SOURCE PARAMETERS OF SMALL EARTHQUAKES RECORDED AT 2.5 KM DEPTH, CAJON PASS, SOUTHERN CALIFORNIA - IMPLICATIONS FOR EARTHQUAKE SCALING

A 2.5 km deep triaxial seismometer at Cajon Pass in southern California has recorded several hundred earthquakes <M(L)4.0 occurring within the San Andreas fault system. At 2.5 km seismic background noise is below amplifier sensitivity and the 2-250 Hz spectral range of recorded seismic motion is wider and higher than that of most natural event catalogs. Compared with downhole recorded motion, seismic amplitudes at the surface are amplified below 10 Hz and severely attenuated above 30 Hz. We estimate that Q(S) is at least 1000 for wave motion at 2.5 km and below and Q(P) is over 2000. The range of source dimensions in the downhole recorded catalog is approximately 10 m to approximately 70 m (M(L) approximately -2.0, M(o) approximately 10(8) Nm to M(L) approximately 2.7, M(o) approximately 10(13) Nm). The plot of log(source-radius) vs log(moment) has a straight line trend compatible with earthquake scaling at constant stress drop; inferred stress drops are scattered between 1 and 500 bars. There is no evidence in the catalog for the proposed minimum source dimension at approximately 100 m. When the Cajon Pass borehole catalog, containing some of the smallest recorded natural earthquakes, is combined with 800 larger events from previous studies, the moment-radius trend suggests that natural earthquakes are self-similar over a magnitude range M approximately -2 to approximately 8. We suggest that inferences of minimum source dimension are more likely due to bias in bandlimited individual catalogs than to properties of the seismic crust.