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.

Observations of creep-related tilt, strain, and water-level changes on the central San Andreas fault

1977 ◽  
Vol 67 (3) ◽  
pp. 641-649 ◽  
Author(s):  
C. E. Mortensen ◽  
R. C. Lee ◽  
R. O. Burford
Keyword(s):  
Water Level ◽  
San Andreas Fault ◽  
Slip Surface ◽  
Lower Boundary ◽  
Water Level Fluctuations ◽  
Fault Creep ◽  
San Andreas ◽  
Strain Data ◽  
Data Source ◽  
Surface Creep

abstract Several simultaneous observations of surface fault creep, tilt, strain, and water-level fluctuations have been recorded along the San Andreas fault in the vicinity of the Almaden-Cienega Winery south of Hollister, California. Creep events recorded on the winery creepmeters on February 16, 1975, and by the winery and Harris Ranch creepmeters on September 17, 1975, were modeled as migrating dislocations with geometries chosen to give results that match the observed tilt and strain data. Source depths for the February 16th and September 17th creep events were found to be relatively shallow, the depth to the lower boundary of the slip surface being 0.4 and 2.0 km, respectively. In both cases slip was found to propagate from the northwest toward the southeast, which is consistent with changes in water level observed in a well near the winery. Since the installation of the tiltmeter and strainmeter 0.8 km northwest of the Cienega Winery, six tilt and strain signals with durations typical of creep events have been related to observed surface creep, while 11 such signals appear unrelated to recorded surface creep. The latter may result from surface creep of limited extent or creep at depth.

A Discussion on the measurement and interpretation of changes of strain in the Earth - Role of pore fluids in faulting

1973 ◽  
Vol 274 (1239) ◽  
pp. 297-304 ◽  
Keyword(s):  
Pore Pressure ◽  
San Andreas Fault ◽  
Fault Creep ◽  
Time Constants ◽  
San Andreas ◽  
The Earth ◽  
Shallow Earthquakes ◽  
Pressure Changes

We consider three in situ processes which involve fluid flow in the crust: fault creep, aftershocks and dilatancy. Measurements of water level in wells suggest that creep events on the San Andreas fault are coupled with pore pressure changes. Readjustment of transient pore pressure, induced by large shallow earthquakes, possess the correct time constants and magnitudes to explain the occurrence of aftershocks. And finally, temporal changes of travel times in the Gram district (U.S.S.R.) imply that dilatancy may occur in situ.

Implication of seismicity for failure of a section of the San Andreas Fault

1980 ◽  
Vol 70 (1) ◽  
pp. 185-201
Author(s):  
W. H. Bakun ◽  
R. M. Stewart ◽  
C. G. Bufe ◽  
S. M. Marks
Keyword(s):  
San Andreas Fault ◽  
P Wave ◽  
Seismogenic Zone ◽  
Wave Radiation ◽  
Focal Region ◽  
Fault Creep ◽  
Aftershock Activity ◽  
Discontinuous Surface ◽  
San Andreas ◽  
Fault Trace

abstract On January 15, 1973, a magnitude ML 4.1 earthquake occurred near Cienega Road on the San Andreas Fault about 20 km south of Hollister, California. A 3-km-long segment of the fault southeast of the earthquake was aseismic for the 7 weeks preceding the event, although microearthquakes occurred at both its ends. The first day's aftershocks occurred at the northwest end of the aseismic segment; later aftershock activity migrated to the southeast, filling the remainder of the segment. If the discontinuous surface trace of the fault can be extrapolated to the focal region of the earthquakes to define fault geometry at depth, then aftershocks occurred primarily on one continuous segment of the fault and epicenter locations and direction of rupture propagation (inferred from the azimuthal pattern of P-wave radiation) of the precursory shocks correlate with the discontinuities in the trace that terminate the segment. The 1970 to 1976 deficit in seismic slip within the segment suggests that fault creep accounts for a significant part of cumulative slip within the segment. The pattern of seismicity is consistent with the hypothesis that creep on the segment before the main shock caused a buildup of stress at the ends of the segment or at the ends of adjacent offset segments. Correlation of seismicity and discontinuities or bends in the mapped fault trace are the basis for an extension and refinement of the “stuck” and “creeping” patch model of the San Andreas Fault in central California. Patch boundaries extend from the free surface down through the seismogenic zone. Creeping patches lie beneath smooth continuous segments of the fault trace. Stuck patches lie beneath discontinuities or bends in the fault trace.

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