The three-dimensional attenuation structures beneath the Philippine archipelago based on seismic intensity data inversion

Attenuation structures were determined for the Philippine archipelago by inversion of earthquake intensity data set from 1957 to 1992. The intensity data based on the Adapted Rossi-Forel Intensity Scale were translated into acceleration values using a modified acceleration-intensity equation. A damped least-squares method was used in our inversion. We correlated the inversion results with other data, such as observed anomalous intensity distribution, present seismicity, and the location of trenches and volcanic centers. Our results strongly indicate the association of low attenuation values with subducted slabs along the known location of trenches in the Philippines. For example, the configuration of the Philippine Trench is suggested by the concentration of low attenuation values along the east side of central and southern Philippines while on the western side of Luzon Island a low attenuation structure is consistent with the location of the Manila Trench. The extent and depth of low attenuation values are also apparent along the eastern side of Luzon. In southern Luzon through Negros Island in the central Philippines, the site where anomalous intensity distribution was observed coincided with the location of a noticeable low-attenuation structure determined in this study. Moreover, possible locations of an upwelling magmatic body along the numerous areas in the Philippines are also indicated by the concentration of high attenuation values. Furthermore, a possible aseismic slab, which probably represents a structure that connects the Manila Trench with the Negros Trench, was also inferred in Eastern Mindoro. However, it is also possible that these low attenuating values are due to the presence of continental material in this area. On the basis of these initial results, we suggest some modifications of the configuration of tectonic structures in several portions of the archipelago. (C) 1997 Elsevier Science B.V.