The Oroville Earthquakes: A study of source characteristics and site effects

Abstract

Half‐space modeling, in the time domain, of broadband seismic records from the Oroville aftershock sequence demonstrates that these events have simple sources. Systematic scaling between the far‐field source duration and seismic moment for the aftershocks studied here and other California earthquakes confirms Brune's (1970) source model and shows that stress drops for California earthquakes range between 10 and 100 bars. These events also show no trend in stress drop as a function of size for 3.0 ≤ ML ≤ 6.4. Analysis of static deformation associated with the main shock shows that slip occurred on a fault 81 km2 in area corresponding to the zone defined by the first 3 days of aftershocks. Teleseismic studies indicate that the seismic source area was 50 km2, corresponding to a zone completely free of aftershocks in the middle of the aftershock zone. Both results imply a stress drop of 29–39 bars and seismic moment between 5.7 and 8.7×1024 dyn cm. The evidence suggests that faulting occurred seismically on the central portion of the fault and propagated out slowly, enlarging the fault plane. Complicated waveforms recorded for the same events at nearby sites on alluvium indicate the critical role that propagation through complex structure plays in determining the character of the observed seismic signal at close‐in stations. Modeling of these records shows that dipping structure may account for the observed waveform complexities.

Estimation of Source Parameters for the Aftershocks of the 2001 Mw 7.7 Bhuj Earthquake, India

“…Two standard approaches have been used to estimate the source parameters of an earthquake-like seismic moment, corner frequency, source-dimension and stress drop (AKI, 1965;BRUNE, 1970;HANKS and KANAMORI, 1979;FLETCHER, 1980;LANGSTON and HELMBERGER, 1975;COHN et al, 1982). In one approach (frequency domain technique), the spectra of P, S or surface waves are used to estimate seismic static moment (Mo) and corner frequency (fc) (AKI, 1965;BRUNE, 1970;HANKS and KANAMORI, 1979;FLETCHER, 1980).…”

“…In one approach (frequency domain technique), the spectra of P, S or surface waves are used to estimate seismic static moment (Mo) and corner frequency (fc) (AKI, 1965;BRUNE, 1970;HANKS and KANAMORI, 1979;FLETCHER, 1980). In another approach (time-domain wave form matching), the Mo and the equivalent to fc are estimated from synthetic waveform amplitudes and the source time function, respectively (LANGSTON and HELMBERGER, 1975;COHN et al, 1982). Another important source parameter is the stress drop that is generally derived from Mo and fc, obtained in frequency or time domain techniques.…”

Seismicity and stress‐drop in the Eastern Transverse Ranges, southern California

“…On average, however, it is acceptable to assume a linear relationship between pulse-width and source dimension. Cohn et al [1982], assuming a circular fault [Brune, 1970], obtained the relation…”

Preliminary Report on the 29 July 2008 Mw 5.4 Chino Hills, Eastern Los Angeles Basin, California, Earthquake Sequence

“…This trend is almost identical to the relationship used by the Harvard and the global CMT groups and is given by τ = × gives Δσ ∝ τ -3 , the data plotted in Figure 5 would suggest that ∆σ of the Chino Hills earthquake is 2.4 3 (~14) times larger than the global average of large earthquakes. The empirical relation given by Cohn et al (1982), Δσ = M 0 τ -3 /10 15.73 (∆σ in MP, M 0 in N-m), leads to ∆σ ~ 30 MP for τ = 1 s. However, since ∆σ depends on other factors such as the rupture speed and geometry, its exact value is subject to some uncertainty, but the very short pulse width of the Chino Hills earthquake suggests that it is most likely a high-stress drop event. Bent and Helmberger (1989) estimated the stress drop of the 1987 Whittier Narrows earthquake (Mw = 5.9) to be about 75 MP.…”