Showing papers on "Hypocenter published in 1976"
TL;DR: In this article, the arrival times from an ensemble of discrete earthquakes independently contain information on hypocenter locations and jointly provide information on the velocity model, and a properly formulated least squares estimation procedure can be used to determine simultaneously both hypocenter and velocity model parameters.
Abstract: Arrival times from an ensemble of discrete earthquakes independently contain information on hypocenter locations and jointly provide information on the velocity model. A properly formulated least squares estimation procedure can be used to determine simultaneously both hypocenter and velocity model parameters. By means of this procedure a sufficiently well distributed set of earthquakes provides information on crustal structure without some of the inherent difficulties associated with explosion refraction interpretation. Useful error estimates and resolving information are provided, and tests on artificial data indicate that successful inversions are obtainable, even for models with low-velocity zones, by using only P wave first arrivals at a number of stations. The method may have application for detecting P wave low-velocity regions resulting from dilatancy and is directly extendable to modeling shear wave velocity structure when S arrivals are used in place of P arrivals.
331 citations
TL;DR: In this article, a nonlinear least squares modeling procedure was developed to estimate simultaneously hypocenter parameters, station corrections, and velocity model parameters by using P wave (or S wave) arrivals from local earthquakes at a regional array.
Abstract: A nonlinear least squares modeling procedure has been developed to estimate simultaneously hypocenter parameters, station corrections, and velocity model parameters by using P wave (or S wave) arrivals from local earthquakes at a regional array. This procedure is applied to P wave data obtained from the 14-station telemetered seismograph network in the Puget Sound region of western Washington. Forty selected earthquakes with a depth distribution from near the surface to over 50 km were used, providing a stable inverted model having a large step increase in velocity in the upper 10 km of the crust and exhibiting a comparatively low velocity gradient between 10- and 40-km depths. Transition to an upper mantle velocity of 7.8 km/s is indicated at approximately the 41-km depth, earlier measurements of an anomalously low upper mantle velocity west of the Cascade Range thus being confirmed. The inverted model indicates the presence of a low-velocity zone at the base of the crust, although the exact configuration of this zone is difficult to determine because of a lack of resolving power at critical depths. Relocation of a number of earthquakes using the derived model indicates a substantial reduction in average residuals and a corresponding increase in the general confidence level of hypocenter locations for the region.
94 citations
TL;DR: The M L 6.0 Point Mugu, California earthquake of February 21, 1973 and its aftershocks occurred within the complex fault system that bounds the southern front of the Transverse Ranges province of southern California as discussed by the authors.
Abstract: The M L 6.0 Point Mugu, California earthquake of February 21, 1973 and its aftershocks occurred within the complex fault system that bounds the southern front of the Transverse Ranges province of southern California. P -wave fault plane solutions for 51 events include reverse, strike slip and normal faulting mechanisms, indicating complex deformation within the 10-km broad fault zone. Hypocenters of 141 aftershocks fail to delineate any single fault plane clearly associated with the main shock rupture. Most aftershocks cluster in a region 5 km in diameter centered 5 km from the main shock hypocenter and well beyond the extent of fault rupture estimated from analysis of body-wave radiation. Strain release within the imbricate fault zone was controlled by slip on preexisting planes of weakness under the influence of a NE-SW compressive stress.
46 citations
TL;DR: The distribution in space and time of foreshocks and aftershocks suggests that rupture began at depth and progressed up dip and to the north and south as discussed by the authors, which is consistent with the distribution of pre-shocks.
Abstract: On August 1, 1975, a magnitude 5.9 ( m b ) earthquake occurred approximately 8 km SSE of the town of Oroville, California. This earthquake and its associated foreshock-aftershock sequence are of particular interest because of their possible relation to the impounding of the 4.3 billion m 3 Lake Oroville. Hypocenter locations for 336 aftershocks that occurred during August define a fault plane striking N3°E and dipping 60° to the west to a depth of 10 km. Dimensions of the epicentral area are approximately 7 km in an east-west direction by 15 km in a north-south direction. The fault plane passes beneath Oroville Dam at 5-km depth, and if projected up dip, would crop out beneath the reservoir to the east. The distribution in space and time of foreshocks and aftershocks suggests that rupture began at depth and progressed up dip and to the north and south.
43 citations
TL;DR: In this paper, the source process of a large deep-focus earthquake (h=643 kin) which occurred in the Sea of Okhotsk in 1970 is studied on the basis of the seismic body waves recorded on the WWSSN long-period seismograms.
Abstract: The source process of a large deep-focus earthquake (h=643 kin) which occurred in the Sea of Okhotsk in 1970 is studied on the basis of the seismic body waves recorded on the WWSSN long-period seismograms. The P-wave source functions are obtained from the long-period P-wave signals by eliminating the effects of the seismograph, crust and mantle. The interpretation of the source functions yields the following results: this shock is a triplet and three events are located at nearly the same hypocenter; the rupture corresponding to each event spreads out separately but finally covers a more or less fan-shaped surface. The source parameters are estimated from the comparison of the P-wave source functions with the calculated P-wave displacements. Seismic moment Mo, rupture velocity c, fault area S, displacement discontinuity D and stress drop da are estimated as follows: M0=1.1×1027 dyne ·cm, c-4.0 km/sec, S=420 km2, D=1.9 m and Δσ= 320 bars. The tectonic implications of the source parameters and the orientation of the earthquake-generating stress are discussed.
10 citations
TL;DR: In this paper, the authors present a P nodal solution with a pressure axis with a north-south orientation and a tension axis with an east-west orientation for the right lateral strike-slip fault on a near vertical fault, striking in a northnorthwest direction.
Abstract: An earthquake occurred at 10 h 16 m 39 s GMT on 5 July 1972 in close proximity to the west coast of Vancouver Island, near 49.5 °N and 127.2 °W. Its magnitude (mb) was 5.7 and the hypocenter was near the middle of the crust. A well defined P nodal solution has a pressure axis with a north-south orientation and a tension axis with an east-west orientation. The preferred nodal plane suggests right lateral strike-slip faulting on a near vertical fault, striking in a north-northwest direction. The other nodal plane suggests that left lateral strike-slip faulting on a near vertical fault striking in an east-northeast direction is also a plausible solution. A field study with portable seismographs revealed that very few aftershocks were associated with this earthquake. The largest aftershock had a magnitude (ML) of 3.4. The distribution of intensity of the mainshock observed on Vancouver Island differs from that predicted by the intensity versus distance relation presently used for western Canada.
7 citations
TL;DR: In this article, the authors analyzed 30 stations in the vicinity of Hollister, California, for P -velocity variations prior to the magnitude 5.1 November 28, 1974, Thanksgiving Day earthquake.
Abstract: Teleseismic arrivals at 30 stations in the vicinity of Hollister, California, have been analyzed for P -velocity variations prior to the magnitude 5.1 November 28, 1974, Thanksgiving Day earthquake. The teleseismic arrivals for large circum-Pacific source events during the period of May through December, 1974, have been analyzed using the method of two-station relative residuals with a correction for observed azimuthal variations. The data contain no significant travel-time delays associated with this event to within 0.1 sec and all 30 stations analyzed exhibited stable travel-time residuals during the study period. In particular, residuals at station OCR for South and Central American sources whose ray paths pass within 2.5 km of the hypocenter exhibit no time changes. Assuming at least a 10 per cent P -velocity charge for anomalous zones, these observations restrict the maximum possible P -velocity anomaly zone that might have been associated with this particular event to within 3 to 4 km of the hypocenter.
7 citations
01 Apr 1976
TL;DR: In this article, historic seismicity maps of the Shumagin seismic gap vicinity show the relatively low level of seismic activity in the gap, a possible offset in the Benioff zone, and a few events associated with the Bering shelf continental margin south of the Pribilof Islands.
Abstract: This report presents: (1) historic seismicity maps of the Shumagin seismic gap vicinity which show the relatively low level of seismic activity in the gap, a possible offset in the Benioff zone, and a few events associated with the Bering shelf continental margin south of the Pribilof Islands; (2) a seismicity map and hypocenter cross-sections based on data from the local Shumagin Islands seismic network which show a very well developed Benioff zone beneath the Shumagin Islands; (3) studies of the focal mechanisms of April 6, 1974 Shumagin Islands earthquake and the February 2, 1975 Near Islands earthquakes which respectively show thrust faulting perpendicular to the arc and right-lateral strike-slip faulting oblique to the arc along the slip direction inferred from the relative motion of the plates; (4) the first strong motion tians, which show accelerograph data from the eastern Aleutians, which show accelerations consistent with those observed for earthquakes in California; (5) this year's geodetic leveling results which when compared with those from 1972 indicate that regional tilting due to tectonic strain accumulation, if it is occuring, is less than 1.9 microradians per year; and (6) seismic observations of two Strombolian eruptive cycles of Pavolf Volcano which reveal monochromaticmore » wave trains which are identical one to the next suggesting a common source region and a harmonic source function or very efficient filtering of body waves by the pyroclastic layers of the volcano.« less
1 citations
31 Oct 1976
TL;DR: In this article, three-dimensional seismic ray tracing leads to improved hypocenter locations and focal mechanism solutions, and relations of events to upper mantle structure to under the Aleutian arc are classified.
Abstract: : Three-dimensional seismic ray tracing leads to improved hypocenter locations and focal mechanism solutions Relations of events to upper mantle structure to under the Aleutian arc are classified Long-period P waves are very weak on HGLP seismograms of explosions, but long-period SV waves are seen Long period body wave spectra can be used to fix the depth of crustal earthquakes Body wave spectra, magnitudes, and focal mechanisms for events in from seismic zones of central Asia have been determined