The Colima, Mexico, earthquake (M (sub w) 5.3) of 7 March 2000; seismic activity along the southern Colima Rift

The 7 March 2000 earthquake (M (sub w) 5.3), located within the southern Colima rift in southwestern Mexico, is the largest crustal earthquake ever recorded within the state of Colima, Mexico. The mainshock was located at 8 km depth, and the aftershocks are distributed at depths between 5 and 10 km. Aftershock distribution follows a normal decay as measured by the modified Omori's law, with a p value of 0.96. Both aftershock locations and focal mechanisms (determined for the mainshock and some of the aftershocks) delineate a fault dipping toward the northwest and striking west-southwest. This fault segment is parallel to the southern end of the Tamazula fault, which ends at Manzanillo Bay. Regional waveform modeling for the mainshock situates the earthquake on a normal fault with a significant left-lateral strike-slip component on a plane striking 260 degrees and dipping toward the northwest at 46 degrees . We use a small aftershock with similar focal parameters as the mainshock to deconvolve P and S waves recorded at the permanent station COIG. From the deconvolution we obtained a source duration of 0.75 sec from the P waves and 0.9 sec from the S waves for the main event and a stress drop of 320 bars. The aftershock area is much larger than the area measured from the apparent source time function. From the aftershock area and the seismic moment obtained by Harvard University (Centroid Moment Tensor Catalog), we compute a static stress drop of 20 bars. Faultplane solutions from the mainshock and some of the aftershocks indicate that the northern, and larger, segment of the Tamazula fault is perpendicular to the extensional axis (directed east-southeast). This study confirms the change in the direction of the extensional axis of the southern Colima rift with respect to the northern Colima rift (extensional axis oriented almost east-west). Here we show that the Tamazula fault zone, at least its eastern segment, is active, and a re-evaluation of the seismic hazard posed by this fault to the state of Colima and the cities of Colima and Manzanillo is urgent.