Keith F. Priestley

Seismicity, secular strain, and maximum magnitude in the Excelsior Mountains area, western Nevada and eastern California

Seismicity in the Excelsior Mountains area appears to have been an order of magnitude higher for at least several decades than that which preceded great earthquakes in central Nevada in 1915 and 1954. A high degree of crustal fracturing is indicated for this area by complex geology and by a scattered distribution of epicenters. A composite fault-plane solution is similar to those for large shocks at Fairview Peak and Rainbow Mountain in 1954, which shows that the same regional stress field is acting to produce earthquakes in both areas. The slope of the recurrence curve, or b value, is higher than average for the Nevada region. Crustal strains recorded at Mina indicate that periods of strain build-up alternate with periods of strain release. Comparison of these characteristics with results of laboratory experiments and observations in other regions suggests that the area is one in which a moderate level of tectonic stress combined with a high degree of crustal fracturing leads to strain release by a continuing series of small-to-moderate earthquakes and fault creep. If so, the magnitude of 6¼ for the 1934 Excelsior Mountains earthquake may represent a maximum magnitude for this area.

The seismic spectrum, radiated energy, and the Savage and Wood inequality for complex earthquakes

Abstract We have integrated velocity squared spectra in order to determine the seismic energy radiated during fault rupture. The high frequency spectral fall-off and the shape of the spectrum at the corner frequency are critical to the energy calculation. High frequency spectral fall-offs of ω−2 beyond the corner frequency, in a Brune (1970) source model, return radiated energies approximately equal to that of an Orowan (1960) type fault failure, where the final stress level is equal to the dynamic frictional stress. Any spectra with an extended intermediate slope of ω−1 would therefore result in higher radiated energies. Savage and Wood (1971) proposed a model in which the final stress level was less than the dynamic stress level and that this was the result of “overshoot”. They based their model on the observation that the ratio of twice the apparent stress to the stress drop was typically around 0.3. We show that for such a ratio to exist high frequency spectral fall-offs of ≈ ω−3 would be required. Composite spectra have been constructed for several moderate to large earthquakes, these spectra have been compared to that predicted by the Haskell (1966) model and velocity squared spectra have been integrated to determine the radiated energy. In all cases this ratio, twice the apparent stress to the stress drop, is greater than or equal to one, violating the Savage and Wood (1971) inequality, and provides evidence against “overshoot” as a source model.

Source parameters of the 1987 Edgecumbe earthquake, New Zealand

Abstract Moment tensor inversions of low-frequency teleseismic surface-wave and body-wave data for the 1987 March 2 Edgecumbe earthquake, New Zealand, give an average seismic moment of 7. 0 × 1018 Nm. Measurements of seismic moment at low frequencies are converted to spectral levels normalised to 10 km distance and combined with the acceleration spectrum from a strong-motion seismogram recorded at about 15 km epicentral distance, to give acomposite source spectrum for the main shock covering more than four decades in frequency. The source spectrum shows a constant low-frequency level, a comer frequency of 0. 22 Hz, and a high-frequency slope of approximately ω-2. The comer frequency implies a fault radius of 6 km giving an average fault displacement of 2 m and a stress drop of 5 MPa. Integration of the velocity-squared spectrum indicates an energy release of 1. 1 × 1015 J.

Crustal structure of Fiordland, southwestern New Zealand, from seismic-refraction measurements

Travel-time data from a seismic refraction profile within western Fiordland, southwest New Zealand, gives no indication of a 6.0 km/s upper-crustal layer typical of most continental crustal sections. The data substantiate the existence of high-velocity material typical of the lower crust, at shallow depths beneath western Fiordland. The velocity gradient below 2.5 km depth is suggestive of that expected from increasing pressure with depth acting on a uniform section of granulite facies rock similar to those cropping out at the surface. The seismic data support the conclusions based on surface geologic observations, that western Fiordland represents a section of uplifted lower crust.

Anomalous crustal structure beneath the Bay of Bengal and passive oceanic sedimentary basins

Recently Brune and Singh (1986) reported evidence for anomalous continent-like crustal structure beneath the Bay of Bengal, possibly caused by perturbation in the temperature-pressure regime and consequent phase chage, partial melting, or mass transport (e.g. convection or underplating). Recent refraction results indicate the existence of an anomalous lower crustal or subcrustal layer ofP-wave velocity about 7·3 km per second along the eastern North America passive margin, possibly a result of underplating of the oceanic crust just after initial rifting. We have searched for other evidence of anomalous crustal structure. The data suggest some mechanism may cause a general increase in the anomalous thickness of the crust with increasing thickness of the accumulated sediments, up to a thickness of about 6–7 km. On the other hand, anamolous crustal structure may in fact be transitional between oceanic and continental, or may have been modified by aseismic ridges, thus requiring no sediment related structure modification mechanism. The explanation for all the data may require more than one mechanism, all probably involving severe temperature perturbations. The general tendency is for perturbations of normal oceanic crust to make it more continent-like, suggesting that normal oceanic crust is an unstable end-member in crustal states.