Showing papers on "Hypocenter published in 1988"

Linearized inversion for fault rupture behavior: Application to the 1984 Morgan Hill, California, earthquake

Abstract: We present a technique to infer the rupture history of an earthquake from near-source records of ground motion. Unlike most previous studies, each point on the fault is assumed to slip only once, when the rupture front passes, with a spatially variable slip intensity. In this parameterization the data are linearly related to slip intensity but nonlinearly related to rupture time. We perform a linearized inversion for slip intensity and rupture time by iteratively perturbing an assumed starting model. The inverse problem for the model perturbation is solved using a tomographic back projection technique. Smoothing and inequality constraints are applied to ensure that the resulting solution is stable and feasible. Asymptotic ray theory is used to calculate theoretical seismograms and partial derivatives with respect to model parameters. In test cases with noisy synthetic data sets we found that it is possible to distinguish between rupture models having variable slip amplitude and models having variable rupture velocity, provided the station coverage is adequate. We apply the technique to recordings of the April 24, 1984, Morgan Hill earthquake. The results indicate that slip amplitude on the fault plane was extremely variable and that the rupture front did not propagate uniformly away from the hypocenter. In particular, rupture was delayed on a 12 km2 section of the fault approximately 14 km to the southeast of the hypocenter. The rupture front surrounded the region, which subsequently failed with a component of rupture propagation back toward the hypocenter. Similar behavior has been observed in dynamic rupture models with stress or strength inhomogeneities. This segment of the fault ruptured with a large slip amplitude, releasing 12% of the total seismic moment from 4% of the total area of the aftershock zone. The surface trace of this section of the fault is characterized by a complex left step that could act to increase the normal stress acting across the fault. However, the distribution of aftershocks suggests that the fault at depth is simpler and that it may bend to the right. In either case, our rupture model suggests that this segment of the fault represents an asperity, which initially resisted rupture but eventually ruptured massively. We estimate the shear fracture energy for this earthquake to be 2×106 J/m2.

Inversion for slip distribution using teleseismic p waveforms: north palm springs, borah peak, and michoacan earthquakes

Abstract: We have inverted the teleseismic P waveforms recorded by stations of the Global Digital Seismograph Network for the 8 July 1986 North Palm Springs, California, the 28 October 1983 Borah Peak, Idaho, and the 19 September 1985 Michoacan, Mexico, earthquakes to recover the distribution of slip on each of the faults using a point-by-point inversion method with smoothing and positivity constraints. In the inversion procedure, a fault plane with fixed strike and dip is placed in the region of the earthquake hypocenter and divided into a large number of subfaults. Rupture is assumed to propagate at a constant velocity away from the hypocenter, and synthetic ground motions for pure strike-slip and dip-slip dislocations are calculated at the teleseismic stations for each subfault. The observed seismograms are then inverted to obtain the distribution of strike-slip and dip-slip displacement for the earthquake. Results of the inversion indicate that the Global Digital Seismograph Network data are useful for deriving fault dislocation models for moderate to large events. However, a wide range of frequencies, which includes periods shorter than those within the passband of the long-period Global Digital Seismograph Network instruments, is necessary to infer the distribution of slip on the earthquake fault. Although the long-period waveforms define the size (dimensions and seismic moment) of the earthquake, data at shorter periods provide additional constraints on the variation of slip on the fault. Dislocation models obtained for all three earthquakes are consistent with a heterogeneous rupture process where failure is controlled largely by the size and location of high-strength asperity regions.

Rupture process of an extended earthquake sequence: Teleseismic analysis of the Chilean Earthquake of March 3, 1985

Abstract: Broadband displacement and velocity records of P waves recorded at teleseismic distances are analyzed to determine the static and dynamic source parameters of the Chilean earthquake of March 3, 1985 (Ms 7.8) and seven large (mb > 5.6) aftershocks. Besides the usual parameters of depth, moment, and focal mechanism, the analysis provides estimates of radiated energy, associated stresses, and source dimension for seven of the eight modeled earthquakes. To insure that the parameters are describing the same physical aspect of the rupture process, the parameters for each shock are computed from the time window in which most of the seismic energy is radiated. The hypocenters of an additional 149 small foreshocks and aftershocks of 4.5 ≤ mb ≤ 5.6 are computed with the method of joint hypocenter determination. The spatial and temporal variation of source parameters and earthquake locations are used to infer a description of the rupture process before, during, and after the main shock. The main shock and modeled aftershocks occurred as thrust faulting on the interface between the Nazca and South American plates. A cluster of foreshocks in the 10 days preceding the main shock also involved thrust faulting on the plate interface. The main shock itself was a complex rupture consisting of three events. The first two events, denoted as ms1 and ms2, released minor amounts of energy, and they occurred on the periphery of the small zone defined by the foreshocks. The major release of energy occurred with the third event, denoted by MS, which nucleated downdip of ms1 and ms2. Within the time window of major energy release, rupture extended approximately 90 km to the south from the point at which MS nucleated. The size of the early aftershock zone, which far exceeds our inferred dimensions for the major shock, and the strong frequency dependence of scalar moment imply that substantial slow slip occurred on the plate interface that was not associated with major energy release. The dip of the seismically active interface increases landward from about 15°, at 20 km depth, to about 35°, at 40 km depth, where the seismogenic interface reaches its maximum observed depth. The vast majority of small and moderate aftershocks occurred in the shallower part of the interface, as did the foreshocks ms1 and ms2. It appears, however, that the asperities that control the rupture of the largest earthquakes are the asperities on the deeper interface. MS and the largest 1985 aftershock, that of April 9 (Ms 7.2), occurred on the deeper interface. The Chilean earthquake of July 9, 1971 (Ms 7.5), which occurred just north of the 1985 earthquake, also involved main shock rupture on the deeper interface and was followed by aftershocks on the shallower interface. Asperities on the deeper interface appear on average to be stronger, larger, and more uniform in size than asperities on the shallow interface. A greater average strength for deeper asperities is suggested by the observation that stress drops and apparent stresses of the main shock and the after-shock of April 9 (Ms 7.2) are higher than those of the large shallow aftershocks and is consistent with arguments that the maximum friction on a fault zone should be near the base of the seismogenic zone. One deep interface aftershock with low-stress drop and apparent stress may have been situated below the point of maximum friction on the interface. The inferred larger and more uniform sizes of the deeper asperities are suggested by the higher proportion of large to small shocks on the deeper interface. The downdip change in the asperity size distribution may reflect increased friction on the plate interace or the coalescence of small asperities into large asperities with increased subduction. The radiated energies of the main shock and large aftershocks that are computed directly from the broad bandwidth data are smaller than those predicted by standard empirical formulas by factors ranging from 4 to 20.

Event location at any distance using seismic data from a single, three-component station

Abstract: This paper demonstrates the usefulness of a novel technique for analysis of single-site, three-component seismogram records for reliable epicenter determinations for even small seismic events at any distance. Parameters that may be extracted by this technique are P -phase identifications in terms of x 2 probabilities and the associated slowness vectors. Using data from short-period and broadband seismometers installed in the new NORESS (Norway) array, single-site event locations are demonstrated: azimuth estimates are found directly from the slowness vector, while epicenter distance estimates are derived from differential arrival times between Pn, Pg, Sn , and Lg ( Sg ). Simultaneous inversion for crustal model and hypocenter data indicate a crustal thickness beneath NORESS of 33.5 km. If Sn and/or Lg are well-recorded, focal depth estimates are often accurate to within a few kilometers. Locating teleseisms is problematic, since secondary phases are not fully readily identifiable, and hence distance can be estimated only by conversion of the measured apparent phase velocity. For distances out to approximately 1000 km, event location errors as compared to network solutions seldom exceed 50 km and are due mainly to errors in azimuth estimates. With Sn phases available, focal depth errors appear to be of the order of 4 km. For teleseismic P waves (short-period), location errors occasionally exceeded 7° and were due mainly to poor distance estimates. Using broadband records from relatively strong events, location errors were about 1°. Further refinements seem feasible by using seismicity information and/or recognition of stationarity in the P -wave decomposition pattern for events within the same region. At least in certain cases, such approaches reflect genuine location refinements as demonstrated.

The response of creeping parts of the San Andreas fault to earthquakes on nearby faults: Two examples

Abstract: Rates of shallow slip on creeping sections of the San Andreas fault have been perturbed on a number of occasions by earthquakes occurring on nearby faults. One example of such perturbations occurred during the 26 January 1986 magnitude 5.3 Tres Pinos earthquake located about 10 km southeast of Hollister, California. Seven creepmeters on the San Andreas fault showed creep steps either during or soon after the shock. Both left-lateral (LL) and right-lateral (RL) steps were observed. A rectangular dislocation in an elastic half-space was used to model the coseismic fault offset at the hypocenter. For a model based on the preliminary focal mechanism, the predicted changes in static shear stress on the plane of the San Andreas fault agreed in sense (LL or RL) with the observed slip directions at all seven meters; for a model based on a refined focal mechanism, six of the seven meters showed the correct sense of motion. Two possible explanations for such coseismic and postseismic steps are (1) that slip was triggered by the earthquake shaking or (2) that slip occurred in response to the changes in static stress fields accompanying the earthquake. In the Tres Pinos example, the observed steps may have been of both the triggered and responsive kinds. A second example is provided by the 2 May 1983 magnitude 6.7 Coalinga earthquake, which profoundly altered slip rates at five creepmeters on the San Andreas fault for a period of months to years. The XMM1 meter 9 km northwest of Parkfield, California recorded LL creep for more than a year after the event. To simulate the temporal behavior of the XMM1 meter and to view the stress perturbation provided by the Coalinga earthquake in the context of steady-state deformation on the San Andreas fault, a simple time-evolving dislocation model was constructed. The model was driven by a single long vertical dislocation below 15 km in depth, that was forced to slip at 35 mm/yr in a RL sense. A dislocation element placed in the seismogenic layer under XMM1 was given a finite breaking strength of sufficient magnitude to produce a Parkfield-like earthquake every 22 years. When stress changes equivalent to a Coalinga earthquake were superposed on the model running in a steady state mode, the effect was to make a segment under XMM1, that could slip in a linear viscous fashion, creep LL and to delay the onset of the next Parkfield-like earthquake by a year or more. If static stress changes imposed by earthquakes off the San Andreas can indeed advance or delay earthquakes on the San Andreas by months or years, then such changes must be considered in intermediate-term prediction efforts.

Earthquakes in the Davie Ridge‐Madagascar region and the southern Nubian‐Somalian plate boundary

Abstract: The largest earthquake sequence (mb up to 6.4 and M0 up to 5 × 1018 Nm) to have struck eastern Africa within the past 60 years occurred in May and June of 1985 off the coast of the Tanzania-Mozambique border. The epicenters are located on the northern extension of the Davie Ridge, a prominent north-south trending bathymetrie feature accompanied by distinct gravity anomalies. A joint hypocenter location of the 14 largest events in this sequence appears as a cluster of epicenters with no apparent lineation. Al teleseismic distances, P and SH waves generated by the two largest events of this sequence have nearly identical waveforms on the long-period records of the WorldWide Standard Seismograph Network. A formal inversion of the data shows a pure normal faulting mechanism on nodal planes striking slightly west of north. At least two point sources are necessary to model each event, one at a depth of 40 ± 10 km and the other at about 20 ± 5 km. On a regional scale, normal faulting seems to characterize the scattered seismicity throughout the region between Madagascar and the eastern arm of the East African rift zone, even though oceanic lithosphere dominates part of the region and no topographic expression of a rift system can be identified. Since normal faulting events of up to 25 ± 5 km deep also characterize the scattered seismicity west of the topographic expression of the western arm of the rift zone at about the same latitude, we propose that the southern termination of the Nubian-Somalian plate boundary consists of a diffuse zone of east-west extension up to 2000 km wide.

Indication of active Faults in the New Madrid Seismic Zone from Precise Location of Hypocenters

Abstract: The hypocenter locations of the larger and better recorded earthquakes of the New Madrid seismic zone are examined in order to determine how closely the hypocenters lie along planar surfaces, thus relating the foci to active fault surfaces. For this purpose more than 500 earthquakes of the region have been selected for study, based on the number (7 or more) of observing stations used in the initial hypocenter location and on the quality of the P-wave onset. These events are relocated using a joint hypocenter-velocity-depth (JHVD) algorithm. The relocated earthquakes are separated geographically into three trends: ARK, the southwest trending zone from Caruthersville, Missouri, to Marked Tree, Arkansas; DWM, the northeast trending zone from New Madrid to Charleston, Missouri; and CEN, the central, left-stepping offset zone from Ridgely, Tennessee, to New Madrid, Missouri. Vertical profiles taken along and across the ARK and DWM trends verify the strike and dip of dominantly strike slip motion on near vertical active faults along these trends. These results agree with previously determined composite focal mechanism solutions for these trends. No coherent picture has been obtained for the CEN trend. As a by-product of the study, velocity models from the JHVD inversion are found to be reasonably uniform throughout the New Madrid seismic zone, and to offer supporting evidence for the presence of a shallow low velocity zone in the central portion of the Mississippi embayment.

The foreshock sequence of the 1986 Chalfant, California, earthquake

Abstract: The ML 6.4 Chalfant, California, earthquake of 21 July 1986 was preceded by an extensive foreshock sequence. Foreshock activity is characterized by shallow clustering activity, including 7 events greater than ML 3, beginning 18 days before the earthquake, an ML 5.7 foreshock 24 hr before the main shock that ruptured only in the upper 10 km of the crust, and an “off-fault” cluster of activity perpendicular to the slip surface of the ML 5.7 foreshock associated with the hypocenter of the main shock. The Chalfant sequence occurred within the local short-period network, and the spatial and temporal development of the foreshock sequence can be observed in detail. Seismicity of the July 1986 time period is largely confined to two nearly conjugate planes; one striking N30°E and dipping 60° to the northwest associated with the ML 5.7 foreshock and the other striking N25°W and dipping 70° to the southwest associated with the main shock. Focal mechanisms for the foreshock period fall into two classes in agreement with these two planes. Shallow clustering of earthquakes in July and the ML 5.7 principal foreshock occur at the intersection of the two planes at a depth of approximately 7 km. The seismic moments determined from inversion of the teleseismic body waves are 4.2 × 1025 and 2.5 × 1025 dyne-cm for the principal foreshock and the main shock, respectively. Slip areas for these two events can be estimated from the aftershock distribution and result in stress drops of 63 bars for the principal foreshock and 16 bars for the main shock. The main shock occurred within an “off-fault” cluster of earthquakes associated with the principal foreshock. This cluster of activity occurs at a predicted local shear stress high in relation to the slip surface of the 20 July earthquake, and this appears to be the triggering mechanism of the main shock. The shallow rupture depth of the principal foreshock indicates that this event was anomalous with respect to the character of main shocks in the region.

Coda-Q Before and After the 1983 Misasa Earthquake of M 6.2, Tottori Prefecture, Japan

Abstract: To detect any temporal change in coda-Q associated with an earthquake of M 6.2, digitized data from analog magnetic records of microearthquakes for a small confined region just around its hypocenter have been processed, with special attention to measuring errors. The M 6.2 earthquake took place at the Misasa town, Tottori Prefecture, in southwest Japan with preceding anomalous seismicity changes. TheQ values were elevated 20% around three years before the M 6.2 event for frequencies of 5 to 20 Hz, and tended to decrease around two years before. Data with high quality show undulated temporal variation with a period of 5–6 years before the earthquake, which is correlated with the regional seismic activity. There is a possibility that the observedQ change stands for precursory effect of the medium-scale earthquake. The aftershock sequence also shows an undulated temporal variation ofQ with a period of 150 days for around 10Hz, correlating also with the seismic activity. The fracturing processes by numerous microearthquakes may be responsible for the attenuation property of randomly scattered seismic waves.

Rupture extent of the 1978 Miyagi‐Oki, Japan, earthquake and seismic coupling in the northern Honshu Subduction Zone

Abstract: Underthrusting at subduction zones can cause large earthquakes at shallow depths, but is always accommodated by aseismic creep below a certain depth. This transition depth is referred to as the depth of seismic coupling and can be directly observed in a subduction zone as the lower depth extent of significant moment release of the deepest large underthrusting earthquakes. In 1978, a large (Ms=7.5) earthquake occurred off the coast of Miyagi Prefecture in northern Honshu. Its focal mechanism represents underthrusting of the Pacific plate beneath Honshu. Since the hypocenter is located 150 km landward from the trench and there are no other large interplate earthquakes further landward from the trench axis, this event defines the maximum depth of the coupled zone. The lower limit of significant moment release of the Miyagi-Oki earthquake is obtained by analysis of the long-period P waves. The deconvolved source time function consists of a dominant single pulse with peak moment release at 12 s and a total duration of 18 s. The rupture extent of this dominant pulse does not extend deeper than 40 km, thus the transition from coupled to uncoupled in northern Honshu occurs at or above 40 km depth.

Apparatus for estimating hypocentral distance

Abstract: PURPOSE:To permit estimation of the depth of hypocenter as well only with the information on the initial motion part at one observation point by measuring the oscillation waveforms of three components-vertical, zonal and meridional - and calculating the amplitude ratio V/H between the vertical and the horizontal oscillation components and also calculating the period, amplitude and max. value of V/H of the initial motion part successively. CONSTITUTION:The incoming of the seismic waves is monitored at all times by measuring the oscillation waveforms of the three components; vertical, zonal and meridional directions at one observation point and calculating the amplitude ratio V/H between the vertical and the horizontal oscillation components. Period, amplitude and the max. value of V/H in the initial motion part are calculated, upon incoming of the seismic waves. The hypocentral distance and the depth of hypocenter are then estimated from three sets of data. The accuracy is thereby improved even in the case of estimating the hypocentral distance of an earthquake having enough distance in excess of 200km. Since the depth of hypocenter is estimable at one observation point, the accuracy in outputting earthquake alarm is improved.