Geology topics

M. Lisowski

Publications and source records attributed to M. Lisowski.

26 records · Page 2Linked to original sources

USGS1996

Crustal velocity field near the big bend of California's San Andreas fault

We use geodetic data spanning the 1920–1992 interval to estimate the horizontal velocity field near the big bend segment of California's San Andreas fault (SAF). More specifically, we estimate a horizontal velocity vector for each node of a two-dimensional grid that has a 15-min-by-15-min mesh and that extends between latitudes 34.0°N and 36.0°N and longitudes 117.5°W and 120.5°W. For this estimation process, we apply bilinear interpolation to transfer crustal deformation information from geodetic sites to the grid nodes. The data include over a half century of triangulation measurements, over two decades of repeated electronic distance measurements, a decade of repeated very long baseline interferometry measurements, and several years of Global Positioning System measurements. Magnitudes for our estimated velocity vectors have formal standard errors ranging from 0.7 to 6.8 mm/yr. Our derived velocity field shows that (1) relative motion associated with the SAF exceeds 30 mm/yr and is distributed on the Earth's surface across a band (>100 km wide) that is roughly centered on this fault; (2) when velocities are expressed relative to a fixed North America plate, the motion within our primary study region has a mean orientation of N44°W ± 2° and the surface trace of the SAF is congruent in shape to nearby contours of constant speed yet this trace is oriented between 5° and 10° counterclockwise relative to these contours; and (3) large strain rates (shear rates > 150 nrad/yr and/or areal dilatation rates < −150 nstr/yr) exist near the Garlock fault, near the White Wolf fault, and in the Ventura basin.

Journal of Geophysical Research B: Solid Earth↗

USGS1994

Geodetic slip rate for the eastern California shear zone and the recurrence time of Mojave desert earthquakes

Where the San Andreas fault passes along the southwestern margin of the Mojave desert, it exhibits a large change in trend, and the deformation associated with the Pacific/North American plate boundary is distributed broadly over a complex shear zone. The importance of understanding the partitioning of strain across this region, especially to the east of the Mojave segment of the San Andreas in a region known as the eastern California shear zone (ECSZ), was highlighted by the occurrence (on 28 June 1992) of the magnitude 7.3 Landers earthquake in this zone. Here we use geodetic observations in the central Mojave desert to obtain new estimates for the rate and distribution of strain across a segment of the ECSZ, and to determine a coseismic strain drop of ~770 ??rad for the Landers earthquake. From these results we infer a strain energy recharge time of 3,500-5,000 yr for a Landers-type earthquake and a slip rate of ~12 mm yr-1 across the faults of the central Mojave. The latter estimate implies that a greater fraction of plate motion than heretofore inferred from geodetic data is accommodated across the ECSZ.

Nature↗

USGS1993

The Cape Mendocino, California, earthquakes of April 1992: Subduction at the triple junction

The 25 April 1992 magnitude 7.1 Cape Mendocino thrust earthquake demonstrated that the North America—Gorda plate boundary is seismogenic and illustrated hazards that could result from much larger earthquakes forecast for the Cascadia region. The shock occurred just north of the Mendocino Triple Junction and caused strong ground motion and moderate damage in the immediate area. Rupture initiated onshore at a depth of 10.5 kilometers and propagated up-dip and seaward. Slip on steep faults in the Gorda plate generated two magnitude 6.6 aftershocks on 26 April. The main shock did not produce surface rupture on land but caused coastal uplift and a tsunami. The emerging picture of seismicity and faulting at the triple junction suggests that the region is likely to continue experiencing significant seismicity.

California↗

USGS1991

Strain measurements and the potential for a great subduction earthquake off the coast of Washington

Geodetic measurements of deformation in northwestern Washington indicate that strain is accumulating at a rate close to that predicted by a model of the Cascadia subduction zone in which the plate interface underlying the continental slope and outer continental shelf is currently locked but the remainder of the interface slips continuously. Presumably this locked segment will eventually rupture in a great thrust earthquake with a down-dip extent greater than 100 kilometers.

Washington↗

USGS1986

Strain accumulation in the Shumagin and Yakataga seismic gaps, Alaska

Strain accumulation during the 1980-85 interval has been measured by means of trilateration surveys in the Shumagin and Yakataga seismic gaps, which are the two regions identified as the most likely sites for the next great thrust earthquakes along the Alaska-Aleutian arc. No significant strain accumulation was detected in the Shumagin gap, but experience at similar subduction zones and simple models of the subduction process suggest that a measurable amount of strain should have accumulated. The most likely explanation of the observation is that subduction mere is either aseismic or episodic. The strain accumulation measured in the Yakataga gap is consistent with that expected for the plate convergence rate, although the direction of maximum compression may suggest a somewhat more oblique convergence than expected.

Alaska↗

USGS1983

Strain accumulation along the San Andreas fault system east of San Francisco Bay, California

The occurrence of several large earthquakes to the east of San Francisco Bay during historical times, and present high levels of microseismicity, indicate that a significant part of the relative plate motion may be occurring east of San Francisco Bay. Furthermore, the Hayward fault is known to be slipping aseismically at the surface, and the Calaveras fault may be slipping aseismically also. These facts raise an important question: Is the observed creep rate accommodating all of the east bay deformation or is there a significant amount of strain accumulating along these faults? Several small survey networks (< 2 km diameter) located along the Hayward and Calaveras faults, have been measured occasionally since 1965. Recent observations of these and other networks have been made by the U.S. Geological Survey. These observations imply a surface slip rate on the Hayward fault at Fremont, Hayward, Berkeley, and Richmond of about 6 mm/yr. On the Calaveras fault, north of the Hayward-Calaveras fault junction, surface slip rates have been determined from only four data sets. Three of which give a rate of 3 mm/yr. The U.S. Geological Survey annually measures 32 longer lines (10–30 km) in the east bay. Observations of these lines extend back to 1977 for most and to 1970 for some of the lines. The observed creep rates and the data for the longer east-bay lines provide constraints on the amount and position of deeper slip on the Hayward and Calaveras faults. After correcting for line-length changes due to fault slip, we calculated the strain accumulation rate. The shear strain rate parallel to east bay faults is 0.07 ± 0.02 μstrain / yr , a rate well below that of other areas along the San Andreas fault system, suggesting that creep is relieving a large part of the strain in this area.

California↗

USGS1981

Strain on the San Andreas fault near Palmdale, California: Rapid, aseismic change

Frequently repeated strain measurements near Palmdale, California, during the period from 1971 through 1980 indicate that, in addition to a uniform accumulation of right-lateral shear strain (engineering shear, 0.35 microradian per year) across the San Andreas fault, a 1-microstrain contraction perpendicular to the fault that accumulated gradually during the interval 1974 through 1978 was aseismically released between February and November 1979. Subsequently (November 1979 to March 1980), about half of the contraction was recovered. This sequence of strain changes can be explained in terms of south-southwestward migration of a slip event consisting of the south-southwestward movement of the upper crust on a horizontal detachment surface at a depth of 10 to 30 kilometers. The large strain change in 1979 corresponds to the passage of the slip event beneath the San Andreas fault.

California↗

USGS1978

Strain in southern California: Measured uniaxial north-south regional contraction

The plate tectonics model of the Pacific moving northwest relative to North America implies that the regional strain in California should be simple shear across a vertical plane striking N45°W or equivalently equal parts of north-south contraction and east-west extension. Measurements of the strain accumulation at seven separate sites in southern California in the interval 1972 through 1978 indicate a remarkably consistent uniaxial north-south contraction of about 0.3 part per million per year; the expected east-west extension is absent. It is not clear whether the period from 1972 through 1978 is anomalous or whether the secular strain in southern California is indeed a uniaxial north-south contraction.

California↗