bluuebunny/crossref_metadata_2025_split huggingface dataset:
Earthquake location based on the gradient method and on minima of beams of directions
Authors: Gershanik Simón 
Published: September 2021

abstract
Earthquake location is usually made by means of the Gauss-Newton iterative method, known in seismology as the Geiger method. As this method may fail to be efficient in some cases, attention is turned to the possibilities offered to the problem by the gradient method, which is always convergent, and, when properly used, may lead to the vicinity of the solution after a few iterations.
But in the vicinity of the solution, the gradient method becomes slowly convergent. Therefore, in addition, another convergent method based on the minimum value, g, of the sum of the squared residuals taken on a beam of directions, is presented. In it the sum g is approximated by a polynomial of Laplace spherical harmonics.
The new method includes the gradient method as a particular case; it is well suited in the vicinity of the solution and may lead quickly to it.
Field reconnaissance of the effects of the earthquake of April 13, 1973, near Laguna de Arenal, Costa Rica
Authors: Plafker George 
Published: September 2021

abstract
The April 13, 1973, earthquake (MS = 6.5) in northwestern Costa Rica caused significant damage and the loss of 23 lives in a rural area of about 150 km2 centered south of Laguna de Arenal. Earthquake-triggered failures of mountain slopes were the cause of all deaths and injuries attributable to the earthquake. Incipient landslides and landslide-dammed lakes pose a continuing hazard to populated areas at Tronadora and Rio Chiquito during the 1973 rainy season.
The modest amount of shaking damage to structures is attributable to the moderate size of the earthquake and to the predominance of small wood-frame buildings with lightweight corrugated roofs that have good shear resistance to lateral forces. Most of the structural damage that did occur was to buildings made of concrete block with little or no reinforcing steel; none of these, however, involved total collapse. In some areas, damage to buildings and roads was also caused by foundation failures involving downslope slumping or sliding, lateral spreading, and differential compaction of unconsolidated or poorly consolidated surficial deposits including thick lateritic soil.
The location of the epicentral region and the nature of the earthquake and aftershock sequence suggest that the earthquake was of tectonic, rather than volcanic, origin. The shape and size of the damage area and the apparent absence of surface faulting are compatible with a moderately shallow source, possibly several kilometers in depth.
Single-station spectral discrimination using coda waves
Authors: Hartse Hans E. , Phillips W. Scott , Fehler Michael C. , House Leigh S. 
Published: September 2021

Abstract
Spectral discrimination methods applied at regional distances are based on the observation that there is a difference in the frequency content of the seismic waves radiated from the source region of earthquakes and explosions. We have investigated a spectral discrimination method that exploits the scattered energy contained in the event codas of seismograms to measure the relative source excitation of earthquakes and explosions as a function of frequency. Key strengths of the coda method include the following: (1) it is not affected by source radiation pattern because the scattering process averages the radiation over the focal sphere, (2) path corrections are not required with the coda method because coda waves are scattered throughout a common crustal volume, (3) it can be successfully applied using data from only a single station, and (4) it can be successfully applied using data with clipped direct arrivals. To implement the coda method, we first estimate a “type curve,” which represents the average shape of all the individual coda decay curves for a particular frequency band. The shape of the type curve (as well as the shape of all individual coda decay curves) is controlled by the scattering and attenuation properties of the crustal volume sampled by the coda waves (a common path effect). The amplitude of the type curve is not defined, and we arbitrarily adjust it to have a zero mean. The amplitude difference between an individual event coda decay curve and the type curve provides a relative measure of the event's source size within the particular frequency band. We define this amplitude difference as the “relative source factor.” Our approach exploits all of an event's usable coda for each spectral measurement, thus providing a more stable measure of relative source size than could be obtained from using a predefined, fixed-length time window. Analysis of 27 Nevada Test Site explosions and 15 southern Great Basin earthquakes (3.1 ≦ M ≦ 4.7), shows that the source factor difference between the 0.5- to 1- and 2- to 4-Hz bands plotted against the 1- to 2-Hz source factor is a stable event discriminant. The method reliably distinguishes small explosions from small earthquakes (3.0 &lt; M &lt; 3.5), even if the explosions are overburied. Because the coda method requires only one seismic station, we believe that it would be especially useful in identifying small, clandestine underground nuclear tests that occur far from established networks.
Vertical and near-vertical incidence of P waves in a layered solid
Authors: Abramovici F. 
Published: September 2021

Abstract
This article gives two analytical expressions of the displacement in a flat structure: one based on Cagniard's method for simpler cases, the other using a Fourier integral for more general cases. The numerical experiments aimed to check the methods by comparing zero-offset cases with cases of near-vertical incidence confirm the existence of longitudinal PS waves as obtained by Hron and his collaborators. In addition, the experiments reveal that the longitudinal motion of the PS waves is the limit of an elliptical-like particle motion associated with the PS waves for near-vertical incidence. It turns out that this unusual particle motion tends to the classical transverse rectilinear motion when the source-receiver offset is large enough. A similar behavior was found for the particle motion associated with the reflected PS, P2S, and PS2 for small offset receivers located on the free surface of a solid consisting of one homogeneous layer over a half-space.
Variation of regional seismic discriminants with surface topographic roughness in the Western United States
Authors: Zhang Tian-Run , Lay Thorne , Schwartz Susan , Walter William R. 
Published: September 2021

Abstract
Regional distance P and S waves behave differently with propagation distance due to complex elastic and anelastic effects in the crustal wave guide and uppermost mantle. It is well recognized that seismological discriminants based on P/S ratios or individual phase spectral ratios must be corrected for regionally dependent path effects for successful discrimination to be achieved. Such corrections usually involve only distance-dependent decay, parameterized to account for geometric spreading and attenuation factors averaged over some geographic region. However, large scatter persists in distance-corrected discriminants, which degrades their performance, and strategies are needed to further reduce the scatter by accounting for individual path variability. We demonstrate that characterizing some of the path variability by the readily measured surface topographic roughness along each path allows reduction of the scatter in some regional discriminants. Pg/Lg and Pn/Lg ratios and Pg, Pn, and Lg low-frequency/high-frequency spectral ratios for 80 earthquakes in the Western United States recorded at four stations of the Livermore NTS Network are analyzed for path effects. While all of these discriminants display distance dependence, log Pg/Lg is also found to be significantly correlated with average surface roughness along each path. The correlation is emphasized when the product of distance and roughness is used as an independent variable, and the data in the frequency range 1 to 6 Hz have the strongest trends. Corrections based on the correlations with distance and roughness can reduce the variance of the log Pg/Lg discriminant by 22 to 25% in the 2- to 4-Hz band. Log Pn/Lg and individual phase spectral ratios have stronger distance dependence than log Pg/Lg, but the correlation with surface roughness is weak for those measurements. It appears that interchange of Pg and Lg energy that reverberates in the crust accounts for the sensitivity to the gross wave-guide properties that are manifested in the surface roughness parameterization. The results support the concept of pursuing path corrections beyond purely range-dependent trends.
Ray theoretical analysis of Lg
Authors: Keers H. , Nolet G. , Dahlen F. A. 
Published: September 2021

Abstract
Ray theory is used to investigate the effect that irregular crustal interfaces have on the different phases that comprise Lg. We concentrate on the effects of a variable Moho in this initial study, while keeping the surface flat. The Moho models considered are a cosinusoid, a model of the Moho below the German part of the European Geo-Traverse (EGT) and a model representing an ocean-continent transition.
It is found that even slight undulations of the Moho give rise to extensive multipathing. This affects the amplitudes of the supercritically reflected crustal multiples comprising Lg in a dramatic way. Two different types of ray behavior are distinguished. The first type, corresponding to regular or weakly chaotic ray behavior, occurs at smaller values of the ray parameter and gives rise to large amplitudes at distinct epicentral distances due to focusing, accompanied by “gaps” containing very little energy due to defocusing in between. The second type, corresponding to strongly chaotic ray behavior, occurs at larger values of the ray parameter. Small changes in the take-off angle in this chaotic regime produce large fluctuations in the epicentral distance of arrivals. The Moho below Germany, for example, produces 140 supercritically reflected arrivals at an epicentral distance of 400 km. Since the amplitudes of arrivals in the chaotic region are smeared out over a large epicentral distance range, they are much less prominent than the arrivals from the regular region, even though they are supercritically reflected.
Travel-time fluctuations are less sensitive to the undulations in the Moho than amplitude variations. They are more affected by the magnitude than by the wavelength of the undulations. Amplitude undulations of more than 3 km produce multiple arrivals that have differences in travel time of up to 10 sec.
Q-phase compensation of seismic records in the frequency domain
Authors: Bano Maksim 
Published: September 2021

Abstract
The attenuation process acts as a low-pass filter that attenuates the high frequencies (absorption) of the signal spectrum and also changes the phase of the seismic wavelet (dispersion). Seismic frequency losses are usually recovered according to an appropriate processing technique (such as deterministic or statistical deconvolution methods), while phase distortions are generally disregarded. Therefore, accurate processing of seismic data requires a careful investigation of the relationship between absorption and phase.
In this article, a procedure is presented to accomplish this goal. To account for anelastic losses, a complex power function of frequency for the phase velocity is introduced into the one-way wave-field equation in 1D. The compensation, for both effects (absorption and dispersion) described here, is analyzed in the context of wave-field extrapolation in one dimension 1D, equivalent to that in the f-k domain as phase-shift and/or Stolt migration. The phase-only inverse Q filtering works in the frequency domain. It provides for dispersion according to a constant-Q (frequency-independent) model and is valid for any positive value of Q. The extension of this algorithm for a Q depth-variable model is also shown. The amplitude compensation is accomplished through the use of a standard statistical approach. Synthetic and real data are shown to illustrate both amplitude and phase inverse Q filtering of seismic reflection records.
Amplification and differential motion due to an antiplane 2D resonance in the sediment valleys embedded in a layer over the half-space
Authors: Moczo Peter , Labák Peter , Kristek Jozef , Hron Franta 
Published: September 2021

Abstract
We investigate an antiplane 2D resonance in a certain class of the sedimentary structures using the finite-difference modeling. Our models are derived from the realistic local geologic conditions beneath the colosseum in Rome and are significantly different from those investigated in the previous theoretical articles on a 2D resonance. They include a trough at the bottom of the horizontal surface layer as well as relatively deep sediment valleys embedded in a single layer over the halfspace.
We present the finite-difference algorithm for SH waves on a combined (h × h and 2h × 2h) rectangular grid. Although being simple, the algorithm allowed us to save up to 75% of the grid points compared with the regular grid h × h that would cover the same computational region.
A 2D resonance may develop in the valleys that do not satisfy Bard and Bouchon's existence condition.
A simple trough at the bottom of the horizontal surface layer can give rise to the fundamental mode of a 2D resonance whose frequency, spectral amplification, and the maximum time-domain differential motion are very close to those in the closed sediment valley.
Our results confirm that the resonance phenomenon is quite robust and that it is to be expected in many configurations of sediment valleys or basins.
Physical significance of earthquake quanta
Authors: Wu Z. L. , Chen Y. T. , Kim S. G. 
Published: September 2021

Abstract
The physical significance of “earthquake quanta” is discussed in the perspective of a proposed concept “seismon.” It is pointed out that the physical significance of earthquake quanta may be understood in the way similar to that of phonon in solid-state physics. Using the statistical properties of the seismons, the property such that for large earthquakes the mean stress drop is approximately constant, while for small events the mean stress drop increases with the seismic moment, may be obtained, without detailed considerations of the failure criteria and the interaction between different earthquake quanta.
What is a reference site?
Authors: Steidl Jamison  H. , Tumarkin Alexei  G. , Archuleta Ralph  J. 
Published: September 2021

Abstract
Many methods for estimating site response compare ground motions at sites of interest to a nearby rock site that is considered a “reference” motion. The critical assumption in these methods is that the surface-rock-site record (reference) is equivalent to the input motion at the base of the soil layers. Data collected in this study show that surface-rock sites can have a site response of their own, which could lead to an underestimation of the seismic hazard when these sites are used as reference sites. Data were collected from local and regional earthquakes on digital recorders, both at the surface and in boreholes, at two rock sites and one basin site in the San Jacinto mountains, southern California. The two rock sites, Keenwild and Piñon Flat, are located on granitic bedrock of the southern California peninsular ranges batholith. The basin site, Garner Valley, is an ancestral lake bed with watersaturated sediments, on top of a section of decomposed granite, which overlies the competent bedrock. Ground motion is recorded simultaneously at the surface and in the bedrock at all three sites. When the surface-rock sites are used as the reference site, i.e., the surface-rock motion is used as the input to the basin, the computed amplification underestimates the actual amplification at the basin site for frequencies above 2 to 5 Hz. This underestimation, by a factor of 2 to 4 depending on frequency and site, results from the rock sites having a site response of their own above the 2-to 5-Hz frequencies. The near-surface weathering and cracking of the bedrock affects the recorded ground motions at frequencies of engineering interest, even at sites that appear to be located on competent crystalline rock. The bedrock borehole ground motion can be used as the reference motion, but the effect of the downgoing wave field and the resulting destructive interference must be considered. This destructive interference may produce pseudo-resonances in the spectral amplification estimates. If one is careful, the bedrock borehole ground motion can be considered a good reference site for seismic hazard analysis even at distances as large as 20 km from the soil site.
The influence of errors in source parameters on phase velocity measurements of surface waves
Authors: Muyzert Everhard , Snieder Roel 
Published: September 2021

AbstractThe phase of both Love and Rayleigh waves can be very sensitive to errors in the source parameters. This unstable behavior of the phase depends on the source parameters and is frequency dependent. Fractional errors larger than 10° in the phase measurements can arise when realistic errors in the source parameters are considered. When these phase measurements are interpreted as phase velocity, uncertainties up to 3% can occur when errors in the source parameters are considered. The frequency bands where the unstable behavior occurs can be found by calculating the partial derivative of the initial phase with respect to the various source parameters. We have tested this method on a Rayleigh wave data set and find a good correlation between outliers in the data set and large partial derivatives of the source parameters.
Interpretation of significant ground-response and structure strong motions recorded during the 1994 Northridge earthquake
Authors: Shakal A. F. , Huang M. J. , Darragh R. B. 
Published: September 2021

Abstract
Some of the largest accelerations and velocities ever recorded at ground-response and structural sites occurred during the Northridge earthquake. These motions are greater than most existing attenuation models would have predicted. Although the motions are large, the correspondence between measured acceleration and damage requires further study, since some sites with high acceleration experienced only moderate damage. Also, some peak vertical accelerations were larger than the horizontal, but in general, they are smaller and fit the pattern observed in previous earthquakes. Strong-motion records processed to date show significant differences in acceleration and velocity waveforms and amplitudes across the San Fernando Valley.
Analysis of processed data from several buildings in the San Fernando Valley indicates that short-period buildings such as shear-wall buildings experienced large forces and relatively low inter-story drift during the Northridge earthquake. However, long-period (1 to 5 sec) steel or concrete moment-frame buildings experienced large inter-story drift. For this earthquake, accelerations did not always amplify from base to roof for flexible structures like the moment-frame buildings, but the displacements were always larger at the roof. The drifts at many of the moment-frame buildings were larger than the drift limit for working stress design in the building code. The records from a base-isolated building indicate that high-frequency motion was reduced significantly by the isolators. The isolators deformed about 3.5 cm, which is much less than the design displacement. The records from a parking structure show important features of the seismic response of this class of structure.
Seismological studies at Parkfield IV: Variations in controlled-source waveform parameters and their correlation with seismicity, 1987 to 1995
Authors: Karageorgi E. , McEvilly T. V. , Clymer R. 
Published: September 2021

AbstractSince June 1987 at Parkfield, California, the 10-station borehole network of three-component sensors has been illuminated 52 times using a shear-wave vibrator in three orientations at up to eight source points, in a search for temporal changes in elastic wave P and S velocities, anisotropy, or attenuation. The monitoring interval includes the beginning and end of a severe 3-yr drought and four earthquake sequences, two of which produced the only A-level alerts to date in the Parkfield Prediction Experiment. A comprehensive study of the entire data set reveals a progressive travel-time advance in the coda of S waves propagating in a localized region southeast of Middle Mountain. The anomalous wave field exhibits high apparent velocities, suggesting deep penetration of the fault zone, although similar changes are not seen in waveforms from repeating similar microearthquakes. Accompanying the changes in travel time were systematic variations in spectral content and polarization of the same segments of the wave field. These variations correlate well in time and space with significant features of seismicity, fault creep, and water levels at Parkfield. A preferred mechanism for the phenomenon is changing hydrologic conditions along the affected stretch of the fault zone, possibly deformation-induced, that perturb the shallow-propagating S coda in the upper few hundred meters of section.
Site amplifications for generic rock sites
Authors: Boore David M. , Joyner William B. 
Published: September 2021

AbstractSeismic shear-wave velocity as a function of depth for generic rock sites has been estimated from borehole data and studies of crustal velocities, and these velocities have been used to compute frequency-dependent amplifications for zero attenuation for use in simulations of strong ground motion. We define a generic rock site as one whose velocity at shallow depths equals the average of those from the rock sites sampled by the borehole data. Most of the boreholes are in populated areas; for that reason, the rock sites sampled are of particular engineering significance. We consider two generic rock sites: rock, corresponding to the bulk of the borehole data, and very hard rock, such as is found in glaciated regions in large areas of eastern North America or in portions of western North America. The amplifications on rock sites can be in excess of 3.5 at high frequencies, in contrast to the amplifications of less than 1.2 on very hard rock sites. The consideration of unattenuated amplification alone is computationally convenient, but what matters for ground-motion estimation is the combined effect of amplification and attenuation. For reasonable values of the attenuation parameter κ0, the combined effect of attenuation and amplification for rock sites peaks between about 2 and 5 Hz with a maximum level of less than 1.8. The combined effect is about a factor of 1.5 at 1 Hz and is less than unity for frequencies in the range of 10 to 20 Hz (depending on κ0).Using these amplifications, we find provisional values of about Δσ = 70 bars and κ0 = 0.035 sec for rock sites in western North America by fitting our empirically determined response spectra for an M 6.5 event to simulated values.The borehole data yield shear velocities (V30) of 618 and 306 m/sec for “rock” and “soil” sites, respectively, when averaged over the upper 30 m. From this, we recommend that V30 equals 620 and 310 m/sec for applications requiring the average velocity for rock and soil sites in western North America.By combining the amplifications for rock sites and the site factors obtained from our analysis of strong-motion data, we derive amplifications for sites with V30 = 520 m/sec (NEHRP class C, corresponding to a mix of rock and soil sites) and V30 = 310 and 255 m/sec (average soil and NEHRP class D sites, respectively). For the average soil site, the combined effect of amplification and attenuation exceeds a factor of 2.0 for frequencies between 0.4 and about 4 Hz, with a peak site effect of 2.4; the peak of the NEHRP class D site effect is 2.7.