Seismicity Patterns: Are they Always Related to Natural Causes?

By analyzing data from two catalogs of seismicity, one local and one regional, examples are drawn that demonstrate artificial seismicity patterns which can arise when changes in magnitude reporting or in detection ability are introduced by changes in operating procedures of a seismic network. In the first case, a catalog which comprises seismic data from Guerrero, Mexico, is revised and a correction is applied to magnitudes suspected of being affected by different operative practices. Next, several time windows are analyzed which compare the seismicity rate within the window to that of the total record mapped as a function of space. The same procedure is applied to the original (i.e., uncorrected) catalog. A significant seismic quiescence apparent in the original data set in the center of the seismic network all but disappears in the corrected data, indicating that this anomaly is not naturally induced. In the second case, we investigate the homogeneity of seismicity reporting for the Interior of Alaska. We computed the standard deviate z as a function of time by comparing the overall seismicity rate with the rate in a 3-year window. The maps z-values were inspected for all times. The most outstanding rate change is found around 1992.5. Since the b-value remained unchanged, the most reasonable explanation for the observed rate change around mid-1992 is a decrease in the detection ability of the network in the Interior of Alaska. Both case studies demonstrate the usefulness of systematic comparisons of the cumulative and noncumulative frequency-magnitude distribution, and of spatial and temporal mapping of the seismicity rates as a tool to investigate the homogeneity of earthquake reporting.