bluuebunny/crossref_metadata_2025_split huggingface dataset:
Northridge aftershocks, a source study withTERRAscope data
Authors: Song Xi J. , Helmberger Donald V. 
Published: September 2021

AbstractBroadband and long-period displacement waveforms from a selection of Northridge aftershocks recorded by the TERRAscope array are modeled to study source characteristics. Source mechanisms and moments are determined with long-period data using an algorithm developed by Zhao and Helmberger (1994). These results are compared with those by Hauksson et al. (1995) and Thio and Kanamori (1996). The width of the direct pulses at the nearest stations PAS and CALB are measured as indications of the source duration. Another measurement of the source-time functions of these earthquakes is obtained by comparing the short-period to long-period energy ratio in the data to that in the synthetics. These measurements are used to estimate the relative stress drop using a formula given by Cohn et al. (1982). The depth distribution of the relative stress drops indicates that the largest stress drops are in the depth range of 5 to 15 km for an aftershock population of 24 events. A correlation of extended surface wave train with source depth is demonstrated for paths crossing the San Fernando basin.
Statistical significance of the seismic coupling coefficient
Authors: McCaffrey Robert 
Published: September 2021

Abstract
Variations in the seismic coupling coefficient (χ), the ratio of the observed to expected seismic moment release rates, at subduction zones are commonly used to make inferences about the physics of subduction. Taking into account the power-law form of the earthquake frequency-moment relation, it is shown that the observed distribution of χ at the world's subduction zones based on the twentieth century's earthquakes can be matched by a single value of χ ≈ 0.3 if, as is likely the case, the 90-yr observation time is less than or comparable to the repeat time of the largest possible earthquake. It is shown that because our reliable earthquake record is short, global variations in χ, based on seismicity alone, are poorly resolved and cannot be used to distinguish among subduction models.
One minute after: Strong-motion map, effective epicenter, and effective magnitude
Authors: Teng Ta-liang , Wu Ludan , Shin Tzay-Chyn , Tsai Yi-Ben , Lee William H. K. 
Published: September 2021

Abstract
This article reports the recent progress on real-time seismic monitoring in Taiwan, particularly the real-time strong-motion monitoring by the Taiwan Central Weather Bureau's telemetered seismic network (CWBSN), which is presently aiming at rapid reporting immediately after a large earthquake occurrence. If rapid reporting can be achieved before the arrival of the strong shaking, earthquake early warning will become possible. CWBSN has achieved the generation of the intensity map, epicenter, and magnitude within 1 min of the occurrence of a large earthquake. Both rapid reporting and early warning are principally applied to large (M ≫ 5) events; the requirement of on-scale waveform recording prompted CWBSN in 1995 to integrate strong-motion sensors (e.g., force-balance accelerometers) into its telemetered seismic monitoring system. Time-domain recursive processing is applied to the multi-channel incoming seismic signals by a group of networked personal computers to generate the intensity map. From the isoseismal contours, an effective epicenter is immediately identified that resides in the middle of the largest (usually the 100-gal) contour curve of the intensity map. An effective magnitude is also defined that can be derived immediately from the surface area covered by the largest (usually the 100-gal) contour curve. For a large event with a finite rupture surface, the epicenter and magnitude so derived are more adequate estimates of the source location and of the strength of destruction. The effective epicenter gives the center of the damage area; it stands in contrast with the conventional epicenter location, which only gives the initial point of rupture nucleation. The effective magnitude reflects more closely the earthquake damage potential, instead of the classical magnitude definition that emphasizes the total energy release. The CWBSN has achieved in obtaining the above crucial source information well within 1 min. This time can further be reduced to better than 30 sec, as illustrated by the example in this article, showing that earthquake early warning is indeed an achievable goal. The rapid reporting and early warning information is electronically transmitted to users to allow rapid response actions, with or without further human intervention.
Dogtails versus rainbows: Synthetic earthquake rupture models as an aid in interpreting geological data
Authors: Ward Steven N. 
Published: September 2021

AbstractGeologists have been collecting, for decades, information from historical and paleoearthquakes that could contribute to the formulation of a “big picture” of the earthquake engine. Observations of large earthquake ruptures, unfortunately, are always going to be spotty in space and time, so the extent to which geological information succeeds in contributing to a grander view of earthquakes is going to be borne not only by the quantity and quality of data collected but also by the means by which it is interpreted. This article tries to understand geological data more fully through carefully tailored computer simulations of fault ruptures. Dogtails and rainbows are two types of fault rupture terminations that can be recognized in the field and can be interpreted through these models. Rainbows are concave down ruptures that indicate complete stress drop and characteristic slip. Rainbow terminations usually coincide with fault ends or strong segment boundaries. Dogtails are concave up ruptures that indicate incomplete stress drop and noncharacteristic slip. Dogtail terminations can happen anywhere along a fault or fault segment. The surface slip pattern of the magnitude 6.6, 1979 Imperial Valley, California, earthquake shows both dogtail and rainbow terminations. The rainbow confirms the presence of a strong fault segment boundary 6 km north of the international border that had been suggested by Sieh (1996). The dogtail implies that the displacement observed in 1979 is not characteristic. By combining paleoseismic information with the surface slip patterns from this event and the magnitude 7.1, 1940 Imperial Valley earthquake, I develop a quantitative Imperial fault model with northern, central, and southern segments possessing 50, 110, and 50 bar strength, respectively. Both the 1940 and 1979 events caused 1-m amplitude dogtailed ruptures of the northern segment; however, characteristic slip of the segment is more likely to be about 3 m. To illustrate the full spectrum of potential rupture modes, models were run forward in time to generate a 2000 year rupture “encyclopedia.” Even with well-constrained segmentation and strengths, modest changes in two friction law parameters produce several plausible histories. Further discrimination awaits analysis of the extensive paleoseismic record that geologists believe exists in the shore deposits of the intermittent lakes of the Salton Trough.
Static stress drop in the 1994 Northridge, California, aftershock sequence
Authors: Hardebeck Jeanne L. , Hauksson Egill 
Published: September 2021

Abstract
We use time-domain pulse widths to estimate static stress drops for 279 ML 2.5 to 4.0 aftershocks of the 17 January 1994, MW 6.7 Northridge, California, earthquake. The stress drops obtained range from 0.02 to 40 bars, with a log average of 0.75 bar. Error bars computed for our estimates are typically a factor of 5, indicating that the three order of magnitude scatter in stress drops is not solely a result of measurement errors and that there is a significant amount of heterogeneity in the static stress drops of the aftershocks. Stress drops might be expected to increase with depth, since a fault can maintain a higher shear load at higher confining pressures. We observe an increase in log average stress drop at about 15 km depth, which is statistically significant at the 80% confidence level. The increase is due primarily to a lack of lower stress-drop events below this depth and may be controlled by material properties since the Northridge aftershocks are observed to intersect an anomalously high-velocity body at around this depth (Hauksson and Haase, 1997). An apparent increase in stress drop with magnitude is also observed over the entire magnitude range of the study, although whether this trend is real or an artifact of attenuation of high frequencies in the upper crust is unresolved.
Study of low-magnitude seismic events near the Novaya Zemlya nuclear test site
Authors: Ringdal Frode 
Published: September 2021

AbstractA study of available seismic data shows that all but one of the 42 known underground nuclear explosions at Novaya Zemlya have been detected and located by stations in the global seismic network. During the past 30 years, only one seismic event in this area has been unambiguously classified as an earthquake (1 August 1986, mb = 4.3). Several other small events, most of which are thought to be either chemical explosions or aftereffects of nuclear explosions, have also been detected. Since 1990, a network of sensitive regional arrays has been installed in northern Europe in preparation for the global seismic monitoring network under a comprehensive nuclear test ban treaty (CTBT). This regional network has provided a detection capability for Novaya Zemlya that is shown to be close to mb = 2.5. Three low-magnitude events have been detected and located during this period, as discussed in this article: 31 December 1992 (mb = 2.7), 13 June 1995 (mb = 3.5), and 13 January 1996 (mb = 2.4). To classify the source types of these events has proved very difficult. Thus, even for the mb = 3.5 event in 1995, we have been unable to provide a confident classification of the source as either an earthquake or explosion using the available discriminants. A study of mb magnitude in different frequency bands shows, as expected, that the calculation of mb at regional distances needs to take into account source-scaling effects at high frequencies. Thus, when comparing a 4 to 8 or 8 to 16 Hz filter band to a “teleseismic” 2 to 4 Hz band, the smaller events have, relatively speaking, significantly more high-frequency energy (up to 0.5 mb units) than the larger events. This suggests that a P-wave spectral magnitude scale might be appropriate. The problem of accurately locating small events using a sparse array network is addressed using the 13 January 1996 event, which was detected by only two arrays, as an illustrative example. Our analysis demonstrates the importance of using accurately calibrated regional travel-time curves and, at the same time, illustrates how array processing can be used to identify an interfering phase from a local disturbance, thereby avoiding location errors due to erroneous phase readings.
Investigation of azimuth residuals observed at stations of the GSETT-3 Alpha network
Authors: Koch Karl , Kradolfer Urs 
Published: September 2021

Abstract
The GSETT-3 experiment was conducted for more than two years and the International Data Center (IDC) reported more than 20,000 events for 1995 in the Reviewed Event Bulletins (REB). We have used this data base to investigate the location bias of arrays and selected three-component stations in the Alpha network, for which the recorded data is automatically processed to determine earthquake locations in near-real time. The Alpha network, which consists of 14 arrays, provides estimates of event azimuths determined by f-k processing. We selected 19 three-component stations that were operated all or most of 1995 and for which event azimuth was estimated by polarization analysis. Only events with epicentral distances of less than 100° were considered in order to exclude phases with steep incidence angles, associated with unreliable azimuth estimates. Teleseismic P phases and the major regional and local phases were examined. Only data for which a signal-to-noise ratio was given were included in this analysis. The signal-to-noise ratios were used as a weighting factor for the statistical parameters and the graphical representation of the azimuth residuals. The preliminary analysis of azimuth residuals identified some arrays that give significantly erroneous azimuths; in some cases, the error is systematic, in other cases, it is random. One array with systematic azimuthal bias is CMAR. This array shows positive residuals for event paths from the south, while phases for events with paths from the north show negative residuals. Similar patterns, which strongly suggest azimuthal bias, are observed for PDAR, TXAR, and SPITS. In the case of MJAR, the large azimuthal errors observed particularly for regional and local distances are not a function of direction. The scatter in the azimuth residuals is much larger for 3-C stations than for the arrays. Clear systematic errors were identified for SCHQ and CPUP, which show constant offsets, and at VNDA, which has negative residuals for events to the north and positive residuals for events from the south. The most significant azimuth bias was observed for PDY, where the azimuth residuals change from about −90° to more than +90° at backazimuths near 120°, meaning that all rays appear from the northwest.
Wavelet transform methods for phase identification in three-component seismograms
Authors: Anant Kanwaldip Singh , Dowla Farid U. 
Published: September 2021

Abstract
We apply the wavelet transform to seismic signals for the purpose of automatically identifying the P and S phase arrivals of seismic events. In this article, an algorithm is presented that locates these arrivals in single-station three-component short-period seismograms using polarization and amplitude information contained in the wavelet transform coefficients of the signals. The main idea is that strong features of the seismic signal appear in the wavelet coefficients across several scales. The first step in the algorithm is the wavelet decomposition of each component of a three-component short-period seismogram. The resulting multi-scalar representation is used to construct “locator” functions that identify the P and S arrivals. The P locator function is constructed by using polarization information across scales, and the S locator function is constructed using transverse over radial amplitude information across scales. These functions prove to be very effective at identifying the important P and S arrivals in the test data. The results are compared with arrival times picked by an analyst.
Observations of high-frequency scattered waves using dense arrays at Teide Volcano
Authors: Del Pezzo E. , La Rocca M. , Ibanez J. 
Published: September 2021

Abstract
A study of the seismic coda of seven small earthquakes recorded on the Teide Volcano-Canary Islands (Spain) was carried out using a temporary, small-aperture, 12-station, seismic array. The purpose was to measure backazimuth, apparent velocity and the type of waves that compose the coda in the frequency range 4 to 6 Hz. We used the zero-lag cross-correlation (ZLC) method to obtain the components of the wave vector and three-component analysis techniques based on the covariance matrix of the signal in the time domain to investigate the polarization properties of the signals. The results show that a great part of the coda signals in the analyzed frequency range are almost uncorrelated, while a low number of isolated correlated arrivals show an apparent slowness between 2 × 10−3 and 2.5 × 10−4 sec/m and an almost random distribution of backazimuths. The correlated arrivals have been interpreted as generated by strong scatterers, probably related to the presence, in the study area, of surface topography irregularities (the volcanic cone and the caldera rim). The wave type varies from pure S-type waves to mixed surface waves with some Rayleigh components.
Do historical rates of seismicity in southern California require the occurrence of earthquake magnitudes greater than would be predicted from fault length?
Authors: Stirling Mark W. , Wesnousky Steven G. 
Published: September 2021

AbstractThe Working Group on California Earthquake Probabilities reported a discrepancy between the historical rates of large earthquakes in southern California and rates predicted from interpretation of geological, geodetic, and historical seismicity data. It was suggested that the discrepancy may be due to the assumption within their analysis that the magnitude of the largest earthquake on a fault is limited by the mapped fault length. Our analysis of the available data does not support the presence of a historical deficit in the rate of seismicity, nor does it require that earthquakes that rupture beyond the lengths of mapped active faults in southern California, or that rupture numerous subparallel faults, are needed to explain the historical distribution of seismicity.
Estimation of attenuation relations for strong-motion data allowing for individual earthquake magnitude uncertainties
Authors: Rhoades David A. 
Published: September 2021

AbstractStandard errors of earthquake magnitudes are routinely calculated and vary appreciably between earthquakes. However, the uncertainties of magnitude determination are usually ignored in regression models of strong ground motion as a function of magnitude and distance from the earthquake source. This practice has the potential to bias estimates of strong ground motion. A method is given for taking account of the uncertainty of each magnitude determination in fitting such models. It extends previous methods in which the error variance is partitioned into between-earthquake and within-earthquake components. It allows for further decomposition of the between-earthquake component into a part attributable to magnitude uncertainties and a part attributable to other causes.The method has been applied to the well-known attenuation data of Joyner and Boore (1981). The Mw determinations in this dataset fall into two subsets with distinctly different levels of precision, namely, those determined directly and those inferred from values of ML. It is shown that most of the between-earthquake component of variance can be attributed to the relatively low precision of the magnitudes in the latter subset.
A simple strategy to examine the sources of errors in attenuation relations
Authors: Lee Yajie , Zeng Yuehua , Anderson John G. 
Published: September 2021

AbstractA new method is presented in this article to visualize two different sources in the uncertainty of attenuation regression relations. The method utilizes the residuals from regression equations, defined as the log of the ratio of observed to predicted ground-motion parameters, from stations that have recorded more than one earthquake. The earthquake-to-earthquake variance is first calculated. Then the residuals are corrected with the mean residual of the corresponding earthquake. The corrected residual from one event is plotted versus the corrected residual from another event for every station that has more than one record. If site effects are perfectly represented in the regression, the resulting scatter plot will show statistically uncorrelated points, while extension along the diagonal and a positive correlation coefficient is the result of a contribution from site effects. This simple strategy allows us to visualize the uncertainty caused by the earthquake source and path in regression relations and indicates quantitatively how much we can improve the prediction by adding additional site information. The results obtained from this method are very similar to those that are calculated directly from the method proposed by Joyner (personal comm., 1997). In southern California, we find that source and path effects dominate the uncertainties at high frequency, while at low frequency, the regression can be still improved more significantly by correcting for the site effects.
A technique for simulating strong ground motion using hybrid Green's function
Authors: Kamae Katsuhiro , Irikura Kojiro , Pitarka Arben 
Published: September 2021

AbstractA method for simulating strong ground motion for a large earthquake based on synthetic Green's function is presented. We use the synthetic motions of a small event as Green's functions instead of observed records of small events. Ground motions from small events are calculated using a hybrid scheme combining deterministic and stochastic approaches. The long-period motions from the small events are deterministically calculated using the 3D finite-difference method, whereas the high-frequency motions from them are stochastically simulated using Boore's method. The small-event motions are synthesized summing the long-period and short-period motions after passing them through a pair of matched filters to follow the omega-squared source model. We call the resultant time series “hybrid Green's functions” (HGF). Ground motions from a large earthquake are simulated by following the empirical Green's function (EGF) method. We demonstrate the effectiveness of the method at simulating ground motion from the 1995 Hyogo-ken Nanbu earthquake (Mw 6.9).
Spectral structure in the earth's microseismic background between 20 and 40 seconds
Authors: Holcomb L. Gary 
Published: September 2021

Abstract
The long-term behavior of a narrow band peak near 26 sec in the microseismic background of the Earth has been studied over many years at several sites. The amplitude of this peak has been determined to be a function of the Earth's weather seasons; it is larger during the southern hemisphere winter. In addition, the microseismic background between 20 and 40 sec has been scanned for peaks in the spectra; three more lower-level peaks have been identified with the possible presence of more.
EURO-SEISTEST strong-motion array near Thessaloniki, Northern Greece: A study of site effects
Authors: Dimitriu P. P. , Papaioannou Ch. A. , Theodulidis N. P. 
Published: September 2021

AbstractThe effects of local geology on the characteristics of strong ground motion have been, and continue to be, a field of active research. Despite the considerable efforts made so far, there are still several unresolved and controversial issues remaining. In particular, debates still continue over the limits of applicability of one-dimensional (1D) wave-propagation models. There are also unresolved questions related to the implementation and reliability of site-response estimation techniques such as the standard spectral ratio (SSR) and the horizontal-to-vertical spectral ratio (HVSR). This study addresses these issues on the basis of data from the EURO-SEISTEST strong-motion array at Volvi, near Thessaloniki, Greece. The data set used consists of accelerograms of 32 earthquakes, almost exclusively local, covering a magnitude range from ML 2.0 to 6.1. The range of recorded accelerations is 0.001 − 0.042 g.We implement two 1D wave-propagation models (the Haskell-Thompson matrix method and Kennett's reflectivity-coefficient method) and the HVSR site-response assessment technique. We test the applicability of the 1D models in a basin environment. We use both Fourier-amplitude and response spectra to compute HVSR and investigate how HVSR is influenced by the choice of the time window and smoothing procedure.We found the HVSR technique and 1D SH-wave modeling to perform reasonably well in a sediment-valley environment (the modeling was performed for a site in the center of the valley).