Showing papers on "Hypocenter published in 1991"

Seismological studies at Parkfield. I. Simultaneous inversion for velocity structure and hypocenters using cubic B-splines parameterization

Abstract: A model for the 3D P - and S -wave velocity structure at Parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface Vibroseis sources, recorded by nearby CALNET stations and by the local borehole-installed 10-station High-Resolution Seismic Network there. A modification of the linear B-splines model parameterization method of Thurber (1983) to a cubic B-splines formulation, through tests inverting synthetic data sets for the Parkfield configuration, is found to produce smooth velocity distributions that still retain some small-scale features. Application of the method to 953 P -wave and 288 S -wave arrivals from 111 microearthquakes and 8 Vibroseis locations throughout the study area yields a velocity structure having variable resolution. A low-velocity zone appears to coincide with the creeping segment of the San Andreas fault zone northwest of the 1966 Parkfield earthquake epicenter. An increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone. The higher velocities to the southwest apparently correspond to the granitic basement. The contrast across the fault is subdued to the southeast along the 1966 rupture zone. A high Vp/Vs ratio of 1.9 is seen in a 2-km-wide volume near the location of the 1966 main shock hypocenter, presumably the nucleation zone for the expected M = 6 Parkfield earthquake.

Simultaneous study of the source, path, and site effects on strong ground motion during the 1989 Loma Prieta earthquake: A preliminary result on pervasive nonlinear site effects

Abstract: We considered the effects of source, propagation path, and local site conditions on strong ground motion from the Loma Prieta earthquake in the following steps. First, the local site effect was removed from the strong motion records applying the weak-motion amplification factors estimated from nearby stations of USGS regional network using the coda method. Then, we confirmed the approximate validity of the 1/ R law of geometrical spreading factor for the crustal model applicable to the region, and we determined the attenuation parameter Q ( f ) and source spectrum S ( f ) empirically from the site-effect — corrected spectral amplitude. Both Q ( f ) and S ( f ) are consistent with the results obtained earlier for major California earthquakes. We then synthesized the time history of ground acceleration by applying the stochastic simulation technique. The agreement between the observed and predicted records was good for duration and spectral content. We found, however, a strong systematic discrepancy with regard to peak acceleration; namely, for distances less than 50 km, the predicted peak acceleration consistently overestimated the observed for sediment sites and underestimated it for the Franciscan formation sites. Since the above systematic discrepancy applies to all the available stations within about 50 km from the hypocenter, it is unlikely that the effect is due to the radiation pattern, near-source structure, or topography. It appears that the strong difference in the amplification factor between the Franciscan and the sediment sites observed for weak motion disappears at acceleration levels higher than about 0.1 to 0.3 g . If this is due to the nonlinear response of the sediment sites, it is in the range expected from geotechnical engineering studies.

Near-source modeling of the Loma Prieta earthquake: Evidence for heterogeneous slip and implications for earthquake hazard

Abstract: I have analyzed records from 20 strong-motion instruments that recorded the 1989 Loma Prieta earthquake ( ML = 6.9) to determine the spatial and temporal distribution of slip using a tomographic back-projection technique. I find that the earthquake ruptured bilaterally at approximately 80% of the local shear-wave velocity over a distance of slightly more than 30 km from 13 km northwest to 20 km southeast of the hypocenter. Slip within the rupture zone was highly variable both in magnitude and direction. Slip at the hypocenter and immediately up dip was relatively low, about 1 m. There were two main areas of concentrated slip, one centered 7 km northwest of the hypocenter at a depth of 14 km and the other centered 6 km southeast of the hypocenter at a depth of 12 km. Peak slip amplitude on these high-slip patches exceeded 4.5 m. A surprising aspect of the rupture model is that rake varies from being predominately strike-slip to the southeast of the hypocenter to being predominately reverse slip to the northwest. Despite this variation in rake, the average rake determined from the strong-motion data is in agreement with that determined teleseismically. There is a correlation between areas of high slip and areas of low aftershock activity. A simple model to explain this observation is that the areas of high slip are also areas of high strength, which slip only during infrequent large mainshocks. Other areas of the fault are weaker and may slip both seismically and aseismically. Most of the slip in the Loma Prieta earthquake occurred between 9 and 16 km depth on a structure that dips to the southwest and runs underneath the surface trace of the San Andreas fault at a depth of 8 to 10 km. This observation complicates the assessment of seismic hazard on the southern Santa Cruz Mountains segment of the San Andreas fault.

Deep structure of an arc-continent collision: Earthquake relocation and inversion for upper mantle P and S wave velocities beneath Papua New Guinea

Abstract: We examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction, by analyzing the seismicity and upper-mantle velocities in the New Guinea arc-continent collision. Earthquakes in the Papua New Guinea (PNG) region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities, using P and S wave arrival times recorded by seismographs in PNG. A total of 957 well-recorded earthquakes were chosen for use in three-dimensional velocity inversions from an initial catalog of 12,960 events, based on the stability of initial hypocenter relocations. Constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters, and to exploit a priori knowledge of the geometry of velocity anomalies. Spherical geometry was used to accurately calculate long ray paths (up to 1500 km in length). Most of the ∼700 best located hypocenters from the PNG data set are in the upper mantle beneath the Finisterre-Huon (FH) ranges (the newly accreted island arc terrane), beneath the Papuan Peninsula, and in the New Britain seismic zone (an oceanic subduction system). These hypocenters show a well-defined seismic zone dipping vertically or steeply to the north beneath the northern FH ranges from 125- to 250-km depth, continuous along strike with the New Britain seismic zone to the east (which shows earthquakes to 600 km depth). The steeply dipping intermediate-depth seismicity flattens near 100 km depth beneath the FH ranges and forms a subhorizontal seismic zone beneath the FH ranges. By contrast, seismicity continues upward to the surface beneath New Britain where the arc has not yet collided with New Guinea. The 100-km-deep flat zone does not continue south of the FH ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane, and no clear evidence for a south-dipping seismic zone could be found. The relationship of the flat part of the slab 100 km beneath the FH ranges and surface faulting is difficult to discern; the apparent surface suture bounding the south side of the FH ranges is 100 km directly above the southern termination of intermediate-depth seismicity. Nowhere in PNG could clear evidence for arc polarity reversal be found from seismicity. All of the well-located subcrustal earthquakes beneath the Papuan peninsula lie in a narrow horizontal line between 125- and 175-km depth that follows the center of the peninsula, and are distinctly separated from the FH seismic zone to the north. These events do not require a southward subducting plate beneath the Papuan peninsula. An alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere. Information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals. Allowing for only crustal heterogeneity reduces travel time residual variances by 40% from a simple one-dimensional structure, while allowing for additional mantle heterogeneity results in ≤ 10% further reduction for the various parameterization s attempted here. Inversions with several different block geometries show low velocities at 35- to 171-km km depth beneath the Bismarck Sea back-arc basin and high velocities just south of the intermediate depth FH seismic zone. The pattern of lateral heterogeneity supports the inference from seismicity of north-dipping slabs beneath both northeast New Guinea and New Britain.

Faulting geometry and slip from co-seismic elevation changes: The 18 October 1989, Loma Prieta, California, earthquake

Abstract: Leveling surveys conducted before and after the Loma Prieta earthquake provide observations of the co-seismic elevation changes. These data are used to determine the faulting geometry and distribution of slip, considering planar, listric, and negatively listric fault shapes. Both the planar and nonplanar models produce elevation changes consistent with the observations. Most of the observed elevation changes can be modeled with a rupture surface that extends from 4- to 15-km depth. If the rupture surface is planar, the observations require 2.4 m of right-lateral strike slip and 1.7 m of reverse slip on a 34-km-long plane that dips 60°SW. The best-fitting model faults lie above and to the southwest of the aftershock zone. A significantly better fit to the observations is obtained when these fault geometries are allowed to have two rake values, with a larger thrust component northwest of the epicenter and a larger strike-slip component southeast of the epicenter. When a low-modulus layer over a half-space is used for consistency with the seismic P -wave velocity structure, the fault deepens, coming within 3 km of the hypocenter, but still locates several kilometers southwest of most aftershocks.

Comparison of L1 and L2 norms in a teleseismic waveform inversion for the slip history of the Loma Prieta, California, earthquake

Abstract: Broadband, digital, teleseismic body waves ( P and SH) are inverted to obtain the history of slip for the 17 October 1989, Loma Prieta earthquake. Comparison of finite-fault, waveform, inversion results using an L 1 norm and an L 2 norm reveal some basic uncertainties in the rupture history that are not usually appreciated in earthquake source studies. However, the more robust features of the source can be identified that are less dependent on the choice of minimization norm. From a consideration of first-motion data and forward modeling of body waves, the strike and dip of our model fault plane are set at 126° and 67°, respectively. The waveform inversion results indicate that the majority of the moment release occurs over a fault length of 35 to 40 km and a depth range of 2 to 18 km. The hypocenter (18 km depth) experienced a relatively small amount of slip. The rupture propagated upward and bilaterally with a peak slip of about 3 m occurring at a depth of 10 km. The average velocity of the rupture propagation is 2.5 km/sec. The details of the slip distribution depend on the minimization norm, but two main sources can be identified with the larger source above and to the southeast of the hypocenter. The rake vector varies considerably over the fault. The southern source is mostly strike slip, while the northern source is approximately equally partitioned between dip slip and strike slip. There is also a trend for more strike-slip motion at shallower depths. The majority of the moment release occurs in the first 7.5 sec with an integrated moment of 2.6 × 10 26 dyne-cm. Including smaller, peripheral sources that are less well resolved raises the moment estimate to 3.0 × 10 26 dyne-cm.

Rupture history of the 1989 Loma Prieta, California, earthquake

Abstract: Strong motion records of the 1989 Loma Prieta earthquake are inverted to determine a model of the rupture history. Uncorrected horizontal and vertical accelerograms are integrated to particle velocity time histories for 38 stations within an epicentral range of 75 km. The time histories are bandpassed filtered with corners at 0.05 and 1.0 Hz. These bandpassed time histories are inverted using a nonlinear method to solve for the distribution of slip amplitudes and rupture times at specified locations on the fault plane. The fault plane is specified a priori : 38 km long and 17 km wide, extending from 3 to 19 km depth at a constant dip of 70°. Starting models have rupture times based on constant rupture velocities of 2.5, 2.8, and 3.0 km/sec and uniform slip with rise times of 0.5, 1.0, 1.5, and 3.0 sec. The waveform inversion results show the strike-slip displacement is concentrated at the southern end of the rupture (rake = 156°) and the dip-slip displacement is concentrated at the northern end of the rupture (rake = 115°). The average total slip is partitioned almost equally between strike slip and dip slip (rake = 137°). The hypocentral area has an unusually small amount of slip with almost no slip in a region just to the north and up dip from the hypocenter. The rupture front is complex, propagating up dip to the south faster than it propagates to the north. The region of maximum strike slip to the southeast radiates simultaneously with the region of maximum dip slip to the northwest. The average rupture velocity is 3.0 km/sec, approximately 0.83 times the local shear wave speed. The calculated seismic moment is 3.5 ± 0.5 × 1026 dyne-cm.

Application of an iterative least-squares waveform inversion of strong-motion and teleseismic records to the 1978 Tabas, Iran, earthquake

Abstract: An iterative least-squares technique is used to simultaneously invert the strong-motion records and teleseismic P waveforms for the 1978 Tabas, Iran, earthquake to deduce the rupture history. The method solves for both slip amplitudes and rupture times on a finite fault. Several inversions of the data are done to determine the best-fitting hypocenter, average rupture velocity, and source-time function, within the range of our parameter search. The effects of using different data sets and different parametrizations of the problem (linear versus nonlinear) are also considered. Teleseismic body waves are shown to be essential to obtain a reliable solution due to the sparse coverage of the fault plane by strong-motion stations. A simple triangular source-time function is compared with a modified Kostrov source-time function for a circular rupture and a constant propagating stress drop. Although the waveforms used in the inversions can not resolve between these two time functions, the Kostrov source-time function gives a moment estimate and maximum fault displacement more consistent with long-period surface waves and field observations. The results of the iterative inversion for slip amplitudes and rupture times are compared with the results of a linear inversion for slip amplitudes in which the rupture velocity is fixed. Flexibility in the sequence of faulting for the linear inversion is obtained by allowing multiple rupture intervals. A consensus of all the inversion runs indicates a complex, multiple source for the Tabas earthquake, with four main source regions over a fault length of 90 km and an average rupture velocity of 2.5 km / sec.

Two-dimensional model of a fault.

Abstract: We present the results of simulations of a two-dimensional mechanical model of a fault, which is a generalization of the one-dimensional model studied previously [Carlson and Langer, Phys. Rev. A 40, 6470 (1989)]. We incorporate both the lateral fault axis and the fault depth, and consider both cases of spatially homogeneous and depth-dependent velocity-weakening friction. Our main result is that in both cases for small- to moderate-sized events, the Gutenberg-Richter scaling exponent b is unity for a wide range of parameters, as observed in the one-dimensional model, and consistent with measurements for real earthquakes. In addition, in the depth-dependent friction model, we study the activity patterns as a function of depth. We observe that smaller events tend to be triggered near the surface, while on relatively shallow faults larger events tend to achieve their first appreciable velocities at depth, in agreement with certain trends observed in seismological studies of the hypocenter distributions of small and large earthquakes.

Earthquake distribution in the subduction zone off eastern Hokkaido, Japan, deduced from ocean-bottom seismographic and land observations

Abstract: SUMMARY In 1986, observations of microearthquakes were conducted with the use of five ocean-bottom seismometers (OBSs) on the continental slope east of the Kushiro Submarine Canyon (KSC) off Hokkaido Island, Japan. The observation area is located about 50-100 km landward of the southern Kuril Trench beneath which the Pacific Plate is subducted toward Hokkaido Island. The OBS data were analysed together with land data on Hokkaido Island to determine the precise earthquake distribution and the geometry of the subducted plate. The OBS data revealed for the first time the seaward seismicity east of KSC. High and shallow seismicity is concentrated in an area between the coast line of Hokkaido Island and the 2000 m isobath. A region between the 2000 and 4000 m isobaths corresponds to the source area of an interplate earthquake occurring in 1973 (the Nemuro Hanto-Oki earthquake, M= 7.4). The OBS array detected more than 10 events in this region. Most of them, however, are classified as earthquakes occurring within the subducted plate, and the remarkable aftershock activity of the 1973 event is not recognized. The seismic activity is extremely low between the 4000 and 6000 m isobaths as is commonly seen in subduction zones. The seismicity is also very low beneath the trench, in contrast with the adjacent trench areas. In a vertical section taken perpendicular to the trench axis, most of the located events form a seismic plane dipping landward. The dip-angle of the plane is 20d-25d beneath the continental slope, while 35d-40d under Hokkaido Island. The change in dip angle occurs 150 km landward of the trench axis. This seismic plane is located 30-40 km below the plate boundary defined from the aftershock distribution of the Nemuro Hanto-Oki event. The seismicity on the plate boundary is correlated with the geometry of the subducted plate. The high activity occurs in a restricted region where the subducted plate shows a remarkable bend. The seismic activity obtained is different from those in the adjacent regions along the southern Kuril Trench. In particular, the regional difference in seismicity is remarkable between the western and eastern sides of KSC. Our result suggests that the subduction zone along the Kuril Trench is divided into several tectonic blocks and KSC is one of their boundaries.

An attenuating structure beneath the Aso Caldera determined from the propagation of seismic waves

Abstract: The attenuation of amplitude is seen in seismic waves which pass through the central region of the Aso caldera, in Kyushu, Japan. It is also recognized from spectral analysis of seismic waves that the higher frequencies of the P-wave are reduced in the waves which pass through the central region of the caldera. It is shown that the relative attenuation increases remarkably for the frequency range of 5 to 10 Hz. The specific attenuation factor Q of the P-wave train is about 100. From the surface projection of the ray paths with low Q values through the Aso caldera to each station, the attenuating region is located beneath the center of the caldera, extending to the north of the central cones. In conjunction with the low Q value of the P-wave and the decreases of S-wave amplitudes, the relative P-wave residual times have comparatively large values for seismic waves passing through the central region beneath the caldera. In order to attempt to provide additional information on the depth configuration of the attenuating material, the ray paths of P-wave's first arrivals are located in three-dimensional space. It indicates that the low-velocity material is located beneath the center of the caldera at depths of about 6 to 9 km. However, lowvelocity anomalies above the depth of 6 km and below the depth of 15 km were not able to be detected, because most of the available seismic ray paths had crossed the caldera at depths of about 6 to 15 km. Furthermore, the relative residual times have numerous errors resulting from incorrect hypocenter locations, origin times, inhomogeneities in the structure and uncertainty of the velocity structure. At shallow depths in the Aso caldera, refraction or reflection studies are required for an accurate estimate of the structure and more detailed properties of the attenuating material.

The temporal and spatial evolution of the 19 September 1985 Michoacán earthquake as inferred from near-source ground-motion records

Abstract: A frequency domain inversion technique is employed to image the rupture process of the 19 September 1985 Michoacan, Mexico, earthquake. A linear system is derived from the elastodynamic representation theorem and is solved for the spatial slip-velocity distribution, in the frequency bandwidth 0.0 to 0.32 Hz. The temporal behavior of slip velocity on the fault plane is then recovered by Fourier synthesis. Model complexity is penalized by seeking a solution that propagates with the most nearly constant rupture velocity and that produces the most nearly constant spatial slip distribution. At 0 Hz, the inversion of surface static displacements, as inferred from strong-motion recordings of the Guerrero accelerograph array, gives an estimate of the static offset on the fault plane. Coastal uplift data inferred from the mortality of intertidal organisms (Bodin and Klinger, 1986) is also used to further constrain the spatial slip distribution. A low-frequency image of the rupture process of the earthquake has been obtained using full-space Green9s functions. Two areas of large slip amplitude are discernible, one located downdip of the reported hypocenter and the other updip of the projection on the fault plane of station La Union. Their separation is approximately 100 km. Assuming that full-space Green9s functions underestimate static offset values at the surface of a half-space by a factor of 0.5, the inferred seismic moment is 10.6 × 10 27 dyne-cm, 90 per cent of which is released in these two areas sustaining the largest slip. The total inferred rupture duration is approximately 56 sec. From the hypocenter, the rupture front proceeds with an average velocity estimated at 2.8 km/sec. There are indications of bilateral propagation both from the hypocentral area and from an area surrounding the projection on the fault plane of the strong-motion station La Union. An upper bound on rupture duration at the two principal subevents is 8 sec.

Inversion for the rupture details of the 1987 East Chiba Earthquake, Japan, using a fault model based on the distribution of relocated aftershocks

Abstract: The rupture of the 1987 east Chiba earthquake was investigated using an empirical Green's function method and a nonlinear inversion technique. In order to discuss what happened on the fault plane in more detail, we relocated the aftershocks as well as the main shock. Relocations were done using a joint hypocenter method in order to increase the accuracy of the relative locations. Based on this aftershock distribution, we propose a double-fault model. The main fault strikes N11°W with nearly vertical dip. A subsidiary fault is perpendicular to the main fault. The results of a source inversion using strong motion waveforms are as follows: (1) moment release was large both in the deep northern area of the main fault and on the subsidiary fault, where aftershock activity was weak; (2) rupture propagated on both main and subsidiary faults simultaneously, and rupture velocity decreased at the depth of around 35 km, near the upper boundary of the Philippine Sea plate; and (3) stress drop was high on the lower northern corner of the main fault where the rupture terminated and on the deep southern area of main fault where the rupture initiated. A negative correlation was found between moment release during the main shock and that during the aftershock sequence, but they did not compensate each other quantitatively. This suggests the existence of a heterogeneous strain distribution in the fault area either before the main shock or after its aftershock sequence.

A Seismo-Tectonic Map of the Gulf and Peninsular Province of the Californias: Chapter 5: Part II. Geological and Geophysical Maps

Abstract: Lineaments in marine gravity anomalies, bathymetry, and magnetic anomalies, along with previously mapped onshore faults, have been used to construct a Seismo-Tectonic Map of the region extending from Lazaro Cardenas in Michoacan, Mexico, to north of Point Conception in Alta California. Earthquake epicenters taken from the NOAA/NGDC Hypocenter Data File for all events with Mb>4.2 occurring between 1963 and 1985 are also shown on the map. We have tried to identify the principal faults and lineaments of the province. The structural pattern that emerges for the Gulf of California, the California Borderland, and the Pacific margin of Baja California is similar to that seen onshore in southern Alta California and northern Baja California. Present-day shear between the Pacific and North American plates is distributed across a fairly wide zone of subparallel crustal slices. In Mexico, where this zone includes most of the Gulf crust, more than one extensional axis commonly occurs at the same plate rotational latitude. Earlier faulting along the Pacific side of the peninsula and in the outer Borderland appears to have occurred in a similar style. In the southernmost Gulf the shear is presently less widely distributed; in this area several horsts of continental crust, which are relicts of early rifting, are now fixed to the North American plate. Gravity anomalies show that the Tosco-Abreojos Fault Zone may have been much wider and more discontinuous than indicated by shallow seismic-reflection data. The eastern part of the Rivera Fracture Zone and the ast Pacific Rise north of lat. 17.5°N are reorienting in a small but very complicated area where the boundaries of five plates and crustal blocks nearly coincide. We identify the seaward extensions of the Colima Graben and the Tepic-Chapala Fault Zone, which bound the Jalisco tectonic block. End_Page 71-------------------------

Relocation of teleseismically recorded earthquakes near Tennant Creek, Australia: Implications for midplate seismogenesis

Abstract: The three MS > 6 Tennant Creek, Australia, earthquakes of January 22, 1988, were preceded by small and moderate earthquakes in 1986 and 1987. We have used the method of joint hypocenter determination to estimate the positions of teleseismically recorded 1986–1987 earthquakes, the 1988 main shocks, and aftershocks with respect to a system of surface fault scarps. The 1986–1987 shocks and the 1988 main shocks nucleated near the center of the zone of surface scarps, where fault-segment boundaries at depth are implied by complexities in the distribution of scarps at the surface. This suggests that the fault segments that ruptured in 1988 were already in existence in 1986–1987, which is consistent with the hypothesis that strong midplate earthquakes occur on preexisting faults. The redetermined 1986–1987 hypocenters are, however, also consistent with the hypothesis that midplate seismicity is localized by stress concentration due to bulk rheological heterogeneity of the crust, because they are situated on a regionally prominent gravity anomaly. The teleseismically recorded seismicity of the Tennant Creek region prior to the 1988 main shocks has the temporal pattern of a swarm followed by a lull. The concentration of swarm earthquakes between two scarps is consistent with models in which precursory-swarm earthquakes correspond to faulting that is spatially distinct from the site of primary main-shock faulting. The time interval between swarm and main shocks is similar to intervals between intermediate-term precursory swarms and main shocks in regions that have much higher rates of tectonic loading; this similarity suggests that the time intervals between precursory swarms and subsequent main shocks are not strongly influenced by the rate of tectonic loading, but are determined primarily by time dependence of the failure process. The spatial distribution of teleseismically recorded aftershocks is in most respects like that of aftershocks recorded by a local network of portable stations in the half year following the main shocks. The set of teleseismically recorded aftershocks, like the set of locally recorded aftershocks, includes some events that occurred well away from the causative faults of the main shocks. At the length scale of the 1988 main-shock rupture, the distribution of aftershocks occurring more than one year after the main shocks is not representative of the distribution of earlier aftershocks.

Hypocenter constraint with regional seismic data: A theoretical analysis for the natural resources defense council network in Kazakhstan, USSR

Abstract: Source depth and epicenter determination remains a simple but key method for discrimination between earthquakes and nuclear explosions. At near-regional distances, a variety of secondary body wave arrivals can be observed that may be useful for constraining source depth and epicenter. In this paper, we study the source location problem using regional arrival time and arrival azimuth data from a sparse seismic network. A theoretical analysis of the source location inverse problem provides estimates of the stability and uncertainty of the solution for a particular suite of data as a function of epicenter and depth, as well as evaluating the importance of each datum to the solution. An empirical analysis with actual data complements the theoretical analysis. Our approach is applied to the three-station Natural Resources Defense Council-Soviet Academy of Sciences network in Kazakhstan. The theoretical analysis indicates that reasonable constraint on depth and epicenter is possible whenever three distinct phases (P, Pn, Pg, PzP, and PmP, or equivalent S phases) are observed at two or three stations. Our theoretical analysis for two-station locations shows that the epicenter constraints are extremely poor for sources along the line passing through the two observing stations. The use of arrival azimuth in two-station locations results in improvement in epicenter constraint for events occurring in the vicinity of the line connecting the two stations, but there is no significant improvement elsewhere. An empirical analysis of data from two of the September 1987 chemical explosions and data from an explosion in an active mining area supports the theoretical analysis, showing that two-station locations (both epicenter and depth) are quite reliable. Furthermore, the theoretical analysis also indicates that, even with data from a single observing station, satisfactory results are obtained if three or more phases plus arrival azimuth are available.

Undersea seismic observation of 1989 volcano-seismic activity, off Ito, Izu Peninsula, central Japan

Abstract: Volcano-related seismicity associated with the July 13, 1989, submarine eruption off Ito, Izu Peninsula, central Japan, began with a swarm activity on June 30. Two ocean bottom seismometers (OBSs) deployed near the future eruptive vent observed several precursor explosive activities during the two days preceding the surface eruption. OBS records show precursor waveforms, called "spindle packets, " which resemble those during the surface eruption. After the eruption, a sonobuoy was repeatedly deployed above the vent and, finally a cable OBS was deployed 1km NW of the vent. Isolated volcanic explosions show high frequency sound arrivals and low frequency (1.2Hz) coda on the vertical seismometer; this coda, which has a group velocity of approximately 330m/s, is identified as a Rayleigh wave. The high frequency arrivals result from the propagation of sound waves through the water column. Hypocenters before the surface eruption trended WNW-ESE and had depths between the ocean bottom and 6km. After the eruption, earthquakes occurred around the vent with depths a few kilometers below the surface. Sonobuoy records show an X phase. Assuming the X phase is generated by the conversion of an S wave to a P wave at the magma surface, the depth to the top of the magma is estimated as 1km below the hypocenter. Quiet vent site hydrothermal activity was observed one and half months after the surface eruption. Although rising bubbles were seen, no earthquake was located beneath the vent site.

Seismicity and focal mechanisms for the southern Great Basin of Nevada and California in 1990

Abstract: For the calendar year 1990, the Southern Great Basin seismic network (SGBSN) recorded about 1050 earthquakes in the SGB, as compared to 1190 in 1989. Local magnitudes, M{sub L}, ranged from 0.0 for various earthquakes to 3.2 for an earthquake on April 3, 1990 5:47:58 UTC, 37.368{degrees} North, 117.358{degrees} West, Mud Lake, Nevada quadrangle. 95% of those earthquakes have the property, M{sub L} {le} 2.4. Within a 10 km radius of the center of Yucca Mountain, the site of a potential national, high-level nuclear waste repository, one earthquake with M{sub L} = 0.6 was recorded at 40-Mile Wash. The estimated depth of focus of this earthquake is 3.8 km below sea level. Other, smaller events may have also occurred in the immediate vicinity of Yucca Mountain, but would have been below the detection threshold (M{sub L}{approx}0.0 at Yucca Mountain). Focal mechanisms are computed for seventeen earthquakes in the Nevada Test Site (NTS) and in the SGB west of the NTS for the year 1990. Solutions are mostly strike-slip, although normal slip is observed for a hypocenter at Stonewall Flat, Nevada, and reverse slip is observed for a hypocenter at Tucki Mountain, California. The average direction of the focal mechanism P-axes is North 47{degrees} East, with nearly horizontal inclination, and the average direction of the T-axes is North 42{degrees} West, with nearly horizontal inclination, consistent with a regional tectonic model of active northwest extension during the Holocene epoch.