A note on relocating the 1963 Watsonville earthquakes

1982 ◽  
Vol 72 (4) ◽  
pp. 1309-1316
Author(s):  
David G. Evans ◽  
Thomas V. McEvilly
Keyword(s):  
San Andreas Fault ◽  
Velocity Model ◽  
Earthquake Sequence ◽  
Dense Network ◽  
Network Coverage ◽  
Site Specific ◽  
San Andreas ◽  
New Locations

abstract Events in the Watsonville earthquake sequence of August and September 1963 were found by Udias (1965) not to concentrate on the main San Andreas fault break. New locations are found using a more site-specific velocity model with carefully determined station adjustments. Resulting relocated hypocenters indicate that this sequence probably does not exhibit the anomalous spatial scatter found previously. The revised locations are found to be consistent with patterns of recent seismicity, based on dense network coverage, which show a tight coincidence with the San Andreas fault.

Interpretation of seismic reflection profiling data for the structure of the San Andreas fault zone

1983 ◽  
Vol 73 (6A) ◽  
pp. 1701-1720
Author(s):  
R. Feng ◽  
T. V. McEvilly
Keyword(s):  
Fault Zone ◽  
Seismic Reflection ◽  
San Andreas Fault ◽  
Velocity Model ◽  
Seismic Reflection Profile ◽  
Lateral Velocity ◽  
Zone Structure ◽  
Ray Method ◽  
Velocity Change ◽  
San Andreas

Abstract A seismic reflection profile crossing the San Andreas fault zone in central California was conducted in 1978. Results are complicated by the extreme lateral heterogeneity and low velocities in the fault zone. Other evidence for severe lateral velocity change across the fault zone lies in hypocenter bias and nodal plane distortion for earthquakes on the fault. Conventional interpretation and processing methods for reflection data are hard-pressed in this situation. Using the inverse ray method of May and Covey (1981), with an initial model derived from a variety of data and the impedance contrasts inferred from the preserved amplitude stacked section, an iterative inversion process yields a velocity model which, while clearly nonunique, is consistent with the various lines of evidence on the fault zone structure.

scholarly journals Location of earthquake swarm events near Palmdale, California, using a linear gradient velocity model

1980 ◽  
Vol 70 (6) ◽  
pp. 2145-2158
Author(s):  
Dayna Salter Drowley ◽  
Karen C. McNally
Keyword(s):  
San Andreas Fault ◽  
Earthquake Swarm ◽  
Nonlinear Least Squares ◽  
Velocity Model ◽  
Time Data ◽  
Crustal Layer ◽  
California Institute Of Technology ◽  
San Andreas ◽  
Travel Time Data ◽  
Institute Of Technology

abstract A series of small earthquakes (0.5 ≦ ML ≦ 3.0) along a 60-km segment of the San Andreas Fault in the vicinity of Palmdale, California, has been recorded since 1976 by an array operated by the California Institute of Technology. The events were analyzed in two steps. First, travel-time data from four regionally well-recorded events (ML = 2.2, 2.8, 3.0, 2.8) were inverted using a nonlinear least-squares algorithm to obtain a local velocity model consisting of an upper crustal layer with linearly increasing velocity in dipping contact with a constant velocity half-space. Hypocenters of over 150 events were relocated using this velocity model. Most of the events are clustered between the mapped traces of the San Andreas and Punchbowl faults; however, there has been a migration of activity along the San Andreas Fault. Activity which began in a 5-km cluster has expanded during a 2-yr period to fill a 60-km segment of the fault.

A slow earthquake sequence on the San Andreas fault

Nature ◽  
1996 ◽  
Vol 383 (6595) ◽  
pp. 65-68 ◽  
Author(s):  
Alan T. Linde ◽  
Michael T. Gladwin ◽  
Malcolm J. S. Johnston ◽  
Ross L. Gwyther ◽  
Roger G. Bilham
Keyword(s):  
San Andreas Fault ◽  
Earthquake Sequence ◽  
Slow Earthquake ◽  
San Andreas

The July 2019 Ridgecrest, California, Earthquake Sequence Recorded by Creepmeters: Negligible Epicentral Afterslip and Prolonged Triggered Slip at Teleseismic Distances

2020 ◽  
Vol 91 (2A) ◽  
pp. 707-720 ◽  
Author(s):  
Roger Bilham ◽  
Bryan Castillo
Keyword(s):  
Surface Waves ◽  
Travel Time ◽  
San Andreas Fault ◽  
Surface Strain ◽  
Earthquake Sequence ◽  
Fault Creep ◽  
San Andreas ◽  
Garlock Fault ◽  
Growing Body ◽  
Epicentral Region

Abstract We report sequential triggered slip at 271–384 km distances on the San Andreas, Superstition Hills, and Imperial faults with an apparent travel-time speed of 2.2 ± 0.1  km/s, following the passage of surface waves from the 4 July 2019 (17:33:49 UTC) Mw 6.4 and 6 July 2019 (03:19:53 UTC) Mw 7.1 Ridgecrest earthquakes. Slip on remote faults was not triggered instantaneously but developed over several minutes, increasing in duration with distance. Maximum slip amplitudes varied from 10  μm to 5 mm within minutes of slip nucleation, but on the southernmost San Andreas fault slip continued for two months and was followed on 16 September 2019 by a swarm of microearthquakes (Mw≤3.8) near Bombay Beach. These observations add to a growing body of evidence that fault creep may result in delayed triggered seismicity. Displacements across surface faults in the southern epicentral region and on the Garlock fault in the months following the Ridgecrest earthquakes were negligible (<1.1  mm), and they are interpreted to characterize surface strain adjustments in the epicentral region, rather than to result from discrete slip on surface faults.

THREE-DIMENSIONAL GROUND MOTION SIMULATIONS FOR LARGE EARTHQUAKES ON THE SAN ANDREAS FAULT WITH DYNAMIC AND OBSERVATIONAL CONSTRAINTS

2001 ◽  
Vol 09 (03) ◽  
pp. 1203-1214 ◽  
Author(s):  
KIM B. OLSEN
Keyword(s):  
Los Angeles ◽  
Ground Motion ◽  
San Andreas Fault ◽  
Three Dimensional ◽  
Velocity Model ◽  
Staggered Grid ◽  
Near Fault ◽  
San Andreas ◽  
3D Velocity Model ◽  
Kinematic Inversion

I have simulated 0–0.5 Hz viscoelastic ground motion in Los Angeles from M 7.5 earthquakes on the San Andreas fault using a fourth-order staggered-grid finite-difference method. Two scenarios are considered: (a) a southeast propagating and (b) a northwest propagating rupture along a 170-km long stretch of the fault near Los Angeles in a 3D velocity model. The scenarios use variable slip and rise time distributions inferred from the kinematic inversion results for the 1992 M 7.3 Landers, California, earthquake. The spatially variable static slip distribution used in this study, unlike that modeled in a recent study,1 is in agreement with constraints provided by rupture dynamics. I find peak ground velocities for (a) and (b) of 49 cm/s and 67 cm/s, respectively, near the fault. The near-fault peak motions for scenario (a) are smaller compared to previous estimates from 3D modeling for both rough and smooth faults.1,2 The lower near-fault peak motions are in closer agreements with constraints from precarious rocks located near the fault. Peak velocities in Los Angeles are about 30% larger for (b) 45 cm/s compared to those for (a) 35 cm/s.

Bear Valley, California, earthquake sequence of February-March 1972

1975 ◽  
Vol 65 (2) ◽  
pp. 483-506 ◽  
Author(s):  
William L. Ellsworth
Keyword(s):  
San Andreas Fault ◽  
Aftershock Sequence ◽  
Earthquake Sequence ◽  
Fault System ◽  
The West ◽  
Rupture Surface ◽  
Source Areas ◽  
Focal Mechanism Solutions ◽  
The North ◽  
San Andreas

abstract The earthquake sequence of late February and March 1972 involved movement along the San Andreas fault and within the crustal wedge enclosed by the branching San Andreas and San Benito faults near Bear Valley, San Benito County, California. Activity was mainly confined to three distinct zones of strike-slip faulting: the short north-trending aftershock zone of the M 3.5 earthquake of February 22, 1972, the aftershock zone of the M 5.0 Bear Valley earthquake of February 24, 1972 located along the San Andreas fault, and the west-trending aftershock zone of the M 4.6 earthquake of February 27, 1972. The north-trending and west-trending zones lie between the two major splays of the branching fault system. Focal mechanism solutions from events in these zones are consistent with the transfer of horizontal, dextral displacement from the San Andreas fault to the San Benito, Paicines and Calaveras faults within the Bear Valley region. During the 18 months preceding the February 1972 sequence, the hypocentral regions of both the M 5.0 and M 4.6 shocks were characterized by concentrations of small earthquakes. Aftershock source areas of these two events progressively expanded during the course of the aftershock sequence. Estimates of the mainshock rupture surface for these events based on the distribution of aftershocks range over a factor of 4 owing to the irregular distribution of aftershocks and the rapid growth of the aftershock zone.

A detailed seismicity study of the Middle Mountain zone at Parkfield, California

1990 ◽  
Vol 80 (3) ◽  
pp. 577-588
Author(s):  
Gail K. Nishioka ◽  
Andrew J. Michael
Keyword(s):  
San Andreas Fault ◽  
Velocity Model ◽  
Strike Slip ◽  
Seismogenic Zone ◽  
Fault Plane Solutions ◽  
Motion Data ◽  
Station Corrections ◽  
The North ◽  
San Andreas ◽  
First Motion

Abstract In order to better understand the preparation zone of the predicted Parkfield earthquake, a detailed study of the seismicity at middle Mountain in the Parkfield, California, area was made using 71 digitally recorded earthquakes that located within, or close to, the Middle Mountain alert box. These earthquakes were retimed on an interactive graphics system. Based on these new arrival times, new station corrections were developed; however the data did not support changing the velocity model developed from refraction and 1966 aftershock data. The process of retiming the earthquakes and using the new station corrections reduced the rms travel-time residuals by 70 per cent to 0.025 sec, halved the location errors, and clustered the earthquakes closer to the surface trace of the San Andreas fault. The seismicity can be approximated by a plane on the scale of several kilometers, but at finer scales two clusters were discovered that show demonstrable width to the seismogenic zone. Previous workers had proposed a 5° bend in the fault at the hypocenter of the 1966 main shock on the basis of patterns in the first motion data in the 1966 aftershocks. We find that this pattern also exists in the first-motion data from 1969 to 1987, but the 5° bend was not evident in the hypocentral distribution. This suggests that a more complicated explanation is needed to explain the first-motion data. Fault plane solutions were determined for the 71 events and 69 of these were compatible with strike-slip motion on a vertical San Andreas fault. An event located in the north end of the study area co-locates with the strike-slip solutions and may be a thrust or oblique solution. The other earthquake, located 2½ kilometers northeast of the fault, has a thrust or NNE-SSW striking right lateral solution but can not be explained by a San Andreas style mechanism. Both possible solutions can be explained by structures observed in the geology.

The San Andreas Fault

1993 ◽  
Author(s):  
Sandra S. Schulz ◽  
Robert E. Wallace
Keyword(s):  
San Andreas Fault ◽  
San Andreas
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