S51C-01
Mexican National Seismic Network: An overview, and its application to the seismic detection in the State of Guerrero, Mexico
The Mexican National Seismic Network (MNSN), has been increasing its coverage in Mexico. At the present we
have about 30 broad-band seismic stations along the country, that transmit in real time its information to the
National Mexican University (UNAM), in Mexico City, home of the MNSN. At the present we are finishing the
installation of six stations in the Northern part of Mexico, a region poorly covered before. Guerrero, is one of the
states with a major seismicity in the country. For this reason we have 4 broad-band seismic stations installed
there, and two more near the borders of the state. This coverage has improved the hypocentral locations in
Guerrero, allowing us look for paterns of seismicity distribution, in important regions such as the Guerrero
Seismic Gap (GSG), a site where no large earthquake (> 7.5), has ocurred since 1911. A close look at the
epicentral distribution in Guerrero, between 2005 and 2006, shows that 270 earthquakes with magnitudes
greater than 4, are located principally between the Middle America Trench and the coast, and a smaller number of
events is located along a section that parallels the coast at about 80-110 Km inland. If we plot the epicenters of
earthquakes with magnitudes greater than 4.5, the number of events becomes 36, and a few of them are located,
again along the inland sections that parallels the coast, and the others shows a pattern that borders, what can be
consider the GSG. Although, with this information it is not possible to evaluate the state of the GSG, it is important
to stablish the behavior of the seismicity before the ocurrance of an expected major earthquake.
http:ssn.unam.mx
S51C-02 INVITED
Receiver Function Modeling of the Central Mexico Slab and Comparison to MASE Data
We have produced an image of the subducted Cocos plate beneath Central Mexico with receiver functions utilizing data from Meso-America Subduction Experiment (MASE). The receiver functions show that the subducting oceanic crust is shallow, dipping to the north at 15 degrees for 80 km from Acapulco and then horizontally underplates the continental crust for 200 km, all the way to the beginning of the Trans Mexico Volcanic Belt (TMVB). The continental Moho is about 40 km deep beneath the TMVB and shallows towards the north. The region under the TMVB and further north is less clear in the images. To further investigate this region we produce synthetic receiver functions with a 2D finite-difference wave propagation program for particular velocity and slab models and compare these to the data. The receiver function image is produced by migration of the receiver functions, with several different velocity functions. Seismic multiples are also included in the image because in some portions of the receiver functions they are stronger than the primary conversions.
S51C-03 INVITED
Attenuation of Seismic Waves Along and Perpendicular to the Pacific Coast of Mexico: Does it Differ?
Does the attenuation of seismic waves along the Pacific coast of Mexico and towards inland, along the dip direction of the subducted slab, differ? Several studies report different attenuation along these two trajectories. Others, however, have not found any difference. To resolve this issue, we analyze an extended dataset of nearly 450 strong-motion recordings from Mexican interplate earthquakes. Our data show that the Fourier amplitude spectra of the horizontal components of S-wave at inland sites are larger than those at coastal sites at similar distances. We explore and discard possible source and site effects. Hence, the observed differences seem to be a reflection of differences on the attenuation function, Γ(f,R) = e-π f R/β Q(f)/G(R), where Q is the quality factor and G(R) is the geometrical spreading term. The strong trade-off between Q and G(R) hinders reliable estimation of Q. Numerical simulations suggest that Lg and SmS phases propagate very efficiently along the direction of the dipping slab, which could explain the observations. We propose new attenuation functions for both trajectories that may be useful in predicting future ground motions.
S51C-04 INVITED
Regional Attenuation, Local Magnitude and Source Excitation in Central Mexico.
Central Mexico is not only of scientific interest but also of great economic and social impact since it is the most populated region of the country of Mexico. Of particular importance to the seismic hazard in this region are intermediate-depth earthquakes, not only because of some highly energetic events, but also because of the proximity to the cities and villages in the Mexican Altiplano. This work is an effort to characterize the seismic-wave propagation parameters, regional attenuation and magnitude estimation in this region. I discuss the major issues regarding ground motion scaling of this challenging region of the world such as the predictive relation within and around the Trans Mexican Volcanic Belt, local magnitude estimations and source excitation characteristics.
S51C-05 INVITED
Body-Wave Attenuation Structure in Central Mexico
Velocity spectra from moderate earthquakes are used to investigate the P-wave attenuation structure in southern Mexico. We include regional events with magnitudes in the range 4.5 < M < 5.0 recorded on the Meso American Subduction Experiment (MASE) array, which consists of 100 broadband sensors from Acapulco to Tampico. By assuming a Brune-type source, a path-averaged frequency- independent Q is obtained for each seismogram in the frequency band 0.5 Hz to 7-30 Hz, depending on the signal quality. These measurements are then inverted for spatial variations in Q. The 1-D tomography result shows a pattern of Q qualitatively similar to other subduction zones, with low attenuation crust (Q ~ 1100), and high attenuation in the mantle wedge beneath the Trans-Mexico-Volcanic-Belt (Q < 250). The 2-D inversion shows a more detailed image of Q variations. The location of the low-Q region and the variation of the Q value also provides some constraints on the geometry of the subducting slab, or with the structure provided by other methods such as receiver functions, The Q estimates will be used to estimate variations in viscosity.
S51C-06
Q of Lg Waves in the Central Mexican Volcanic Belt
From seismograms of shallow, coastal earthquakes recorded at a pair of broadband stations, we estimate Q of Lg waves in the part of central Mexican Volcanic Belt (MVB) that includes the Valley of Mexico. The two stations straddle the central MVB and are located on Cretaceous limestone. A weighted least-square fit to the Q-1(f) data in the frequency range 0.25 to 8 Hz yields Q(f)=98f0.72. This estimate of Q is lower than the corresponding Q in the forearc region which is given by Q(f)=273f0.66. We note that our estimate of Q(f) corresponds to a 200 km-wide zone of the MVB. The result of this study sheds light on the characteristics of seismic waves as they traverse through the MVB where they undergo dramatic amplification in the Valley of Mexico. It also provides one of the critical elements needed in the estimation of expected ground motions at sites to the north of the MVB from future coastal earthquakes. Lower Q of Lg waves in the MVB as compared to the forearc region seems correlated with lower resistivity reported in the MVB relative to the forearc region.
S51C-07
Upper Mantle Shear Wave Anisotropy for Stations in Mexico and its Relationship to Subduction at the Middle America Trench
We have calculated the splitting parameters that describe upper mantle shear wave anisotropy under stations in continental Mexico located over the subducting Cocos Plate. SKS and SKKS arrivals recorded on both the radial and transverse horizontal components were used. The splitting parameters which quantify anisotropy are the delay time (δt) and the fast polarization direction (φ). The anisotropy is calculated using the approach by Silver and Chan [1991]. A time segment containing the SKS arrival is selected from both horizontal components. The space of possible solutions is then searched in one-degree intervals with φ ranging between 0 and 180°. Specifically, the coordinate axes are rotated every 1 degree increment and the autocorrelation and crosscorrelation between the components is calculated. For each value of φ, the solution space is also searched in 0.05 s increments. Next the eigenvalues corresponding to each δt and φ combination are calculated. In the presence of noise, the desired solution will be given by the matrix which is most nearly singular. In order to check our results, we apply a correction in the amount of the measured δt and φ to the original records and then rotate them to make sure that the anisotropy disappears. The shapes and the difference in the arrival times of the fast and slow waves are compared to make sure that the result is robust. As a further check, the polarization of the particle motion for the radial and transverse components before and after correction is plotted. The records used were taken from Mexico's Servicio Sismológico Nacional broadband network [Singh et al., 1997]. The orientation of the fast polarization direction, φ, can be explained by the absolute motion of the North American plate for some of the stations. Most of the stations, however, require a different explanation for the orientation of φ. For example, the orientation of φ for stations Platanillo (PLIG), Yautepec (YAIG), and Popocatépetl (PPIG) is aligned with the direction of absolute motion for the North American plate [Van Benthem, 2005]. The orientation of φ under Cayaco (CAIG) is close to the orientation of the relative plate motion vector between the Cocos and North American plates. The only measurement available under Ciudad Universitaria (CUIG) indicates that the orientation of φ is consistent with the extensional regime in the Trans- Mexican Volcanic Belt, with normal faults running east-west.
S51C-08
How Large can Mexican Subduction Earthquakes be? Evidence of a Very Large Event in 1787 (M~8.5)
A sequence of very strong earthquakes occurred from 28 March to 18 April, 1787. The first earthquake on 28 March, appears to be the largest of the sequence followed by three strong events on 29 and 30 March, and 3 April; strong aftershocks continued to be reported until 18 April. The event of 28 March was strongly felt and caused damage in Mexico City, where several buildings were reported to suffer. The strongest effects, however, were observed on the southeastern coast of Guerrero and Oaxaca. Intensities greater than 8 (MMI) were observed along the coast over a distance of about 400 km. The towns of Ometepec, Jamiltepec and Tehuantepec reported strong damage to local churches and other apparently well-constructed buildings. In contrast to the low intensities observed during the coastal Oaxaca earthquakes of 1965, 1968 and 1978, Oaxaca City reports damage equivalent to intensity 8 to 9 on 28 March, 1787. An unusual effect of this earthquake on the Mexican subduction zone was the presence of a very large tsunami. Three different sources report that in the area known as the Barra de Alotengo (16.2N, 98.2 W) the sea retreated for a distance of about one Spanish league (4.1 km). A large wave came back and invaded land for approximately 1.5 leagues (6.2 km). Several local ranchers were swept away by the coming wave. Along the coast near the town of Tehuantepec, about 400 km to the southeast of Alotengo a tsunami was also reported to have stranded fish and shellfish inland; in this case no description of the distance penetrated by the tsunami is reported. It is also described that in Acapulco, some 200 km to the northwest of Alotengo, a strong wave was observed and that the sea remained agitated for a whole day. Assumming that the subduction zone ruptured from somewhere near Alotengo to the coast Tehuantepec, the resulting fault lenght is about 400 to 450 km. This large fault rupture contrasts with the seismic cycle of the Oaxaca coast observed during this century where apparently earthquakes with magnitudes Mw > 7.4 ruptured asperities with a return period of about 32 to 56 years. The presence of a great earthquake rupturing approximately 450 km of the Mexican subduction zone with an equivalent magnitude of about 8.5 is a phenomenon not observed during instrumental times. The presence of such a large rupture process, as the one suspected on 28 March, 1787, raises the question whether great events like this one may take place in other segments of the Mexican subduction zone.