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On the southern part of the San Francisco Peninsula, the San Andreas fault traverses the actively uplifting Santa Cruz Mountains. The field guide is comprised of a hiking tour along the fault in Sanborn County fault, a visit to a winery and vineyards traversed by the fault, and visits to two wineries that provide vistas of the San Andreas Rift Valley and surroundings.
Numerous small earthquakes occur each day in the Coast Ranges of central California. The detailed study of these earthquakes provides a tool for gaining insight into the tectonic and physical processes responsible for the generation of damaging earthquakes. This catalog contains the fundamental parameters for earthquakes located within and adjacent to the seismograph network operated by the U.S. Geological Survey (USGS), during the second quarter of 1977.
Numerous small earthquakes occur each day in the Coast Ranges of central California. The detailed study of these earthquakes provides a tool for gaining insight into the tectonic and physical processes responsible for the generation of damaging earthquakes. This catalog contains the fundamental parameters for earthquakes located within and adjacent to the seismograph network operated by the U.S. Geological Survey(USGS), during the fourth quarter of 1977.
The slip model for the 2011 M w 5.6 Prague, Oklahoma, earthquake is inferred using a linear least squares methodology. Waveforms of six aftershocks recorded at 21 regional stations are used as empirical Green's functions (EGFs). The solution indicates two large slip patches: one located around the hypocenter with a depth range of 3–5.5 km; the other located to the southwest of the epicenter with a depth range from 7.5 to 9.5 km. The total moment of the solution is estimated at 3.37 × 10 24 dyne cm (M w 5.65). The peak slip and average stress drop for the source at the hypocenter are 70 cm and 90 bars, respectively, approximately one half the values for the M w 5.8 2011 Mineral, Virginia, earthquake. The stress drop averaged over all areas of slip is 16 bars. The relatively low peak slip and stress drop may indicate an induced component in the origin of the Prague earthquake from deep fluid injection.
Numerous small earthquakes occur each day in the Coast Ranges of central California. The detailed study of these earthquakes provides a tool for gaining insight into the tectonic and physical processes responsible for the generation of damaging earthquakes
The Bartlett Springs Fault (BSF), the easternmost branch of the northern San Andreas Fault system, creeps along much of its length. Geodetic data for the BSF are sparse, and surface creep rates are generally poorly constrained. The two existing geodetic slip rate inversions resolve at least one locked patch within the creeping zones. We use the 3-D finite element code FaultMod to conduct dynamic rupture models based on both geodetic inversions, in order to determine the ability of rupture to propagate into the creeping regions, as well as to assess possible magnitudes for BSF ruptures. For both sets of models, we find that the distribution of aseismic creep limits the extent of coseismic rupture, due to the contrast in frictional properties between the locked and creeping regions.
A reverse-right-oblique surface rupture, associated with a M L 2.5 earthquake, formed in a diatomite quarry near Lompoc, California, in the northwesternmost Transverse Ranges on April 7, 1981. The 575-m-long narrow zone of ruptures formed in clay interbeds in diatomite and diatomaceous shale of the Neogene Monterey Formation. The ruptures parallel bedding, dip 39°–59°S, and trend about N84°E on the north limb of an open symmetrical syncline. Maximum net slip was 25 cm; maximum reverse dip slip was 23 cm, maximum right-lateral strike slip was about 9 cm, and average net slip was about 12 cm. The seismic moment of the earthquake is estimated at 1 to 2 × 10 18 dyne/cm and the static stress drop at about 3 bar. The removal of an average of about 44 m of diatomite resulted in an average load reduction of about 5 bar, which decreased the normal stress by about 3.5 bar and increased the shear stress on the tilted bedding plane by about 2 bar. The April 7,1981, event was a very shallow bedding-plane rupture, apparently triggered by crustal unloading.
Strong aftershocks following major earthquakes present significant challenges for infrastructure recovery as well as for emergency rescue efforts. A tragic instance of this is the 22 February 2011 M w 6.3 Christchurch aftershock in New Zealand, which caused more than 100 deaths while the 2010 M w 7.1 Canterbury mainshock did not cause a single fatality (Figure 1). Therefore, substantial efforts have been directed toward understanding the generation mechanisms of aftershocks as well as mitigating hazards due to aftershocks. Among these efforts are the prediction of strong aftershocks, earthquake early warning, and aftershock probability assessment. Zhang et al. (1999) reported a successful case of strong aftershock prediction with precursory data such as changes in seismicity pattern, variation of b-value, and geomagnetic anomalies. However, official reports of such successful predictions in geophysical journals are extremely rare, implying that deterministic prediction of potentially damaging aftershocks is not necessarily more scientifically feasible than prediction of mainshocks.
The 1700 Cascadia earthquake attained moment magnitude 9 according to new estimates based on effects of its tsunami in Japan, computed coseismic seafloor deformation for hypothetical ruptures in Cascadia, and tsunami modeling in the Pacific Ocean. Reports of damage and flooding show that the 1700 Casscadia tsunami reached 1-5 m heights at seven shoreline sites in Japan. Three sets of estimated heights express uncertainty about location and depth of reported flooding, landward decline in tsunami heights from shorelines, and post-1700 land-level changes. We compare each set with tsunami heights computed from six Cascadia sources. Each source is vertical seafloor displacement calculated with a three-dimensional elastic dislocation model, for three sources the rupture extends the 1100 km length of the subduction zone and differs in width and shallow dip; for the other sources, ruptures of ordinary width extend 360-670 km. To compute tsunami waveforms, we use a linear long-wave approximation with a finite difference method, and we employ modern bathymetry with nearshore grid spacing as small as 0.4 km. The various combinations of Japanese tsunami heights and Cascadia sources give seismic moment of 1-9 ?? 1022 N m, equivalent to moment magnitude 8.7-9.2. This range excludes several unquantified uncertainties. The most likely earthquake, of moment magnitude 9.0, has 19 m of coseismic slip on an offshore, full-slip zone 1100 km long with linearly decreasing slip on a downdip partial-slip zone. The shorter rupture models require up to 40 m offshore slip and predict land-level changes inconsistent with coastal paleoseismological evidence. Copyright 2003 by the American Geophysical Union.
In the version of this article originally published, describing the location of the earthquakes using their Universal Transverse Mercator (UTM) coordinates introduced a bias in the inversion used to derive the two-dimensional (2D) strain fields, affecting the sign (positive versus negative) of the strain results.
Two events dominated the January 1994, Wyomissing, PA earthquake sequence, an M w 4.0 foreshock, followed by an M w 4.6 mainshock. We modeled regional waveforms to estimate the event depth and the moment tensors for the two largest events in the sequence, and examine teleseismic waveforms recorded on the ARCESS short-period seismic array to estimate the depth and source time function of the mainshock. Our data constrain the depth of the events to be shallower than 5 km, and prefer a depth of 3–5 km. For an assumed depth of 3 km, the mainshock moment tensor is 75% double couple, with (the major double couple) planes striking at 135°N, 347°N, dips of 49°, 46°, and rakes of 68°, 114°. The estimated moment is 8.9 × 10 22 dyne-cm. The P axis strikes 241°N and plunges 2°, the Tension axis strikes 336°N and plunges 73°. The foreshock inversion results are virtually identical to the mainshock results; for a source depth of three km, we find a major double couple with a strike, dip, and rake of 121°N, 60°, and 66°, respectively. The seismic moment for the foreshock is 1.2 × 10 22 dyne-cm, which is approximately 13% of the mainshock moment release. These events did not excite high-frequency Lg waves as effectively as typical eastern North American events, and the mainshock had a stress drop in the range of 25–50 bars.
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The West Valley fault zone (WVFZ) and Salt Lake City segment (SLCS) of the Wasatch fault zone comprise Holoceneactive normal faults that bound an intrabasin graben in northern Salt Lake Valley, Utah. Both fault zones have evidence of recurrent Holocene surface-faulting earthquakes. A topic of recent research is the seismogenic relation of the antithetic (subsidiary) WVFZ to the Wasatch fault zone—specifically, to what degree are WVFZ earthquakes independent of slip on the SLCS, or other adjacent segments, of the Wasatch fault zone. To improve paleoseismic data for the WVFZ and better understand the seismogenic relation between the WVFZ and Wasatch fault zone, we conducted a fault-trench investigation at the Airport East site, developed new earthquake recurrence and fault sliprate estimates for the WVFZ, and compared WVFZ earthquake timing data with data from the Wasatch fault zone. The Airport East site is near the northern end of the easternmost traces of the WVFZ, collectively referred to as the Taylorsville fault. At this site, we excavated two parallel trenches across a small (~0.5-m high) east-facing fault scarp. Shallow groundwater severely limited trench depth, and we were able to expose deposits only as old as mid-Holocene. However, the late Holocene section contained evidence for three surfacefaulting earthquakes on the Taylorsville fault, as well as earthquake-related deformation (liquefaction and folding) from a fourth earthquake that was likely sourced elsewhere. Based on OxCal modeling of radiocarbon and optically stimulated luminescence ages, the most recent earthquake, AE1, occurred at 0.4 ± 0.2 ka (mean modeled time ± 2σ), earthquake AE2 occurred at 0.6 ± 0.2 ka, and earthquake AE3 occurred at 2.0 ± 0.3 ka. Timing data from the Airport East site indicate the liquefaction and folding event (LE1) occurred at 5.1 ± 0.3 ka. Net vertical displacement across the fault is 0.6–1.1 m, and calculations of mean per-event displacement range from 0.20 to 0.37 m. Inter-event recurrence intervals for the Taylorsville fault at the Airport East site vary from 200 to 1400 yr, and the mean late Holocene (post-2 ka) recurrence interval is 800 yr. Paleoseismic (closed-interval) slip rates range from 0.23 to 1.4 mm/yr, and geologic (open-interval) slip rates range from 0.1–0.2 mm/yr over the past ~5000 yr to 0.2–0.4 mm/yr over the past ~2500 yr. Combining our new Airport East data with previous paleoseismic data for the WVFZ shows that during individual late Holocene earthquakes, surface faulting has occurred on either the Taylorsville or Granger fault, but not both faults at the same time (at least not on the parts of the faults that have been trenched). However, five mid- to late Holocene WVFZ earthquakes have mean modeled times that are either very similar or identical to mean modeled times of SLCS and Weber-segment earthquakes, and temporal correlations of two additional late Pleistocene WVFZ earthquakes with SLCS earthquakes cannot be ruled out. When comparing the earthquake chronologies of the WVFZ and Wasatch fault zone, our new data lend support to the idea that, more often than not, some part of the WVFZ moves in response to, and possibly synchronously with, slip on the Wasatch fault zone.