The 2003 Armen'a, M6xico Earthquake (M w 7.4): Mainshock and Early Aftershocks Fco. Javier N fiez-Corn 1, Gabriel A. Reyes-D vila 2, Marta Rutz L6pez I Elizabeth lrejo G6mez I Miguel A Camarena-Garcia I and C Ariel Ramirez-Vazquez 2 INTRODUCTION On 22 January 2003 an earthquake of M w 7.4 occurred off the Pacific coast of the state of Colima, Mdxico, near the town of Armerfa. The damage pattern and effects of this earthquake in the nearby areas of Colima and Jalisco were dif- ferent and more severe than those from recent previous big earthquakes in this region. The mainshock and its aftershocks were recorded by local seismic networks. The Jalisco region is one of the most active seismic regions in Mexico and is characterized by complex seismotec- tonics. The largest earthquake (M 8.2) of the twentieth cen- tury in Mdxico occurred in 1932 in this region, followed fifteen days later by another large event (M 7.8). Singh et al. (1985) studied these earthquakes and concluded that their composite rupture area extended along the whole coast of Jalisco and Colima; they also proposed a recurrence time of 77 years for this region. In 1995, a M w 8.0 earthquake occurred near Manzanillo but did not rupture the whole 1932 rupture area, leaving unruptured regions in the north, the Vallarta gap, and to the south, the Colima gap (Figure 1). In spite of the occurrence of these large earthquakes and the high hazard associated with the seismic activity in the region, only one permanent sta- tion~at Chamela (CJIG), from the Mexican Seismic Net- work (SSN)~and the Red Sfsmica Telem~trica de Colima (RESCO)~an analog, 1 Hz vertical sensor network~were sited on the Colima graben to monitor seismicity in that area and at the Colima volcano (Figure 1). By the end of 2001 the Jalisco Civil Defense and the Centro de Sismologfa y Volca- nologfa de Occidente had begun to deploy the Red Sfsmica Digital Telem~trica de Jalisco (RESJAL) (Nfifiez-Cornt~ et al., 2001). Currently, RESJAL operates six telemetered and five autonomous stations, all with 24-bit data loggers and three- channel LE3D 1 Hz sensors (Figure 1). Using data from RESCO and from microearthquakes studies carried out in the Jalisco region between 1996 and 1998, Ntifiez-Cornti et al. (2002) found that the subduction of the Rivera Plate under the North America Plate is associ- 1. SisVOc, Universidad de Guadalajara 2. RESCO, Universidad de Colima ated with a double seismic zone, with a subduction angle smaller than 15 ~ up to 160 km from the trench. N~fiez- Corntl et al. (2002) also identified various seismogenic zones and showed that the data from SSN alone are not sufficient to study the seismic patterns in the Jalisco region accurately. The seismicity recorded throughout 2002 by RESJAL and RESCO confirm these results (Ntifiez-Corntl et al., 2003). TECTONICSETTING In the Western Mexican Volcanic Belt, where the North American, Pacific, Cocos, and Rivera lithospheric plates interact and several triple junctions have been proposed (Fig- ure 2), the seismotectonic setting is poorly understood. A tec- tonic unit known as the Jalisco Block has been proposed in this region (Luhr et al., 1985; Bourgois et al., 1988; DeMets and Stein, 1990; Allan et al., 1991; Gardufio and Tibaldi, 1991; Ferrari et al., 1994; Ferrari and Rosas-Elguera, 1996). The Jalisco Block is limited to the east by the Colima rift zone (CRZ), which extends northward from the Pacific coast and connects at its northern end with two other major extensional structures: the Tepic-Zacoalco rift zone (TZRZ) (trending roughly northwest-southeast), which is defined as the north boundary of the Jalisco Block, and the Chapala rift zone (trending roughly east-west). The connection between the northwest border of the Jalisco Block and the continent (the Tamayo Fault zone) is not well defined. Previous studies have related this border to the San Blas Fault (SBF) as a continua- tion of the TZRZ (Gastil et al., 1978), or to the Islas Marias escarpment (IME) (west of Islas Marias) and the Banderas Fault (BF), which crosses the Bahfa de Banderas and contin- ues along the Vallarta graben to join the TZRZ (Figure 2) (Johnson and Harrison, 1990). Another possible connection is to the Islas Marias Fault (IMF). The last two possibilities suggest the existence of an additional small block, the Tres Marias Block. The BaMa de Banderas area may be experienc- ing strong crustal stresses as a result of the convergence direc- tion of the Rivera Plate (Kostoglodov and Bandy, 1995). There is general agreement that the Jalisco Block is start- ing to separate from the continent, moving in a west-north- west direction away from mainland central Mexico (Luhr et 734 Seismological Research Letters Volume75,Number6 November/December2004
Transcript
The 2003 Armen'a, M6xico Earthquake (M w 7.4): Mainshock and Early Aftershocks Fco. Javier N fiez-Corn 1, Gabriel A. Reyes-D vila 2, Marta Rutz L6pez I Elizabeth lrejo G6mez I Miguel A Camarena-Garcia I and C Ariel Ramirez-Vazquez 2
INTRODUCTION
On 22 January 2003 an earthquake of M w 7.4 occurred off the Pacific coast of the state of Colima, Mdxico, near the town of Armerfa. The damage pattern and effects of this earthquake in the nearby areas of Colima and Jalisco were dif- ferent and more severe than those from recent previous big earthquakes in this region. The mainshock and its aftershocks were recorded by local seismic networks.
The Jalisco region is one of the most active seismic regions in Mexico and is characterized by complex seismotec- tonics. The largest earthquake (M 8.2) of the twentieth cen- tury in Mdxico occurred in 1932 in this region, followed fifteen days later by another large event (M 7.8). Singh et al.
(1985) studied these earthquakes and concluded that their composite rupture area extended along the whole coast of Jalisco and Colima; they also proposed a recurrence time of 77 years for this region.
In 1995, a M w 8.0 earthquake occurred near Manzanillo but did not rupture the whole 1932 rupture area, leaving unruptured regions in the north, the Vallarta gap, and to the south, the Colima gap (Figure 1). In spite of the occurrence of these large earthquakes and the high hazard associated with the seismic activity in the region, only one permanent sta- t ion~at Chamela (CJIG), from the Mexican Seismic Net- work (SSN)~and the Red Sfsmica Telem~trica de Colima (RESCO)~an analog, 1 Hz vertical sensor network~were sited on the Colima graben to monitor seismicity in that area and at the Colima volcano (Figure 1). By the end of 2001 the Jalisco Civil Defense and the Centro de Sismologfa y Volca- nologfa de Occidente had begun to deploy the Red Sfsmica Digital Telem~trica de Jalisco (RESJAL) (Nfifiez-Cornt~ et al.,
2001). Currently, RESJAL operates six telemetered and five autonomous stations, all with 24-bit data loggers and three- channel LE3D 1 Hz sensors (Figure 1).
Using data from RESCO and from microearthquakes studies carried out in the Jalisco region between 1996 and 1998, Ntifiez-Cornti et al. (2002) found that the subduction of the Rivera Plate under the North America Plate is associ-
1. SisVOc, Universidad de Guadalajara 2. RESCO, Universidad de Colima
ated with a double seismic zone, with a subduction angle smaller than 15 ~ up to 160 km from the trench. N~fiez- Corntl et al. (2002) also identified various seismogenic zones and showed that the data from SSN alone are not sufficient to study the seismic patterns in the Jalisco region accurately. The seismicity recorded throughout 2002 by RESJAL and RESCO confirm these results (Ntifiez-Corntl et al., 2003).
TECTONIC SETTING
In the Western Mexican Volcanic Belt, where the North American, Pacific, Cocos, and Rivera lithospheric plates interact and several triple junctions have been proposed (Fig- ure 2), the seismotectonic setting is poorly understood. A tec- tonic unit known as the Jalisco Block has been proposed in this region (Luhr et al., 1985; Bourgois et al., 1988; DeMets and Stein, 1990; Allan et al., 1991; Gardufio and Tibaldi, 1991; Ferrari et al., 1994; Ferrari and Rosas-Elguera, 1996). The Jalisco Block is limited to the east by the Colima rift zone (CRZ), which extends northward from the Pacific coast and connects at its northern end with two other major extensional structures: the Tepic-Zacoalco rift zone (TZRZ) (trending roughly northwest-southeast), which is defined as the north boundary of the Jalisco Block, and the Chapala rift zone (trending roughly east-west). The connection between the northwest border of the Jalisco Block and the continent (the Tamayo Fault zone) is not well defined. Previous studies have related this border to the San Blas Fault (SBF) as a continua- tion of the TZRZ (Gastil et al., 1978), or to the Islas Marias escarpment (IME) (west of Islas Marias) and the Banderas Fault (BF), which crosses the Bahfa de Banderas and contin- ues along the Vallarta graben to join the TZRZ (Figure 2) (Johnson and Harrison, 1990). Another possible connection is to the Islas Marias Fault (IMF). The last two possibilities suggest the existence of an additional small block, the Tres Marias Block. The BaMa de Banderas area may be experienc- ing strong crustal stresses as a result of the convergence direc- tion of the Rivera Plate (Kostoglodov and Bandy, 1995).
There is general agreement that the Jalisco Block is start- ing to separate from the continent, moving in a west-north- west direction away from mainland central Mexico (Luhr et
734 Seismological Research Letters Volume75, Number6 November/December2004
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A Figure 2. Tectonic framework of western Mexico. SBF: San Bias Fault; BF: Banderas Fault; IMF: Islas Marias Fault; IME: Islas Marias escarpment; trian- gles: active volcanoes; ?: proposed tectonic features.
Seismological Research Letters November/December2004 Volume75, Number6 735
TABLE 1 First Solutions Reported for Armeria Earthquake Locations
Source OriginTime Lat(N) Long (W) Depth (km) Magnitude Mo (N-m) Strike Dip Rake
RESCO 02:06:33.8 18.625 ~ -104.125 ~ 10
SSN 02:06:34.6 18.60 ~ -104.22 ~ 9.3 M e 7.6
USGS 02:06:34 18.807 ~ -103.886 ~ 5 M,, 7.8
CMT 02:06:47.3 18.77 ~ -103.89 ~ 32.6 M,, 7.4
Yagi 18.625 ~ -104.125 ~ 20 Mw7.4
1.20 x 1020 267 ~ 8 ~ 50 ~
1.62 x 10 20 305~ 17 ~ 103 ~
1.45 x 1020 300 ~ 20 ~ 93 ~
al., 1985; Bourgois et aL, 1988; Allan et al., 1991; Gardufio and Tibaldi; 1991). Alternative models have also been pro- posed, however (DeMets and Stein, 1990; Ferrari et al., 1994; Rosas-Elguera et al., 1996).
To date, the geometry of the Rivera Plate below the Jalisco Block is also not clear. Eissler and McNally (1984) and Singh et al. (1985) suggest a dip angle of 20 ~ A dip angle of 12 ~ was obtained from local seismicity studies at the south- east border by Ntifiez-Corn6 and Sanchez-Mora (1999), and similar values were obtained from recent reflection-refraction studies carried out in central and northern parts of the Jalisco coast (Dafiobeitia et al., 1997). More controversial is the exact location and shape of, and relative motion along, the subducted Rivera-Cocos Plate boundary. Does it lie below the CRZ (Nixon, 1982) or to the north (Eissler and McNally, 1984); is it a convergent (DeMets and Stein, 1990) or a diver- gent (Bandy, 1992) boundary? The origin of the CRZ, which is roughly delimited by the rivers Armerfa and Cohuayana, has been related to this boundary (Nixon, 1982; Bandy, 1992; Ferrari el al., 1994), to a spreading center location (Luhr et al., 1985), and to the southeastward motion of the forearc region southeast of the CRZ (DeMets and Stein, 1990). It is thus not clear what underlies the CRZ: the Rivera or the Cocos Plate or both; a contact zone between both plates, a tear zone, a spreading center, or something else.
FIRST REPORTS
On 22 January 2003, a M w 7.4 earthquake took place off- shore the town of Armerla on the Pacific coast of Colima, M4xico, within the Jalisco Block. This earthquake produced severe damage in the states of Colima and Jalisco in a strip from the epicentral coastal region to the north, including Colima City and Zapotitlan de Vadillo Jal. The damage at Ciudad Guzman was minimal. The intensity and distribution of the damage pattern reported by Colima and Jalisco Civil Defense are different from those reported for the 1995 earth- quake. Small tsunami were reported along the coast of Colima and Jalisco.
Initial locations for the mainshock from SSN and world data centers suggested that the event broke the Colima gap and was a typical shallow thrust earthquake with a dip angle between 2o-20 ~ and a strike between 267~ ~ (Table 1 and Figure 2). Yagi et al. (2003) modeled this earthquake assum-
ing a particular plate boundary geometry (Pardo and Suarez, 1993) and a fault mechanism as reported in Table 1.
In Figure 3 we also show the preliminary locations (pro- cessed in near real-time and not revised) reported by RESCO for 82 aftershocks that occurred during the first 72 hours after the mainshock. Figures 4A and 4B show east-west and north-south sections of the hypocentral distribution. From these figures we observe two main groups of aftershocks, one near the RESCO mainshock epicenter and another to the northwest of this epicenter. Figure 5 shows a 3D projection of a profile perpendicular to the trench along line A-A' (Figure 3). In these figures, the aftershocks appear to be distributed in narrow band approximately parallel to the trench axis at a dis- tance of 75 km from the trench, near the shoreline. All of the aftershocks are located west of the Armerfa River, which marks the western limit of the CRZ. The distribution of hypocenters in depth is almost vertical. Most of the events are between 0-18 km deep; a few events also occur between 20 and 25 km. Few of the aftershocks seem to be located on the subduction interface area (Figure 5). Assuming a 12 ~ dip angle, the locations of some of the deeper events do fall on the plate boundary. None of the hypocenters is below the pre- sumed subduction interface as observed in other studies (Pacheco et al., 1997; Ntifiez-Corntl et al., 2002). The pat- terns observed in these figures do not support a rupture of the Colima gap.
LOCAL DATA
Since most RESCO records were saturated, we combined data recorded by the RESCO and RESJAL networks to improve the quantity and quality of the data and the event locations. We used data from 18 to 20 stations to locate most aftershocks.
The RESCO P-wave velocity model is determined from the seismicity around the volcano. For this study we created a regional velocity model using a local velocity model obtained from a seismic refraction experiment carried out in the Colima region in 1991 using quarry blasts and sea shots (Ntifiez-Cornfi et al., 1994). The P-wave velocity structure in the shallow layers is very well controlled from these results; we adjusted the VJB01 model used by RESJAL for the bot- tom layers. The model obtained (VJB02) for the region is show in Figure 6.
736 Seismological Research Letters Volume75, Number6 November/December2004
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A Figure 3. Previous earthquakes in the area: (1) 19951009, M 8.0 (2) 2000306, Mw5.3; (3)20010429, M 5.8; (4)20010508, M 5.4; (5) 20010519, M 6.3. Epicenters reported for the 2003 mainshock: (star) RESCO; (a) SSN (b) NEIC/USGS; (c) CMT. Preliminary aftershock locations reported by RESCO for the first 72 hours. CG: Colima Gap.
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A Figure 4. (A) East-west and (B) north-south profiles for aftershocks (RESCO preliminary locations).
Seismological Research Letters November/December2004 Volume75, Number6 737
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A total of 72 hypocenters was relocated for the first 72 hours using Hypo71 (Lee and Lahr, 1978) and the VJB02 velocity model; all events were processed using ten different initial trial depths within 3 and 40 km. Optimum residual values were rms < 0.50 s, erh and erz < 2.0 km. Local magni- tude was evaluated using S-wave maximum amplitude (Lay and Wallace, 1995) at RESJAL stations. The relocated events are the largest in this period; the best solution for the main- shock hypocenter has a depth of 5 km.
Our relocations improve the results of the RESCO pre- liminary reports. The epicenters of relocated aftershocks are shown in Figure 7, in which two clusters are clearly seen. One cluster (95C) lies to the west, in the 1995 earthquake epicen- tral zone; the two larger aftershocks (20030122, 19:41, 18.8225~ 104.4866~ -7.1 km, M~ 5.7; 20030122, 20:15, 18.7619~ 104.4797~ -6.5 km, M~ 5.3) occurred in this zone almost 20 hours after the main event. The second cluster (AC) lies around the mainshock epicenter, mainly to the north. The epicenters are located west of the Armerfa River and/or a submarine feature that we named Armerfa Canyon (reported as Canyon B by Khutorskoy et al., 1994), which seems to be the continuation of this river in the ocean floor but is definitely out of the CRZ. Only four events are
located inland with a north-south alignment. In the east-west section (Figure 8A) the 95C and AC clusters are clearly iden- tified; the events are distributed in two vertical bands between 2 and 15 km depth, with only a few below 16 km. In the north-south section (Figure 8B) we observe again a narrow vertical distribution as shown in Figure 4B. In a profile per- pendicular to the trench (Figure 9) we observe two tenden- cies, an almost vertical one (empty boxes) between 60 and 70 km from the trench axis corresponding to the 95C events, and the AC hypocenters (empty circles), which appear to be distributed in a path with a dip angle that varies from 40 ~ to 12 ~ but east of the trench axis and above a hypothetical inter- plate plane dipping 12 ~ .
In the longitude vs. time plot (Figure 10A) we observe additional differences between the 95C and AC clusters. Activity in the 95C cluster started 18 hours after the main- shock and took place only during two periods of about 12 hours each, while at the AC cluster the activity occurred within the 72-hour period. On 22 January between 1342 and 1806 UT a swarm occurred in the AC cluster area; these epi- centers are located from 104.00 ~ in the east (mouth of Arm- erfa River) to 104.35 ~ in the west (Manzanillo), in the area known as "Cafiones de Manzanillo" (Figure 7). In contrast,
738 Seismological Research Letters Volume 75, Number6 November/December2004
0
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0
5,,
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A Fioure 7. Aftershocks relocated using RESJAL and RESCO data. Focal mechanism solutions for mainshock and bigger aftershocks (joint solution). Dashed line: reference line for Figure 11.
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A Fioure 8. Profiles (A) east-west and (B) north-south for relocated aftershocks. Mainshock is also shown (star).
Seismological Research Letters November/December 2004 Volume 75, Number 6 739
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A Figure 10. (A) Sequence of the aftershocks along east-west projection. Fault length proposed for rupture area between dashed lines. Dotted line marks the mouth of Armerfa River. (B) Sequence of AC aftershocks along depth projection. Fault width proposed for rupture area between dashed lines.
the activity associated with the AC clusters in the first 72 hours is mainly concentrated between dashed lines in Figure 10A; with these data we mark the limits for the AC cluster and interpret it as a projection of the fault length.
In the depth vs. time plot (Figure 10B), where only the AC events are shown, it can be seen that the swarm took place between 2 and 16 km depth. After this swarm most of the activity remains between 2 and 11 km depth (dashed lines, Figure 10B). We interpret this fact as the projection of the fault width.
In Figure 11, a 3D projection of a 352 ~ azimuth section of AC events, we observe an almost linear distribution, sug- gesting a planar distribution, dipping about 40 ~
FOCAL MECHANISM
To evaluate focal mechanisms we used polarities of first arriv- als at RESJAL and RESCO stations plus CJIG and MOIG from SSN. We used the MEC93 program (Ntifiez-Cornti and Sanchez-Mora, 1999), which is based on the algorithm pro-
740 Seismological Research Letters Volume 75, Number6 November/December2004
O t * ............................................. ---= :" r~ ; ~ ' ............................................................... ["
A: ~ 41 M P: t M M A~ 312 40 M P: 22,3 77 411 M 1": 354 4 B: t,33 SO 88 2':. 48 .5
, i Figure 12. (A) Focal mechanism for mainshock from local data. (B) Composite first-motion focal mechanism solution for two principal aftershocks.
posed by Brillinger et al. (1980) and uses output from Hypo71. The solution obtained for the main event is shown in Figure 12A. The A plane is well constrained by RESJAL stations and CJIG, but coverage is not sufficient to constrain the B plane adequately. This solution represents the first stage of the rupture and indicates reverse faulting with a strike of 261~ and a dip angle of 41 o, which does not agree with solu- tions proposed by other sources using teleseismic data. A joint solution for the two principal aftershocks (Figure 12B) agrees with the solutions reported by Singh et al. (2003) for these aftershocks.
DISCUSSION AND CONCLUSIONS
In the first 72 hours after the mainshock no detectable seis- micity was located on or below the presumed subduction contact in the epicentral region. Our results thus do not sup- port the hypothesis of a subduction earthquake breaking the
Colima gap, and the first-motion focal mechanism does not agree with teleseismic solutions proposed. Other cases are reported for M~xico (Cocco et al., 1997; Quintanar et al., 1999) where teleseismic solutions do not agree with local first-motion solutions. Yagi et al. (2003) reported difficulty identifying the P-wave first motions at teleseismic data. To model the earthquake, however, he used a different focal mechanism (Table 1) and an old, poorly constrained plate- boundary geometry model for the region.
Preliminary RESCO locations suggest no plate-bound- ary activity: All of the relocated aftershocks lie within the continental plate along two well defined zones. We relate one of these zones to the rupture area of the mainshock, while the 95C cluster appears to be a reactivation, 18 hours after the mainshock, of a feature possibly associated with the 1995 earthquake. A study of the 1995 earthquake aftershocks (Pacheco et al., 1997) and Ntifiez-Corntl and Sanchez-Mora (1999) also report activity in the AC area.
Seismological Research Letters November/December2004 Volume75, Number6 741
These are preliminary results, which should be revised once results are ready from the aftershock study with a porta- ble station deployed 72 hours after the mainshock, and fur- ther analysis of the mainshock is carried out. Analysis of the space and time hypocentral distributions of the AC cluster indicates that most of the aftershocks are located roughly in a plane with a N80~ orientation and about 40 ~ dip. From Fig- ures 7, 8A, 9, 10A, and 10B we estimate a 35 km length and a 10.5 km width for this plane, which results in a 370 km 2 area. The focal mechanism solution for the mainshock (plane A) agrees with the orientation of the rupture area as inferred from the aftershock analysis. These results indicate that the mainshock was a continental intraplate reverse-fault earth- quake. No event of this type has been reported before for the Jalisco coast. The origin of this earthquake is not clear, but it could be associated with oblique subduction. This type of earthquake may represent an additional seismic hazard to the region that must be studied. E:I
ACKNOWLEDGMENTS
We are grateful to Susan Hough for her comments and sug- gestions that helped us improve the manuscript. We espe- ciaUy acknowledge E A. Nava for comments and suggestions, and J. Pacheco for providing data from CJIG and MOIG. We thank an anonymous reviewer for his suggestions. We also thank Melchor Ursua, Protecci6n Civil Colima, for logistical support.
REFERENCES
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Bandy, W. L. (1992). Geological and Geophysical Investigation for the Riv- era-Cocos Plate Boundary: Implications for Plate Fragmentation, Ph.D. dissertation, Texas A&M University.
Bourgois, J., D. Renard, J. Auboin, W. Bandy, E. Barrier, T. Calmus, J. C. Carfantan, J. Guerrero, J. Mammerickx, B. Mercier de Lepinay, E Michaud, and R. Sosson (1988). Fragmentation en cours du bord ouest du continent nord americain: Les fronti~res sous- marines du Bloc Jalisco (Mexique), Comptes Rendus de l'Acaddmie des Sciences 307(II), 1,121-1,133.
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Centro de Sismologla y Volcanologla de Occidente (SisVOc) Universidad de Guadalajara
Puerto Vallarta, Jal. MExico
E-mail: Ieornu @pv.udg.mx (EJ.N.-C., M.R.L., E. T. G., M.A. C.-G.)
RESCO Universidad de Colima
Colima, Col. MExico
(G.A.R.-D., C.A.I~-V.)
Seismological Research Letters November/December2004 Volume75, Number6 743
