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Lewis, MA, Peng Z, Ben-Zion Y, Vernon FL.  2005.  Shallow seismic trapping structure in the San Jacinto fault zone near Anza, California. Geophysical Journal International. 162:867-881.   10.1111/j.1365-246X.2005.02684.x   AbstractWebsite

We analyse fault zone trapped waves, generated by similar to 500 small earthquakes, for high-resolution imaging of the subsurface structure of the Coyote Creek, Clark Valley and Buck Ridge branches of the San Jacinto fault zone near Anza, California. Based on a small number of selected trapped waves within this data set, a previous study concluded on the existence of a low-velocity waveguide that is continuous to a depth of 15-20 km. In contrast, our systematic analysis of the larger data set indicates a shallow trapping structure that extends only to a depth of 3-5 km. This is based on the following lines of evidence. (1) Earthquakes clearly outside these fault branches generate fault zone trapped waves that are recorded by stations within the fault zones. (2) A traveltime analysis of the difference between the direct S arrivals and trapped wave groups shows no systematic increase (moveout) with increasing hypocentral distance or event depth. Estimates based on the observed average moveout values indicate that the propagation distances within the low-velocity fault zone layers are 3-5 km. (3) Quantitative waveform inversions of trapped wave data indicate similar short propagation distances within the low-velocity fault zone layers. The results are compatible with recent inferences on shallow trapping structures along several other faults and rupture zones. The waveform inversions also indicate that the shallow trapping structures are offset to the northeast from the surface trace of each fault branch. This may result from a preferred propagation direction of large earthquake ruptures on the San Jacinto fault.

Lewis, JL, Day SM, Magistrale H, Eakins J, Vernon F.  2000.  Regional crustal thickness variations of the Peninsular Ranges, southern California. Geology. 28:303-306.   10.1130/0091-7613(2000)28<303:rctvot>;2   AbstractWebsite

We used the teleseismic receiver function technique to obtain a profile of the crustal thickness of the northern Peninsular Ranges, California. Depth to the Moho varies from similar to 37 inn beneath the western Peninsular Ranges batholith to similar to 27 km at the western edge of the Salton trough, an average apparent dip of similar to 10 degrees to the west over a lateral distance of 60 km, We previously obtained a similar result for a profile similar to 100 km to the south (a Moho dip of similar to 20 degrees over 30 km lateral distance). In both cases, the Moho depth variations do not correlate with topography of the eastern batholith, but rather appear to parallel the trend of a boundary that separates compositionally distinct eastern and western terranes, These observations suggest that a steeply dipping Moho is a regional feature beneath the eastern Peninsular Ranges, and that compensation is through lateral variations in crustal or upper mantle density rather than through an Airy root.

Lewis, JL, Day SM, Magistrale H, Castro RR, Astiz L, Rebollar C, Eakins J, Vernon FL, Brune JN.  2001.  Crustal thickness of the peninsular ranges and gulf extensional province in the Californias. Journal of Geophysical Research-Solid Earth. 106:13599-13611.   10.1029/2001jb000178   AbstractWebsite

We estimate crustal thickness along an east-west transect of the Baja California peninsula and Gulf of California, Mexico, and investigate its relationship to surface elevation and crustal extension. We derive Moho depth estimates from P-to-S converted phases identified on teleseismic recordings at 11 temporary broadband seismic stations deployed at similar to 31 degreesN latitude. Depth to the Moho is similar to 33 (+/-3) km near the Pacific coast of Baja California and increases gradually toward the east, reaching a maximum depth of similar to 40 (+/-4) km beneath the western part of the Peninsular Ranges batholith, The crust then thins rapidly under the topographically high eastern Peninsular Ranges and across the Main Gulf Escarpment, Crustal thickness is similar to 15-18 (+/-2) km within and on the margins of the Gulf of California. The Moho shallowing beneath the eastern Peninsular Ranges represents an average apparent westward dip of similar to 25 degrees. This range of Moho depths within the Peninsula Ranges, as well as the sharp similar to east-west gradient in depth in the eastern part of the range, is in agreement with earlier observations from north of the international border. The Moho depth variations do not correlate with topography of the eastern batholith, These findings suggest that a steeply dipping Moho is a regional feature beneath the eastern Peninsular Ranges and that a local Airy crustal root does not support the highest elevations. We suggest that Moho shallowing under the eastern Peninsular Ranges reflects extensional deformation of the lower crust in response to adjacent rifting of the Gulf Extensional Province that commenced in the late Cenozoic, Support of the eastern Peninsular Ranges topography may be achieved through a combination of flexural support and lateral density variations in the crust and/or upper mantle.

Li, YG, Aki K, Vernon FL.  1997.  San Jacinto fault zone guided waves: A discrimination for recently active fault strands near Anza, California. Journal of Geophysical Research-Solid Earth. 102:11689-11701.   10.1029/97jb01050   AbstractWebsite

We deployed three 350-m-long eight-element linear seismic arrays in the San Jacinto Fault Zone (SJFZ) near Anza, California, to record microearthquakes starting in August through December 1995. Two arrays were deployed 18 km northwest of Anza, across the Casa Loma fault (CLF) and the Hot Springs fault (HSF) strands of the SJFZ. The third array was deployed across the San Jacinto fault (SJF) in the Anza slip gap. We observed fault zone guided waves characterized by low-frequency, large amplitudes following S waves at the CLF array and the SJF array for earthquakes occurring within the fault zone. However, we did not observe guided waves at the HSF array for any events. The amplitude spectra of these guided waves showed peaks at 4 Hz at the CLF and 6 Hz at the SJF, which decreased sharply with the distance from the fault trace. In contrast, no spectral peaks at frequency lower than 6 Hz were registered at the HSF array. We used a finite difference method to simulate these guided modes as S waves trapped in a low-velocity waveguide sandwiched between high-velocity wall rocks. The guided mode data are adequately fit by a waveguide on the CLF with the average width of 120 m and S velocity of 2.5 km/s, about 25% reduced from the S velocity of the surrounding rock; this waveguide becomes 40 to 60 m wide with the S velocity of 2.8 km/s in the Anza slip gap. On the other hand, there is not a continuous waveguide on the HSF at depth. Locations of the events with guided modes suggest that the fault plane waveguide extends along the CLF between the towns of San Jacinto and Anza, dipping northeastward at 75 degrees-80 degrees to a depth of about 18 km; it becomes nearly vertical in the Anza gap. We speculate that the existence of a continuous low-velocity waveguide on the CLF can be caused by the rupture of the magnitude 6.9 earthquake on April 21, 1918,occurring near the towns of San Jacinto and Hemet. Further, the lack of a clear waveguide on the HSF suggests that it was not ruptured in this event.

Li, ZF, Peng ZG, Ben-Zion Y, Vernon FL.  2015.  Spatial variations of shear wave anisotropy near the San Jacinto Fault Zone in Southern California. Journal of Geophysical Research-Solid Earth. 120:8334-8347.   10.1002/2015jb012483   AbstractWebsite

We examine crustal anisotropy at several scales along and across the San Jacinto Fault Zone (SJFZ) by systematically measuring shear wave splitting (SWS) parameters. The analyzed data are recorded by 86 stations during 2012-2014, including five linear dense arrays crossing the SJFZ at different locations and other autonomous stations within 15 km from the main fault trace. Shear phase arrivals and SWS parameters (fast directions and delay times) are obtained with automated methods. The measurement quality is then assessed using multiple criteria, resulting in 23,000 high-quality measurements. We find clear contrast of fast directions between the SW and NE sides of the SJFZ. Stations on the SW side have fast directions consistent overall with the maximum horizontal compression direction (SHmax), while stations on the NE side show mixed patterns likely reflecting lithological/topographic variations combined with fault zone damage. The fast directions in the Anza gap section with relatively simple fault geometry agree with the inferred SHmax, and the delay times at an array within that section are smaller than those observed at other across-fault arrays. These indications of less pronounced damage zone in the Anza section compared to other segments of the SJFZ are correlated generally with geometrical properties of the surface traces. Significant variations of fast directions on several across-fault arrays, with station spacing on the orders of a few tens of meters, suggest that shallow fault structures and near-surface layers play an important role in controlling the SWS parameters.

Li, YG, Vernon FL.  2001.  Characterization of the San Jacinto fault zone near Anza, California, by fault zone trapped waves. Journal of Geophysical Research-Solid Earth. 106:30671-30688.   10.1029/2000jb000107   AbstractWebsite

We installed three 350-m-long seismic arrays, each array consisting of 12 three-component stations, across the Coyote Creek fault (CCF), Clark Valley fault (CVF), and Buck Ridge fault (BRF) of the San Jacinto fault zone (SJFZ) near Anza, California, to record fault zone trapped waves from microearthquakes. We observed trapped waves with relatively large amplitudes and long duration at stations close to the fault traces for earthquakes occurring within the fault zone. The coda-normalized amplitude spectra of trapped waves showed peaks at 4-7 Hz, which decreased sharply with the distance from the fault trace. Observations and three-dimensional finite difference simulations of trapped waves revealed low-velocity and low-Q waveguides on these active faults with the width of 75- 100 m in which shear velocities are reduced by 25-30% from wall rock velocities and Q values are 40-90 at depths between the surface and 18 km. The locations of earthquakes for which we observed trapped waves delineate the most seismically active fault strands of the SJFZ in a region with complicated slip planes near Anza. The low-velocity waveguides inferred from trapped waves extend 15 to 20 km in the length on these active faults and are segmented by the fault discontinuities. The waveguide on the BRF dips southwestward to connect the waveguide on the CVF, which dips northeastward. This waveguide extends at the seismogenic depth through Anza slip gap to another low-velocity waveguide on the Casa Loma fault (CLF), which has been delineated in our previous study of the SJFZ using trapped waves [Li et al., 1997]. The waveguide on the CCF in Coyote Mountain is nearly vertical and disconnected from the CLF at the south edge of Anza gap. We interpret the low-velocity waveguides on these active strands to partly result from recent prehistoric significant earthquakes on them and evaluate the future earthquake in the Anza region.

Li, YG, Vernon FL, Aki K.  1997.  San Jacinto fault zone guided waves: A discrimination for recently active fault strands near Anza, California (vol 102, pg 11689, 1997). Journal of Geophysical Research-Solid Earth. 102:20437-20437.   10.1029/97jb02128   Website
Lindquist, KG, Newman RL, Vernon FL.  2007.  The antelope interface to PHP and applications: Web-based real-time monitoring. Seismological Research Letters. 78:663-670.   10.1785/gssrl.78.6.663   Website