bluuebunny/crossref_metadata_2025_split huggingface dataset:
Modeling strong motions produced by earthquakes with two-dimensional numerical codes
Authors: Helmberger Donald V. , Vidale John E. 
Published: July 2021

AbstractWe present a scheme for generating synthetic point-source seismograms for shear dislocation sources using line source (two-dimensional) theory. It is based on expanding the complete three-dimensional solution of the wave equation expressed in cylindrical coordinates in an asymptotic form which provides for the separation of the motions into SH and P-SV systems. We evaluate the equations of motion with the aid of the Cagniard-de Hoop technique and derive close-formed expressions appropriate for finite-difference source excitation.
The foreshock sequence of the 1986 Chalfant, California, earthquake
Authors: Smith Kenneth D. , Priestley Keith F. 
Published: July 2021

AbstractThe ML 6.4 Chalfant, California, earthquake of 21 July 1986 was preceded by an extensive foreshock sequence. Foreshock activity is characterized by shallow clustering activity, including 7 events greater than ML 3, beginning 18 days before the earthquake, an ML 5.7 foreshock 24 hr before the main shock that ruptured only in the upper 10 km of the crust, and an “off-fault” cluster of activity perpendicular to the slip surface of the ML 5.7 foreshock associated with the hypocenter of the main shock. The Chalfant sequence occurred within the local short-period network, and the spatial and temporal development of the foreshock sequence can be observed in detail. Seismicity of the July 1986 time period is largely confined to two nearly conjugate planes; one striking N30°E and dipping 60° to the northwest associated with the ML 5.7 foreshock and the other striking N25°W and dipping 70° to the southwest associated with the main shock. Focal mechanisms for the foreshock period fall into two classes in agreement with these two planes. Shallow clustering of earthquakes in July and the ML 5.7 principal foreshock occur at the intersection of the two planes at a depth of approximately 7 km. The seismic moments determined from inversion of the teleseismic body waves are 4.2 × 1025 and 2.5 × 1025 dyne-cm for the principal foreshock and the main shock, respectively. Slip areas for these two events can be estimated from the aftershock distribution and result in stress drops of 63 bars for the principal foreshock and 16 bars for the main shock. The main shock occurred within an “off-fault” cluster of earthquakes associated with the principal foreshock. This cluster of activity occurs at a predicted local shear stress high in relation to the slip surface of the 20 July earthquake, and this appears to be the triggering mechanism of the main shock. The shallow rupture depth of the principal foreshock indicates that this event was anomalous with respect to the character of main shocks in the region.
High-frequency spectra observed at Anza, California: Implications forQstructure
Authors: Hough S. E. , Anderson J. G. 
Published: July 2021

AbstractData from the Anza array in southern California have been analyzed to yield a model for the depth dependence of attenuation. The result is obtained from a formal inversion of the distance dependence of the spectral decay parameter, κ, observed from sources at a wide range of distances from single stations.The inversion procedure assumes constant Qi in plane layers and finds models which are as nearly constant with depth as possible. We find that the data cannot be explained by a model in which Qi is constant with depth and that the data generally require three-layer models. The resulting models typically give Qi for P waves between 300 and 1000 in the top 5 km, rising to 1000 to 3000 at greater depths, and decreasing to 700 to 1000 around 12 km depth. Qi for S waves is slightly higher in most cases. Because this depth dependence of Qi is generally correlated with the depths of earthquake epicenters, we suggest that Qi may be due to a pressure and temperature-controlled intrinsic attenuation mechanism.
Spectral characteristics of regional phases recorded at Noress
Authors: Suteau-Henson Anne , Bache Thomas C. 
Published: July 2021

AbstractThe spectral characteristics of Pn and Lg are studied for regional events recorded at the NORESS array in Norway. The emphasis is on the potential value of spectral ratios for identifying events as earthquake, chemical explosions, and nuclear explosions. The events studied include a suite of explosions from the Titania Mine in southwest Norway, a suite of events of unknown source type from a site offshore about 90 km from this mine, suites of explosions from several mines in the Soviet Union about 1000 km from NORESS, events from several locations along the 90° azimuth from NORESS, an apparent earthquake in the North Atlantic, and a nuclear explosion (PNE) at a range of 1560 km. The event identification issues addressed are as follows: (1) Can earthquakes and explosions be identified based on the ratio of high- and low-frequency energy in their signal spectra? (2) Do spectral ratios separate mine blasts from earthquakes for all source areas? (3) Will spectral ratio discriminants be effective for identifying decoupled underground nuclear explosions? We conclude that spectral ratios can sometimes separate events. An example is separation of the Titania mine blasts from the events at the nearby offshore locations, although we cannot be sure how much this is due to path differences. However, in general, spectral ratios vary as much within classes of events as they do among different classes. The PNE in our data set allows the simulation of spectra from a decoupled nuclear explosion by simply applying the frequency-dependent decoupling factor to the observed Pn spectrum for this event. After applying a distance correction, the spectrum for this simulated decoupled nuclear explosion is similar to those for the Soviet Union mine blasts in our data set.
Mini-sosie high-resolution reflection survey of the Cottonwood Grove fault in northwestern Tennessee
Authors: Sexton John L. , Jones Paul B. 
Published: July 2021

Abstract
The Cottonwood Grove fault is located within a portion of the New Madrid seismic zone in northwestern Tennessee. Focal mechanism studies indicate that this area is a seismic transition zone. To the southwest is a southwest-northeast seismic trend in which movements along deeper seated faults is predominantly right-lateral strike-slip. To the north is a southeast-northwest seismic trend in which reverse and normal faulting predominate.
The Cottonwood Grove fault is buried beneath the poorly consolidated sediments of the Mississippi embayment. The fault, as identified by an earlier Vibroseis ®* survey is a northeast-southwest trending, eastward-dipping reverse fault with approximately 75 m (245 ft) of displacement on the Paleozoc-Cretaceous boundary.
A Mini-Sosie™† high-resolution seismic reflection survey was conducted through the village of Cottonwood Grove along the previously surveyed Vibroseis line to improve estimates of the age, geometry, and displacements of the Cottonwood Grove fault. Results of the Mini-Sosie survey reveal that displacements across the major fault are relatively consistent within Cretaceous, Paleocene, and middle Eocene sedimentary rocks. In upper Eocene and younger rocks, however, there is no evidence for faulting. Our interpretation includes a previously undetected secondary fault at the boundary between upper Cretaceous and Paleocene rocks. Also included in our interpretation of the subsurface profile through Cottonwood Grove is an Eocene age channel feature located 2 km east of the Cottonwood Grove fault. In addition, the Paleozoic-Cretaceous boundary is interpreted to be an erosional surface with no intrusives included in the Paleozoic rocks. Synthetic seismogram modeling, detailed gravity survey data, and theoretical gravity calculations support this interpretation, and indicate that shallow intrusive bodies within Paleozoic rocks are not needed to explain the observed data. Seismic reflections which would be expected if the intrusives were present are not observed, and the observed Bouguer gravity anomaly can be explained by use of irregularities on the erosional Paleozoic bedrock surface along with sedimentary features within the post-Paleozoic sediments.
These data suggest that Cottonwood Grove fault formed during middle Eocene time and that since that time, any major movement on deeper faults has been predominantly strike-slip with little or no vertical reactivation. This interpretation is consistent with the prevailing hypotheses relating current seismicity of the New Madrid seismic zone to the contemporary regional compressive stress field acting on zones of weakness associated with the Precambrian Reelfoot Rift Complex.
®* Registered trademark of Conoco, Inc. ™† Registered trademark of Société Nationale Elf Aquitane (Production).
Stochastic geologic effects on near-field ground motions in alluvium
Authors: Reinke Robert E. , Stump Brian W. 
Published: July 2021

Abstract
Analysis of accelerograms recorded at the 20 m range from a buried 5-lb detonation in alluvium revealed wide (as large as 20 dB in the amplitude modulus of the Fourier transform) variations in response for frequencies above 35 Hz. Additional experiments were performed which ruled out source asymmetry or instrumental irregularity as the cause of these variations. The observations suggest that scattering by geologic inhomogeneity is responsible for the frequency-dependent spatial variability in ground motion. A thorough understanding of the physical processes responsible for this variability requires that a quantitative relationship be established between the subsurface material property information and the observed ground motion characteristics. An attempt was made to do this using available standard penetration test data from the test bed where the initial experiment was detonated. Autocorrelations of the blow count data from the standard penetration test were computed and compared with theoretical exponential and Gaussian distributions. The exponential distribution with a scale length between 2.0 and 3.0 m best matches the data. Assuming the Born approximation, a scale length of 2.0 to 3.0 m implies that significant scattering should occur above 10 Hz. The recorded ground motions are, however, coherent out to about 35 Hz, suggesting a scale length on the order of 0.5 to 1.0 m, which is beyond the resolution of the standard penetration test technique. This scale length is, however, not unreasonable in light of the general geologic characteristics of the test area.
Experimental confirmation of superposition from small-scale explosions
Authors: Stump Brian W. , Reinke Robert E. 
Published: July 2021

AbstractAn in situ experimental program in alluvium is implemented and analyzed to test linear superposition. After separating stochastic and deterministic propagation path effects, direct superposition is experimentally validated at 20 m for two 5-lb charges spaced as close as 2 m in alluvium. The charges are separated by the scaled range of 147 m/kt1/3 and observed at the scaled range of 1470 m/kt1/3. Finite-spatial source effects are observed and simulated in the plane passing through two charges separated by 2 to 10 m. The deterministic single-burst waveforms are used to model the multiple-burst data. The effects observed and simulated include direct superposition below the corner frequency, shift to lower corner frequency with increasing charge separation, and spectral scalloping. For charges closely spaced (up to 4 m, observed at 20 and 24 m), the primary effect on the waveform is replicated by a constant delay time between two identical waveforms. For charges spaced by 10 m (observed at 20 and 30 m), the effects of propagation path differences must be included. These effects result in smoothed spectra.
Comments on the joint determination of hypocenters and station corrections
Authors: Pujol Jose 
Published: July 2021

AbstractSome aspects of the joint determination of hypocenters and station corrections (JHD) are discussed. An alternative solution to one of the methods currently in use (Pavlis and Booker, 1983) is proposed. It requires only a modest computational effort and is very fast. This simplification is not achieved at the expense of numerical stability and is backed by a theoretical study of the properties of the matrices involved. These matrices are positive semidefinite, and this allows a general analysis of the JHD solution. Several facts, some of them known and some new, are formally proved. It is possible, for example, to carry out a detailed study of the eigenvalues of the matrix from which the station corrections are derived. It is also shown that the mean value of the initial estimate of station corrections are retained as a bias in the final solution. The relationship between the approximate solution of Frohlich (1979) and the exact JHD solution can also be studied, and the reason for its success in certain cases is explained.
A fast, two-point, three-dimensional raytracing algorithm using a simple step search method
Authors: Prothero W. A. , Taylor W. J. , Eickemeyer J. A. 
Published: July 2021

Abstract
The simplex method of function minimization is used to find the ray that follows the path of minimum time through an arbitrary three-dimensionally varying medium. The advantage of this method is that it is extremely fast, always converges, and lends itself to a variety of velocity parameterizations. The first step in the algorithm is to find the minimum time circular arc path between the source and receiver using an exhaustive search method. Then perturbations, consisting of a sum of N sine wave harmonics with their zero crossings at the endpoints, are made to the circular arc path. The simplex method searches for the amplitude coefficients that produce the path of least time. On a VAX-750 running the VMS operating system, the first arriving ray for a station receiver distance of 96 km is computed to an accuracy of 0.02 sec, computed in a southern California velocity structure using less than 26 sec of computer time. The required computer time decreases as the ray path distance decreases.
Reduced seismic velocities in the source zone of New Madrid earthquakes
Authors: Al-Shukri Haydar J. , Mitchell Brian J. 
Published: July 2021

AbstractA three-dimensional inversion of P-wave travel-time residuals from local earthquakes reveals a remarkable pattern of low seismic velocities in crustal rocks immediately adjacent to the active portions of the New Madrid fault system. Seismic velocities are lowest in regions of greatest concentration of earthquake activity near two intersections of linear trends in seismicity. The maximum reduction in compressional wave velocity is at least 7 per cent in the upper 5 km of the crust and at least 4 per cent in the depth range of 5 to 14 km. The reductions are consistent with a velocity decrease which would be expected if crustal rocks in the source zone contain fluid-filled cracks in which pore pressure is a substantial fraction of external pressure. The presence or absence of such fluids may explain why some portions of the faults in and surrounding the upper Mississippi Embayment are active while others are not.
Alarm systems based on a pair of short-term earthquake precursors
Authors: Grandori Giuseppe , Guagenti Elisa , Perotti Federico 
Published: July 2021

Abstract
A statistical analysis of the foreshock-main shock correlation for a seismically active region in Italy is presented. It is found that the probability that a weak shock be followed within 2 days by a main shock is of the order of 2 per cent, while the probability that a main shock be preceded by a foreshock is of the order of 50 per cent. These results are quite similar to those found by L. Jones (1985) for southern California.
The effectiveness of alarm systems based on a pair of short-term earthquake precursors is then analyzed. In particular, the analysis shows under what conditions the precursor, consisting of potential foreshocks, could be combined with another precursor to provide a reasonably effective alarm system.
Studies of high-frequency seismic noise in eastern Kazakhstan
Authors: Berger J. , Eissler H. K. , Vernon F. L. , Nersesov I. L. , Gokhberg M. B. , Stolyrov O. A. , Tarasov N. T. 
Published: July 2021

Abstract
Three seismographic stations have been established in the region of the principal underground nuclear weapons test site in the republic of Kazakhstan in the Soviet Union. Instrumentation includes both surface and borehole seismometers with a bandwidth of 100 Hz and a system dynamic range of 180 dB. Power spectra of the regional ambient ground acceleration peak between 4 and 6 Hz, and are, on average, frequency independent above 10 Hz, where levels are approximately equal to 10−15 to 10−16 (m sec−2)2Hz−1. Kazakh noise levels are roughly equivalent to those observed by the Regional Seismic Test Network between 1 and 20 Hz, and about 10 to 20 dB higher than the very low noise model for Lajitas, Texas, in the same band. Noise observed by the borehole sensors is about 10 dB less than that observed by the surface instruments above about 20 Hz under low wind conditions at two of the three Kazakh sites; borehole improvement is greater under high winds. The third Kazakh site, Karasu, is characterized by much higher surface noise levels (15 to 20 dB higher than the borehole levels from 4 to 40 Hz) under both low or high wind conditions, which is attributed to amplification by local site conditions and the exposed terrain. Power spectra of individual runs of ambient vertical ground acceleration measured down the borehole show that high-frequency slopes (between 10 and 80 Hz) vary between f−0.60 and f0.22, depending on wind and site conditions.
Application of Nuttli's method to estimate yield of Nevada Test Site explosions recorded on Lawrence Livermore National Laboratory's Digital Seismic System
Authors: Patton H. J. 
Published: July 2021

Abstract
Methods utilizing Lg waves for estimating yields of underground nuclear explosions have attracted considerable attention for their accuracy, claims of portability, and suitability for low yield test ban verification. In this study, the method developed by Nuttli is applied to a large dataset of Nevada Test Site explosions recorded on Livermore's Digital Seismic System. The purpose was to check the reproducibility of Nuttli's results and to verify the accuracy of mb(Lg) to estimate yield at high and low yield levels. Portability of the method is examined from the perspective of variability of the magnitude-yield relations as a function of station site and source geology. The results of this study show accuracies of about a factor of 1.6 in yield at the two sigma level for shots below the water table in source regions exhibiting more or less uniform coupling. At Nevada Test Site, there are three such source regions: Pahute Mesa (areas 19 and 20), northern Yucca including areas 2 and 9, and southern Yucca including areas 3, 4, and 7. There is no apparent degradation in accuracies at low yields (1 to 10 kt). Variations in source coupling were observed as a function of shot location and as a function of the medium's gas porosity for shots in tuff. The effect of gas porosity was not observed for shots in alluvium, which have uniformly low coupling compared to shots in tuff. Variations in magnitude-yield relations were observed from station to station, while the four-station network-average magnitudes are more robust and agreed well with Nuttli's magnitude-yield relations. Thus, portability to other test sites should be considered guardedly optimistic, requiring geologic and seismic data related to coupling in each source region and sufficient numbers of stations (four or five) to estimate a robust network magnitude. Taken all together, the findings of this study lend support to this method as a viable means of monitoring a threshold test ban or low yield threshold test ban.
The Superstition Hills, California, earthquakes of 24 November 1987
Authors: Magistrale Harold , Jones Lucile , Kanamori Hiroo 
Published: July 2021

Abstract
The 24 November 1987 Superstition Hills earthquakes occurred on the conjugate northwest-striking right-lateral Superstition Hills fault and a previously unknown northeast-striking left-lateral structure defined by a lineation of hypocenters extending from the Superstition Hills fault to the Brawley seismic zone. Master event locations of the earthquakes using Caltech-USGS seismic network data reveal the following. The first main shock (33° 4.9′ N, 115° 47.7′, h = 10.6 km, MS = 6.2, 0154 GMT), on the northeast striking structure and its foreshocks co-locate. Events on the northeast structure cluster in space and time, and some aftershocks occur in the Brawley seismic zone. The second main shock (33° 0.9′ N, 115° 50.9′ W, h = 1.9 km, MS = 6.6, 1315 GMT), on the Superstition Hills fault, was 12 hr after the first main shock and initiated at shallow depth where the two trends join. Aftershocks of the second main shock lie in a northwest trend between the Superstition Hills and Superstition Mountain faults and have a sharply defined western edge. The extent of the northwest-trend aftershocks is not coincident with surface rupture on the Superstition Hills fault. In general, the earthquakes in the northeast trend are deep and the earthquakes in the northwest trend are shallow. We compare the earthquake distribution to the distribution of crystalline basement rocks defined from a refraction study of Fuis et al. (1982). The earthquake locations and extent of aftershock activity appear to be controlled by the presence of crystalline basement rocks.
Slip on the Elmore Ranch fault during the past 330 years and its relation to slip on the Superstition Hills fault
Authors: Hudnut K. W. , Seeber L. , Rockwell T. 
Published: July 2021

AbstractThe Elmore Ranch fault is a left-lateral fault that strikes northeast within the right-lateral transform boundary that strikes northwest through southern California. It lies transverse and adjacent to the segment of the Superstition Hills fault that ruptured in 1987. Rupture of the Elmore Ranch fault (MS = 6.2) preceded rupture of the Superstition Hills fault (MS = 6.6) by about 11.4 hr. The Elmore Ranch fault slipped at the surface in 1987 with left-lateral displacements of up to 130 mm. Geological data indicate that it had slipped prehistorically, sometime after about 1660 A.D., probably in a single event. Excavations at three sites enable the following comparisons: November 1987 (mm) ~1660-October 1987 (mm) Main Strand 70 ± 5 230 ± 20 West Strand 30 ± 5 90 ± 10 East Strand 50 ± 10 &lt; 25(?) Total 150 ± 20 320 ± 30 At these sites, the only significant component of dip slip (down to the southeast) was found on the west strand for slip previous to 1987.The Superstition Hills fault has also been documented to have experienced one slip event between ∼1660 and 1987. Thus these slip events on the Elmore Ranch and Superstition Hills faults may have occurred in a sequence similar to that in 1987. Neither the main fault nor the cross-fault, however, appear to have exactly duplicated their previous surficial slip. Previous slip was probably smaller on the Superstition Hills fault and larger on the Elmore Ranch fault zone than in the 1987 event. Because the temporal correlation between previous slip events is not proven, rupture sequences other than a doublet in which main fault rupture follows cross-fault rupture are possible.