Showing papers in "Bulletin of the Seismological Society of America in 1986"

Three-dimensional velocity structure in northern California Coast Ranges from inversion of local earthquake arrival times

Abstract: The three-dimensional P -wave velocity structure in the upper crust in the northern California Coast Ranges from Santa Rosa to Ukiah is obtained by simultaneous inversion of local earthquake and refraction travel-time data for velocity and hypocentral parameters. In this region, where there are lateral heterogeneities, the three-dimensional inversion provides important results not found in two-dimensional refraction studies. The medium is parameterized with velocities at the intersections (gridpoints) of a nonuniform three-dimensional grid. Travel times are calculated with approximate ray tracing along arcuate paths, which are reasonable for lateral distances less than 45 km. Inversions with 5 to 10 and 2 to 3 km gridspacing achieved 80 per cent decreases in data variance. The inversion with 2 to 3 km gridspacing reduced the rms residual level to approximately the level of timing error. Anomalously low velocities to at least 6 km depth are found along the Maacama and Healdsburg-Rodgers Creek (HRC) fault zones. The low-velocity region is widest at the en-echelon junction of the Maacama and HRC fault zones. Low velocities are associated with the Clear Lake basin where there is a thick sequence of volcanics interbedded with sedimentary deposits. There is a high-velocity body, located southeast of The Geysers between the Maacama and Collayomi fault zones, which may be associated with igneous intrusives or high-grade metamorphism related to an underlying inferred partial-melt body, modeled in previous teleseismic travel-time studies. There is some indication in the vapor-dominated geothermal reservoir of lower velocities relative to surrounding water-saturated rock, as has been shown in some laboratory experiments. Hypocenter locations obtained in the inversion show little variation from hypocenters obtained through inversion for one-dimensional velocity model and station corrections. For the inversion with 2 to 3 km gridspacing, however, some groups of events become more tightly clustered.

Complex polarization analysis of particle motion

Abstract: Knowledge of particle motion polarization aids in identifying phases on three-component seismograms. The scheme of Montalbetti and Kanasewich (1970) is extended to analytic three-component seismograms, where the imaginary part of the signal is the Hilbert transform of the real part. This scheme has only one free parameter, the length of the time window over which the polarization parameters are estimated, so it can be applied in a routine way to three-component data. The azimuth and dip of the direction of maximum polarization and the degree of elliptical polarization as a function of time for the seismograms are obtained. Polarization analysis of strong motion data from the 1971 San Fernando earthquake aids in the discrimination between wave types, which is important for the understanding of the complicated earthquake-induced shaking observed in basins. Most arrivals are incident on the receivers in the direction of the back-azimuth to the epicenter, which suggests that despite the complicated motions, two-dimensional finite difference methods are sufficient to understand the effect on seismic waves of the Los Angeles and San Fernando basins (Vidale and Helmberger, 1986b).

HYPOCENTER: An earthquake location method using centered, scaled, and adaptively damped least squares

Abstract: We present an earthquake location method, HYPOCENTER, which combines features of the two well-known algorithms HYPO71 and HYPOINVERSE, with a new technique which we term adaptive damping. Each column of the linearized condition matrix T , which relates changes in arrival time to changes in hypocentral position, is centered and scaled to have zero mean and a norm of one. Origin time is defined as the mean arrival time minus the mean travel time. The three least-squares normal equations for hypocentral coordinates, with diagonal terms equal to one, are then solved iteratively by adding a variable damping factor, θ 2, to their diagonal terms before inversion. If the residual sum of squares increases, we return to the previous iteration, increase θ 2, then try again. This procedure, which we term adaptive damping, always results in residuals which are less than or equal to the HYPO71 or HYPOINVERSE residuals. We demonstrate HYPOCENTER by comparing it to HYPO71 and HYPOINVERSE using synthetic and real arrival time data for four- and eight-station seismic arrays.

Site amplification of coda waves from local earthquakes in central California

Abstract: The goals of this study are two-fold: (1) to examine the site effect of coda waves, believed to be the average site effect of shear waves, in order to understand its spatial and frequency-dependent behavior, and (2) to learn what we can about the processes that generate the coda itself. We use digital data from over 90 earthquakes, each recorded by a subset of some 150 stations in the Coast Ranges of central California between San Francisco and San Luis Obispo. Results from the band 1.5 to 24 Hz indicate that site amplification depends strongly on surface geology and frequency. At low frequencies (1.5 to 3.0 Hz), changes in amplification up to a factor of 20 are observed; amplification generally varies inversely with deposit age at sediment sites and is lowest at granite and Franciscan (basement) sites. At high frequencies (6 to 24 Hz), the pattern changes. Many granite sites, the majority in the Gabilan range, exhibit increasing amplification with frequency relative to the average station. This behavior differs strikingly from that observed at sediment and Franciscan sites which roll-off at an intermediate rate, and many sediment sites adjacent to the San Andreas fault zone which decay away even more rapidly with frequency. These results can be explained qualitatively by appealing to variable near-site impedance and attenuation, however, complicating phenomena such as mode excitation in valley sediments must exist at low frequencies. Peaked behavior at a few hard rock sites at high frequency (12 Hz) currently lacks explanation. At a few sediment sites, the low-frequency excitation is so strong that the coda envelopes begin to take on different shapes. This contradicts the assumption based on previous observations that the coda shape is independent of source-receiver location, thus, some site measurements may be artificially high. The working model of coda waves as body-wave energy backscattered from randomly distributed inhomogeneities must be modified to include the possibility of efficient near-site resonance excited by incident direct and coda wave energy.

Scaling differences between large interplate and intraplate earthquakes

Abstract: A study of large intraplate earthquakes with well determined source parameters shows that these earthquakes obey a scaling law similar to large interplate earthquakes, in which M sub o varies as L sup 2 or u = alpha L where L is rupture length and u is slip. In contrast to interplate earthquakes, for which alpha approximately equals 1 x .00001, for the intraplate events alpha approximately equals 6 x .0001, which implies that these earthquakes have stress-drops about 6 times higher than interplate events. This result is independent of focal mechanism type. This implies that intraplate faults have a higher frictional strength than plate boundaries, and hence, that faults are velocity or slip weakening in their behavior. This factor may be important in producing the concentrated deformation that creates and maintains plate boundaries.

Rupture history of the 1984 morgan hill, california, earthquake from the inversion of strong motion records

Abstract: Near-source strong motion velocity records and teleseismic short-period P waveforms are modeled to obtain the spatial and temporal distribution of slip for the 1984 Morgan Hill earthquake. Both forward modeling and constrained, least-squares inversion techniques are used to interpret the strong motion velocity waveforms in the frequency range of approximately 0.2 to 2.0 Hz. These data support a nearly unilateral rupture to the southeast with a rupture propagation velocity of nine-tenths of the local S-wave velocity. The majority of the slip occurs over a fault length of 25 km and to a first approximation can be interpreted as two main source regions, each with an extent of about 5 km with their centers separated by about 12 km. However, each of the sources has detailed structure of its own, and a simple two-point-source model is not an accurate representation of the Morgan Hill earthquake. The second source occurs about 4.5 sec after the first and is approximately 3 times larger. The maximum dislocation on the fault plane is about 1 m. The total moment of the earthquake is estimated to be 2.1 × 10^(25) dyne-cm. The Morgan Hill earthquake offers convincing evidence for very inhomogeneous slip and stress distributions on shallow strike-slip faults.

A fourth-order accurate finite-difference scheme for the computation of elastic waves

Abstract: A finite difference for elastic waves is introduced. The model is based on the first order system of equations for the velocities and stresses. The differencing is fourth order accurate on the spatial derivatives and second order accurate in time. The model is tested on a series of examples including the Lamb problem, scattering from plane interf aces and scattering from a fluid-elastic interface. The scheme is shown to be effective for these problems. The accuracy and stability is insensitive to the Poisson ratio. For the class of problems considered here it is found that the fourth order scheme requires for two-thirds to one-half the resolution of a typical second order scheme to give comparable accuracy.

Appraising earthquake hypocenter location errors: A complete, practical approach for single-event locations

Abstract: For conventional single-event, nonlinear, least-squares hypocentral estimates, I show that the total error is expressible as a linear combination of three terms: (1) measurement error; (2) modeling errors caused by inadequacy of the travel-time tables; and (3) a nonlinear term. Errors in calculating travel-time partial derivatives are shown to have no effect, provided a stable solution can be found. This is in contrast to linear problems where errors in calculating matrix elements can distort the solution drastically. The error appraisal technique developed here examines each of the three error terms independently. The first can be analyzed by standard confidence ellipsoids with critical values based on measurement error statistics. The second can cause conventional error ellipsoid calculations that derive a critical value from an estimate based on rms residuals, to give misleading results. I introduce an alternative extremal bound procedure for appraising such errors. Travel-time modeling errors are bounded as the product of ray arc length and an estimate of the nominal scale of slowness errors along the ray path. These are used to derive an upper bound on systematic errors in each hypocentral coordinate based on a novel bounding criteria. Finally, I show that, for errors of a reasonable scale, the nonlinear error term can be estimated adequately using a second-order approximation. Given an upper bound on the total location error, bounds on the travel-time error induced by nonlinearity can be calculated from the spectral norm of the Hessian for each measured arrival time. The systematic errors in each hypocentral coordinate due to nonlinearity can then be bounded using the same criteria used for constructing modeling error bounds. This overall procedure is complete because it allows one to independently appraise the relative importance of all sources of hypocentral errors. It is practical because the required computational effort is small.

Lg waves and structural boundaries

Abstract: The propagation of Lg waves in complex media can be described either by means of a modal superposition scheme with numerical integration through the heterogeneity or by using ray diagrams. Rays are set off at equal horizontal intervals in a stratified zone adjacent to the heterogeneity, with phase velocities appropriate to particular modes. The constructive interference pattern in the stratified medium is modified in the horizontally varying region to give a graphic illustration of the propagation effects. The ray method agrees well with the modal calculations but may be conveniently applied in more general circumstances, e.g., to include surface topography or permanent changes in crustal structure. Structural boundaries which involve sudden thinning of the crustal wave guide are particularly disruptive to the Lg train, as at a pinch in crustal thickness or the continent-ocean transition. The effects of localized thickening are more subtle. The relatively sharp cutoff for Lg waves in some structures, e.g. in the Tibetan Plateau, can be explained by the source being no longer able to couple into the crustal wave guide at the receiver.

Short-period P- and S-wave radiation from large earthquakes: Implications for spectral scaling relations

Abstract: Recent measurements of peak P-wave amplitudes on World Wide Standardized Seismographic Network short-period instruments by Houston and Kanamori (1986) provided the opportunity to investigate source radiation from great earthquakes at higher frequencies than have previously been available. The dependence on moment magnitude (M) of the amplitude measurements (A) and the dominant period (T) in the P-wave seismograms are compared to predictions from several source-scaling relations. For all of the relations, the radiated energy was assumed to be randomly distributed over a duration proportional to the inverse corner frequency. An w-square source-scaling relation with a constant stress parameter of 50 bars gives a good fit to both observed quantities (A and T) for earthquakes up to M 9.5. This model, with the same stress parameter, also fits peak acceleration and peak velocity data for earthquakes with moment magnitude as low as 0.5. Predictions using the source spectra derived by Gusev (1983), which are representative of several published relations featuring regions of reduced spectral decay after an initial ~-2 attenuation beyond the corner frequency, do not fit the various high-frequency observations quite as well as do those using the ~-square model, although the differences between the predicted motions are generally within a factor of 2 to 3. Although the w-square model successfully predicts a wide variety of time-domain measures over an extraordinary magnitude range, it fails to fit the Ms, M correlation for large earthquakes; Gusev's spectral scaling relation, on the other hand, fits this correlation, but was constrained in advance to do so. This failure of the w-square model is of little practical concern, occurring as it does at periods longer than those of usual engineering importance. An ~-cube model fails completely to explain the seismic moment dependence of the observations.

Estimation of earthquake source parameters by the inversion of waveform data: Global seismicity, 1981-1983

Abstract: A waveform inversion algorithm, based on optimal filter theory, has been applied to the P waves from 260 of the largest earthquakes occurring during the years 1981 through 1983. Estimates of average focal depth, scalar moment, and deviatoric source mechanism have been obtained. For all except the largest events ( M 0 > 10 27 dyne-cm), the scalar moments obtained in this study are close to, but somewhat larger than, the Harvard centroid-moment tensor (CMT) scalar moments. The CMT estimates of scalar moment are probably biased to low values due to the way unmodeled lateral heterogeneity affects the fitting procedure. For the largest events, however, source complexity can bias the scalar moments determined in this study to lower values, and the CMT scalar moments are probably more accurate. The moment tensor, CMT, and U.S. Geological Survey first-motion source mechanisms have been objectively compared by computing the locations of the vector representations of the mechanisms on the unit sphere. We find that the similarities and differences between these mechanisms can be related to the uncertainties inherent in each method for certain types of earthquakes: (1) lack of constraint on one of the nodal planes for dip-slip type mechanisms from first-motion analysis; (2) lack of resolution of the moment tensor elements defining the dip-slip component of faulting for shallow-focus earthquakes in the CMT method; and (3) lack of resolution of the moment tensor elements defining the strike-slip component of faulting in the P -wave inversion method used here. Thus, on the average, the first-motion and CMT methods yield the more reliable mechanisms for strike-slip-type earthquakes, and the method used in this study gives a more reliable result for shallow-focus earthquakes with dip-slip-type mechanisms. Finally, both of the moment tensor methods should yield reliable solutions for intermediate- and deep-focus earthquakes.

Source spectra of great earthquakes: Teleseismic constraints on rupture process and strong motion

Abstract: Short-period body waves recorded at teleseismic distances from great earthquakes provide information about source rupture processes and strong motions First, we examine mostly WWSSN records of 19 earthquakes of moment magnitude M_w of 65 to 95 Four parameters are measured from the short-period P-wave train: the maximum amplitude; the period at maximum amplitude; the time between the first arrival and when the maximum amplitude is attained; and coda length An extension, m_b, of the teleseismic magnitude, m_b, is defined using the maximum amplitude of the entire short-period P-wave rather than the amplitude achieved in the first few P-wave cycles A least-squares fit to the data yields the following relationship between m_b and M_w: m_b = 053 M_w + 270 for M_w 65 to 95 The time from the first arrival until the maximum amplitude is achieved and the coda length are roughly proportional to M_w, but are further interpreted by a simple asperity model of the rupture process These data support that short-period waves are, on average, generated preferentially in the same regions of the fault plane as long-period waves (with periods of 10 to 50 sec) We analyze the spectra of short- and intermediate-period teleseismic GDSN records for seven earthquakes with M_w's of 64 to 78 and hand-digitized short-period WWSSN records of the 1971 San Fernando earthquake Significant differences exist between the spectra of different events, due partly to variations in tectonic setting or seismic coupling Using the digital data, we also investigate the relationship between time-domain amplitude and spectral amplitude for short-period P waves From our empirical relation between spectral amplitude and time-domain amplitude, we estimate the spectral amplitudes implied by the m_b data We compare our results to the ω^(−2) and Gusev spectral models Neither model can completely represent the data Nevertheless, we consider the ω^(−2) model a useful reference model for comparing different events The average source spectrum of six large events with M_w 74 to 78 does not have the spectral structure suggested by Gusev An application to strong motion modeling is presented in which a 1971 San Fernando teleseismic short-period record is summed up to simulate teleseismic records produced by five great earthquakes The summation procedure matches the moment of the event to be simulated, and includes rupture propagation, fault plane roughness, and randomness The m_b data provide an important constraint on the summation procedures Thus constrained, this summation procedure can be more confidently used with near-field strong motion records as Green's functions to predict strong motions from great earthquakes

Lateral velocity variations in Southern California. II. Results for the lower crust from Pn waves

Abstract: The plate boundary and major crustal blocks in southern California are imaged by a tomographic backprojection of the Pg first arrivals recorded by the Southern California Array. The method, formulated specifically for local earthquake arrival times, is a fast, iterative alternative to direct least-squares techniques. With it, we solve the combined problem of determining refractor velocity perturbations and source and station delays. Resolution and variance are found empirically by using synthetic examples. A map showing lateral velocity variations at a depth of approximately 10 km is presented. The results show a strong correlation with surface tectonic features. Clear velocity contrasts exist across the San Andreas, the San Jacinto, and the Garlock faults. The Mojave region has the slowest velocities while the Peninsula Ranges have the highest. The San Jacinto block has velocities intermediate between Mojave and Peninsula Range velocities, and also has early station delays. This may indicate that the San Jacinto block has overridden Mojave material on a shallow detachment surface. No velocity variations are found associated with the Transverse Ranges, which we interpret to mean that the surface batholithic rocks in this area do not extend to Pg depths.

Observations and analysis of B-type earthquakes, explosions, and volcanic tremor at Pavlof Volcano, Alaska

Abstract: An eight-station network of short-period seismometers has been operated near Pavlof Volcano (55°25′N, 161°54′W) Alaska Peninsula since 1976. High-quality analog-to-digital and direct digital data were recorded during eruptions in 1980 through 1983. Data are analyzed from explosions with distinct air phases, B-type volcanic earthquakes (shallow, low-frequency events lacking clear S phases), and volcanic tremor to determine source and propagation effects. Between zero and several hundred B-type events and explosions per day are recorded, with higher numbers during eruptions. Magnitudes range between −0.5 and 1.3, and b -values range between 1.9 ± 0.1 and 2.6 ± 0.1 with no systematic relation to eruptive activity. Six results are most important: (1) data from a recently installed three-component station reveal that the highest-amplitude waves of the B-type events show retrograde elliptical particle motion and normal dispersion, and are hence probably Rayleigh waves; (2) stacking of the events enhances the P wave, which has a velocity of 3.1 ± 0.1 km sec−1; (3) the explosions and B-type events have similar hypocenters, as evidenced by signal characteristics and wave arrival times; (4) the B-type events contain virtually no high-frequency energy, even when allowing for attenuation effects. In particular, we estimate the anelastic attenuation coefficient Q to have a value of 45 ± 20 for Rayleigh waves in the shallow part of the volcano; (5) volcanic tremor spectra exhibit evenly spaced narrow peaks, which we model as eigenvalues of organ-pipe resonance in the magma conduit; we estimate the length of the conduit to be a maximum of 1.6 km; (6) the volcano shows evidence of extreme lateral variation of velocity and attenuation in its shallow structure.

Rupture process of the 1983 Japan Sea (Akita-Oki) earthquake using a waveform inversion method

Abstract: The rupture process of the 1983 Japan Sea earthquake ( MJMA = 7.7) is determined by applying waveform inversion of displacement-type, strong motion records based on the Bayesian method. The synthesis of the seismic motions from the main shock is made using the records from the two aftershocks ( MJMA = 7.1 and MJMA = 6.1) as empirical Green's functions. The main shock fault consists of two subfaults. The aftershock ( M = 6.1) occurring south of the main shock fault is used for the southern subfault empirical Green's function and the aftershock ( M = 7.1) occurring north of the main shock fault is used for the northern subfault. In this synthesis, it is not necessary to calculate propagation path and local site effects. Therefore, the information about the source process can be extracted from the comparison between observed seismic motions and the synthetic ones without detailed information on the velocity structure along the whole path from the source to each station. The inversion for the rupture process is attempted for both a line source model and an area source model. In both models, the main shock fault surface is divided into several elements whose sizes are determined from the scaling relations between the main shock and its aftershocks. The slip displacement and the rupture starting time on each element are estimated as model parameters in the inversion. A good convergence is obtained after about 10 iteration steps for the line source case and after 5 iteration steps for the area source case. The results are summarized: (1) the average rupture velocity is approximately 2.5 km/sec for the southern subfault and 2.0 km/sec for the northern subfault, and (2) there is a large slip displacement near the rupture initiation point and the north edge of the fault. Thus, there is considerable heterogeneity in rupture propagation and slip distribution over the fault plane.

The seismic response of two-dimensional sedimentary deposits with large vertical velocity gradients

Abstract: An extension of the Aki-Larner technique to vertically inhomogeneous media is presented, so that vertical velocity gradients may be taken into account in two-dimensional models. Only SH waves are considered here, but the theory is valid for P and SV waves as well. This method is applied to investigate the seismic response of two-dimensional sedimentary deposits with large velocity gradients. Three different cases are considered: a shallow, high-contrast valley, a deep, high-contrast valley, and a deep, low-contrast valley. In each case, a comparison is performed with the results of a two-dimensional model taking into account only homogeneous sediments and a one-dimensional model taking into account the vertical inhomogeneity of sediments. The presence of a large velocity gradient does not change a lot the qualitative behavior of a two-dimensional deposit: local surface waves and/or two-dimensional resonance patterns are observed as in the case of homogeneous sediments. Nevertheless, the surface waves are much more dispersive with lower phase and group velocities and larger amplitudes. Only very deep valleys give rise to the development of two-dimensional resonance. On the other hand, the amplifications obtained in such deposits may reach much larger values than those predicted with either two-dimensional, homogeneous models or one-dimensional, inhomogeneous models. Since these results have been obtained for realistic values of valley geometrical and mechanical considerations, they should find some application in earthquake engineering or seismic microzonation studies.

Analysis of seismic discrimination capabilities using regional data from western United States events

Abstract: Additional studies have been conducted to help verify the findings of previous investigations (cf. Murphy and Bennett, 1982) which indicated that spectral differences in short-period regional phases may be diagnostic of source type for Nevada Test Site (NTS) explosions and nearby earthquakes recorded at Tonto Forest Observatory in Arizona. Regional phase signals from two additional stations, the Uinta Basin Observatory array in Utah and the Blue Mountains Observatory array in Oregon, and from supplemental events, including aftershocks of the 1966 Caliente, Nevada, earthquake, have been analyzed. These stations are at epicentral distances of 430 to 530 km for Tonto Forest Observatory, 520 to 670 km for Uinta Basin Observatory, and 870 to 880 km for Blue Mountains Observatory, with the near ranges representing the distances to the Caliente source area and the far ranges to the average NTS source/receiver distances. This enhanced data base included 50 earthquakes within about 150 km of NTS and 35 NTS explosions recorded at one or more stations. The events cover a magnitude range from about 2.8 to 5.2 ( mb ), with the majority of earthquakes concentrated in the lower half of that range. The current investigation essentially corroborated our previous findings that comparison of simple, peak-amplitude measurements of regional P and Lg phases did not consistently discriminate between earthquakes and explosions. This discriminant breakdown appears to be related to variability in excitation of regional phases from similar sources which has been dramatically illustrated by observations from the 1966 Caliente aftershocks; these events produced large variations in the relative amplitudes of Pg and Lg signals even though the events were spatially restricted to a small zone. In contrast, the Lg spectral ratio discriminant measure, defined in the previous study, continued to provide reliable distinction between the earthquake and explosion sources for data observed at all three stations. The discrimination capability of the spectral ratio prevails in spite of the evident mechanism variability between earthquakes and shifts in the discrimination threshold between stations apparently related to attenuation differences along the propagation paths.

Crustal structure of Oaxaca, Mexico, from seismic refraction measurements

Abstract: Seismic refraction and gravity data have been analyzed to obtain a model of the compressional-wave structure of the ocean-to-continent transition in the State of Oaxaca in southwestern Mexico. Crustal thickness on the continent at the latitude 18°N is 45 _ 4 km, based on reflected phases from the Moho discontinuity. The crust has been modeled with three layers, with velocities of 4.3 to 4.6, 5.0 to 5.7, and 6.85 to 7.0 km/sec, each with positive velocity gradient. The crust thins to 10 km at the coast near Pinotepa Nacional, where Precambrian metamorphic rocks are exposed 45 km from the mid-America trench. Offshore, the oceanic structure consists of an 8-km-thick crust with a normal crustal velocity structure (Spudich and Orcutt, 1980). The apparent dip of the subducting plate beneath western Mexico is 10 °, On the oceanic side, strong reflections suggest a minimum depth of 35 km for the lithosphere-asthenosphere boundary. The asthenosphere has a seismic velocity of 7.6 km/sec, and a thin lid in which the velocity is 8.6 km/sec.

The evolution of seismic barriers and asperities caused by the depressuring of fault planes in oil and gas fields of South Texas

Abstract: Fluid pressures within some oil and gas fields of South Texas have dropped to less than 20 per cent of their original values, producing earthquakes with magnitudes up to 3.9 in recent years. Differential compaction of the depressurized region may be sufficient to result in the number and size of earthquakes generated, or the faults may have been creeping prior to depressurization. In either model, the depressuring of fluids strengthens a fault and at first produces a “barrier” to slip. As strain accumulates due to compaction or the continued aseismic slip of nearby portions of the fault, stress builds up along the locked portions, eventually forming high-stress regions or “asperities.” The asperities ultimately fail and earthquakes occur. The process is repeated as long as the faults are active. As the fluid pressures continue to decrease, the barriers and subsequent asperities may increase in size and strength, resulting in increasingly large and frequent earthquakes.

Thermoelastic strain in a half-space covered by unconsolidated material

Abstract: An algorithm to predict crustal thermoelastic strain from observed local atmospheric temperature is given and applied to a 24-month crustal strain record of one test strainmeter site located near Bouquet Reservoir in southern California. We use a crustal model that consists of an elastically decoupled layer overlying a uniform elastic half-space, and a thermal source that is given by a stationary temperature wave whose wavelength is related to local topography and/or lateral material heterogeneity. The decoupled layer delays, attenuates, and low-pass filters the source temperature field. The thermoelastic strain in the underlying half-space, resulting from the temperature variations at the base of the decoupled layer, is calculated using the Berger (1975) solution for thermoelastic strain in a uniform half-space. Applying our model to the test data, we obtain a good fit between predicted and observed strains if we filter the surface thermal signal through a 63-cm-thick decoupled layer. Much of the remaining strain variations clearly correspond to other environmental sources (reservoir loading and rainfall). Our analysis suggests that the horizontal thermoelastic strain is inversely proportional to the wavelength of local topography and/or lateral material heterogeneity. Thus, the horizontal thermoelastic strain will be greater in areas of local topography and/or lateral material heterogeneity and smaller in more homogeneous and flat areas. An upper layer of loose material, natural or artificial, acts as a thermoelastic strain insulator. Burial of strainmeters in places where such a layer exists can reduce the thermoelastic strain noise considerably even for shallow strainmeter emplacements.

Observation of 1- to 5-second microtremors and their application to earthquake engineering. Part III. A two-dimensional study of site effects in the San Fernando Valley

Abstract: A two-dimensional study of the influence of deep sediment on seismic ground motions was conducted by recording and analyzing long-period microtremors in the San Fernando Valley, California. The recordings were made at 50 regularly distributed sites in the valley. Three reference sites were employed at the baserock outcrop around the valley so as to observe the time-dependent characteristics of the microtremors . 1. Amplitudes in a period range of several seconds correlate with the thicknesses of the sedimentary layers . 2. A site amplification effect, which was evaluated in terms of sediment-to-rock spectral ratios, is qualitatively consistent with available geological and strong motion data . 3. The practical field observation procedure designed for this study, the two-dimensional approach, can be easily used in other surveys for estimating the spatial characteristics of earthquake ground motions in the period range of several seconds .

Source characteristics of hypothetical subduction earthquakes in the northwestern United States

Abstract: Historic earthquake sequences on subduction zones that are similar to the Cascadia subduction zone are used to hypothesize the nature of shallow subduction earthquakes that might occur in the northwestern United States. Based on systematic comparisons of several physical characteristics, including physiography and seismicity, subduction zones that are deemed most similar to the Cascadia subduction zone are those in southern Chile, southwestern Japan, and Colombia. These zones have all experienced very large earthquake sequences, and if the Cascadia subduction zone is also capable of storing elastic strain energy along its greater than 1000 km length, then earthquakes of very large size (M_w > 8 1/2) must be considered. Circumstantial evidence is presented that suggests (but does not prove) that large subduction earthquakes along the Cascadia subduction zone may have an average repeat time of 400 to 500 yr.

Fault scarps related to the 1739 earthquake and seismicity of the Yinchuan graben, Ningxia Huizu Zizhiqu, China

Abstract: Surface faulting accompanying the great Yinchuan-Pingluo earthquake of 1739 in Ningxia Huizu Zizhiqu (Ningsia Hui Autonomous Region) produced two sections of fault scarps 3.5 and 16.5 km long and separated from one another by 65 km along strike. The scarps are on the west side of the Yinchuan graben along the east flank of the Helan Shan (Holan Mountains). The east side of the faults is downthrown, and surface offsets at the fault are as much as 5.3 m on the Hongguozigou (northern) section, and 4.6 m on the Suyukou (southern) section. Actual net displacement may be slightly less. Near the north end of the set of faults, the Great Wall is offset by about 2.7 m vertically and about 3 m right laterally. On scarps more than 2 m high, a free face has persisted for the 245 yr since the scarps were formed in 1739; free faces commonly are 2 to 3 m high. The 1739 fault displacement occurred, at least in part, along an older fault scarp that is estimated from profile analysis to be about 12,000 yr old. The historical record of destructive earthquakes in the Yinchuan graben since 1010 A.D. includes only one near M 8 in 1739, and only two of approximately M 6.5, one in 1143 and the other in 1477. Average recurrence intervals for major earthquakes comparable to that of 1739 in the Yinchuan graben probably are measured in thousands and possibly ten thousand or more years.

Statistical analysis and clustering features of the Phlegraean Fields earthquake sequence (May 1983-May 1984)

Abstract: The seismic activity at the Phlegraean Fields volcanic area (Southern Italy) during the period May 1983-May 1984 (5,301 selected events) has been analyzed from a statistical point of view. The analysis of the time series associated to the microearthquakes shows a high degree of interdependence among the events, the main feature being the occurrence of dense swarms. The Generalized Poisson Process provides a good fit for the sequence. The clustering of the events has not substantially changed during the analyzed period, while the mean rate of occurrence of the swarms has generally increased. The distribution of the magnitudes within the swarms shows pronounced maxima of energy around the center of the subsequences, in agreement with theoretical models of fractures in highly heterogeneous media.

Continent-like crustal thickness beneath the Bay of Bengal sediments

Abstract: Observed dispersion of fundamental mode Rayleigh and Love waves across the Bay of Bengal to WWSSN and Indian seismograph stations suggests increasing crustal thickness northward, from an approximate 15-km-thick oceanic crust south of the latitude of the southern tip of India, to a more continent-like thickness of approximately 25 km at 20°N, and over 35 km at the northernmost part of the Bengal Fan. The dispersion data are not consistent with models which simply add the increasing sedimentary thickness on top of a normal oceanic crust. The sediment thicknesses and crust velocities are controlled by extensive seismic refraction results and are in agreement with clearly observed sedimentary higher mode waves recorded at the Indian seismograph stations, Madras and Vishakapatnam. Several hypotheses might explain this surprising result. 1. 1. The blanketing effect of the sediments, with consequent temperature rise, has brought about differentiation of basalt to increase the crustal thickness (R. N. Singh, personal communication). 2. 2. The change in Moho depth represents dynamic isostatic adjustment such as a phase change boundary which is lowered because of temperature (or pressure?) perturbations from the sedimentary blanket, or from pressure perturbations of plate collision. 3. 3. The collision has underthrust a wedge of low-velocity material beneath the oceanic crust. 4. 4. The northern Bay of Bengal is actually fortuitiously continental, or continent-like, possibly as a result of the influence of the 90° ridge (especially in the northern part). If the explanation is (1), (2), or (3), the result is important to our understanding of crust and upper mantle dynamics. It is suggested that further controlled studies be performed to verify the result.

Interplate coupling and temporal variation of mechanisms of intermediate-depth earthquakes in Chile

Abstract: We investigated the temporal variation of the mechanism of large intraplate earthquakes at intermediate depths in relation to the occurrence of large under-thrusting earthquakes in Chile. Focal mechanisms were determined for three large events (1 March 1934: M = 7.1, d = 120 km; 20 April 1949: M = 7.3, d = 70 km; and 8 May 1971: M_W = 7.2, d = 150 km) which occurred down-dip of the great 1960 Chilean earthquake (M_W = 9.5) rupture zone. The 1971 event is down-dip compressional: θ (strike) = 12°, δ (dip) = 80°, and λ (rake) = 100°. The 1949 earthquake focal mechanisms is θ = 350°, δ = 70°, and λ = −130°. The data available for the 1934 event are consistent with a down-dip tensional mechanism. Thus, the two events which occurred prior to the great 1960 Chilean earthquake are down-dip tensional. Published fault plane solutions of large intermediate-depth earthquakes (28 March 1965 and 7 November 1981) which occurred down-dip of the Valparaiso earthquakes of 1971 (M_W = 7.8) and 1985 (M_W = 8.0) are also down-dip tensional. These results suggest that before a major thrust earthquake, the interplate boundary is strongly coupled, and the subducted slab is under tension at intermediate depths; after the occurrence of an interplate thrust event, the displacement on the thrust boundary induces transient compressional stress at intermediate depth in the down-going slab. This interpretation is consistent with the hypothesis that temporal variations of focal mechanisms of outer-rise events are due to changes of interplate coupling.

Seismic quiescence precursor to the 1983 Kaoiki (MS = 6.6), Hawaii, earthquake

Abstract: A remarkable seismicity rate decrease of 65 per cent occurred in most of the aftershock volume 2.4 yr before the 1983 Kaoiki MS = 6.6 main shock. If the background seismicity rate (1978 to mid-1981) is extrapolated to the time of the main shock, more than 300 earthquakes of ML ≧ 1.8 are missing because of the quiescence. Subvolumes measuring 5.5 km on a side, and located within the aftershock zone, showed seismicity rate decreases ranging from 0 to 90 per cent. The volume of no change had dimensions of approximately 10 × 3 km. It contained the main shock hypocenter, and was located near the center of the 10 km radius aftershock area. The seismic quiescence in different subvolumes of the aftershock volume surrounding this central nonquiet zone varied somewhat in starting time, amount of rate decrease, and statistical significance. According to the standard deviate z-test, the reported rate decrease is the most pronounced example of precursory quiescence defined to date. The seismicity pattern before the 1983 Kaoiki shock conformed to the quiescence hypothesis proposed on the basis of the precursors to the 1975 Hawaii main shock: no quiescence was observed within the immediate surrounding of the hypocenter, while strong rate decreases occurred in most of the rest of the main shock source volume. It is thus hypothesized that major asperities which contain high stress levels, and from which main ruptures can emanate, may be recognizable as volumes of constant seismicity rate within surrounding volumes of quiescence, provided that decreases of seismicity rate as a function of time can be defined quantitatively.

Seismic strain rates in the Central and Eastern United States

Abstract: Seismic strain rates for the entire Central and Eastern United States are estimated from historical seismicity and from several seismicity models. Typical strain rates are estimated to be the on the order of 10−12 to 10−11 per year, except near historical sites of large earthquakes where higher rates pertain. Uncertainties in strain rates are large. Deformation caused by strain rates less than 10−10 yr−1 is very small compared to average denudation rates. Denudation and deposition may control the rate of earthquake occurrence in some parts of the Central and Eastern United States by modifying the vertical stress due to load. A simple quantitative model predicts the earthquake occurrence rate in the Appalachian Mountains from an estimate of the regional denudation rate.

Application of the principal parameters method to the 1983 Coalinga, California, aftershock sequence

Abstract: A new technique, called the method of the principal parameters (Ebblin and Michelini, in press), to infer the orientations of active fault planes of an aftershock sequence has been applied to the sequence that followed the Coalinga, California, earthquake of 2 May 1983 ( M L = 6.7). The method is based on observed clustering in time and space. Clustering is a characteristic feature of aftershock sequences, and it suggests interdependence of the events. It follows that the spatial locations of time-successive foci may provide additional information about the geometries of the rupturing fault system. The method involves sliding of a temporal window of a fixed number of foci along the sequence and estimating the eigenvalues and eigenvectors of a spatial matrix for each window-set. This matrix can be interpreted as an ellipsoid which is fitted through the foci. During the earthquake sequence, the different trends of the seismicity pattern can be isolated by selecting and averaging the greatly flattened ellipsoids. In the case of the Coalinga sequence, the trends are generally consistent with one of the focal planes obtained from the fault-plane solutions given in earlier published studies. The method appears to offer a simple way to infer average active fault geometries in complex areas from hypocentral locations only.