Showing papers on "Hypocenter published in 1981"

Teleseismic location techniques and their application to earthquake clusters in the South-Central Pacific

Abstract: Improved methods for single-and multiple-event hypocenter determinations are developed and applied to the problem of locating earthquake clusters in the South-Central Pacific Ocean. Bayesian statistical methods are used to incorporate a priori information about arrival-time variance into the derivation of hypocenter confidence ellipsoids, permitting a more realistic calculation of critical parameters in the case where the number of stations is small. The diagonal elements of certain projection operators, called “data importances” by Minster et al. (1974), are used to evaluate network balance. The hypocentroid of an event cluster is defined to be the average location of events within the cluster, and the deviations of individual hypocenters from the hypocentroid are called cluster vectors . The problem of estimating the cluster vectors can be decoupled from the problem of estimating the hypocentroid by a simple but fundamental mathematical result, here termed the hypocentroidal decomposition theorem. The algorithm based on this analysis appears to have many advantages over other published methods for multiple-event location, both in its efficient use of available information and its computational speed. The application of this method to three clusters of shallow intraplate seismicity in the South-Central Pacific, designated Regions A, B, and C, demonstrates that the seismicity within each cluster is very localized; the rms lengths of the cluster vectors for each group of epicenters are estimated to be only 9, 6, and 12 km, respectively. Estimates of the epicentroids are

Source inversion of seismic waveforms: The Koyna, India, earthquakes of 13 September 1967

Abstract: The treatment of the seismic source inverse problem, when diverse forms of waveform data are available, is simple and elegant using a moment tensor formalism. If earth structure is known and its effects predictable in terms of vertically inhomogeneous elastic-layered models, then all types of wave phenomena (e.g., surface waves, body waves, leaky modes, etc.) for a purely deviatoric moment tensor point source may be represented by, at most, a sum of three Green9s functions. For an arbitrary symmetric moment tensor point source, one additional Green9s function is needed for the P - SV system. However elegant this formalism may be for posing the linear inverse problem, the major difficulties lie in earth structure unknowns and resultant nonlinearities in the Green9s functions which can cause significant trade-offs with source parameters. A hybrid inversion procedure is set up to gain insight into the probable unknowns in particular problems by incorporating both a linearized least-squares gradient method for the moment tensor or double couple, and smoothed time function parameters, and a nonlinear systematic trial-and-error search for moment tensor or double couple parameters for several assumptions of Green9s function. The inversion technique is applied to near-regional waveform data from a small earthquake associated with the Koyna Reservoir which occurred 13 September 1967, as the second in a group of three events with very similar waveshapes, but differing amplitudes. The magnitudes of the second and third events are smaller by −0.2 and −0.8 units, respectively, compared to the first. The absolute magnitude for the first event is poorly constrained but is estimated to be 4.0 to 4.5 rather than the previously published value of 5.5 to 6.0. From the similarity of waveshapes, all three events are inferred to have the same mechanism and occurred within about 2 km of the same hypocenter. The results from moment tensor and double couple inversions for event 2 data indicate that source depth was 5 km and that left-lateral faulting occurred on a plane with a strike of N20°E ± 5°, dip of 90° ± 15°, and a rake of 0° ± 35°. The inferred far-field time function is approximately 3 sec in duration, unusually long for the seismic moment of 9 × 10 22 dyne-cm, yielding a possible stress drop of about 0.05 bars. A fault map was constructed from LANDSAT image interpretation and shows a predominance of NNW to NNE striking faults in the Koyna area which is used to infer the appropriate nodal plane in the inversion results. These faults tend to define a broad en-echelon zone which parallels the Western Ghats in this area.

The denver earthquakes of 1967-1968

Abstract: A detailed study of the earthquakes associated with the Rocky Mountain Arsenal disposal well is presented. Long-period surface-wave studies are used together with P -wave first motions to show that the 10 April, 9 August, and 27 November 1967 earthquakes occurred at depths of 3 to 5 km and were characterized by normal faulting along a northwest striking fault plane. A joint hypocenter relocation of 103 microearthquakes of a data set of 279 recorded between 1967 and 1968 shows a hypocenter pattern striking N50°W, with most of the events located about 5 km northwest of the disposal well at depths between 3 to 8 km. A fault plane dipping southwest is tenuously suggested by those earthquakes with depths less than 5 km. Modeling of near-field seismoscope observations lend support to the focal mechanisms derived.

Procedures for computing focal mechanisms from local (SV/P)z data

Abstract: A procedure for calculating the focal mechanism of an earthquake from the distribution of the ratio of the amplitudes of SV to P waves has been developed and tested. The input consists of information taken from the hypocenter solution for the earthquake, a factor for each station that corrects for the effect of the free surface on the observed wave amplitudes and the observed values of the ratio of the vertical component of P to the vertical component of SV . The procedure starts with the selection of a slip direction. The program then finds the best fault strike and dip corresponding to that slip direction, with “best” measured by the smallest scatter of the dip for any strike. All three fault parameters are then adjusted by an iterative least-squares adjustment. ( SV/P ) z is a strongly nonlinear function of the fault parameters, so the solution found by the procedure is inherently nonunique. The acceptable solutions can be quite well constrained if the slip direction can be estimated initially on the basis of independent information. A variety of tests of the procedure have been carried out on real and synthetic data. Given a set of amplitude data for an earthquake in Bear Valley, California, and told only that the mode of slip was predominantly strike-slip, the program “found” the San Andreas fault, i.e., converged to a fault with strike and dip close to the known values. Further work on a German earthquake served to bring out some of the ambiguities of the solutions. Analysis of data from a brief swarm of earthquakes in southwest Germany showed that the method yields useful mechanisms for small events and revealed a rotation of the focal mechanisms during the swarm. Processing of amplitudes from synthetic seismograms gave a fault plane close to the known input, but marked disagreement between the SV amplitudes on the synthetic seismograms and the amplitudes predicted by simple dislocation theory is unresolved.

Quasi-dynamic models of simple earthquakes: Application to an aftershock of the 1975 Oroville, California, earthquake

Abstract: A class of kinematic rupture models for simple earthquakes is proposed; these models incorporate results from both theoretical and numerical solutions to the mixed boundary value problem of a three-dimensional frictional rupture. The analytic form for the slip velocity is divided into two phases: the rupture growth, during which the slip distribution is “self-similar” (Kostrov, 1964); and the healing, during which the slip velocity decreases monotonically to zero. The arrival of a P -wave stopping phase, propagating into the interior of the rupture area from the fault perimeter, initiates the healing. The duration of healing may be varied to produce a gradual or abrupt stopping of the rupture, which permits modeling a wide range of high-frequency spectral behavior. These models are used to generate synthetics which are fit to the velocity and the velocity-squared pulse shapes obtained from strong motion accelerograms of the 0103 aftershock of the 1975 Oroville, California, earthquake. The aftershock ( ML = 4.6) has a moment of 1.8 × 1023 dyne-cm and a total radiated energy of 2.7 × 1019 dyne-cm. The fitted source model propagates updip with a rupture velocity of 0.85 β and a dynamic stress drop of 210 bars. The arrival times of five “stopping phases”, correlated across the suite of accelerograms, are used to determine the relative locations of the hypocenter and five stopping events. The description of rupture determined from these stopping events both corroborates the source model obtained from the waveform modeling and establishes an important counterpoint to the model by providing a dynamic interpretation of the unmodeled complexity of the waveforms.

The Bermuda earthquake of March 24, 1978: A significant oceanic intraplate event

Abstract: The Bermuda earthquake (M ∼ 6) occurred near the westerly extension of the Kane Fracture Zone roughly 370 km southwest of the island of Bermuda. It is one of the largest oceanic intraplate earthquakes to occur off the eastern coast of North America. Because of its size and location, it has provided an excellent set of WWSSN body waves. They can be used to infer its depth and faulting parameters by waveform modeling techniques. The results indicate a north-northwest striking thrust mechanism (strike = N20°W, dip = 42°NE, rake = 90°) with the hypocenter located at a depth of 11 km, which for an oceanic crust places it predominantly in the mantle. The event had a seismic moment of 3.4 × 1025 dyne cm, and its time history was modeled with a symmetric trapezoidal time function 3 s in duration. The north-northwest strike of the event is in good agreement with the bathymetry of the area, the epicenter being close to the southwestern edge of the Bermuda Rise. The strike of the event is also close to that of the inferred extensions of the present ridge fracture zones in the region. The presence of such fracture zones is indicative of local weak zones in the lithosphere. The Bermuda earthquake most likely is associated with one of these zones of weakness and is the result of the application of present day stress imposed on the region by the North American plate in the direction of its absolute motion. This is an important event in terms of understanding and estimating seismic hazard on the eastern seaboard of North America.

Patterns of local seismicity preceding the petatlan earthquake of 14 March 1979

Abstract: Local seismic activity ( M > 2.3) during the 4 1 2 -day period preceding the Petatlan earthquake of 14 March 1979 ( Ms = 7.6) was monitored by a seismic network deployed by the Hawaii Institute of Geophysics. These data allow us to revise the location of that event, and to study its pattern of foreshocks. The foreshock hypocenters are concentrated above the ocean/continent plate boundary and landward of the hypocenter of the main shock. The spatial distribution of foreshocks suggests that the foreshocks occurred along existing zones of weakness within the continental plate. During the 1 1 2 -day period preceding the Petatlan event, foreshocks were concentrated within 100 km of the epicenter of that earthquake. The pattern of precursory seismic activity we observed during that period is similar to that observed during the same time period prior to the Oaxaca earthquake of 29 November 1978. However, the 17-hr period of seismic quiescence (for events with M > 2.8) preceding the Oaxaca earthquake is not found in our data where a magnitude 4 foreshock occurs within 28 min of and 2 km from the Petatlan event. This suggests that the spatially larger set of foreshocks may have greater diagnostic value than does the sequence of precursory events within 30 km of the main shock.

Results of recent South Carolina seismological studies

Abstract: Understanding of the South Carolina seismic zone and the 1886 Charleston earthquake is much improved today as a result of seismological studies which began in 1973. A permanent seismic network was installed in 1974 in South Carolina between Charleston on the Atlantic Coast and Columbia in the central part of the state. Temporary field networks of portable instruments were established in the epicentral areas of earthquakes near reservoirs in the northwest part of the state. The seismic network data have provided precise hypocenter determinations in previously known zones of continuing activity near Middleton Place and Summerville, Bowman, and in a newly identified source of seismicity near Adams Run. Several new composite focal mechanisms were determined, and abundant aftershock data were acquired. Results of recent geophysical and geological studies, when combined with the seismological results, indicate two seismic regimes in South Carolina. The first regime covers the buried basement structure of the middle and lower Coastal Plain province, which has been shown by geophysical studies to be quite unlike Piedmont province structures to the northwest. The Middleton Place-Summerville and Bowman seismic zones are associated with buried early Mesozoic basins and large, elongate mafic intrusives inferred from coincident positive aeromagnetic and gravity anomalies. The current seismicity is concentrated in clusters rather than distributed along a northwest-trending fault zone as suggested earlier by various workers. The second regime covers the exposed Piedmont province and the upper Coastal Plain. Earthquake activity may be associated with strain release on or near mapped faults or contacts between metamorphic belts. In particular, studies of aftershock and microearthquake activity in the vicinity of reservoirs indicates that in areas of high ambient stress, the seismicity appears to be induced by fluctuations in the impounded water and pore pressure at depth.

Microearthquake activity on the Orozco Fracture Zone: Preliminary results from Project ROSE

Abstract: We present preliminary hypocenter determinations for 52 earthquakes recorded by a large multi-institutional network of ocean bottom seismometers and ocean bottom hydrophones in the Orozco Fracture Zone in the eastern Pacific during late February to mid-March 1979. The network was deployed as part of the Rivera Ocean Seismic Experiment, also known as Project ROSE. The Orozco Fracture Zone is physiographically complex, and the pattern of microearthquake hypocenters at least partly reflects this complexity. All of the well-located epicenters lie within the active transform fault segment of the fracture zone. About half of the recorded earthquakes were aligned along a narrow trough that extends eastward from the northern rise crest intersection in the approximate direction of the Cocos-Pacific relative plate motion; these events appear to be characterized by strike-slip faulting. The second major group of activity occurred in the central portion of the transform fault; the microearthquakes in this group do not display a preferred alignment parallel to the direction of spreading, and several are not obviously associated with distinct topographic features. Hypocentral depth was well resolved for many of the earthquakes reported here. Nominal depths range from 0 to 17 km below the seafloor.

Joint analysis of seismological data by the USSO and DSS stations in the Caucasus region

Abstract: This paper deals with a procedure of a joint analysis of seismic data from earthquakes and those obtained by DSS. The DSS data are used as a first approximation to construct a two-dimensional model of the medium made up of individual blocks. These models serve as a basis when constructing specific three-dimensional travel-time curves. These travel-time curves are further used for the calculation of hypocenter parameters in a laterally inhomogeneous block medium. The hypocenter field and the travel times obtained are input data for the computation of three-dimensional fields of velocities in earthquake focal zones. Results of applying the proposed procedure to the Caucasus region are presented.