Precursory Seismicity Patterns

Mature seismic gaps are defined here as gaps in which a possible precursor to a future event has been identified. The seismic potential of these seismic gaps is enhanced relative to immature seismic gaps. The seismic gap hypothesis by itself provides an excellent means of narrowing spatial limits along plate boundaries within which the great earthquakes of the next several decades are expected. To narrow the temporal limits on these forecast events, identified gaps must be monitored for signs of maturity. Decreased seismicity rates (number of events reported/unit time) have been reported before over fifty large earthquakes. Seismicity rates measured teleseismically are tested as a possible indicator of maturity by examining data from the NOAA HDF in the regions of eleven recent large events. These events were chosen as test cases because they occurred in previously recognized seismic gaps. Many previous seismicity studies lack quantitative evaluation of patterns or changes of patterns which are proposed as precursors. This makes many of the results unconvincing. In this work seismicity data is quantified by studying seismicity rates, the number of events per month. Changes in rates are evaluated using the normal deviate (z) test for a difference between two means. Several techniques for appling this test to seismicity data are proposed. These techniques offer various approaches to quantitatively defining seismicity rate changes and evaluating their temporal and spatial uniqueness.