Directivity in the high-frequency radiation of small earthquakes

On December 12, 1972 at 0351 and 0355 GMT, two earthquakes with magnitudes equal to 3.0 and 2.8, respectively, occurred on the Cienega Road section of the San Andreas fault in central California. The two events have the same hypocenter location and fault-plane soultion. Observed seismograms for these two events at 28 stations within about 65 km of and surrounding the epicenters are systematically different in a pattern that is consistent with different directions of rupture expansion for the two events. The 0351 GMT event preferentially radiated high-frequency (f ⪚ 10 Hz) body waves to the southeast consistent with unilateral rupture propagation toward the southeast while the 0355 GMT event rupture expanded more toward the northwest.