bluuebunny/crossref_metadata_2025_split huggingface dataset:
Limits of sensitivity of inertial seismometers with velocity transducers and electronic amplifiers
Authors: Riedesel Mark A. , Orcutt John A. , Moore Robert D. 
Published: September 2021

Abstract
Portable instruments such as ocean bottom seismographs and the PASSCAL recorders often use rugged, portable geophones. The desire to use such sensors for relatively low-frequency work has raised questions about the limits of their sensitivity. The lower and upper frequency limits of performance of seismic sensors are determined by the sensor's mass, period, and Q, and by the amplifiers used with those sensors. We have tested Mark Products 1 Hz, 2 Hz, and 4.5 Hz velocity transducers against Streckeisen seismometers in order to examine the limits of their performance in measuring ground noise, particularly at low frequencies. Among the velocity transducers, only the 1 Hz Mark Products L-4 sensor provided good resolution of the 6-sec microseism peak. For this sensor, the lower limits of sensitivity was at approximately 0.06 Hz, although this depends on the amplifier used and the noise level at a given site. The amplifiers examined included conventional, low power, and commutating auto-zero operational amplifiers. It was found that the noise levels of the amplifiers intersected the ground noise level at frequencies ranging between 0.06 and 0.2 sec, depending on the amplifier and the exact circuit design. Measurements indicated that by modeling the amplifier noise for a given circuit correctly, the performance of an amplifier can be predicted with a high degree of accuracy, obviating the need for actual circuit construction to determine performance in the field. Given the very steep slope of the ground noise spectrum between 0.05 and 0.1 Hz and the rapid fall off in a seismometer's output below its resonant frequency, it would require a lowering of amplifier noise by more than an order of magnitude to be able to resolve ground noise at frequencies lower than 0.05 Hz using relatively small geophones such as the L-4. To resolve ground noise at lower frequencies, it is necessary to use a seismometer with a displacement transducer to sense the mass position, such as Guralp or Streckeisen sensors.
Initial results from the Intelligent Monitoring System
Authors: Bratt Steven R. , Swanger Henry J. , Stead Richard J. , Ryall Floriana , Bache Thomas C. 
Published: September 2021

AbstractThe Intelligent Monitoring System (IMS) integrates advanced technologies in a knowledge-based distributed system that automates most of the seismic data interpretation process. Results from IMS during its first 8 weeks of operation (1 October through 25 November 1989) are analyzed to evaluate its performance. During this test period, the IMS processed essentially all data recorded by the NORESS and ARCESS high-frequency arrays in Norway. The emphasis was on detection and location of regional events within 2,000 km of these arrays. All events were reviewed and corrected if necessary by a skilled analyst. The final IMS Bulletin for the period includes 1,580 regional events (∼280 events/day). Approximately 55 per cent were smaller than MLg 1, with the largest just over MLg 3.Comparison of IMS locations in southern Finland and northwestern USSR (800 to 900 km from both arrays) with event locations from the University of Helsinki's local network bulletin are used to assess the detection and location capabilities of the system. Two or more phases (minimum needed to locate) were detected for 96 per cent of the events with magnitude greater than 2.5. The median separation between the IMS and Helsinki locations for all common events was 23.5 km. A consistent bias in arrival-time and azimuth residuals was observed for events in small geographic areas, indicating that refined travel-time models and path corrections could further improve location accuracy.The knowledge base in this first version of IMS was based on analysis of NORESS data, and many of the errors in interpretation corrected by the analysts can be attributed to differences encountered when this knowledge is used to interpret ARCESS data. Nevertheless, nearly 60 per cent of the events appearing in the final bulletin are automatic solutions approved without change or moved (by analyst corrections) less than 25 km from the automatic locations. The IMS had the most difficulty interpreting the overlapping signals generated by closely spaced explosions commonly detonated at mines in the Kola Peninsula and northern Sweden. Using the knowledge acquisition facilities included in the system, the deficiencies responsible for these and other errors are isolated, leading to development of new knowledge to be incorporated in the next version of the IMS knowledge base.
Azimuth estimation capabilities of the NORESS regional seismic array
Authors: Bame Dorthe A. , Walck Marianne C. , Hiebert-Dodd Kathie L. 
Published: September 2021

Abstract
We have investigated the regional Pn backazimuth estimation capabilities of the NORESS seismic array as a function of element spacing, frequency band, and time window to determine which parameters are optimal for reducing azimuth errors. We used a broadband frequency-wavenumber estimator to calculate backazimuths from the Pn arrival for each of 274 regional events recorded at NORESS for 126 parameter combinations. The large data base provides a wide range of signal-to-noise ratio (SNR) (0 to 70 dB), distance (up to 10.5°), and azimuth characteristics, and includes identified earthquakes and explosions as well as “unknown” sources. Most of the errors in backazimuth are less than 20° when appropriate parameters are used, and mean backazimuth errors are close to zero. The best results are obtained using a 13-element array configuration that has a 1.4 km aperture and a maximum station spacing of about 600 m. With the 13-element configuration and the data filtered to include frequencies between 3 and 10 Hz, the mean errors for the 274 event data set are less than 1.4°, and standard deviations are as small as ± 11.1°. The entire array also produces good results for 3 to 10 Hz, and a 9-element configuration (two inner rings) performs well at high frequencies. The five-element B-ring (600 m aperture) appears to be important in obtaining good backazimuth estimates for regional Pn waves. Of the frequency bands considered in this study, the 3 to 6 Hz, 4 to 8 Hz, and 5 to 10 Hz bands yield the most reliable backazimuth estimates, even better than an “optimal” band that calculates the azimuth in the fixed frequency band that has the largest average SNR. The time-window length has little effect on the backazimuth estimates. Other factors investigated include SNR, source region, and phase type. We found that event backazimuth accuracy degrades if the SNR of a beamed Pn signal is less than 5 dB in the frequency band of interest. Conversely, the backazimuth estimation statistics improve if only events with 5 dB of SNR are included in the event set. These data sets yield near-zero mean errors and smaller standard deviations than the entire data set. For specific source regions, standard deviations are as low as 2° for some parameter combinations, but there can also be large biases in the backazimuth estimates. Events farther than 500 km from NORESS tend to have larger azimuth errors than the closer events, but combinations of small aperture configurations and middle (3 to 10 Hz) frequency bands work well for events over the entire distance range of 40 to 1200 km. The Pn arrival and RONAPP azimuths calculated from the Lg phase have similar accuracy statistics for a 220 event common data base, implying that the two phases work equally well for regional event backazimuth estimation. In fact, averaging of the Lg and Pn estimates provides the most accurate backazimuths relative to PDE reference information.
An on-line analysis system for three-component seismic data: Method and preliminary results
Authors: Kedrov O. K. , Ovtchinnikov V. M. 
Published: September 2021

Abstract
This paper presents the basic method and algorithm of a system for automatic on-line analysis of seismic data at three-component stations and gives preliminary experimental results. Data processing consists of four steps: (1) preliminary detection and estimation of the signal arrival time by using the energy-based γ-detector; (2) refinement of signal parameters (arrival time, amplitude, period, and coda duration); (3) estimation of the azimuth and incident angle of the seismic ray by using polarization analysis; and (4) approximate estimation of event parameters (epicenter coordinate, origin time, and magnitude) from teleseismic P waves for a constrained focal depth.
The software package, which is called “SEISMOSTANSIA,” has been tested in an on-line environment at an experimental seismic station in Eastern Kazakhstan. Testing results show that the system can handle an input flow of up to 100 signals in a 24-hr period. Comparison of the results of automatic detection with data of the station bulletin obtained by visual analysis of analog seismograms shows that: (1) with the present threshold setting, the automatic system detects practically all teleseismic signals seen by the analyst together with typically 10 to 15 signals per day not confirmed by visual analysis; (2) in 70 per cent of the cases, the residual between the arrival time obtained from the automatic and subjective procedures is 0.5 sec or less for SNR &gt; 10 and 1 sec or less for SNR &lt; 10; (3) in 70 per cent of the cases, the residual between the signal period obtained from the automatic and subjective procedures is 0.2 sec or less. Analysis of data indicates that an effective means of identifying false alarms with low SNR can be the level of polarization rectilinearity of P and S waves. By analyzing azimuth estimates for a suite of explosions at the Nevada Test Site, it is shown that a systematic bias forms the main contribution to the residual relative to true azimuth. In the general case, the azimuth residuals are due to the features of the medium both at the epicenter and at the station, and consequently depend on both azimuth and epicentral distance. An analysis of azimuth residuals points to a periodic dependence on azimuth. The value of the period is about 120°. Additional information (azimuth and angle of incidence) reported from three-component stations to data centers is shown to improve significantly the reliability of phase association in the processing of data from a global seismic network.
The GEOSCOPE program: Present status and perspectives
Authors: Romanowicz Barbara , Karczewski Jean François , Cara Michel , Bernard Pascal , Borsenberger Jean , Cantin Jean-Michel , Dole Bernard , Fouassier Danielle , Koenig Jean-Claude , Morand Michel , Pillet Robert , Pyrolley Annick , Rouland Daniel 
Published: September 2021

Abstract
GEOSCOPE is a global three-component digital broadband seismological network deployed by France. It presently counts 22 stations, with a final goal of 25 stations, representing the French contribution to the new generation global seismological network, an international effort coordinated through the Federation of Digital Seismic Networks, and the basic tool for global seismology of the 1990s.
Polarization analysis of high-frequency, three-component seismic data
Authors: Bataille K. , Chiu J. M. 
Published: September 2021

AbstractWe present a method to determine the polarization of body waves from three-component, high-frequency data and examples of its application. The method is based on the principal component approach. One advantage of this approach is that the polarization state can be determined for small time windows compared with the predominant period of the wave. This is particularly useful for identifying converted waves within the crust. The stability of the result is analyzed with synthetic cases by adding simultaneous arrivals from waves and random noise. The method works well with both synthetic and local data in the detection of the polarization of the wave by separating arrivals from different directions. From the local data, some seismic phases related to crustal conversions are observed that require strong lateral variations.
Modeling of wave propagation in northern Los Angeles basin
Authors: Sen Mrinal K. 
Published: September 2021

AbstractThe Los Angeles basin is characterized by deep structural complexity and is the site of numerous earthquakes. We analyze the amplitude and waveform data of earthquakes recorded in the Los Angeles basin in order to identify the structurally controlled path effects. This has been done in order to determine which features of the structure affect the observations and to generate improved models in different parts of the basin increasing our capabilities of predicting seismic hazard. Several structural models of the L.A. basin that are based on regional subsurface studies, exposed sections, scattered oil wells, and other limited geophysical data have recently become available. In this study, we have used seismic modeling techniques together with geologic models to attempt to explain site variations in some important data bases. We have modeled waveforms of (1) the 19 January 1989 Malibu earthquake data recorded in Pasadena, (2) an M = 2.8 earthquake recorded by the USC downhole array in the Baldwin Hills, southern California, and (3) two aftershocks of the Whittier Narrows earthquake recorded at five stations along a profile.The modeling of the Malibu earthquake results in a layered one-dimensional model representing 6.5 km of layered sediment underlain by a high-velocity basement. Direct arrivals, basin reflected phases, and multiple reverberations are modeled fairly well by this one-dimensional model. The downhole array data in the Baldwin Hills show a significant near-surface amplification that can be modeled by very low-velocity near-surface materials. A series of aftershocks of the Whittier Narrows earthquake recorded at close distances at temporary recording sites shows a wide variation in peak accelerations due both to radiation pattern and propagation effects. The effect of lateral heterogeneity is evident in the travel time, where stations within the basin show later arrival times than those away from the basin for the same epicentral distance. Seismograms recorded at different sites show variation in waveforms due to multi-pathing of rays. We use such triplicated arrivals to constrain the structure of different interfaces. Using these criteria and the synthetic seismograms calculated by ray theory and finite difference methods, we have been able to model the tangential seismograms due to two of these events recorded at five stations along a two-dimensional profile.All three modeling exercises helped us understand wave propagation in the basin environment. A basin-reflected phase was identified in the seismogram from the Malibu earthquake, which was used to derive an effective depth of the basement. Near-surface amplification of the waves observed in the downhole data could be explained by constructive interference of the multiply reflected waves through the thin near-surface low-velocity layer. Similarly, site-dependent variation of waveform and travel time shown by the seismograms from the aftershocks of the Whittier Narrows, California, earthquake could be accounted for when a laterally varying structure was used.
Origin of precursors to teleseismic S waves
Authors: Vinnik Lev. P. , Romanowicz Barbara A. 
Published: September 2021

Abstract
In the literature, there are descriptions of precursors to teleseismic S waves that are polarized as teleseimic P. They are commonly regarded as phases converted from SV to P underneath the seismographic stations. We present observations of precursors at the broadband digital GEOSCOPE stations in the distance range from 45° to 95° and at periods around 10 sec. These precursors are polarized as the teleseismic P but cannot be interpreted in terms of conversion underneath the station. We propose that they are formed by conversion and scattering from S to P at the free surface and scattering of the resulting P in the lithosphere of the region between the source and the receiver. The apparent velocity of S in the region of scattering is usually around 7 km/sec, which implies a long wavepath of the converted P in the continental crust. Apparently, S-to-P and P-to-P scattering play an important role in forming teleseismic wave fields even in the relatively long-period band around 10 sec. A possibility of observing true S-P converted phases originating in the lithosphere underneath the station is certainly not ruled out. However, our analysis is a warning that scattered waves sometimes can be erroneously taken for converted phases.
The EARSS digital seismograph: System description and field trials
Authors: Gledhill K. R. , Randall M. J. , Chadwick M. P. 
Published: September 2021

AbstractAn earthquake detection and recording system known as EARSS has been developed for permanent seismograph stations and temporary field installations. It records three components of ground motion with a dynamic range of 120 dB. A frequency-domain algorithm detects earthquakes and initiates the recording of data on magnetic tape. Alternatively, EARSS can record data continuously, for preselected periods of time, or recording can be triggered by a time-domain phase picker. Up to 1500 earthquakes (25.5 Mbytes) can be recorded on each magnetic tape cartridge. The field version of EARSS supplies power to the tape drive only when data is being written to tape, thus reducing the normal power consumption of 12 watts (at 12 volts) to 2.5 watts. A field trial using a network of eight EARSS seismographs resulted in 1020 successful station-days of operation from a possible total of 1098 station-days (3 years). Of the 78 lost days of operation, 23 were due to power supply problems external to EARSS, and 52 were caused by a low-temperature failure of the recording system, which has since been corrected. A total of 442 Mbytes of data were recorded, of which about 250 Mbytes were useful data.
Rupture model of the 1989 Loma Prieta earthquake from the inversion of strong-motion and broadband teleseismic data
Authors: Wald David J. , Helmberger Donald V. , Heaton Thomas H. 
Published: September 2021

Abstract
We have used 24 broadband teleseismic and 48 components of local strong-motion velocity records of the 1989 Loma Prieta earthquake in a formal inversion to determine the temporal and spatial distribution of slip. Separate inversions of the teleseismic data (periods of 3 to 30 sec) or strong-motion data (periods of 1 to 5 sec) result in similar models. The data require bilateral rupture with relatively little slip in the region directly updip from the hypocenter. Slip is concentrated in two patches: one centered 6 km northwest of the hypocenter at a depth of 12 km and with a maximum slip of 350 cm, and the other centered about 5 km southeast of the hypocenter at a depth of 16 km and with a maximum slip of 460 cm. The bilateral nature of the rupture results in large amplitude ground motions at sites located along the fault strike, both to the northwest and the southeast. However, the northwestern patch has a larger moment and overall stress drop and is, consequently, the source of the largest ground motion velocities, consistent with the observed recordings. This bilateral rupture also produces relatively modest ground motion amplitudes directly updip from the hypocenter, which is in agreement with the velocity ground motions observed at Corralitos. There is clear evidence of a foreshock (magnitude between 3.5 and 5.0) or a slow rupture nucleation about 2 sec before the main part of the rupture; the origin time implied by strong-motion trigger times is systematically 2 sec later than the time predicted from the high-gain regional network data. The seismic moment obtained from either of the separate data sets or both sets combined is about 3.0 × 1026 dyne-cm and the potency is 0.95 km3.
Ground motion at the San Francisco international airport from the Loma Prieta earthquake sequence, 1989
Authors: McGarr A. , Çelebi M. , Sembera E. , Noce T. , Mueller C. 
Published: September 2021

AbstractFollowing the Loma Prieta earthquake, the U.S. Geological Survey installed four portable digital seismic recorders at the San Francisco International Airport (SFO) for one week to study aftershock ground motion at this important Bay area “lifeline.” This study was motivated largely by the need to anticipate strong ground motion from future major earthquakes affecting the Bay area and, to a lesser extent, by the fact that SFO was shut down for 13 hours owing to damage from the Loma Prieta shock. Accordingly, the recording sites were chosen so as to elucidate the effects of varying thicknesses of low-velocity surficial alluvium on the ground motion. Three large aftershocks with magnitudes ranging from 4.2 to 4.5 each produced ground motion that was recorded at all four SFO stations. One of our stations was collocated with a permanent ground motion recorder that indicated a peak horizontal velocity of 29 cm / sec and a peak horizontal acceleration of 0.33 g during the 18 October mainshock. From the aftershock data and one mainshock record, it is possible to extrapolate approximately the mainshock ground motion to other locations at SFO and, more generally, to assess the effects of low-velocity sedimentary cover, including artificial fill material, on the character of the ground motion. The main-shock ground motion recorded at the permanent station was apparently typical for most of SFO where the near-surface alluvium resulted in peak horizontal ground velocity, in the frequency band 0.1 to 3 Hz, amplified by a factor of about 2.5 relative to that recorded at bedrock sites. Observations, in the epicentral distance range 59 to 95 km, including SFO, of the moho-reflected phases PmP and SmS from the aftershocks support the hypothesis, presented elsewhere, that the phase SmS accounted for much of the peak ground motion throughout most of the San Francisco Bay area.
Evaluation of the failure of the Cypress Viaduct in the Loma Prieta earthquake
Authors: Miranda Eduardo , Bertero Vitelmo V. 
Published: September 2021

Abstract
This article describes analytical studies performed to explain the collapse of the Cypress Street Viaduct. The modes of failure of four types of bents are described. A strength analysis is presented, which gives estimates of the forces that were necessary to produce the failure of these bents. These estimates are computed based on the observed failure mechanism of each type of bent, on measured mechanical characteristics of the materials, and on simplified structural models. The overstrength of these bents, above the level of forces for which they were designed, is evaluated. Levels of acceleration required to produce the failure are given. From comparison of these levels of accelerations with those resulting from the elastic as well as the inelastic response spectra corresponding to ground motions recorded near the Viaduct, it is concluded that the demanded shear strength was considerably higher than the supplied strength and that bent type B8 was the weakest one and where the failure was probably initiated.