Characteristics of seismicity in the coast and north of Jalisco Block, Mexico

Physics of the Earth and Planetary Interiors 132 (2002) 141–155

Characteristics of seismicity in the coast and north of Jalisco Block, Mexico Francisco J. Núñez-Cornú a,∗ , Rutz L. Marta a , F. Alejandro Nava P. b , Gabriel Reyes-Dávila c , Carlos Suárez-Plascencia a a

Centro de Sismolog´ıa y Volcanolog´ıa de Occidente (SisVOc), Universidad de Guadalajara, Puerto Vallarta, Jal. 48280, Mexico b Depto. Ciencias de la Tierra, Cto. de Investigación y Educ, Superior de Ensenada, Ensenada BC, Mexico c RESCO Universidad de Colima, Colima, Col., Mexico

Abstract Several studies of local seismicity were carried out between 1996 and 1998 on the coast and north of the Jalisco Block (JB) using a portable seismic network consisting of five Lennartz M88 seismographs with Le 3D (1 Hz) seismometers. The data from these studies, complemented with data from RESCO (Colima Seismic Network), is used to characterize three seismogenic zones in the region. One seismicity zone, located east of the Middle America Trench (MAT), under the continental plate, is associated with the interaction of the Rivera and North America plates, and can be characterized as a Double Seismic Zone (DSZ) with geometry suggesting a bending plate and, probably, oblique subduction; this pattern is clearer south of the 20◦ N parallel. A second zone of continental intraplate seismicity is located between Ameca and Amatlan de Cañas, in the northeastern part of the study region, where the source depths vary from very shallow to about 35 km. The third zone is Bah´ıa de Banderas where the stress pattern seems to be very complex; here we observe different types of seismic events: the first type consists of shallow events, located in the southern part of the Bay and a under continental area, that seems to be associated with the interplate (Rivera/North America) activity. A second event type with depths between 10 and 30 km, seems to be related to features that cross the Bay in an EW direction. In this zone, there is also a high shallow continental seismicity, which agrees with continental topographic features. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Megathrust earthquakes; Intraslab earthquakes; Subduction; Rivera plate; Jalisco Block; Double Seismic Zone

1. Introduction The largest Mexican earthquake of the 20th century occurred near the coast of Jalisco on 3 June (MS = 8.2) 1932, and was followed by another on 18 June (MS = 7.8) 1932. These earthquakes were studied by Singh et al. (1985), who concluded that both events broke the Rivera–North America (RIVE–NOAM) plate boundary (Fig. 1a) and proposed a recurrence ∗ Corresponding author. E-mail address: [email protected] (F.J. N´uñez-Corn´u).

period of 77 years for the region. In 1995, an Mw = 8.0 earthquake, ruptured an area approximately half of that proposed for the 1932 earthquakes. The lack of other significant earthquakes in the region implies the existence of an important seismic gap on the northern coast of Jalisco, the Puerto Vallarta Gap (Fig. 1a). Another smaller gap is at the eastern border of Jalisco Block (JB), the Colima Gap. Large inland earthquakes also occur in the study region, such as the 27 December 1568 (Suarez et al., 1994) and 11 February 1872 (De la Barcena, 1872) events, which makes the Jalisco region one of the most active seismic regions in Mexico.

0031-9201/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 9 2 0 1 ( 0 2 ) 0 0 0 4 9 - 3

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Fig. 1. (a) Seismic gaps for the Jalisco Block region (modified from Singh et al., 1985), Colima Gap at south and Puerto Vallarta Gap at north. (b) Distribution of seismographic stations in the Jalisco Block: in the north a typical configuration of the portable network, on the coast the Chamela station (CJIG) from Servicio Sismol´ogico Nacional (SSN), at east and south of the RESCO network. Towns cited in text.

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However, its seismicity has been poorly documented because the only permanent seismographic coverage consists, to date, of a single station from the national seismic network (SSN, which began only recently to operate continuously) located near the coast at Chamela (CJIG), and of the Colima Seismic Network (RESCO), which is an analog telemetered network, sited near the southeastern edge of the JB, on the Colima Graben, and processing only data from events associated with said graben and that the Colima Volcano (Fig. 1b). Nevertheless, many theories and hypotheses have been proposed to explain details of local tectonics using only world-wide seismicity data and a few regional data (e.g. Bevis and Isacks, 1984; Burbach et al., 1984; Eissler and McNally, 1984; Pardo and Suárez, 1993), in spite of the fact that hypocenters are mislocated due to the lack of stations and to poor azimuthal coverage. Pardo and Suárez (1993) use regional data from SSN, data from a small temporal array near Chapala lake in the east corner of JB and some epicenters reported from RESCO to the Colima Civil Defense (since at that time data files from RESCO were not available) to propose their model. For the southern part of the Middle America Trench (MAT), along the states of Guerrero and Oaxaca, where more local stations operate, this mislocation was calibrated by Singh and Lermo (1985) who found a 35 km shift towards the northeast. The purpose of this study is to get a more detailed knowledge of the regional seismicity.

2. Tectonic setting In the western Mexican Volcanic Belt, where the North American, Pacific, Cocos, and Rivera lithospheric plates interact and several triple points have been proposed (Fig. 2), the seismotectonics are not well understood. The existence of a tectonic unit known as the JB has been proposed in this region (Luhr et al., 1985; Bourgois et al., 1988; Garduño and Tibaldi, 1991; Allan et al., 1991; DeMets and Stein, 1990; Ferrari et al., 1994; Rosas-Elguera et al., 1996). The JB is bounded to the east by the Colima Rift Zone which extends northward from the Pacific coast and connects at its northern end with two other major extensional structures: the Tepic-Zacoalco Rift Zone

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(TZRZ, trending roughly NWSE) which is defined as the north boundary of the JB, and the Chapala Rift Zone (trending roughly EW). The connection between the northwest border of the Jalisco Block and the continent (the Tamayo Fault System) is not well defined. It has been proposed to relate this border to the San Blas Fault (SBF) as the continuation of the TZRZ, or to the Islas Marias Escarpment (IME, west of Tres Marias islands) and the Banderas Fault (BF), which crosses the Bah´ıa de Banderas and continues along the Vallarta Graben to join the TZRZ (Fig. 2). Another possible connection is to the Islas Marias Fault (IMF). The last two possibilities suggest the existence of a small block (the Tres Marias Block, ?). The Bah´ıa de Banderas area may be experiencing strong crustal stresses as result of the convergence direction of the Rivera plate (Kostoglodov and Bandy, 1995). The existence of shallow submarine hydrothermal activity in the Bah´ıa de Banderas (Núñez-Cornú et al., 2000) could be a result of these stresses. There is general agreement that the JB is starting to separate from the continent, moving in a WNW direction away from mainland central Mexico (Luhr et al., 1985; Bourgois et al., 1988; Garduño and Tibaldi, 1991; Allan et al., 1991). However, alternative models have also been put forward (DeMets and Stein, 1990; Ferrari et al., 1994; Rosas-Elguera et al., 1996) where a movement to the SW is proposed. Although the relative velocity vector of RIVE–NOAM plate movement in the last 0.78 Ma are perpendicular to the MAT (DeMets and Wilson, 1997) and this data agrees with seismic data of recent big earthquakes (Escobedo et al., 1998), to date the geometry and length of the slab below the JB is 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 SW border by Núñez-Cornú and Sánchez (1999), and similar values were obtained from recent reflection–refraction studies carried out in the central and northern parts of the Jalisco coast (Dañobeitia et al., 1996).

3. Regional seismicity Between 1996 and 1998, the Centro de Investigación y Estudios Superiores de Ensenada (CICESE) and the University of Guadalajara operated a portable

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Fig. 2. Tectonic frame of Jalisco Block: SBF, San Blas Fault; IMF, Islas Marias Fault; IME, Islas Marias Escarpment; BF, Banderas Fault; 1, Volcan de Fuego; 2, Ceboruco Volcano; 3, Sangangüey Volcano; 4, Bah´ıa de Banderas.

network with five Lennartz Mars88 recorders and LE3D three component 1 Hz seismometers, which recorded over short time periods, in different array configurations, for a total of approximately 4 months continuous recording time. Triggered recording was based on a Short-Time-Average/Long-Time-Average algorithm; data were recorded on 3.5 in. floppies, and processed at the Centro de Sismolog´ıa y Volcanolog´ıa de Occidente located in the University of Guadalajara “Puerto Vallarta” Campus. After building a data base with all the events recorded by two or more portable stations, complementary data has been searched for at RESCO. However, since RESCO does not routinely process information for events outside the network, only a few time series of the larger earthquakes occurring outside of the network before 1998 are available. Data for other events were obtained from analog drum

records. All data since 1998 are kept in the digital continuous recording files of RESCO. For the CJIG station, only few data were available. Hypocenters were located using the Hypocenter computer program (Lienert and Havskov, 1995), which allow us to include azimuth, and refracted phases in the location process. All events were processed using seven different initial trial depths within 5–50 km and the best solution was chosen. Generally, the best solutions were obtained with shallower trials but, not always. Initial criteria to accept the location of an event as reliable was to be recorded in at least two or three stations from the portable array and four to five stations from RESCO (mainly ZLGC, ESPC, EZV2, EZV6, EZV3 and COL, the only station with three components) with the following residuals: root mean square (rms) = 0.50 s; standard error in epicenter (erh) =

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10.0 km; standard error in focal depth (erz) = 10 km. However, the mean values of rms, erh and erz, obtained were less than 0.40, 5.0 and 5.0, respectively, and a minimum of 20 phase readings; the only exception was the area of Bah´ıa de Banderas, as discussed below. A velocity structure model was obtained using data from Dañobeitia et al. (1996) for shallow layers and adjusting residuals for direct and refracted P phases (Table 1). Focal mechanism were obtained for some of the larger events, but due to the distribution of stations solutions obtained were ambiguous. To evaluate local magnitude several relations were tested using data recorded at the portable array. We obtained the magnitude for the same event at different distances (only Lennartz stations), the most consistent relation was that proposed by Lay and Wallace

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Table 1 Structure velocity model for Jalisco Block (VJB01) Depth (km)

VP (km/s)

VS (km/s)

0 3.6 19.0 28.0 35

5.7 6.4 6.9 7.6 8.0

3.2 3.6 3.9 4.3 4.5

(1995), showed below, for which the differences in magnitude values obtained were less than ±0.2 units: ML = log A − 2.48 + 2.76 log D The epicentral distribution along the coast shown in Fig. 3 shows more activity in the southern half of

Fig. 3. Epicentral map of seismicity located in this study over a digital elevation model. Main epicentral groups are marked by dashed lines.

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the Jalisco coast, a pattern similar to that reported for the aftershocks of the 1995 Jalisco MW = 8.0 earthquake (Garc´ıa-Arthur et al., 1996; Pacheco et al., 1997). This suggests that the northern half is at the moment less active than the southern half, at least during the period of the portable network deployment. From this figure, it is apparent that coastal seismic activity ceases north of the 20.75◦ parallel, which may be interpreted as marking a tectonic boundary: the coastal northern edge of the Jalisco Block which we will call the Banderas Boundary (BB). Seismicity is lower in the JB’s interior, and the scarcity of coastal seismicity between latitudes 19.5 and 20.5◦ may be of

significance as belonging to a yet unruptured part of the oceanic–continental plate boundary. Epicenters form three main groups according to their geographical distribution. One group is in the Bah´ıa de Banderas (BAB) area with 43 events, 30 of which were located using only the portable stations nearby Bah´ıa de Banderas including microearthquakes with magnitude less than 2.0; for these events we have only 6–12 phase readings. Another group is in the Amatlán de Cañas–Ameca area (ACA) in the northeastern part of the study region with 48 events, and a third one in the coastal area with 120 events. All the events of the third group are located east of the MAT

Fig. 4. Seismic traces for stations TUJ and COL, where differences in wave forms can be seen for oceanic and continental events, arrows mark refracted phases (Pn), in the OC event, reflected phases (PmP) and a possible converted phase (?) in continental events. The time scale is in seconds, on the left side are indicated the type of event, station and component, initial second, maximum and minimum amplitude of the trace in counts (not calibrated). On the right side appear epicentral distance and year, Julian day, hour, and minute. Hypocenters of events are: 970708 0653, 19.121N, 105.1947W, −5.05, 4.45; 980605 1210, 19.1088N, 104.903W, −20.00, 4.25.

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and reach depths of 35 km at a distance of some 50 km from the trench, which at a first glance, could suggest very steep subduction. For this last group, conspicuous waveform differences are observed by using records from stations TUJ located in the north and

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COL located in the south. These differences suggest that these should be separated into two groups: oceanic (OC) and continental (CC), as we shall see below. Fig. 4 illustrates the above mentioned differences; the N, Z, and W components are plotted for

Fig. 6. Groups of seismic events located between 1996 and 1998 in the Jalisco Block: (a) epicentral map; (b) east-west profile; (c) north-south profile.

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Fig. 6. (Continued ).

stations TUJ and COL, with the station name and the earthquake’s oceanic/continental classification specified on each trace. It can be seen that the beginning of the P phase is quite different for earthquakes at similar epicentral distances but barely different (less than 15 km difference) source depths; OC events are characterized by energetic P arrivals at COL (which is located inland) and also at TUJ when the distance is less than 85 km (Fig. 5), which is typical of signals arriving in a direct path from a focus under the station. Clear Pn phases at distances greater than 90 km can be observed in record sections for TUJ in Fig. 5, with most of these paths being parallel to the trench. Meanwhile CC events do not show refracted phases at that distance but do have phases that we interpret as reflected waves (PmP), and perhaps converted phases (PmS, ?), which are not observed in oceanic events at similar distance (Fig. 4). This indicates the presence of the Moho or the existence of a thin layer with very high contrast velocity. Differences in the P and S waveforms between OC and CC events at less than 100 km from TUJ are clear in the record sections of Fig. 5, where a very energetic impulsive P wave for OC events is observed as is observed for records in COL (Fig. 4). Traces from CC events are typical

of local seismic waves traveling through continental crust. A total of 211 events were located, all them with ML < 5.50. In Table 2 we compare our hypocenters with those reported by the SSN. It is clear that the sample is not statistically representative, but some facts can be observed. Only 20 events of our sample were reported by the SSN, and in most cases the magnitude reported by the SSN is inferior to ours. For CC earthquakes 7 events of 17 possible are reported by the SSN (41%), on the basis of the minimum magnitude reported by SSN compared to our results. These events are mislocated (in relation to this study) mainly in the NW direction, by about 15–20 km. Depths are similar, but differences in magnitudes are greater than 1.0 unit. In the case of OC events, we have four events of six possible (66%). These events are mislocated to the SW by about 30 km. Depths are similar and differences in magnitude are between 0.5 and 1.0 units. For BAB earthquakes, we have two events of four possible (50%). These events are mislocated to the SW by about 60 km, SSN events deeper and with differences in magnitude between 0.3 and 1.0 units. For ACA earthquakes we have seven events of nine possible (77%). These events are mis-

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located to the NW by about 16 km. Depths of SSN events are greater than the maximum depths obtained in this work for this area; differences in magnitude are less than 0.5 units. Between May and October 1998, one station was deployed at Ceboruco Volcano (CBJ) which allowed us to improve the control of the seismicity in this area. The differences we obtained in event locations could not be due only to station distribution, crustal and source effects are also present. In Fig. 6 the earthquakes are plotted as an epicentral map (a) and as EW (b) and NS (c) profiles, with symbols indicating their classification: CC (circles), OC (crosses), BAB (diamonds) and ACA (squares). Fig. 7 shows a 3D view, from a point 20 km away at an azimuth of 266◦ and a vertical angle of 33◦ , of the hypocentral distribution where the four groups can be easily distinguished. We associated the CC events to the seismogenic interface between Rivera plate and the JB. From the lower limit of the CC events in Fig. 6b it

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is possible to infer a gentle subduction angle (10–15◦ ) in the EW direction. North of 19.5◦ latitude, continental seismicity decreases to the NW (AA , Fig. 6a); the deepest CC event located was at 32 km depth and 160 km from the trench. The depth of the OC events ranges from 5 to 35 km, and they are located mainly between the trench and the coast line (Fig. 6a) and below the CC events (Fig. 6b). These OC events occur both within and below (near the mantle–plate interface) the oceanic plate. This feature can be seen more clearly in Fig. 7, were the trench line is the eastern boundary for the OC (tectonically, it should be the upper limit). There is a gap between the OC and the CC events; if a projection is made at the same distance with an azimuth = 266◦ and a vertical angle = 48◦ , the trench line fits the western (lower) limit of the CC events. The OC events take place below the coupled plate interface where normally there exists a paucity of

Fig. 7. Projection of hypocenters for seismicity in the Jalisco Block between 1996 and 1998. From this point of view the four groups are easily distinguished.

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events. This fact could suggest the possibility of a major intraslab earthquake such as the “Oct 4 type” (Ruff, 1996); this type of large event has occurred in the Cocos plate in the south of Mexico (Singh et al., 2000). The OC events define a shallow Double Seismic Zone (DSZ) in a slab of oceanic crust only 10 Ma old, which is unusual at this very shallow depth (Seno and Yamanaka, 1996). In order to try to get an image of the geometry under the JB, quadratic spline surfaces have been fitted to the OC and CC hypocenters in Fig. 8; under the supposition that the coastal events are related to the subduction process, the seismogenic plate interface roughly suggested by the CC surface appears to go down obliquely at the trench, and to increase in dip both with distance from the coast and, in a gentle way, from southwest to northeast, the surface defined by OC events looks more irregular than the CC surface, as expected since they are located inside and below the slab.

Since the direction of subduction suggested by tectonic and seismic data is perpendicular to the MAT (DeMets and Wilson, 1997; Escobedo et al., 1998), projections of the OC and CC events on a profile perpendicular to the trench (line AA , Fig. 6a) are shown in Fig. 9. Different interpretations can be made of this pattern, including very steep subduction angles. We choose as subduction angle the 12◦ dip of the dashed line that roughly separates CC from OC events. This pattern and subduction angle are similar to those reported by Núñez-Cornú and Sánchez (1999) for the nearby Jalisco–Colima coast. From Fig. 6c it is clear that seismicity suddenly stops at Latitude 20.75◦ N; the BAB and ACA areas are aligned along the BB direction. Seismic records (not shown) from stations located north (SPJ) and south (TUJ) of this alignment suggest a strong change in the crustal structure. This supports the existence of the BB, although we do not have enough data to determine its nature, whether it is the border of the Rivera plate

Fig. 8. Quadratic spline surfaces fitted to the CO and CC hypocenters. Both surfaces are deeper to the northeast. The upper surface (generated with CC events) could be considered as the contact between plates.

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Fig. 9. Profile perpendicular to the trench, CC and OC events are projected onto the AA direction (Fig. 6a). Dashed lines indicate possible subduction angles.

Fig. 10. Seismicity in the northern part of the Jalisco Block, with the ACA region to the east: AC, Amatl´an de Cañas; A, Ameca. West of 105◦ BAB events; black diamonds, shallower seismos; black circles, deeper seismos; white diamonds, symbols in Bah´ıa de Banderas are events located using data from one or two portable, three component, stations.

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and/or a continental feature. Seismic patterns can be correlated with topographic features as can be seen in Fig. 10. The ACA area is the intersection of the BB and the TZRZ. The seismicity here, with source depths ranging from 0 to 35 km, presents two main directions: the first one is aligned with the TZRZ along the Amatlán de Cañas and Ameca half grabens (Rosas-Elguera et al., 1996) and implies an active fault joining both features; the second one, where a seismic swarm took place in March 1998, oriented towards the Bah´ıa de Banderas in the same direction as the BB. These can be observed more clearly in Fig. 10. The BAB group contains two subgroups: crustal shallow events, black diamonds in Fig. 10 (already described by Núñez-Cornú et al., 1997) and deeper events (10–35 km) that appear to be related to the BB which appear as black circles west of 105◦ in Fig. 10. In Fig. 10 we also plot all the seismicity (white diamonds) located with one or two stations in the Bah´ıa de Banderas area; it can be seen that microseismicity is high, although the magnitude level is lower than that observed in areas to the south in the region of the Cocos plate.

4. Conclusions As expected from the tectonics of the region the seismicity in the JB is diverse, showing regional differences in depth distribution and waveform, which define three seismogenic zones and five earthquake types: OC and CC events in the coastal region; in Bah´ıa de Banderas area two main types, shallow events associated with crustal stresses and deep events related to the BB; the ACA area with crustal events with depths between 0 and 35 km. Due to the distribution of our stations it was not possible to locate seismicity in the area east of the JB. Mislocations inferred for SSN hypocenters are different for each area, which supports our zonation. Only 55% of the events that should be detected by the SSN are reported. It is important to remark that the differences of the depth for the ACA events between our study and those reported by SSN are as large as 99 km. According to our results all the events are located in the continental crust, and the depths reported by SSN could suggest a intermediate depth Wadati–Benioff

seismicity (Ruff, 1996). This fact is important since previous models use these data, so in our opinion it is not possible to compare since the quality and quantity of local data used by different authors is very different. The dip of the slab is less than 15◦ until 150 km from the trench, however, the length and geometry are not possible to define with these data. Although Fig. 8 suggests that the slab is bending, and probably dipping obliquely, this is not sufficient to establish a model for the morphology of the Rivera plate. The scarce seismicity in the northern area of the coast supports the hypothesis of the existence of the Puerto Vallarta Gap in that region. The OC events, at depths between 7 and 35 km, are clearly occurring in the oceanic lithosphere below the coupled interface, and events of this type have not been previously reported for the subducting Rivera and Cocos plates along the coast of Mexico. The OC events define a DSZ, which is unusual at this very low depth. This may suggest the possibility of the occurrence of large oceanic intraslab earthquakes for the Rivera plate, as occurs in the Cocos plate (Singh et al., 2000). The ACA events with two tendencies, one in the direction of TZRZ and the other in the direction of the Bah´ıa de Banderas, which reflects the active NE border of the JB. The BAB deeper events appear to be aligned with the ACA events along the Banderas Boundary and the northern end of the MAT. The BB seems either to be the limit of Rivera plate, or a big discontinuity in the continental crust, more studies are necessary to confirm this. A line joining the Marias Islands, Bah´ıa de Banderas and Colima Volcano (Fig. 3) appears to be the NE limit for the coastal events. No deep seismicity was located east of this line. The results of this study confirm the necessity of a permanent seismic network in the region in order to define a seismotectonic model.

Acknowledgements We thank our anonymous reviewers for helpful suggestions and comments and to Phil Cummings for improving our English. Workfield and data processing were funded by the following projects: CORTES 96 by CiCYT (ANT94-0182-C02-01/02) Spain; BLOJAL by CONACyT (0894PT), LICOJAL by CONACyT (4144PT) and SIMORELOS (19990306004) Mexico.

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