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对菲律宾皮纳图博火山区直到山顶以下14km深度通过b值测定的地震震级-频度分布绘制成二维和三维图。我们采用最大似然法,分析了1991年6月下旬至 8月记录的1406次MD≥0．73的定位较好的地震。我们发现b值高于正常值(b=1．0), 其范围在b=1．0至b=1．8之间。在邻近火山口西南方偏西地区计算出较低b值,而又发现两个显著高b值的异常区(b-1．7),一个位于火山口东北2 km处,深度为0～ 4 km,第二个位于火山口东南5 km处,深度在8 km以下。所选的低b值和高b值区域之间的统计学差异其置信水平确定为99%。高b值异常与早期从P波走时层析成像法研究得出的低速异常具有较好的空间相关性。我们的数据配置不适合把b值作为时间的函数的变化分析。我们推断,皮纳图博火山周围的高b值异常为裂纹密度增加的区,和／或高孔隙压力区,与邻近存在的岩浆体有关。
Seismic magnitude-frequency distribution measured by b-value for the 14-km depth below Pinotti volcano area in the Philippines is plotted as 2D and 3D maps. Using the maximum likelihood method, we analyzed 1406 well-located earthquakes with MD ≥0.73 recorded in late June to August 1991. We found that b values were higher than normal (b = 1.0) and ranged from b = 1.0 to b = 1.8. The lower b value is calculated in the westwest of the volcano near the crater, and two abnormal regions (b-1.7) with significant b value are found. One is located 2 km to the northeast of the volcano and has a depth of 0-4 km, the second at 5 km southeast of the crater with a depth below 8 km. The statistical difference between the selected low b-value and high b-value regions was confirmed at 99% confidence level. The high b value anomaly has a good spatial correlation with the low speed anomalies derived from the P wave travel tomography. Our data configuration is not suitable for the b value analysis as a function of time. We conclude that the high b-value anomaly around Mount Pinatubo is a zone of increased crack density and / or a zone of high pore pressure associated with the presence of nearby magma.