Attenuation of modified Mercalli intensity with distance in Mexico

Two relations are proposed to predict the attenuation of Modified Mercalli Intensity (I) with distance (D) for Mexican earthquakes, i.e. ln I = B 0 + B 1 ln ( D / D ' ) + B 2 ( D − D ' ) + B 3 ln M s ln I = B 0 + B 1 ( D / D ' ) + B 2 ln ( D − D ' ) + B 3 ln M s Ms is the earthquake surface-wave magnitude, D′ is a distance related to the maximum I mapped for an earthquake, I′ or to Ms. The coefficients Bi, i = 0, 1, 2, 3 were obtained by fitting in a least-square sense the information contained in the intensity maps of 32 events to the relations. Those events were classified in three groups according to their epicentral location, focal mechanism, and depth, i.e., events related to the subduction-zone intermediate-depth earthquakes in south-central Mexico and to shallow crustal events along the Trans-Mexican Volcanic Belt. The I predicted by the proposed relations compare well with the I observed for historical earthquakes not included in the fitting. Results obtained from a parametrical study showed that the attenuation of I with D is different for each of the three types of earthquakes. For distances of less than about 200 km, the earthquakes associated with the subduction zone have a larger attenuation than the ones originating in the south-central region of Mexico; for greater distances (D > 200 km), the opposite behavior is observed. The events located in the Trans-Mexican Volcanic Belt have a larger attenuation with distance than that of events in the other two regions. From these results, it seems advisable in Mexico to use several attenuation relations to estimate the seismic hazard at a site, depending on the particular tectonic setting and the path of the events under consideration.