摘要:

We have located and estimated source parameters of 109 earthquakes (0 to 5M) using seismograms recorded at 2.5 km depth in the Cajon Pass borehole, southern California. The borehole is about 4 km from the San Andreas fault, at the boundary between the locked, almost aseismic Mojave and San Bernardino segments, where the San Jacinto fault approaches the San Andreas. This area is of interest both on account of its tectonic complexity and its high potential seismic hazard. The clear, relatively unattenuated downhole recordings are rotated to determine the incoming azimuth of thePwave, and the delay time between thePandSarrivals is used to estimate the hypocentral distance. The difference between the hypocenters located this way and the Southern California Seismic Network (SCSN) locations of the same earthquakes increases with hypocentral distance from the borehole. Of the earthquakes within 20 km of the borehole, 95% are within 1–2 km of the SCSN epicenters and all are within 5 km of the SCSN depths. All but three of the 58 earthquakes located here which the SCSN did not record were within 20 km of the borehole and so the location errors are similar to those of SCSN A and B quality events. Most of the small earthquakes occurred within the relatively active San Jacinto fault zone, but at least eight are located close to or within the San Andreas fault zone. The stress drops for these events range from 0.1 to 18 MPa. These earthquakes appear similar to the other events in this study suggesting that the San Andreas fault is similar to other faults at the scale of these small earthquakes. The variation in stress drop and slip orientation of these small earthquakes suggests that larger scale heterogeneity continues to these small scales. Also, the location of these small earthquakes in major established fault zones implies that earthquake source dimensions are not geometrically controlled by fault zone widt

ISSN:0148-0227    

DOI:10.1029/95JB02396

年代:1995

数据来源: WILEY

摘要:

The scaling relationships of earthquake sources less than about magnitude 3 have been the subject of considerable controversy over the last two decades. Studies of such events have shown a tendency for the constant stress drop, self similarity of larger earthquakes to breakdown at small magnitudes, and an apparent minimum source dimension of about 100 m has been observed. Other studies showed that this apparent breakdown in scaling could be an artifact of severe near‐surface attenuation, limiting the spatial resolution of surface data. In this study, source parameters are determined for over 100 nearby, tectonic earthquakes, from recordings at a depth of 2.5 km (in granite) in the Cajon Pass scientific drill hole, southern California. Comparison of the seismograms recorded at this depth with those at the wellhead clearly demonstrates the effect of the severe attenuation in the upper kilometers of the Earth's crust. Source parameters are calculated by spectral modeling of three‐component P and S waves, assuming four source models based on the Brune ω−2(n= 2) model. In model l,n= 2 is fixed, andQofPandSwaves is determined to be 912 (581–1433) and 1078 (879–1323), respectively (the numbers in parentheses are ±1 standard deviation). In model 2,QP=QS= 1000 is assumed andnis allowed to vary. The ω−2model is a good average for the data set, but there is some real scatter supported by the data. In model 3,QP=QS= 1000 is also assumed and ω−2is constrained, and in model 4, attenuation is ignored andnis allowed to vary. Source dimensions of less than 10m are observed for all four models, 10 times smaller than the proposed “minimum”. No breakdown in constant stress drop scaling is seen in the downhole data (approximatelyML‐1 to 5.5,M0= 109‐ 1016Nm). The ratio between radiated seismic energy (estimated by integrating the velocity squared spectra with adequate signal bandwidth) and seismic moment appears to decrease gradually with decreasing moment in the magnitude range −1 to 7. This is not incompatible with a constant stress drop but could result from errors in calculating energy. The ratio of theSwave energy to that radiated by thePwaves is about 14, after correction for attenuation. This low value is consistent with the corner frequency shift of about 1.3. This corner frequency shift is observed for all four source models and therefore is interpreted

ISSN:0148-0227    

DOI:10.1029/95JB02397

年代:1995

数据来源: WILEY

摘要:

On the basis ofP, PP, S, SSarrival times andSS‐S410S,SS‐S660Sdifferential times, we construct models of mantlePandSvelocity structure and boundary topography of the 410‐km and 660‐km discontinuities. Events from the catalog of the International Seismological Centre (ISC) are relocated relative to the International Association of Seismology and Physics of the Earth's Interior 1991 (IASP91) velocity model using bothPandSarrival times. The arrival times are corrected for ellipticity and thePPandSSresiduals are corrected for the topography at the bounce point. The cap‐averagedPP‐PandSS‐Sdifferential time residuals, plotted at thePPandSSsurface reflection points, form broad coherent patterns. The geographic distribution of the cap averaged residuals agrees quite well withPP‐PandSS‐Sdifferential time residuals derived from long period Global Digital Seismograph Network (GDSN) data. A robustlpinversion scheme is used to infer global mantle structure. Synthetic tests indicate that for regions well sampled bySS‐S410SandSS‐S660Sdifferential times, the velocity estimates are not seriously contaminated by the topography of the 410‐ and 660‐km discontinuities. However, estimates of boundary deflections may be influenced by extensivePandSvelocity variations of 3% or greater. We find the 410‐km discontinuity to be depressed by as much as 24 km beneath North America. Conversely, the discontinuity is deflected upward underneath Eurasia. In some regions the topography of the 660‐km discontinuity is quite distinct from that of the 410‐km discontinuity, but the two appear to be positively correlated. A series of depressions are found at several intersections of the 660‐km discontinuity with known subduction zones. The elevated topography in the 410‐km discontinuity beneath Europe is underlain by a trough in the 660‐km discontinuity. A number of subduction zones are characterized by a thinning of the transition zone. NegativePandSvelocity anomalies, underlying back‐arc basins and tectonically active continental regions, encircle the Pacific. Where they are resolved, the stable continental cratons are systematically positive velocity features that extend below 200 km. With the inclusion ofPPandSStravel time residuals we are better able to constrain midmantle structure. Most notably, in the depth range 35–660 km beneath the Northwest Pacific we observe highPvelocity. Where they are resolved, mid‐ocean ridges are most clearly imaged as low velocity features in theSmodel. The northern portion of the Mid‐Atlantic Ridge is underlain by negativeSvelocity anomalies. In the Pacific, the East Paci

ISSN:0148-0227    

DOI:10.1029/95JB00150

年代:1995

数据来源: WILEY

摘要:

An algorithm is presented which uses the surface waveforms originating from one event recorded in two stations which are located close to each other. The waveforms are inverted for local laterally homogeneousSvelocity structure or for path‐averaged horizontalSvelocity gradients. In the first case the stations are situated on the same great circle to the source. In the second case the two stations lie at approximately the same epicentral distance, but with a slightly different azimuth. In both cases, the frequency‐dependent phase differences between the two recordings are used as data for an inversion forSvelocity values. No a priori knowledge about the source parameters and the medium between the source and the nearest receiver is needed. These factors are estimated in the first step of the inversion (linear). In the second step of the inversion (nonlinear) theSvelocity information is obtained. Synthetic tests show that the algorithm is a powerful tool to image structure with only two fundamental mode surface wave recordings as data. In addition, they show that for the case where a horizontal gradient is determined, higher modes can be included in the analysis to display gradients at larger depths, without the need to identify the individual modes separately. The power of the method is illustrated with some real data examples that are all related to a pronounced tectonic boundary in western Eurasia: the Tornquist‐Teisseyre zone. The results of the experiments with real data coincide well with previously obtained models for this

ISSN:0148-0227    

DOI:10.1029/95JB02505

年代:1995

数据来源: WILEY

摘要:

We construct an earthquake instability model to estimate precursory faulting and ground deformation before the next moderate (M5.5–6) earthquake on the San Andreas fault near Parkfield, California. The quasi‐static model simulates fault slip, fault shear stress, and ground deformation for all stages of repeated earthquake cycles. Unstable slip, which is the analog of a moderate Parkfield mainshock, is caused by overall failure of a postulated strong area or patch of the fault zone. The brittle patch and surrounding weaker viscous fault are defined by the sign of a spatially varying coefficient of a slip rate dependent fault law. Stable failure of the patch leading up to mainshock failure causes slip rate to increase at depth. Amplitudes and timescales of the resulting preseismic deformation anomalies are found for the current earthquake cycle at Parkfield, starting just after the 1966 mainshock, in two ways. In the first way we vary parameter values in a sequence of simulations so as to find the simulation giving best agreement with locations, moments, and recurrence times of past moderate earthquakes and also with fault creep and trilateration line length changes since the 1966 mainshock. In the second way we assign values of most parameters from laboratory friction experiments and a temperature‐depth profile inferred from heat flow data. Overall agreement between model results and observations is comparable for the two methods. In all simulations, preseismic deformation anomalies arise from slip rate increase near the model earthquake focus and start several years or less before instability. Preseismic anomalies in surface fault creep and line length are always too small to detect in current measurements. However, for some simulations constrained mainly by field data the predicted anomalies in borehole dilatation are large enough to d

ISSN:0148-0227    

DOI:10.1029/95JB02517

年代:1995

数据来源: WILEY

摘要:

A theoretical piezomagnetic field is calculated as a function of time and ground position for an earthquake instability model applied to moderate (M∼ 6) earthquakes on the San Andreas fault near Parkfield, California. The instability model simulates fault slip and stress for all parts of a 32‐year earthquake cycle, including unstable (mainshock) slip. Mainshock slip occurs when a nearly locked patch of the fault becomes sufficiently loaded by surrounding aseismic slip that it reaches a condition of rapid failure. The piezomagnetic field is calculated directly from fault slip using analytic solutions based on piezomagnetic nuclei that are analogous to strain nuclei. During the interseismic interval, increasing stress concentration on the locked patch is accompanied by a gradual evolution of the magnetic anomaly field, leading to maximum cumulative changes of about ±2 nT in the vicinity of the pending epicenter. Mainshock slip cancels about half of the interseismically accumulated changes, and rapid post seismic relaxation cancels most of the rest. At certain sites near the locked patch the rate of change of the field progressively increases during the interseismic interval, while at others it progressively decreases. This effect is most pronounced during the last 3–4 years before the mainshock and, at suitably chosen pairs of sites, results in reversal of the temporal trend of the differential field. Such reversals, if separable from geomagnetic variation due to other sources, would constitute precursory indicators of approaching f

ISSN:0148-0227    

DOI:10.1029/95JB02516

年代:1995

数据来源: WILEY

摘要:

We consider a two‐dimensional model for quasi‐static evolution of four parallel faults. The model is based on the physics of continuum, with slips along the faults, long‐range interactions, a distribution of barriers and asperities, a velocity dependent friction, healing and reactivation of cohesive forces, and a mutual causality between faulting and tectonic forces all taken into account. Both the long‐term time series of seismic activity and details of seismic and aseismic crustal deformations during each phase of the earthquake cycle are produced by the model. It is shown that the system whose evolution is described by a set of deterministic differential equations may exhibit periodic or quite irregular (despite its assumed symmetry) behavior, depending on model par

ISSN:0148-0227    

DOI:10.1029/95JB02624

年代:1995

数据来源: WILEY

摘要:

The Rose Canyon fault zone in San Diego, California, has many well‐expressed geomorphic characteristics of an active strike‐slip fault, including scarps, offset and deflected drainages and channel walls, pressure ridges, a closed depression, and vegetation lineaments. Geomorphic expression of the fault zone from Mount Soledad south to Mission Bay indicates that the Mount Soledad strand is the most active. A network of trenches excavated across the Mount Soledad strand in Rose Creek demonstrate a minimum of 8.7 m of dextral slip in a distinctive early to middle Holocene gravel‐filled channel that crosses the fault zone. The gravel‐filled channel was preserved within and east of the fault but was removed west of the fault zone by erosion or possibly grading during development. Consequently, the actual displacement of the channel could be greater than 8.7 m. Radiocarbon dates on detrital charcoal recovered from the sediments beneath the channel yield a maximum calibrated age of about 8.1±0.2 kyr. The minimum amount of slip along with the maximum age yield a minimum slip rate of 1.07±0.03 mm/yr on this strand of the Rose Canyon fault zone for much of Holocene time. Other strands of the Rose Canyon fault zone, which are east and west of our site, may also have Holocene activity. Based on an analysis of the geomorphology of fault traces within the Rose Canyon fault zone, along with the results of our trenching study, we estimate the maximum likely slip rate at about 2 mm/yr and a best estimate of about 1.5 mm/yr. Stratigraphie evidence of at least three events is present during the past 8.1 kyr. The most recent surface rupture displaces the modern A horizon (topsoil), suggesting that this event probably occurred within the past 500 years. Stratigraphie and structural relationships also indicate the occurrence of a scarp‐forming event at about 8.1 kyr, prior to deposition of the gravel‐filled channel that was used as a piercing line. A third event is indicated by the presence of several fault strands that displace the channel but did not move during the most recent event. Other events may also have occurred, but these data suggest that the return time for surface‐rupturing earthquakes is no more t

ISSN:0148-0227    

DOI:10.1029/95JB02627

年代:1995

数据来源: WILEY

摘要:

TheSwave velocity structure of the lithopheric mantle and asthenosphere beneath Iberia is displayed by means of tomographic images obtained from dispersion data of Rayleigh waves propagating across the Iberian region. We have used long‐period data recorded at the broadband stations of the Network of Autonomously Recording Seismographs (NARS) installed in the Iberian Peninsula on the occasion of the Iberian Lithosphere Heterogeneity and Anisotropy (ILIHA) project. A total of 64 teleseismic events provided by the ILIHA array and 143 seismic paths have been studied. Surface wave dispersion analysis is carried out using various methods: from methods for a correct acquisition of data and subsequent two‐station surface wave velocity measurements to inversion methods for velocity structure and methods for verifying the reliability of the inversion results. The phase and group velocity dispersion curves of the fundamental mode Rayleigh waves are the basic information from which we have obtained several Earth models represented by shear wave velocity distributions with depth. Using these inversion results, we display the most conspicuous features of the velocity stnicture of the Iberian lithosphere‐asthenosphere system and propose a new regional model, the Iberian Lithosphere‐Asthenosphere (ILA) model, for the deep structure of the Iberian Peninsula down to a depth of 200 km. We use a representation technique based on an iterative Laplacian interpolation method for obtaining tomographic images of the subcrustal lithosphere and asthenosphere of Iberia. We find significant lateral velocity variations in the peninsula, though these differences vary with depth interval. A low‐velocity channel in the lithosphere (41–51 km) appears with nonuniform velocity structure. In contrast, at the greatest lithospheric depths (51–81 km), almost the entire peninsula shows a rather uniform velocity structure. The asthenosphere (81–181 km) is clearly a nonhomogeneous gross layer both laterally and with depth. The relatively higher velocities span the shallower depths within the asthenosphere, whereas the lower ones span

ISSN:0148-0227    

DOI:10.1029/95JB00979

年代:1995

数据来源: WILEY

摘要:

We describe a spectral technique to measure the apparent attenuation of compressional waveforms recorded during an active seismic tomography experiment centered at 9°32′N on the East Pacific Rise. Over 3500 estimates oft* are obtained from 0.4‐ to 0.7‐s‐long windows aligned with crustalPphases, including diffractions above and below a midcrustal magma chamber and Moho reflections which cross the rise axis at a range of lower crustal depths. We apply a smoothest model inversion algorithm to thet* measurements to derive images of apparent crustalQ−1both 20 km off axis and within a 16×16 km area centered on the rise crest. The models resolve regions of high attenuation in the uppermost crust and in a low‐Qzone which extends from midcrustal to lower‐crustal depths beneath the rise axis. Off axis,Qvalues in the upper 1 km average 35–50, while at depths greater than 2–3 kmQis at least 500–1000. The high levels of attenuation in the uppermost crust probably result from the combined effect of factional, fluid flow, and scattering mechanisms. Within 1–3 km of the rise axis,Qincreases markedly in the uppermost 1 km to about 65. If the increase in attenuation off axis is entirely due to the ∼300‐m increase in the thickness of layer 2A extrusives required by seismic velocity measurements, thenQin layer 2A must be about 10–20. No measurements ofQare obtained in the immediate vicinity of the 1.6‐km‐deep axial magma lens because no wave paths cross the rise axis through this region. The diffractions beneath the magma chamber and the Moho reflections require a low‐Qregion, with minimumQvalues of 20–50, which extends from no more than 2.5 km depth to the base of the crust. These values are similar to laboratory measurements ofQobtained at solidus temperatures and constrain the low‐Qregion to contain no more than a few percent melt. The axial magma chamber, which comprises a melt lens and an underlying crystal mush zone, must be confined to a narrow, 1‐km‐thick region through which no rise‐crossing paths pass. Inversions for along‐axis structure in the low‐Qanomaly show a 20–25% increase in attenuation at 2–3 km depth north of 9°34′N but resolve no such trend at 4–6 km depth. The along‐axis variations may reflect the recent history of volcanic eruptions and hydrothermal cooling and do

ISSN:0148-0227    

DOI:10.1029/95JB02280

年代:1995

数据来源: WILEY