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Spatial Variations in the Frequency-Magnitude Distribution of Earthquakes at Mount Pinatubo Volcano

Geophysical Institute
University of Alaska, Fairbanks
903 N. Koyukuk Dr.
Fairbanks, Alaska 99775
(J.J.S., S.R.M.)
Alaska Volcano Observatory
U.S. Geological Survey
4200 University Dr.
Anchorage Alaska 99508
(J.A.P.)
World Agency for Planetary Monitoring and Earthquake Risk Reduction,
Geneva
Route de Malagnou 36A
CH-1208 Geneva, Switzerland
(M.W.)

Manuscript received 16 December 2002.

The frequency-magnitude distribution of earthquakes measured by the b-value is mapped in two and three dimensions at Mount Pinatubo, Philippines, to a depth of 14 km below the summit. We analyzed 1406 well-located earthquakes with magnitudes M_D 0.73, recorded from late June through August 1991, using the maximum likelihood method. We found that b-values are higher than normal (b = 1.0) and range between b = 1.0 and b = 1.8. The computed b-values are lower in the areas adjacent to and west-southwest of the vent, whereas two prominent regions of anomalously high b-values (b 1.7) are resolved, one located 2 km northeast of the vent between 0 and 4 km depth and a second located 5 km southeast of the vent below 8 km depth. The statistical differences between selected regions of low and high b-values are established at the 99% confidence level. The high b-value anomalies are spatially well correlated with low-velocity anomalies derived from earlier P-wave travel-time tomography studies. Our dataset was not suitable for analyzing changes in b-values as a function of time. We infer that the high b-value anomalies around Mount Pinatubo are regions of increased crack density, and/or high pore pressure, related to the presence of nearby magma bodies.

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