The 1984 Round Valley, California Earthquake Sequence

SUMMARY The 1984 November 23 ML 5.8 Round Valley earthquake is one in the.series of moderate earthquakes to have occurred in the Bishop-Mammoth Lakes, California area since 1978. This event and its aftershock sequence are particularly well recorded in that they occurred within a dense, local high frequency seismic network, and strong motion accelerograms, and regional and teleseismic digital seismograms are available for the main shock. We have derived the fault plane solution from the local and regional first motion data, inverted the teleseismic body wave data for the moment tensor solution, and fit regional surface waves for the low frequency moment. Unlike results for several of the earlier moderate events in the Bishop-Mammoth Lakes region, all results for the Round Valley main shock data sets give solutions consistent with left-lateral strike slip faulting on a near vertical N30"E striking plane. The seismic moment of the main shock from the regional surface wave analysis is 7.9 X 101'Nt m. Measurements of the seismic moment from regional surface waves and teleseismic body waves have been converted to spectral level and combined with the acceleration spectra to give a composite source spectrum for the main shock. The spectrum shows a lower corner frequency at 0.2Hz associated with the overall faulting event, and a higher comer frequency at 4.0 Hz which may be associated with the subevents composing the but at a lower slope of w-l between the low and high frequency corners as postulated in models for complex or partial stress drop events. The Round Valley main shock was followed by a widespread and prolonged aftershock sequence. Because of the dense local, high gain permanent seismograph network operated jointly by the University of Nevada-Reno and the United States Geological Survey, accurate locations and fault plane solutions could be determined for the aftershocks beginning immediately after the main shock, and the temporal and spatial growth of the aftershock sequence could be followed in detail. The Round Valley sequence is characterized by the development of two conjugate planes of aftershock activity; one, a near vertical plane striking N30"E associated with the main shock, and another which developed in the first 24 h of activity striking N40"W and dipping 55"NE. This shallow plane conforms to an extension of the Hilton Creek fault postulated by Malcome Clark, and may be the first indication of activity on this major Holocene structure since the recent period of earthquake activity began in 1978. Aftershocks defining the deeper near vertical plane tend to concentrate around the periphery of a 36-49 km2 area which appears to represent the slip surface of the main shock. Relating the seismic moment determined from regional surface waves to this slip area results in an average dislocation of 0.54-0.73 m and a stress drop of 1.5-2.3 MPa. Nearly 800 fault plane solutions for aftershocks were determined. A smaller subset which samples the aftershock zone both spatially and temporally shows that during the initial hours of the sequence, aftershocks occuring on the deeper, near vertical plane were predominantly left-lateral strike slip. Motion on the N40"W plane was dominantly right-lateral to right-lateral oblique slip with a tendency for shallow normal displacement at its western extent. Fault plane solutions in the shallow crust above the two conjugate planes are more variable, suggesting complex brecciation of the shallow crustal block. Although the main shock and the bulk of aftershock activity is confined to the foot wall block of the Round Valley range front fault, this activity at the north end of the sequence area may represent activation of the hanging wall block of the range front fault.