Spatial variation of seismic b-values beneath Makushin Volcano, Unalaska Island, Alaska

Mathematics – Logic

Scientific paper

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Scientific paper

The frequency magnitude distribution was spatially mapped beneath Makushin Volcano, Unalaska Island, Alaska using an earthquake catalog of 491 events that occurred between July 2001 and April 2005. An area of high seismic b-values (˜ 2.0) is found ˜ 4 km east of Makushin's main vent at a depth between 4 and 7 km. The anomaly is statistically significant based on Utsu's p-test [T. Utsu, On seismicity, in Report of the Joint Research Institute for Statistical Mathematics, Tokyo (1992) 139 157], and is not data processing method or parameter dependent. Interestingly, a recent InSAR interferometric study [Z. Lu, J.A. Power, V.S. McConnell, C. Wicks Jr., D. Dzurism, Preeruptive inflation and surface interferometric coherence characteristics by satellite radar interferometry at Makushin volcano, Alaska: 1993 2000, J. Geophys. Res. 107 (2002) 2266, doi:10.1029/2001JB000970] inferred a surface uplift of about 7 cm during the two-year period prior to October 1995, centered approximately in the area with the observed anomalous b-values. The uplift was caused by the volume increase of an inferred magma chamber at a depth of about 7 km. The close correspondence of the seismic and InSAR observations suggests that the heterogeneous area associated with the observed high b-values is most likely the result of increased crack density associated with the magma chamber. This study demonstrates the effectiveness of combining InSAR and seismological observations in locating magma chambers and areas of high heterogeneity in the crust.

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