Geophys. J. Int. (1997) 130,220-228
The rupture history of the M , 8.0 Jalisco, Mexico, earthquake of
1995 October 9
Vyacheslav M. Zobin
Observatorio Vulcanologico, Unioersidad de Colima, Colima, Col., 28045, Mexico. E-mail: [email protected]
Accepted 1997 March 12. Received 1997 March 11; in original form 1996 July 12
SUMMARY
Finite-fault, broad-band teleseismic P waveform inversion was applied to the large M ,
8.0 Jalisco, Mexico, earthquake of 1995 October 9. The earthquake hypocentre was
located at shallow depths just near the boundary zone between two lithospheric plates,
Rivera and Cocos, and was felt with a maximum intensity of 9 M M on the Pacific
coast. It was the first earthquake of magnitude greater than 6 to occur in this region
since the two great Jalisco earthquakes ( M , 8.0 and 7.6) of 1932.
For inversion, we used vertical records from 14 digital stations situated 30" to 90"
from the epicentre. We used the fixed rake of the thrust-type focal mechanism given
by the preliminary Harvard CMT. The low-angle dipping fault plane was taken as a
180 x 90 km2 rectangle divided into 162 subfaults of 10 x 10 km2. Each subfault had
five intermediate point sources along-strike, and five intermediate point sources downdip. The size of the fault plane was changed from an initial size of 110 x 60 km',
estimated by RESCO as an aftershock area, to the final size of 180x90km2, which
included all significant displacements along the fault. The hypocentre was embedded
40 km from the left edge of the 180 km long fault, 40 km down-dip from the top of the
fault which was situated practically along the trench axis. A boxcar source time function
of width 2.0 s was used for each discrete rupture interval of each subfault. The rupture
velocity was taken to be 2.8 km s-l which is approximately 80 per cent of the shearwave speed in the layer containing the hypocentre.
The following results on the rupture history were obtained. (1) The rupture duration
was about 55 s, and slip occurred within an area of about 180 x 90 km2 along the
Middle American trench in the depth interval from 9 to 33 km. (2) There were two
main stages in the rupture history: during the first 18 s two main asperities at a distance
of about 30 km from the earthquake hypocentre were ruptured with maximum slip up
to 610 cm at a depth of 12-15 km; then during the next 37 s rupturing was observed
to the north along the trench.
The slip distribution obtained shows that the faulting, originating near the boundary
between the Rivera and Cocos plates, went mainly along the Rivera-North American
plate boundary and is consistent with the subduction of the Rivera plate beneath
North America.
Key words: earthquakes, Mexico, P waves, Rivera plate, subduction.
INTRODUCTION
The earthquake of 1995 October 9 occurred within the Jalisco
block, which represents the northern part of the Mexican
subduction zone where the Rivera plate subducts beneath the
North American plate along the Middle American trench
(Fig. la). Detailed studies of the seismicity and tectonics of the
Rivera plate (Eissler & McNally 1984; Pardo & Suarez 1995)
have demonstrated that after the two great Jalisco earthquakes
220
of 1932 (June 3, M , 8.0 and June 18, M , 7.6, magnitudes from
the catalogue of Pacheco & Sykes 1992) this zone of interaction
between the Rivera and the North American plates was
characterized by low seismicity. This is shown in Fig. l(b),
where the epicentres of earthquakes recorded from 1964 to
1993 are plotted, together with the area of the 1932 June 3
Jalisco earthquake. During 1974 to 1993 no earthquakes of
magnitude 5.5 or greater were recorded in this area (Zobin
1996).
ORAS 1997
Rupture history of the Jalisco, Mexico, earthquake
221
a
I
j
i
PACIFIC OCEAN
Figure 1. Tectonic setting (a) and seismicity (b) of the region of study. (a) The main elements of tectonic setting of the Jahsco block (JB) are
shown. Four lithospheric plates are indicated: North America, Pacific, Rivera and Cocos. For the Cocos and Rivera plates, relative convergence
rates (cm yr-') between the oceanic and continental plates are indicated by open arrows. The abbreviations are EPR, East Pacific Rise; RFZ,
Rivera Fracture Zone; EG, El Gordo Graben; MAT, Middle American Trench. The epicentre of the Jalisco earthquake of 1995 October 9 is shown
by a black circle and indicated by an arrow. (b) The epicentres of earthquakes recorded during 1964-93 by USGS, magnitude greater than 3.5,
depth from 0 to 60 km,are shown. The aftershock area of the Jalisco earthquake of 1932 June 3, magnitude M , 8.0, is shown by dashed lines with
the epicentre indicated by a cross.
The Jalisco earthquake of 1995 October 9 interrupted this
calm period. It occurred at shallow depths in the southern
part of the Rivera subduction zone and was felt with maximum
intensity of 9 MM (Modified Mercalli) on the Pacific coast.
Over 60 people were killed, and some structures collapsed in
the coastal towns. This earthquake was followed by intensive
aftershock activity, mainly during 9-16 October. More than
20 aftershocks had magnitude mb> 4.0; the largest aftershock
occurred on October 12 to the south of the main-shock
epicentre and had a magnitude mb 5.5 (M, 6.0). The aftershock
area spread mainly to the northwest from the mainshock epicentre, along the slope of the trench, and was
estimated to be about 110 x 60 km2 (Ramirez Vazquez et a/.
1995). The earthquake was followed by a tsunami of up to
5 m in the Manzanillo area, which was also recorded over
many other parts of the southern Pacific.
According to the Harvard centroid moment tensor (CMT)
solution, this event was characterized by a thrusting movement
along the low-angle dipping fault plane orientated parallel to
the trench.
This paper is devoted to the study of the rupture history of
the main shock. The finite-fault, waveform inversion allows
the reconstruction of the temporal moment-release function of
an earthquake and the spatial distribution and size of asperities
within the fault plane (e.g. Hartzell, Langer & Mendoza 1994).
We have applied this method to the Jalisco earthquake using
the broad-band records of P waves.
METHOD
The fault parametrization and modelling procedure for the
finite-fault, waveform inversion technique is fully described by
0 1997 RAS, GJI 130, 220-228
Hartzell & Heaton (1983) and was later developed in numerous
papers by Hartzell with colleagues. The complete list of
references used for calculations of synthetics for teleseismic
records was published by Hartzell et al. (1994). We present
here only a short description of this method. The procedure
consists of several stages:
(1) The fault parametrization and the velocity structure
model are chosen. The method proposes that the fault plane
of fixed dimensions and orientation is embedded at the appropriate depth in the crustal structure of the earthquake source
region. The fault parametrization includes the choice of the
focal depth, the coordinates of the epicentre, the fault length
and width, the fault strike, dip and rake, the rupture velocity,
and the shape and duration of the source time function. Also,
we have to introduce the optimal number of rectangular,
equal-area subfaults with uniformly distributed point sources
over each subfault which would model the seismic radiation
produced by the fault. The subfault dimensions and the total
number of subfaults may be chosen after consideration of the
following factors: the bandwidth of the data, the size of the
fault plane, and the computational limitations.
( 2 ) The synthetic seismograms are calculated as the
responses of the total point sources. The ground motion for a
unit amount of slip on each subfault is calculated by a timedomain sum of uniformly spaced point sources. We obtain a
response of a finite fault in a layered hemisphere. This response
is convolved later with a time function, and the files of
synthetics ready for the inversion are prepared.
(3) The non-negative least-squares inversion generates the
solution vector, which then gives the possibility of obtaining
the moment-release function and the slip distribution for a
222
V. M . Zobin
Table 2. Velocity structure.
number of time windows. The synthetic waveforms and
observed data comprise an overdetermined system of linear
equations of the form AX =b, where A is an m-by-n matrix
of the synthetics, and b is a vector of order m containing the
seismic observations. The vector X, of order n, contains the
subfault dislocation weights required of each component to
reproduce the observed data. The main constraints on solution
used to minimize instability are requiring a non-negative
solution and a smooth variation of slip across the fault. We
find the smoothest solution with the smallest moment that fits
the data.
Note. This velocity model of the Jalisco-Colima region was proposed
by 2. Jimenez (UNAM, Mexico) for the routine locating of the JaliscoColima coastal earthquakes using the Sesmic Network of Colima
University (RESCO).
DATA
Table 3. Parametrization of the model.
The parameters of the main shock of the Jalisco earthquake
estimated by different seismic surveys are given in Table 1. The
location used in this paper comes from the local seismic
network RESCO of the University of Colima, Mexico, situated
along the coast close to the epicentre. This epicentre differs
slightly from one obtained by USGS, but RESCO used nearby
stations and estimated the depth of the focus (USGS gave only
a fixed depth of 33 km), which made its location preferable.
The local velocity structure used for locating the epicentre and
for our inversion is shown in Table 2.
The parametrization of the model is presented in Table 3.
We used the fixed rake of the focal mechanism given by the
preliminary Harvard CMT. The final Harvard CMT is not
significantly different from the preliminary solution. The fault
plane was taken as a 180 x 90 km2 rectangle divided into 162
subfaults of 10 x 10 km2. Each subfault had five intermediate
point sources along-strike and five intermediate point sources
down-dip. The size of the fault plane was changed from an
initial size of 110 x 60 km2, estimated by RESCO as an aftershock area (Ramirez Vazquez et al. 1995), to the final size of
180 x 90 km', which included all the significant displacements
along the fault. The hypocentre was embedded 40 km from
the left edge of the 180 km long fault, 40 km down-dip from
the top of the fault, which was situated practically along the
trench axis. A boxcar source time function of width 2.0 s was
used for each discrete rupture interval of each subfault.
Mendoza & Hartzell ( 1988a) pointed out that rupture-velocity
values generally remain within 0.8 to 0.9 of the shear-wave
velocity for most earthquakes. Therefore, the rupture velocity
was taken equal to 2.8 km SS', which is ~ 8 per
0 cent of the
shear-wave speed in the layer containing the hypocentre.
We had a set of 30 broad-band and long-period digital
records of this earthquake from USGS containing waveforms
of P waves. For inversion, we selected 14 good-quality vertical
teleseismic broad-band displacement records from stations of
the Global Digital Seismograph Network (GDSN) situated
30" to 90" from the epicentre (Table 4). These provided good
azimuthal coverage of our event. We used the first 70 s samples
Fault length
Fault width
Focal depth
Fault strike
Fault dip
Rupture velocity
Vp,km s-'
K,km s-'
Density, g cm-3
2.6
4.6
5.8
6.0
7.5
1.5
2.7
3.4
3.5
4.3
2.1
2.3
2.5
2.7
2.9
180 km
90 km
20 km
390"
16"
2.8 km s - '
Thickness, km
0.5
5.5
6.0
11.0
30.0
Subfault length
10 km
Subfault width
10 km
Number of subfaults
162
Number of time windows
3
Fault rake
104"
Note. Fault strike, dip and rake were taken from the preliminary
Harvard CMT solution.
Table 4. Broad-band records used in source inversion.
Station
Code
Distance, deg
Azimuth, deg
ElY
Newport
Yorkshire
Lakeside
Harvard
Lisbon
La Paz
Villa Florida
Adak
Brasilia
Paso Flores
Rarotonga
Borgarnes
Afiamalu
EYMN
NEW
YSNY
LSCT
HRV
LBNH
LPAZ
CPUP
ADK
BDFB
PLCA
RAR
BORG
AFI
30.94
31.11
32.27
35.06
36.55
37.22
49.99
64.14
64.44
65.32
67.13
67.13
70.27
73.78
17.16
- 16.37
37.40
42.67
42.53
39.82
131.91
132.96
-40.84
117.82
152.55
- 123.70
26.91
-111.16
of P waves (Fig. 2). This sample length was chosen because of
computational limitations.
The synthetic P waves from the subfault containing the
hypocentre were then aligned with the initial observed P waves
for each station. We had no short-period records of this event
to compare the timing of our data with that of synthetic
records, and could only partially compensate it using the time
of arrivals obtained from the long-period records. Nevertheless,
Fig. 3 shows a good coincidence of the calculated synthetics
with the observed records. Some of the data records (CPUP,
LPAZ, PLCA, AFI, and BDFB) show late complexity, which
the model attempts to fit. This results in low synthetic-toobserved peak amplitude ratios, with similar waveforms. Four
Table 1. Parameters of the earthquake of 1995 October 9
Origin time
hh mm ss
Lat.
N
Long.
W
Depth
km
M,
mb
Mo
E20 N m
M,
Energy
Nm
Ref.
15 35 53.9
15 36 28.8
15 35 50.6
19.05
19.34
18.81
104.20
104.80
104.54
33
15
20
7.4
6.6
9.1
11.0
7.9
8.0
1.8 E l 5
1
2
3
Note. 1, EDR USGS; 2, Harvard CMT; 3, RESCO.
0 1997 RAS, GJI 130, 220-228
Rupture history of the Jalisco, Mexico, earthquake
1
-
A = 73.78"
AF1
I
70 SEC
Figure2. Examples of the broad-band displacement records of P
waves (vertical component) for the nearest (EYMN) and farthest (AFI)
seismic stations. These seismograms were digitized with a sampling
interval of 0.1 s. Arrows indicate the end of the 70 s samples taken
into account during the inversion.
223
R U P T U R E MODEL O F THE JALISCO
EARTHQUAKE
The rupture model is presented in terms of a contour map of
the final slip on the fault, and the integrated moment-release
function (Figs 4 and 5).
The contour map of the final slip on the fault (Fig. 4) was
obtained using three 2 s time windows and represents all fault
displacements occurring within 6 s of the passage of a rupture
front propagating at 2.8 km s-' away from the hypocentre. We
can see three main zones of maximum coseismic slip, or
asperities, destroyed during the earthquake. The first of them,
with a maximum slip of 610cm, was situated around the
hypocentre. Its maximum was situated about 20 km up-dip
and to the south of the hypocentre. The area that had a slip
of more than 200 cm was about 1000 km'. Maximum slip was
observed at a depth between 12 and 17 km.
The second largest asperity was situated opposite the first,
down-dip and to the north of the hypocentre. Its maximum
slip was 500cm, and the area of maximum slip at depths of
20 to 25 km was about 500 km2.
The last significant asperity had a maximum slip of 400 cm,
an area of maximum dislocations of about 300 km2, and was
situated at depths of 25 to 28 km, about 100 km to the north
of hypocentre.
ADK
LBNH
1.0920
0.8966
AFI
LPAZ
0.7869
0.4957
BDFB
LSCT
0.806 1
0.8353
BORG
NEW
0.8529
1.230 1
CPUP
PLCA
0.6070
0.55 10
EYMN
RAR
0.9497
0.8485
HRV
YSNY
1.0352
0.7061
Figure 3. Comparison of the observed (solid line) and synthetic (dashed line) P waveforms corresponding to t he slip distribution inferred for the
Jalisco earthquake. The ratio of synthetic-to-observed peak amplitude is indicated for each seismogram pair.
of these five stations were situated in the South American
region and may show a regional effect in the formation of the
P-wave train. This effect was later compensated for by the
introduction of the amplitude scaling factor while computing
the dislocation model. The prescribed moment underestimated
the actual value only by a factor of about 1.3.
0 1997 RAS, GJI 130, 220-228
This description of asperity distribution is valid in the
context of our chosen constraints of the fault model and may
be dependent upon the choice of rupture velocity. At the same
time, we expect the relative position and number of the main
asperities to be sufficiently stable.
The moment-release function (Fig. 5) shows that the ruptur-
224
V. M . Zobin
mum at the 40th second, and then a sharp decrease was
observed.
E
2
RUPTURE HISTORY
&.
Hartzell et al. (1994) demonstrated that a presentation of the
evolution of the rupture in a series of plots that contour the
slip within successive time intervals after the origin time may
be a useful supplement to a contour map of the final slip on
the fault. Fig. 6 shows the spatio-temporal development of the
rupture for 60s; it shows six 10s intervals of the rupture
history. It can be seen that during the first 10s the faulting
occurred around the hypocentre, breaking up-down elongated
asperity. Then, during the next lOs, both upper and lower
ends of the initial rupture continued to grow bilaterally,
breaking two main asperities of the fault plane with a displacement of up to 600 cm within the upper asperity at a depth of
12 to 16 km and up to 500 cm within the lower asperity at a
depth of 20 to 25 km. The first stage of the rupture history
was thus over, and during the next 40 s the frontal development
of the rupture to the north could be seen. During this frontal
movement, relatively high slips were observed only during the
interval between 31 and 40 s, when displacement reached up
to 300-400 cm, marking a system of two asperities-the upper
one being a vertical narrow asperity of about 50 km length
with up to 300 cm of slip, and other one having a smaller size
but with higher slip (up to 400 cm).
A comparison with the moment release shown in Fig. 5
EE
0
40
20
TIME, SEC
Figure 4. The moment release as a function of time. The maximum
value of moment release is marked.
ing of the main shock with a total seismic moment 5.85 x 10''
N m continued for 55 s. There are at least two stages in the
rupture development. First, there was a sharp increase in
moment release for 10 s, reaching a maximum of 3.05 x l O I 9
N m , and then a sharp decrease of the moment release back
to the initial level. The total time for the first stage was about
18s. The second stage of the rupturing (about 37s) was
characterized by a small moment increase with a slight maxi-
SE
9
0
NW
E
!& 30
&
E
A
20
E
8
c6"
c1
PI
W
n
w
60
2
31
0
60
90
120
150
DISTANCE ALONG STRIKE, K M
30
180
Figure 5. The contours of the final slip in centimetres on the fault plane. The hypocentre is indicated by a black hexagon. Cumulative slip is
contoured at 100 cm intervals beginning from 100 cm.
0 1997 RAS, GJI 130, 220-228
Rupture history of the Jalisco, Mexico, earthquake
225
0-10 SEC
Bl
31-40 SEC
41-50 SEC
.
,
_ y , i
.
I
/
i
i
_
Y
DISTANCE ALONG STRIKE, KM
Figure 6. The evolution of the rupture model. The time-frames show fault-slip distribution at discrete 10 s intervals beginning at the onset of
moment release. Slip greater than 100 cm is contoured at 100 m intervals. The hypocentre is indicated by a black hexagon.
shows that the first large impulse in moment release during
the first 18 s might be related to the breaking of the two large
asperities above and below the hypocentre. The following
smoother impulse of the moment release corresponds to the
frontal movement of the rupturing to the north, breaking two
asperities about 90 km to the north of the hypocentre at the
40th second of the rupturing.
COMPARISON O F T H E R U P T U R E M O D E L
W I T H GPS OBSERVATIONS
We can compare our results with observations from 11 GPS
stations situated on the coast of Jalisco, compiled by
Melbourne et al. (1997). Their records show a consistent
vertical subsidence, verified by tide gauge data and southwestdirected extension decreasing landwards (Fig. 7). The direction
and amplitude of the horizontal and vertical offsets recorded
at the GPS stations are shown in Fig. 7, together with the
distribution of the main asperities with a slip area of more
than 200 cm corresponding to our model. It can be seen that
the vectors of displacement recorded at the nearest stations
are directed to our three main asperities. The maximum
horizontal displacement, of 910 mm, was recorded at station
CHAM, situated a reasonable distance from the epicentre, but
just over the asperity at a depth of 25-28 km with slip of up
to 400 cm.
The’inversion of GPS data by Melbourne et al. (1997)
allowed them to place some constraints on fault displacements.
The earthquake source was characterized by up to 500 cm of
slip and occurred within the upper 15 km. This reconstruction
was made using the centroid position of the source as the
hypocentre, which was about 60km to the north of our
hypocentre. As a result, the source reconstructed by inversion
of the GPS data is also situated about 60 km to the north of
our epicentre (Fig. 7, isolines drawn by dashed lines).
It can be seen that this model differs from ours. The GPS
0 1997 RAS, GJI 130, 220-228
inversion did not show the maximum slip zone near the El
Gordo Graben and concentrated the slip within one central
region instead of our two northern zones of large slip (they
noted a 200 cm slip source 90 km to the south, but it is too
small). However, similar values of maximum slip were observed
on the earthquake fault in the centre of the fault. This difference
in reconstruction could be a result of the fact that the distribution of GPS stations is one-sided with respect to the
epicentre, while the inversion of the broad-band seismic records
was done with a better distribution of seismic stations. It could
also be related to the choice of the centroid epicentre in the
GPS model, which is more northerly than the regional network
location of the epicentre used in our model. Geodetic data
give a model with smoother slip compared to seismic data;
this was also noted for the 1994 Northridge earthquake,
California (Wald, Heaton & Hudnut 1996), where one central
asperity was obtained from geodetic data compared with two
separate asperities obtained from teleseismic and strongmotion data.
SLIP D I S T R I B U T I O N A N D AFTERSHOCKS
Fig. 8 shows the distribution of aftershocks recorded during
the first 20 days after the main shock, together with the
distribution of the main fault asperities. 26 events of magnitude
mb from 3.7 to 5.5 published by the USGS Earthquake Data
Reports (EDR) are plotted. One can see that all the aftershocks
occurred between the main asperities broken during the main
shock. This scheme supports the earlier result obtained by
Mendoza & Hartzell(1988b) for a few Californian earthquakes,
that aftershocks occur mostly outside or near the edges of the
source areas indicated by the pattern of the main-shock slip.
Mendoza & Hartzell have proposed that the spatial distribution of aftershocks reflects either a continuation of slip in
the outer regions of the areas of maximum coseismic displace-
226
V. M . Zobin
20 N
19
~~
106
104 W
Figure 7. A comparison of the obtained asperities characterized by maximum slip and the data of GPS stations (Melbourne et af. 1997). Black
circles show the seven GPS stations nearest to the epicentre from the total of 11 stations operating during the earthquake. Vectors obtained by
GPS observations are shown without scaling, only indicating the direction offset. The horizontal and vertical offsets in mm recorded at each station
are written in parentheses. A projection of the fault model is shown by the rectangle. The distribution of slip obtained by P-wave inversion is
shown for zones of maximum slip beginning from 200 cm (or 33 per cent of the maximum recorded slip) with an interval of 200 cm. The distribution
of slip obtained by GPS record inversion (Melbourne et al. 1996) is shown by dashed lines for isolines of 200 cm and 500 cm. The epicentre of the
Jalisco earthquake is shown by a black hexagon, and the epicentre of CMT is shown by a white hexagon. Bathymetry is shown for each 1000 m
of the ocean floor.
ment or the activation of subsidiary faults within the volume
surrounding the boundaries of the main-shock rupture.
The comparative distribution of the model asperities and
aftershocks may indirectly support our model.
SEISMOTECTONIC IMPLICATIONS OF THE
EARTHQUAKE MODEL
Now let us return to Fig. 1. This figure shows the plate
tectonics in the region of the Jalisco block. The earthquake
epicentre was located near the boundary zone between two
lithospheric plates, Rivera and Cocos. This boundary zone is
supposedly marked by the El Gordo Graben cutting the
Middle American trench (Bourgois et al. 1988). A high seismic
activity for the Cocos plate, subducting with a relative velocity
of 5.2cmyr-', is out of question, whereas the low seismic
activity of the .Rivera plate subducting with a velocity of
2.0 cm yr-' has been postulated as aseismic (Nixon 1982).
Therefore, it is very important to study the rupture process of
the Jalisco earthquake of 1995 October 9.
Fig. 6 sheds some light on this question. It is clearly seen
from Fig. 6 that, during the first 20 s, rupturing was observed
from the hypocentre to the surface and to the south (or to the
Rivera-Cocos boundary) as well as to the north and deeper.
Afterwards, however, there was only slip to the north along
the subducting area of the Rivera plate.
At the same time, it is important to note that the slip at the
Rivera-Cocos boundary zone was also very active. The largest
slip, up to 610 cm, was observed along this zone. Fig. 8 shows
that the two strongest aftershocks with magnitudes mb 5. 1
and 5.5 also occurred along this boundary line.
RESULTS A N D DISCUSSION
The finite-fault, broad-band teleseismic P waveform inversion
was applied to the large Jalisco, Mexico earthquake of
1995 October 9. This inversion allowed a description the
spatio-temporal rupture history of this event.
(1) The rupture duration is about 55 s, and slip occurred
within an area of about 1 8 0 x 9 0 km2 along the Middle
American trench in the depth interval from 9 to 33 km.
(2) There were two main stages in the rupture history:
during the first 18 s two main asperities at a distance of about
30 km from the earthquake hypocentre were ruptured with a
maximum slip of up to 610 cm at depths of 12-15 km; during
the next 37 s there was rupturing to the north along the trench.
The slip distribution shows that the faulting originating near
the boundary between the Rivera and Cocos plates (Fig. la)
occurred mainly along the Rivera-North American plate
boundary and might be related to the subduction activity of
the Rivera plate.
0 1997 RAS, GJI 130, 220-228
Rupture history of the Jalisco, Mexico, earthquake
227
20 r
19
106
105
104 W
Figure 8. Slip distribution for the main shock and the aftershock pattern. A projection of the fault model is shown by the rectangle. The
distribution of slip obtained by P-wave inversion is shown for zones of maximum slip beginning from 200 cm (or 33 per cent of the maximum
recorded slip) with an interval of 200 cm. The epicentre of the Jalisco earthquake is shown by a black hexagon, and the epicentres of aftershocks,
by black triangles. The two strongest aftershocks, of magnitude mb 5.1 and 5.5, are shown by triangles in circles. Bathymetry is shown for each
1000 m of the ocean floor.
The model obtained consists of two main elements: the time
history of moment release and the slip distribution. The first
element, the time history of moment release, is usually well
resolved in teleseismic-based studies. However, problems might
occur because of a dependence of the spatial slip location upon
the assumed rupture velocity. Of course, any change in the
rupture velocity could move the position of slip peaks.
Nevertheless, it was shown by Mendoza (1996) that a change
of rupture velocity from 2.5 to 3.5 km SC' provided similar
patterns of coseismic slip on the fault for the 1995 Guerrero,
Mexico, earthquake, with a change in slip-peak position of no
more than 10 km.
The results were obtained using only the P-waveform inversion. Of course, the addition of S-wave and strong-motion
data could provide more constraints on our model.
Nevertheless, the practice of inverting the teleseismic P waveforms alone is widely used (e.g. Mendoza 1996). The dislocation
model obtained from the teleseismic P-wave inversion for the
1985 Michoacan, Mexico, earthquake was found to be similar
to models inferred from the strong-motion data alone and
from simultaneous inversion of both data sets (Mendoza &
Hartzell 1989). The similarity of dislocation models derived
from teleseismic P and S waveforms was shown for the 1989
Loma Prieta, California, earthquake by Hartzell, Stewart &
Mendoza (1991).
The results obtained are similar to those by Tanioka & Ruff
(1996), who analysed the source process of the Jalisco earthquake using teleseismic waveforms recorded at IRIS stations
and a tsunami waveform. Their body-wave inversion indicated
that the total duration of the rupture was about 70 s with at
0 1997 RAS, G J l 130, 220-228
least two significant subevents. The estimates of the rupture
length (from epicentre) vary from 140 km (body waves) to
250 km (tsunami waveforms). Directivity analysis of the body
waves showed that the rupture propagated from the epicentre
in the direction N6O0-70"W, i.e. along the Rivera-North
America boundary. They suggested that the 1995 Jalisco
earthquake is the characteristic earthquake in the Jalisco
region.
A comparison with the source process of the 1932 June 3
Jalisco earthquake (Eissler & McNally 1984; Singh, Ponce &
Nishenko 1985) shows that the large events both developed in
a similar way. Their aftershock areas were situated along the
Rivera-North America boundary, and their rupturing had a
complex character (Singh & Mortera 1991). We could also
propose that rupturing along the Cocos-Rivera boundary zone
was observed during the event of 1932. Singh et al. (1985)
noted that the reports about tsunami in Manzanillo originating
after the event of 1932 may indicate that the rupture involved
a substantial portion of the sea floor. The probable position
of this sea-floor portion could be opposite Manzanillo, where
the maximum slip was recorded in 1995, or along the RiveraCOCOS
boundary.
ACKNOWLEDGMENTS
The broad-band digital seismic records of the Global Digital
Seismograph Network were supplied by C. Mendoza from the
US Geological Survey. Steve Hartzell and Carlos Mendoza
provided me with their programs and their kind assistance in
studying the finite-fault waveform inversion method during
228
V. M . Zobin
my short stay a t Golden, US Geological Survey a n d later
during m y work on this paper. T h e comments of t wo anonymous reviewers were very valuable. I thank Gabriel Reyes for
the d at a ab o u t the Jalisco earthquake location by RESCO.
The preprint of the paper by Melbourne et al. (1997)containing
the GPS d a t a was very useful for my study. I thank C. Famozo,
A. Santillan, G . Marmolejo a n d M. Mayoral for help in the
computational problems.
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