Mexican Subduction Zone Research Articles

Space-Time Clustering of Large Thrust Earthquakes along the Mexican Subduction Zone: An Evidence of Source Stress Interaction

The spatiotemporal plot of epicenters of large ( M s ≥6.9) subduction earthquakes in Mexico (1900 to present day) suggests that these earthquakes cluster in space and time. In this work we test the hypothesis that the coseismic stress transfer may lead to this clustering. For the analysis we estimate the spatial extent of the coseismic Coulomb stress change for these large events and then perform a statistical analysis using the χ 2 goodness-of-fit test for the interevent time intervals. We find that there are, at least, two groups of time intervals where the observed frequencies are much higher than that expected from a Poisson model, indicating a bimodal pattern. For the first mode, the observed frequencies for the 0- to 5-year interval becomes about 2.1 times the expectation, with a probability of occurrence of about 30%. These results show that large thrust Mexican earthquakes between 1900 and 2003 are clustered in space and time probably due to stress interactions among them. The second mode includes the time interval of 30–50 years. In the interval of 30–40 years, the observed frequencies become about 1.7 times the expectation and about 1.2 times the expectation for the 40- to 50-year interval. This second mode could be associated with a reloading interval of tectonic stress due to the plate convergence and appears consistent with the long-term recurrence periods of large thrust earthquakes in the Mexican subduction zone.

Thermo-mechanical model of the mantle wedge in Central Mexican subduction zone and a blob tracing approach for the magma transport

The origin of the Central Mexican Volcanic Belt (CMVB) and the influence of the subducting Cocos plate on the CMVB volcanism are still controversial. In this study, the temperature and mantle wedge flow models for the Mexican subduction zone are developed using the finite element method to investigate the thermal structure below CMVB. The numerical scheme solves a system of 2D Stokes equations and 2D steady-state heat transfer equation. Two models are considered for the mantle wedge: the first one with an isoviscous mantle wedge and the second one with strong temperature-dependent viscosity. The first model reveals a maximum temperature of ∼830 °C in the mantle wedge, which is not sufficient for melting of wet peridotite. Also, the geotherm of the subducting plate upper surface does not intersect the dehydration-melting solidus for mafic minerals. The second model predicts temperatures of more than 1200 °C beneath the CMVB for a wide range of rheological parameters (reference viscosity and activation energy). Up to 0.6 wt.% H 2O can be released down to 60 km depth through metamorphic changes in the oceanic crust of the subducting slab. The melting of this oceanic crust apparently occurs in a narrow depth range of 50–60 km and also melting of the mantle wedge hydrated peridotite is now expected to take place beneath CMVB. Considering that the melting processes on and in the vicinity of the subducting plate surface generate the most of the volcanic material, a dynamic model for the blob tracers is developed using Stokes flow at infinite Prandtl number. The blobs of 0.2–10.0 km in diameter migrate along very different trajectories only at low wrapping viscosities ( η w = 10 14–5 × 10 17 Pa s). The modeling results show that the “fast” trajectories terminate at the same focus location at the base of the continental crust, while the arrival points of “slow” trajectories, which are common for the blobs of smaller size (∼0.4–0.5 km), are scattered away from the average focus location. This observation may give us a hint on a possible mechanism of strato and mono volcanoes genesis. The rise time, which the blob detached from the subducted plate, needs to reach the bottom of the continental crust, is from 0.001 up to 14 million years depending on the blob diameter and surrounding viscosity.

Interplate coupling and a recent aseismic slow slip event in the Guerrero seismic gap of the Mexican subduction zone, as deduced from GPS data inversion using a Bayesian information criterion

A large-scale slow slip event with a magnitude equivalent to Mw = 7.4 has been observed at seven continuous GPS stations located in the Guerrero–Oaxaca region, southern Mexico, for several months from late 2001 to middle 2002 [Geophys. Res. Lett. (2003) 30: doi: 10.1029/2003GL017219]. We re-analyzed the GPS time series data to estimate the displacements associated with the slow slip event and also the displacement rates during the interseismic period prior to this event, by applying an inversion technique using a Bayesian information criterion (ABIC). We then carried out GPS data inversion again with ABIC and some detailed forward modeling to obtain spatial distributions of the back-slip rates and the slow slip on a 3-D curved plate interface between the subducting Cocos and the overriding North America plates. The results show that the average direction of the back-slip rates is N31.3°E ± 5.6°, which is congruent with that of plate convergence in this region. Interplate coupling is found to be very strong down to a depth of about 45 km, indicating a coupling ratio of 0.83–0.86 except for a couple of segments on the model region, and decreases dramatically down below. The average direction of the slow slip is oriented about 20°–35° counterclockwise from the opposite direction of the plate convergence. The slow slip detected on the mid-depth segment to the upper rim of the model region reaches about 9–18 cm in the landward region of the Guerrero seismic gap from inversion and forward modeling. From these results, we conclude it is possible that the slow slip may have invaded a deeper part of the strongly coupled, seismogenic zone at least up to a depth of about 25 km. We also estimate stress changes due to the slow slip event. Some decrease in shear stress on the plate interface at these depths suggests that if such a slow slip event occurred episodically, it is possible that the time of occurrence of a forthcoming large earthquake in the Guerrero gap would be delayed to some extent.

Thermal structure, coupling and metamorphism in the Mexican subduction zone beneath Guerrero

Temperature is one of the most important factors that controls the extent and location of the seismogenic coupled and transition, partially coupled segments of the subduction interplate fault. The width of the coupled fault inferred from the continuous GPS observations for the steady interseismic period and the transient width of the last slow aseismic slip event (Mw~ 7.5) that occurred in the Guerrero subduction zone in 2001-2002 extends up to 180-220 km from the trench. Previous thermal models do not consider this extremely wide coupled interface in Guerrero subduction zone that is characterized by shallow subhorizontal plate contact. In this study, a finite element model is applied to examine the temperature constraints on the width of the coupled area. The numerical scheme solves a system of 2-D Stokes equation and 2-D steady-state heat transfer equations. The updip limit of the coupling zone is taken between 100 and 150 °C, while the downdip limit is accepted at 450 °C as the transition from partial coupling to stable sliding. From the entire coupled zone, the seismogenic zone extends only up to ~82 km from the trench (inferred from the rupture width of large subduction thrust earthquakes), corresponding to the 250 °C isotherm. Only a small amount of frictional heating is needed to fit the intersection of the 450 °C isotherm and the subducting plate surface at 180-205 km from the trench. The calculated geotherms in the subducting slab and the phase diagram for MORB are used to estimate the metamorphic sequences within the oceanic subducting crust. A certain correlation exists between the metamorphic sequences and the variation of the coupling along the interplate fault.

Slip-weakening distance in dynamic rupture of in-slab normal-faulting earthquakes

We apply the above method to three large in-slab, normal-faulting earthquakes in the Mexican subduction zone. To do this, we calculate the spatial distribution of slip-velocity functions and final slip from kinematic waveform inversion of strong-motion and teleseismic records, and the stress history and final stress change from dynamic rupture calculations. By integrating the slip-velocity functions obtained from the inversion, from the rupture arrival time to the time of peak slip velocity, we obtain slip Dat T pv and then correct it for Dc at T b through dynamic calculations. We also estimate the lowest resolvable limit and probable errors of Dc from the slip-velocity functions, and its upper bound from a theoretical constraint between the dynamic stress drop and Dc. We found that the slip-weakening distance Dc estimated in the frequency band between 0.05 and 0.5 Hz ranges between 40 and 120 cm on the in-slab fault of the 1999 Oaxaca earthquake (Mw = 7.5). The largest Dc is detected in the central fault and in part of the deeper sections, and Dc in the zone around the hypocentre ranges between 50 and 70 cm. The estimated Dc values appear to be less depth-dependent but are rather more dependent on the local maximum slip. This possible slip dependence might be interpreted by the degree of fault roughness, in addition to stress heterogeneities. The fracture energy G in the central section and in the hypocentral zone are roughly estimated to be of the order of 10-15 and 5-8 MJ m −2 , respectively. Both of the estimated Dc and G values are somewhat larger than those on the vertical fault of two recent, shallow strike-slip earthquakes in western Japan.

Reconciling Teleseismic and Regional Estimates of Seismic Energy

Estimates of the radiated seismic energy based on teleseismic and regional data often differ by up to an order of magnitude, with a tendency for regional estimates to be larger than teleseismic estimates for the same event. In this study we compare the velocity spectrum determined from teleseismic data after correction for radiation pattern and propagation effects, with the velocity spectrum determined from regional data, after the corresponding corrections, for nine earthquakes in the Middle America subduction zone of Mexico. This comparison of the corrected spectra is used to identify and reduce the sources of the regional versus teleseismic energy discrepancy, which is about an order of magnitude for these events. We find that the teleseismic attenuation operator needs to be calibrated. In our case, for the tectonic environment of the Mexican subduction zone, we need a teleseismic attenuation operator that is stronger at high frequencies than the global average. A larger factor, however, is the correction needed to account for site amplification. This correction has an impact on both regional and teleseismic data, but it has a larger influence on the regional estimates because the angle of incidence for teleseismic waves is steep and the stations are located on more competent rock. By modifying the teleseismic attenuation operator and applying site corrections based on a generic site model, we essentially eliminate the order-of-magnitude discrepancy between teleseismic and regional estimates of the radiated seismic energy for these events.

Probabilistic prediction of the next large earthquake in the Michoacán fault-segment of the Mexican subduction zone

La estimación del intervalo de tiempo Δt hasta el siguiente sismo fuerte, que es esperado en una región sísmica, es un problema difícil. Dentro del método más convencional de predicciones basadas en el intervalo de tiempo (Δt), dada una distribución probabilística de los intervalos de tiempo observados y el tiempo transcurrido (t) desde el último sismo fuerte, podemos estimar la probabilidad de un nuevo evento sísmico en un intervalo de tiempo, digamos los siguientes 25 años. En este artículo, invertimos la aproximación y estimamos el intervalo de tiempo (Δt) durante el cual ocurrirá el nuevo sismo fuerte, usando como criterio que la probabilidad condicional de ocurrencia sísmica sea un máximo, dada que el nuevo sismo fuerte no ha ocurrido en el tiempo (t) transcurrido desde el último sismo fuerte. Adoptamos como distribuciones probabilísticas la distribución de Weibull, la distribución de Rayleigh y la distribución de Pareto (power-law).Seleccionamos una región limitada por 17.50 - 18.50° N y 101.7 - 103° W para definir el significado de Michoacán segmento de fractura. Usando una lista de sismos históricos (incluido el sismo de Petatlán) ocurridos en esta área, encontramos que usando el modelo de Pareto (power-law), se estima que un sismo destructor (M ≥ 7) puede ocurrir antes del año 2014.99, o equivalentemente antes de diciembre de 2014.

Regional Wave Propagation from Mexican Subduction Zone Earthquakes: The Attenuation Functions for Interplate and Inslab Events

The seismic waves from subduction zone earthquakes are significantly affected by the presence of 3D variation in crust and upper-mantle structure around the source area. These heterogeneous structures also profoundly modify the character of seismic waves as they propagate from the source area to regional distances. This is illustrated by studying shallow, interplate earthquakes along the Mexican subduction zone, and deeper, inslab, normal-faulting earthquakes in the subducted Cocos plate beneath Mexican mainland. The strong-motion recordings of these earthquakes are used to evaluate the character of wave propagation along the path between the source region and Mexico City. We compare the wavefield from two large earthquakes of different source type. During the shallow ( H = 17 km), interplate, 1995 Guerrero earthquake ( M w 7.3), the Lg phase is the most prominent feature at regional distances of about 150 to a few hundred kilometers from the source. The presence of a lateral velocity gradient in the crust, caused by the subduction of the Cocos plate, enhances the Lg -wave amplitude, which is then amplified further in the Mexican volcanic belt by amplification in the low-velocity volcanic rocks. Both effects lead to very large ground motions along the path from the coast to the Mexican inland, in the frequency band from 0.2 to 4 Hz. However, for the deeper ( H = 40 km), inslab, normal-faulting, 1999 Oaxaca earthquake ( M w 7.5), the amplitude of the Lg phase is too small to produce the abnormal wave propagation, and the direct S wave and its multiple SmS reflections between the free-surface and Moho show a simple attenuation with increasing distance. We compare these observations with numerical simulations of seismic-wave propagation using the Fourier spectral method. The results provide a key to the understanding of seismic-wave field generated by shallow interplate and deeper inslab earthquakes in a realistic 3D heterogeneous structure.

A Source and Wave Propagation Study of the Copalillo, Mexico, Earthquake of 21 July 2000 (Mw 5.9): Implications for Seismic Hazard in Mexico City from Inslab Earthquakes

The Copalillo earthquake of 21 July 2000 ( M w 5.9) is the closest, well-located inslab event to Mexico City ever to be recorded. In this study, we analyze local and regional broadband and accelerometric recordings to determine the source parameters of the earthquake and the attenuation of ground motion with distance and to obtain a preliminary estimate of the seismic hazard posed to the city by such events. Our results show that the earthquake occurred at a depth of about 50 km, most probably in the subducted oceanic crust. The waveform inversion discriminates between the two nodal planes; the fault plane defined by the following: strike, 305°; dip, 32°; and rake, -80°. The rupture propagated nearly unilaterally along the strike toward northwest with a small downdip component. The observed source spectrum can be well explained by an ω 2 -source model with M 0 = 6.0 × 10 25 dyne cm and a stress drop of 360 bar. We find that high-frequency ground motion ( f > 3 Hz), which is related to A max during inslab earthquakes, is not amplified at Ciudad Universitaria (CU), a hill-zone site in the Valley of Mexico that is known to suffer amplification at low frequencies (0.1 f M w 7.0 event could give rise to an A max value of 30-40 gal. The CU recordings indicate that the A max value of 30 gal could have a return period of about 40 yr, about the same as from shallow-dipping thrust earthquakes along the Mexican subduction zone, which have been regarded as posing the highest hazard for the city. An inslab earthquake with an A max value of about 40 gal could cause heavy damage to small buildings at certain locations of the city. We conclude that seismic hazard from inslab earthquakes to Mexico City has so far been underestimated.

A possible stress interaction between large thrust and normal faulting earthquakes in the Mexican subduction zone

AbstractA large, nearly vertical, normal-faulting earthquake (Mw = 7.1) took place in 1997 in the subducting Cocos plate just beneath the ruptured fault zone of the 1985 Michoacan, Mexico, earthquake (Mw = 8.1). We investigate the possibility of stress interaction between the two large events through a 3D analysis of coseismic-stress change that was due to the first event, taking into consideration the postseismic change and the dynamic rupture process of the second event. In the middle portion of the subducting plate at depths below 30 km, the calculated coseismic increase in the vertical-shear stress and in the Coulomb-failure stress beneath the high stress-drop zones of the 1985 earthquake is in the order of 0.4 to 0.8 MPa. It was also found that the 1997 earthquake took place in the zone of maximum coseismic-stress increase. Possible postseismic-stress changes due to the subduction process or to the loading of the overriding continental lithosphere and from aseismic slip would enhance the coseismic-stress change and hence the possibility of occurrence of a normal-faulting earthquake in the subducting plate. The dynamic rupture pattern of the 1997 event seems to be consistent with the inferred stress interactions.

On the nature of regional seismic phases-III. The influence of crustal heterogeneity on the wavefield for subduction earthquakes: the 1985 Michoacan and 1995 Copala, Guerrero, Mexico earthquakes

The most prominent feature of the regional seismic wavefield from about 150 to over 1000 km is usually the Lg phase. This arrival represents trapped S-wave propagation within the crust as a superposition of multiple reflections, and its amplitude is quite sensitive to the lateral variation in the crust along a propagation path. In an environment where the events occur in a subduction zone, such as the western coast of Mexico, quite complex influences on the character of the regional wavefield arise from the presence of the subduction zone. The great 1985 Michoacan earthquake (MW=8.1), which occurred in the Mexican subduction zone, was one of the most destructive earthquakes in modern history and its notable character was that at Mexico City, located over 350 km from the epicentre, there was strong ground shaking almost comparable to that in the epicentral region that lasted for several minutes. Considerable effort has been expended to explain the origin of the unusual observed waves that caused the severe damage in the capital city during the destructive earthquake. The nature of the propagation process in this region can be understood in part by using the detailed strong-motion records from the 1995 Copala, Guerrero (MW=7.4) earthquake near the coast to the south of Mexico City, which also had an enhanced amplitude in the Valley of Mexico. Numerical modelling of both P and S seismic waves in 2-D and 3-D heterogeneous crustal models for western Mexico using the pseudospectral method provides direct insight into the nature of the propagation processes through the use of sequences of snapshots of the wavefield and synthetic seismograms at the surface. A comparison of different models allows the influences of different aspects of the structure to be isolated. 2-D and 3-D modelling of the 1985 Michoacan and 1995 Copala earthquakes clearly demonstrates that the origin of the long duration of strong ground shaking comes from the Sn and Lg wave trains. These S-wave arrivals are produced efficiently from shallow subduction earthquakes and are strongly enhanced during their propagation within the laterally heterogeneous waveguide produced by the subduction of the Cocos Plate beneath the Mexican mainland. The amplitude and duration of the Lg coda is also strongly reinforced by transmission through the Mexican Volcanic Belt from the amplification of S waves in the low-velocity surficial layer associated withS-to-P conversions in the volcanic zone. The further amplification of the large and long Lg wave train impinging on the shallow structure in the basin of Mexico City, with very soft soil underlain by nearly rigid bedrock with a strong impedance contrast, gives rise to the destructive strong ground shaking from the Mexican subduction earthquakes.

Dynamic rupture of asperities and stress change during a sequence of large interplate earthquakes in the Mexican subduction zone

Abstract We investigate the spatial and temporal variations of shear stress due to the successive failures over an extensive segment of the Mexican subduction zone during a sequence of large interplate earthquakes that occurred over a period of 13 yr. For this purpose, we develop 3D dynamic rupture models incorporating a shallowly dipping fault located above the subducting plate. The spatial distribution of dynamic stress drop over the fault has been estimated for each of the events, through an inversion procedure using some of the previously derived kinematic fault parameters as observational constraints. The results revealed quite heterogeneous stress changes during these earthquakes coming from medium to high dynamic stress drop due to the rupture of a few patch-like asperities and from stress increase in between and around them. Two weak asperities located southeast of the Michoacan segment were ruptured first by the 1979 Petatlan event. The 1981 Playa Azul event ruptured two asperities in the central zone with a stress drop higher than 80 bars. The largest 1985 Michoacan earthquake resulted from the rupture of two large-size, strong asperities located at both sides of the 1981 fault zone with high stress drop of 80 to 100 bars and from another two asperities at depth. Two days after this largest event, two asperities were broken during the Zihuatanejo aftershock in the southeastern adjacent zone. Many aftershocks of these large events tend to be distributed in the zones of stress increase outside the asperities, while only small numbers of aftershocks have been observed within these asperity zones. It appears that several major asperities that existed in this extensive segment have been ruptured successively so as to fill unbroken gaps on the plate interface. Thus, the stress change left over from the previous earthquake has dominant effects on the next event in this subduction zone.

The macroseismic field generated by the Mw 8.0 Jalisco, Mexico, earthquake of 9 October 1995

Abstract The Mw 8.0 earthquake of 9 October 1995 occurred within the Jalisco block that represents the northern part of the Mexican subduction zone where the Rivera plate subducts beneath the North America plate along the Middle American trench. The source rupture of the 1995 low-angle thrust-type event was complicated. The studies of rupture history had shown that the slip occurred within an area of about 180 × 90 km2 along the Middle American trench at the depth interval from 9 to 33 km. The rupture duration was estimated in interval from 55 to 62 sec. At least three main asperities ruptured along the fault plane. The earthquake was felt along the 600-km coast of the Mexican states of Colima, Jalisco, and Michoacan and in the continental part of Mexico. The macroseismic study presented in this article was carried out in March through July 1997, more than one year after the earthquake. We had about 300 interviews with people who felt the earthquake in their houses located in 56 cities and towns of the states of Colima, Jalisco, and Michoacán. All our estimates of the earthquake intensity were referred to the intermediate type of masonry situated on the intermediate type of soils. For this purpose, corrections were introduced for type of soil and masonry. The study of macroseismic effects related to the 1995 Jalisco earthquake allows description of some properties of the macroseismic field generated by the earthquake. (1) Three zones of intensity 4, 5, and 6 to 7 grades of MM (Modified Mercalli) scale were distinguished. (2) The zone of maximum intensity of 6 to 7 MM was heterogeneous. This heterogeneity was in accordance with the rupture asperities distribution. (3) The study of intensity attenuation along the coast had demonstrated the asymmetry in intensity distribution according to the epicenter. The maximum intensities were observed for the sites that were situated to the northwest of the epicenter. (4) A good correlation was observed between the observed intensities and the values of GPS displacements.

Fractal geometry and seismicity in the Mexican subduction zone


 
 
 By means of the box counting method we calculate the fractal dimension D of the distribution of faults, fractures and lineaments in a region of the southern Mexican Pacific coast. The results suggest that in the spatial sense the b-value of the Gutenberg-Richterlaw and the fractal dimension are positively correlated. We also suggest that D depends on the depth of the seismogenic zone.
 
 

Seismic Isolation of Buildings Subjected to Typical Subduction Earthquake Motions for the Mexican Pacific Coast

An analytical study on the application of different base isolation systems for original design or retrofit of typical building structures of the Mexican Pacific Coast is presented. The subject hypothetical buildings are located on hard soil conditions at Acapulco. Typical accelerograms for the Mexican subduction zone recorded during recent earthquakes were used for 3-D time-history analyses. Bidirectional input was used for the time-history analyses. The studied base isolation systems reported in this study are lead-rubber bearings (LRB) and steel-hysteretic dampers (SHD). For the original design case studies, the superstructures were designed: a) according to the seismic provisions of the building code of Guerrero state (RCGS-90) for the fixed-base condition and, b) according to an elastic design based upon a 3-D lateral force distribution consistent with dominant mode shapes for the isolated structure to yield the peak dynamic base shear transmitted by the isolation system. Material volumes for the superstructure were estimated for both the fixed-base code designs and the base-isolation designs. Important savings on the volume of concrete and steel reinforcement can be attained for the base isolated designs with respect to their counterpart fixed-base designs. Dynamic responses for the isolated structures compare favorably with those for the fixed-base structures. The study confirms many findings published in the literature regarding the effectiveness of base isolation and the effect of torsional responses. However, the study also shows that the dynamic stability of isolators is not always achieved using rational design procedures. The dynamic stability and design of the studied base isolators can be controlled by acceleration records associated to moderate earthquakes when these records are near the fault plane and by torsional responses.

Gravity and seismicity over the Guerrero Seismic Gap, Mexico

Four detailed (average station interval = 5 km) gravity transects were recently conducted in the Pacific coastal region of Mexico. A differential GPS technique was used to determine the elevation and coordinates of the gravity stations. The profiles are oriented northeast‐southwest and extend from the coast up to ∼60 km inland. The Bouguer gravity anomaly is decreasing consistently along every profile from 60–80 mGal at the coast with an approximately constant regional gradient of −2.2 mGal/km normal to the trench. A plot of the gravity anomaly against the distance from the trench axis demonstrates that the regional slope in the gravity anomaly is shifting gradually (20–25 mGal) inland along the coast of Guerrero from the southeast (Atoyac) to the northwest (Petatlán ‐ Zihuatanejo). A model cross section of the Mexican subduction zone (MSZ) based on the tomography inversion for the Guerrero region shows that the gravity anomaly values and the regional anomaly trend can be explained mostly by the effect of the density contrast between the slab and the continental crust. The upper surface of the subducted slab (USS) and the seismogenic contact zone between the upper plate and the slab is traced clearly in several seismicity cross sections based on the data of the regional seismic network in Guerrero. The depth and shape of the USS revealed from the seismicity and gravity anomaly data for the same profiles are in good agreement. This correlation may be fairly useful when applied to gravity profiles in order to estimate the depth of the USS and the seismogenic contact in other parts of the MSZ which lack reliable seismicity data.

Shallow depth of seismogenic coupling in southern Mexico: implications for the maximum size of earthquakes in the subduction zone

Studies of locally recorded microearthquakes and the centroidal depths of the largest earthquakes analyzed using teleseismic data show that the maximum depth of thrust faulting along the Mexican subduction zone is anomalously shallow. This observed maximum depth of about 25 ± 5 km is about half of that observed in most subduction zones of the world. A leveling line that crosses the rupture zone of the 19 September 1985 Michoacan event was revisited after the earthquake and it shows anomalously low deformation during the earthquake. The comparison between the observed coseismic uplift and dislocation models of the seismogenic interplate contact that extend to depths ranging from 20 to 40 km shows that the maximum depth at which seismic slip took place is about 20 km. This unusually shallow and narrow zone of seismogenic coupling apparently results in the occurrence of thrust events along the Mexican subduction zone that are smaller than would be expected for a trench where a relatively young slab subducts at a rapid rate of relative motion. A comparison with the Chilean subduction zone shows that the plate interface in Mexico is half that in Chile, not only in the down-dip extent of the seismogenic zone of plate contact, but also in the distance of the trench from the coast and in the thickness of the upper continental plate. It appears that the narrow plate contact produced by this particular plate geometry in Mexico is the controlling variable defining the size of the largest characteristic earthquakes in the Mexican subduction zone.

Report on the Earthquake of September 14, 1995, in Guerrero, Mexico

On September 14, 1995, at 8:04 AM, local time (14:04, GMT), an earthquake of magnitude Ms = 7.2 (Mw = 7.3) took place in the Pacific coast of the State of Guerrero, Mexico, close to the town of Copala, with epicenter located at 16.7°N and 98.5°W. The shallow focal depth (17.5 km), the focal mechanism (thrust) and the low dip angle indicate that it is a typical earthquake of the Mexican subduction zone, in the interface between the North American and Cocos plates. Table 1 gives some basic seismological parameters, as well as some values of recorded peak ground acceleration (PGA) that might be of interest. The earthquake produced high intensities in the epicentral area, where it caused considerable damage in some villages in South Guerrero and Southwest Oaxaca. The region is sparsely populated, so only six people died, although 2,000 people remained homeless and more than 5,000 houses were...

Times of increased probability of large earthquakes ( Ms ≥ 7.5) along the Mexican subduction zone : D. A. Novelo-Casanova & J. Alvarez-Moctezuma, Physics of the Earth & Planetary Interiors, 87(3–4), 1995, pp 279–286

Times of increased probability of large earthquakes ( Ms ≥ 7.5) along the Mexican subduction zone : D. A. Novelo-Casanova & J. Alvarez-Moctezuma, Physics of the Earth & Planetary Interiors, 87(3–4), 1995, pp 279–286

Beginnings of earthquakes in the Mexican subduction zone on strong-motion accelerograms

Abstract The Guerrero, Mexico, accelerograph network has recorded, at short distances, earthquakes with magnitudes from 3.0 to 8.1. In this article, the initial 3.5 sec of the P waves, on the vertical component, are compared. In addition to unfiltered accelerograms, several causal bandpass filters are applied. We do not see significant differences, in either original or filtered records, between the initial (e.g., 0.5 sec) seismograms of moderate and large earthquakes. In the original and 2- to 5-Hz passband, the main differences among events with magnitudes over about 4.5 is in the duration of shaking. The initial ground motions of all the large events have very small amplitudes, which gradually get larger. At lower frequencies, seismograms of the largest events show a complex series of multiple pulses, assumed related to failure of numerous asperities. In our lowest frequency band (0.2 to 0.5 Hz), most of the large events that we examined begin with rupture of asperities that are not necessarily larger, and are sometimes smaller, than the asperities that fail during smaller events. The observations are consistent with a model of faulting in which the largest asperities are located at random on the eventual fault plane, and fail when the rupture front, propagating from the hypocenter, reaches them. This allows the largest asperities to occasionally be at the hypocenter and to fail immediately, but for a large fault, it is more likely that they will be located elsewhere and fail later.