Determination of the Attenuation Equation of Strong Motion in the Michoacán State

Several attenuation relationships have been developed to Mexico, mostly after the September 19, 1985 earthquake which has meant a watershed in the development of Mexican seismological engineering. Since 1985, the number of seismic stations has increased significantly especially between the Coast of Guerrero and Mexico City because of the large amplifications that have acurrect on lake zone and hard ground sites in Mexico City. Some studies have analyzed how the seismic waves are attenuated or amplified from the Pacific coast towards the continent. The attenuation relationship used for seismic hazard assessment in Mexico is due to Ordaz (1989) this was obtained from data from the Guerrero acceleration network. Another recent study is that conducted by (Garcia et al., 2005) with recent data from the Guerrero acceleration network considering intraplate earthquakes. It is important to note that all these relations cover to only part of the Mexican subduction zone, and for some types of seismic sources it may be not suitable to study the earthquake risk in other regions of Mexico.

For this work we consider the state of Michoacán, because it has one of the most important seismogenic zones in Mexico. Within the state there are three different kinds of seismic sources: and volcanic tectonic earthquakes and those caused by local faults in the region. Then it is a vital issue to study the seismic wave propagation within the state. We installed a temporary network with 9 accelerographic stations, located at Faro de Brucerías, Aguililla, Apatzingán, Taretán, Uruapan, Nueva Italia Pátzcuaro, Morelia and Maravatío, Michoacán. The stations formed a perpendicular line to the coast, with a total length of 366 km, the distance between stations varies from 60 to 80 km.

Among the total seismic events recorded, we selected 7 seismic events located in the Michoacán coastline, from 4.1 to 5.1 Mw. With those records, Q quality factor (107.215 f ^0.74) was calculated for frequencies between 0.1 and 10 Hz, since those are the important frequencies for Earthquake Engineering. The preliminary results show a significantly larger attenuation compared with the attenuation laws for the states of Guerrero and Colima.