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Haase, JS, Hauksson E, Vernon F, Edelman A.  1996.  Modeling of ground motion from a 1994 Northridge aftershock using a tomographic velocity model of the Los Angeles Basin. Bulletin of the Seismological Society of America. 86:S156-S167. AbstractWebsite

The 1994 Northridge mainshock and its aftershocks show a complex pattern of peak accelerations at stations located in the Los Angeles Basin. The waveforms contain multiples of body-wave phases and extensive surface waves at frequencies mostly below 1 Hz. In particular, for stations at distances greater than 18 km, secondary arrivals show larger accelerations than the direct S-wave arrivals. The mainshock waveforms are further complicated by irregularities of the source rupture. We use 2D finite difference to evaluate the effect of lateral variations in seismic velocity on the amplitude of shear-wave energy and to distinguish the effects of source and propagation path. We model waveforms from one aftershock recorded at nine stations deployed along a 60-km-long profile extending into the Los Angeles Basin. We use a two-dimensional slice through the 3D tomography model of the Los Angeles Basin in the 2D finite-difference calculations. These synthetic waveforms fit the aftershock waveforms significantly better than corresponding waveforms determined from simple 1D velocity models. With the addition of a thin low-velocity surface layer above the tomography model, the finite-difference synthetics reproduce most of the important features of the recorded data, in particular, the large-amplitude arrivals 7 to 10 sec following the direct S arrival. These arrivals correspond to the SS arrival, which is sharply refracted at the basin edge, and the S-wave with multiple legs trapped by the dipping near surface gradient. For large earthquakes located either inside or outside the basin, these phases can be the cause of the largest and hence potentially most hazardous shaking in the Los Angeles Basin.

Hartse, HE, Fehler MC, Aster RC, Scott JS, Vernon FL.  1994.  Small-Scale Stress Heterogeneity in the Anza Seismic Gap, Southern California. Journal of Geophysical Research-Solid Earth. 99:6801-6818.   10.1029/93jb02874   AbstractWebsite

Focal mechanism inversions reveal significant lateral variations in stress orientations along the Anza segment of the San Jacinto fault zone. The most notable stress anomaly is within the 20-km aseismic (seismic gap) portion of the fault zone, where sigma1, the maximum compressive stress, is nearly horizontal and is oriented at 74-degrees +/- 13-degrees relative to the fault strike. This contrasts with orientations ranging from 62-degrees +/- 11-degrees to 49-degrees +/- 7-degrees along the more seismically active portions of the fault zone immediately to the northwest and southeast of the seismic gap. Regional stress results, found by inverting all focal mechanisms simultaneously, indicate that sigma1 is horizontal and trends north-south, while sigma3 is horizontal and trends east-west. Approximately, 15 km west of the seismic gap, in the off-fault Cahuilla swarm area, sigma1 and sigma3 solutions are rotated clockwise by about 25-degrees relative to the regional model. Roughly, 10 km southeast of the seismic gap near the Buck Ridge fault, sigma1 and sigma3 are rotated counterclockwise by about 10-degrees relative to the regional solution. Northwest of the seismic gap along the fault zone, sigma3 plunges about 30-degrees from the horizontal, correlating with a local increase in reverse faulting between the Hot Springs and San Jacinto faults. Southeast of the seismic gap, sigma1 plunges about 45-degrees from the horizontal, correlating with a local increase in normal faulting in the trifurcation region of the Buck Ridge, Clark, and Coyote Creek faults. We propose a simple mechanical model in which a block rotation superimposed on the dominant right-lateral strike-slip motion of the fault zone satisfies the first-order observations of stress orientation, faulting, and horizontal surface strain. Under this model the Anza seismic gap is the region of zero convergence between the northeast and southwest sides of the fault, and the fault zone strength within the seismic gap is either comparable to or exceeds the fault zone strength adjacent to the gap.

Hedlin, MAH, Berger J, Vernon FL.  2002.  Surveying Infrasonic Noise on Oceanic Islands. Pure and Applied Geophysics. 159:1127-1152., Germany   10.1007/s00024-002-8675-1   AbstractWebsite

-An essential step in the establishment of an International Monitoring System (IMS) infrasound station is the site survey. The survey seeks a location with relatively low infrasonic noise and the necessary logistical support. This paper reports results from our surveys of two of the oceanic sites in the IMS - the Azores and Cape Verde. Each survey sampled infrasonic noise, wind velocity, air temperature and humidity for 3 weeks at 4 sites near the nominal IMS locations. The surveys were conducted on Sao Miguel (the main island in the Azores) and Maio (Cape Verde). Infrasonic noise was measured using the French MB2000 microbarometer.During our 3-week experiment in January the trade winds at Cape Verde varied little from an azimuth of 63 degree . Because of the unvarying wind azimuth, the experiment gave us an opportunity to examine the effectiveness of a forest at reducing both wind speed and infrasonic noise. We find that the thick Acacia forest on Maio reduces wind speeds at a 2m elevation by more than 50% but does not reduce infrasonic noise at frequencies below 0.25Hz. This forest serves as a high-frequency filter and clearly does not reduce long-period noise levels which are due to large-scale turbulence in the atmospheric boundary layer above the forest. This is consistent with our observations in the Azores where the relationship between infrasonic noise and wind speed is more complex due to frequent changes in wind azimuth.In Cape Verde, wind speed and infrasonic noise are relatively constant. The diurnal variations are clearly seen however the microbarom is only rarely sensed. In the Azores, during our 3-week experiment in November and December of 1998, wind speed and infrasonic noise change rapidly. At this location, daily noise level swings of 40 to 50dB at 0.1Hz are not uncommon in the early winter and are due to changes in wind speed and atmospheric turbulence. The effectiveness of an infrasound station in the Azores will be strongly dependent on time during the winter season.The two surveys illustrate some of the difficulties inherent in the selection of sites for 1 to 3km aperture arrays on oceanic islands. Due to elevated noise levels at these sites, 8 element, 2km aperture arrays are strongly preferred.

Heuze, F, Archuleta R, Bonilla F, Day S, Doroudian M, Elgamal A, Gonzales S, Hoehler M, Lai T, Lavallee D, Lawrence B, Liu PC, Martin A, Matesic L, Minster B, Mellors R, Oglesby D, Park S, Riemer M, Steidl J, Vernon F, Vucetic M, Wagoner J, Yang Z.  2004.  Estimating site-specific strong earthquake motions. Soil Dynamics and Earthquake Engineering. 24:199-223.   10.1016/j.soildyn.2003.11.002   AbstractWebsite

The Campus Earthquake Program (CEP) of the University of California (UC) started in March 1996, and involved a partnership among seven campuses of the UC-Berkeley, Davis, Los Angeles, Riverside, San Diego, Santa Barbara, Santa Cruz-and the Lawrence Livermore National Laboratory (LLNL). The aim of the CEP was to provide University campuses with site-specific assessments of their earthquake strong motion exposure, to complement estimates they obtain from consultants according to the state-of-the-practice (SOP), i.e. Building Codes (UBC 97, IBC 2000), and Probabilistic Seismic Hazard Analysis (PSHA). The Building Codes are highly simplified tools, while the more sophisticated PSHA is still somewhat generic in its approach because it usually draws from many earthquakes not necessarily related to the faults threatening the site under study. Between 1996 and 2001, the site-specific studies focused on three campuses: Riverside, San Diego, and Santa Barbara. Each campus selected 1-3 sites to demonstrate the methods and procedures used by the CEP: Rivera Library and Parking Lots (PL) 13 and 16 at UCR, Thornton Hospital, the Cancer Center, and PL 601 at UCSD, and Engineering I building at UCSB. The project provided an estimate of strong ground motions at each selected site, for selected earthquake scenarios. These estimates were obtained by using an integrated geological, seismological, geophysical, and geotechnical approach, that brings together the capabilities of campus and laboratory personnel. Most of the site-specific results are also applicable to risk evaluation of other sites on the respective campuses. The CEP studies have provided a critical assessment of whether existing campus seismic design bases are appropriate. Generally speaking, the current assumptions are not acknowledging the severity of the majority of expected motions. Eventually, both the results from the SOP and from the CEP should be analyzed, to arrive at decisions concerning the design-basis for buildings on UC campuses. Published by Elsevier Ltd.