bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

abstractPKiKP 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

Abstract
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

AbstractWorld 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

Abstract
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

AbstractP- 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

Abstract
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

Abstract
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

Abstract
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 &lt; 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

AbstractThe 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

AbstractMicroseism 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

Abstract
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

Abstract
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

Abstract
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

AbstractProbabilistic 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

Abstract
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

Abstract
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.