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 Californi a has recorded several hundred earthquakes <M(L)4.0 occurring within t he San Andreas fault system. At 2.5 km seismic background noise is bel ow amplifier sensitivity and the 2-250 Hz spectral range of recorded s eismic motion is wider and higher than that of most natural event cata logs. Compared with downhole recorded motion, seismic amplitudes at th e surface are amplified below 10 Hz and severely attenuated above 30 H z. We estimate that Q(S) is at least 1000 for wave motion at 2.5 km an d below and Q(P) is over 2000. The range of source dimensions in the d ownhole recorded catalog is approximately 10 m to approximately 70 m ( M(L) approximately -2.0, M(o) approximately 10(8) Nm to M(L) approxima tely 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 b etween 1 and 500 bars. There is no evidence in the catalog for the pro posed minimum source dimension at approximately 100 m. When the Cajon Pass borehole catalog, containing some of the smallest recorded natura l earthquakes, is combined with 800 larger events from previous studie s, 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.