Showing papers on "Hypocenter published in 1989"

The Irpinia (Italy) 1980 earthquake: Detailed analysis of a complex normal faulting

Abstract: A detailed analysis of near-source strong motion and leveling data, together with the results of teleseismic waveform modeling by Westaway and Jackson (1987) and aftershock studies by Deschamps and King (1984), allows a satisfactory kinematic description of the complex normal faulting associated with the magnitude Ms = 6.9, November 23,1980, Irpinia earthquake (southern Italy). The three main rupture episodes, starting at about 0, 20 and 40 s, are here associated with better constrained source parameters than in previous studies. We first evaluated the triggering time for the 11 closest accelerometers by using the well-recorded 40-s strong motion phases. This gave the absolute time and location of the dominant episodes of faulting. The first rupture propagated mainly toward the northwest on a NE dipping normal fault, at a mean velocity close to 3 km/s for about 20 km, and continued 15 km further on smaller subfaults. It was associated with surface breakage in the southeastern part. The second rupture started from the southeastern end of the first rupture, about 18 s after it, and propagated about 20 km toward the southeast on a low-angle normal fault dipping 20°NE. It was associated with secondary faulting on steeper planes reaching the surface. The third and last episode at 39 s nucleated near the first hypocenter, at shallower depth, and the rupture possibly propagated on a 10- to 15-km-long normal fault, striking SE, antithetic to the first activated fault. A clear correlation appears between the strength of the geological formations and the existence of surface breakage and shallow aftershock activity.

Slip distribution of the 19 September 1985 Michoacan, Mexico, earthquake: near-source and teleseismic constraints

Abstract: We simultaneously invert the strong-motion velocity records and the long- and intermediate-period teleseismic P waveforms of the 19 September 1985 Michoacan, Mexico, earthquake to recover the distribution of slip on the fault using a point-by-point constrained and stabilized, least-squares inversion method. A fault plane with strike fixed at 300° and dip fixed at 14° is placed in the region of the earthquake hypocenter and divided into 120 subfaults. Rupture is assumed to propagate at a velocity of 2.6 km/sec away from the hypocenter. Synthetic near-source ground motions and teleseismic P waveforms for pure strike-slip and dipslip dislocations are calculated for each subfault. The observed data are then inverted to obtain the amount of strike-slip and dip-slip displacement required of each subfault. We also invert the data sets using a time-window procedure where the subfaults are allowed to slip up to three times. This approach relaxes the constraint of fixed subfault rupture time imposed by a constant rupture velocity. Inversion of the strong-motion data alone yields a slip model similar to the solution previously obtained using only teleseismic waveforms. This result supports the use of teleseismic waveform data for the derivation of fault dislocation models in the absence of strong-motion recordings. Our simultaneous inversion of both data sets suggests that rupture during the Michoacan earthquake was controlled largely by the failure of three major asperities located along the length and down the dip of a 150-km segment of the Cocos-North America plate boundary. The solution contains three major source regions including an 80 km by 55 km source near the hypocenter with a peak slip of 6.5 meters. Two additional sources are present on the southeast portion of the fault about 70 km away from the hypocenter. One of these sources, with a peak slip of 5 meters, covers a 45 km by 60 km area and extends downdip from a depth of about 10 km to 24 km. The third source region is somewhat smaller (30 km by 60 km area, 3.1-meters peak slip) and extends further downdip at depths between 27 km and 39 km. Aftershock activity following the earthquake was associated mainly with the two shallow sources. These two sources are separated by the aftershock zone of the 1981 Playa Azul earthquake.

The 1987 Whittier Narrows earthquake sequence in Los Angeles, southern California: Seismological and tectonic analysis

Abstract: The October 1, 1987, Whittier Narrows earthquake (M_L = 5.9) was located at 34°2.96′N, 118°4.86′W, at a depth of 14.6±0.5 km in the northeastern Los Angeles basin. The focal mechanism of the mainshock derived from first motion polarities shows pure thrust motion on west striking nodal planes with dips of 25°±5° and 65°±5°, respectively. The aftershocks define an approximately circular surface that dips gently to the north, centered at the hypocenter of the mainshock with a diameter of 4–6 km. Hence the spatial distribution of the mainshock and aftershocks as well as the focal mechanisms of the mainshock indicate that the causative fault was a 25° north dipping thrust fault striking west and is confined to depths from 10 to 16 km. Although most of the 59 aftershock focal mechanisms presented here document a complex sequence of faulting, they are consistent with deformation of the hanging wall caused by the thrust faulting observed in the mainshock. A cluster of reverse faulting events on north striking planes occurred within hours after the mainshock, 2 km to the west of the mainshock. The largest aftershock (M_L = 5.3) occurred on October 4 and showed mostly right-lateral faulting on the same north-northwest striking plane within the hanging wall. Similarly, several left-lateral focal mechanisms are observed near the eastern edge of the mainshock rupture. The earthquake and calibration blast arrival time data were inverted to obtain two refined crustal velocity models and a set of station delays. When relocating the blast using the new models and delays, the absolute hypocentral location bias is less than 0.5 km. The mainshock was followed by nearly 500 locatable aftershocks, which is a small number of aftershocks for this magnitude mainshock. The decay rate of aftershock occurrences with time was fast, while the b value was low (0.67±0.05) for a Los Angeles basin sequence.

Coseismic folding, earthquake recurrence, and the 1987 source mechanism at Whittier Narrows, Los Angeles basin, California

Abstract: Static deformation associated with the October 1, 1987, Whittier Narrows, California, M = 6.0 earthquake was detected by geodetic elevation changes. The earthquake uplifted a 1.5-km-high Quaternary fold (the Santa Monica Mountains anticlinorium) by 50 mm but caused no fault rupture at the ground surface. This suggests that folding and faulting of the Los Angeles basin sediments are coincident and continuing. After correction for surveying errors and nontectonic subsidence, we model the 214 geodetic observations with a simple dislocation in an elastic half-space. A thrust fault with reverse slip of 1.1±0.3 m and dipping 30°±4°N, with an upper edge at a depth of 12±1 km and a lower edge at 17±1 km, fits the geodetic data best and is consistent with the main shock hypocenter, fault plane solution, and initial aftershock distribution. The upper limit on the static stress drop, Δσ, is 17.5±5.0 MPa (175±50 bar). The lower limit on the geodetic moment is 1.0±0.2 × 1025 dyne cm, in accord with seismic estimates, indicating that most of the slip took place during seismic rupture. If earthquakes of M ≤ 6 characterize the blind thrust fault on which the earthquake occurred, we estimate a 200 year repeat time at Whittier Narrows, and a 5–18 year rate of earthquake occurrence within a 150-km-long band in the northern Los Angeles basin and Santa Monica Bay to the west. The 25-year rate of historical occurrence since 1860 is less than this prediction. The deficit in moment release implies that either vigorous aseismic slip or infrequent larger earthquakes occur here.

Mapping high Pn velocity beneath the Colorado Plateau constrains uplift models

Abstract: The massive International Seismological Centre data set of the past 20 years and the two-station method are used to determine Pn velocities in the mantle lid beneath the Colorado plateau. In this method the event is located at distances where Pn is the first arrival (2°–16°) and the path is in or very near the azimuth of the two-station pair and crosses the plateau. This technique to a large extent minimizes the hypocenter mislocation effect and possible errors due to variations in the crustal structure near the source, since only the difference in travel times at the two stations is used. However, this technique has a few underlying assumptions and possible sources of errors (such as the quality of the Pn data base and station delays caused by varying crustal structure) that require an extremely careful application of the method. A detailed study of the source of errors and a methodology of selection of the data are presented. Application of this method to the Colorado plateau using all possible two-station pairs from 53 stations located within or along the margin of the plateau yields an average high Pn velocity of 8.12±0.09 km/s. This value is considerably larger than the average value of 7.83 km/s based on available but very limited seismic refraction profiles but is remarkably similar to the average value of 8.1 km/s for the relatively stable midcontinent region. Our new Pn velocity for the Colorado plateau eliminates the paradox in the literature that emphasizes the rather close similarity between average Pn velocities beneath the Colorado plateau and the Basin and Range Province while their tectonic and magmatic Cenozoic history is dramatically different. Previous models for the structure and evolution of the plateau have used the low Pn velocity as an important constraint on density and thermal state of the lithosphere. Hence such models should be reexamined on the basis of this new uppermost mantle Pn velocity determination. There are two main models that have been proposed to explain the 2-km uplift of the Colorado plateau. One is based on a combination of thermal thinning of the lithosphere and crustal thickening, and the other involves a combination of the delamination of the subducted, subhorizontal Farallon oceanic plate from the overriding North American plate and crustal thickening. We show that the delamination model is more readily consistent not only with our velocity determination and the elevation of the plateau but also with varied geological observations reported in the literature that concern the Cenozoic evolution of western North America.

Influence of fault plane heterogeneity on the seismic behavior in the Southern Kurile Islands Arc

Abstract: The seismic behavior in the southern Kurile Islands Arc is strongly influenced by spatial heterogeneity in the mechanical properties of the plate interface. Maps showing sites of maximum seismic moment release for the great underthrusting earthquakes of 1958, 1963, 1969, and 1973 provide a first-order image of the major regions of enhanced strength, or asperities. Hypocenters of these and an additional 83 smaller earthquakes with mb ≥ 5.3 and known underthrusting mechanisms were recomputed using the method of joint hypocenter determination in order to assess better the influence that the strength distribution exerts on the interplate seismicity patterns. Nearly all of the smaller-magnitude earthquakes locate outside of the principal asperities of the great earthquakes. This observation is consistent with substantial release of accumulated strain energy in asperity regions during the great earthquakes and a redistribution of stress in adjacent areas following their rupture. An along-dip, depth dependent component of strength heterogeneity appears to influence the seismicity patterns. The asperities of the 1958 and 1973 great earthquakes and hypocenters of most of the Ms ≥ 7 earthquakes occur near the downdip edge of the 100-km-wide seismically active plate interface. This would be expected if shear resistance on the interface increases monotonically with lithostatic pressure up to the brittle-ductile or stick slip-stable slip transition that defines the base of the seismogenic zone. The asperities of the 1963 and 1969 earthquakes, however, appear to be located well above the downdip edge of the coupled plate interface. The occurrence of large asperities in middepth ranges of the seismogenic interface suggests the influence of along-dip changes in other parameters, besides lithostatic pressure, that control shear resistance. Along-strike segmentation of the Kurile Islands thrust zone may be a consequence of along-strike alternation in the location of the largest asperities between the deep interface and the midinterface.

Active faulting and deformation of the Coalinga Anticline as interpreted from three-dimensional velocity structure and seismicity

Abstract: This work gives a clear picture of the geometry of aftershock seismicity in a large thrust earthquake. Interpretation of hypocenters and fault plane solutions, from the 1983 Coalinga, Coast Range California, earthquake sequence, in combination with the three-dimensional velocity structure shows that the active faulting beneath the fold primarily consists of a set of southwest dipping thrusts uplifting blocks of higher-velocity material. Above the main listric blind thrust there is a conjugate fault, steeply northeast dipping, that provides the western limit of the aftershocks within the Coalinga Anticline and that corresponds in location and spatial extent with the adjacent Pleasant Valley syncline. The character of the seismicity varies with the degree of previous deformation on each section of the anticline. Where the previous uplift was largest, the shallow seismicity shows secondary faulting on either side of the fold with orientations that correspond to the preexisting geologic structure. Diffuse seismicity characterizes the area with the least previous deformation. The mainshock rupture terminated where the fold trend was no longer uniform but had competing north and west trending features. The upward extent of the mainshock rupture ended at the approximate boundary between Franciscan and Great Valley Sequence rocks. Above that depth the main thrust appears to splay into a steeper segment and a near-horizontal segment. Thus the extent of rupture area is limited by the area of uniform structural orientation and by the variation in the type of material. With the three-dimensional velocity model each individual hypocenter moved slightly (0–2 km) in accord with the details of the surrounding velocity structure, so that secondary features in the seismicity pattern are more detailed than with a local one-dimensional model and station corrections. The overall character of the fault plane solutions was not altered by the three-dimensional model, but the more accurate ray paths did result in distinct changes. In particular, the mainshock has a fault plane dipping 30° southwest instead of the 23° obtained with the one-dimensional model.

The Southeastern Illinois Earthquake of 10 June 1987

Abstract: The June 10 southeastern Illinois earthquake was the 11th largest earthquake felt in the central U.S. during this century. Source parameters of the main shock were estimated from an analysis of surface-wave amplitude spectra. The source that best fit the observed data has focal depth of 10 ± 1 km; mechanism with strike= 40.6°± 5.9°, dip= 76.2° ± 5.6°, slip= 159.7° ± 6.0°; tension and pressure axes of (T) trend= 357°, plunge= 24°, (P) trend= 89°, plunge= 4°; and a seismic moment of 3.1 * 10 23 dyne-cm. With the combined efforts of six institutions, a 24-station analog microearthquake network was deployed around the main shock epicenter. One hundred eighty-five aftershocks were recorded in the first week of monitoring, providing 144 hypocenter determinations. A subset of 51 well recorded events was used for joint relocation and calculation of station corrections for the stations within 100 km of the main shock epicenter. Joint hypocenter locations differ only slightly from the original locations. The spatial distribution of well located aftershocks indicates that the rupture was confined to a pencil-like zone within the Precambrian basement, extending from 7 to 11 km depth.

Rupture process of the Ms 6.6 Superstition Hills, California, earthquake determined from strong-motion recordings: Application of tomographic source inversion

Abstract: We analyze strong-motion recordings of the M s 6.6 Superstition Hills earthquake to determine the timing, location, spatial extent, and rupture velocity of the subevents that produced the bulk of the high-frequency (0.5 to 4 Hz) seismic energy radiated by this shock. The earthquake can be characterized by three principal subevents, the largest ones occurring about 3 and 10 sec after initiation of rupture. Timing relationships between pulses on the seismograms indicate that the three subevents are located within 8 km of each other along the northern portion of the Superstition Hills fault. The two largest subevents display different directivity effects. We apply a tomographic source inversion to the integrated accelerograms to determine the slip acceleration on the fault as a function of time and distance, based on a one-dimensional fault model. The azimuthal distribution of amplitudes for the second subevent can be largely explained by a rupture that propagated about 2 km to the southeast along the Superstition Hills fault at a velocity about equal to the P -wave velocity. An alternative model with rupture propagating to the northeast along a conjugate fault plane can also account for the observed directivity of this subevent, but it is not supported by the aftershock distribution. The third subevent ruptured to the southeast along an 8-km long portion of the Superstition Hills fault at about the shear-wave velocity. This rupture propagation caused the relatively large accelerations and velocities observed in strong-motion records for stations southeast of the hypocenter. The long time intervals between the subevents and their relative proximity to each other indicate a very slow component to the rupture development. The southern half of the Superstition Hills fault did not generate significant high-frequency strong ground motion, although it showed substantial co-seismic surface displacement. The subevents are situated along the same northern portion of the fault where most of the aftershocks are located. The locations of the subevents appear to be controlled by bends in the fault mapped at the surface and by changes in basement structure at depth.

Source parameters of the October 1, 1987 Whittier Narrows Earthquake from crustal deformation data

Abstract: Offsets in the regional strain field, generated by the October 1, 1987, Whittier Narrows earthquake, were recorded with large amplitudes on two deep-borehole dilational strainmeters at distances of 46.7 and 65.5 km from the hypocenter and marginally on instruments at greater distances in the Parkfield area and at Pinon Flat, where laser extensometers also recorded small offsets. These data are insufficient to solve for the location and physical parameters of the earthquake, but by also using the measured elevation changes in the epicentral area, we are able to invert for source models consistent with all available observations of crustal deformation. The source models obtained indicate that slip extends to a depth of about 30 km, well below the recorded aftershock zone. The requirement for deeper (and presumably aseismic) slip derives from the large negative dilatation experienced by the nearest strainmeter (PUBS), but high-frequency data from the same site exclude any significant slow component of moment release. By ignoring PUBS we obtain a moment of about 0.7 × 1018 N m on a fault that has a strike and dip of about N60°W and 40° down to the north, respectively, and extends to a depth of about 16 km. The most likely reason for the anomalous offset at PUBS appears to be sympathetic slip triggered on a nearby fault by the main shock. Precursive strain during the month prior to the earthquake is not apparent at the nanostrain level in the data from the closest instrument, but the event was accompanied by a change in strain rate at the two nearer sites.

Determination of fault planes at Coalinga, California, by analysis of patterns in aftershock locations

Abstract: The May 2, 1983, Coalinga, California, earthquake was not anticipated because it took place along a previously unrecognized fault concealed beneath an anticline, in a region that had little historic seismicity. The main shock hypocenter was at 10 km depth, and faulting did not break the surface. There has been considerable controversy about which of the nodal planes from the main shock fault plane solution was the slip plane. Conflicting results from geodetic, geologic, and aftershock data imply that slip occurred along one or the other or both nodal planes. The aftershock zone is large; several planar features within the aftershock zone indicate that faulting was complex. To delineate the planes of slip, we applied the three-point method, a statistical technique by which event planes can be determined from aftershock locations. We obtained several important results from our study. First we identified a number of planes using locations of aftershocks, some of which have not been previously identified from visual inspection of the aftershock locations. Second, the method allows us to choose slip planes by associating three-point planes with fault plane solutions based on proximity and similar orientation. Finally, three of the planes identified by the method intersect near themore » main shock hypocenter, making it difficult to determine which plane is the main shock slip plane. Nevertheless, aftershocks during the first 24 hours after the main shock clearly define the high-angle NE dipping nodal plane, whereas few aftershocks were located along the conjugate plane. Assuming that the aftershocks took place along the rupture zone, we conclude that slip during the main shock occurred predominantly along the high-angle NE dipping plane from the fault plane solution for this event. /copyright/ American Geophysical Union 1989« less

The 1984 southern Yellow Sea earthquake of Eastern China: Source properties and seismotectonic implications for a stable continental area

Abstract: On 21 May 1984, a strong shock ( MS = 6.3, mb = 5.7) occurred in the southern Yellow Sea which shook the densely populated coastal area of Eastern China. About 1 min before the main shock, a foreshock of mb 5.4 occurred. Because of interference from the foreshock, no reliable P -wave first motions or focal mechanisms have been obtained for the main shock from local seismic data. In this paper, long-period teleseismic data have been used to investigate the source mechanism. Fault plane solutions determined from first motions as well as P and SH waveforms show a predominant strike-slip motion with a small dip-slip component on a steeply dipping plane. The tectonic environment of this event supports the observation that margins of former rifts tend to be sites of midplate seismic source regions. The Yellow Sea developed in a cratonic environment due to early Cenozoic rifting. The major graben faults strike NNE-SSW to NE-SW and are intersected by second-order faults trending WNW-ESE. Aftershock distribution suggests that the fault which generated the main shock probably strikes WNW-ESE. The mechanism determined in this paper strikes 120°, dips 88°, and slips 28°. This solution suggests that the earthquake ruptured along a secondary fault near the intersection of a more major NNE-SSW trending fault. The epicentral area is subject to to an ENE-WSW compression and a NNW-SSE extension. The seismic moment and focal depth are determined to be 1.09 × 1025 dyne-cm and 12 ± 4 km, respectively. Relative hypocentral locations indicate that the focal depth of the foreshock is similar to that of the main shock, and the hypocenter of the foreshock is located very close to the WNW-ESE striking nodal plane of the main shock mechanism and is about 20 km away from the main shock hypocenter. Source properties of the southern Yellow Sea earthquake have been compared with those of three major earthquakes of North China. The stress drop of the Yellow Sea earthquake is determined to be about 42 bars. This value is much lower than the stress drops computed for the 1966 Hsingtai and the 1975 Haicheng earthquakes of North China. The southern Yellow Sea is characterized by short recurrence intervals, while the Hsingtai and Haicheng areas have very long recurrence intervals. The short recurrence intervals and low stress drop may reflect a lower material strength at the source region in the Yellow Sea.

Source parameters of the Cache Valley (Logan), Utah, earthquake of 30 August 1962

Abstract: The Cache Valley, Utah, earthquake of 30 August 1962 ( M s 5.7) has been reexamined. Our revised focal mechanism, with dip 43°, strike 193° and rake −101°, confirms oblique normal faulting with nodal planes striking approximately north and south. This mechanism is based mainly on polarities from records at U.S. Air Force LRSM stations, which are a valuable scientific resource. Inversion of long-period teleseismic body waveforms indicates centroid focal depth 10 km and seismic moment 3.1 × 10 24 dyne cm (0.31 × 10 18 Nm) corresponding to M w 5.6. The Cache Valley, like other north-south-trending extensional features in the epicentral region, is bounded on its east side by a major active normal fault, the East Cache fault, which has several km of Neogene throw. However, our revised source coordinates for the Cache Valley earthquake indicate it occurred too far east to have involved slip on this fault. Its hypocenter was near a downdip projection of the subparallel Temple Ridge fault, a less prominent feature with only ∼500 m of Neogene throw, oriented subparallel to the west-dipping nodal plane of our focal mechanism. We suggest that this fault is continuous and approximately planar between ∼10 km depth and the earth9s surface with ∼40° dip, and moved in the Cache Valley earthquake with a component of right-lateral strike-slip, with slip vector azimuth ∼N 65°W.

Hypocenter Determination Procedure of Japan University Network Earthquake Catalog

Abstract: The Earthquake Prediction Data Center (EPDC) of the Earthquake Research Institute, University of Tokyo, has been receiving the hypocentral parameters and arrival time data acquired through the University Information System for Earthquake Prediction Research, which is operated by Japanese national universities under the national program for earthquake prediction. Through the cooperation of these universities, the data and hypocenters were compiled and stored in the database system of the EPDC. There are two types of database; one is the real-time database and the other is the revised database which is sent by magnetic tapes from each regional center. EPDC has prepared to open these database for every seismologists to use and now the real-time database can be used by the real-time monitoring system and the revised database is open to be public as the Japan University Network Earthquake Catalog. The hypocentral coordinates and orgin times listed in the catalog are redetermined by EPDC using the arrival time data of the revised database. Although, the minimun magnitude of the earthquakes listed in the catalog is 2.0, the earthquakes listed in the catalog covers the microearthquake activities in Japan. In the present paper, we discuss the hypocenter determination procedure of the catalog and also the characteristics of the hypocenters listed in the catalog.

P-wave deflection or off-azimuth arrivals in the Charlevoix Seismic Zone

Abstract: During the months of August 1984 and October 1985, experiments were undertaken in the Charlevoix Seismic Zone to record high-frequency body waves from earthquakes. The seismograms were recorded by digital three-component seismographs. The experiments were conducted on the north shore of the St. Lawrence River only and covered nine sites. All but the two most northeasterly recording sites exhibited P -wave arrivals that arrived from azimuths that differed from the theoretical azimuth based on the computed hypocenter-station relationship. These differences amounted to several tens of degrees. Neither the errors in hypocenter location nor in the orientation of the horizontal seismometers are believed to be the cause of this difference. Theoretical computations based on a two-dimensional model with strong lateral velocity gradients or a discontinuity in velocity produce deviations of the direction of arrival of the P wave in a horizontal plane normal to the anomaly that can amount to several tens of degrees. Both the lateral gradient and discontinuity models satisfy the observed deviation of azimuth versus azimuth deflection. However, only the discontinuity satisfies the deflection versus hypocentral distance distribution, at least for distances to 20 km. Comparing the observed data with the theoretical values suggests that the anomaly in velocity occurs in a near-vertical plane striking parallel to the St. Lawrence River. This is also the direction of ancient extension faults in the region. Other structures were introduced 350 m.y. ago when the Charlevoix impact structure was formed. A number of anorthosite bodies with very high seismic velocities also outcrop in the region. Because of the high frequencies involved, the scale of the anomaly may well be only a few hundred meters, and the anomaly may most likely be located very close to, even directly beneath, the station.

The Darrington seismic zone in northwestern Washington

Abstract: Earthquakes occurring between 1971 and 1988 are evidence for a small zone of crustal seismicity under the western North Cascades near Darrington, Washington. Better-quality hypocenters imply the activity occurs on a fault or fault zone striking N80°W ± 20°, dipping nearly south at 40° ± 15°, with a length along strike of at least 10 km and possibly 20 km or more. We term this feature the Darrington Seismic Zone (DSZ). Focal depths range between 3 and 15 km. A single-event and a composite focal mechanism show nearly pure thrust faulting with one nodal plane in agreement with the hypocenter pattern. P axes strike N20°W to N25°W, in accord with a regional stress direction due to relative motion of the Pacific and North American Plates. No mapped fault can be identified as the surface expression of the zone. The area of the DSZ is adequate to generate a magnitude 5+ earthquake should it rupture in a single event, and an M L 5.6±earthquake on 29 April 1945 in the Cascades ESE of Seattle demonstrates that crustal earthquakes having such magnitudes are possible beneath the western North Cascades. The DSZ is the first crustal seismogenic structure to be identified beneath the North Cascades.

A Bayesian approach to the detection of temporal changes in P wave velocity

Abstract: On the basis of Akaike's Bayesian information criterion (ABIC), a new method is proposed for detecting a temporal change in a seismic velocity in a source region. The method of joint hypocenter determination was modified in order to determine a seismic velocity in a source layer as a function of time together with hypocenters and station corrections. Arrival times of initial waves of shallow earthquakes in a small area are analyzed in this method. The smoothness of the estimated temporal variation in the velocity is guaranteed by the introduction of a prior distribution of the parameter. The hyperparameter of the prior distribution of the velocity, the reading error of arrival times, and the initial velocity in the source layer are chosen to minimize ABIC. This procedure was applied to the 1983 eastern Yamanashi M = 6.0 earthquake in central Japan. We analyzed P arrival times of 374 earthquakes observed at 12 stations in the network of the National Research Center for Disaster Prevention by dividing the whole period (from October 1981 to May 1987) into 12 six-month subperiods. Calculating ABICs for different combinations of the three parameters above, we searched for the minimum value of ABIC and found two minima. The first one corresponds to a model of a constant velocity in time, and the other corresponds to a model of a variable velocity with 5% velocity change at maximum. However, since ABIC in the former is 10 smaller than that in the latter, the former constant velocity model is statistically more suitable than the latter. Furthermore, generating artificial data with the same reading errors as the actual data, we used computer simulation to examine the lower limit of the velocity change detectable for this data set. In conclusion, the velocity in the source region is 6.24±0.18 km/s, and the velocity change exceeding 6–7% at maximum did not exist during the 6 years before and after the M = 6.0 earthquake.

Complex source process of a small earthquake with m 4.8

Abstract: We observed the waveforms from a small earthquake (M=4.8), which corresponds to the largest aftershock of the 1983 Tottori earthquake, just above its hypocenter. The records have significant information on complicated source features. The P-waveforms recorded by a velocity seismograph are decomposed into a longer- (4 Hz) and a shorter-period (10-20 Hz) component. The source of the longer-period P-waves and S-waves can be theoretically explained by a uniform dislocation over a rectangular fault plane (1, 200 m × 800 m). Aftershocks accompanying this earthquake are distributed around the periphery of this fault plane. The source of the shorter-period P-waves corresponds to a multiple shock composed of two subevents whose source dimensions are each about 150m. Stress drops of the two subevents are about 10 to 100 times as large as that of the longer-period process. The distribution of aftershocks has a clustering structure. Events in each cluster appear to take place on a common fault plane appropriate to each cluster with a dimension of 100-200 m. The clustering structure shows heterogeneous distribution of fracture strength in the aftershock area. We may suppose such heterogeneities composed of pre-existing small weak zones also in the source area of this earthquake (M=4.8), in order to explain its complex source process. Namely, the rupture is considered to have extended over the rectangular fault including several weak zones, and the shorter-period waves with high stress drop mentioned above are expected to have been radiated from the fractures of strong patches among weak zones. The source process of micro to small earthquakes in a heterogeneous crust may be systematically explained as follows: The growth of rupture in the case of a microearthquake is confined within one or a few neighboring pre-existing weak zones (NISHIGAMI, 1987). On the other hand, the rupture in a small earthquake grows more extensively by fracturing not only several weak zones but also strong patches among them.