Intraslab Mexican earthquakes of 27 April 2009 (Mw5.8) and 22 May 2009 (Mw5.6): a source and ground motion study

Two types of intraslab earthquakes in the subducted Cocos plate occur below Guerrero, Mexico, and adjacent areas: (A) steeply-dipping thrust earthquakes, and (B) normal-faulting earthquakes. Type A events are located ~10-35 km from the coast at a depth of ~35 km and reveal down-dip compression in the slab, most probably a consequence of unbending of the slab. Type B earthquakes are only slightly deeper than type A events when they occur near the coast, but if they occur farther inland where the slab becomes horizontal, they reach a depth of 40-50 km. They reveal down-dip extension in the subducted slab. Analysis of earthquakes of 27 April 2009 and 22 May 2009 reveals that they were intraslab events in the subducted Cocos plate of type A and B, respectively. The source spectra retrieved from local and regional data yield Brune stress drop, Δσ, of ~49 and 34 MPa, respectively, somewhat greater than the median Δσ of 30 MPa previously reported for intraslab Mexican earthquakes. Our estimates of the radiated energy, E R, are 3.55x10(13) J and 2.29x10(13) J, which gives E R/M0 of 5.63x10-5 and 6.54x10-5, and apparent stress, σa, of 3.9 MPa and 4.6 MPa, respectively (corresponding to M0 reported in Global CMT catalog), reasonable values for intraslab earthquakes. Peak ground accelerations (PGA) as a function of distance are in fairly good agreement with previously-derived attenuation relations for Mexican intraslab earthquakes. Ground motions in the Valley of Mexico from intraslab earthquakes are enriched at high frequency as compared to those from interplate events, especially in the hill zone. This reflects both a more energetic nature of the intraslab sources and, relatively, closer location of many of these earthquakes. The results obtained in this study give us some confidence in our knowledge of the nature of intraslab sources and our ability to estimate ground motions from future events.