bluuebunny/crossref_metadata_2025_split huggingface dataset:
Underground and ridge site effects: A comparison of observation and theory
Authors: Bard Pierre-Yves , Tucker Brian E. 
Published: July 2021

AbstractThis paper compares experimental observations and theoretical predictions of the effects of underground and ridge sites on the amplitude of seismic waves.Observations by Tucker et al. (1984) of narrow-band, earthquake-independent site effects of a factor of 3 at underground sites down to a depth of 40 m can be predicted well using a one-dimensional theoretical model and information about seismic velocity as a function of depth. The observations can be explained as being due to the interference of upcoming, incident, and downgoing, surface-reflected waves within a thin, low-velocity, near-surface layer. An important effect of this layer is to make the interference pattern at shallow depths (i.e., within the layer) insensitive to the incidence angle of the incoming signal and, thus, stable from earthquake to earthquake. Our model suggests that at depths of about 100 m, site effects as great as a factor of 6 would be observed at about 5 Hz, while at much greater depths, site effects would be small. Currently available theoretical techniques are sufficient to assess the expected ground motion at underground sites provided that the velocity profile is known. This is useful for buried pipes, tunnels, and critical storage facilities.Observations by Tucker et al. (1984) and others of narrow-band, earthquake-independent site effects of about a factor of 8 on ridges cannot be predicted completely using theoretical techniques currently available to us. Accurate predictions could probably be made with a model that included the effects of ridge topography, neighboring topography, and near-surface velocity gradients. Use of such a model, if it were available, would require detailed, expensive knowledge of regional topography and velocity structure. The observed insensitivity of ridge site effects on ridges to incidence angle can, however, be explained with existing theoretical models as being due to a near-surface, low-velocity layer. The size of site effects on ridges and the stability of these effects from earthquake to earthquake suggests that they should be considered when interpreting seismograms obtained from ridge sites and when designing structures, such as communication antennae and power transmission towers, that will be located on mountain ridges. Accurate determination of the site effects of ridges and mountain tops can probably best be done using experimental techniques.
Three-dimensional ultrasonic modeling of Rayleigh wave propagation
Authors: Bullitt John T. , Toksöz M. Nafi 
Published: July 2021

AbstractThe effects of topographic features on Rayleigh wave propagation and scattoring are investigated in the laboratory using three-dimensional ultrasonic models. Starting from simple steps, different topographic features are modeled. The effects of these features on Rayleigh wave transmission and scattering are examined as a function of wavelength and as a function of angle of incidence. In general, backscattered or reflected Rayleigh waves are small compared to transmitted waves. A significant fraction of the Rayleigh wave energy is scattered into body waves. Transmission and reflection coefficients (transmitted or reflected energy/incident energy) computed from spectral ratios vary strongly with incidence angle. At wavelengths equal to twice the step height, the fraction of incident energy scattered into body waves ranges from more than 90 per cent at normal incidence to about zero at near-grazing incidence. At each angle, transmission coefficients vary strongly with frequency. Because of frequency-dependent phase shifts, the transmitted and reflected waves are distorted.The effect of the steps on the propagation of Rayleigh waves is demonstrated by convolving synthetic dispersed wave trains with the impulse response of the scale models. The ocean-continent margin of the Western United States is modeled as a 60° ramp scaled to 60 km height. The Tibetan Plateau is modeled as a broad mesa scaled to 40 km height. In both models, azimuthal dependence of transmitted Rayleigh waves is similar to that observed at WWSSN stations for Rayleigh waves crossing the modeled terrestrial structures.
Stress trajectories across the Northeast Alaska Range
Authors: Gedney Larry 
Published: July 2021

AbstractA single earthquake on the southeast portion of the Denali fault in 1981 and two earthquake swarms occurring in 1981 and 1984 on the northern flanks of the Alaska Range south of Fairbanks indicate that the axis of maximum horizontal compressive stress is oriented in a NW-SE direction in the region. This is in accord with earlier findings by Nakamura et al. (1980) that tectonic trends signal a similar orientation of maximum horizontal compressive stress trajectories here.
Some observations on strong-motion earthquake measurement using a digital accelerograph
Authors: Iwan Wilfred D. , Moser Michael A. , Peng Chia-Yen 
Published: July 2021

Abstract
This paper presents results of a study of some of the characteristics of a PDR-1 digital strong-motion accelerograph. Results are presented for laboratory tests of the background noise level of the instrument, and these results are compared with previously reported observations for optical instruments. Noise levels for the digital instrument are found to be one or two orders of magnitude lower than for an analog optimal instrument. The paper discusses determination of displacement from acceleration data, and results of laboratory tests are presented. An instrument anomaly in the FBA-13 transducer is identified, a simple data correction algorithm proposed, and examples given. The paper also presents detailed results of a comparison of earthquake records obtained from side-by-side digital and optical analog instruments during an aftershock of the 1983 Coalinga earthquake.
The great Jalisco, Mexico, earthquakes of 1932: Subduction of the Rivera plate
Authors: Singh S. K. , Ponce L. , Nishenko S. P. 
Published: July 2021

AbstractWe have analyzed seismograms of the 3 June 1932 (Ms = 8.2) and the 18 June 1932 (Ms = 7.8) Jalisco earthquakes and their aftershocks at Manzanillo (MNZ), Guadalajara (GUM), and Tacubaya (TAC). The aftershocks locations, the first motions at MNZ, and the isoseismic maps of the two main shocks strongly suggest that: (a) the 3 June 1932 earthquake initiated NW of but close to MNZ and propagated NW for an estimated length of rupture of 220 km; (b) the 18 June 1932 earthquake nucleated SW of MNZ (offshore) and perhaps ruptured a length of about 60 km; and (c) the width of rupture was approximately 80 km. There is no evidence of events occurring SE of MNZ even up to 1 1/2 yr after the first main shock. A gap of about 60 km remains between the aftershock areas of the 1932 Jalisco and the 1973 Colima earthquakes whose seismic potential is unknown. Although the boundary between the Rivera and Cocos plates is uncertain, there is little doubt that the 1932 earthquakes broke the shallow part of the Rivera subduction zone. Our results may help in quantifying seismic potential of tectonically similar areas such as the Juan de Fuca subduction zone in the NW United States.
Laboratory test of apparent attenuation formulas
Authors: Menke William , Witte Dean , Chen Rolin 
Published: July 2021

Abstract
We measure the apparent attenuation of P waves propagating through a three-dimensional scattering medium (a three-dimensional aluminum block containing a random assemblage of cylindrical voids). The observed apparent attenuation is compared to that predicted by a single scattering approximation. At low frequencies, both the amount of attenuation and its variation with frequency are well predicted. At intermediate and high frequencies, the agreement is poorer. Observed phase shifts are very different from those predicted by the Kramers-Krönig relations.
Analysis of the trade-off between hypocentral depth and source time function
Authors: Christensen Douglas H. , Ruff Larry J. 
Published: July 2021

AbstractFocal depth determination for shallow (≤60 km) intermediate-size (Ms ≈ 6.5 to 7.5) earthquakes continues to be problematic. For these events, the depth phases (i.e., pP and sP) are not distinctly separated in time, and thus synthetic modeling of wave shapes becomes necessary. Unfortunately, the uniqueness of the solution is not easily determined because of the trade-off between various focal parameters. We have investigated the trade-off between depth and source function both theoretically and experimentally. The changes in the Green's function caused by an incorrect assumed depth can almost always be compensated by changes in the source function, producing the same “observed” seismogram. Thus, there is considerable trade-off between depth and the source function. Perturbing the depth from the “true” depth leads to a more “complicated” source function. The “simplest” source function then pinpoints the true depth. We have used two different measures of simplicity, the varimax norm (Vmax) and the half-absolute moment time (T12); both measures successfully characterize the complications that appear in the source functions for incorrect assumed depths. The true depth of an event can be interpreted from the resultant Vmax versus depth and T12 versus depth curves. Thus, we have a technique that determines the depth range of faulting in a systematic quantitative fashion. Our observations from the synthetic study have been applied to an outer-rise event off the coast of central Chile (16 October 1981; Ms = 7.2), to an event in southern Mexico (24 October 1980; Ms = 7.0), and to the Borrego Mountain event in southern California (9 April 1968; Ms = 6.4) resulting in faulting depth ranges of 0 to 20 ± 5 km, 60 to 70 ± 5 km, and 10 to 14 ± 5 km, respectively.
Effects of magnitude uncertainties on estimating the parameters in the Gutenberg-Richter frequency-magnitude law
Authors: Tinti Stefano , Mulargia Francesco 
Published: July 2021

AbstractThe evaluation of the parameters in the Gutenberg-Richter (GR) frequency-magnitude law log Nr = a − bm is shown to be strongly affected by magnitude uncertainties. If the magnitude errors are assumed to be distributed normally with standard deviation σ, the observed magnitude that we call the “apparent magnitude,” becomes a random variable, and the frequency-apparent magnitude law differs from the GR relation. We show that there is a range of magnitudes within which this law may be approximated as log Na = a − bm + γ2log(e). Here, γ2 = β2σ2/2, [β = b/log(e)], and Na stands for the apparent number of the earthquakes. As both the true and the apparent magnitude curves have the same slope on a logarithmic graph, the estimators usually employed for β remain valid. However, the usual estimators for a are biased by a quantity which is a quadratic function of the error standard deviation σ and of β. This implies that the apparent number, Na, of earthquakes exceeding a given magnitude tends to be larger than the real number Nr and for realistic values of a and b (or β), Na is expected to be even as large as twice Nr. The implications of the magnitude variability on the seismic risk analysis are also considered: in particular, the evaluations of the attenuation law parameters and of the exceedance probability of the ground motion peak acceleration are shown to be affected.
An earthquake detector employing frequency domain techniques
Authors: Gledhill K. R. 
Published: July 2021

AbstractA single-station real-time earthquake detector has been developed which operates in the frequency domain. Frequency domain operation is possible because the method of calculation of the discrete Fourier transform and the data format have been adapted to suit the architecture of the microprocessor used. A test period of 10 days resulted in a detection reliability of greater than 95 per cent, with a false detection rate of less than 10 per cent (241 earthquakes total sample). The spectral detector outperforms time domain detectors in two respects. It is much more likely to identify the more distant, emergent earthquakes, and those recorded with a low signal-to-noise ratio. The algorithm operates on a Motorola MC6809 microprocessor and uses less than a quarter of the available time at a 50-Hz sampling frequency.
The “Report” of the 1868 Haywards earthquake
Authors: Aldrich Michele L. , Bolt Bruce A. , Leviton Alan E. , Rodda Peter U. 
Published: July 2021

Abstract
An important question on the major 1868 northern California earthquake centered near Hayward is whether a comprehensive field report was ever written, and if so, was the report suppressed. Documentation bearing on this question includes contemporary minutes and proceedings of the California Academy of Sciences, a report in 1870 by James Blake on the work of a subcommittee of the Committee on Earthquake Topics, contemporary newspaper accounts, a treatise published in 1868 by Thomas Rowlandson, and a letter written by George Davidson in 1908, the contents of which were made public in 1982. George Davidson claimed that a report had been prepared but was suppressed. The claim is not supported by either James Blake or Thomas Rowlandson, who indicate that lack of funds and internal dissension prevented completion of the investigation or presentation of a final report.
Estimation of scalar moments from explosion-generated surface waves
Authors: Stevens Jeffry L. 
Published: July 2021

AbstractRayleigh waves from underground nuclear explosions are used to estimate scalar moments for 40 Nevada Test Site (NTS) explosions and 18 explosions at the Soviet East Kazakh test site. The Rayleigh wave spectrum is written as a product of functions that depend on the elastic structure of the travel path, the elastic structure of the source region, and the Q structure of the path. We use our results to examine the world-wide variability of each factor and the resulting variability of surface wave amplitudes. The path elastic structure and Q structure are found by inversion of Rayleigh wave phase and group velocities and spectral amplitudes. The Green's function derived from this structure is used to estimate the moments of explosions observed along the same path.This procedure produces more consistent amplitude estimates than conventional magnitude measurements. Network scatter in log moment is typically 0.1. In contrast with time domain amplitudes, the elastic structure of the travel path causes little variability in spectral amplitudes. When the mantle Q is constrained to a value of approximately 100 at depths greater than 120 km, the inversion for Q and moment produces moments that remain constant with distance. Based on the best models available, surface waves from NTS explosions should be larger than surface waves from East Kazakh explosions with the same moment. Estimated scalar moments for the largest East Kazakh explosions since 1976 are smaller than the estimated moments for the largest NTS explosions for the same time period. This contrasts dramatically with mb values for the same events which show the opposite behavior.
Seismicity of the Sudan
Authors: Ambraseys N. N. , Adams R. D. 
Published: July 2021

abstract
This paper presents a record of information on the seismicity of the Sudan. In addition to events for which only felt information exists, there are a number of early teleseismic locations of earthquakes which have been reexamined using information from contemporary station bulletins. Most activity is close to the southern borders of Sudan, near the African rift system, but the northeast is affected by earthquakes in the Red Sea, and the central intraplate region has also experienced earthquakes capable of producing damage.
Lateral velocity variations in Southern California. II. Results for the lower crust from Pn waves
Authors: Hearn Thomas M. , Clayton Robert W. 
Published: July 2021

abstract
A tomographic inversion of the Pn arrivals in Southern California yields new information about wave velocities and topography on the Moho discontinuity. We produce maps of Pn velocity and Pn station delays. The Pn velocities do not show the dramatic correlation with surface faults that is found for the shallower Pg arrivals (Hearn and Clayton, 1986). This implies that the lower crust and mantle are largely decoupled from the upper crust. Undoubtedly, this is due to the different responses of the brittle upper crust and the ductile lower crust to tectonic and isostatic stresses. Detachment faults must play an important role in separating the crust.
In general, velocities on the American plate are higher than on the Pacific plate, but no distinct transition is observed. The Colorado River region has extremely thin crust due to basin-and-range type extension. The Transverse Ranges have a small root as seen in the station delays and which also results in slightly lower Pn velocities there. The Peninsula Ranges also have slow Pn velocities, but they do not have late station delays. Any root to the Peninsula Ranges must be very narrow. Isostatic balance must be maintained primarily through lateral density contrasts.
Site amplification of coda waves from local earthquakes in central California
Authors: Phillips W. Scott , Aki Keiiti 
Published: July 2021

AbstractThe goals of this study are two-fold: (1) to examine the site effect of coda waves, believed to be the average site effect of shear waves, in order to understand its spatial and frequency-dependent behavior, and (2) to learn what we can about the processes that generate the coda itself. We use digital data from over 90 earthquakes, each recorded by a subset of some 150 stations in the Coast Ranges of central California between San Francisco and San Luis Obispo. Results from the band 1.5 to 24 Hz indicate that site amplification depends strongly on surface geology and frequency. At low frequencies (1.5 to 3.0 Hz), changes in amplification up to a factor of 20 are observed; amplification generally varies inversely with deposit age at sediment sites and is lowest at granite and Franciscan (basement) sites. At high frequencies (6 to 24 Hz), the pattern changes. Many granite sites, the majority in the Gabilan range, exhibit increasing amplification with frequency relative to the average station. This behavior differs strikingly from that observed at sediment and Franciscan sites which roll-off at an intermediate rate, and many sediment sites adjacent to the San Andreas fault zone which decay away even more rapidly with frequency. These results can be explained qualitatively by appealing to variable near-site impedance and attenuation, however, complicating phenomena such as mode excitation in valley sediments must exist at low frequencies. Peaked behavior at a few hard rock sites at high frequency (12 Hz) currently lacks explanation. At a few sediment sites, the low-frequency excitation is so strong that the coda envelopes begin to take on different shapes. This contradicts the assumption based on previous observations that the coda shape is independent of source-receiver location, thus, some site measurements may be artificially high. The working model of coda waves as body-wave energy backscattered from randomly distributed inhomogeneities must be modified to include the possibility of efficient near-site resonance excited by incident direct and coda wave energy.
Interstation surface wave analysis by Frequency-domain Wiener deconvolution and modal isolation
Authors: Hwang H. J. , Mitchell B. J. 
Published: July 2021

AbstractA new technique which combines frequency-domain Wiener filtering and modal isolation is developed for determining interstation phase velocities, group velocities, and attenuation coefficients for seismic surface waves. Frequency-domain Wiener filtering is more effective than time-domain Wiener filtering for the determination because it uses a smaller window lag which produces a smoother interstation Green's function. This leads to greater accuracy and stability when noise-contaminated data are analyzed. We optimize Wiener filtering in the frequency domain by applying two trapezoidal windows of different lags to the cross-correlation function between two stations and to the autocorrelation function of the first station, respectively. The windowed correlation functions are then transformed to the frequency domain. The interstation Green's function in this technique is the ratio of the smoothed cross-spectrum to the smoothed auto-spectrum of the first station. Frequency-domain Wiener filtering is equivalent to time-domain Wiener filtering when the same rectangular window is applied to both the correlation functions.Wiener filtering, however, cannot efficiently remove higher mode interference when the higher modes are superimposed on the fundamental mode in the correlation functions. To more thoroughly eliminate the effects of such interference, phase-matched filtering or time-variable filtering can be employed to isolate one particular mode at each of two stations. Frequency-domain Wiener deconvolution is then applied to calculate the Green's function. The interstation group velocites can be obtained by applying the multiple filter technique to the Green's function, and can be refined by phase-matched filtering. The amplitude and phase spectra of the Green's function are used to calculate attenuation coefficients and phase velocities, respectively, for the interstation medium.This new technique is compared with other methods by applying them to both noise-contaminated synthetic seismograms and real data. The proposed technique is found to be superior, particularly in period ranges where the signal-to-noise is low.
Surface displacement on the Imperial and Superstition Hills faults triggered by the Westmorland, California, earthquake of 26 April 1981
Authors: Sharp Robert V. , Rymer Michael J. , Lienkaemper James J. 
Published: July 2021

AbstractParts of the Imperial and the Superstition Hills faults moved right laterally at the ground surface at the time of or shortly following the ML 5.6 Westmorland earthquake of 26 April 1981. The displacements probably occurred before any significant aftershocks on either fault and thus are classed as triggered slips. Although the main shock was located in an exceptionally seismogenic part of Imperial Valley, about 20 km distant from either fault, no clear evidence of past surface faulting is known in the epicentral area. Horizontal displacement on the Imperial and Superstition Hills faults, southeast and southwest of the epicenter, respectively, reached maxima of 8 and 14 mm, and the discontinuous surface ruptures formed along approximately equal lengths of northern segments of the two structures (16.8 and 15.7 km, respectively). The maximum vertical component of slip measured on surface cracks on the Imperial fault (6 mm) was located on the west side of Mesquite basin near Harris Road. Leveling data at Harris Road suggest slightly larger vertical movement at the surface there. Fault dislocation modeling of this leveling data further suggests that the maximum shallow subsurface dip-slip component of the triggered movement was as much a factor of 4 larger than that measured at the fault trace. The map pattern of the rupture may reveal the principal strand among the several near the north end of the Imperial fault.No surface displacement was found along traces of the Brawley fault zone, the San Andreas fault, or the part of the Coyote Creek fault that slipped during the 1968 Borrego Mountain earthquake. Ground search in the epicentral area of the main shock and in the widely dispersed aftershock region in northern Imperial Valley failed to locate any definite evidence of new surface faulting.