SPATIAL VARIATION OF SEISMIC B-VALUES BENEATH MAKUSHIN VOLCANO, UNALASKA ISLAND, ALASKA

The frequency-magnitude distribution was spatially mapped beneath Makushin Volcano, Unalaska Island, Alaska. The final earthquake catalog used in this study consists of 491 events that occurred between July 2001 and April 2005. Two high b-value anomalous zones were found, one ~4 km east of the Makushin's main vent area at a depth of 3 km to 7 km (b ~ 1.8) and a second one located 7 km southeast of the vent (b ~ 1.6). The significance of this second volume could not be tested statistically due to its small dimensions and the low number of seismic events in its proximity. The first anomaly is robust statistically based on Utsu's (1992) p-test and is not method or parameter dependent. This anomaly suggests the presence of crustal heterogeneity and/or high pore pressure, such as that typically found near magma chambers, at ~6-7 km depth. Our results agree with the findings of an InSAR interferometric study which suggest the existence of a magma chamber at ~7 km depth, 5 km east of Makushin's vent.