5.5 (Okada, 1983). Available
fault-plane solutions (e.g., Montero and Dewey, 1982;
White and Harlow, 1993) (Fig. 2) show a strike-slip
faulting mechanism, either right-lateral along a NW–
SE plane or left-lateral along a NE–SW plane. The
former would mean along-the-arc faulting while the
latter would indicate that the fault plane is oriented
perpendicular to the volcanic chain.
In most cases, there is no direct evidence that either
of the planes is the fault plane because there is no
surface faulting that could be associated with the
earthquake. Indirect evidence suggests that some of
the events have a right-lateral, along-arc rupture
whereas for others faulting is left-lateral, perpendicular
to the arc.
For the following events there is evidence that
suggests right-lateral, along-the-arc, faulting: Earthquakes in Costa Rica in 1839 and 1841 show
isoseismals for intensities VII and VIII noticeably
elongated in a NW–SE direction (Peraldo and
Montero, 1999); for the El Salvador event of 1854
there are reports that damage occurred along a
narrow zone stretching from SE to NW (Peraldo
and Montero, 1999), additionally, isoseismals (Harlow et al., 1993) are also significantly elongated in
this direction. The two Costa Rica earthquakes of
1910 show isoseismals in a NW–SE direction
(Montero and Dewey, 1982; White and Harlow,
1993). Three events in El Salvador from 1917 to
1919, very close in time and space, progressed from
W to E, suggesting faulting in this direction (White et
al., 1987; Harlow et al., 1993; White and Harlow,
1993). The foreshock–aftershock sequence of the
1965 El Salvador earthquake is oriented in a NW–
SE direction (Lomnitz and Schultz, 1966; White et al.,
1987). The distribution of aftershocks suggests a
NW–SE faulting plane for the February 2001, El
Salvador earthquake (Centro de Investigaciones Geotécnicas, 2001).
According to available evidences, left-lateral faulting, perpendicular to the arc is most probable for the
next events: Cracks in a N–S direction were reported
for the 1857 El Salvador earthquake (Peraldo and
Montero, 1999); surface faulting for the 1931 and
245
1972 Nicaragua earthquakes took place in a N–S
direction (e.g., White and Harlow, 1993); alignment of
aftershocks suggests a NNE–SSW faulting plane for
the 1982 Gulf of Fonseca earthquake; from aftershock
distribution, the 1986 El Salvador earthquake was
caused by a N25E-trending fault (e.g., White et al.,
1987; Harlow et al., 1993) but a foreshock swarm just
east of the epicentral area had an E–W distribution
(White et al., 1987).
4. Method and data
The method of Kostrov (1974) is now the standard
tool to determine seismic strain rate within a volume in
the Earth. It has been used to calculate deformation in
several areas, for example: The Mediterranean and
Middle East (Jackson and McKenzie, 1988), continental regions (Ekström and England, 1989), central
Greece (Papapzachos and Kiratzi, 1992), the Aegean
(Papazachos et al., 1992), the Anadaman Sea (Guzmán
Speziale and Ni, 1993), the North and East Anatolian
faults (Kiratzi, 1993), Japan (Kiratzi and Papazachos,
1996) and the grabens of Central America (GuzmánSpeziale, 2001a), among other areas.
The average seismic strain rate from N earthquake
moment tensors M ij within a volume V and a time
period s is given by (Kostrov, 1974):
ėē¯ ij ¼
N
1 X
Mn
2lV s n¼1 ij
ð1Þ
l is the modulus of rigidity (3 1010 N/m2).
The sum of seismic moment tensors in the righthand side of Eq. (1) may be expressed as (e.g.,
Papapzachos and Kiratzi, 1992):
!
N
N
X
X
n
n
Mij ¼
M0 F̄ij
ð2Þ
n¼1
n¼1
where the average shape tensor F̄ is given by
(Papapzachos and Kiratzi, 1992; Kiratzi and Papazachos, 1996):
F̄ij ¼
N Mn
1 X
ij
N n¼1 M0n
ð3Þ
In this manner, a tensor is obtained which
represents each of the seismic moment tensors
246
M. Guzmán-Speziale et al. / Tectonophysics 400 (2005) 241–254
segments on a sphere may be obtained in spherical
coordinates by:
Z h2 Z /2 Z q2
V ¼
q2 sin/dqd/dh
ð5Þ
involved, with equal weight. Historic earthquakes
(for which the seismic moment tensor is not known)
may be included in the calculation if Eq. (2), instead
of Eq. (1), is used The average shape tensor is first
calculated with available moment tensors and then
scalar moments are added. Knowing the surface-wave
magnitude Ms, a scalar seismic moment Mo may be
obtained by (Ekström and Dziewonski, 1988):
logMo ¼ 12:24 þ Ms
logMo ¼ 23:20 h1
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
92:45 11:40Ms
ð4bÞ
5:3VMsV6:8
logMo ¼ 1:5Ms þ 9:14
ð4cÞ
MsN6:8
q1
where the radius q goes from 6321 km (Earth radius
minus 50 km) to 6371 km, / is the angular distance,
and h is the azimuth from the center of curvature of
the segment. Parameters for each of the segments are
given in Table 1. Centers of curvature for each of the
segments are based on those given in GuzmánSpeziale (1995b). The total volume of the convergent
zone is 6.93 1015 m3.
We calculate the volume in the volcanic arc in a
similar manner. We take three segments along the arc
using the same centers of curvature, with a width of
0.68, which covers the deformation (seismically
active) region. White (1991) and White and Harlow
(1993) argue that the vast majority of earthquakes
along the volcanic arc take place in depths between 3
and 15 km, so we use 15 km as the seismogenic depth.
The total volume thus calculated is 8.94 1014 m3.
The volume for the grabens is taken directly from
Guzmán-Speziale (2001a): 7.5 1014 m3.
Centroid moment-tensor solutions (CMTs)
reported by Harvard University (e.g., Dziewonski
and Woodhouse, 1983; Harvard University, 2004) are
used to calculate the average shape tensor associated
to the interface between the Cocos and Caribbean
plates, as well as the Central America volcanic arc,
and the grabens of Central America. For the convergent margin, tensors with scalar seismic moment of
at least 2.5 1017 N m and T axis plunging 458 or
more (thrust-faulting mechanism, according to Frohlich and Apperson, 1992) are chosen, whereas B axis
must plunge at least 458 (strike-slip faulting earthquakes) for earthquake moment tensors along the
ð4aÞ
Msb5:3
/1
We obtained the average seismic strain rate tensor
for shallow crustal earthquakes along the Central
America volcanic arc and interplate earthquakes along
the Cocos-Caribbean interface. Based on the work of
Guzmán-Speziale (2001a), we also updated the
average seismic strain rate along the grabens of
Central America.
For each of the elements involved (i.e., the
volcanic arc, the convergent margin, and the backarc grabens), the volume V must be determined. In
the case of the volcanic arc, only the volume where
there is seismic activity is considered (from latitude
908 to 83.58, approximately). The interplate
region is that in front of the active part of the
volcanic arc (Fig. 2).
We consider the interplate region along the CocosCaribbean interface to be 50 km deep and 1.48 (about
155 km) wide (e.g., Pacheco et al., 1993). Because of
the change in azimuth and curvature of the Middle
America Trench, three separate segments are considered (Fig. 2). From elementary Calculus (e.g., Munem
and Foulis, 1978), the volume of these circular
Table 1
Parameters for the volume segments along the Cocos-Caribbean margin and the Central America volcanic arc
Segment
1
2
3
a
b
Center of curvature
Distance (degrees)a
Distance (degrees)b
Azimuth
Lat.
Lon.
Min.
Max.
Min.
Max.
Min.
Max.
32.59
7.37
13.58
80.74
90.63
81.69
20.61
4.84
4.52
22.01
6.24
5.92
20.21
6.04
4.12
20.81
6.64
4.72
202.25
18.25
187.50
214.25
54.00
235.50
Distances for convergent margin.
Distances for volcanic arc.
M. Guzmán-Speziale et al. / Tectonophysics 400 (2005) 241–254
247
Table 2
Significant earthquakes along the Central America volcanic arc 1700–1978
N
Date
Latitude
Longitude
Msa
Msb
Mo (N m)c
Mo (N m)d
Ref.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
1701.00.00
1712.12.14
1739.00.00
1748.03.03
1765.04.14
1772.02.15
1783.11.29
1798.02.02
1821.04.10
1835.06.10
1838.12.00
1839.03.22
1839.10.01
1841.09.02
1842.03.21
1851.03.18
1853.08.24
1854.04.16
1854.06.11
1857.11.06
1860.06.21
1860.12.03
1867.03.21
1872.12.30
1873.03.04
1873.03.19
1878.10.03
1888.12.30
1896.04.20
1899.03.25
1910.04.03
1910.05.04
1911.08.28
1912.06.06
1912.07.19
1917.06.08
1917.06.08
1919.04.28
1931.03.31
1936.12.20
1937.12.25
1938.04.25
1938.05.06
1951.05.06
1951.05.06
1951.05.07
1951.08.02
1951.08.03
1952.12.30
1955.04.04
1955.04.30
1955.09.01
11.95
13.58
11.80
13.60
13.70
10.00
13.60
13.65
9.83
9.92
13.50
13.82
13.66
10.00
9.97
10.13
10.42
13.68
13.65
13.63
13.62
13.78
13.76
13.62
13.71
13.71
13.28
10.13
9.88
13.60
9.85
9.85
10.25
10.23
13.87
13.70
13.75
13.66
12.15
13.72
13.93
12.45
12.53
13.52
13.52
13.48
13.00
13.00
10.05
12.75
12.38
10.25
86.05
88.83
86.20
89.10
89.00
84.13
88.80
89.25
84.08
84.17
88.40
89.25
89.22
83.92
84.12
84.19
84.90
89.18
88.83
89.00
88.91
89.33
89.50
88.66
89.20
89.20
88.25
84.20
83.92
88.80
83.92
84.33
84.32
84.28
89.57
89.50
89.27
89.17
86.17
88.93
89.78
86.85
86.87
88.40
88.40
88.45
87.50
87.50
83.92
87.17
86.52
84.25
5.4
5.4
5.4
6.4
6.0
5.7
5.4
5.7
5.0
5.4
5.4
6.0
5.4
6.5
5.4
6.0
5.4
6.0
5.4
6.0
6.0
6.0
5.4
5.4
6.4
6.2
6.0
6.0
5.0
6.1
5.8
6.4
6.0
6.5
5.9
6.5
6.4
6.0
6.0
6.1
5.8
5.9
6.1
6.0
6.2
5.8
5.8
5.9
5.9
6.2
6.0
5.8
5.4
6.2
5.4
6.4
6.1
5.7
6.0
6.2
5.9
5.4
6.0
6.2
5.9
6.5
5.4
6.9
6.0
6.6
6.2
6.4
6.1
6.0
5.8
5.8
6.4
6.5
6.0
6.0
5.9
6.1
5.8
6.4
6.0
6.5
5.9
6.5
6.4
6.0
6.0
6.1
5.8
5.9
6.1
6.0
6.2
6.0
5.9
6.0
5.9
6.2
6.0
6.0
0.4387e + 18
0.4387e + 18
0.4387e + 18
0.6099e + 19
0.1977e + 19
0.9095e + 18
0.4387e + 18
0.9095e + 18
0.1738e + 18
0.4387e + 18
0.4387e + 18
0.1977e + 19
0.4387e + 18
0.8247e + 19
0.4387e + 18
0.1977e + 19
0.4387e + 18
0.1977e + 19
0.4387e + 18
0.1977e + 19
0.1977e + 19
0.1977e + 19
0.4387e + 18
0.4387e + 18
0.6099e + 19
0.3421e + 19
0.1977e + 19
0.1977e + 19
0.1738e + 18
0.2592e + 19
0.1171e + 19
0.6099e + 19
0.1977e + 19
0.8247e + 19
0.1517e + 19
0.8247e + 19
0.6099e + 19
0.1977e + 19
0.1977e + 19
0.2592e + 19
0.1171e + 19
0.1517e + 19
0.2592e + 19
0.1977e + 19
0.3421e + 19
0.1171e + 19
0.1171e + 19
0.1517e + 19
0.1517e + 19
0.3421e + 19
0.1977e + 19
0.1171e + 19
0.4387e + 18
0.3421e + 19
0.4387e + 18
0.6099e + 19
0.2592e + 19
0.9095e + 18
0.1977e + 19
0.3421e + 19
0.1517e + 19
0.4387e + 18
0.1977e + 19
0.3421e + 19
0.1517e + 19
0.8247e + 19
0.4387e + 18
0.3090e + 20
0.1977e + 19
0.1126e + 20
0.3421e + 19
0.6099e + 19
0.2592e + 19
0.1977e + 19
0.1171e + 19
0.1171e + 19
0.6099e + 19
0.8247e + 19
0.1977e + 19
0.1977e + 19
0.1517e + 19
0.2592e + 19
0.1171e + 19
0.6099e + 19
0.1977e + 19
0.8247e + 19
0.1517e + 19
0.8247e + 19
0.6099e + 19
0.1977e + 19
0.1977e + 19
0.2592e + 19
0.1171e + 19
0.1517e + 19
0.2592e + 19
0.1977e + 19
0.3421e + 19
0.1977e + 19
0.1517e + 19
0.1977e + 19
0.1517e + 19
0.3421e + 19
0.1977e + 19
0.1977e + 19
2
2,
2
3
1,
1,
2,
2,
4
2
2
1,
1,
1,
2
1,
2
1,
1,
1,
1,
1,
2,
1,
1,
1,
1,
2
4
3
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
3
2, 3
2
3
3
2, 3
2, 3
2
4
2,
2,
2,
2,
2
3
2,
2,
2
2
3
3
3
3
3
3
(continued on next page)
248
M. Guzmán-Speziale et al. / Tectonophysics 400 (2005) 241–254
Table 2 (continued)
N
Date
Latitude
Longitude
Msa
Msb
Mo (N m)c
Mo (N m)d
Ref.
53
54
55
1965.05.03
1972.12.23
1973.04.14
13.72
12.15
10.47
89.12
86.27
84.97
6.0
6.2
6.5
6.0
6.2
6.5
A
0.1977e + 19
0.3421e + 19
0.8247e + 19
1.27853e + 20
0.1977e + 19
0.3421e + 19
0.8247e + 19
1.984193e + 20
5
5
5
References: 1. Carr and Stoiber (1977); 2. Peraldo and Montero (1999); 3. Harlow et al. (1993); 4. Montero-Pohly (1989); 5. White and Harlow
(1993) (and references therein).
We exclude events west of 908.
a
Minimum value reported.
b
Maximum value reported.
c
Calculated from minimum magnitude.
d
Calculated from maximum magnitude.
volcanic arc, and a normal faulting mechanism
(plunge of P axis z 458) is required for earthquakes
in the region of the grabens. In all, 77 CMTs met the
requirements for the convergent margin, 23 for the
volcanic arc, and 5 for the grabens. Using only CMTs
to determine the average shape tensor ensures a
uniform data set, although focal mechanisms in
Central America are reported by other workers (e.g.,
Molnar and Sykes, 1969; Dean and Drake, 1978;
Burbach et al., 1984; White and Harlow, 1993).
Large historic earthquakes for Central America are
reported in various sources. We consider historic
those earthquakes that occurred prior to 1978, when
systematic reporting of CMTs by Harvard University
began, and on or after 1700.
Several authors (e.g., Carr and Stoiber, 1977;
Montero-Pohly, 1989; Harlow et al., 1993; White
and Harlow, 1993; Peraldo and Montero, 1999) report
historic earthquakes along the Central America
volcanic arc. We use 55 of these events (Table 2).
Table 3
Significant thrust-faulting earthquakes along the Cocos-Caribbean plate margin, 1700–1977
N
Date
Latitude
Longitude
Msa
Msb
Mo (N m)c
Mo (N m)d
Ref.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
17190305
17520507
17760530
18150820
18260403
18310207
18331002
18591208
18670630
18690301
18820303
18851012
19000621
19160227
19210328
19260208
19391221
19561024
13.00
12.30
13.18
12.75
10.00
13.20
10.00
13.20
13.20
13.00
9.20
12.08
10.00
10.70
12.50
13.00
10.00
11.50
89.50
87.50
90.08
89.00
85.50
89.70
85.50
90.00
89.16
90.00
84.20
87.03
85.50
85.98
87.50
89.00
85.00
86.50
7.2
6.7
7.2
7.2
6.7
7.0
7.0
7.0
6.5
7.0
6.7
6.7
7.1
7.3
7.2
7.0
7.1
7.2
7.4
6.7
7.5
7.2
6.7
7.1
7.2
8.0
7.1
7.0
7.2
7.7
7.1
7.3
7.2
7.0
7.1
7.2
A
0.8710e + 20
0.1553e + 20
0.8710e + 20
0.8710e + 20
0.1553e + 20
0.4365e + 20
0.4365e + 20
0.4365e + 20
0.8247e + 19
0.4365e + 20
0.1553e + 20
0.1553e + 20
0.6166e + 20
0.1230e + 21
0.8710e + 20
0.4365e + 20
0.6166e + 20
0.8710e + 20
9.70437e + 20
0.1738e + 21
0.1553e + 20
0.2455e + 21
0.8710e + 20
0.1553e + 20
0.6166e + 20
0.8710e + 20
0.1380e + 22
0.6166e + 20
0.4365e + 20
0.8710e + 20
0.4898e + 21
0.6166e + 20
0.1230e + 21
0.8710e + 20
0.4365e + 20
0.6166e + 20
0.8710e + 20
3.2126e + 21
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
References: 1. Peraldo and Montero (1999); 2. Pacheco and Sykes (1992).
a
Minimum value reported.
b
Maximum value reported.
c
Calculated from minimum magnitude.
d
Calculated from maximum magnitude.
M. Guzmán-Speziale et al. / Tectonophysics 400 (2005) 241–254
249
Table 4
Significant earthquakes along the grabens of Central America 1570–1978
N
Date
Latitude
Longitude
Msa
Msb
Mo (N m)c
Mo (N m)d
Ref.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
1586.12.23
1607.10.09
1651.02.18
1689.02.12
1717.09.29
1733.04.00
1733.05.00
1743.10.15
1765.06.02
1773.07.29
1773.12.14
1774.10.14
1809.06.20
1820.10.09
1830.04.21
1851.11.14
1885.12.18
1854.04.16
1917.12.26
1917.12.29
1918.01.04
1918.01.25
1934.02.03
14.60
14.50
14.52
14.55
14.52
14.20
14.42
15.00
14.83
14.50
14.50
14.50
14.40
16.00
14.47
14.50
14.41
13.68
14.53
14.55
14.58
14.50
14.85
90.75
90.50
90.68
90.75
90.80
88.40
89.28
89.50
89.50
90.80
90.80
87.66
87.66
87.85
90.60
87.70
90.62
89.18
90.53
90.53
90.53
90.53
89.15
5.4
5.4
5.4
6.0
6.5
4.9
5.4
6.7
6.0
6.5
5.7
5.4
5.0
6.0
6.3
6.0
6.3
6.0
5.8
5.7
6.1
6.2
6.2
6.0
6.2
5.4
6.0
6.5
5.4
7.5
6.7
7.6
6.5
5.7
6.0
5.7
6.5
6.3
6.5
6.4
6.6
5.8
5.7
6.1
6.2
6.2
A
0.4387e + 18
0.4387e + 18
0.4387e + 18
0.1977e + 19
0.8247e + 19
0.1380e + 18
0.4387e + 18
0.1553e + 20
0.1977e + 19
0.8247e + 19
0.9095e + 18
0.4387e + 18
0.1783e + 18
0.1977e + 19
0.4550e + 19
0.1977e + 19
0.4387e + 18
0.4550e + 19
0.1171e + 19
0.9095e + 18
0.2592e + 19
0.3421e + 19
0.3421e + 19
6.59383e + 19
0.1977e + 19
0.1977e + 19
0.4387e + 18
0.1977e + 19
0.8247e + 19
0.4387e + 18
0.2455e + 21
0.1553e + 20
0.3467e + 21
0.8247e + 19
0.9095e + 18
0.1977e + 19
0.9095e + 18
0.8247e + 19
0.4550e + 19
0.8247e + 19
0.1977e + 19
0.6099e + 19
0.1171e + 19
0.9095e + 18
0.2592e + 19
0.3421e + 19
0.3421e + 19
6.770012e + 20
1,
1,
1,
1,
5
3,
3,
1,
1,
1,
1,
2,
2,
2,
1,
1,
1,
1,
1,
1,
4
4
1,
5
5
5
5
5
5
3,
5
5
5
3,
3,
5
5
2
5
3,
4
4
5
5
5
4
4
Table taken from Guzmán-Speziale (2001a,b).
References: 1. Carr and Stoiber (1977); 2. Osiecki (1981); 3. White (1991); 4. White and Harlow (1993); 5. Peraldo and Montero (1999).
a
Minimum value reported.
b
Maximum value reported.
c
Calculated from minimum magnitude.
d
Calculated from maximum magnitude.
Data for 18 historic earthquakes along the convergent
margin come from Pacheco and Sykes (1992) and
Peraldo and Montero (1999) (Table 3). The 19 historic
events for the grabens come from Guzmán-Speziale
(2001a) (Table 4).
5. Results
5.1. Cocos-Caribbean convergent margin
moments calculated from the smallest and largest
magnitudes reported, and moments from CMTs.
These are: 1.51173 1021 N m and 3.75389 1021
N m, with an average of 2.63281 1021 N m, which
include the values reported in Table 3 and from the
77 CMTs. In a coordinate system where x 1 is north,
x 2 is east, and x 3 is down, considering the average
sum of scalar seismic moments, the corresponding
volume (see above), and 304 years of data, the
average seismic strain-rate tensor calculated using
Eqs. (1) (2) and (3) is:
The average shape tensor F from Harvard CMTs
is:
2
0:4954
F ¼ 4 0:2842
0:6027
0:2842
0:0746
0:4081
3
0:6027
0:4081 5
0:5698
2
ð6Þ
We obtained a minimum and a maximum value
for the sum of scalar seismic moments, adding the
1:0320
ē˙ij ¼ 4 0:5921
1:2554
108 yr1
3
0:5921 1:2554
0:1554 0:8501 5
0:8501 1:1868
ð7Þ
250
M. Guzmán-Speziale et al. / Tectonophysics 400 (2005) 241–254
This tensor has the following eigenvalues, arranged in
decreasing order:
2
3
1:9067
¼ 4 0:1285 5 108 yr1
ð8aÞ
2:0358
and associated eigenvectors, in columnar form:
2
3
0:3296 0:5155
0:7909
U ¼ 4 0:2774
0:8536
0:4409 5
ð8bÞ
0:9024 0:0740 0:4244
The eigenvector associated to the largest (extensive) eigenvalue is mostly vertical, with a small
horizontal component oriented in a 408 azimuth.
The intermediate eigenvector is practically horizontal
and oriented in a S298E direction while the smallest
(compressive) eigenvector has a large horizontal
component in a N298E direction and a small vertical
component plunging 258.
5.2. Central America volcanic arc
For the volcanic arc we obtained an average shape
tensor:
2
3
0:5844
0:0885 0:1265
F ¼ 4 0:0885
0:7093 0:0463 5
ð9Þ
0:1265 0:0463 0:1249
The minimum, maximum, and average sums of scalar
moments are 1.50396 1020 N m, 2.20963 1020 N
m, 1.85679 1020 N m, respectively. Taking the
average and 304 years as the time, the average seismic
strain-rate yields:
2
3
6:6555
1:0081 1:4401
ēė¯ ij ¼ 4 1:0081
8:0780 0:5273 5
1:4401 0:5273 1:4226
109 yr1
with eigenvalues and eigenvectors:
2
3
0:8188
¼ 4 0:1115 5 108 yr1
0:7073
2
0:0740
U ¼ 4 0:9951
0:0656
0:2368
0:0815
0:9681
3
0:9687
0:0560 5
0:2417
ð10Þ
In this case, the largest eigenvector (extensive) is
horizontal and oriented in an E–W direction, the
intermediate eigenvector is mostly vertical, and the
compressive (smallest) eigenvector is almost horizontal (a small vertical component with a 148 plunge),
oriented in the N direction.
5.3. Grabens of Central America
The shape tensor is:
2
0:0942 0:2727
F ¼ 4 0:2727
0:8037
0:0177
0:0620
3
0:0177
0:0620 5
0:8979
ð12Þ
6.6545 1019 N m, 6.7761 1020 N m, and
3.7209 1020 N m, are the minimum, maximum,
and average sums of scalar moments. The average
strain-rate yields:
2
3
0:2562 0:7417 0:0480
ēė¯ ij ¼ 4 0:7417
2:1860
0:1686 5
0:0480
0:1686 2:4422
108 yr1
ð13Þ
The eigenvalues and eigenvectors are:
2
3
2:4444
¼ 4 0:0041 5 108 yr1
2:4485
2
0:3215
U ¼ 4 0:9462
0:0358
0:9469
0:3216
0:0036
3
0:0081
0:0351 5
0:9994
ð14aÞ
ð14bÞ
The grabens of Central America show eigenvectors
of the seismic strain-rate tensor oriented in an azimuth
of 1098 (extensive or largest one), and an almost
vertical smallest (compressive) one.
6. Discussion
ð11aÞ
ð11bÞ
Several authors (Fitch, 1972; Harlow and White,
1985; Guzmán-Speziale, 1995a; DeMets, 2001) have
suggested that earthquakes along the Central America
volcanic arc are due to oblique subduction of the
Cocos Plate. Recently, however, Guzmán-Speziale
and Gómez (2002) pointed out that this model
M. Guzmán-Speziale et al. / Tectonophysics 400 (2005) 241–254
presents several problems, such as very small alongarc components of relative plate motion (Fig. 1),
earthquake faulting planes perpendicular to the
volcanic arc for some of the earthquakes, and
buttressing of the supposedly detached forearc at its
northwestern end.
If not oblique subduction, what is the mechanism
that triggers these tectonic earthquakes? GuzmánSpeziale (2001b) suggested that a combination of
compression along the subduction zone and extension
in the back-arc region might yield strike-slip faulting
along the volcanic arc. We retake this idea here.
Earthquakes along the volcanic arc yield an
average seismic strain-rate tensor for which the largest
(extensive, or least compressive) and smallest (compressive) eigenvectors are horizontal and oriented E–
W and N–S, respectively. Counterparts for these
eigenvectors may be found along the convergent
margin and in the zone of grabens.
The compressive (smallest) eigenvector along the
convergent margin plunges 258 and is oriented in a
298 azimuth. Its magnitude is 2.0 108 year1. Its
-95 o
-90 o
251
horizontal component along a NS direction is
1.6 108 year1. The same eigenvector for earthquakes along the volcanic arc is oriented N–S with a
magnitude of 0.7 108 year1. The NS, horizontal
component of the compressive vector is only two
times in magntitude, compared to the compressive
eigenvector of earthquakes along the volcanic arc. In
the case of the grabens, the extensive eigenvector is
oriented in a 1098 azimuth, with a magnitude of
2.4 108 year1, compared to the 0.8 108 year1,
that is, only about three times and a very similar
orientation.
Compressive strain-rate along the convergent
margin and along the volcanic arc are similar, in
direction and in magnitude. So are extensive strainrates along the grabens and the volcanic arc (Fig. 3).
This suggests to us that compression along the
convergent margin and extension along the grabens
are transmitted to the volcanic arc. We propose that
this strain combination is resolved along the volcanic
arc because it is a zone where lithospheric strength is
decreased due to a higher thermal gradient and a small
-85 o
15o
15o
10o
10 o
-95o
-90 o
-85o
Fig. 3. Horizontal direction of extensive (white arrows) and compressive (dark arrows) eigenvectors along tectonic elements of Central America.
252
M. Guzmán-Speziale et al. / Tectonophysics 400 (2005) 241–254
thickness. Extension from the convergent margin or
compression from the back-arc region does not
contribute to the state of stress along the volcanic
arc because both components are vertical.
Evidently, seismic activity along the convergent
margin is larger than either along the volcanic arc or
the grabens, both in number of earthquakes and in
magnitudes. Yet, seismic strain-rate is similar because
in the convergent margin it is distributed along a much
larger volume.
7. Conclusion
Evidence presented elsewhere (Guzmán-Speziale
and Gómez, 2002) suggests that oblique plate
convergence may not be the driving mechanism for
tectonic earthquakes along the Central America
volcanic arc. The model first suggested by GuzmánSpeziale (2001b), in which the earthquakes are due to
a combination of compression from the CocosCaribbean convergent margin and back-arc extension,
is shown here to be well supported by calculations of
the strain-rate tensor in all three tectonic elements. In
other words, our results suggest that, indeed, seismic
activity along the Central America volcanic arc is
related to subduction of the Cocos plate along the
Cocos-Caribbean interface.
Acknowledgements
This work was possible thanks to grants GEOF
3.4.2.42 from Instituto Panamericano de Geografı́a e
Historia (IPGH), to Guzmán-Speziale and Valdés,
and 36449-T from Consejo Nacional de Ciencia y
Tecnologı́a (Conacyt), Mexico, to Guzmán-Speziale.
We are grateful to the two referees, Carlos Mendoza
and Marino Protti, for their comments, which greatly
improved the manuscript. Centro de Geociencias,
UNAM, contribution 905.
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