bluuebunny/crossref_metadata_2025_split huggingface dataset:
Source inversion of the 1988 Upland, California, earthquake: Determination of a fault plane for a small event
Authors: Mori Jim , Hartzell Stephen 
Published: September 2021

Abstract
We examined short-period P waves to investigate if waveform data could be used to determine which of two nodal planes was the actual fault plane for a small (ML 4.6) earthquake near Upland, California. We removed path and site complications by choosing a small aftershock (ML 2.7) as an empirical Green function. The main shock P waves were deconvolved by using the empirical Green function to produce simple far-field displacement pulses. We used a least-squares method to invert these pulses for the slip distribution on a finite fault. Both nodal planes (strike 125°, dip 85° and strike 221°, dip 40°) of the first-motion focal mechanism were tested at various rupture velocities. The southwest trending fault plane consistently gave better fitting solutions than the southeast-trending plane. We determined a moment of 4.2 × 1022 dyne-cm. The rupture velocity, and thus the source area could not be well resolved, but if we assume a reasonable rupture velocity of 0.87 times the shear wave velocity, we obtain a source area of 0.97 km2 and a stress drop of 38 bars. Choice of a southwest-trending fault plane is consistent with the trend of the nearby portion of the Transverse Ranges frontal fault zone and indicates left-lateral motion. This method provides a way to determine the fault plane for small earthquakes that have no surface rupture and no obvious trend in aftershock locations.
SH-wave scattering from a sinusoidal grating: An evaluation of four discrete wavenumber methods
Authors: Axilrod H. D. , Ferguson J. F. 
Published: September 2021

Abstract
A useful test of discrete wavenumber modeling techniques is to model scattering from a sinusoidal free surface, while varying the maximum slope of the interface. Four discrete wavenumber methods, the Aki-Larner, the Waterman, the Waterman-Fourier, and the Campillo-Bouchon, are evaluated by testing for energy conservation and comparing displacement. Contrary to the claim of some authors (Varadan et al. 1987), the Waterman-Fourier shows no advantage over the Aki-Larner method for steep slopes. With the FFT to calculate the scattering matrix, the Waterman-Fourier method is as fast as Aki-Larner. The Campillo-Bouchon method is superior to the other methods in its ability to handle steep slopes, but it requires more wavenumber samples and is an order of magnitude slower.
Calculation of zero-offset vertical seismic profiles generated by a horizontal point force acting on the surface of an anelastic half-space
Authors: Liu Hsi-Ping 
Published: September 2021

AbstractImpulse responses including near-field terms have been obtained in closed form for the zero-offset vertical seismic profiles generated by a horizontal point force acting on the surface of an anelastic half-space. The method is based on the correspondence principle. Through transformation of variables, the Fourier transform of the elastic impulse response is put in a form such that the Fourier transform of the corresponding anelastic impulse response can be expressed as elementary functions and their definite integrals involving distance, angular frequency, phase velocities, and attenuation factors. These results are used for accurate calculation of shear-wave arrival rise times of synthetic seismograms needed for data interpretation of anelastic-attenuation measurements in near-surface sediment.
Fault zone trapped seismic waves
Authors: Li Y.-G. , Leary P. C. 
Published: September 2021

AbstractTwo instances of fault zone trapped seismic waves have been observed. At an active normal fault in crystalline rock near Oroville in northern California, trapped waves were excited with a surface source and recorded at five near-fault borehole depths with an oriented three-component borehole seismic sonde. At Parkfield, California, a borehole seismometer at Middle Mountain recorded at least two instances of the fundamental and first higher mode seismic waves of the San Andreas fault zone. At Oroville recorded particle motions indicate the presence of both Love and Rayleigh normal modes. The Love-wave dispersion relation derived for an idealized wave guide with velocity structure determined by body-wave travel-time inversion yields seismograms of the fundamental mode that are consistent with the observed Love-wave amplitude and frequency. Applying a similar velocity model to the Parkfield observations, we obtain a good fit to the trapped wavefield amplitude, frequency, dispersion, and mode time separation for an asymmetric San Andreas fault zone structure—a high-velocity half-space to the southwest, a low-velocity fault zone, a transition zone containing the borehole seismometer, and an intermediate velocity half-space to the northeast. In the Parkfield borehole seismic data set, the locations (depth and offset normal to fault plane) of natural seismic events which do or do not excite trapped waves are roughly consistent with our model of the low velocity zone. We conclude that it is feasible to obtain near-surface borehole records of fault zone trapped waves. Because trapped modes are excited only by events close to the fault zone proper—thereby fixing these events in space relative to the fault—a wider investigation of possible trapped mode waveforms recorded by a borehole seismic network could lead to a much refined body-wave/tomographic velocity model of the fault and to a weighting of events as a function of offset from the fault plane. It is an open question whether near-surface sensors exist in a stable enough seismic environment to use trapped modes as an earth monitoring device.
The effect ofS-wave arrival times on the accuracy of hypocenter estimation
Authors: Gomberg Joan S. , Shedlock Kaye M. , Roecker Steven W. 
Published: September 2021

AbstractWell-constrained hypocenters (latitude, longitude, depth, and origin time) are required for nearly all studies that use earthquake data. We have examined the theoretical basis behind some of the widely accepted “rules of thumb” for obtaining accurate hypocenter estimates that pertain to the use of S phases and illustrate, in a variety of ways, why and when these “rules” are applicable. Results of experiments done for this study show that epicentral estimates (latitude and longitude) are typically far more robust with respect to data inadequacies; therefore, only examples illustrating the relationship between S phase arrival time data and focal depth and origin time estimates are presented. Most methods used to determine earthquake hypocenters are based on iterative, linearized, least-squares algorithms. Standard errors associated with hypocenter parameters are calculated assuming the data errors may be correctly described by a Gaussian distribution. We examine the influence of S-phase arrival time data on such algorithms by using the program HYPOINVERSE with synthetic datasets. Least-squares hypocenter determination algorithms have several shortcomings: solutions may be highly dependent on starting hypocenters, linearization and the assumption that data errors follow a Gaussian distribution may not be appropriate, and depth/origin time trade-offs are not readily apparent. These shortcomings can lead to biased hypocenter estimates and standard errors that do not always represent the true error. To illustrate the constraint provided by S-phase data on hypocenters determined without some of these potential problems, we also show examples of hypocenter estimates derived using a probabilistic approach that does not require linearization. We conclude that a correctly timed S phase recorded within about 1.4 focal depth's distance from the epicenter can be a powerful constraint on focal depth. Furthermore, we demonstrate that even a single incorrectly timed S phase can result in depth estimates and associated measures of uncertainty that are significantly incorrect.
The Intelligent Monitoring System
Authors: Bache Thomas C. , Bratt Steven R. , Wang James , Fung Robert M. , Kobryn Cris , Given Jeffrey W. 
Published: September 2021

Abstract
The Intelligent Monitoring System (IMS) is a computer system for processing data from seismic arrays and simpler stations to detect, locate, and identify seismic events. The first operational version processes data from two high-frequency arrays (NORESS and ARCESS) in Norway. The IMS computers and functions are distributed between the NORSAR Data Analysis Center (NDAC) near Oslo and the Center for Seismic Studies (Center) in Arlington, Virginia. The IMS modules at NDAC automatically retrieve data from a disk buffer, detect signals, compute signal attributes (amplitude, slowness, azimuth, polarization, etc.), and store them in a commercial relational database management system (DBMS). IMS makes scheduled (e.g., hourly) transfers of the data to a separate DBMS at the Center. Arrival of new data automatically initiates a “knowledge-based system (KBS)” that interprets these data to locate and identify (earthquake, mine blast, etc.) seismic events. This KBS uses general and area-specific seismological knowledge represented in rules and procedures. For each event, unprocessed data segments (e.g., 7 min for regional events) are retrieved from NDAC for subsequent display and analyst review. The interactive analysis modules include integrated waveform and map display/manipulation tools for efficient analyst validation or correction of the solutions produced by the automated system. Another KBS compares the analyst and automatic solutions to mark overruled elements of the knowledge base. Performance analysis statistics guide subsequent changes to the knowledge base so it improves with experience.
The IMS is implemented on networked Sun workstations, with a 56 kbps satellite link bridging the NDAC and Center computer networks. The software architecture is modular and distributed, with processes communicating by messages and sharing data via the DBMS. The IMS processing requirements are easily met with major processes (i.e., signal processing, KBS, and DBMS) on separate Sun 4/2xx workstations. This architecture facilitates expansion in functionality and number of stations.
The first version was operated continuously for 8 weeks in late-1989. The Center functions were then transferred to NDAC for subsequent operation. Later versions will be distributed among NDAC, Scripps/IGPP (San Diego), and the Center to process data from many stations and arrays. The IMS design is ambitious in its integration of many new computer technologies, but the operational performance of the first version demonstrates its validity. Thus, IMS provides a new generation of automated seismic event monitoring capability.
Three-component analysis of regional seismograms
Authors: Jepsen D. C. , Kennett B. L. N. 
Published: September 2021

Abstract
Both phased array techniques for single-component sensors and vectorial analysis of three-component recordings can provide estimates of the azimuth and slowness of seismic phases. However, a combination of these approaches provides a more powerful tool to estimate the propagation characteristics of different seismic phases at regional distances.
Conventional approaches to the analysis of three-component seismic records endeavor to exploit the apparent angles of propagation in horizontal and vertical planes as well as the polarization of the waves. The basic assumption is that for a given time window there is a dominant wavetype (e.g., a P wave) traveling in a particular direction arriving at the seismic station. By testing a range of characteristics of the three-component records, a set of rules can be established for classifying much of the seismogram in terms of wavetype and direction. It is, however, difficult to recognize SH waves in the presence of other wavetypes. Problems also arise when more than one signal (in either wavetype or direction) arrive in the same window. The stability and robustness of the classification scheme is much improved when records from an array of three-component sensors are combined.
For a set of three-component instruments forming part of a larger array, it is possible to estimate the slowness and azimuth of arrivals from the main array and then extract the relative proportions of the current P-, SV-, and SH-wave contributions to the seismogram. This form of wavetype decomposition depends on a model of near-surface propagation. A convenient choice for hard-rock sites is to include just the effect of the free surface, which generates a frequency-independent operation on the three-component seismograms and which is not very sensitive to surface velocities. This approach generates good estimates of the character of the S wavefield, because the phase distortion of SV induced by the free surface can be removed. The method has been successfully applied to regional seismograms recorded at the medium aperture Warramunga array in northern Australia, and the two small arrays NORESS and ARCESS in Norway, which were designed for studies of regional phases.
The new wavefield decomposition scheme provides results in which the relative proportions of P, SV, and SH waves as a function of time can be compared without the distortion imposed by free surface amplification. Such information can provide a useful adjunct to existing measures of signal character used in source discrimination.
Upper mantle structure along a profile from Oslo (NORESS) to Helsinki to Leningrad, based on explosion seismology
Authors: Ryaboy Vladislav 
Published: September 2021

Abstract
Waveforms from the NORESS array were analyzed for 147 industrial explosions during the 1985 to 1988 period, along a profile running east from Oslo (NORESS) to Helsinki to Leningrad (OHL profile). The events were 250 to 1300 km from NORESS and had local magnitude in the range 2.0 to 3.5. Event locations and origin times constrained by the University of Helsinki's regional seismic network provide a reliable basis for travel-time estimation at NORESS. We also used data recorded by NORSAR in 1979 for three shots on the FENNOLORA north-south, long-range seismic profile, which were near the OHL profile. Analysis of mantle P-wave signals from the explosions showed that first arrivals could be traced continuously to a distance of 750 to 800 km, where there is a cutoff and shift of approximately 2.0 to 2.5 sec in the travel-time curve and an increase in average apparent velocity. Interpretation of the observed travel times and waveforms for this profile suggests a low-velocity zone from approximately 105 to 135 km depth. Combined analysis of the seismic data with a Bouguer gravity map indicates the presence in the upper mantle of a high-velocity, high-density body of linear extent approximately from 200 to 300 to 500 to 600 km east of the NORESS array. It is postulated that this body may represent the root of an ancient volcanic system, in which lighter, silicic constituents were depleted from the upper mantle during the eruptive phase.
F-K analysis of NORESS array and single station data to identify sources of near-receiver and near-source scattering
Authors: Gupta I. N. , Lynnes C. S. , McElfresh T. W. , Wagner R. A. 
Published: September 2021

Abstract
F-k analyses of short-period recordings of both explosion and earthquake sources at the high-frequency NORESS array indicate secondary arrivals from a near-receiver source about 25 to 30 km southwest of the array. Use of residual seismograms, derived by subtracting the beamed record from each array channel, improved the identification of the same scatterer. F-k power difference plots, obtained by subtracting (after normalization) the f-k power versus slowness estimates for the initial P window from those for the latter windows, provided nearly identical results. The secondary seismic source appears to be short-period surface waves, Rg, generated by the scattering of incident P waves in the region of Lake Mjosa, 27 km southwest of NORESS, where there is about 1 km of relief from the bottom of the lake to the top of an adjacent hill. Polarization analysis of an explosion recorded on three-component elements of NORESS also suggested a secondary phase with polarization characteristic of Rg and arrival time and azimuth consistent with those derived from the f-k analysis. Near-source scattering is investigated by f-k analysis of data from reciprocal arrays formed by interchanging the roles of source and receiver. Application to closely spaced Yucca Flat (NTS) shots recorded at several stations provides evidence for Rg → P scattering near the northwestern and other basin boundaries of the Yucca Valley. It seems that deterministic locations of near-receiver P → Rg and near-source Rg → P scattering can be obtained by f-k techniques.
Simultaneous inversion forQand source parameters of microearthquakes accompanying hydraulic fracturing in granitic rock
Authors: Fehler Michael , Phillips W. Scott 
Published: September 2021

AbstractAn inversion that fits spectra of earthquake waveforms and gives robust estimates of corner frequency and low-frequency spectral amplitude has been used to determine source parameters of 223 microearthquakes induced by hydraulic fracturing in granodiorite. Assuming a ω−2 source model, the inversion fits the P-wave spectra of microearthquake waveforms to determine individual values of corner frequency and low-frequency spectral amplitude for each event and one average frequency-independent Q for all source-receiver paths. We also implemented a constraint that stress drops of all microearthquakes be similar but not equal and found that this constraint did not significantly degrade the quality of the fits to the spectra. The waveforms analyzed were recorded by a borehole seismometer. The P-wave Q was found to be 1070. For Q values as low as 600 and as high as 3000, the misfit between model and spectra increased by less than 5 per cent and the average corner frequency changed by less than 15 per cent from those obtained with a Q of 1070. Average stress drop was 3.7 bars. Seismic moments obtained from spectra ranged from 1013 to 1018 dyne-cm. The low stress drops are interpreted to result from underestimation of the actual stress drops because of a nonuniform distribution of stress drop and slip along the fault planes. Spatially varying stress drops and slips result from the strong rock heterogeneity due to the injection of fluid into the rock. Stress drops were found to be larger near the edges of the seismic zone, in regions that had not been seismically active during previous injections.The seismic moments determined from spectra were used to obtain a coda length-to-moment relation. Then, moments were estimated for 1149 events from measurements of coda lengths from events whose moments could not be measured from spectra because of saturation or a low signal-to-noise ratio. The constant of proportionality between cumulative number of events and seismic moment is higher than that found for tectonic regions. The slope is so high that the seismic energy release is dominated by the large number of small events. In the absence of information about the number of events smaller than we studied, we cannot estimate the total seismic energy released by the hydraulic injection.
High-frequency seismic noise as a function of depth
Authors: Carter Jerry A. , Barstow Noel , Pomeroy Paul W. , Chael Eric P. , Leahy Patrick J. 
Published: September 2021

Abstract
Evidence is presented supporting the view that high-frequency seismic noise decreases with increased depth. Noise amplitudes are higher near the free surface where surface-wave noise, cultural noise, and natural (wind-induced) noise predominate. Data were gathered at a hard-rock site in the northwestern Adirondack lowlands of northern New York. Between 15- and 40-Hz noise levels at this site are more than 10 dB less at 945-m depth than they are at the surface, and from 40 to 100 Hz the difference is more than 20 dB. In addition, time variability of the spectra is shown to be greater at the surface than at either 335- or 945-m depths. Part of the difference between the surface and subsurface noise variability may be related to wind-induced noise. Coherency measurements between orthogonal components of motion show high-frequency seismic noise is more highly organized at the surface than it is at depth. Coherency measurements between the same component of motion at different vertical offsets show a strong low-frequency coherence at least up to 945-m vertical offsets. As the vertical offset decreases, the frequency band of high coherence increases.
Reporting history of the Central Aleutians Seismograph Network and the quiescence preceding the 1986 Andreanof Island earthquake
Authors: Wyss Max 
Published: September 2021

Abstract
The earthquake catalog of the Central Aleutian Seismic Network (CASN) was searched systematically for significant rate changes in the period from 1976 through 1985. The magnitude signatures for the nine changes were modeled by synthetic signatures. By this method, six of the rate changes could be identified as artificial, based on the criteria that the rate change was confined to small magnitude events and that magnitude shifts were present. For each of these changes an artificial cause existed that affected the network within one to two months of the time of change as estimated from the catalog analysis. One of these cases was a rate decrease by a factor of 2 associated with a storm that had put out of operation two thirds of the network. Another change coincided with the introduction of digital data processing. The four remaining cases were artificial reporting rate increases due to installation of several new stations or to repair of several stations that had failed. The three rate changes that could not be identified unambiguously as artificial or natural on the basis of magnitude signatures were all rate decreases by factors of approximately 2. Two of these were clearly due to disasters that had put out of operation more than half of the network. The third was the proposed precursory quiescence to the 1986 Andreanof Island (M = 7.9) earthquake. From this we conclude that, without knowledge of the network history, many of the artificial reporting rate changes in earthquake catalogs can be identified as artificial and that their onset time can be estimated to within one to two months.
An earthquake count, independent from the Adak catalog, was obtained from the Helicorder records of the station Adak for the years 1978 through 1985 for the vicinity of Adak island. Of a total of 1831 events, 335 were not in the Adak catalog and 981 were located within the 1986 aftershock area. The Helicorder earthquake count showed a pronounced decrease by about 50 per cent, similar in amount to that in the Adak catalog. However, the decrease started in 1980, at a time when in the central subregion covered by the Adak catalog only M ≧ 2.9 events decreased, while the smaller magnitude bands show a maximum decrease in 1982. We conclude that the possibility could not be ruled out that electronics changes contributed to the reporting rate decrease starting in the years 1980 to 1982. However, it seems very unlikely that a network that was at its healthiest during the precursor time would be reduced by an improvement in the electronics to a detection level approximately equal to that in two short periods when only one third of the stations were operating. Therefore, the seismic quiescence reported and interpreted as a precursor by Kisslinger et al. (1985) was most likely a natural phenomenon.