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Bilham, R.

Publications and source records attributed to Bilham, R..

8 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Sawtooth segmentation and deformation processes on the southern San Andreas fault, California

Five contiguous 12-13 km fault segments form a sawtooth geometry on the southernmost San Andreas fault. The kinematic and morphologic properties of each segment depend on fault strike, despite differences of strike between segments of as little as 3 degrees. Oblique slip (transpression) of fault segments within the Indio Hills, Mecca Hills and Durmid Hill results from an inferred 8:1 ratio of dextral slip to convergence across the fault zone. Triggered slip and creep are confined almost entirely to transpressive segments of the fault. Durmid Hill has been formed in the last 28 + or - 6 ka by uplift at an average rate of 3 + or - 1 mm/a.

Bilham, R.↗
NASA NTRS1984

Comparing tiltmeters for crustal deformation measurement - A preliminary report

Attention is given to 1.2 years of data from two Michelson-Gale long fluid tiltmeters, one long center pressure tiltmeter, and a shallow borehole tiltmeter, which have been in operation at Pinon Flat Observatory in Southern California. The most stable tilt record shows a secular rate of 0.28 microrad/a, which may be real. Comparisons undertaken among instruments have identified more and less successful measurement techniques. It is concluded that even the best existing sensors must be improved in order to measure continuous tectonic motions.

Wyatt, F.↗
NASA NTRS1984

Coherent tilt signals observed in the Shumagin seismic gap Detection of time-dependent subduction at depth?

Repeated surveys of short level lines in the Shumagin Islands, Alaska, reveal coherent tilt signals associated with subduction of the Pacific plate beneath the North American plate in the Shumagin seismic gap. Ten years of steady tilt down toward the trench is interrupted during 1978-1980 by a rapid episode of reverse tilt. The 'normal' tilt represents surface deformation as subduction occurs, with the plate boundary locked to at least 60 km depth. Using all available tilt, sea level, and seismic data, the tilt reversal is interpreted as due to an episodic reverse slip of about 80 cm magnitude on the plate boundary between about 70 km and 20 km depth, downdip from the main seismogenic zone, which remains locked. This event causes an increase of stress on the locked main thrust zone. It is speculated that such events may be a regular process at subduction zones, that great plate boundary earthquakes may be more common during their occurrence, and that their onset may be detectable early enough to give warning of an increase in probability for the occurrence of a great earthquake.

Beavan, J.↗
NASA NTRS1981

Strain buildup and release, earthquake prediction and selection of VBL sites for margins of the north Pacific

During the past year, the grant supported research on several aspects of crustal deformation. The relation between earthquake displacements and fault dimensions was studied in an effort to find scaling laws that relate static parameters such as slip and stress drop to the dimensions of the rupture. Several implications of the static relations for the dynamic properties of earthquakes such as rupture velocity and dynamic stress drop were proposed. A theoretical basis for earthquake related phenomena associated with slow rupture growth or propagation, such as delayed multiple events, was developed using the stress intensity factor defined in fracture mechanics and experimental evidence from studies of crack growth by stress corrosion. Finally, extensive studies by Japanese geologists have established the offset across numerous faults in Japan over the last one hundred thousand years. These observations of intraplate faulting are being used to establish the spatial variations of the average strain rate of subregions in southern Japan.

Scholz, C. H.↗
NASA NTRS1979

Surface deformation and elasticity studies in the Virgin Islands

The report consists of four sections. The first section describes tilt and leveling measurements on Anegada, the most northerly of the British Virgin Islands; the second section contains a discussion of sea-level measurements that were initiated in the region and which played a significant role in the development of a network of sea-level monitors now telemetered via satellite from the Alaskan Shumagin Islands. The third part of the report is a brief description of surface deformation measurements in Iceland using equipment and techniques developed by the subject grant. The final part of the report describes the predicted effects of block surface fragmentation in tectonic areas on the measurement of tilt and strain.

Bilham, R.↗
NASA NTRS1977

A sea-level recorder for tectonic studies

In the past tide gauges have provided valuable information concerning the vertical ground deformation associated with major earthquakes. Although tide-gauge data contains numerous sources of noise, a spacing of less than 40 km between gauges is indicated for a useful study of dilatant behavior, and a spacing of less than 80 km may be adequate for the study of crustal downwarping in island arcs. An inexpensive tide gauge which is designed to provide a continuous record of sea level with a measurement precision of 1 mm is described. Hydraulic filtering is incorporated into the instrument to attenuate daily tides relative to longer period variations of sea level. The instrument is designed to operate from flashlight batteries for a year unattended and to withstand temporary submersion as might be caused by tsunamis. Several of these sea-level recorders have been installed in seismic gaps in the Aleutians and in the Caribbean.

Bilham, R.↗