Showing papers in "Bulletin of the Seismological Society of America in 1994"

New Empirical Relationships among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement

Abstract: Source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude ( M ), surface rupture length, subsurface rupture length, downdip rupture width, rupture area, and maximum and average displacement per event. The resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment, moment magnitude, subsurface rupture length, downdip rupture width, and average surface displacement. Each source parameter is classified as reliable or unreliable, based on our evaluation of the accuracy of individual values. Only the reliable source parameters are used in the final analyses. In comparing source parameters, we note the following trends: (1) Generally, the length of rupture at the surface is equal to 75% of the subsurface rupture length; however, the ratio of surface rupture length to subsurface rupture length increases with magnitude; (2) the average surface displacement per event is about one-half the maximum surface displacement per event; and (3) the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement. Thus, for most earthquakes in this data base, slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface. Log-linear regressions between earthquake magnitude and surface rupture length, subsurface rupture length, and rupture area are especially well correlated, showing standard deviations of 0.25 to 0.35 magnitude units. Most relationships are not statistically different (at a 95% significance level) as a function of the style of faulting: thus, we consider the regressions for all slip types to be appropriate for most applications. Regressions between magnitude and displacement, magnitude and rupture width, and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area. The large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data. Separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions. Regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults, and to estimate the expected surface displacement along a fault for a given size earthquake.

Static stress changes and the triggering of earthquakes

Abstract: To understand whether the 1992 M = 7.4 Landers earthquake changed the proximity to failure on the San Andreas fault system, we examine the general problem of how one earthquake might trigger another. The tendency of rocks to fail in a brittle manner is thought to be a function of both shear and confining stresses, commonly formulated as the Coulomb failure criterion. Here we explore how changes in Coulomb conditions associated with one or more earthquakes may trigger subsequent events. We first consider a Coulomb criterion appropriate for the production of aftershocks, where faults most likely to slip are those optimally orientated for failure as a result of the prevailing regional stress field and the stress change caused by the mainshock. We find that the distribution of aftershocks for the Landers earthquake, as well as for several other moderate events in its vicinity, can be explained by the Coulomb criterion as follows: aftershocks are abundant where the Coulomb stress on optimally orientated faults rose by more than one-half bar, and aftershocks are sparse where the Coulomb stress dropped by a similar amount. Further, we find that several moderate shocks raised the stress at the future Landers epicenter and along much of the Landers rupture zone by about a bar, advancing the Landers shock by 1 to 3 centuries. The Landers rupture, in turn, raised the stress at site of the future M = 6.5 Big Bear aftershock site by 3 bars. The Coulomb stress change on a specified fault is independent of regional stress but depends on the fault geometry, sense of slip, and the coefficient of friction. We use this method to resolve stress changes on the San Andreas and San Jacinto faults imposed by the Landers sequence. Together the Landers and Big Bear earthquakes raised the stress along the San Bernardino segment of the southern San Andreas fault by 2 to 6 bars, hastening the next great earthquake there by about a decade.

Spatial and temporal distribution of slip for the 1992 Landers, California, earthquake

Abstract: We have determined a source rupture model for the 1992 Landers earthquake (M_W 7.2) compatible with multiple data sets, spanning a frequency range from zero to 0.5 Hz. Geodetic survey displacements, near-field and regional strong motions, broadband teleseismic waveforms, and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface. Our fault parameterization involves a variable-slip, multiple-segment, finite-fault model which treats the diverse data sets in a self-consistent manner, allowing them to be inverted both independently and in unison. The high-quality data available for the Landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern. In all models, consistent features include the following: (1) similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 × 10^(26) dyne-cm (seismic potency of 2.3 to 2.7 km^3); (2) very heterogeneous, unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km, though slip is limited to shallower regions in some areas; (3) a total rupture duration of 24 sec and an average rupture velocity of 2.7 km/sec; and (4) substantial variations of slip with depth relative to measured surface offsets. The extended rupture length and duration of the Landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter-duration, strike-slip events. Our imaging allows visualization of the rupture evolution, including local differences in slip durations and variations in rupture velocity. Rupture velocity decreases markedly at shallow depths, as well as near regions of slip transfer from one fault segment to the next, as rupture propagates northwestward along the multiply segmented fault length. The rupture front slows as it reaches the northern limit of the Johnson Valley/Landers faults where slip is transferred to the southern Homestead Valley fault; an abrupt acceleration is apparent following the transfer. This process is repeated, and is more pronounced, as slip is again passed from the northern Homestead Valley fault to the Emerson fault. Although the largest surface offsets were observed at the northern end of the rupture, our modeling indicates that substantial rupture was also relatively shallow (less than 10 km) in this region.

Are microtremors useful in site response evaluation

Abstract: We have reviewed the applicability of microtremor measurements to evaluate site response of soft soils. To this end, we evaluated three different techniques commonly used to estimate site effects from microtremor measurements: interpretation of Fourier amplitude spectra, computation of spectral ratios relative to a firm reference station, and, finally, computation of spectral ratios of horizontal components relative to the vertical component of ground motion (Nakamura's technique). These techniques are applied to microtremor records obtained in three cities in Mexico: Mexico City, Oaxaca, and Acapulco. These cities differ in their local geological conditions and in their seismotectonic environment. In order to evaluate the results obtained from microtremor measurements, we compare them with standard spectral ratios of the intense, S -wave part of weak or strong motion records obtained at the same sites. Our results showed that microtremor measurements can be used to estimate the dominant period of a site with very acceptable reliability in the range 0.3 to 5 Hz. The best results were obtained with Nakamura's technique, which also gives a rough estimate of amplification of seismic waves when the local geology is relatively simple. Simple numerical simulations indicate that the underlying assumptions of Nakamura's technique are consistent with the propagation of Rayleigh waves. These simple numerical simulations also explain why different researchers have been able to successfully characterize 1D site effects using microtremor records, regardless of whether they consider microtremors to consist of surface or body waves. Our results strongly suggest that the technique by Nakamura effectively compensates for source effects in microtremor measurements, which eliminates a major limitation to their application in earthquake engineering.

The Gutenberg-Richter or characteristic earthquake distribution, which is it?

Abstract: Paleoearthquake and fault slip-rate data are combined with the CIT-USGS catalog for the period 1944 to 1992 to examine the shape of the magnitude-frequency distribution along the major strike-slip faults of southern California. The resulting distributions for the Newport-Inglewood, Elsinore, Garlock, and San Andreas faults are in accord with the characteristic earthquake model of fault behavior. The distribution observed along the San Jacinto fault satisfies the Gutenberg-Richter relationship. If attention is limited to segments of the San Jacinto that are marked by the rupture zones of large historical earthquakes or distinct steps in fault trace, the observed distribution along each segment is consistent with the characteristic earthquake model. The Gutenberg-Richter distribution observed for the entirety of the San Jacinto may reflect the sum of seismicity along a number of distinct fault segments, each of which displays a characteristic earthquake distribution. The limited period of instrumental recording is insufficient to disprove the hypothesis that all faults will display a Gutenberg-Richter distribution when averaged over the course of a complete earthquake cycle. But, given that (1) the last 5 decades of seismicity are the best indicators of the expected level of small to moderate-size earthquakes in the next 50 years, and (2) it is generally about this period of time that is of interest in seismic hazard and engineering analysis, the answer to the question posed in the title of the article, at least when concerned with practical implementation of seismic hazard analysis at sites along these major faults, appears to be the “characteristic earthquake distribution.”

Source Estimation from Broadband Regional Seismograms

Abstract: Recently developed source inversion techniques do not take full advantage of the broadband nature of regional seismograms. The reason is that Green's functions generated from flat-layered models are not correct for the complicated propagational phenomena associated with realistic crustal structure. The inversion is usually performed by removing short periods and inverting only selected portions of the records. In this article, we introduce a source estimation technique that allows for better use of the entire broadband record when only imperfect Green's function are available. The procedure desensitizes the timing between the principal crustal arrivals by fitting portions of the Green's functions independently. The preferred source is the orientation that minimizes the average L1 and L2 norms in term of the four parameters, M_0, strike, dip, and rake. The L2 error emphasizes the longer period surface waves while the L1 error weights the shorter period body waves. We obtain estimates of duration by modeling long period and broadband signals. The depth determination can be obtained directly from the surface reflected phases sP_mP or sS_mS or by cycling through the depth-dependent Green's functions. In addition to the source parameters, we obtain “δt” phase alignments shifts that can be used as Green's function corrections for relocating other events or as a guide to deriving new crustal models.

Characterization of global seismograms using an automatic-picking algorithm

Abstract: An automatic phase picker is useful for quickly identifying and timing phase arrivals in large seismic data bases. We have developed an automatic phase picker that is sensitive to small changes in amplitude and applied it to over 7 yr of global data distributed by the National Earthquake Information Center (NEIC). Our phase-picking algorithm is based on a short-term-average to long-term-average ratio (STA/LTA) taken along an envelope function generated from the seismogram. The algorithm returns arrival times and corresponding pick qualities. The procedure requires few input parameters and is easily adapted to various types of data. We produce global travel-time plots from both high-frequency (20- or 40-Hz sample rate) and low-frequency (1-Hz sample rate) data. These plots clearly image the predominant high- and low-frequency phases in the NEIC data base. Picks made from the long-period seismograms are less precise, but they reveal far more phase arrivals than the short-period picks. A number of phases resulting from reflections and phase conversions at upper mantle discontinuities can be identified in the low-frequency picks; however, a search of the short-period picks for upper mantle discontinuity phases, between P and PP and prior to P′P′ , has so far been unsuccessful. In the long-period S and SS picks, we observe a discrepancy in SV and SH travel times, a possible result of upper mantle anisotropy. To check the accuracy and consistency of our algorithm, we present comparisons between hand-picked times and automatic-picked times for identical seismograms. Travel-time residuals from the short-period automatic picks and data reported to the International Seismological Centre (ISC) picks exhibit a comparable amount of scatter. Histograms of the ISC residuals and automatic-pick residuals are similar in shape and width for P and PcP . These observations suggest that human picking errors are not a major contributor to the scatter observed in ISC travel times, although direct comparisons between ISC reported picks and automatic picks on particular seismograms occasionally identify operator mispicks.

A kinematic self-similar rupture process for earthquakes

Abstract: The basic assumption that the self-similarity and the spectral law of the seismic body-wave radiation (e.g., ω-square model) must find their origin in some simple self-similar process during the seismic rupture led us to construct a kinematic, self-similar model of earthquakes. It is first assumed that the amplitude of the slip distribution high-pass filtered at high wavenumber does not depend on the size of the ruptured fault. This leads to the following “ k -square” model for the slip spectrum, for k > 1/ L : Δ ~ u L ( k ) = C Δ σ μ L k 2 , where L is the ruptured fault dimension, k the radial wavenumber, Δσ the mean stress drop, μ the rigidity, and C an adimensional constant of the order of 1. The associated stress-drop spectrum, for k > 1/ L , is approximated by Δ ~ σ L ( k ) = Δ σ L k . The rupture front is assumed to propagate on the fault plane with a constant velocity v , and the rise time function is assumed to be scale dependent. The partial slip associated to a given wavelength 1/ k is assumed to be completed in a time 1/ kv , based on simple dynamical considerations. We therefore considered a simple dislocation model (instantaneous slip at the final value), which indeed correctly reproduces this self-similar characteristic of the slip duration at any scale. For a simple rectangular fault with isochrones propagating in the x direction, the resulting far-field displacement spectrum is related to the slip spectrum as u ˜ ( ω ) = F Δ ~ u ( k x = 1 C d ω v , k y = 0 ) , where the factor F includes radiation pattern and distance effect, and C d is the classical directivity coefficient 1/[1 − v/c cos (θ)]. The k -square model for the slip thus leads to the ω-square model, with the assumptions above. Independently of the adequacy of these assumptions, which should be tested with dynamic numerical models, such a kinematic model has several important applications. It may indeed be used for generating realistic synthetics at any frequency, including body waves, surface waves, and near-field terms, even for sites close to the fault, which is often of particular importance; it also provides some clues for estimating the weighting factors for the empirical Green9s function methods. Finally, the slip spectrum may easily be modified in order to model other power-law decay of the radiation spectra, as well as composite earthquakes.

An efficient method for computing Green's functions for a layered half-space with sources and receivers at close depths (part 2)

Abstract: In this study, we improve Hisada's (1994) method to efficiently compute Green's functions for viscoelastic layered half-spaces with sources and receivers located at equal or nearly equal depths. Compared with Hisada (1994), we can significantly reduce the range of wavenumber integration especially for the case that sources and receivers are close to the free surface or to boundaries of the source layer. This can be done by deriving analytical asymptotic solutions for both the direct wave and the reflected/transmitted waves from the boundaries, which are neglected in Hisada (1994). We demonstrate the validity and efficiency of our new method for several cases. The FORTRAN codes for this method for both point and dipole sources are open to academic use through anonymous FTP.

Sources of primary and secondary microseisms

Abstract: Low-frequency (0.01 to 0.2 Hz) seismic noise, arising from pe- lagic storms, is commonly observed as microseisms in seismic records from land and ocean bottom detectors. One principal research objective, in the study of microseisms, has been to locate their sources. This article reports on an anal- ysis of primary and secondary microseisms (i.e., near and double the frequency of ocean swell) recorded simultaneously on three land-based long-period arrays (Alaskan Long Period Array, Montana Large Aperture Seismic Array, and Nor- wegian Seismic Array) during the early 1970s. Reliable microseism source lo- cations are determined by wide-angle triangulation, using the azimuths of ap- proach obtained from frequency-wave number analysis of the records of microseisms propagating across these arrays. Two near-shore sources of both primary and secondary microseisms appear to be persistent in the sense that they are associated with essentially constant near-shore locations. Secondary microseisms are observed to emanate from wide-ranging pelagic locations in addition to the same near-shore locations determined for the primary micro- seisms.

Postseismic deformation following the Landers earthquake, California, 28 June 1992

Abstract: Accelerated strain followed the Landers and Big Bear earthquakes, returning to the normal rate only after a period of several months. We observed this strain throughout most of southern California using the Global Positioning System (GPS). Three GPS receivers operating continuously in fixed positions at Pinyon Flat, Jet Propulsion Laboratory (Pasadena), and Goldstone all recorded postseismic deformation in a relative sense. In addition, we established 16 sites where we deployed portable receivers occasionally over a period of about 6 months near the rupture zones of the earthquakes. Anomalous postseismic displacements ranged from 55 mm near the epicenter to a few millimeters far from the fault. We modeled the displacements, using dislocation theory, as due to variable slip on the faults that were displaced at the times of the earthquakes. The model suggests that the postseismic strain released the equivalent of about 15% of the seismic moment of the mainshock. While the strain released from the upper 10 km is about the same as what can be explained by direct effects of aftershocks, the major contribution of strain release comes from the lower layer, below 10-km depth. Significant afterslip or viscous relaxation must have occurred below 10-km depth to explain the observed deformation more than 100 km from the fault. One interpretation is that high stress on the margin of the co-seismic rupture zone drives the rupture to extend itself into urbroken rock below and along the initial rupture zone.

Slip distribution of the 1992 Landers earthquake and its implications for earthquake source mechanics

Abstract: We use near-source (10 to 164 km) displacement seismograms to image the slip distribution and rupture history of the 28 June 1992 Mw = 7.3 Landers, California, earthquake. Aftershock seismograms from similar distances are modeled to find the velocity model and frequency range (0.05 to 0.25 Hz) over which theoretical Green's functions are most accurate, and the measure of fit is used as an upper bound on theoretical error in the mainshock inversion. We represent the rupture surface with three planar segments divided into 3 by 3 km elements extending from the surface to 18-km depth, and solve for the slip distribution and the rupture time model that minimizes misfit to both recorded seismograms, and mapped surface displacement in a least-squares sense. We investigate two faulting models, one with a uniformly short (<3 sec) rise time everywhere on the fault surface and the other with a variable rise time (2 to 6 sec). We perform sensitivity tests using synthetic data from a hypothetical earthquake to evaluate model resolution and solution stability. In the sensitivity tests, both fault models recover similar slip and rupture features, but neither is capable of imaging details of the rise time. For the Landers earthquake, we find an average rupture velocity of 2.5 km/sec and use this average for a starting model in a linearized inversion for rupture time. In our solutions of slip-amplitude distribution, the southernmost, Johnson Valley fault segment has 20% of the total seismic moment (6 to 8 × 1019 N-m) with small displacements near the hypocenter; the Homestead Valley segment contributes half of the moment with the largest slip amplitudes 25 to 35 km northwest of the hypocenter at 4 to 12-km depth; and the Camp Rock-Emerson segment contributes the remaining moment with the largest slip amplitude 35 to 50 km northwest of the hypocenter in the shallow crust (<9 km). There is some evidence that the rupture front is delayed as it encounters high-slip regions, suggesting that prior to the mainshock these areas were further from failure owing either to greater strength or lower prestress.

Seismic quiescence before the landers (M = 7.5) and big bear (M = 6.5) 1992 earthquakes

Abstract: The Landers earthquake of 28 June 1992 was preceded by a seismicity rate decrease of 75% in a volume approximately 11 by 23 by 15 km, located adjacent to and north of the epicenter. This anomaly started, for all magnitude bands, in January 1988, lasting 4.5 yr up to the mainshock. A smaller volume (7 by 14 by 15 km), to the south of the epicenter, showed a seismicity rate decrease of 75% starting in November 1989. The Big Bear earthquake of the same day was preceded by a 100% decrease of the seismicity rate, within a volume of approximately 13 by 22 by 15 km around the hypocenter. This volume contained 192 earthquakes of M ≧ 1.6 during the background period of 9 yr, but none during the 1.6 yr from December 1990 to 27 June 1992. The standard deviate z values characterizing these rate changes are very high, 6.3 and 12.3, respectively, and the anomalies are unique, not surpassed in significance by any other rate decrease in portion of the southern California earthquake catalog examined (117°06′W to 115°40′W and 33°45′N to 35°10′N). No seismicity rate change was found in the vicinity of the M = 6.1 Joshua Tree earthquake of 23 April 1992. Based on magnitude signatures and frequency-magnitude analyses, we conclude that the two quiescence anomalies could not have been due to a shift or a compression of the magnitude scale, and we interpret them as precursor anomalies. This analysis was carried out using the declustered earthquake catalog for the Landers area with corrections of 0.2 and 0.1 (August 1985 and May 1990) for two suspected magnitude shifts. With a new visualization technique we can investigate the stability, and changes, of the seismicity rate as a function of time and space. By mapping z values for rate changes at every time interval, and in volumes centered on a dense grid of points (latitude, longitude), this computer code furnishes an almost continuous sequence of smooth contour maps of the degree of rate changes. Any anomalies, artificial or real, may thus be defined accurately.

Characteristics of long-period microtremors and their applicability in exploration of deep sedimentary layers

Abstract: Applicability of long-period microtremors in inferring subsurface structure is examined using measurements of microtremors in the northwestern part of the Kanto Plain in Japan. Short-term continuous measurements of long-period microtremors at both sediment and basement sites were taken. A spectral peak at a period of 4 to 5 sec is stable with time, while peaks at periods less than 2 sec are time variant, suggesting a variation of microtremor sources. However, it was found that the spectral ratio between vertical and horizontal microtremors (ellipticity) at each site is stable with time. Good agreement was found between ellipticities of microtremors at the sediment site and those computed for Rayleigh waves in which the structure of the sediments beneath the site was taken into account. We also found that the ellipticities of Rayleigh waves in earthquake ground motions were consistent with those of the microtremors. These comparisons provide strong evidence that long-period microtremors in the area studied consist mainly of Rayleigh waves. The ellipticity of microtremors was investigated by observing microtremors at temporary observation sites in the Kanto Plain where the sediment thickness varied from 0 to 1 km. The subsurface structures were deduced by trial-and-error fitting of observed ellipticities with theoretical ellipticities that were calculated assuming Rayleigh waves. These results show that ellipticity of long-period microtremors is effective for deducing structure from microtremor data at a single site.

Ground-motion amplitude across ridges

Abstract: Local amplification and wave diffraction on an elongated ridge near Sourpi in central Greece were studied by the analysis of seismic records of local and regional earthquakes. Data were obtained during field work especially designed for this purpose. These data were analyzed in the frequency and time domains. In the frequency domain, spectral ratios show amplifications of 1.5 to 3 at the ridge top relative to the base of the ridge. The horizontal components of motion are more amplified than the vertical component and the observed spectral ratios seem stable for different earthquake locations. Theoretical spectral ratios, calculated by the indirect boundary element method, are dependent on earthquake location but are in general agreement with the observed spectral ratios. Another dataset, from Mont St. Eynard in the French Alps, showed similar characteristics with spectral amplitudes on the top of the ridge up to four times those on the flank. These relative amplifications are within the range predicted by numerical simulations. The numerical simulations also show that the topographic effect involves the emission of diffracted waves propagating from the top toward the base of the ridge. The use of a seven-station array on the ridge at Sourpi made it possible to identify such waves. The analysis was performed with wave separation methods using singular value decomposition and spectral matrix filtering. Our results show agreement between experimental data and theoretical results supporting the use of numerical simulations for estimation of purely topography-induced amplification on ridge tops. Our results also show that such amplification is moderate for the ridges under study.

Six-degree-of-freedom ground-motion measurement

Abstract: True six-degree-of-freedom (6DOF) measurement of free-field strong ground motion has been accomplished using a prototype 6DOF accelerograph system. This system consists of a traditional triaxial translational accelerometer, three new rotational velocity sensors, and a digital data logger. Rotational and translational ground motions at a single free-field location were measured successfully during the recent NPE event, a very large (1 kton) chemical explosion. Peak vertical acceleration at the near-field measurement site exceeded 1 g for this event; the peak measured rotational velocity was 2.2°/sec. Earthquake strong-ground-motion measurements are currently in progress.

A multidisciplinary approach to seismic hazard in southern California

Abstract: A serious obstacle facing seismic hazard assessment in southern California has been the characterization of earthquake potential in areas far from known major faults where historical seismicity and paleoseismic data are sparse. This article attempts to fill the voids in earthquake statistics by generating “master model” maps of seismic hazard that blend information from geology, paleoseismology, space geodesy, observational seismology, and synthetic seismicity. The current model suggests that about 40% of the seismic moment release in southern California could occur in widely scattered areas away from the principal faults. As a result, over a 30-yr period, nearly all of the region from the Pacific Ocean to 50 km east of the San Andreas Fault has a greater than 50/50 chance of experiencing moderate shaking of 0.1 g or greater, and about a 1 in 20 chance of suffering levels exceeding 0.3 g . For most of the residents of southern California, thelion9s share of hazard from moderate earthquake shaking over a 30-yr period derives from smaller, closer, more frequent earthquakes in the magnitude range (5 ≦ M ≦ 7) rather than from large San Andreas ruptures, whatever their likelihood.

State of Stress from Focal Mechanisms Before and After the 1992 Landers Earthquake Sequence

Abstract: The state of stress in the Eastern California Shear Zone (ECSZ) changed significantly because of the occurrence of the 1992 M_w 6.1 Joshua Tree and the M_W 7.3 Landers earthquakes. To quantify this change, focal mechanisms from the 1975 Galway Lake sequence, the 1979 Homestead Valley sequence, background seismicity from 1981 to 1991, and the 1992 Landers sequence are inverted for the state of stress. In all cases, the intermediate principal stress axis (S2) remained vertical, and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes (S_1 and S_3) and small variations in the value of the relative stress magnitudes (ϕ). In general, the stress state in the ECSZ has S_1 trending east of north and ϕ = 0.43 to 0.65, suggesting that the ECSZ is a moderate stress refractor and the style of faulting is transtensional. South of the Pinto Mountain fault, in the region of the 1992 Joshua Tree earthquake, the stress state determined from the 1981 to 1991 background seismicity changed on 23 April and again on 28 June 1992. In the central zone, S_1 rotated from N14° ± 5°E to N28° ± 5°E on 23 April and back again to N16° ± 5°E on 28 June. Thus, the Landers mainshock in effect recharged some of the shear stress in the region of the M_w 6.1 Joshua Tree earthquake. Comparison of the state of stress before and after 28 June 1992, along the Landers mainshock rupture zone, showed that the mainshock changed the stress orientation. The S1 trend rotated 7° to 20° clockwise and became progressively more fault normal from south to north. Along the Emerson-Camp Rock faults, the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor. The complex distribution of P and T axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock. The San Bernardino Mountains region of the M_w 6.2 Big Bear earthquake has a distinctively different state of stress, as compared to the Landers region, with S_1 trending N3° ± 5°W. This region did not show any significant change in the state of stress following the 1992 M_w 6.2 Big Bear sequence.

Seismicity in the Western Great Basin Apparently Triggered by the Landers, California, Earthquake, 28 June 1992

Abstract: Within 24 hr after the Landers earthquake, there were three magnitude 3.4+ events in western Nevada and an unexpected, widespread increase in the rate of small events. Based on combined catalogs for northern Nevada, southern Nevada, and southern California, and a model that assumes statistical independence of events in these regions, the probability of this happening in a 24-hr period by random chance is less than ∼10−12 per day. Therefore, there is high statistical confidence that the increased seismicity was triggered by the Landers event. Based on the statistical model, we develop a list of 227 earthquakes in the first 83 days following the Landers earthquake, each of which has no more than 10% probability of occurring by random chance. The suspect events are broadly distributed in regions that correlate with historical activity in the Great Basin. The events are not uniquely associated with known volcanic activity, or with zones that were previously active with microearthquakes or aftershock sequences. The magnitudes of the largest triggered events appear to decrease with distance. With time, the number of suspect events decreases at a rate comparable to the rate of decrease of aftershocks of the Landers and Big Bear earthquakes. Three of the most significant triggered events that that occurred were as follows: Mina, 500 km from Landers, M 4, 36 min after Landers; Smith Valley, 590 km from Landers, M 3.4, 56 min after Landers; and Little Skull Mountain, 280 km from Landers, M 5.6, 22.3 hr after Landers. The evidence for triggering is particularly strong in the case of the Little Skull Mountain event, where an increased rate of microseismicity was evident as soon as small events could be identified in the coda of the Landers earthquake. We evaluate the seismic history for the past 3 yr to understand what was unique about the Landers earthquake. This includes identifying the previous earthquakes most likely to have caused the strongest shaking in various frequency bands. Based on a simple screening model, the strains from the Landers earthquake were uniquely large at low frequencies, but at high frequencies they were exceeded frequently by small, more local events. Thus, we hypothesize that the cause of triggering is low-frequency dynamic strains, at periods of about 10 sec or greater, and that there is a regional threshold that must be exceeded, since previous events with calculated strains that were almost as strong were not causes of widespread triggering. This mechanism for triggering satisfies the criterion of being a relatively rare phenomenon, since it is likely to occur only when a large long-period wave is radiated into an area where strain has been building slowly toward the point where faults are unstable.

Seismic energy release in Mexican subduction zone earthquakes

Abstract: We estimated radiated seismic energy ( Es ) from 18 shallow, thrust Mexican subduction zone earthquakes (4 × 1022 ≦ M ≦ 1.1 × 1028 dyne-cm; 11 ≦ H ≦ 37 km) using digital accelerograms from the Guerrero Accelerograph Array and neglecting stations with large site effects. Es is computed by integrating squared velocity spectra, after applying geometrical spreading and Q corrections. We discarded epicentral recordings for the largest Michoacan event. We find that log ( Es / M ) = −4.152 ± 0.275, which gives a median value of Es / M and apparent stress (σ a ) of 7.1 × 10−5 and 24 bars, respectively. The median Es / M value is in accordance with Gutenberg and Richter's (G-R) (1956) formula for Es , in which Es / M = 5 × 10−5 is implicit. Worldwide Es / M data, where Es is computed from local records, mostly fall between 5 × 10−5 and 5 × 10−4 for events with M ≧ 1022 dyne-cm. On the other hand, Es / M values, generally, lie between 5 × 10−6 and 5 × 10−5, if Es is estimated from teleseismic records. Especially anomalous are the Es / M data from Kikuchi and Fukao (1988) for large and great earthquakes, which fall near 5 × 10−6. Thus, while Es from local data suggests that the G-R relation seldom overestimates seismic energy release, the teleseismic data point to the contrary. The cause of this discrepancy may lie in the difficulty of resolving incoherent radiation from the fault and inappropriate choice of t * in the analysis of teleseismic data.

Fault slip rates and earthquake histories for active faults in southern California

Abstract: Within the context of the historical record of seismicity, we review the geological data bearing on the Quaternary slip rates and paleoearthquake histories of active faults in southern California.

A hybrid method for the estimation of ground motion in sedimentary basins: Quantitative modeling for Mexico city

Abstract: To estimate the ground motion in two-dimensional (2D), laterally heterogeneous, anelastic media, a hybrid technique has been developed that combines modal summation and the finite-difference method. In the calculation of the local wave field owing to a seismic event, both for small and large epicentral distances, it is possible to take into account the source, path, and local soil effects. As a practical application, we have simulated the ground motion in Mexico City caused by the Michoacan earthquake of September 19, 1985. By studying the one-dimensional (1D) response of the two sedimentary layers present in Mexico City, it is possible to explain the difference in amplitudes observed between records for receivers inside and outside the lake-bed zone. These simple models show that the sedimentary cover produces the concentration of high-frequency waves (0.2 to 0.5 Hz) on the horizontal components of motion. The large amplitude coda of ground motion observed inside the lake-bed zone and the spectral ratios between signals observed inside and outside the lake-bed zone can only be explained by 2D models of the sedimentary basin. In such models, the ground motion is mainly controlled by the response of the uppermost clay layer. The synthetic signals explain the major characteristics (relative amplitudes, spectral ratios, and frequency content) of the observed ground motion. The large amplitude coda of the ground motion observed in the lake-bed zone can be explained as resonance effects and the excitation of local surface waves in the laterally heterogeneous clay layer. Also, for the 1985 Michoacan event, the energy contributions of the three subevents are important to explain the observed durations.

Initial investigation of site and topographic effects at Robinwood Ridge, California

Abstract: Following the 1989 Loma Prieta earthquake, a dense array of seven digitally recorded, three-component seismograph stations was deployed on Robinwood Ridge 7.3 km northwest of the epicenter. The purpose of this array was to investigate the cause of high levels of structural damage and ground cracking observed on the ridge crest. Aftershocks recorded by the array allow a comparison of ground motion up the slope of the ridge from the base to the crest. The data present an extremely complicated pattern of ground motion that demonstrates the importance of the three-dimensionality of the problem. Slowness analysis of P wave trains show initial arrivals propagating away from the source with small angles of incidence and large apparent velocities, consistent with direct arrivals. After 0.5 sec, propagation azimuths become more random and apparent velocities drop, indicating nearly horizontal wave propagation and multiply reflected and diffracted phases within the ridge. Slowness analysis and particle motion diagrams of horizontal components of motion show dramatic variations in ground motion with changes in azimuth of the source and a complicated interaction between body waves and Rayleigh and Love waves. Results suggest that the larger amplitude, more coherent arrivals at the array stations favor a propagation direction parallel to the ridge axis. An amplification factor of from 1.5 to 4.5 is seen for frequencies from 1.0 to 3.0 Hz with wavelengths comparable to the base of the ridge, part of which may be caused by local site effects and part by topographic amplification. In addition, amplifications of up to a factor of 5 are seen at higher frequencies and are attributed to local site effects. These effects are most notable from 4 to 8 Hz on the vertical components, and from 6 to 9 Hz on the horizontal components. The entire Robinwood Ridge area may also have been situated in a region of heightened mainshock ground motion due to source directivity and radiation pattern effects.

Three-dimensional scattering by two-dimensional topographies

Abstract: Three-dimensional seismic responses of two-dimensional topographies are studied by means of the indirect boundary element method (IBEM). The IBEM yields, in the presented form, very accurate results and has the advantage of low computational cost. In IBEM, diffracted waves are constructed in terms of single-layer boundary souces. The appropriate Green's functions used are those of a harmonic point foce moving along the axis of the topography in a full space. Obtained reults are compared against those published by other authors. Examples of simulations are presented for different geometries, for different types of incident wave fields, and in particular, for different arrival angles to the topography to quantitatively study three-dimensional effects of the scattering. The accuracy of the results makes it possible to analyze them in both the time and drequency domaisn. Frequency-space representations allow identification of difraction and interference patterns in the seismic response of the topography. Synthetic seismograms are obtained by Fourier analysis. Using timespace domain representations, the nature of each of the scattered waves are identified in terms of, for example, creeping waves and reflected compressional waves.

A reappraisal of large earthquake scaling

Abstract: Twelve years ago I pointed out that observations indicate that the slip in large earthquakes scales with their length, rather than their width, as expected from the canonical model. Romanowicz (1992) has recently argued that more recent data show that the opposite is true, which has obliged me to re-examine the question. I point out here a defining flaw in her analysis, in that she allowed Mo*, the moment at the cross-over from small to large earthquakes, to be a free variable in her curve fitting, whereas this parameter can be defined independently. When this parameter is independently fixed, I find that the up- dated data set confirms my earlier conclusion that for large crustal earthquakes M0 scales with L 2. A little over a decade ago I pointed out that obser- vations indicate that the slip in large earthquakes scales with their rupture length (Scholz, 1982). Several sub- sequent papers provided additional data supportive of this view (Chen and Molnar, 1983; Shimazaki, 1986; Scholz et al., 1986). Large earthquakes are defined in this con- text as those that rupture the entire seismogenic thick- ness W0 and are thus constrained to propagate farther in a horizontal direction only. For crustal earthquakes, W0 is nearly constant -15 km; so this scaling conflicts with conventional models of earthquakes, such as dislocation models, because they predict that slip scales with width, which is invariant for these earthquakes. I offered two alternative ways to interpret these observations. One is that such conventional W models are correct but that stress- drop could no longer be regarded as scale-invariant but must scale with fault length. As an alternative, I sug- gested the possibility of what I termed an L model, in which slip intrinsically scales with fault length. This created a minor crisis in seismology, which has yet to be resolved. It is no exaggeration to say that my L model suggestion has not attracted a noticeable coterie of admirers, nor has my other suggestion, and there has been no other model proposed in the interim that ex- plains the observed L scaling [although Heaton (1990) has suggested a way in which length may scale with stress- drop, a variation of my W model scaling alternative]. In the face of this, an alternative is to question the obser- vations, which is what Romanowicz (1992) has recently chosen to do. In her interpretation of an updated data set, she concluded that seismic moment, M0, scales lin- early with rupture length L for large earthquakes, which is consistent with W models and implies that slip, u, is scale invariant for large crustal earthquakes. This great discrepancy with the earlier result, she implies, is due to the presence of superior modern measurements in the updated data set. This issue has major implications, both practical and theoretical, and merits some debate. Here I present my own reappraisal of the data, in which the conclusions are very different. Reappraisal of the Data Romanowicz's conclusions were based on a log-log Plot of M0 versus L for a data set of strike-slip earth- quakes. She showed that the data in this figure could be better fit by two straight lines, one with slope 3 in the lower scale range and one with slope 1 in the upper scale range, than with a single line of slope 2. For small earth- quakes, which grow in both dimensions with L ~ W, M0 should scale with L 3. For large earthquakes, where W has saturated at W0 and growth only occurs in the L di- rection, M0 should scale with L according to conven- tional models; hence the problem produced by my ob- servation that M0 scales with L 2 for those events. Superficially, then, it appears that she has resolved the issue. The problem, however, is that she has allowed the cross-over length, L*, which separates small from large earthquakes, to be a free variable in her curve-fitting ex- ercise. She thus obtains a cross-over moment of 0.6 to 0.8 x 1020 N-m, which is appropriate for a rupture length of 60 to 70 km. By adding an additional free parameter to the curve fitting, one would expect a much better fit to be achieved; the question is whether such a step is justified. If small earthquakes grow with L ~ W, then the transition to large earthquakes (defined as an event that ruptures the entire seismogenic thickness) would be L* ~ W0 "~ 10 to 20 km for the data being studied. This may be checked observationally; because large earth- quakes almost invariably nucleate near the base of the seismogenic layer (Das and Scholz, 1983), it follows that the transition to large earthquakes is at the length at which one begins to consistently see surface ruptures. Inspec- tion of the earthquakes in Romanowicz's (1992) Figure 215

Errors in hypocenter location: Picking, model, and magnitude dependence

Abstract: The location procedures of seismic events are influenced by two major classes of errors, the error in picking individual seismic phases and modeling error due to the departure of the real Earth from the reference model used in the location. Both classes of error influence the estimate of location and it is difficult to separate them. The role of picking errors can be assessed by a nonlinear analysis using a Monte Carlo procedure. Arrivals times are perturbed with random numbers drawn from a normal distribution, and the event is relocated using these perturbed arrival times. By repeating the procedure many times, a cluster of locations is obtained, which can be used to investigate the effects of picking errors on the hypocenter. This analysis is insensitive to velocity-model errors as these are fixed for a given combination of stations and phases. Some care must be exercised when analysing multidimensional distributions in two-dimensional slices because of a projection effect. The modeling error due to the influence of lateral heterogeneity in the Earth is examined by comparing the locations of the same event using different combinations of phases and network geometries, which reinforces the need to use arrivals other than P for accurate depth resolution. The sensitivity of P arrivals to changes in depth are swamped by model errors, and inclusion of depth-sensitive phases such as pP is highly recommended. The effect of picking errors on location is found to be much smaller than the mislocation caused by neglecting lateral heterogeneity when only P arrivals are used. Consequently, the Monte Carlo analysis, which is primarily aimed at picking errors only, is most appropriate when multiple phases have been used to more accurately constrain the hypocenter, especially for the depth component. Altering the type of phase data used in the location plays a similar role in changing the network geometry, in that both are mechanisms that influence the nature of the constraint on the hypocenter. By relocating events with network geometries corresponding to the different magnitudes, it is found that the location of the event can be affected significantly by the magnitude, and when using robust statistics to describe earthquake residuals, the mislocation can occur in a systematic manner. The effect is marked in regions with significant lateral variations in seismic velocities. For example, low-magnitude events in the Flores Sea are found to be dragged toward Australia as a result of the fast paths to Australian stations relative to the iasp 91 reference velocity model.

Magnitude-distance relations for liquefaction in soil from earthquakes

Abstract: The review of a large number of historical documents and scientific publications revealed that at least 30 cases of liquefaction in soil from earthquakes of Ms = 5.8 to 7.2 have been observed in Greece since 1767. Liquefaction usually occurs in the epicentral area of earthquakes. However, maximum epicentral and fault distances, Re and Rf , generally increase with the earthquake magnitude, M , which is consistent with similar increase observed in other parts of the world. We propose equations approximating the limiting distances Re and Rf as a function of M . By supplementing the Greek liquefaction data with a worldwide compilation of Ambraseys (1988) and using published observations for recent liquefaction cases in New Zealand, California, Venezuela, Iran, and the Philippines we also propose a slight modification of the M/Re and M/Rf relations suggested by Ambraseys (1988).

Co-seismic displacements of the 1992 landers earthquake sequence

Abstract: We present co-seismic displacement vectors derived from Global Positioning System (GPS) measurements of 92 stations in southern California. These GPS results are combined with five well-determined GPS displacement vectors from continuously tracking stations of the Permanent GPS Geodetic Array, as well as line-length changes from USGS Geodolite and two-color laser trilateration observations, to determine a self-consistent set of geodetic data for the earthquake. These combined displacements are modeled by an elastic dislocation representation of the primary fault rupture planes. On average, the model residuals are about twice the estimated measurement errors.

Triggering of the Ms = 5.4 Little Skull Mountain, Nevada, earthquake with dynamic strains

Abstract: We have developed an approach to test the viability of dynamic strains as a triggering mechanism by quantifying the dynamic strain tensor at seismogenic depths. We focus on the dynamic strains at the hypocenter of the M s = 5.4 Little Skull Mountain (LSM), Nevada, earthquake. This event is noteworthy because it is the largest earthquake demonstrably triggered at remote distances (∼280 km) by the M s = 7.4 Landers, California, earthquake and because of its ambiguous association with magmatic activity. Our analysis shows that, if dynamic strains initiate remote triggering, the orientation and modes of faulting most favorable for being triggered by a given strain transient change with depth. The geometry of the most probable LSM fault plane was favorably oriented with respect to the geometry of the dynamic strain tensor. We estimate that the magnitude of the peak dynamic strains at the hypocentral depth of the LSM earthquake were ∼4 μstrain (∼.2 MPa) which are ∼50% smaller than those estimated from velocity seismograms recorded at the surface. We suggest that these strains are too small to cause Mohr-Coulomb style failure unless the fault was prestrained to near failure levels, the fault was exceptionally weak, and/or the dynamic strains trigger other processes that lead to failure.

Local Observations of the Onset of a Large Earthquake: 28 June 1992 Landers, California

Abstract: The Landers earthquake ( MW 7.3) of 28 June 1992 had a very emergent onset. The first large amplitude arrivals are delayed by about 3 sec with respect to the origin time, and are preceded by smaller-scale slip. Other large earthquakes have been observed to have similar emergent onsets, but the Landers event is one of the first to be well recorded on nearby stations. We used these recordings to investigate the spatial relationship between the hypocenter and the onset of the large energy release, and to determine the slip function of the 3-sec nucleation process. Relative location of the onset of the large energy release with respect to the initial hypocenter indicates its source was between 1 and 4 km north of the hypocenter and delayed by approximately 2.5 sec. Three-station array analysis of the P wave shows that the large amplitude onset arrives with a faster apparent velocity compared to the first arrivals, indicating that the large amplitude source was several kilometers deeper than the initial onset. An ML 2.8 foreshock, located close to the hypocenter, was used as an empirical Green's function to correct for path and site effects from the first 3 sec of the mainshock seismogram. The resultant deconvolution produced a slip function that showed two subevents preceding the main energy release, an MW 4.4 followed by an MW 5.6. These subevents do not appear anomalous in comparison to simple moderate-sized earthquakes, suggesting that they were normal events which just triggered or grew into a much larger earthquake. If small and moderate-sized earthquakes commonly “detonate” much larger events, this implies that the dynamic stresses during earthquake rupture are at least as important as long-term static stresses in causing earthquakes, and the prospects of reliable earthquake prediction from premonitory phenomena are not improved.