The 1984 Round Valley, California Earthquake Sequence

Abstract

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...

Regional Magnitude Scaling, Transportability, and Ms:mb Discrimination at Small Magnitudes

“…Amplitudes were measured o simulated WWSSN short-period records using Nuttli's practice of taking the 3rd peak for both upswings and downswings and averaging the two amplitudes. This worked well for explosions at NTS, but PRIESTLEY and PATTON (1997) found that adopting the same Q 0 values obtained by Nuttli and Patton led to overestimation of m b (Lg) for earthquakes located near the test site. In Vol.…”

Choosing the ‘best’ double couple from a moment-tensor inversion

“…Following this line of reasoning, it will be shown that the CLVD decomposition is to be preferred in the absence of compelling source complexity or other situations which would lead to major and minor couples. Conversely, it can be argued that reporting the source in terms of major + minor double couples as done recently by Priestley, Smith & Cockerham (1988) is as reasonable as reporting a double couple + CLVD, as is more common, in the absence of the kind of mathematical arguments presented here.…”

Source parameters of the 1987 Edgecumbe earthquake, New Zealand

“…The additional high-frequency energy causing the spectrum to drop off with a slope less than co-2 past the comer frequency can arise due to fault roughness (Gusev 1983). Such effects have been observed for earthquakes over a wide magnitude range Brune et al 1986;Smith et al 1988). The lack of this roughness energy in the spectrum of the Edgecumbe earthquake indicates a relatively uniform rupture process.…”

“…This presumably represented the upper limit for the surface which slipped during the main shock. In several cases where dense seismograph networks were in place at the time of a main shock, allowing accurate aftershock locations immediately following a main event, it has been observed that the aftershock zone grew in size by 10-20 % in the first few hours (e.g., Smith & Priestley 1988;Priestley et al 1988). This is to be expected since aftershocks should be most frequent at the boundaries of the slip zone where the main shock has increased the stress.…”

“…The shape of the spectrum beyond the corner frequency is important in the calculation of the total radiated energy; if the amplitude spectrum falls off as co-2 , the velocity spectrum falls off as CO-I and the velocity squared spectrum, which is proportional to energy, falls off as co-2 , thus, there is relatively little contribution to the total energy for frequencies beyond the corner frequency. However, if the amplitude spectrum falls off as CO-I beyond the comer frequency, the velocity spectrum and the velocity squared spectrum are constant, and the contribution to the total radiated energy is proportional to the bandwidth of that portion of the spectrum (Smith et al 1988). The additional high-frequency energy causing the spectrum to drop off with a slope less than co-2 past the comer frequency can arise due to fault roughness (Gusev 1983).…”