Showing papers on "Hypocenter published in 1983"

Inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 Imperial Valley, California, earthquake

Abstract: A least-squares point-by-point inversion of strong ground motion and teleseismic body waves is used to infer the fault rupture history of the 1979 Imperial Valley, California, earthquake. The Imperial fault is represented by a plane embedded in a half-space where the elastic properties vary with depth. The inversion yields both the spatial and temporal variations in dislocation on the fault plane for both right-lateral strike-slip and normal dip-slip components of motion. Inversions are run for different fault dips and for both constant and variable rupture velocity models. Effects of different data sets are also investigated. Inversions are compared which use the strong ground motions alone, the teleseismic body waves alone, and simultaneously the strong ground motion and teleseismic records. The inversions are stabilized by adding both smoothing and positivity constraints. The moment is estimated to be 5.0 × 10^(25) dyne-cm and the fault dip 90° ± 5°. Dislocation in the hypocentral region south of the United States-Mexican border is relatively small and almost dies out near the border. Dislocation then increases sharply north of the border to a maximum of about 2 m under Interstate 8. Dipslip motion is minor compared to strike-slip motion and is concentrated in the sediments. The best-fitting constant rupture velocity is 80 per cent of the local shear-wave velocity. However, there is a suggestion that the rupture front accelerated from the hypocenter northward. The 1979 Imperial Valley earthquake can be characterized as a magnitude 5 earthquake at the hypocenter which then grew into or triggered a magnitude 6 earthquake north of the border.

Interpretation of seismic reflection profiling data for the structure of the San Andreas fault zone

Abstract: A seismic reflection profile crossing the San Andreas fault zone in central California was conducted in 1978. Results are complicated by the extreme lateral heterogeneity and low velocities in the fault zone. Other evidence for severe lateral velocity change across the fault zone lies in hypocenter bias and nodal plane distortion for earthquakes on the fault. Conventional interpretation and processing methods for reflection data are hard-pressed in this situation. Using the inverse ray method of May and Covey (1981), with an initial model derived from a variety of data and the impedance contrasts inferred from the preserved amplitude stacked section, an iterative inversion process yields a velocity model which, while clearly nonunique, is consistent with the various lines of evidence on the fault zone structure .

The near-source ground motion of the 6 August 1979 Coyote Lake, California, earthquake

Abstract: A finite fault striking N24°W and extending to a depth of 10 km is proposed to explain the strong ground motion data for the 6 August 1979 Coyote Lake, California, earthquake (M_L = 5.9). Our source model suggests that right-lateral faulting initiated at a depth of 8 km and ruptured toward the south with a velocity of 2.8 km/sec. This unilateral rupture can explain the large displacement recorded south of the epicenter. However, the waveform coherency across an array south and southwest of the epicenter suggests that the rupture length is less than 6 km. The maximum dislocation is about 120 cm in a small area near the hypocenter, and the total moment is estimated to be 3.5 × 10^(24) dyne-cm. An abrupt stopping phase which corresponds to a deceleration of right-lateral motion can explain the high peak acceleration recorded at array station 6. The stress drop in the hypocentral area is about 140 bars; the average stress drop over the entire rupture surface is 30 bars. The preferred finite-source model can predict the P_(n1) waveforms and the beginning features in the teleseismic seismograms. No clear arrivals can be observed in the near-source data for the possible second and third smaller events suggested by Nabelek (personal communication).

The 23 November 1980 southern Italy earthquake

Abstract: The seismic activity associated with the catastrophic southern Italy earthquake was monitored by 11 seismic stations operating before this event, within an epicentral distance of 200 km, and by 32 additional short-period seismometers installed soon after the main shock. The hypocenter of this event was located at 40°46′N and 15°18′E, at 16 km depth. The fault-plane solution reveals normal faulting, with tensile axis dipping 18° and oriented orthogonal to the axis of the Apennines chain. This mechanism is in good agreement with the stress pattern inferred from some previous earthquakes and the local seismotectonics. The hypocenter locations of more than 600 aftershocks, with local magnitudes greater than 2.4, show a pronounced alignment extending for about 70 km, oriented north 120° and scattered laterally less than 15 km. These events are mostly concentrated between 8 and 16 km depth. A cluster of aftershocks occurred close to the hypocenter of the main shock covering a region elongated 25 km which corresponds also to the highly damaged area. No significant spreading of the aftershock area with time is observed, but one of the events with higher magnitude ( M L = 4.8, 14 February 1981) is displaced 20 km NW from the tip of the aftershock region. The time evolution of the number of aftershocks fits well Omori9s hyperbolic law with a decay coeffcient of 1.07 ± 0.06. The possibility of a future delayed multiple sequence of large events, as already observed in the past along the central and southern Apennines, is discussed. In particular, a relatively high seismic potential seems to exist along the northern boundary of the 1980 rupture segment.

A teleseismic analysis of the New Brunswick Earthquake of January 9, 1982

Abstract: The analysis of the New Brunswick earthquake of January 9, 1982, has important implications for the evaluation of seismic hazards in eastern North America. Although moderate in size (mb 5.7), it was well-recorded teleseismically. Source characteristics of this earthquake have been determined from analysis of data that were digitally recorded by the Global Digital Seismograph Network. From broadband displacement and velocity records of P waves, we have obtained a dynamic description of the rupture process as well as conventional static properties of the source. The depth of the hypocenter is estimated to be 9 km from depth phases. The focal mechanism determined from the broadband data corresponds to predominantly thrust faulting. From the variation in the waveforms the direction of slip is inferred to be updip on a west dipping NNE striking fault plane. The steep dip of the inferred fault plane suggests that the earthquake occurred on a preexisting fault that was at one time a normal fault. From an inversion of bodywave pulse durations, the estimated rupture length is 5.5 km. Average properties of the rupture process were examined by a moment tensor analysis of long-period P and SH body waves. The long-period moment of this earthquake was 5.3 × 1024 dyne cm. The static and dynamic stress drops are 41 and 65 bars, respectively, similar to those of many earthquakes with similar moment in regions that are more seismically active. The joint epicenter determination algorithm was used to locate, relative to the mainshock, the three teleseismically recorded aftershocks that occurred through March 31, 1982. The relocated hypocenters of the aftershocks are significantly different from each other and from that of the mainshock; they provide additional support for the source dimensions inferred from the waveform analysis.

Seismic rupture patterns in Oaxaca, Mexico

Abstract: The spatio-temporal patterns of seismic activity for events with body wave magnitude mb ≥ 4.0 are investigated for the three most recent large earthquakes in the Oaxaca region of southern Mexico (August 23, 1965, MS = 7.6, MW = 7.5; August 2, 1968, MS = 7.1, MW = 7.3; November 29, 1978, MS = 7.8, MW = 7.6). A master catalogue of earthquakes is compiled for the analyses, using the International Seismological Center (ISC) catalogue supplemented by the National Earthquake Information Service (NEIS) catalogue; the events are then relocated with the Joint Hypocenter Determination (JHD) method. At this magnitude threshold, we find that the aftershock zone of the 1965 earthquake was seismically quiescent for at least 20 months prior to the main event; the 1968 earthquake was preceded by 1 year of foreshock type activity clustered in the aftershock zone; the 1978 earthquake was preceded by 43 months of quiescence and one event (mb = 4.7) within the aftershock zone 4 months prior to the mainshock. We also find that a segment of the subduction zone in Oaxaca remains unbroken by these earthquakes. In addition, some catalogue problems are pointed out which are important to interpretations of seismicity patterns.

Large earthquakes, mean sea level, and tsunamis along the Pacific Coast of Mexico and Central America

Abstract: Along the Pacific Coast of Mexico and Central America, 26 local tsunamis have been reported during the period 1732 to 1973. Nine of these were caused by earthquakes with teleseismic hypocenters, all of which were located well inland. If these epicenters were correct, these earthquakes could not have generated tsunamis. Under the assumption that the true epicenters must have been located at the coast or off shore, it was estimated that teleseismic hypocenters in this area are mislocated by about 75 km toward the northeast, and 20 km toward greater depth. We propose that most teleseismic locations in this area are afflicted by this same error. The most likely cause for the mislocations are shorter than expected travel times for rays in the down-dip direction of the subducted lithospheric slab. These rays travel to North American stations which contribute strongly to hypocenter locations in Middle America. The annual mean sea level of 13 tide gauge stations along the Pacific coast of Mexico and Central America were examined for evidence of vertical crustal deformation changes that could have been associated with earthquakes along this coast. Only one coseismic change could be identified in the annual mean sea level data. It occurred at Acapulco, Mexico, during the 11 May ( M s = 7.0) and 19 May ( M s = 7.2) 1962 earthquakes. The crustal uplift was about 22 cm, estimated from the difference of the 10-yr sea level means before and after the events. By comparing annual mean with daily mean sea level data, it appears that about 23 per cent of the permanent uplift observed at Acapulco was due to aseismic slip or aftershocks in this area. If tide gauge data in this area are kept current, long-term precursory crustal movements might be detectable if they exceed several centimeters.

Local seismicity preceding the March 14, 1979, Petatlan, Mexico Earthquake (Ms = 7.6)

Abstract: Local seismicity surrounding the epicenter of the March 14, 1979, Petatlan, Mexico earthquake was monitored by a network of portable seismographs of the Hawaii Institute of Geophysics from 6 weeks before to 4 weeks after the main shock. Prior to the main shock, the recorded local seismic activity was shallow and restricted within the continental plate above the Benioff zone. The relocated main shock hypocenter also lay above the Benioff zone, suggesting an initial failure within the continental lithosphere. Four zones can be recognized that showed relatively higher seismic activity than the background. Activity within these zones has followed a number of moderate earthquakes that occurred before or after the initial deployment of the network. Three of these moderate earthquakes were near the Mexican coastline and occurred sequentially from southeast to northwest during the three months before the Petatlan earthquake. The Petatlan event occurred along the northwestern extension of this trend. We infer a possible connection between this observed earthquake migration pattern and the subduction of a fracture zone because the 200-km segment that includes the aftershock zones of the Petatlan earthquake and the three preceding moderate earthquakes matches the intersection of the southeastern limb of the Orozco Fracture Zone and the Middle America Trench. The Petatlan earthquake source region includes the region of the last of the three near-coast seismic activities (zone A). Earthquakes of zone A migrated toward the Petatlan main shock epicenter and were separated from it by an aseismic zone about 10 km wide. We designate this group of earthquakes as the foreshocks of the Petatlan earthquake. These foreshocks occurred within the continental lithosphere and their observed characteristics are interpreted as due to the high-stress environment before the main shock. Pre-main shock seismicity of the Petatlan earthquake source region shows a good correlation with the aftershocks in their spatial distribution. This suggests that an asperity existing along the Benioff zone may have affected both the pre-main shock activity in the continental lithosphere and the aftershocks along the Benioff zone. Although major thrust earthquakes at trenches occur along Benioff zones, in the present study we find little activity on this interplate boundary before the Petatlan earthquake. The overlying continental block, on the contrary, is very active seismically. Our data suggest that the activity is probably governed by the stress transmitted from below due to coupling between two plates and the heterogeneity within the continental lithosphere. The continental material is probably the more likely place for precursors.

Aftershock distribution of the 1982 urakawa-oki earthquake determined by ocean bottom seismographic and land observations

Abstract: The Urakawa-Oki earthquake (M=7.1) occurred offshore of Urakawa, Hokkaido, Japan, on March 21, 1982. In order to investigate the aftershock activity of this event, we deployed four ocean bottom seismographs (OBS's) off Urakawa. During an observation period from March 29 to April 6, the OBS network detected more than 4, 500 earthquakes. Magnitude range of these events is from -2.0 to 5.0. Aftershock distribution of the Urakawa-Oki earthquake was determined on the basis of the OBS and land observations. In the analysis, we selected about 250 aftershocks with M≥2.0, because the seismic signals from these events were recorded clearly both by the OBS's and the land stations around the aftershock area. Hypocenter determination was carried out using a method of inversion analysis. Most of the aftershocks were located in a region of 42°00'-42°20'N and 142°25'-142°40'E. These events were distributed in a depth range of 10-30km. The characteristic dimensions of the aftershock area were estimated as 35km×25km. The events in the southern part(south of 42°15'N) were distributed on a plane with an area of 20-25km×10-15km dipping 20-30° northward. In the northern part of the aftershock area, we found another trend of aftershocks which inclined southward with a high angle of 60-70°. The events in this group were located on a plane with a relatively narrow area of 10km×10-15km. The aftershock distribution obtained in the present analysis suggests that the fracture mechanism of the Urakawa-Oki earthquake was very complicated.

Detailed study of the earthquake sequence in 1980 off the east coast of the izu peninsula, japan

Abstract: The earthquake sequence which occurred off the east coast of the Izu Peninsula, Japan, in 1980 is investigated in detail on the basis of a homogeneous hypocenter file (M≥2.9) carefully constructed for this purpose. Absolute and relative precisions of hypocenters are improved to be 1-2km and several hundred meters, respectively, by applying two kinds of correction to arrival times. Furthermore spectra of seismic waves recorded at OYM (Δ-50km) are examined to detect possible changes in spectral features in the course of the development of the sequence. The present sequence is the superposition of two types of activities, the swarm activity (M≤5.1, MJMA≤4.9) and the mainshock (M6.5, MJMA 6.7)-aftershock sequence. The swarm activity consisted of 28 bursts of activity, each of which lasted for about 1 to 2hr after a quiescent period of six to ten-several hours. Such swarm activity occurred within a narrow area of 6km by 2km throughout the whole sequence. The depth was also limited from 9 to 11km. Each burst of activity moved from place to place in this limited area without any significant relation to the largest shock of the sequence, occupying an area of about 1-2km square each time. It only seems to show the local fracture strength in the swarm area. The estimated fault of the largest shock (MJMA 6.7) is a vertical plane of 20km in length and 8-10km in width with a strike of N20°W. Its location agrees well with an evident cliff of the submarine topography. The hypocenter of the largest shock is 3km south of the swarm area. This place had been quiescent during the early stage of the sequence, but about 20hr before the largest shock, an M4.0 (MJMA 3.9) shock occurred there.

Interpretation of near-source ground motion and implications

Abstract: This thesis presents some deterministic modeling and interpretation of various aspects of observed near-source ground motions. In Chapter 1, finite source parameters determined from waveform modeling studies are presented for two California earthquakes; the 1979 Coyote Lake event and the 1966 Parkfield event. These events were recorded by strong motion arrays with similar station to fault rupture geometries. Thus it is possible to demonstrate that differences in the ground motions recorded within 30 km of the epicenter are indeed due to the differences in rupture fault length and dislocation distribution. Details of the waveform modeling for the August 6, 1979 Coyote Lake earthquake are described in part 1-A. A finite fault striking N24°W and extending to a depth of 10 km is proposed to model the strong-ground motion data. The source model suggests that right-lateral faulting initiated at a depth of 8 km and ruptured towards the south with a velocity of 2.8 km/sec. This unilateral rupture can explain the large displacements recorded south and southwest of the epicenter. However, the waveform coherency observed across an array south and southwest of the epicenter suggests that the rupture length is less than 6 km. The maximum dislocation is about 120 cm in a small area near the hypocenter and the total moment is estimated to be 3.5 x 1024 dyne-cm. An abrupt stopping phase, which corresponds to a cessation of right-lateral motion, can explain the high peak acceleration recorded at array station 6. The stress drop in the hypocentral area is about 140 bars; although the average stress drop over the entire rupture surface is 30 bars. This preferred finite source model can predict observed Pni waveforms as well as the beginning features of teleseismic body waves. In part 1-B, a similar waveform modeling technique is used to interpret the ground motions recorded during the June 28, 1966 Parkfield earthquake. The preferred model suggests that the earthquake involved two fault segments; one is the NE branch which extends 22 km southward from epicenter and has an average slip of 45 cm, another is the SW branch which ruptured less than 10 km and has an average slip of about 22 cm. The total moment indicated by this model is 1.25 x 1025 dyne-cm. The anomalous large amplitude ground displacement seen at station Cholame No. 2 is modeled as a local amplification effect rather than a source effect due to significant dislocation near this station. Direct waveform comparisons between recordings of the Parkfield event and the Coyote Lake event also support the conclusion that the rupture length of the Coyote Lake earthquake is much shorter than that of the Parkfield event. The waveform modeling also emphasizes the importance of using array data to constrain source parameters. The solution derived from a single station's recording, which in many cases is the only available information, may often produce misleading results. In Chapter 2, high-frequency ground motions (ground velocity and acceleration) recorded at less than 30 km epicentral distances are studied for two aftershocks of the 1979 Imperial Valley, California earthquake. In the past, little has been done to understand these high frequency waves through a deterministic modeling approach. The waveform modeling technique and the source mechanisms of these two aftershocks are described in sections 2-A and 2-B. An important feature of the ground motions recorded during the October 15, 1979 Imperial Valley earthquake sequence is the strong high frequency waves observed on the vertical components. This feature is also seen in recordings of the aftershock of October 16, 23:16, 1979, which is described in section 2-A. This polarization feature is easily explained by the basin velocity structure which bends rays towards the vertical at the free surface. Short S-P times are observed at the three closest stations (epicentral distances of 3 km to 5 km) suggesting that this aftershock occurred at a very shallow depth of about 2 km. A fault plane orientation (strike = N20°E, dip = 30°SE, and rake = -80°) obtained from a first P-motion study, generates synthetic waveforms of the strong ground velocities which are similar to those observed at three closest stations. The source time duration is determined to be 1.0 second and the moment is 1.6x1023 dyne-cm. Synthetics for a number of line source models are compared with the observations. These comparisons lead to two basic mechanisms that are necessary to explain the frequency content of the observed P- and S- waves. One is that the source process is characterized by irregular rupture. It is postulated that the heterogeneous stiffness in the layered medium is the basic cause of the irregular rupture. Heterogeneous rupture generates both high-frequency P- and S-waves. In order to explain the contrast in observed frequency content it is also necessary that there is a mechanism for attenuating S-waves much stronger than P-waves. The aftershock that occurred about 3 minutes after the mainshock, at 23:19 October 15, 1979 is presented in section 2-B. This aftershock was located on the Imperial fault near Highway 8 and close to the zone of high frequency energy release of the main event. The impulsive seismograms for 16 array stations, ranging from 8 km to 26 km in epicentral distance, are well suited for source parameter inversion studies to obtain an optimal solution for ground velocity and acceleration. The earthquake source is approximated by a model consisting of several point dislocation sources separated in space and time and having different dislocation orientations and moments. These source parameters were deduced by trial and error modeling as well as by applying inversion procedures. The waveforms and amplitudes of horizontal ground velocities are well modeled by two predominantly strike-slip point sources; the first source (strike = N41°W, dip = 42°NE and rake = 174°) has a moment of 0.7 x 1024 dyne-cm, the second source (strike = N36°W, dip = 82°SW and rake = 181°) lies about 1 km to the north of the first and has a seismic moment of about twice that of the first source. It is suggested that the higher-frequency ground motions, such as accelerations, can be derived from very irregular source processes, whereas the longer-period ground motions, such as ground displacements, can be well modeled by simpler planar source. A Futterman attenuation operator with a t*β of about 0.08 to 0.1 and a t*α of about 0.001 in the sedimentary region produces longer period S waves and the proper amplitude ratio between P and S waves. In Chapter 3, the ground motion data from the 1971 San Fernando earthquake recorded at epicentral distances of less than 100 km are presented. Three long profiles (> 50 km ) and three short profiles ( Although there is considerable variation in waveforms and peak amplitudes observed along the long profiles, there are also many examples of coherent phases seen at adjacent stations. Ground velocity profiles show striking differences in amplitude and duration between stations located on hard rock sites and stations located within the sedimentary basins. The San Fernando basin, in which the source is located, seems to respond quite differently from the Los Angeles basin which is about 30 km from the earthquake source area. Ground acceleration profiles show that there is little change in the duration of high-frequency shaking along the long profiles. The three short profiles, which are all located within the Los Angeles basin, demonstrate that ground velocity waveforms are nearly identical along these profiles. Although greater variation of waveforms and amplitudes are seen for ground acceleration along these short profiles, strong phase coherence is still observed. The 2D acoustical finite difference method is used to compute the effects on SH-waves of irregular velocity structures believed to exist along Profile I and Profile II. Profile I extends 65 km southward from the epicenter across the San Fernando and Los Angeles basins to a station on the Palos Verdes Peninsula. Profile II extends 95 km S 40° E along the front of the San Gabriel mountains and across the San Gabriel and Los Angeles basins. These numerical models consist of low-velocity sedimentary basins (β = 2.1 km/sec) of irregular shape which are imbedded in high-velocity basement rock (β = 3.5 km/sec). Heaton's (1982) finite source model derived from modeling the five nearest stations for the San Fernando event, is also incorporated in the interpretation. The resulting simulation suggests that the smaller S! phases in both Profile I and Profile II are direct S waves from the deep source region (13 km). The shallow source region (at 1 km) dominates high amplitude later arrived phases observed along Profile I and are due to the complicated basin path along this profile. The shallower source region, however, contributes little to the ground motions along Profile II due to the lack of thick sediments near the source region along this azimuth.

The 1981 earthquake swarm off the kii peninsula observed by the ocean bottom seismometer array

Abstract: An earthquake swarm that occurred east of the Kii Peninsula near the Nankai trough was investigated using seismic data from four pop-up type OBSs, four telemetering OBSs, and one land seismic observation station. The addition of the pop-up type OBSs was most effective in the analysis of the swarm. We analysed the daily frequency of earthquakes, the b-value, S-P time distribution, and hypocenter distribution. Here we discuss the swarm activity with regard to recent seismicity around the area, to submarine active tectonic lines, to oceanic topography, and to block boundaries along the Nankai trough. The swarm activity was largely divided into two periods, and seemed to consist of six or seven sequences of foreshocks-mainshock-aftershocks or mainshock-aftershocks. The earlier activity occurred in a deeper region (at a depth of about 17km), the later one in a shallower region (about 5km). The epicenter was distributed in a region no larger than 18km×8km. The b-value seems to have decreased before the largest earthquake. The swarm seems to have occurred at a submarine active tectonic line near the Nankai trough.

Does hydraulic-fracturing theory work in jointed rock masses

Abstract: The hypocenter locations of micro-earthquakes (acoustic emissions) generated during fracturing typically are distributed three-dimensionally suggesting that fracturing stimulates a volumetric region, rather than the planar fracture theoretically expected. The hypocenter maps generated at six operating, or potential, HDR reservoirs in the US, Europe and Japan are examined in detail and the fracture dimensions are correlated with fracture injection volumes and formation permeability. Depsite the volumetric appearance of the maps we infer that the induced fractures are mainly planar and may propagate aseismically. The induced seismicity stems from nearby joints, which are not opened significantly by fracturing, but are caused to shear-slip because of local pore pressure.

Analysis of hypocenter location capability of a regional seismic network

Abstract: A general procedure has been developed for analyzing the hypocenter location capability of a regional seismic network. In this analysis the fact is considered that as the earthquake magnitudes and locations vary within the region, the arrival times of P and S phases are measured by different subnets of stations.As an example, the Beijing Telemetered Seismic Network has 19 stations covering an area of about 300 km x 400 km; empirical formulae are established for determining the measurement capability of each station as a function of earthquake magnitude and instrument magnification. Then, for any possible event with a given magnitude and location, a specific geometrical configuration of a 'subnetwork' can be determined. Based on the subnetwork configurations, the resulting distributions of errors in the hypocenter coordinates and the condition numbers of the linearized condition equation systems are estimated by the singular value decomposition technique. For various assumed earthquake magnitudes, say, 1.0, 2.0, and 3.0, the results are plotted in the form of a contour map with respect to an assumed standard error of the arrival time data.For comparison, a planned enlarged Beijing network with 61 stations is also analyzed, giving a preview of the location capability of the future network.

Some Remarks on the Contribution of Geotechnical Engineering to an Estimation of Seismic Risk

Abstract: In this paper an outline will be given of the contribution which geotechnical engineering can give to an estimation of the seismic risk of a site. Fig. 1. gives a very schematic picture of some phenomena which can occur during an earthquake. The earthquake originates in the hypocenter and in relation with this the first question can be posed: what is the magnitude of the earthquake to be expected?

The joint determination of hypocenter parameters and velocity structure in the beijing area of china

Abstract: A new seismic velocity model was constructed for improving routine determination of hypocenter parameters in the Beijing Telemetered Seismological Network (BTSN). This model describes a region of approximately 350 km X 450 km and consists of 4 layers over a half space, with the first layer divided into 3 blocks to include lateral variation of the shallow structure and topographic relief effects. The adjustable parameters for the velocity model in the joint determination include the average P-wave velocities in each layer, the block thicknesses, the layer depths, and the average ratio of P- and S-wave velocities. By separating the velocity model parameters from the hypocenter parameters, the solution of a large set of equations for simultaneous determination of these parameters is avoided.836 P- and S-arrival times from 43 events recorded by the BTSN in 1979 are used for the joint determination. The result indicates that in the area around Beijing there is a lateral inhomogeneity of velocity structure; the bottom depth of the blocks in the first layer increases by 3.5 km from NW to SB. The newly determined hypocenters are generally located closer to mapped surface fault traces than hypocenters for the same events previously determined by the BTSN.