HR: 1400h
AN: S33B-01    [Abstracts]
TI: Microtremors study applying the SPAC method in Colima state, Mexico.
AU: * Vázquez Rosas, R
EM: rvazquezr@iingen.unam.mx
AU: Aguirre González, J
EM: joagg@pumas .unam.mx
AF: Universidad Nacional Autonoma de Mexico, Instituto de Ingeniería,Coordinación de Ingeniería Sismológica, Circuito Escolar SN Edificio Torre de Ingeniería,Ciudad Universitaria, Mexico, DF 04510, Mexico
AU: Mijares Arellano, H
EM: hma@pumas.iingen.unam.mx
AB: One of the main parts of seismic risk studies is to determine the site effect. This can be estimated by means of the microtremors measurements. From the H/V spectral ratio (Nakamura, 1989), the predominant period of the site can be estimated. Although the predominant period by itself can not represent the site effect in a wide range of frequencies and doesn't provide information of the stratigraphy. The SPAC method (Spatial Auto-Correlation Method, Aki 1957), on the other hand, is useful to estimate the stratigraphy of the site. It is based on the simultaneous recording of microtremors in several stations deployed in an instrumental array. Through the spatial autocorrelation coefficient computation, the Rayleigh wave dispersion curve can be cleared. Finally the stratigraphy model (thickness, S and P wave velocity, and density of each layer) is estimated by fitting the theoretical dispersion curve with the observed one. The theoretical dispersion curve is initially computed using a proposed model. That model is modified several times until the theoretical curve fit the observations. This method requires of a minimum of three stations where the microtremors are observed simultaneously in all the stations. We applied the SPAC method to six sites in Colima state, Mexico. Those sites are Santa Barbara, Cerro de Ortega, Tecoman, Manzanillo and two in Colima city. Totally 16 arrays were carried out using equilateral triangles with different apertures with a minimum of 5 m and a maximum of 60 m. For recording microtremors we used short period (5 seconds) velocity type vertical sensors connected to a K2 (Kinemetrics) acquisition system. We could estimate the velocities of the most superficial layers reaching different depths in each site. For Santa Bárbara site the exploration depth was about 30 m, for Tecoman 12 m, for Manzanillo 35 m, for Cerro de Ortega 68 m, and the deepest site exploration was obtained in Colima city with a depth of around 73 m. The S wave velocities fluctuate between 230 m/s and 420 m/s for the most superficial layer. It means that, in general, the most superficial layers are quite competent. The superficial layer with smaller S wave velocity was observed in Tecoman, while that of largest S wave velocity was observed in Cerro de Ortega. Our estimations are consistent with down-hole velocity records obtained in Santa Barbara by previous studies.
DE: 0935 Seismic methods (3025, 7294)
DE: 3025 Marine seismics (0935, 7294)
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Seismology [S]
MN: 2007 Joint Assembly