North Palm Springs Earthquake Research Articles

Remotely Triggered Earthquakes Following Moderate Mainshocks (or, Why California Is Not Falling into the Ocean)

On several occasions in recent memory California has experienced apparent clusters of earthquake activity that are too far apart to be considered related according to a classic taxonomy that includes foreshocks, mainshocks, and aftershocks. During a week-long period in July 1986, California experienced the M 6.0 North Palm Springs earthquake, the M 5.5 Oceanside earthquake, and a swarm of smaller events beneath San Diego Bay. The recent M 6.0 Parkfield earthquake was followed approximately 30 hours later by the M 5.0 Arvin event, which was located well outside the traditional aftershock zone for a M 6.0 mainshock. These periods of apparently heightened activity lead to understandable consternation among California residents, who wonder if activity will build further. The recent, memorably dramatic television mini-series, 10.5, was based on what might be considered an end-member doomsday scenario, culminating in a large part of California literally falling into the ocean. While the public did seem to recognize the gross liberties that were taken with science in this movie, old myths die hard, and seismicity maps showing activity in different parts the state are not reassuring. Neither is what used to be conventional wisdom on the part of the experts, that far-flung earthquakes are not related (even though this might remain a possibility). Since 1992, however, scientists have come to understand that earthquakes can be related over greater distance and time scales than previously recognized. In addition to developing theories of static stress transfer ( e.g. , Das and Scholz, 1981; King et al. , 1994; Toda and Stein, 2003), remotely triggered earthquakes were first identified in 1992 (Hill et al. , 1993) and have subsequently been observed following large (generally M > 7) earthquakes in California as well as in other regions. Investigations of “earthquake interactions” remain at the forefront of earthquake …

Complexities of the San Andreas fault near San Gorgonio Pass: Implications for large earthquakes

Geologic relationships and patterns of crustal seismicity constrain the three‐dimensional geometry of the active portions of San Andreas fault zone near San Gorgonio Pass, southern California. Within a 20‐km‐wide contractional stepover between two segments of the fault zone, the San Bernardino and Coachella Valley segments, folds, and dextral‐reverse and dextral‐normal faults form an east‐west belt of active structures. The dominant active structure within the stepover is the San Gorgonio Pass‐Garnet Hill faults, a dextral‐reverse fault system that dips moderately northward. Within the hanging wall block of the San Gorgonio Pass‐Garnet Hill fault system are subsidiary active dextral and dextral‐normal faults. These faults relate in complex but understandable ways to the strike‐slip faults that bound the stepover. The pattern of crustal seismicity beneath these structures includes a 5–8 km high east‐west striking step in the base of crustal seismicity, which corresponds to the downdip limit of rupture of the 1986 North Palm Springs earthquake. We infer that this step has been produced by slip on the linked San Gorgonio Pass‐Garnet Hill‐Coachella Valley Banning (SGP‐GH‐CVB) fault. This association enables us to construct a structure contour map of the fault plane. The large step in the base of seismicity downdip from the SGP‐GH‐CVB fault system probably reflects a several kilometers offset of the midcrustal brittle‐plastic transition. (U/Th)/He thermochronometry supports our interpretation that this south‐under‐north thickening of the crust has created the region's 3 km of topographic relief. We conclude that future large earthquakes generated along the San Andreas fault in this region will have a multiplicity of mostly specifiable sources having dimensions of 1–20 km. Two tasks in seismic hazard evaluation may now be attempted with greater confidence: first, the construction of synthetic seismograms that make useful predictions of ground shaking, and second, theoretical investigations of the role of this complexity in retarding the propagation of future seismic ruptures.

Nonstationary analysis of geomagnetic time sequences from Mount Etna and North Palm Springs earthquake

Volcanomagnetic and/or seismomagnetic effects are geomagnetic variations generated before eruptions and/or seismic events. Our aim is to analyze geomagnetic time series to detect the volcanomagnetic and/or seismomagnetic effects among a number of other variations. Two advanced signal‐processing techniques are proposed to analyze the geomagnetic time series. The first technique, called Continuous Wavelet Transform Singularity Analysis (CWTSA), is based on the Continuous Wavelet Transform; the second, called Time‐Variant Statistical Analysis of Nonstationary Signals (TVANS), is based on a time‐varying adaptive algorithm (Recursive Least Squares). Both techniques are very effective in detecting the geomagnetic variations at the time instants likely linked to volcanic and/or seismic activity. The application of these methodologies to geomagnetic time sequences, respectively, recorded on Mount Etna during the volcanic activity of 1981 and in North Palm Springs during the seismic events of 8 July 1986 yields a good correspondence between events detected by both techniques and volcanic end seismic events. The statistical significance of geomagnetic time series was also assessed to verify the obtained results from CWTSA and TVANS. It was defined at significance level of 95% in the wavelet power spectrum for the difference of the geomagnetic time series aiming at distinguishing the most “significant” events when they are upon this one.

A Transportable System for Monitoring Ultralow Frequency Electromagnetic Signals Associated with Earthquakes

Claims that electromagnetic (EM) signals occur associated with some earthquakes, typically prior to but sometimes during seismic activity, have appeared in the literature for several decades (Parrot and Johnston, 1989; Park, et al., 1993; Park, 1996; Johnston, 1997). Such claims cover an exceptionally broad range of phenomena. For example, Gokhberg et al. (1982) and Yoshino (1991) observed increases in signal amplitude at 81 kHz from minutes to hours prior to earthquakes and up to hundreds of kilometers away from epicenters, and attributed these signals to seismoelectric emissions. Seismoelectric signals (SES) of a different nature have been suggested to precede some earthquakes in Greece (Varotsos et al., 1993a; Varotsos et al., 1993b). These SES are transients of amplitudes of 20 mv/km and durations of several minutes recorded on multiple dipoles of different length, and their mechanism of generation is subject to debate (Lighthill, 1996; Pham et al., 1999). Reported claims of EM anomalies associated with earthquakes also extend over a large frequency range, from megahertz down to quasi-dc. At the low end of the frequency range, Johnston and Mueller (1987) observed magnetic field offsets coinciding with the 1986 North Palm Springs earthquake, which occurred in Southern California close to the San Andreas Fault, and Johnston et al. (1994) also observed offsets at the time of the 1992 Landers earthquake in the same region. At the high-frequency end, radio emissions at 18 MHz were recorded on multiple northern hemisphere receivers for about fifteen minutes before the 1960 great Chilean earthquake (Warwick et al., 1982). In the ULF (0.01-10 Hz) frequency range, Fraser-Smith et al. (1990) recorded anomalous magnetic field fluctuations prior to the 17 October 1989 Loma Prieta Ms = 7.1 earthquake in central California. In particular, there was an amplitude increase in activity about …

Modelling the compliance of crustal rock-II. Response to temporal changes before earthquakes

SUMMARY There have been several claims that seismic shear waves respond to changes in stress before earthquakes. The companion paper develops a stress-sensitive model (APE) for the behaviour of low-porosity low-permeability crystalline rocks containing pervasive distributions of fluid-filled intergranular microcracks, and this paper uses APE to model the behaviour before earthquakes. Modelling with APE shows that the microgeometry and statistics of distributions of such fluid-filled microcracks respond almost immediately to changes in stress, and that the behaviour can be monitored by analysing seismic shear-wave splitting. The physical reasons for the coupling between shear-wave splitting and differential stress are discussed. In this paper, we extend the model by using percolation theory to show that large crack densities are limited at the grain-scale level by the percolation threshold at which interacting crack clusters lead to pronounced increases in rock-matrix permeability. In the simplest formulation, the modelling is dimensionless and almost entirely constrained without free parameters. Nevertheless, APE modelling of the evolution of fluid-saturated rocks under stress reproduces the observed fracture criticality and the narrow range of shear-wave azimuthal anisotropy in crustal rocks. It also reproduces the behaviour of temporal variations in shear-wave splitting observed before and after the 1986, M= 6, North Palm Springs earthquake, Southern California, and several other smaller earthquakes. The agreement of APE modelling with a wide range of observations confirms that fluid-saturated crystalline rocks are stress-sensitive and respond to changes in stress by critical fluid-rock interactions at the microscale level. This means that the effects of changes in stress and other parameters can be numerically modelled and monitored by appropriate observations of seismic shear waves.

Seismic behavior of the southern San Andreas fault zone in the northern Coachella Valley, California: Comparison of the 1948 and 1986 earthquake sequences

Abstract Moderate earthquakes in 1948 and 1986 that occurred along the southern San Andreas fault zone are examined to help understand the subsurface geometry and seismic behavior of active fault segments in the northern Coachella Valley. The 1986 North Palm Springs earthquake occurred between the Banning and Mission Creek traces of the San Andreas fault zone. Based on data from both portable and permanent stations, aftershocks of the 1986 event define a nearly planar surface that strikes about N60° to 70°W, is about 15-km long, and dips northeast from near the Banning surface trace at about 45° to 50°. Relocation of the mainshock using revised station corrections and a local velocity model inverted from the aftershock arrival times indicates a location at 34°N00.26′, 116°W36.34′, and a focal depth of 10.4 km. The first-motion focal mechanism for the mainshock indicates pure right-slip on a plane dipping northeast at 40° to 45° with a strike of N60°W. Previous moment-tensor solutions (e.g., Hartzell, 1989) exhibit significant oblique-reverse slip on a plane with a more westerly strike and a seismic moment of 1.4 to 1.7 × 1018 N-m (Mw = 6.1). Actual and synthetic Wood-Anderson recordings indicate a local magnitude of ML 6.0. The 1948 Desert Hot Springs earthquake produced waveforms similar to 1986, although amplitudes for 1948 are typically 20 to 30% larger. The 1948 mainshock is relocated at 33°N55.2′, 116°W28.9′, with a focal depth of 12 km, a revised magnitude of ML 6.3, and an estimated seismic moment of 2 to 3 × 1018 N-m. Based on arrival-time information from close (<20 km) portable stations deployed in 1948 to 1949, the 1948 aftershock distribution is about 15-km long, 8- to 10-km wide, and abuts the 1986 aftershock sequence to the southeast along the Banning fault. The 1948 aftershock zone is consistent with a focal mechanism that exhibits predominantly right-slip motion (rake 169°) on a plane that strikes N55°W and dips more steeply northeast at 60° to 70°. In 1986, the horizontal extent of the rupture may have been controlled by secondary cross faults and possible changes in fault geometry, while the down-dip extent may have been controlled by the presence at depth of a regional detachment. In 1948, the rupture likely had a steeper dip than in 1986 and extended southeast as far as the northern Indio Hills, where the surface trace of the Banning fault undergoes an approximate 7° change in strike. These results indicate that the Banning fault is nonvertical, is likely segmented according to fault dip, as well as fault strike, and has been the primary locus of recent moderate-sized earthquake activity in the northern Coachella Valley.

Magnetic field observations in the near-field the 28 June 1992 Mw 7.3 Landers, California, earthquake

Abstract Recent reports suggest that large magnetic field changes occur prior to, and during, large earthquakes. Two continuously operating proton magnetometers, LSBM and OCHM, at distances of 17.3 and 24.2 km, respectively, from the epicenter of the 28 June 1992 Mw 7.3 Landers earthquake, recorded data through the earthquake and its aftershocks. These two stations are part of a differentially connected array of proton magnetometers that has been operated along the San Andreas fault since 1976. The instruments have a sensitivity of 0.25 nT or better and transmit data every 10 min through the GOES satellite to the USGS headquarters in Menlo Park, California. Seismomagnetic offsets of −1.2 ± 0.6 and −0.7 ± 0.7 nT were observed at these sites. In comparison, offsets of −0.3 ± 0.2 and −1.3 ± 0.2 nT were observed during the 8 July 1986 ML 5.9 North Palm Springs earthquake, which occurred directly beneath the OCHM magnetometer site. The observations are generally consistent with seismomagnetic models of the earthquake, in which fault geometry and slip have the same from as that determined by either inversion of the seismic data or inversion of geodetically determined ground displacements produced by the earthquake. In these models, right-lateral rupture occurs on connected fault segments in a homogeneous medium with average magnetization of 2 A/m. The fault-slip distribution has roughly the same form as the observed surface rupture, and the total moment release is 1.1 × 1020 Nm. There is no indication of diffusion-like character to the magnetic field offsets that might indicate these effects result from fluid flow phenomena. It thus seems unlikely that these earthquake-generated offsets and those produced by the North Palm Springs earthquake were generated by electrokinetic effects. Also, there are no indications of enhanced low-frequency magnetic noise before the earthquake at frequencies below 0.001 Hz.

The seismic coda, site effects, and scattering in alluvial basins studied using aftershocks of the 1986 North Palm Springs, California, earthquake as source arrays

Abstract We use single-station recordings of widely distributed earthquake sources to identify heterogeneities in the Earth9s crust that scatter energy into the early part of the observed S -wave codas. We assume that ray theory accurately describes the propagation of body waves from the earthquake sources to the scatterers, but we assume no propagation mechanism between the scatterers and the seismic station. A grid of hypothetical scatterers is assumed to cover the Earth9s surface, and the strength of each scatterer is determined by an iterative inversion method. Our data are recordings of aftershocks of the 1986 North Palm Springs earthquake recorded at a seismic station in the Coachella Valley, SMP, and at a station in the Morongo Valley, SMC. The technique is applied separately to each horizontal component of motion at each station. We identify a particular spot at the northwest edge of the Quaternary alluvium in the Coachella Valley that scatters waves into the S coda observed at SMP 8 km distant in the valley. The geologic structure that causes the strong scattering seems to be the basin-edge structure rather than the mountainous terrain 3 km farther west. The early coda at SMC is dominated by scattering within a few kilometers of SMC, either in the basin itself or in the Morongo Valley fault zone. This study shows that laterally propagating waves are sometimes observable in the early coda, casting doubt on the use of 1-D techniques for estimating site responses. The northwest edge of the Coachella Valley seems to be an especially strong scatterer of incident waves. If other basins have similar loci of strong scattering, perhaps these places can be identified using microearthquake seismograms and used to predict strong ground motions in the basins.

On the inference of absolute stress levels from seismic radiation

Spudich, P.K.P., 1992. On the inference of absolute stress levels from seismic radiation. In: T. Mikumo, K. Aki, M. Ohnaka, L.J. Ruff and P.K.P. Spudich (Editors), Earthquake Source Physics and Earthquake Precursors. Tectonophysics, 211: 99–106. This paper determines the conditions under which it is possible to learn the absolute stress level at a subset of points on a fault from observation of the fault kinematics. Specifically, the points on a rupturing fault can be divided into two groups, those points at which the rake rotates during the rupture process, and those points at which the rake does not rotate. If it is true that sliding frictional traction is collinear with the instantaneous velocity of one side of a fault, then at the former group of points (having rotating rakes) there is a unique absolute stress consistent with the motion, while at the latter group of points (having rakes that do not rotate) the absolute stress is unspecified by the motion. Published dislocation solutions for the 1979 Imperial Valley, 1986 North Palm Springs, 1987 Whittier Narrows, and 1989 Loma Prieta, California, earthquakes are examined for evidence of temporal rotations of rake at the hypocenters and largest asperities. The 1986 North Palm Springs earthquake shows evidence of a possible rake rotation at the hypocenter. This may imply a low absolute stress level.

Travel‐time tomography in the northern Coachella Valley using aftershocks of the 1986 ML 5.9 North Palm Springs Earthquake

Tomographic inversion is applied to delay times from aftershocks of the 1986 ML 5.9 North Palm Springs (NPS) earthquake to image 3‐D velocity variations within the northern Coachella Valley. P‐wave arrival times from 1074 earthquakes, with depths ranging from 3 to 20 km, were used as sources recorded by 12 portable and 4 permanent stations. Preliminary results show well‐defined high‐ and low‐velocity anomalies (2–7%) that correlate with the rupture distribution of the 1986 mainshock. At depths less than 8 km, a low‐velocity anomaly predominates between the two NE‐dipping Banning and Mission Creek faults. From 8 to 12 km in depth, where the NPS mainshock and most of the aftershocks occur, a high‐velocity anomaly is observed. This high‐velocity feature is interpreted as imaging the asperity responsible for the 1986 rupture; and suggests that velocity information may help to define important elements, such as asperities, that control fault rupture, and thus, may help to predict the location and size of future events.

Travel‐time tomography in the northern Coachella Valley using aftershocks of the 1986 ML 5.9 North Palm Springs Earthquake

Travel‐time tomography in the northern Coachella Valley using aftershocks of the 1986 ML 5.9 North Palm Springs Earthquake

Observations and implications of water well and creepmeter anomalies in the Mojave segment of the San Andreas fault zone

Four years of continuous water level data from a 1,900 m deep well near the San Andreas fault in southern California show a 21-month period of unusual rapid fluctuations. The anomalous water level activity ceases with the occurrence of the 8 July 1986, ML = 5.6 North Palm Springs earthquake on the San Andreas fault, approximately 120 km southeast of the well site. The water level data correlate with signals from two San Andreas fault zone strain sensors. The long-term water level variations are similar in shape to those recorded by a borehole dilatometer 30 km from the well site, while the rapid water level fluctuations correlate in time with anomalies in the record of a fault zone creepmeter located 35 km from the water level monitor. The observations are interpreted in terms of strain occurring in a horizontally and vertically heterogeneous crust. Tectonic deformation is concentrated along weak block-bounding fault zones. The blocks are sufficiently rigid to generate correlated displacement phenomena along their weak boundaries. As a consequence, fault zone strain events can be related over large distances as in the case of the North Palm Springs earthquake and the coincident creep and water level anomalies. The observed creep and water level events occur in a weak aseismic shallow (<3 km) material overlying stronger asperities along which the fault is locked and regional strain loading occurs. Local failure of the fault within the weak shallow material has no far-field effects and is not important in the crustal stress budget, but may signal accelerated strain accumulation at depth. In this context, the reported observations demonstrate the possibility of joint detection of fault zone anomalies that relate to, and possibly anticipate, major tectonic strain events on large faults.

Seismic site effects and the spatial interpolation of earthquake seismograms: Results using aftershocks of the 1986 North Palm Springs, California, earthquake

Abstract We address the following two questions. Can a microearthquake's ground motions be modeled by incident P and S waves that excite a site transfer-function that is a smooth function of incidence angle? Given recorded ground motions from a set of earthquakes having known locations and mechanisms, can we derive such a site transfer-function and use it to obtain the ground motions that would result from an earthquake source occurring somewhere in the same volume but having a location and mechanism that are different from the recorded events? Although many factors will cause two distinct microearthquake sources to have different seismograms at a common station, in this paper we concentrate only upon the differences caused by source mechanisms, P- and S-wave travel-time variations and by variations in the site transfer-function. We specifically exclude the effects of waves scattered from heterogeneities in the geologic structure away from the seismic site. We express the site transfer-function as a sum of several terms having simple dependences upon incidence angle and azimuth. Each term is an independent function of time. Given a set of seismograms observed at the site, we solve a linear system of equations for the time dependences of each term. These time series may be used to calculate the seismograms that would have resulted from an earthquake having arbitrary mechanism and location. This step is an interpolation. We have applied this technique to seismograms after aftershocks of the 1986 North Palm Springs earthquake. Our interpolation technique works fairly well within the volume occupied by the recorded events, but the method is not very successful at providing accurate seismograms for sources located outside the aftershock volume. The primary causes of the inaccuracy are the inadequacy of our chosen angular functions to model the site response fully and the likely scattering of seismic waves by geological heterogeneities (in this case, the Banning and Mission Creek faults) near the seismic stations. Our methods could be used to determine the effects of single scattering from lateral heterogeneities in geologic structure.

Quantitative measurements of shear wave polarizations at the Anza Seismic Network, southern California: Implications for shear wave splitting and earthquake prediction

We analyze shear wave polarizations from local earthquakes recorded by the Anza network in southern California, using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival (the linearity interval). Initial shear wave particle motions are strongly aligned at four stations, a feature that is not predicted by focal mechanisms. The particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations, a result supported by the clear shear wave splitting seen in a borehole recording near one of the Anza stations. These results are consistent with an earlier analysis of these data by Peacock et al. [1988]. However, our analysis does not support claims by Crampin et al. [1990] that shear wave splitting delay times at station KNW exhibit temporal variations which can be correlated with the occurrence of the North Palm Springs earthquake (ML=5.6) of July 8, 1986. Automatically determined linearity intervals scatter widely from 0.02 to 0.15 s and exhibit no clear temporal trends. We find a correlation between earthquake moment and the linearity interval, possibly a result of longer effective source time functions for the larger events. The inability to identify a distinct slow quasi‐shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion, even in the very early shear wave coda. Analysis of earthquake clusters with similar waveforms recorded at KNW shows that seismic Green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events. If shear wave splitting is causing the observed delay times between horizontal components, the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5–10%.

Changes in shear wave splitting at Anza near the time of the North Palm Springs Earthquake

Changes in shear wave splitting are observed at KNW before and after the M = 6 North Palm Springs earthquake of July 8, 1986. KNW is a station of the Anza seismic network monitoring the Anza seismic gap on the San Jacinto fault, southern California. The gradual increase in the delays between the split shear waves over 3 years at KNW, reported by Peacock et al. (1988), ended in June 1986. The further 2 years of observations analyzed here show that the behavior of the delays changed abruptly near the time of the North Palm Springs earthquake, 33 km north of KNW. Peacock et al. demonstrated that the increase in delays could be simulated by increasing the aspect ratio of stress‐aligned fluid‐filled inclusions, and speculated that this increase might be the result of a build up of stress before an impending earthquake. The new data appear to confirm this speculation, but the temporal variations require a more complex interpretation, although they still suggest that the changes in shear wave splitting are caused by earthquake‐induced stress changes to the fluid‐filled inclusions throughout the rockmass. Central to our interpretation of temporal changes in shear wave splitting is the well established existence, throughout at least the uppermost 10 to 20 km of the crust, of small fluid‐filled cracks, microcracks, and pores. The existence of such inclusions introduces a compliant quality to otherwise stiff crustal rock. We term these distributions of inclusions extensive dilatancy anisotropy or EDA, and the individual inclusions EDA cracks because, although they may include a wide range of shapes, many of the seismic properties can be simulated by distributions of thin parallel cracks.

Source parameters for small events associated with the 1986 North Palm Springs, California, earthquake determined using empirical Green functions

Using small events as empirical Green functions, which include path, site, and instrument effects on the P waveforms, source parameters were estimated for 25 ML 3.4 to 4.4 events associated with the 1986 North Palm Springs earthquake. The static stress drops ranged from 3 to 80 bars, for moments of 0.7 to 11 × 1021 dyne-cm. There was a spatial pattern to the stress drops of the aftershocks which showed increasing values along the fault plane toward the northwest compared to relatively low values near the hypocenter of the mainshock. The highest values were outside the main area of slip, and are believed to reflect a loaded area of the fault that still has an higher level of stress which was not released during the main shock. The relatively high stress drop of a preshock in 1983, located close to the hypocenter of the main shock, is also interpreted as evidence that stress drops of magnitude 3 and 4 events can be used as indicators of the stress level in a particular area.

Temporal and spatial variation on codaQ ?1 associated with the North Palm Springs earthquake of July 8, 1986

A temporal and spatial change of codaQ−1 associated with the occurrence of the North Palm Springs earthquake of July 8, 1986 was studied by using 242 small local earthquakes in the vicinity of the mainshock. We found that the codaQ−1 of earthquakes which occurred before the mainshock was significantly higher than that of the aftershocks in the mainshock area while the codaQ−1 for the surrounding area remained almost constant throughout 1986. CodaQ−1 was determined separately for the lapse time windows of 10 to 20 sec. and 15 to 40 sec. for the period from 1981 to 1987. The result for the time window 10 to 20 sec. showed a peak in codaQ−1 before the time of mainshock at all frequencies. The peak appeared earlier at lower frequencies. There was no significant change in codaQ−1 for the time window 15 to 40 sec., probably because the change was restricted to a small area.

ON THE USE OF VOLUMETRIC STRAIN METERS TO INFER ADDITIONAL CHARACTERISTICS OF SHORT-PERIOD SEISMIC RADIATION

Volumetric strain meters (Sacks-Evertson design) are installed at 15 sites along the San Andreas fault system, to monitor long-term strain changes for earthquake prediction. Deployment of portable broadband, high-resolution digital recorders (GEOS) at several of the sites extends the detection band for volumetric strain to periods shorter than 5 x 10 -2 sec and permits the simultaneous observation of seismic radiation fields using conventional short-period pendulum seismometers. Simultaneous observations establish that the strain detection bandwidth extends from periods greater than 107 seconds to periods near 5 x 10 -2 sec with a dynamic range exceeding 140 dB. Measurements of earth-strain noise for the period band, 107 to 10 -2 sec, show that ground noise, not instrument noise, currently limits the measurement of strain over a bandwidth of more than eight orders of magnitude in period. Comparison of the short-period portion of earth-strain, noise spectra (20 to 5 x 10 -2 sec) with average spectra determined from pendulum seismometers, suggest that observed noise is predominantly dilatational energy. Recordings of local and regional earthquakes indicate that dilatometers respond to P energy but not direct shear energy and that straingrams can be used to resolve superimposed reflected P and S waves for inference of wave characteristics not permitted by either sensor alone. Simultaneous measurements of incident P- and S-wave amplitudes are used to introduce a technique for single-station estimates of wave field inhomogeneity, free-surface reflection coefficients and local material P velocity. Estimates of these parameters derived for the North Palm Springs earthquake (Mw 5.9) respectively for an incident P wave of 29 ° are -85 °, 1.71, 2.9 km/sec, and for an incident S wave of 17 ° are 79 °, 0.85, 2.9 km/sec. The empirical estimates of reflection coefficients are consistent with model estimates derived using an anelastic half-space model with incident inhomogeneous wave fields.

Comparison of seismic waveform inversion results for the rupture history of a finite fault: Application to the 1986 North Palm Springs, California, earthquake

The July 8, 1986, North Palm Springs earthquake is used as a basis for comparison of several different approaches to the solution for the rupture history of a finite fault. The inversion of different waveform data is considered; both teleseismic P waveforms and local strong ground motion records. Linear parametrizations for slip amplitude are compared with nonlinear parametrizations for both slip amplitude and rupture time. Inversions using both synthetic and empirical Green's functions are considered. In general, accurate Green's functions are more readily calculable for the teleseismic problem where simple ray theory and flat‐layered velocity structures are usually sufficient. However, uncertainties in the variation in t* with frequency most limit the resolution of teleseismic inversions. A set of empirical Green's functions that are well recorded at teleseismic distances could avoid the uncertainties in attenuation. In the inversion of strong motion data, the accurate calculation of propagation path effects other than attenuation effects is the limiting factor in the resolution of source parameters. The assumption of a laterally homogeneous velocity structure is usually not a good one, and the use of empirical Green's functions is desirable. Considering the parametrization of the problem, any degree of fault rupture complexity can be described in terms of a linear parametrization for slip amplitudes. However, a nonlinear parametrization for rupture times and slip amplitudes can have a distinct advantage over a simple linear one by limiting the number of unknown parameters. Regardless of the choice of data or the type of parametrization, the model or solution will be affected by the choice of minimization norm and the type of stabilization used.

Shear-Wave VSP'S: A Powerful New Tool for Fracture and Reservoir Description

Summary Seismic shear waves contain much more information than P waves about the internal structure of the rock along the ray path, which can be interpreted in terms of the crack and stress geometries within the rock mass. Recent field data sets show that shear-wave vertical seismic profiles (VSP's), where a surface source is recorded on geophones downwell, are powerful new tools for detecting fracture orientation and reservoir description. They can provide detailed estimates of the internal crack and stress structure of the reservoir rock. Introduction Several major oil companies recently presented substantial shearwave data sets from reflection surveys gathered for hydrocarbon E and P purposes that reveal crack alignment in the rock mass. Similar oil company data from VSP surveys in the Paris basin, France, and the Silo field, WY, have been reported, Earthquake seismologists have seen the same type of phenomenon, called shearwave splitting, above small earthquakes in many parts of the world (see Ref. 10). There is a now a substantial body of observational evidence that shear-wave splitting is caused by propagation through stress-aligned fluid-filled inclusions. We know that there are fluid-filled fractures, cracks, microcracks, and pores within most rocks in the crust. These fluid-filled inclusions are the most compliant parts of the rock mass and will be preferentially oriented by the current stress field acting on the rock mass. These aligned inclusions are effectively anisotropic to seismic waves, so shear waves that are most sensitive to anisotropy display shear-wave splitting. Confirmation of this interpretation has been provided by recent observations of temporal changes in shear-wave splitting before and after the North Palm Springs earthquake in California and before and after hydraulic fracturing, as reported in this paper. No other explanation for the observed anisotropy displays changes with time in response to comparatively small changes of low-level stress in the comparatively cool upper crust. Shear-wave splitting indicates that the "cracks" are typically aligned by stress into nearly parallel, nearly vertical orientations, striking parallel to the horizontal direction of the current maximum compressional stress (or, more correctly, striking perpendicular to the horizontal direction of current minimum compressional stress). These results are particularly important for reservoir and production engineering. Analysis of shear-wave VSP's can provide information about the internal structure of the reservoir. In particular, analysis of shear-wave splitting yields estimates of the in-situ crack and stress geometries within the reservoir. This improved understanding of the physics of the rock mass is expected to result in improved hydrocarbon recovery. Shear-Wave Splitting In the past, most seismic investigations used seismic P waves to examine the rock mass. P waves arrive before the shear waves because shear waves travel about half as fast as P waves. P waves have longitudinal polarization parallel to the ray path, as illustrated schematically in Fig. 1a. The arrival time, amplitude, and frequency spectrum are the only information in the P wave train. In contrast, shear waves have transverse polarizations (transverse particle motion, Fig. 1b), which carry much more information. In rock without any internal structure (isotropic, uncracked rock, where the properties are the same in all directions), any shear-wave polarization radiated by the source will propagate without significant change of waveform (Fig. 1b). Any two orthogonal polarizations radiated from the same source position will propagate at the same velocity (in isotropic uncracked rock) and the initial polarization of the shear wave will be essentially preserved.