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W.L. Ellsworth

Publications and source records attributed to W.L. Ellsworth.

34 records · Page 2Linked to original sources
USGS1998

Observation of the seismic nucleation phase in the Ridgecrest, California, earthquake sequence

Near-source observations of five M 3.8–5.2 earthquakes near Ridgecrest, California are consistent with the presence of a seismic nucleation phase. These earthquakes start abruptly, but then slow or stop before rapidly growing again toward their maximum rate of moment release. Deconvolution of instrument and path effects by empirical Green's functions demonstrates that the initial complexity at the start of the earthquake is a source effect. The rapid growth of the P -wave arrival at the start of the seismic nucleation phase supports the conclusion of Mori and Kanamori [1996] that these earthquakes begin without a magnitude-scaled slow initial phase of the type observed by Iio [1992, 1995].

Geophysical Research Letters↗
USGS1997

Fault-zone guided waves from explosions in the San Andreas fault at Parkfield and Cienega Valley, California

Fault-zone guided waves were successfully excited by near-surface explosions in the San Andreas fault zone both at Parkfield and Cienega Valley, central California. The guided waves were observed on linear, three-component seismic arrays deployed across the fault trace. These waves were not excited by explosions located outside the fault zone. The amplitude spectra of guided waves show a maximum peak at 2 Hz at Parkfield and 3 Hz at Cienega Valley. The guided wave amplitude decays sharply with observation distance from the fault trace. The explosion-excited fault-zone guided waves are similar to those generated by earthquakes at Parkfield but have lower frequencies and travel more slowly. These observations suggest that the fault-zone wave guide has lower seismic velocities as it approaches the surface at Parkfield. We have modeled the waveforms as S waves trapped in a low-velocity wave guide sandwiched between high-velocity wall rocks, resulting in Love-type fault-zone guided waves. While the results are nonunique, the Parkfield data are adequately fit by a shallow wave guide 170 m wide with an S velocity 0.85 km/sec and an apparent Q ∼ 30 to 40. At Cienega Valley, the fault-zone wave guide appears to be about 120 m wide with an S velocity 0.7 km/sec and a Q ∼ 30.

California↗
USGS1996

Detailed observations of California foreshock sequences: Implications for the earthquake initiation process

We find that foreshocks provide clear evidence for an extended nucleation process before some earthquakes. In this study, we examine in detail the evolution of six California foreshock sequences, the 1986 Mount Lewis ( M L = 5.5), the 1986 Chalfant ( M L = 6.4), the 1986 Stone Canyon ( M L = 4.7), the 1990 Upland ( M L = 5.2), the 1992 Joshua Tree ( M W = 6.1), and the 1992 Landers ( M W = 7.3) sequence. Typically, uncertainties in hypocentral parameters are too large to establish the geometry of foreshock sequences and hence to understand their evolution. However, the similarity of location and focal mechanisms for the events in these sequences leads to similar foreshock waveforms that we cross correlate to obtain extremely accurate relative locations. We use these results to identify small-scale fault zone structures that could influence nucleation and to determine the stress evolution leading up to the mainshock. In general, these foreshock sequences are not compatible with a cascading failure nucleation model in which the foreshocks all occur on a single fault plane and trigger the mainshock by static stress transfer. Instead, the foreshocks seem to concentrate near structural discontinuities in the fault and may themselves be a product of an aseismic nucleation process. Fault zone heterogeneity may also be important in controlling the number of foreshocks, i.e., the stronger the heterogeneity, the greater the number of foreshocks. The size of the nucleation region, as measured by the extent of the foreshock sequence, appears to scale with mainshock moment in the same manner as determined independently by measurements of the seismic nucleation phase. We also find evidence for slip localization as predicted by some models of earthquake nucleation.

Journal of Geophysical Research B: Solid Earth↗
USGS1996

Properties of the seismic nucleation phase

Near-source observations show that earthquakes begin abruptly at the P-wave arrival, but that this beginning is weak, with a low moment rate relative to the rest of the main shock. We term this initial phase of low moment rate the seismic nucleation phase. We have observed the seismic nucleation phase for a set of 48 earthquakes ranging in magnitude from 1.1–8.1 The size and duration of the seismic nucleation phase scale with the total seismic moment of the earthquake, suggesting that the process responsible for the seismic nucleation phase carries information about the eventual size of the earthquake. The seismic nucleation phase is characteristically followed by quadratic growth in the moment rate, consistent with self-similar rupture at constant stress drop. In this paper we quantify the properties of the seismic nucleation phase and offer several possible explanations for it.

California↗
USGS1995

Seismic evidence for an earthquake nucleation phase

Near-source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event. This phase was observed for the 30 earthquakes having moment magnitudes 2.6 to 8.1, and the size and duration of this phase scale with the eventual size of the earthquake. During the nucleation phase, moment release was irregular and appears to have been confined to a limited region of the fault. It was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone. These observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake.

Science↗
USGS1994

Variations in rupture process with recurrence interval in a repeated small earthquake

In theory and in laboratory experiments, friction on sliding surfaces such as rock, glass and metal increases with time since the previous episode of slip. This time dependence is a central pillar of the friction laws widely used to model earthquake phenomena. On natural faults, other properties, such as rupture velocity, porosity and fluid pressure, may also vary with the recurrence interval. Eighteen repetitions of the same small earthquake, separated by intervals ranging from a few days to several years, allow us to test these laboratory predictions in situ. The events with the longest time since the previous earthquake tend to have about 15% larger seismic moment than those with the shortest intervals, although this trend is weak. In addition, the rupture durations of the events with the longest recurrence intervals are more than a factor of two shorter than for the events with the shortest intervals. Both decreased duration and increased friction are consistent with progressive fault healing during the time of stationary contact.In theory and in laboratory experiments, friction on sliding surfaces such as rock, glass and metal increases with time since the previous episode of slip. This time dependence is a central pillar of the friction laws widely used to model earthquake phenomena. On natural faults, other properties, such as rupture velocity, porosity and fluid pressure, may also vary with the recurrence interval. Eighteen repetitions of the same small earthquake, separated by intervals ranging from a few days to several years, allow us to test these laboratory predictions in situ. The events with the longest time since the previous earthquake tend to have about 15% larger seismic moment than those with the shortest intervals, although this trend is weak. In addition, the rupture durations of the events with the longest recurrence intervals are more than a factor of two shorter than for the events with the shortest intervals. Both decreased duration and increased friction are consistent with progressive fault healing during the time of stationary contact.

Nature↗
USGS1993

Seismicity remotely triggered by the magnitude 7.3 Landers, California, earthquake

The magnitude 7.3 Landers earthquake of 28 June 1992 triggered a remarkably sudden and widespread increase in earthquake activity across much of the western United States. The triggered earthquakes, which occurred at distances up to 1250 kilometers (17 source dimensions) from the Landers mainshock, were confined to areas of persistent seismicity and strike-slip to normal faulting. Many of the triggered areas also are sites of geothermal and recent volcanic activity. Static stress changes calculated for elastic models of the earthquake appear to be too small to have caused the triggering. The most promising explanations involve nonlinear interactions between large dynamic strains accompanying seismic waves from the mainshock and crustal fluids (perhaps including crustal magma).

California↗
USGS1984

The 1984 Morgan Hill, California, earthquake

The Morgan Hill, California, earthquake (magnitude 6.1) of 24 April 1984 ruptured a 30-kilometer-long segment of the Calaveras fault zone to the east of San Jose. Although it was recognized in 1980 that an earthquake of magnitude 6 occurred on this segment in 1911 and that a repeat of this event might reasonably be expected, no short-term precursors were noted and so the time of the 1984 earthquake was not predicted. Unilateral rupture propagation toward the south-southeast and an energetic late source of seismic radiation located near the southeast end of the rupture zone contributed to the highly focused pattern of strong motion, including an exceptionally large horizontal acceleration of 1.29g at a site on a dam abutment near the southeast end of the rupture zone.

California↗
USGS1983

Triggered reverse fault and earthquake due to crustal unloading, northwest Transverse Ranges, California

A reverse-right-oblique surface rupture, associated with a M L 2.5 earthquake, formed in a diatomite quarry near Lompoc, California, in the northwesternmost Transverse Ranges on April 7, 1981. The 575-m-long narrow zone of ruptures formed in clay interbeds in diatomite and diatomaceous shale of the Neogene Monterey Formation. The ruptures parallel bedding, dip 39°–59°S, and trend about N84°E on the north limb of an open symmetrical syncline. Maximum net slip was 25 cm; maximum reverse dip slip was 23 cm, maximum right-lateral strike slip was about 9 cm, and average net slip was about 12 cm. The seismic moment of the earthquake is estimated at 1 to 2 × 10 18 dyne/cm and the static stress drop at about 3 bar. The removal of an average of about 44 m of diatomite resulted in an average load reduction of about 5 bar, which decreased the normal stress by about 3.5 bar and increased the shear stress on the tilted bedding plane by about 2 bar. The April 7,1981, event was a very shallow bedding-plane rupture, apparently triggered by crustal unloading.

Geology↗
USGS1977

Search for seismic forerunners to earthquakes in central California

The relatively high seismicity of the San Andreas fault zone in central California provides an excellent opportunity to search for seismic forerunners to moderate earthquakes. Analysis of seismic traveltime and earthquake location data has resulted in the identification of two possible seismic forerunners. The first is a period of apparently late (0.3 sec) P-wave arrival times lasting several weeks preceding one earthquake of magnitude 5.0. The rays for these travel paths passed through — or very close to — the aftershock volume of the subsequent earthquake. The sources for these P-arrival time data were earthquakes in the distance range 20–70 km. Uncertainties in the influence of small changes in the hypocenters of the source earthquakes and in the identification of small P-arrivals raise the possibility that the apparantly delayed arrivals are not the result of a decrease in P-velocity. The second possible precursor is an apparent increase in the average depth of earthquakes preceding two moderate earthquakes. This change might be only apparent, caused by a location bias introduced by a decrease in P-wave velocity, but numerical modeling for realistic possible changes in velocity suggests that the observed effect is more likely a true migration of earthquakes. To carry out this work — involving the manipulation of several thousand earthquake hypocenters and several hundred thousand readings of arrival time — a system of data storage was designed and manipulation programs for a large digital computer have been executed. This system allows, for example, the automatic selection of earthquakes from a specific region, the extraction of all the observed arrival times for these events, and their relocation under a chosen set of assumptions.

California↗
USGS1974

Variation of P-wave velocity before the Bear Valley, California, earthquake of 24 February 1972

Residuals for P-wave traveltimes at a seismograph station near Bear Valley, California, for small, precisely located local earthquakes at distances of 20 to 70 kilometers show a sharp increase of nearly 0.3 second about 2 months before a magnitude 5.0 earthquake that occurred within a few kilometers of the station. This indicates that velocity changes observed elsewhere premonitory to earthquakes, possibly related to dilatancy, occur along the central section of the San Andreas fault system.

California↗
USGS1973

Point Mugu, California, earthquake of 21 February 1973 and its aftershocks

Seismological investigations show that the Point Mugu earthquake involved north-south crustal shortening deep within the complex fault zone that marks the southern front of the Transverse Ranges province. This earthquake sequence results from the same stress system responsible for the deformation in this province in the Pliocene through Holocene and draws attention to the significant earthquake hazard that the southern frontal fault system poses to the Los Angeles metropolitan area.

California↗