bluuebunny/crossref_metadata_2025_split huggingface dataset:
Discrimination using spectral slopes at frequencies up to 50 hz
Authors: Bame Carr Dorthe 
Published: September 2021

Abstract
The spectral characteristics of regional Pn waves recorded at the High Frequency Seismic Element (HFSE) at the NORESS array in southern Norway are investigated to determine if spectral slope can be used to discriminate between earthquakes and chemical explosions. Displacement spectra are calculated for 134 events in southern Norway that include 36 known earthquakes and 36 known chemical explosions. Events range in distance from 230 to 620 km, and most have signal above the noise at frequencies over 30 Hz. The average frequency at the peak signal-to-noise ratio (SNR) is 15.5 Hz. Spectral slopes are measured on a displacement spectrum calculated with a 2-sec window. The slopes range between ω−1.8 and ω−5.1, and there is no strong evidence that the slope steepens with distance for this data set. According to one theory, earthquakes should have steeper slopes than explosions, but when this data set is separated into earthquakes and explosions we do not see separation between sources with respect to the slope of the spectrum. Both source types have events with the same range of spectral slopes, and the mean slopes and standard deviations are similar. Spectra of events at the same locations are often similar. However, explosions from the Blåsjø Mine in western Norway show variability in the spectral slope. Since the path and distance to HFSE are the same for these events, the variability must be due to differences in the source mechanism. For a set of small earthquakes, however, the observed variability in spectral slopes appears to be more related to distance and path effects. Therefore, we observe as much variation in spectral slope for one source type as there is between source types. The time window over which the spectrum is calculated is varied from 1 to 8 sec to determine if a time window consisting of a pure Pn wave was required to discriminate between source types. Spectral slope usually decreases as the window extends deeper into the coda and the energy content of the signal changes. The slopes also become more stable and more reliable as longer windows are used. However, source discrimination is not possible with any time window.
On the characteristics of local geology and their influence on ground motions generated by the Loma Prieta earthquake in the San Francisco Bay region, California
Authors: Borcherdt Roger D. , Glassmoyer Gary 
Published: September 2021

AbstractStrong ground motions recorded at 34 sites in the San Francisco Bay region from the Loma Prieta earthquake show marked variations in characteristics dependent on crustal structure and local geological conditions. Peak horizontal acceleration and velocity inferred for sites underlain by “rock” generally occur on the transverse component of motion. They are consistently greater with lower attenuation rates than the corresponding mean value predicted by empirical curves based on previous strong-motion data. Theoretical amplitude distributions and synthetic seismograms calculated for 10-layer models suggest that “bedrock” motions were elevated due in part to the wide-angle reflection of S energy from the base of a relatively thin (25 km) continental crust in the region. Characteristics of geologic and geotechnical units as currently mapped for the San Francisco Bay region show that average ratios of peak horizontal acceleration, velocity and displacement increase with decreasing mean shear-wave velocity. Ratios of peak acceleration for sites on “soil” (alluvium, fill/Bay mud) are statistically larger than those for sites on “hard rock” (sandstone, shale, Franciscan Complex). Spectral ratios establish the existence of predominant site periods with peak amplifications near 15 for potentially damaging levels of ground motion at some sites underlain by alluvium and fill/bay mud. Average spectral amplifications inferred for vertical and the mean horizontal motion are, respectively, (1,1) for sites on the Franciscan Complex (KJf), (1.4, 1.5) for sites on Mesozoic and Tertiary rocks (TMzs), (2.1, 2.0) for sites on the Santa Clara Formation (QTs), (2.3, 2.9) for sites on alluvium (Qal), and (2.1, 4.0) for sites on fill/Bay mud (Qaf/Qhbm). These mean values are not statistically different at the 5% significance level from those inferred from previous low-strain data. Analyses suggest that soil amplification and reflected crustal shear energy were major contributors to levels of ground motion sufficient to cause damage to vulnerable structures at distances near 100 km in the cities of San Francisco and Oakland.
Physical interpretation and stability of paraxial boundary conditions
Authors: Kausel E. 
Published: September 2021

AbstractWe derive in this paper the dynamic impedance matrices associated with some paraxial boundary conditions for wave motions in unbounded homogenous elastic media and use them to establish the existence of directions of propagation of waves for which the boundaries supply rather than dissipate energy. Also, we explore the existence of solutions of exponential growth and discuss conditions under which they can arise. These considerations may be used to provide a physical interpretation to the paraxial boundaries and to understand possible sources of instability that can develop in time-domain implementations of these schemes with finite differences.
A lithospheric velocity anomaly beneath the Shagan river Test Site. Part 2. Imaging and inversion with amplitude transmission tomography
Authors: McLaughlin K. L. , Murphy J. R. , Barker B. W. 
Published: September 2021

AbstractA linear inversion procedure is introduced that images weak velocity anomalies using amplitudes of transmitted seismic waves. Using projection operators from geometrical ray theory, an image of an anomaly is constructed from amplitudes recorded at arrays of receivers using arrays of sources. The image is related to the velocity anomaly by a second-order partial-differential equation that is inverted using 2-D discrete Fourier transforms.As an example of the inversion procedure, magnitude residuals for European stations recording Shagan River explosions are used to image the deep lithospheric anomaly beneath the Shagan River test site described in Part 1. This formal inversion analysis confirms the existence of a small-scale lateral heterogeneity located 50 km west-northwest of the test site at a probable depth between 80 and 100 km and indicates that it is consistent with a deterministic 1.5% peak-to-peak (or 0.5% rms) velocity anomaly with a scale length of about 3 km. 3-D dynamic raytracing is then used to verify that the inferred laterally varying structure produces amplitude fluctuations consistent with observations.
Computation of multi-attribute seismic wavefields by solution of the elastodynamic equations
Authors: Chen How-Wei , McMechan George A. 
Published: September 2021

AbstractBy using the elastodynamic equations rather than wave equations for seismogram synthesis, multi-attribute wave fields can be computed in a single execution of one program. In the present implementation, for 2-D models, the wave fields and seismograms produced include any or all of the following: two components of each of particle acceleration, particle velocity, and particle displacement; two components of normal strain; shear strain; two components of normal stress; shear stress; and the dilatation and curl of the particle displacement. If flexible source and receiver configurations are also included, a single piece of software can be used for most seismic modeling applications. This significantly reduces the need for development and maintenance of separate programs. The algorithm is illustrated using waves synthesized for an explosive source in a layered medium. Snapshots and seismograms at both surface and borehole arrays are presented.
Seismic gaps in Hawaii
Authors: Wyss Max , Koyanagi R. Y. 
Published: September 2021

Abstract
We propose that two seismic gaps exist in southern Hawaii. From their dimensions and tectonic setting, we estimate that the south Kona gap may be capable of a mainshock in the magnitude range of 714 to 712, while the east Kona gap will probably not produce earthquakes larger than M=612. During historic time (1832 to the present), no mainshocks have occurred in these gaps. Since there is no evidence that fault creep relieves stress in these areas, we expect earthquakes to fill these gaps. The boundaries of the south Kona gap are defined by the approximate rupture extent of the earthquakes of 2 April 1868 (M = 7.9) and 21 August 1951 (M = 6.9), those of the east Kona gap by the latter mainshock and the extent of the seismogenic zone.
The seismotectonic model on which we base our gap hypothesis defines a major seismic zone in the southern half of the island of Hawaii. We propose that most major and large earthquakes within this zone are of a decollement type, in which the upper 10 km of the crust is pushed seaward along a near-horizontal plane of weakness, provided by the oceanic sediments buried beneath the volcanic edifice. The direction of slip is away from the volcano summits and perpendicularly away from the volcano rifts.
Several recurrence intervals are known for the Kaoiki area, where M = 6 ± 0.6 mainshocks occur every 10.4 ± 1.5 years. For the Kalapana area, one recurrence interval of 108 years is known for earthquakes with 7 &lt; M &lt; 8. In most of the major seismic zone in Hawaii, the recurrence times are not known, and no information on strain accumulation rate is available. It is therefore not possible to estimate when the portion of the 1868 rupture that has not re-ruptured yet (southern Kau) will produce another large mainshock. The potential for a large earthquake in this area is estimated to be intermediate. The potential in the Kalapana, the central Kona, and northern Kona areas are estimated to be low for the next several decades.
Numerical experiments in broadband receiver function analysis
Authors: Cassidy J. F. 
Published: September 2021

AbstractThe use of broadband receiver function analysis to estimate the fine-scale S-velocity structure of the lithosphere is becoming increasingly popular. A series of numerical experiments shows several important aspects of this technique, with emphasis on estimation of dipping interfaces. The recent modification introduced to the receiver function analysis technique that preserves absolute amplitudes (Ammon, 1991) is more robust than the previous technique of modeling receiver functions that were normalized to unit amplitude. Using the latter method, shallow (e.g., depths less than ∼2 km) high-velocity contrast interfaces may alter the apparent amplitudes of Ps phases and produce inaccuracies in the Earth model developed. The use of absolute amplitudes minimizes this potential for error. When research targets include deep dipping structure, tight stacking bounds (e.g., ≦ 10° in backazimuth (BAZ) and epicentral distance (Δ)) should be applied to avoid attenuating Ps phases and to aid in the identification of reverberations or scattered energy. Reverberations sample a relatively large lateral range about the recording site (e.g., a radius of 1 to 1.5 times the depth of the reflecting interface) and in the presence of dipping interfaces exhibit drastic variations in amplitude and arrival time as a function of BAZ and Δ. Thus, they cannot readily be used to provide constraints on the Earth structure. Formal inversion techniques, which attempt to match all arrivals in the waveform, must be used with caution when modeling receiver functions from complex regions. Only those phases whose amplitude and arrival-time variations as a function of BAZ and Δ are consistent with those of Ps conversions should be modeled. Forward modeling may resolve, depending upon the data quality and noise level, S-velocity contrasts greater than ∼ 0.2 to 0.4 km / sec. Layers of thickness 2 to 5 km may be accurately imaged, and transition zones may be examined by considering various frequency bands of the data. In order to better understand the resolving power of the data, the averaging functions associated with the receiver functions may be calculated from the observed data and, if desired, used in the forward modeling process.
Comparison of various inversion techniques as applied to the determination of a geophysical deformation model for the 1983 Borah Peak earthquake
Authors: Du Yijun , Aydin Atilla , Segall Paul 
Published: September 2021

Abstract
A number of techniques are employed to overcome nonuniqueness and instability inherent in linear inverse problems. To test the factors that enter into the selection of an inversion technique for fault slip distribution, we used a penalty function with smoothness (PF + S), a damped least-squares method (DLS), damped least-squares method with a positivity constraint (DLS + P), and a penalty function with smoothness and a positivity constraint (PF + S + P) for inverting the elevation changes for slip associated with the 1983 Borah Peak earthquake. Unlike solving an ill-posed inverse problem using a gradient technique (Ward and Barrientos, 1986), we have restored the well-posed character between the elevation changes and normal slip distribution. Studies showed that the constraints based on sound understanding of the physical nature of the problem are crucial in the derivation of a meaningful solution and dictates primarily the selection of a particular inversion technique. All available geological and geophysical information were used to determine a geophysical deformation model for the earthquake. It is suggested that the PF + S + P solution for a fault length of 75 km is the preferred model. The long wavelength features in the estimated slip distribution are similar to those obtained by Ward and Barrientos (1986), whereas the shorter wavelength features differ between two solutions. The fault dips 49° to the southwest. The slipped zones deepen from the surface at the northwest to about 20-km downdip depth at the southeast. The fault extends to the southeast beyond the epicenter of the mainshock. It is also shown that only the long wavelength features of the slip distribution are well resolved. The resolution is better at shallower levels than at deeper levels. The resolution deteriorates when the deformation sources are away from the leveling lines. Smoothness constraints provide better resolution than damping does at depth. The addition of a positivity constraint significantly improves the model resolution.
A slow earthquake in the Santa Maria basin, California
Authors: Kanamori Hiroo , Hauksson Egill 
Published: September 2021

AbstractAn ML = 3.5 earthquake near Santa Maria, California, was recorded by the Southern California Seismic Network and a TERRAscope station at Santa Barbara (SBC) on 31 January 1991. The waveform of this event is dominated by 2- to 5-sec waves, and is different from that of ordinary events with similar size. Inquiries into operations in several oil fields in the area revealed that hydro-fracturing at a pressure of about 80 bars was being done at a depth of 100 to 300 m in the Orcutt oil field in the Santa Maria basin from about 9 to 11 a.m. on 31 January and the earthquake occurred in the afternoon. Field evidence of 30-cm displacement to a depth of 300 m was reported. The field evidence as well as the first-motion data indicates that the event had a thrust mechanism with the P axis in the NNE - SSW direction, which is in agreement with the regional stress field. From the analysis of the SBC record and the field evidence, we conclude that the source must be shallower than 1 km and the ratio of the radiated energy to the seismic moment is about 6.2 × 10−7, one to two orders of magnitude smaller than that of ordinary earthquakes. The occurrence of this earthquake demonstrates that release of regional tectonic stress in shallow sediments can yield significant seismic radiation at periods of a few seconds, the period range of engineering importance for large structures, and has important implications for excitation of long-period ground motions from large earthquakes in sedimentary basins.
Site response estimation from earthquake data
Authors: Hartzell Stephen H. 
Published: September 2021

AbstractAftershocks of the 1989 Loma Prieta, California, earthquake are used to estimate site response along the San Francisco Peninsula. A total of 215 shear-wave records from 24 sources and 21 sites are used in a linear inversion for source and site response spectra. The methodology makes no assumptions about the shape of the source spectrum. However, to obtain a stable, unique inverse a Q model and geometrical spreading factor are assumed, as well as a constraint on site response that sets the site response averaged over two specific stations to 1.0. Site responses calculated by this formulation of the problem are compared with other studies in the same region that use different methodologies and / or data. The shear-wave site responses compare favorably with estimates based on an ω2-constrained source model. Comparison with coda amplification factors is not as close, but still favorable considering that the coda values were determined for nearby locations with similar geology, and not the same sites. The degree of agreement between the three methods is encouraging considering the very different assumptions and data used.
The Parkfield prediction fallacy
Authors: Savage J. C. 
Published: September 2021

Abstract
The Parkfield earthquake prediction is generally stated as a 95% probability that the next moderate earthquake there should occur before January 1993. That time limit is based on a two-sided 95% confidence interval. Because at the time of the prediction (1985) it was already clear that the earthquake had not occurred prior to 1985, a one-sided 95% confidence interval would have been more appropriate. That confidence interval ended in October 1991. The Parkfield prediction was based on an extrapolation of five of the six events in the 1857 to 1966 earthquake sequence; the 1934 event was omitted because it did not fit the regularity exhibited by the other data. The fallacy in the prediction is that it did not take account of other less-contrived explanations of the Parkfield seismicity (e.g., not excluding the 1934 event). Even if the Parkfield earthquake should occur in the near future, it would be better explained by less-contrived hypotheses.
An automaticS-phase picker
Authors: Cichowicz Artur 
Published: September 2021

AbstractAn algorithm has been developed for the automatic picking of the S phase from three-component seismic data. Three parameters of the signal are calculated in the program: deflection angle, degree of polarization, and the ratio between transverse energy and total energy. The S phase is declared when the product of the three parameters increases above the reference level. Most parameters are computed automatically and modified if necessary. The S-phase picker is used to analyze data from a local underground mine seismic network.
A microprocessor-based ocean-bottom seismometer
Authors: Willoughby David F. , Orcutt John A. , Horwitt David 
Published: September 2021

AbstractFor over 12 years, the Scripps Institution of Oceanography has operated a fleet of microprocessor-based ocean-bottom seismometers. These instruments free-fall to the seafloor and release their anchors and rise to the surface either at preset times or on receipt of an acoustic command. The instruments are contained in a single spherical pressure case and include geophones with a 1-Hz natural period, and differential pressure gauges responsive to acoustic signals between 0.003 and 30 Hz.Recent improvements described in detail here include the implementation of a C-44 bus 80C88 microprocessor and cassette recorders capable of storing up to 10 days of data digitized at 128 samples/sec, or 40 days at 32 samples/sec. In addition, tiltmeters have been installed in the instruments.Serial links to the processor and release timers provide for instrument checkout and the setting of time and data parameters from outside the pressure case. A portable laboratory also described here is used to prepare the instruments for deployment at sea.
Source inversion of the 1987 Whittier Narrows earthquake, California, using the isochron method
Authors: Zeng Yuehua , Aki Keiiti , Teng Ta-Liang 
Published: September 2021

Abstract
We determine the earthquake source process from an isochron synthetic method and the recursive stochastic inverse algorithm. Using the strong-motion records from 16 stations within 15 km of the epicenter, we determined the earthquake slip and rupture time distribution on the fault with different fault geometries for the Whittier Narrows earthquake. In our study, we have chosen two best-fit fault geometries, with strike direction and dip angle of 270° and 35°, and 280° and 30°, respectively.
The inversion results indicated dominantly two regions of slip on the fault plane for the Whittier Narrows mainshock, one located immediately below the hypocenter and the other near the lower east corner of the fault. In addition, there are three or four other smaller localized source areas near the hypocenter. Together, they indicate a highly heterogeneous fault slip distribution for the Whittier Narrows mainshock. The great majority of seismic moment release occurs in a 10 by 4 km region, which is consistent with the geodetic modeling results. The inversion result for the model allowing variable slip and rupture time model gave the best fit in terms of both the Euclidean norm and the weighted Euclidean norm, suggesting that the rupture velocity may be not constant. But we find no simple causal relation between these rupture velocity variations and the slip amplitude distributions. On the other hand, by comparing the mainshock slip pattern of our inversion results with the early aftershock, we found that the aftershocks tend to occur over patches on the rupture surface deficient in mainshock slip. This observation can be well explained by the barrier model of fault heterogeneity.
Seismic hazard induced by mechanically interactive fault segments
Authors: Allin Cornell C. , Wu Shen-Chyun , Winterstein Steven R. , Dieterich James H. , Simpson Robert W. 
Published: September 2021

Abstract
This paper presents a phenomenological stochastic model for earthquake recurrence processes involving physical interaction among fault segments. Slip on one segment may reduce (or increase) the time to the next event on another segment or possibly induce an immediate slip on that segment as well. The gross behavior of this model is first observed through simulations; temporal and spatial disorder are observed even when the stochastic aspects are minimized.
To estimate the strength of these interactions, we derive factors from the output of three-dimensional elastic dislocation analyses, relating induced stress changes to temporal changes in next-event dates.
In a final section, we derive approximate analytical expressions and numerical results for future probabilistic earthquake risk and site hazard, conditional on the elapsed times since events on all relevant fault segments and on the number of events since that may have caused stress changes (interactions).