Geology topics

Jerry L. Svarc

Publications and source records attributed to Jerry L. Svarc.

31 records · Page 2Linked to original sources

USGS2012

A possible geodetic anomaly observed prior to the Loma Prieta, California, Earthquake

Monthly measurements since mid‐1981 of distance from a geodetic station located 11 km from the epicenter of the Loma Prieta earthquake ( M s = 7.1; October 17, 1989) to three stations 30 to 40 km distant provides an unusually complete record of deformation in the epicentral region in the years prior to an earthquake. Roughly 1.3 years before the earthquake, at about the time of the first magnitude‐5 foreshock, the rate of change in line length for two of the lines appears to change; the rate for the third line does not change. Other similar, though smaller, changes in rate are apparent in the eight‐year record. Thus, there is marginal evidence for a change in deformation rate about one year before the Loma Prieta earthquake, but that change need not be a precursor.

California↗

USGS2012

Postseismic deformation following the 1989 (M = 7.1): Loma Prieta, California, earthquake

Postseismic deformation along a 90‐km profile bisecting the projected surface trace of the coseismic rupture of the 1989 Loma Prieta earthquake has been monitored by frequent GPS surveys for 3.3 years following the earthquake. In addition to the expected deformation associated with secular strain accumulation on the San Andreas and Calaveras faults, deformation associated with postseismic readjustment has been detected. Most of that deformation can be attributed to 1.5 m right‐lateral and 0.9 m reverse postseismic slip on a 5‐km‐wide downdip extension of the Loma Prieta rupture. In addition, there seems to be a 0.1 m postseismic collapse of the Loma Prieta rupture zone in the direction perpendicular to the plane of the rupture. The fault‐normal (N48°E) surface displacements plotted as a function of time exhibit a curvature suggesting a relaxation time of about 1.4 years. Similar plots of the fault‐parallel (N42°W) displacement components do not exhibit significant curvature. Presumably, the deformation shown in those plots is dominated by secular strain accumulation along the San Andreas and Calaveras faults rather than postseismic relaxation.

California↗

USGS2011

Global Positioning System constraints on crustal deformation before and during the 21 February 2008 Wells, Nevada M6.0 earthquake

Using Global Positioning System (GPS) data from permanent sites and U.S. Geological Survey (USGS) campaign data we have estimated co-seismic displacements and secular background crustal deformation patterns associated with the 21 February 2008 Wells Nevada earthquake. Estimated displacements at nearby permanent GPS sites ELKO (84 km distant) and GOSH (81 km distant) are 1.0±0.2 mm and 1.1±0.3 mm, respectively. The magnitude and direction are in agreement with those predicted from a rupture model based on InSAR measurements of the near-field co-seismic surface displacement. Analysis of long GPS time series (>10 years) from the permanent sites within 250 km of the epicenter indicate the eastern Nevada Basin and Range undergoes steady tectonic transtension with rates on the order of 1 mm/year over approximately 250 km. The azimuth of maximum horizontal crustal extension is consistent with the azimuth of the Wells earthquake co-seismic slip vector. The orientation of crustal shear is consistent with deformation associated with Pacific/North America plate boundary relative motion seen elsewhere in the Basin and Range. In response to the event, we deployed a new GPS site with the capability to telemeter high rate, low latency data that will in the future allow for rapid estimation of surface displacement should aftershocks or postseismic deformations occur. We estimated co-seismic displacements using campaign GPS data collected before and after the event, however in most cases their uncertainties were larger than the offsets. Better precision in co-seismic displacement could have been achieved for the campaign sites if they had been surveyed more times or over a longer interval to better estimate their pre-event velocity.

Nevada↗

USGS2010

Postseismic relaxation following the 1989 MS7.1 Loma Prieta earthquake, central California

The postseismic relaxation (postseismic displacement less displacement that would have occurred at the preseismic rate) measured by GPS and leveling following the 1989 M S 7.1 Loma Prieta earthquake is reexamined. The temporal dependence of the relaxation over the first 1200 days postseismic is well described by 1 − e − t / τ , where τ = 414 ± 92 days. (Quoted uncertainties are standard deviations.) That temporal dependence appears to be a linear function of the cumulative number of M > 2.5 aftershocks that have occurred. The relaxation is attributed to afterslip (1.56 ± 0.20 m dextral strike slip and 0.60 ± 0.04 m reverse slip) on the downdip extension (depth, 16–21 km) of the coseismic rupture plus a collapse (0.11 ± 0.02 m fault-normal displacement) of the rupture zone (depth, 5–16 km). Because the postseismic uplift was determined by leveling over a route with relief in excess of 1000 m, an allowance (18 ± 2 ppm of height above the base elevation) for excess (beyond corrections already applied) height-dependent error in the measured uplift was estimated simultaneously with the afterslip and collapse parameters. This new solution for afterslip and collapse on the plane of the rupture provides an alternative explanation to the suggestion by Bürgmann et al. (1997) that the postseismic deformation was due to afterslip on the coseismic rupture and on a nearby, shallow, 30° dipping thrust.

California↗

USGS2001

Strain accumulation near Yucca Mountain, Nevada, 1993-1998

A 50‐km aperture geodetic network centered on the proposed high‐level radioactive waste disposal site at Yucca Mountain, Nevada, was surveyed with GPS in 1993 and 1998. The average deformation rate across the area is described by the principal strain rates 22.8±8.8 nstrain yr N77.6°W±13.5° and −8.8±11.9 nstrain yr N12.5°E±13.5° (extension reckoned positive) and a clockwise rotation rate about a vertical axis of 9.6±7.4 nrad yr relative to fixed North America. Quoted uncertainties are standard deviations. Those strain rates are consistent with the geodetic strain rates (2±12 nstrain yr N87°±12°W and −22±12 nstrain yr N03°±12°E) previously reported by . [1999] for the 1983–1998 interval and with the low extension rate (5–20 nstrain yr) [., 1998] inferred from the geologic record. None of those strain rates is consistent with the 50±9 nstrain yr N65°W extension rate for the area reported by Wernicke et al.

Nevada↗

USGS2000

Strain accumulation across the Eastern California Shear Zone at latitude 36°30'N

The motion of a linear array of monuments extending across the Eastern California Shear Zone (ECSZ) has been measured from 1994 to 1999 with the Global Positioning System. The linear array is oriented N54°E, perpendicular to the tangent to the local small circle drawn about the Pacific-North America pole of rotation, and the observed motion across the ECSZ is approximated by differential rotation about that pole. The observations suggest uniform deformation within the ECSZ (strike N23°W) (26 nstrain yr −1 extension normal to the zone and 39 nstrain yr −1 simple right-lateral shear across it) with no significant deformation in the two blocks (the Sierra Nevada mountains and southern Nevada) on either side. The deformation may be imposed by right-lateral slip at depth on the individual major fault systems within the zone if the slip rates are: Death Valley-Furnace Creek fault 3.2±0.9 mm yr −1 , Hunter Mountain-Panamint Valley fault 3.3±1.6 mm yr −1 , and Owens Valley fault 6.9±1.6 mm yr −1 . However, this estimate of the slip rate on the Owens Valley fault is 3 times greater than the geologic estimate.

Journal of Geophysical Research B: Solid Earth↗

USGS1998

Deformation across the rupture zone of the 1964 Alaska earthquake, 1993–1997

A linear array of 15 geodetic monuments was installed in 1993 across the rupture zone of the 1964 Alaska earthquake ( M w = 9.2). The array extends from Middleton Island (at the edge of the continental shelf and 80 km from the Alaska‐Aleutian trench) to north of Palmer, Alaska (380 km from the trench), in the approximate direction of Pacific‐North American plate convergence (N15.5°W). The array was surveyed in June 1993, May 1995, and June 1997. The changes between surveys are a measure of the deformation of the continental margin across the subduction zone in southern Alaska. Measured relative to the interior of the North American plate, the horizontal velocities on the outer plate margin are parallel to the direction of plate convergence (N15.5°W ) and reach a maximum (58 mm yr −1 ) about 150 km from the trench. Beyond about 300 km from the trench the observed horizontal velocities are small. A narrow (halfwidth 50 km) zone of significant uplift (10 mm yr −1 maximum) is observed about 300 km from the trench, coinciding roughly with the locus of maximum coseismic subsidence associated with the 1964 Alaska earthquake. Although the deformation is roughly described by the conventional model of deformation at a subduction zone (deformation due to virtual back slip on the main thrust zone at the 55 mm yr −1 plate convergence rate), a better fit is given with a 65 mm yr −1 virtual back (normal) slip rate. This higher rate is attributed to continued postseismic relaxation. The model does not explain the relatively high uplift rate and low N15.5°W velocity observed at Middleton Island. That anomalous motion is attributed to continued thrusting on postulated upward trending splays from the subduction zone beneath the island.

Alaska↗

USGS1998

Deformation following the 1994 Northridge Earthquake (M=6.7), Southern California

Following the 1994 M w =6.7 Northridge earthquake, a 65‐km‐long, north‐south array of 11 geodetic monuments was established across the rupture. The array was surveyed with GPS ten times in the 4.25 yr after the earthquake. Although there is evidence for modest nonlinear postseismic relaxation in the first few weeks after the Northridge earthquake, the deformation in the subsequent four years can be adequately described by constant station velocities. The observed S70°E velocity components are consistent with the deformation expected from steady strain accumulation on the San Andreas fault. The N20°E velocity components indicate that the southern Northridge fault block is moving almost as a unit N20°E with repect to the northern fault block, the motion being accommodated by a zone of convergence (width 20 km) at the north end of the Northridge rupture.

California↗

USGS1997

Post seismic deformation associated with the 1992 Mω = 7.3 Landers earthquake, southern California

Following the 1992 M ω =7.3 Landers earthquake, a linear array of 10 geodetic monuments at roughly 5‐km spacing was established across the Emerson fault segment of the Landers rupture. The array trends perpendicular to the local strike of the fault segment and extends about 30 km on either side of it. The array was surveyed by Global Positioning System 0.034, 0.048, 0.381, 1.27, 1.88, 2.60, and 3.42 years after the Landers earthquake to measure both the spatial and temporal character of the postearthquake relaxation. The temporal behavior is described roughly by a short‐term (decay time 84±23 days) exponential relaxation superimposed upon an apparently linear trend. Because the linear trend represents motions much more rapid than the observed preseismic motions, we attribute that trend to a slower (decay time greater than 5 years) postseismic relaxation, the curvature of which cannot be resolved in the short run (3.4 years) of postseismic data. About 100 mm of right‐lateral displacement and 50 mm of fault‐normal displacement accumulated across the geodetic array in the 3.4‐year interval covered by the postseismic surveys. Those displacements are attributed to postseismic, right‐lateral slip in the depth interval 10 to 30 km on the downward extension of the rupture trace. The right‐lateral slip amounted to about 1 m directly beneath the geodetic array, and the fault‐normal displacement is apparently primarily a consequence of the curvature of the rupture. These conclusions are based upon dislocation models fit to the observed deformation. However, no dislocation model was found with rms residuals as small as the expected observational error.

California↗

USGS1997

The Loma Prieta, California, earthquake of October 17, 1989: Aftershocks and postseismic effects

While the damaging effects of the earthquake represent a significant social setback and economic loss, the geophysical effects have produced a wealth of data that have provided important insights into the structure and mechanics of the San Andreas Fault system. Generally, the period after a large earthquake is vitally important to monitor. During this part of the seismic cycle, the primary fault and the surrounding faults, rock bodies, and crustal fluids rapidly readjust in response to the earthquake's sudden movement. Geophysical measurements made at this time can provide unique information about fundamental properties of the fault zone, including its state of stress and the geometry and frictional/rheological properties of the faults within it. Because postseismic readjustments are rapid compared with corresponding changes occurring in the preseismic period, the amount and rate of information that is available during the postseismic period is relatively high. From a geophysical viewpoint, the occurrence of the Loma Prieta earthquake in a section of the San Andreas fault zone that is surrounded by multiple and extensive geophysical monitoring networks has produced nothing less than a scientific bonanza. The reports assembled in this chapter collectively examine available geophysical observations made before and after the earthquake and model the earthquake's principal postseismic effects. The chapter covers four broad categories of postseismic effect: (1) aftershocks; (2) postseismic fault movements; (3) postseismic surface deformation; and (4) changes in electrical conductivity and crustal fluids.

California↗

USGS1996

The Loma Prieta, California, Earthquake of October 17, 1989: Main shock characteristics

The October 17, 1989, Loma Prieta, Calif., earthquake (0004:15.2 G.m.t. October 18; lat 37.036º N., long 121.883º W.; 19-km depth) had a local magnitude (M L ) of about 6.7, a surface-wave magnitude (M S ) of 7.1, a seismic moment of 2.2x10 19 N-m to 3.5x10 19 N-m, a source duration of 6 to 15 s, and an average stress drop of at least 50 bars. Slip occurred on a dipping fault surface about 35 km long and was largely confined to a depth of about 7 to 20 km. The slip vector had a large vertical component, and slip was distributed in two main regions situated northwest and southeast of the hypocenter. This slip distribution caused about half of the earthquake's energy to be focused toward the urbanized San Francisco Bay region, while the other half was focused toward the southeast. Had the rupture initiated at the southeast end of the aftershock zone, shaking in the bay region would have been both longer and stronger. These source parameters suggest that the earthquake was not a typical shallow San Andreas-type event but a deeper event on a different fault with a recurrence interval of many hundreds of years. Therefore, the potential for a damaging shallow event on the San Andreas fault in the Santa Cruz Mountains may still exist.

California↗

USGS1993

The Loma Prieta, California, earthquake of October 17, 1989: Preseismic observations

The October 17, 1989, Loma Prieta, Calif., Ms=7.1 earthquake provided the first opportunity in the history of fault monitoring in the United States to gather multidisciplinary preearthquake data in the near field of an M=7 earthquake. The data obtained include observations on seismicity, continuous strain, long-term ground displacement, magnetic field, and hydrology. The papers in this chapter describe these data, their implications for fault-failure mechanisms, the scale of prerupture nucleation, and earthquake prediction in general. Of the 10 papers presented here, about half identify preearthquake anomalies in the data, but some of these results are equivocal. Seismicity in the Loma Prieta region during the 20 years leading up to the earthquake was unremarkable. In retrospect, however, it is apparent that the principal southwest-dipping segment of the subsequent Loma Prieta rupture was virtually aseismic during this period. Two M=5 earthquakes did occur near Lake Elsman near the junction of the Sargent and San Andreas faults within 2.5 and 15 months of, and 10 km to the north of, the Loma Prieta epicenter. Although these earthquakes were not on the subsequent rupture plane of the Loma Prieta earthquake and other M=5 earthquakes occurred in the preceding 25 years, it is now generally accepted that these events were, in some way, foreshocks to the main event.

California↗

USGS1989

Global positioning system measurements for crustal deformation: Precision and accuracy

Analysis of 27 repeated observations of Global Positioning System (GPS) position-difference vectors, up to 11 kilometers in length, indicates that the standard deviation of the measurements is 4 millimeters for the north component, 6 millimeters for the east component, and 10 to 20 millimeters for the vertical component. The uncertainty grows slowly with increasing vector length. At 225 kilometers, the standard deviation of the measurement is 6, 11, and 40 millimeters for the north, east, and up components, respectively. Measurements with GPS and Geodolite, an electromagnetic distance-measuring system, over distances of 10 to 40 kilometers agree within 0.2 part per million. Measurements with GPS and very long baseline interferometry of the 225-kilometer vector agree within 0.05 part per million.

Science↗