Francisco Nuñez-Cornu

Seismic energy attenuation in the region between the coast of Guerrero and Mexico City: differences between paths along and perpendicular to the coast

Attenuation of seismic energy in Central Mexico from earthquakes generated along the South Coast has been discussed in several works. The attenuation interpretations falls broadly in two categories: the first, lower attenuation in paths going inland relative to paths going along the Coast; the second, attenuation follows the same model regardless of the path. In this paper we analyze data from a seismic refraction experiment between the Coast of Guerrero and the Valley of Mexico, as well as data from an earthquake with its epicenter in the Coast of Guerrero in order to study seismic wave attenuation along the two paths. We found a lower attenuation of S waves going toward the continent than along the coast; this fact is clear from refraction data and can be concluded from our earthquake data and by reinterpreting information from previous works. We find Q values that indicate the attenuation relations, based mainly in anelastic dissipation, currently used for the region between the coast and the Valley of Mexico are not adequate; other factors should be considered to explain the inland observations. This regional effect should also be considered in the study of the large amplification of seismic signals in the Valley of Mexico and, also for other cities in Mexico.

S wave attenuation in the coastal region of Jalisco–Colima, México

Abstract Aftershock data from the October 9, 1995, M =7.6 earthquake, that occurred in the coastal subduction region of Colima–Jalisco, Mexico, are used to obtain estimates of the frequency independent quality factor Q s and spectral decay parameter κ , in the approximate frequency range 3 Hz≤ f ≤40 Hz, as functions of hypocentral distance r . A regression analysis results in the relations for S waves: Q s =261.397+3.198r±15.536 κ=0.009651+0.000462r±0.0012. The observed distance dependence of Q s and κ is similar to that reported for the Mexican regions of Oaxaca and Guerrero. Our Q s values agree with those reported for Oaxaca and northern Baja CA for frequencies around 8 Hz, and are somewhat lower (a factor of ∼0.8 to ∼0.5) for higher frequencies around 20 Hz; however, they are significantly lower than all but the lowest values reported for the Guerrero region, attaining factors ∼0.25 and ∼0.13 for frequency ranges around 8 and 20 Hz, respectively. It thus appears that the Guerrero region has an anomalously low attenuation compared with the flanking coastal regions of Oaxaca–Chiapas to the SE and Jalisco–Colima to the NW.