Showing papers on "Hypocenter published in 1982"

Three-dimensional finite difference simulation of fault dynamics: Rectangular faults with fixed rupture velocity

Abstract: We analyze three-dimensional finite difference solutions for a simple shear-crack model of faulting to determine the effects of fault length and width on the earthquake slip function. The fault model is dynamic, with only rupture velocity, fault dimensions, and dynamic stress-drop prescribed. The numerical solutions are accurate for frequencies up to 5 Hz, and are combined with asymptotic results for shear cracks in order to characterize the slip function at higher frequencies. Near the hypocenter, the slip velocity exhibits a square-root singularity whose intensity increases with hypocentral distance. At distances greater than the fault width, w , growth of the velocity intensity ceases, and the slip function becomes nearly invariant with distance along the fault length. Closed-form expressions are developed for the dependence of static slip ( s ∞), slip rise time ( TR ), and slip velocity intensity ( V ) on fault geometry. Along the center line of a long, narrow fault, at hypocentral distances exceeding w , these expressions reduce to s ∞ ≈ w Δτ/μ, TR ≈ 0.5 w/vR , and V ≈ √ w /2 vR Δτ/μ, where Δτ is the dynamic stress drop, μ the shear modulus, and vR the rupture velocity. The numerical results imply that uniform-dislocation kinematic earthquake models in which slip is represented by a ramp time function will underpredict high-frequency ground motion relative to low-frequency ground motion. A further implication of the numerical solutions is that the nature of inelastic processes at the advancing edge of a long fault will depend on fault width, but will be independent of rupture length.

Analysis of near-source static and dynamic measurements from the 1979 Imperial Valley earthquake

Abstract: Gross features of the rupture mechanism of the 1979 Imperial Valley earthquake ( ML = 6.6) are inferred from qualitative analysis of near-source ground motion data and observed surface rupture. A lower bound on the event's seismic moment of 2.5 × 1025 dyne-cm is obtained by assuming that the average slip over the whole fault plane equals the average surface rupture, 40.5 cm. Far-field estimates of moment suggest an average slip over the fault plane of 105 cm, from which a static stress drop of 11 bars is obtained. An alternative slip model, consistent with the far-field moment, has 40.5 cm of slip in the upper 5 km of the fault and 120 cm of slip in the lower 5 km. This model suggests a static stress drop of 39 bars. From the lower estimate of 11 bars, an average strain drop of 32 µstrain is derived. This strain drop is four times greater than the strain that could have accumulated since the 1940 El Centro earthquake based on measured strain rates for the region. Hence, a major portion of the strain released in the 1979 main shock had been accumulated prior to 1940. Unusually large amplitude (500 to 600 cm/sec2) vertical accelerations were recorded at stations E05, E06, E07, E08, EDA of the EI Centro array, and the five stations of the differential array near EDA. Although the peak acceleration of 1705 cm/sec2 at E06 is probably amplified by a factor of 3 due to local site conditions, these large amplitude vertical accelerations are unusual in that they are evident on only a few stations, all of which are near the fault trace and at about the same epicentral range. Two possible explanations are considered: first, that they are due to a direct P wave generated from a region about 17 km north of the hypocenter, or second, that they are due to a PP phase that is unusually strong in the Imperial Valley due to the large P -wave velocity gradient in the upper 5 km of the Imperial Valley. Based on the distribution of both the horizontal and vertical offsets, it is likely that the rupture went beyond stations E06 and E07 during the main shock. By exploiting the antisymmetry of the parallel components of particle velocity between E06 and E07 and by examining polarization diagrams of the particle velocity at E06 and E07, an average rupture velocity in the basement of 2.5 to 2.6 km/sec between the hypocenter and station E06 is obtained. In addition, several lines of evidence suggest that the Imperial fault dips about 75° to the NE.

The foreshock sequence of the February 4, 1975, Haicheng Earthquake (M = 7.3)

Abstract: We have examined the locations and radiation patterns of the foreshocks of the February 4, 1975, Haicheng earthquake (M = 7.3). Using arrival times from six local seismic stations, the foreshocks and mainshock were located relative to a master event. The foreshocks occurred in a tight cluster that elongated with time. Before the largest foreshock, the activity was located within a small, approximately equidimensional volume with a diameter of about 2 km. After the largest foreshock, the activity spread northwest and southeast forming a 6-km-long, northwest trending zone. First motions and ratios of P to S amplitudes indicate that two different faulting mechanisms occurred during the foreshock sequence. The two radiation patterns can tentatively be correlated with different parts of the zone. The hypocenter of the mainshock was not located on the same fault as that defined by the foreshocks' hypocenters but rather was located 6 km south of and several kilometers shallower than the foreshock cluster. We think this large separation between foreshocks and mainshock in a direction perpendicular both to the plane of rupture of the mainshock and to the trend of the foreshocks might be the result of an en echelon step in the fault that slipped during the mainshock. An analysis of the change in stress due to slip during the foreshocks shows that the increase in shear stress on the mainshock fault caused by the foreshocks is very small and that direct triggering of the mainshock by the foreshocks is unlikely.

Models of the complex source of the El Asnam earthquake

Abstract: The El Asnam earthquake of 10 October 1980 was originated by a thrust across a SW-NE-oriented fault 36 km long and 15 km wide. It is a complex event with a rupture propagating from the SW to the NE. Fault mapping, hypocenter distribution, and b factor studies indicate fault branching to the NE. A dislocation model is used to explain the branch with normal faulting at Beni-Rached as a consequence of the thrust across a main fault that changes orientation. The mechanism of the 1954 earthquake must be similar even though only normal faulting was observed at the surface.

The rupture mechanism of the Coyote Lake earthquake of 6 August 1979 inferred from near-field data

Abstract: The Coyote Lake earthquake ( ML = 5.7) occurred within the dense network of seismic instruments of central California and was recorded by numerous accelerograph stations. Two of these stations, located within the fault zone itself, provide particularly remarkable records of the rupture process. We use these data obtained in the immediate vicinity of the fault and the broadband seismograms recorded at Berkeley, about 100 km away from the epicenter, to infer the velocity of propagation of the rupture, the extent of the fractured area, the rupture front geometry, and other characteristics of the fracture mechanism. In order to extract this information from the data, we model the earthquake as a propagating dislocation starting at the hypocenter and spreading radially. We compute the resulting ground motion at the receiver sites using the discrete wavenumber representation of Green's functions (Bouchon, 1981). The results yield a rupture velocity of 2.6 km/sec, a fault length of about 14 km and a fault slip of 15 to 20 cm. They also show that the fault extends to shallower depth than indicated by the aftershocks. The most important result is the finding that slip, at any given location on the fault, takes place in a very short time span and that simple uniform dislocation models give a good description of the rupture process.

Three-dimensional velocity structure beneath the Kanto district, Japan.

Abstract: Three-dimensional velocity structure under the Kanto district, Japan, is determined by inversion of the P-wave arrival time data from the earthquakes occurring under this district using the method originally due to AKI and LEE (1976). It is found that the crustal structure (0-32km depth) is dominated by a low velocity zone centered in Tokyo Bay, which coincides with the low Bouguer anomaly as well as the thick surficial layer. In the mantle just beneath the crust (32-65km depth), the velocity is high in northeastern and southwestern Kanto and is low in northwestern Kanto and the northeast part of Chiba prefecture. The velocity variation in this depth range is estimated to be 6-7% (±2.5%). At depths of 32-65km, the northeast high velocity zone corresponds to the subduction of the Pacific plate. The southwest high velocity zone correlates with the seismic zone inclining from the Sagami trough toward the northeast. It has been suggested that this seismic zone corresponds to the subduction of the Philippine Sea plate from studies on hypocenter distribution, seismic intensity distribution and focal mechanism solutions. The high velocity characteristic found in the present study confirms this suggestion. The low velocity zone beneath the northeast part of Chiba prefecture roughly coincides with the low-Q zone found in the spectral study of shear waves. We suggest that this low-V, low-Q zone is caused by the downward bending of the Pacific plate.

Source dynamics of the 1979 Imperial Valley earthquake from near-source observations (of ground acceleration and velocity)

Abstract: Two sets of observations obtained during the 15 October 1979 Imperial Valley earthquake, MS 6.9, are presented. The data suggest different dynamic characteristics of the source when viewed in different frequency bands. The first data set consists of the observed residuals of the horizontal peak ground accelerations and particle velocity from predicted values within 50 km of the fault surface. The residuals are calculated from a nonlinear regression analysis of the data (Campbell, 1981) to the following empirical relationships, PGA = A 1 ( R + C 1 ) − d 1 , PGV = A 2 ( R + C 2 ) − d 2 in which R is the closest distance to the plane of rupture. The so-calculated residuals are correlated with a positive scalar factor signifying the focusing potential at each observation point. The focusing potential is determined on the basis of the geometrical relation of the station relative to the rupture front on the fault plane. The second data set consists of the acceleration directions derived from the windowed-time histories of the horizontal ground acceleration across the El Centro Differential Array (ECDA). The horizontal peak velocity residuals and the low-pass particle acceleration directions across ECDA require the fault rupture to propagate northwestward. The horizontal peak ground acceleration residuals and the high-frequency particle acceleration directions, however, are either inconclusive or suggest an opposite direction for rupture propagation. The inconsistency can best be explained to have resulted from the incoherence of the high-frequency radiation which contributes most effectively to the registration of PGA. A test for the sensitivity of the correlation procedure to the souce location is conducted by ascribing the observed strong ground shaking to a single asperity located 12 km northwest of the hypocenter. The resulting inconsistency between the peak acceleration and velocity observations in relation to the focusing potential is accentuated. The particle velocity of Delta Station, Mexico, in either case appears abnormally high and disagrees with other observations near the southeastern end of the fault trace. From the observation of a nearly continuous counterclockwise rotation of the plane of P -wave particle motion at ECDA, the average rupture velocity during the first several seconds of source activation is estimated to be 2.0 to 3.0 km/sec. A 3 km upper bound estimate of barrier dimensions is tentatively made on the basis of the observed quasiperiodic variation of the polarization angles.

A direct measurement of the distance between a hypocenter in a Benioff‐Wadati Zone and the Slab‐Asthenosphere contact

Abstract: We made a direct measurement of the distance from a hypocenter in the Benioff-Wadati zone to the boundary between the downgoing slab and overlying asthenosphere. This was accomplished by identifying a low amplitude P wave reflection off the slab-asthenosphere contact which arrives at teleseismic stations several seconds after the initial P arrival. The measured delay time (after P) of the reflected phase shows a consistent azimuthal variation, thus eliminating possible source or receiver effects. The relative amplitude, polarity, and delay time of this observed phase are consistent with a model in which the distance between hypocenter and slab-asthenosphere contact at this depth is about 38±5 km. The event studied was an intermediate depth (181 km) earthquake in the southern Kuriles which occurred in the lower layer of a double seismic zone. Our measured distance, when compared to the distance between the two seismic zones, indicates that the upper layer of seismicity is within 15 km of the slab-asthenosphere contact. Appendix is available with entire article, on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, U.C. 20009. Document JB1-010; $1.00. Payment must accompany order.

Hypocenter for the 1979 Imperial Valley Earthquake

Abstract: Using P- and S-wave arrival times with the laterally varying P-wave velocity structure derived from analysis of a refraction survey of the Imperial Valley, a hypocenter is ascertained for the October 15, 1979, Imperial Valley earthquake: Latitude 32/sup 0/39.50' N, Longitude 115/sup 0/19.80' W, Depth 8.0 km, Time 23:16:54.40 GMT.

P-velocity models and earthquake locations in the Livermore Valley Region, California

Abstract: P-wave travel times from 223 well-recorded earthquakes and five timed explosions recorded by the US Geological Survey-Lawrence Livermore National Laboratory seismic network are inverted simultaneously for one- and three-dimensional velocity structures and hypocenter locations. The stability of earthquake locations between the one- and three-dimensional inversions demonstrates the adequacy of a layered earth model combined with station corrections for routine hypocenter locations in the Livermore Valley. Initial one-dimensional velocity inversions indicate a fairly strong positive velocity gradient in the upper crust of about 6 km. Three-dimensional models show that the strongest velocity contrasts occur along the eastern and southern edges of the valley where lower velocity down-dropped Tertiary sediments and basin-full alluvium are found in structural contact with higher velocity Late Mesozoic and Tertiary sediments of the Altamont anticline and the Diablo Range. The low-velocity basin structure is truncated along the western margin of the Livermore Valley by the Calaveras fault across which higher velocity aftershocks from the Livermore are exposed. Some of the relocated Cretaceous and Tertiary formations earthquake sequence show a good correlation with surface trace of the northwest-trending Greenville fault. In the southeast corner of the Livermore Valley, a number of short (5 km or less), linear, andmore » discontinuous epicentral zones are observed. These seismic lineations appear to define a conjugate set of right-stepping faults in a right-lateral shear zone. Focal mechanisms for these aftershocks show many complexities, but are generally consistent with this interpretation.« less

Digital processing of regional network data

Abstract: Much more than hypocenter location can be done with digital time histories obtained from regional seismic networks. Examples are given of how these data can be routinely processed to provide readily accessible intermediate results for subsequent studies, among which are velocity inversion using teleseismic P residuals, composite focal mechanism studies, and spectral analysis. The processing must be designed to be as routine and as complete as possible. Only with these two objectives achieved can the seismic networks be as productive as they should be.

Determination of the three-dimensional seismic structure of the lithosphere using the seismic data

Abstract: A new method of inversion of the three-dimensional seismic structure of the litho-sphere using teleseismic data is developed in this paper Ray tracing is introduced into this method which shows some advantages This method can also be used to inverse simultaneously the structure of crust and the hypocenter parameters by using data from local earthquakesAn experiment using synthetic data shows that this method is efficient

Review of earthquake activity and current status of seismic monitoring in the region of the Bradley Lake Hydroelectric Project, southern Kenai Peninsula, Alaska; December 1981 - May 1983

Abstract: A project investigating seismic hazards in the Bradley Lake area of Kenai Peninsula is summarized. Hypocenters of 91 shallow (depth less than 20 km) earthquakes that occurred between December 1981 and May 1983 indicate that the pattern of recent crustal activity around the southern Kenai Peninsula has remained stable relative to the data prior to December 1981. The earthquakes are generally smaller than about magnitude 2.5. Most of the activity occurred east of the Border Ranges fault, and several concentrations can be observed in an otherwise diffuse distribution of activity. In general there is a poor correlation of the shallow activity with mapped fault traces. A more reliable estimate of the depth to the Benioff zone beneath Bradley Lake can be made from the greater number of available hypocenters of well-recorded earthquakes now available. Using the current velocity model, the depth to the top of the Benioff zone is 37 +- 5 km. A strong-motion record was obtained from the SMA-1 instrument co-located at the site of the central high-gain station BRLK, about 2 km from the proposed dam site. A preliminary estimate of 0.14 g (1 g = 980 cm/sec/sup 2/) was obtained for the maximum peak-to-peak horizontal accelerationmore » on the record, but at present the event which caused the trigger is uncertain. Two new stations were installed southeast of Kachemak Bay in June 1983 in order to improve the accuracy of hypocenter determinations for continuing shallow earthquake activity that is observed in this area. 12 refs., 9 figs., 3 tabs.« less