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Joe B. Fletcher

Publications and source records attributed to Joe B. Fletcher.

27 records · Page 2Linked to original sources
USGS1999

Stress drop for three M∼4.3-4.7 (1992-1994) Parkfield, CA, earthquakes

In a previous paper (Fletcher and Spudich, 1998) we determined the slip distribution for the three M∼4.3 to 4.7 Parkfield events that occurred between Oct. 20, 1992 and Dec. 20, 1994. The rupture characteristics of these events are of particular interest because they are the largest events to occur at Parkfield since the 1966 mainshock. To further investigate the rupture process of these events, we compute the static stress change or stress drop caused by the slip using a boundary integral method from Quin and Das (1989) adapted for the forward modeling case of computing stresses from a static slip distribution. We find that the initial phase at or very near the hypocenter has the largest stress change in all cases. This peak has a value of about 3.8 MPa for the Oct. 20, 1992 event (which also has the smallest magnitude of the three at 4.3) and about 45 MPa for the other Nov. 14, 1993 and 60 MPa for the Dec. 20, 1994 event. The rest of the stress release is comparatively small and smooth for the Nov. 14, 1993 event, which is the deepest event and the most impulsive. The pattern of stress release of the other two events (Oct. 20, 1992 and Dec. 20, 1994) is more complex with a greater spatial extent. The spatial distribution of stress is approximately similar to the spatial distribution of slip, but with the larger initial peak in stress drop at the hypocenter.

Geophysical Research Letters↗
USGS1995

Source parameters and crustal Q for four earthquakes in South Carolina

Two three-component seismometers (one surface and one borehole) were re-installed on the Savannah River Site (SRS), South Carolina in July 1992 to determine attenuation in the Coastal Plain sediment wedge and source parameters of local earthquakes. Four earthquakes M ∼ 1.8 to 3.6 were recorded during the next 6 months. The largest event was located near Summerville within the meizoseismal area for the 1886 Charleston earthquake. Two shocks were located 50 km to the east near Neeses, and one was located 20 km north of the SRS near Aiken. Although source parameters have been determined from strong motion data and short-period regional networks for east coast earthquakes, such as the Saguenay, Nahanni, and Mt. Laurier earthquakes (e.g., Atkinson, 1993 ), these are some of the first source parameters determined from broad-band digital recorders. Seismograms for the Summerville event are also available from Chapel Hill, North Carolina and Blacksburg, Virginia, providing estimates of t * beyond 200 km. Here we determine source parameters such as moment, stress drop, and the attenuation parameter t * using a non-linear least-squares algorithm. We do not correct for site response because the deepest borehole is not below the Coastal Plain sediments (about 300m thick at this site) and because only one station is available for most of the data. Values of t* are marginally higher from seismograms recorded at the surface when compared to records from the 91m depth borehole seismograph. A value of 170-200 bars was determined for the Brune stress drop of the Summerville event using the borehole data, which is high compared to a value of 50 bars usually specified for modeling strong motion in western North America, but similar to other estimates for eastern North America. A higher stress drop leads to a higher seismic risk because peak acceleration is approximately proportional to stress drop. Moreover, mid- to upper-crustal Qs are in the range of 2,000 to 3,000, which would permit the propagation of high frequency seismic waves. A comparison of the surface records from the SRS with a record from the USGS dense array at Parkfield, CA for an event at about the same distance range and moment as the Summerville event-SRS case shows that the peak acceleration of the Summerville event is 16 times higher than that for the event from California (stress drop of 21 bars).

South Carolina↗
USGS1990

Near-surface velocities and attenuation at two boreholes near Anza, California, from logging data

To investigate near-surface site effects in granite rock, we drilled 300-m-deep boreholes at two sites which are collocated with stations from the digital array at Anza, California. The first borehole was sited at station KNW (Keenwild fire station), which is located along a ridge line about 8.7 km east of the San Jacinto Fault zone. Station PFO (Piñon Flat Observatory), chosen for the second site, is another 6 km further to the east of station KNW and is located on a gently sloping hillside. We logged each borehole for P- and S-wave velocities, as well as for crack density and orientation.

California↗
USGS1985

A general earthquake-observation system (GEOS)

Microprocessor technology has permitted the development of a General Earthquake-Observation System (GEOS) useful for most seismic applications. Central-processing-unit control via robust software of system functions that are isolated on hardware modules permits field adaptability of the system to a wide variety of active and passive seismic experiments and straightforward modification for incorporation of improvements in technology. Various laboratory tests and numerous deployments of a set of the systems in the field have confirmed design goals, including: wide linear dynamic range (16 bit/96 dB); broad bandwidth (36 hr to 600 Hz; greater than 36 hr available); selectable sensor-type (accelerometer, seismometer, dilatometer); selectable channels (1 to 6); selectable record mode (continuous, preset, trigger); large data capacity (1. 4 to 60 Mbytes); selectable time standard (WWVB, master, manual); automatic self-calibration; simple field operation; full capability to adapt system in the field to a wide variety of experiments; low power; portability; and modest costs. System design goals for a microcomputer-controlled system with modular software and hardware components as implemented on the GEOS are presented. The systems have been deployed for 15 experiments, including: studies of near-source strong motion; high-frequency microearthquakes; crustal structure; down-hole wave propagation; teleseismicity; and earth-tidal strains.

Bulletin of the Seismological Society of America↗
USGS1981

Digital seismograms of aftershocks of the Imperial Valley, California, earthquake of October 15, 1979

This report presents the digital seismograms collected for the aftershocksof the Imperial Valley, California earthquake (M L = 6.6) that occurred onL October 15, 1979. A short summary follows that describes the field procedures, instrumentation, and routine computer analysis that was used to produce the ground motion time histories. Boore and Fletcher (1981) may be referenced for selected fault plane solutions, spectra and moments. Requests for digital time histories on magnetic tape may be made to the Branch of Ground Motion and Faulting, Menlo Park, California.

California↗