The magnitude-frequency distribution of earthquakes recorded with deep seismometers at Cajon Pass, southern California

Abstract

The cumulative b-value (the slope of the Gutenberg-Richter relationship between earthquake occurrence rate and magnitude) is commonly found to be constant (∼1). Network catalogues, however, reveal a decrease at small magnitudes (<3). Some recent studies have suggested that this decrease in b-value is not just an artifact of catalogue incompleteness, but that small earthquakes are really not as numerous as a constant b-value extrapolated from larger events would predict. In the Cajon Pass area, southern California, the b-value of seismicity recorded by the local network (SCSN) appears to decrease below about ML 1.6. In order to investigate whether this decrease is real or simply represents the network detection threshold, we use seismicity recorded by the deep (1.5 and 2.5 km) seismometers deployed in the Cajon Pass Scientific Drillhole between April 1992 and October 1994. The maximum amplitudes recorded downhole are compared to SCSN magnitudes for events recorded by the network, to determine the relationship between amplitude and ML as a function of hypocentral distance from the borehole. Magnitudes are then calculated for 1300 earthquakes which occurred within 40 km of the borehole. Magnitude-frequency curves are calculated for those events within 18 km of the borehole, and a constant b-value is observed to ML 0.5.