A study of the strong ground motion of the Borrego Mountain, California, earthquake

Several synthetic models are constructed to fit the first 40 sec of the transversely polarized displacement, as recorded at El Centro, of the April 9, 1968 Borrego Mountain earthquake. The modeling is done in the time domain using the response computed for a distributed set of point shear dislocations embedded in a layered half-space. The beginning 10 sec of the observed record is used to model the spatial and temporal distribution of faulting whereas the remaining portion is used to determine the upper crustal structure based on surface-wave periodicity. A natural depth criterion was provided by comparing the amplitude of the direct arrival with the surface-wave excitations. Trade-offs are found to exist between source models and velocity structure models. Within the framework of a layer over a half-space model, faulting of finite vertical extent is required, whereas the horizontal dimensions of faulting are not resolvable. A model which is also consistent with the teleseismic results of Burdick and Mellman indicates massive faulting near a depth of 9 km with a fast rise time producing a 10-cm displacement pulse of 1 sec duration at El Centro. The faulting appears to slow down approaching the surface. The moment is calculated to be approximately 7 × 10^(25) dyne-cm which is somewhat smaller than the moment found by Burdick and Mellman (1976)

Generalized ray models of the San Fernando Earthquake

The exact Cagniard-de Hoop solutions for a point dislocation in half-space are used to construct models of the strong ground motion observed during the February 9, 1971 San Fernando earthquake (M_L = 6.4). By summing point dislocations distributed over the fault plane, three-dimensional models of a finite fault located in a half-space are constructed to study the ground motions observed at JPL (Pasadena), Palmdale, Lake Hughes, and Pacoima Dam. Since the duration of faulting is comparable to the travel times for various wave types, very complex interference of these arrivals makes a detailed interpretation of these wave forms difficult. By investigating the motion due to small sections of the fault, it is possible to understand how various wave types interfere to produce the motion due to the total fault. Rayleigh waves as well as S to P head waves are shown to be important effects of the free surface. Near-field source effects are also quite dramatic. Strong directivity is required to explain the difference in amplitudes seen between stations to the north and stations to the south. Faulting appears to have begun north of Pacoima at a depth of 13 km. The rupture velocity, which is near 2.8 km/sec in the hypocentral region, appears to slow to 1.8 km/sec at a depth of 5 km. Displacements on the deeper sections of the fault are about 2.5 m. Fault offsets become very small at depths near 4 km and then grow again to 5 m near the surface rupture. The large velocity pulse seen at Pacoima is a far-field shear wave which is enhanced by directivity. Peak accelerations at Pacoima are probably associated with the large shallow faulting. The total moment is 1.4 × 10^(26) ergs

Predictability of strong ground motion in the Imperial Valley: Modeling the M4.9, November 4, 1976 Brawley earthquake

Strong-motion displacements, recorded at 33 km (IVC) and 36 km (ELC) from the November 4, 1976 Brawley earthquake, are modeled using the Cagniard-deHoop technique. The IVC record consists almost entirely of transversely polarized motion, whereas the ELC record contains an approximately equal proportion of transversely and radially polarized motion. A simplified shear-wave velocity model was determined from the compressional wave refraction studies of Biehler, Kovach, and Allen (1964). The epicentral location and focal mechanism computed from P-wave first-arrival studies were used to locate and orient a double-couple point source within the layered half-space. The far-field time function and source depth were the only parameters without good independent constraints. A triangular far-field time function with a duration of 1.5 sec and a source depth of 7 km were sufficient to model the first 25 sec of tangential ground motion. It appears that the effects of velocity structure on the propagation of long-period SH waves are predictable in the Imperial Valley. A study of the synthetic Fourier amplitude spectra indicates that wave propagation effects should be included in studies of source spectra and seismic wave attenuation

Strong-Motion and Broadband Teleseismic Analysis of the Earthquake for Rupture Process and Hazards Assessment

We have used broadband records from 18 teleseismic stations and three-component records from 16 local strongmotion stations in a formal inversion to determine the temporal and spatial distribution of slip during the earthquake. Separate inversions of the teleseismic (periods, 3-30 s) and strong-motion (periods, 1-5 s) data sets result in similar source models. The data require bilateral rupture, with relatively little slip in the region directly updip from the hypocenter. Slip is concentrated in two patches: one centered 6 km northwest of the hypocenter at 12-km depth with an average slip amplitude of 250 cm, and the other centered about 5 km southeast of the hypocenter at 16-km depth with an average slip amplitude of 180 cm. This bilateral rupture results in large-amplitude ground motions at sites both to the northwest and southeast along the fault strike. The northwestern patch, however, has a larger seismic moment and overall stress drop and thus is the source of the highest ground-motion velocities, a result consistent with observations. The bilateral rupture also results in relatively moderate ground motion directly updip from the hypocenter, in agreement with the ground motions observed at Corralitos, Calif. Furthermore, there is clear evidence of a foreshock (M~4.5-5.0) or slow rupture nucleation about 2 s before the main rupture; the origin time implied by strong-motion trigger times is systematically nearly 2 s later than that predicted from the high-gain regional-network data. The seismic moment obtained from either or both data sets is about 3.0x10^(26) dyne-cm, and the seismic potency is 0.95 km^3. Our analysis indicates that the rupture model determined from the teleseismic data set alone, independent of the strong-motion data set, is adequate to predict many characteristics of the local-strong-motion recordings

Source study of the 1906 San Francisco earthquake

All quality teleseismic recordings of the great 1906 San Francisco earthquake archived in the 1908 Carnegie Report by the State Earthquake Investigation Commission were scanned and digitized. First order results were obtained by comparing complexity and amplitudes of teleseismic waveforms from the 1906 earthquake with well calibrated, similarly located, more recent earthquakes (1979 Coyote Lake, 1984 Morgan Hill, and 1989 Loma Prieta earthquakes) at nearly co-located modern stations. Peak amplitude ratios for calibration events indicated that a localized moment release of about 1 to 1.5 × 10^(27) dyne-cm was responsible for producing the peak the teleseismic body wave arrivals. At longer periods (50 to 80 sec), we found spectral amplitude ratios of the surface waves require a total moment release between 4 and 6 × 10^(27) dyne-cm for the 1906 earthquake, comparable to previous geodetic and surface wave estimates (Thatcher, 1975). We then made a more detailed source analysis using Morgan Hill S body waves as empirical Green's Functions in a finite fault subevent summation. The Morgan Hill earthquake was deemed most appropriate for this purpose as its mechanism is that of the 1906 earthquake in the central portion of the rupture. From forward and inverse empirical summations of Morgan Hill Green's functions, we obtained a good fit to the best quality teleseismic waveforms with a relatively simple source model having two regions of localized strong radiation separated spatially by about 110 km. Assuming the 1906 epicenter determined by Bolt (1968), this corresponds with a large asperity (on the order of the Loma Prieta earthquake) in the Golden Gate/San Francisco region and one about three times larger located northwest along strike between Point Reyes and Fort Ross. This model implies that much of the 1906 rupture zone may have occurred with relatively little 10 to 20 sec radiation. Consideration of the amplitude and frequency content of the 1906 teleseismic data allowed us to estimate the scale length of the largest asperity to be less than about 40 km. With rough constraints on the largest asperity (size and magnitude) we produced a suite of estimated synthetic ground velocities assuming a slip distribution similar to that of the Loma Prieta earthquake but with three times as much slip. For purposes of comparison with the recent, abundant Loma Prieta strong motion data set, we “moved” the largest 1906 asperity into Loma Prieta region. Peak ground velocity amplitudes are substantially greater than those recorded during the Loma Prieta earthquake, and are comparable to those predicted by the attenuation relationship of Joyner and Boore (1988) for a magnitude M_W = 7.7 earthquake

Synthesis of San Fernando strong-motion records

Three-dimensional models of a finite fault located in a half-space are constructed to study the ground motions from the 9 February 1971 earthquake as observed at JPL, Palmdale, and Lake Hughes (Array Station #4). The Cagniard-De Hoop Technique is used to compute the ground motions due to infinitesimal point sources which are evenly distributed (0.5 km spacing) on the fault plane. The responses are summed with time lags determined by the assumed hypocentral solution and rupture velocity. Nonuniform fault displacement is modeled by varying the weights of individual point sources. By investigating the motion due to small sections of the fault it is possible to understand how various wave types interfere to produce the motion due to the total fault. Recent modeling of teleseismic body waves by Langston has indicated that the fault changes dip from 50° to 30° at a depth of approximately 5 km. This feature has been incorporated into our models. The assumed fault geometry and station locations are shown in Figure 1. In Figure 2, we display assumed fault displacements for a preliminary model which is used to explain the motions at JPL, PLM, and LKH. The overall moment for this model is 1.5 x 10^(26) ergs. The hypocenter is assumed to lie in the region of maximum displacement and a rupture velocity of 1.8 km/sec (as suggested by Langston) is also assumed. Although stations LKH and JPL are situated at roughly equal epicentral distances, there appears to be a dramatic difference in the character and amplitudes of ground motion seen for these stations. This can be seen in Figures 3 and 4. In these figures, the synthetic ground motions for the fault model described above are compared with the integrated accelerograms for these stations. Because the integrated accelerograms have been filtered with an 8 sec. Ormsby filter, the synthetics are displayed both with and without the inclusion of this filter. Although it appears that the particular fault model used for Figures 3 and 4 is not, in detail, correct, it does well at explaining the differences in character and amplitude of ground motions as seen between JPL and LKH. An examination of Figure 5 helps one to appreciate the complex interplay between source and wave propagational effects. In this figure the fault is subdivided into 5 strips each of which has a width of 4 km. Also shown are synthetic motions (JPL, North) for a single point source located in the middle of each subfault. Although these point sources produce easily interpreted specific arrivals, it is clear that the JPL record results from complex and not easily interpreted interaction of both source and propagation effects. These synthetics also demonstrate the dramatic effect of the free-surface. Rayleigh wave and sP head wave contributions are of great importance

Southern California Seismographic Network; report to the U.S. Geological Survey, August 21, 1990

On August 21, 1990, the U. S. Geological Survey held a meeting to review the status of regional seismic networks in the United States. The purpose of the meeting was to provide information to the U.S.G.S. to assist them in setting priorities for future funding of seismic networks in a time of increasingly tight budgets. Each of the networks was therefore asked to prepare a report describing their goals and accomplishments. Three specific questions were raised: how the objectives of the network have been met, the potential for future productivity and opportunities for additional funding