bluuebunny/crossref_metadata_2025 huggingface dataset:
The Mississippi Valley earthquakes of 1811 and 1812: Intesities, ground motion and magnitudes
Authors: Nuttli Otto W.
Published: September 2021

abstract

Contemporary newspaper accounts of the 1811-1812 Mississippi Valley earthquake sequence are used to construct a generalized isoseismal map of the first of three principal shocks of the sequence, that of December 16, 1811. The map is characterized by an unusually large felt area, with MM intensities of V as far away as the southeast Atlantic coastal area.

By correlating the isoseismal map with that of recent earthquakes for which ground motion data are available, the body-wave magnitude of the December 16, 1811 earthquake is estimated to be 7.2. The other principal shocks, on January 23, 1812 and February 7, 1812, had estimated mb values of 7.1 and 7.4, respectively. The total energy released by the principal shocks and their larger-magnitude aftershocks is estimated to be equivalent to that of an mb = 7.5 (or Ms = 8.0) earthquake.

The anomalously large areas of damage and of perceptibility of the principal shocks result from both the surficial geological conditions of the Mississippi Valley and the relatively low attenuation of surface-wave energy in eastern North America.

Estimates of the vertical component of ground motion, for an earthquake of mb = 7.2 occurring in eastern North America, are given. These include values for particle velocity, displacement, and acceleration at frequencies of about 3, 1 and 0.3 Hz.
A note on elastic relative displacement of soils during earthquakes
Authors: Stephenson W. R.
Published: September 2021

abstract

The behavior of models of soil deposits has shown the importance of small scale boundary variations in generating elastic relative displacement of soils during earthquakes. These effects can be explained in terms of phase differences between adjacent masses of resonating soil. Such effects can result in the failure of extended structures during earthquakes.
Effect of symmetric undulations of the surface on incident elastic waves
Authors: Bhattacharyya Sudarsan
Published: September 2021

The effect on a vertically incident P wave of circularly symmetric undulations of the surface has been studied. It has been found that for certain wavelengths of the undulations, there is a phenomenon of resonance. Also, for certain amplitudes of the waves, there is concentration of energy in the valleys.
Amplitude-yield scaling for underground nuclear explosions
Authors: Springer D. L. , Hannon W. J.
Published: September 2021

abstract

About 60 sets of seismic amplitude-yield data were examined using standard regression techniques to determine slopes of amplitude-yield scaling relations for explosions in water-saturated tuffs and rhyolites. Both P-wave amplitudes and Rayleigh-wave amplitudes were studied at selected stations located at regional and teleseismic distances. The source population included only those underground nuclear explosions fired near or below the level of the static water table at Pahute Mesa, Nevada Test Site, and covered about three orders of magnitude in yield.

Statistical tests applied to the slope parameter (b) indicate that the slopes at regional and teleseismic distances are different. An estimated mean value of b for P-wave amplitude/period (A/T) was slightly greater than 0.6 for regional distances but was almost 1.0 for teleseismic distances. The estimated mean value of b for Rayleigh-wave A/T data was about 1.1. At a given distance the slopes seem to be independent of the yield range considered for both P-waves and Rayleigh-waves.
Variation of the frequency-magnitude relation during earthquake sequences in New Zealand
Authors: Gibowicz S. J.
Published: September 2021

abstract

Seven New Zealand earthquake sequences are studied statistically. These comprise six aftershock sequences and one earthquake swarm. The magnitude-stability law of Lomnitz does not hold. During the aftershock sequences the coefficient b, governing the frequency-magnitude relationship, is found to increase rapidly after the main shock, and then to decrease until the occurrence of the largest aftershock, when it again begins to increase. During the earthquake swarm, the coefficient b decreases logarithmically with time. This can be explained in terms of stress changes and is consistent with laboratory studies on rock deformation.
One-array and two-array location capabilities
Authors: Gjøystdal H. , Husebye E. S. , Rieber-Mohn D.
Published: September 2021

abstract

Event location capabilities of a single array and of two arrays in terms of random and biased observational errors have been simulated on the computer. The approach was to obtain a Gaussian distribution of the parameters azimuth, slowness (DT/DΔ) and arrival-time differences which are used in one- and two-array epicenter determinations. Specifying their mean values and variances, a large number of paired values of these parameters are simulated by using a random number generation routine. These observations determine a distribution of event locations in geographic space, from which the axes of the 95 per cent confidence ellipse were calculated. In case of one array, the semi-axes of the confidence ellipse are tabulated as a function of epicentral distance, and fictive variances of azimuth and slowness. The joint event location capability for two arrays is easily computed but not tabulated because of the regional changes in the shape and orientation of the confidence ellipses. The above analysis was supplemented with epicenter determinations using real NORSAR and LASA data. In the West Indies area, the average location error was found to be 310, 161 and 139 km for NORSAR, LASA, and NORSAR and LASA combined.

A model for individual subarray time corrections for the NORSAR array is used for simulating biased location errors, which would be very large if left uncorrected. The results obtained roughly indicate that the array siting area may account for about 45 per cent of the observed slowness anomalies, but no improvement was obtained in the azimuth anomalies. The slowness bias makes it difficult to utilize the NORSAR DT/DΔ measurements for investigations of heterogeneities in the mantle.
Determination of source parameters of small earthquakes from P-wave rise time
Authors: O'Neill M. E. , Healy J. H.
Published: September 2021

abstract

A simple method of estimating source dimensions and stress drops of small earthquakes is presented. The basic measurement is the time from the first break to the first zero crossing on short-period seismograms. Graphs relating these measurements to rise time as a function of Q and instrument response permit an estimate of earthquake source parameters without the calculation of spectra. Tests on data from Rangely, Colorado, and Hollister, California, indicate that the method gives reasonable results.
Numerical seismograms of long-period body waves from seventeen to forty degrees
Authors: Helmberger Donald V.
Published: September 2021

abstract

Long-period wave propagation in the upper mantle is investigated by constructing synthetic seismograms for proposed models. A model consisting of spherical layers is assumed. Generalized ray theory and the Cagniard-de Hoop method is used to obtain the transient response. Preliminary calculations on producing the phases P and PP by ray summation out to periods of 50 sec is demonstrated, and synthetic seismograms for the long-period World Wide Standard Seismograph Network (WWSSN) and Long Range Seismic Measurement (LRSM) instruments are constructed.

Models containing prominent transition zones as well as smooth models predict a maximum in the P amplitude near 20°. The LRSM synthetics are quite similar for the various models because the instrument is relatively narrow-band, peaked at 20 sec. The upper mantle appears smooth at wavelengths greater than 200 km. On the other hand, the WWSSN synthetics are very exciting for models containing structure. The triplications are apparent and the various pulses contain different periods.

The amplitude of the P phase at 30° is down to about 25 per cent of its 20° maximum. The amplitude of the PP phase at 35° is comparable to P. Near 37°, the PP phase grows rapidly reaching about twice the P phase amplitude near 40°. Models containing sharp transition zones produce high-frequency interferences at neighboring ranges. A profile of observations is presented for comparison.
A magnitude domain study of the seismicity of Papua, New Guinea, and the Solomon Islands
Authors: Curtis John W.
Published: September 2021

abstract

A detailed magnitude domain study of the distribution of earthquakes occuring in the Papua-New Guinea-Solomon Islands region between 1959 and 1970 is described. The primary results are as follows:
A simple model of the crust and upper mantle in central Chile
Authors: Chuaqui L.
Published: September 2021

abstract

A simplified model of the crust and upper mantle of central Chile is developed with P- and S-wave arrival times and is compared with previous gravimetric work on the area. The following structural parameters were determined: crustal P-wave velocity, upper mantle P-wave velocity, crustal thickness and orientation of the plane separating crust and upper mantle. The model obtained here agrees well with those calculated in the gravimetric study.
Surface-wave attenuation and crustal anelasticity in Central North America
Authors: Mitchell Brian J.
Published: September 2021

abstract

The southeastern Missouri earthquake of October 21, 1965 generated fundamental- and higher-mode Love and Rayleigh waves which were recorded at numerous North American stations. Love-wave amplitude radiation patterns were determined and found to be consistent with theoretical patterns predicted by a fault-plane solution previously inferred from Rayleigh-wave data.

The radiation patterns were used to estimate the source spectrum and values for Love-wave attenuation coefficients for the mid-continent of North America by a least-squares iterative process. The source spectrum derived from Love-wave amplitudes exhibits a peak at periods between 5 and 9 sec and decreases to a lower DC level at longer periods, in agreement with the source spectrum determined previously for Rayleigh waves. The Love-wave attenuation coefficients decrease rapidly from about 0.0018 km−1 at a period of 4 sec to about 0.0001 km−1 at a period of 20 sec. At periods between 20 and 40 sec the values seem to remain nearly constant.

The crust in the mid-continent of North America is characterized by relatively low Qβ values, 75 to 300, in its upper portion. At depths between 15 and 20 km, Qβ increases sharply and decreases again at greater depths. The decrease can be explained as being due to increasing temperature in a homogeneous material, but the sharp increase requires a change in the chemical constitution of the material at mid-crustal depths.
Revised velocities in the Earth's core
Authors: Qamar Anthony
Published: September 2021

abstract

Travel times and amplitudes of PKP and PKKP from three earthquakes and four underground nuclear explosions are presented. Observations of reflected core waves at nearly normal angles of incidence provide new constraints on the average velocities in the inner and outer core. Interpretation of these data suggests that several small but significant changes to Bolt's (1962) core velocity model (T2) are necessary. A revised velocity model KOR5 is given together with the derived travel times that are consistent with the 1968 tables for P. Model KOR5 possesses a velocity in the transition zone which is 112 per cent lower than that in model T2. In addition, KOR5 has a velocity jump at the transition zone boundary (r = 1782 km) of 0.013 km/sec and a jump at the inner core boundary (r = 1213 km) of 0.6 km/sec. These values are, respectively, 1/20 and 2/3 of the corresponding model T2 values.
The spectral theory of seismic sources
Authors: Randall M. J.
Published: September 2021

abstract

The far-field results of Brune's spectral theory are shown to be largely independent of his source model; this implies that the theory has even greater power than it seemed, but that its success in explaining the observed spectra does not in itself justify a dislocation model. Expressions are derived for seismic energy and characteristic stress which are independent of assumptions as to source model. For several models, the characteristic stress is found to be a good approximation to the stress-drop.

A theoretical relationship between earthquake magnitude ML, stress-drop, and fault size is derived. This provides a means of estimating the stress-drop for earthquakes associated with aftershock sequences, using estimates of ML and fault size, and is consistent with empirical relationships between magnitude and fault size, and between seismic energy and magnitude.
Dilatancy and premonitory variations of P, S travel times
Authors: Gupta Indra N.
Published: September 2021

abstract

Nur's (1972) explanation of the observed premonitory decrease in the travel-time ratio of shear and compressional waves, ξ, in terms of dilatancy is modified by considering the anisotropic characteristics of dilatant rock in the focal region. The expected change in ξ is influenced by the orientation and type of geologic fault as well as the geometry of source and observation points. The shear wave will in general split into two components traveling with somewhat different velocities.
Dynamic structure-soil-structure interaction
Authors: Luco J. Enrique , Contesse Luis
Published: September 2021

abstract

A study is made of the dynamic interaction, through the soil, between two parallel infinite shear walls placed on rigid foundations. The steady-state response of both structures for a vertically incident SH wave is obtained and compared with the corresponding values resulting from consideration of only one structure.

It is found that the additional interaction effects caused by the presence of a second structure are especially important at low frequencies and in the neighborhood of the fixed-base natural frequencies of the second structure. For high frequencies it is sufficient to consider the interaction between each structure and the soil, ignoring the presence of other structures.
Low-frequency earth motion generated by slowly propagating partially organized pressure fields
Authors: Sorrells G. G. , Goforth T. T.
Published: September 2021

abstract

General expressions for the transfer function and the coherence, relating earth motion and atmospheric pressure variations observed at the surface, are derived for the case where the pressure field is a random process which is stationary in both time and space. It is found that as long as the phase velocities of the pressure field are low in comparison to seismic wave velocities, this transfer function will be a smoothed version of the Earth's response to a point pressure load applied at the surface. The form of the smoothing operator is determined by the correlation structure of the pressure field. In particular, it was found that the transfer function of an analytical model which possesses all of the observed statistical properties of the wind-generated pressure field closely resembles those associated with plane pressure waves.
Observations ofPKiKPat distances less than 110°
Authors: Buchbinder Goetz G. R. , Wright C. , Poupinet G.
Published: September 2021

abstract

PKiKP phases have been identified at the Yellowknife Array from one nuclear explosion at a distance of 44°, and 18 earthquakes over the distance range 70° to 100°. In addition, PKiKP phases from two nuclear events were observed at 8 single seismometer stations at distances between 21° and 43°. The measured travel times and slownesses agree satisfactorily with those calculated using recent core models. The observed amplitudes are also close to the predicted values when compared with PKiKP amplitudes determined at LASA in Montana. Because of the large scatter of the observed PKiKP amplitudes and the complexities associated with the reflection of PcP, the amplitude ratio PKiKP/PcP does not readily yield detailed information on the inner core boundary.
Surface instability due to initial compressive stress
Authors: Brunelle E. J.
Published: September 2021

Utilizing a linearized theory of isotropic, homogeneous elasticity that includes the effects of initial stress, it is demonstrated that Rayleigh and Love surface instabilities occur when the compressive stress reaches appropriate critical values. The Rayleigh surface instability exists for a half-space, as well as for a finite slab, and the critical stress value depends only on Poisson's ratio. The Love surface instability exists for a layer and a substratum, and the critical stress value depends on two parameters: the ratio of the shear moduli and a nondimensional number related to the geometry of the layer. It is suggested that these two instabilities may offer possible, although highly idealized, mechanisms for earthquake initiation and prehistoric land-mass formations such as mountain chains.
Accuracy of determination of epicenter and origin time of small-magnitude earthquakes in the Indian subcontinent
Authors: Gupta Harsh K. , Skoko Dragutin , Satô Yasuo
Published: September 2021

World seismicity maps prepared by plotting epicenters without considering their magnitude and the distribution of seismic stations may not represent true seismicity, since many earthquakes of small magnitude are not reported from regions having poor seismic networks. This situation is particularly noticeable in the Asian parts of the Alpide belt.

Using the Monte Carlo method, errors of epicenter location and origin-time determination by the Indian network of seismic stations have been estimated for earthquakes in the Indian subcontinent. A simplified earth model has been used, and the calculations have been carried out for mb values of 5.0, 4.5, and 4.0. The results are compared with similar error estimations for the Romanian region where a better network of seismic stations enables location of all earthquakes of mb ≧ 4.2. This comparison shows that the errors for the central and the south Indian regions are comparable to those of Romania. However, the much larger errors in the north and northeastern regions show the necessity of establishing seismological observatories equipped with sensitive seismographs close to the foothills of the Himalayas.
SH travel times and lateral heterogeneities in the lower mantle
Authors: Niazi Mansour
Published: September 2021

The horizontal long-period seismograms of two shallow earthquakes in Turkey and Iran recorded in selected azimuths are combined for travel-time studies of the SH wave beyond the angular distance of 40°. The observed travel times along two profiles which sample the deep mantle in the vicinity of Iceland and the North Pole show monotonically increasing differences beyond 65°, indicating lateral heterogeneity in the lower mantle. The travel-time difference becomes as large as 7 sec at 95°, implying a variation as much as 0.06 km/sec, or about 1 per cent, in the shear-wave velocity near 2,500 km depth. Inversion of observations, adjusted to surface foci, results in an average lower mantle structure with lower shear velocities than those given by Jeffreys. The difference exceeds 0.1 km/sec at the core boundary.

The arrival time and signature of S waves recorded in Greenland show anomalous features which may be related to deep seated anomalous zones associated with the Mid-Atlantic Ridge system.
Corner frequencies ofPandSwaves and models of earthquake sources
Authors: Molnar Peter , Tucker Brian E. , Brune James N.
Published: September 2021

P- and S-wave spectra of 144 aftershocks 12≦M≦412 of the February 9, 1971 San Fernando earthquake corroborate previous work showing that the corner frequencies for P waves in general are greater than those for S waves. This observation is consistent not only with models that treat earthquakes as volume sources, but also with physically reasonable dislocation models for which (1) the source is approximately equidimensional, (2) both the duration of slip at each point on the fault and the time for the ruptured area to develop are not long compared with the time for seismic waves to cross the ruptured area, and (3) much of the source radiates essentially simultaneously. There may be other physically reasonable dislocation models compatible with the observations. Savage's calculations indicate that models that involve propagating dislocations on long thin faults are not adequate for describing most moderate and small earthquakes studied.
Stress relaxation in a semi-infinite viscoelastic earth model
Authors: Rosenman Martin , Singh Sarva Jit
Published: September 2021

Expressions for quasi-static surface stresses resulting from a finite, rectangular, vertical, strike-slip fault in a Maxwellian viscoelastic half-space are derived. Variation of the stresses with time and epicentral distance is studied. Contour maps are obtained in some representative cases. It is found that all nonvanishing stress components at the free surface die exponentially with time. This is in contrast to the behavior of the displacements and strains which, in general, do not vanish for large times.
Earthquakes as critical phenomena: Implications for probabilistic seismic hazard analysis
Authors: Main Ian G.
Published: September 2021

Earthquake populations have recently been postulated to be an example of a self-organized critical (SOC) phenomenon, with fractal spatial and temporal correlations and a power-law distribution of seismic energy or moment corresponding to the Gutenberg-Richter (G-R) frequency-magnitude law. In fact, strict SOC behavior is not seen in all models and is confined to those with weak annealed (permanent) heterogeneity and an intermediate tectonic driving velocity or strain energy rate. Depending on these conditions, distributions may also occur that are subcritical, where the largest events have a reduced probability of occurrence compared to the G-R trend, or supercritical, where the largest events have an elevated probability of occurrence, leading to “characteristic” earthquakes. Here we show type examples of all three types of behavior, lending support to a generalization of the Gutenberg-Richter law to a modified gamma distribution (a power law in energy or moment with an exponential tail with positive, zero, or negative argument). If earthquakes are an example of a critical phenomenon, then the a priori assumption of the G-R law in probabilistic hazard analysis is no longer valid in all cases. The appropriate distribution may also depend systematically on the size of the area, with smaller areas concentrating on individual fault segments more likely to produce a characteristic distribution. This previously unexpected effect of Euclidean zoning is an example of the degree of preconditioning inherent in some of the fundamental assumptions of seismic hazard analysis. Other assumptions, such that of stationarity in the process over long time periods, are borne out by SOC. The assumption of a random Poisson process is firmly at odds with SOC as an avalanche process involving strong local and weaker long-range interactions between earthquakes. The gamma distribution for the case of subcritical behavior predicts a maximum “credible” magnitude that may be independently determined from long-term slip rates, defined as the magnitude where the contribution to the total moment or intensity is negligible though finite. This soft maximum replaces the need to independently impose a hard, though somewhat artificial, maximum in distributions such as the G-R law. The same approach can be taken for the overall seismic hazard expressed by negligible contribution to ground motion, with similar results.
Attenuation and source parameters of earthquakes in the Cascadia region
Authors: Atkinson Gail M.
Published: September 2021

Digital ground-motion data recorded by the Western Canada Telemetered Network (WCTN) are used to examine the attenuation and source parameters of earthquakes in the Cascadia region of southwestern British Columbia and northwestern Washington. The data base is comprised of over 1000 vertical-component Fourier spectra, from earthquakes of magnitude 3 to 7 at distances from 10 to 500 km. Regression analyses determine the shape and coefficients of the regional attenuation curve and source spectra for 68 earthquakes. Seismic moment and stress drop are inferred from the amplitudes of the source spectra.

Shallow (h < 10 km) earthquakes have an attenuation curve with a complex shape, exhibiting significant flattening (no apparent geometric spreading) in the distance range from 75 to 230 km; this shape is probably a result of strong reflected phases from mid-crustal discontinuities and the subducting slab. Events within the subducting slab and the lower crust exhibit a simple R−1 attenuation curve. The anelastic attenuation coefficient for the region as a whole is given by Q = 380f0.39.

The duration of motion for each WCTN record is determined as the value that yields the observed relationship between time-domain and spectral-domain amplitudes, according to random process theory. These durations are approximately constant within 50 km of the source, then increase with distance as 0.07R.

The high-frequency ground motions from Cascadia earthquakes are relatively weak. Cascadia source spectra are characterized by an average Brune stress drop of about 30 bars. This is significantly lower than the average California stress drop of 70 to 100 bars, and dramatically lower than the average eastern stress drop of 150 to 200 bars. It is concluded that there are significant regional variations in source parameters. Hazard estimates for the Cascadia region based on California groundmotion relations may be overly conservative.
An efficient approach to the seismogram synthesis for a basin structure using propagation invariants
Authors: Takenaka H. , Ohori M. , Koketsu K. , Kennett B. L. N.
Published: September 2021

The Aki-Larner method is one of the cheapest methods for synthetic seismograms in irregularly layered media. In this article, we propose a new approach for a two-dimensional SH problem, solved originally by Aki and Larner (1970). This new approach is not only based on the Rayleigh ansatz used in the original Aki-Larner method but also uses further information on wave fields, i.e., the propagation invariants. We reduce two coupled integral equations formulated in the original Aki-Larner method to a single integral equation. Applying the trapezoidal rule for numerical integration and collocation matching, this integral equation is discretized to yield a set of simultaneous linear equations. Throughout the derivation of these linear equations, we do not assume the periodicity of the interface, unlike the original Aki-Larner method. But the final solution in the space domain implicitly includes it due to use of the same discretization of the horizontal wavenumber as the discrete wavenumber technique for the inverse Fourier transform from the wavenumber domain to the space domain.

The scheme presented in this article is more efficient than the original Aki-Larner method. The computation time and memory required for our scheme are nearly half and one-fourth of those for the original Aki-Larner method. We demonstrate that the band-reduction technique, approximation by considering only coupling between nearby wavenumbers, can accelerate the efficiency of our scheme, although it may degrade the accuracy.
Repeatability of microseism measurements in Port Hueneme case study
Authors: Ferritto John M.
Published: September 2021

Microseism measurements can assist in prediction of ground-motion amplification and microzonation. A series of array measurements were conducted in the Port Hueneme, California, area. The measurements were repeated, and the error of microseism measurements is discussed. From the seven individual trials, it was possible to show the convergence of microseism measurements and their repeatability. The series of array measurements of microseisms used both a soil and a rock reference site. The soil site was a site having a well-defined boring log. A comparison is made between results obtained using a rock and a soil reference site. It was found that qualitatively the general dimensionless shape of relative amplification contour plots was similar using both reference sites. Both reference sites showed the same regions as having higher amplification compared with other lower regions.
Array observations of the shear-coupled PL wave
Authors: Zhang Jie , Langston Charles A.
Published: September 2021

Observations from the NARS array in western Europe of a shallow earthquake in northwest Africa are used to study shear-coupled PL-wave (SPL) excitation near the source and receiver. Numerical experiments suggest that average crustal thickness and velocity are important in defining the frequency content of SPL. If crustal structures in the source and receiver areas are similar and the upper mantle velocity gradients are positive, then large SPL waves can be generated. Using the near-receiver velocity structure that was determined from previous NARS studies, we model the near-source structure for an earthquake in northwest Africa. Although the model is subject to significant trade-offs in layer thicknesses and velocities in the near-source structure, SPL waveforms could be a useful auxiliary data set to other seismological structure studies.
Numerical seismograms obtained using ω-κ integrals, for a point P source in a vertically inhomogeneous anelastic model
Authors: Abramovici F. , Le L. H. T. , Kanasewich E. R.
Published: September 2021

This article presents some numerical experiments in using a computer program for calculating the displacements due to a P source in a vertically inhomogeneous structure, based on the Fourier-Bessel representation. The structure may contain homogeneous, inhomogeneous, elastic, or viscoelastic layers. The source may act in any type of sublayer or in the half-space. Synthetic results for the simple case of a homogeneous layer overlaying a homogeneous half-space compare favorably with computations based on the Cagniard method. Numerical seismograms for an elastic layer having velocities and density varying linearly with depth were computed by integrating numerically the governing differential systems and compared with results based on the Haskell model of splitting the linear layer in homogeneous sublayers. Even an adaptive process with a variable step size based on the Haskell model has a poorer performance on the accuracy-cpu time scale than numerical integration.
Moderate-magnitude seismicity remotely triggered in the Taiwan region by large earthquakes around the Philippine Sea plate
Authors: Wen Kuo-Liang , Beresnev Igor A. , Cheng Shih-Nan
Published: September 2021

Seismicity in the Taiwan region was studied before and after the 12 largest seismic events with body-wave magnitudes exceeding 6.5, occurring around the Philippine Sea plate in the 1973 to 1994 period. The total local seismicity rate involving events with magnitudes as small as 2.0 was not affected by remote large events. However, the number of local earthquakes with magnitude ML ≧ 4.5 in the 15 days following a large event exceeded the number before it in 9 out of 12 cases studied. When earthquakes with ML ≧ 4.0 were considered, then 10 cases showed an increase in seismic activity after the big event. This suggests a 75 to 83% probability of remote triggering.
A simple algorithm for tracing synthetic isoseismals
Authors: Sirovich Livio
Published: September 2021

Probabilistic calculation of regional seismic hazard maps also requires the use of the so-called “attenuation relations,” which give the reference “shake-ability” at certain distances from the earthquake sources. This article achieves progress in this area. In fact, the present tests on a series of earthquakes in California (San Fernando, 1971; Whittier Narrows, 1987; Northridge, 1994) suggest that in some regions the areal shapes of the territories damaged by past earthquakes may be synthetically traced—sometimes amazingly well—with a simple algorithm that considers some gross features of the sources, and this is compatible with theory. It seems that this algorithm gives rather stable results. Moreover, when the detailed modeling techniques available nowadays are inapplicable due to lack of data, or for purpose of saving time and money, it might be useable for improving seismic hazard calculations and, conversely, for retrieving information about sources of earthquakes from the preinstrumental era.
On the use of simulated annealing method and cross-validation theory for deconvolution of seismograms
Authors: Courboulex F. , Virieux J. , Gibert D.
Published: September 2021

In order to retrieve the apparent source time function (ASTF) seen at a given station, one must take into account propagation effects, site, and instrumental influences. Removing these effects can be performed by a deconvolution of the mainshock seismogram by a seismogram of a smaller event recorded at the same station. This smaller event must occur nearby the mainshock, and the associated seismogram is considered as an empirical Green's function. We propose a deconvolution based on the simulated annealing method, and we compare it with the often-used spectral division technique.

We show on both synthetic and real signals that the simulated annealing deconvolution (SAD) provides stable and positive ASTF, whereas results from spectral division are very sensitive to an ad hoc parameter called water level.

Finally, the application of cross-validation analysis between the three components of the seismogram in addition to the (SAD allows us to estimate errors on the ASTF.
How well do we utilize global seismicity?
Authors: Wysession Michael E.
Published: September 2021

This article describes a method for quantifying the ability to record teleseismic phases at particular epicentral distance ranges, given the geographical history of global seismicity. With the use of geographical sampling maps, we identify the regions of the Earth that are best suited to record the greatest numbers of earthquakes at particular distances. Since seismic studies of the Earth's interior use teleseismic phases that have unique ranges, this information can be useful in the planning of future permanent and temporary deployments of seismometers. Deployment of ocean-bottom seismometers would be required for recording large numbers of earthquakes in the 40° to 80° range, corresponding to phases like ScS and PcP, and in the 140° to 170° range, important for investigations of the PKP branches. An examination of existing analog and digital networks shows that they do either better or worse than a hypothetical grid of evenly spaced seismometers, depending upon the distance range examined. The use of temporary deployments of seismometers, perhaps even in the oceans, may be the best way to significantly sample poorly examined regions of the Earth's interior.
Long-period ground motions and dynamic strain field of Los Angeles basin during large earthquakes
Authors: Teng T. L. , Qu J.
Published: September 2021

During a big earthquake along the San Andreas fault in southern California, high excitation and low attenuation of long-period (3 to 10 sec) strong ground motions will cause wave motions to propagate efficiently far from the epicentral area. These ground motions could potentially be destructive to large-dimension structures in the Los Angeles basin. We performed calculations using the surface-wave Gaussian beam method for a 3D southern California crustal structure. Displacement field as well as the associated dynamic strain field produced by large propagating ruptures along the San Andreas fault are evaluated. Results indicate that in the presence of lateral heterogeneity, focusing and multipathing interference contribute significantly to a complex pattern of the displacement field and the associated dynamic strain field. For a big event on the San Andreas fault with a seismic moment of 1.8 × 1028 dyne-cm, long-period displacement in the Los Angeles basin could reach a maximum amplitude of meters in places. Since this calculation is fast, we have evaluated the displacement field for a dense grid of points; a differentiation gives the corresponding effective horizontal dynamic strain field. At times, the maximum effective dynamic strains may reach mid-10−3 to even 10−3—high enough to be of engineering concern. This computational result probably gives the upper bound values due to the large source assumed. For events of smaller seismic moment release along less extensive ruptures, these results can easily be scaled down proportionally. Different scenarios are considered in this study with different slip distributions. It is found that with a given seismic moment, a more evenly distributed fault slip over the rupture surface will result in lower peak values on both displacements and dynamic strains. Our displacement results give similar values to those obtained by Kanamori using empirical Green's functions but substantially higher than Bouchon and Aki's results.
High-frequency analysis of seismic background noise as a function of wind speed and shallow depth
Authors: Withers Mitchell M. , Aster Richard C. , Young Christopher J. , Chael Eric P.
Published: September 2021

We used a deep (1500 m) cased borehole near the town of Datil in west-central New Mexico to study high-frequency (>1 Hz) seismic noise characteristics. The remote site had very low levels of cultural noise, but strong winds (winter and spring) made the site an excellent candidate to study the effects of wind noise on seismograms. Along with a three-component set of surface sensors (Teledyne Geotech GS-13), a vertical borehole seismometer (GS-28) was deployed at a variety of depths (5, 43, and 85 m) to investigate signal and noise variations. Wind speed was measured with an anemometer. Event-triggered and time-triggered data streams were recorded on a RefTek 72-02 data acquisition system located at the site. Our data show little cultural noise and a strong correlation between wind speed and seismic background noise. The minimum wind speed at which the seismic background noise appears to be influenced varies with depth: 3 m/sec at the surface, 3.5 m/sec at 43 m in depth, and 4 m/sec at 85 m in depth. For wind speed below 3 to 4 m/sec, we observe omni-directional background noise that is coherent at frequencies below 15 Hz. This coherence is destroyed when wind speeds exceed 3 to 4 m/sec. We use a test event (Md ∼ 1.6) and superimposed noise to investigate signal-to-noise ratio (SNR) improvement with sensor depth. For the low Q valley fill of the Datil borehole (DBH) site, we have found that SNR can be improved by as much as 20 to 40 dB between 23 and 55 Hz and 10 to 20 dB between 10 and 20 Hz, by deploying at a 43-m depth rather than at the surface. At the surface, there is little signal above noise in the 23- to 55-Hz frequency band for wind speeds greater than 8 m/sec. Thus, high-frequency signal information that is lost at the surface can be recorded by deploying at the relatively shallow depth of 40 m. Because we observe only minor further reductions in seismic background noise (SBN) at deeper depths, 40 m is likely to be a reasonable deployment depth for other high-frequency-monitoring sites in similar environmental and geologic conditions.
A statistical technique for validating velocity models
Authors: Iversen Edwin  S. , Lees Jonathan  M.
Published: September 2021

This study investigates the use of a station influence statistic to identify velocity model shortcomings in the earthquake hypocenter location problem. Two groups of microearthquake events are examined. The first is a group of 81 events from the Mount St. Helens region that occurred between November 1987 and September 1991; the second, 110 well-located events from the 1992 Joshua Tree after-shock sequence. We describe a method for validating a postulated earth model. Let λ denote the hypocenter estimates that Geiger's method obtains. Systematically remove each station observation from the location problem, and recompute the location estimate. Call this estimate λ(i) when the ith station is removed. For a single event, define a station's influence (SI) as a weighted difference between λ and λ(i). Distributional summaries of SI statistics across events are used to identify model shortcomings: Given a specified velocity model, SI distributions that are not homogeneous across stations provide evidence of model inadequacies and/or failures in the weighting scheme. We show that velocity model shortcomings detected using SI statistics for the Mount St. Helens sequence under a one-dimensional model appear to correlate with known physical anomalies; while SI distributions evaluated under a three-dimensional model are more homogeneous and reflect a modest improvement over the one-dimensional model. SI distributions provide evidence of model failure for the Joshua Tree sequence under a one-dimensional model, but no evidence of failure under a three-dimensional model. Finally, the weighting scheme's validity is verified for the Joshua Tree sequence under the three-dimensional model.
Predictability of strong motions from the Northridge, California, earthquake
Authors: Anderson John G. , Yu Guang
Published: September 2021

The composite source model for generating synthetic strong ground motions is tested for its ability to predict the statistical characteristics of Northridge accelerograms recorded in or adjacent to the San Fernando valley. The general problem is prediction of strong motions at a site of engineering interest with sufficient realism to be useful for engineering applications. The strongest test of any proposed method is a blind prediction. For this study, a completely blind test was not possible. Our objective was to use only a preliminary description of fault geometry and magnitude and previously published velocity models and, without iteration to improve the quality of fit, to evaluate the differences between predicted and observed accelerograms.

The parameters that we predict are peak acceleration, peak velocity, peak displacement, Fourier spectra at seven frequencies, and pseudorelative velocity response (5% damping) at seven periods. Our results are given for 14 stations. For the horizontal components, these parameters are all predicted with a maximum bias of under 50% and an average bias of observations exceeding predictions by 6%. For peak acceleration and some response spectral periods, the bias for this model is smaller than at least some regressions, when applied to this specific earthquake. On the vertical component, the maximum bias is a factor of 2, and the average gives predictions exceeding observations by 25%. Standard deviations of the common logarithm of the ratio of observed-to-predicted parameters are typically about 0.3, which is perhaps 50% greater than the standard deviations typical of regressions but comparable to standard deviations of observations from this earthquake compared to regressions. In the future, it is likely that, in some cases, traditional regressions will be replaced with synthetic calculations of some type, such as the method used here. Based on the results of this study, the amount of progress that has been made in obtaining that goal is very encouraging.
Simulation of long-period ground motion near a large earthquake
Authors: Takeo Minoru , Kanamori Hiroo
Published: September 2021

We estimated the possible range of long-period ground motion for sites located on a soft sedimentary basin in the immediate vicinity of a large earthquake. Since many large cities in the world (e.g., Los Angeles, San Francisco, and Tokyo) where many large structures have been recently constructed are located in this type of environment, a better understanding of long-period ground motion is becoming increasingly important. Our objective is to estimate the possible range of long-period ground motion, rather than ground motion for a specific fault model. We computed ground-motion time series and pseudo-velocity response spectra (PVS) for more than 5,000 models for the 1923 Kanto, Japan, earthquake (MW = 7.9) using 180 slip distributions, eight rupture geometry, and rupture velocities ranging from 1.5 to 3.0 km/sec. Two seismograms recorded in Tokyo during the 1923 Kanto earthquake are used for comparison. The response spectra computed using seismologically reasonable sets of source parameters for the 1923 Kanto earthquake vary by more than an order of magnitude. At periods of 10 to 13 sec, they range from 25 to 170 cm/sec in Tokyo. For some combinations of model parameters, the response spectra exhibit peaks in the range of 10 to 13 sec. Many of the computed response spectra have peaks at periods longer than 10 sec, which is considerably longer than the dominant period (6 to 8 sec) estimated from studies of small earthquakes and microtremor measurements. Thus, the dominant period of the subsurface structure determined locally may not be representative of the dominant period of ground motion from a nearby large earthquake, which is controlled by rupture directivity and source depth. We performed a similar simulation for a hypothetical MW = 7.5 earthquake located beneath the Los Angeles basin. For a site just above the center of the fault, the ground-motion spectral amplitude at a period of 10 sec can vary from 50 to 350 cm/sec. This range, though very large, is what is expected for a seismologically plausible range of source parameters.
An automated Rayleigh-wave detection algorithm
Authors: Chael Eric P.
Published: September 2021

The desire to operate denser networks in order to monitor seismic activity at lower thresholds leads to greater emphasis on automated data processing. An algorithm for detecting and characterizing long-period Rayleigh-wave arrivals has been developed and tested. The routine continuously monitors all directions of approach to a station, in a manner similar to beamforming. The detector is based on cross-powers between the Hilbert-transformed vertical and rotated horizontal signals, so it is sensitive to both the power and polarization properties of the three-component wave field. Elliptically polarized Rayleigh arrivals are enhanced, while linearly polarized Love waves and body phases are suppressed. A test using one month of data from station ANMO demonstrated that this technique can, with high reliability, detect Rayleigh arrivals that are visible on the records. The measured arrival times and azimuths are accurate enough to permit automated association of the detections to events in a bulletin.