Spatial variations of b-values in the subduction zone of Central America

Abstract

Frequency-magnitude distribution along the Mid-American Trench (MAT) has been studied by means of 2345 earthquakes during the period 1964-1994. We used the regional MIDAS catalogue with a magnitude of completeness of 4.2. To resolve the b- value as a function of depth (one dimensional approach), we applied vertically sliding windows containing a constant number of events. To obtain more details in the b distribution, we projected catalogue hypocenters in three selected regions (approximately Guatemala and El Salvador, Nicaragua, Costa Rica), onto planes perpendicular to the trench. The b-values were calculated in sliding cylindrical volumes (two-dimensional approach) containing a constant number of earthquakes and centered at nodes of a 5 km x 5 km grid. The b-value varies significantly along a large part of MAT. High b-values were identified in the upper part of the slab at depths of 80-110 km beneath Guatemala-El Salvador and at depths 130-170 km beneath Nicaragua. Anomalous (high) b- values in the lower part of the slab were located at depths of 50-90 km and 50-160 km beneath Guatemala-El Salvador and Nicaragua, respectively. Anomalies observed at the upper part of the slab may be related to dehydration and successive increase in pore pressure in the down-going lithosphere, which may generate volcanism above the anomalies in the upper part of the slab. Anomalies on the lower surface of the Wadati-Benioff zone are likely to be associated with high thermal gradients between the slab and mantle.