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A Report on the Atoyac, Mexico, Earthquake of 13 April 2007 (M_w 5.9)

L. A. Aguilar², M. Ambriz², J. G. Anderson³, M. Ayala², C. Cárdenas¹, G. Castro², J. L. Cruz¹, R. Delgado², L. Domínguez⁴, J. Estrada¹, S. I. Franco¹, L. E. Flores⁴, C. Gutierrez⁴, M. A. Macías², I. Molina², C. Morquecho⁴, J. Ortíz¹, M. A. Pacheco-Martínez⁴, A. Quezada¹, R. Quaas⁴, L. Quintanar¹, C. Pérez², J. Perez¹, A. L. Ruiz², H. Sandoval², M. Torres², R. Vázquez², J. M. Velasco², J. Velázquez², and M. Velázquez²

Instituto de Geofísica
Universidad Nacional Autónoma de México
C.U. 04510. México D.F.
krishna{at}ollin.igeofcu.unam.mx
(S. K. S.)

View larger version (34K): [in this window] [in a new window]   Figure 1. (A) Locations and focal mechanisms of Mw 4.0 earthquakes in the Guerrero region relevant to this study. M denotes Atoyac mainshock and 1, 3, and 4 refer to the aftershocks (table 1). Letters associated with the events are keyed to table 2. Except for events a, b, and f, all others occurred between 31 March and 30 April 2007. Rectangle: accelerographic station, triangle: broadband station. Some critical stations are identified. (B) Section along AA'. Dashed line indicates plate interface. Locations, focal mechanisms and associated stress axes of inslab earthquakes are indicated. Atoyac mainshock and its aftershocks (h 37 km) represent downdip compression. The earthquakes that occur below this zone of compression exhibit downdip tension. The stress axis of a typical such event is taken from Pacheco and Singh (2007).

View larger version (34K): [in this window] [in a new window]   Figure 2. Observed (continuous) and synthetic (dashed) displacement seismograms of the Atoyac mainshock and three of its aftershocks at stations ATYC and CAIG (figure 1A). Observed seismograms have been inverted for seismic moments and focal mechanisms (see text).

View larger version (21K): [in this window] [in a new window]   Figure 3. Average source displacement, 0(f), and acceleration, f20(f), spectra (mean and ±1 standard deviation curves). (Left) mainshock, (right) largest aftershock. Superimposed on the spectra are predicted curves from an 2 source model (smooth curves).

View larger version (17K): [in this window] [in a new window]   Figure 4. Horizontal quadratic mean PGA (top) and PGV (bottom) at hard sites as a function of hypocentral distance. Superimposed are predicted curves (mean and ±1 standard deviation) for normal-faulting Mexican inslab earthquakes (García et al. 2005). Triangles: soft sites in Acapulco.

View larger version (22K): [in this window] [in a new window]   Figure 5. Location of strong-motion stations in Acapulco. VNTA and ACAJ are situated on hard sites, while all other stations are located on soft sites.

View larger version (11K): [in this window] [in a new window]   Figure 6. Simulated PGA at ACAJ (hard site) and ACAD (soft site) in Acapulco for postulated 5.5 Mw 6.5 events of the same type as the Atoyac earthquake. Solid symbols: event c (table 2) used as EGF, open symbol: event d (table 2) used as EGF. Predicted PGA is greater when event d is used as EGF. Note that data at ACAD for event d was not available.

View larger version (11K): [in this window] [in a new window]   Figure 7. Observed PGA (ACU) versus expected PGA (Ared) at station CU. ACU and Ared are based on bandpass-filtered CU and nearsource recordings, respectively (see text). The plot includes data since the early 1960s and forms the basis of an alternative seismic alert algorithm for Mexico City (Iglesias et al. 2007). In the proposed scheme, an alert will be issued if the expected PGA at CU, estimated from near-source recording, is 2 cm/s2. If we require that the probability of missed alert at ACU 2 cm/s2 be 1% and that of false alert be 30%, then an alert must be triggered if Ared 0.8 cm/s2. The data for 30 March-30 April 2007 sequence (tables 1 and 2) are shown by dots. Only the Atoyac mainshock merits an alert.

View larger version (58K): [in this window] [in a new window]   Figure 8. Map of Mexico City showing its geotechnical subdivision in hill, transition, and lake-bed zones. Rectangles, dots, and triangles are accelerographic stations operated by three different institutions. Relative spectral amplifications of these sites with respect to CU (large dot) are known. Interpolated relative spectral amplifications are available at 2,500 grid points. Real-time accelerograms at CU are used to generate expected ground-motion maps of the city via random vibration theory.

View larger version (50K): [in this window] [in a new window]   Figure 9. (A) Distribution of estimated PGA (in cm/s2) in Mexico City for the earthquake of 19 September 1985. (B) Distribution of estimated spectral acceleration at 2 s (in cm/s2) in Mexico City for the earthquake of 19 September 1985.

View larger version (25K): [in this window] [in a new window]   Figure 10. (A) Distribution of estimated PGA (in cm/s2) in Mexico City for the Atoyac earthquake of 13 April 2007, computed and distributed in near real time. (B) Distribution of estimated spectral acceleration at 2 s (in cm/s2) in Mexico City for the Atoyac earthquake of 13 April 2007, computed and distributed in near real time.