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

Instituto de Geofísica
UNAM, Ciudad Universitaria
04510 México
D.F., Mexico
(A.I., S.K.S., J.F.P.)
Instituto de Ingeniería
UNAM, Ciudad Universitaria
04510 México
D.F., Mexico
(S.K.S., M.O.)

Manuscript received 21 March 2001.

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 ²-source model with M₀ = 6.0 x 10²⁵ 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 < 2.0 Hz). Simulations using the recording at CU of the Copalillo earthquake as an empirical Green's function suggests that a 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.

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