bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of amplitude and duration magnitudes and site residuals from short-period seismographs in northern California
Authors: Eaton Jerry P.
Published: September 2021

Equations for determining amplitude magnitude (MX) and duration magnitude (MF) that employ all calibrated instruments in the USGS short-period telemetered seismic network in northern California (NCSN) were developed and tested against a set of 1276 earthquakes from 1986 to 1990 that were analyzed on the Caltech-USGS processing system (CUSP). The expressions for decay of amplitude and record duration in these equations are functions of distance alone. Sensitivity corrections for both MX and MF are simply the logarithms of the ratios of the magnification of the reference instrument to that of the instrument actually used. Component corrections were chosen so as to minimize the dependence of instrument site residuals on instrument component. MF site residuals were found to be closely linked to MX site residuals in a manner that suggests both depend primarily on site amplification. Both MX and MF site residuals vary systematically with bedrock lithology: older well-consolidated rocks produce negative residuals (smaller amplitudes and shorter durations) and younger unconsolidated rocks produce positive residuals (larger amplitudes and longer durations).

Average station magnitude residuals are virtually independent of distance from the epicenter to at least 800 km; and MX-MF, averaged over 0.5 unit magnitude intervals, is less than 0.05 from M0.5 to M5.5.

Comparison of MX and MBK (ML, UC Berkeley) for 293 events in both the CUSP data set and the Berkeley catalog shows that calculated MX s are marginally larger than the corresponding MBK s. MX-MBK averages about + 0.04.

The characteristics of the standard Wood-Anderson seismograph employed to calculate MX are: free period 0.8 sec, damping constant 0.8, and static magnification 2080.
A site effect study in Acapulco, Guerrero, Mexico: Comparison of results from strong-motion and microtremor data
Authors: Gutierrez Carlos , Krishna Singh Shri
Published: September 2021

The city of Acapulco is located near or above the mature seismic gap of Guerrero along the Mexican subduction zone. With the purpose of studying the character of strong ground motion on soft sites, four digital accelerographs have been installed in the city on such sites. These instruments have been in operation since 1988. Two additional instruments, part of the Guerrero Accelerograph Array, are located on hard sites in the area. One of these, VNTA, has been in operation since 1985 and the other, ACAN, since 1989. These stations have recorded several earthquakes. We use data from eight events (4.2 ≤ M ≤ 6.9) to study spectral amplification of seismic waves at the soft sites with respect to VNTA. The S waves are amplified by a factor of 6 to 25 at the soft sites in a fairly broad range of frequencies; both the amplification and the frequency band over which it occurs depend upon the site. Although the largest earthquake in our data set (M = 6.9) gave rise to a peak horizontal acceleration exceeding 0.3 g at one of the soft sites, no clear evidence of nonlinear behavior of the subsoil is found. Spectral amplifications of S-wave coda are very similar to those of S waves. We also measured microtremors at the strong-motion sites. The microtremor spectra were interpreted, using reasonable assumptions, to test the feasibility of this technique in reproducing the spectral amplifications observed during earthquakes. Our results show that only a rough estimate of site response can be obtained from this technique, at least in Acapulco; caution is warranted in its use elsewhere.
Internal deformation due to shear and tensile faults in a half-space
Authors: Okada Yoshimitsu
Published: September 2021

A complete set of closed analytical expressions is presented in a unified manner for the internal displacements and strains due to shear and tensile faults in a half-space for both point and finite rectangular sources. These expressions are particularly compact and systematically composed of terms representing deformations in an infinite medium, a term related to surface deformation and that is multiplied by the depth of observation point. Several practical suggestions to avoid mathematical singularities and computational instabilities are also presented. The expressions derived here represent powerful tools both for the observational and theoretical analyses of static field changes associated with earthquake and volcanic phenomena.
Seismic moment catalog of large shallow earthquakes, 1900 to 1989
Authors: Pacheco Javier F. , Sykes Lynn R.
Published: September 2021

We compile a worldwide catalog of shallow (depth < 70 km) and large (Ms ≥ 7) earthquakes recorded between 1900 and 1989. The catalog is shown to be complete and uniform at the 20-sec surface-wave magnitude Ms ≥ 7.0. We base our catalog on those of Abe (1981, 1984) and Abe and Noguchi (1983a, b) for events with Ms ≥ 7.0. Those catalogs, however, are not homogeneous in seismicity rates for the entire 90-year period. We assume that global rates of seismicity are constant on a time scale of decades and most inhomogeneities arise from changes in instrumentation and/or reporting. We correct the magnitudes to produce a homogeneous catalog. The catalog is accompanied by a reference list for all the events with seismic moment determined at periods longer than 20 sec. Using these seismic moments for great and giant earthquakes and a moment-magnitude relationship for smaller events, we produce a seismic moment catalog for large earthquakes from 1900 to 1989. The catalog is used to study the distribution of moment released worldwide. Although we assumed a constant rate of seismicity on a global basis, the rate of moment release has not been constant for the 90-year period because the latter is dominated by the few largest earthquakes. We find that the seismic moment released at subduction zones during this century constitutes 90% of all the moment released by large, shallow earthquakes on a global basis. The seismic moment released in the largest event that occurred during this century, the 1960 southern Chile earthquake, represents about 30 to 45% of the total moment released from 1900 through 1989. A frequency-size distribution of earthquakes with seismic moment yields an average slope (b value) that changes from 1.04 for magnitudes between 7.0 and 7.5 to b = 1.51 for magnitudes between 7.6 and 8.0. This change in the b value is attributed to different scaling relationships between bounded (large) and unbounded (small) earthquakes. Thus, the earthquake process does have a characteristic length scale that is set by the downdip width over which rupture in earthquakes can occur. That width is typically greater for thrust events at subduction zones than for earthquakes along transform faults and other tectonic environments.
Seismological studies at Parkfield. II. Search for temporal variations in wave propagation using vibroseis
Authors: Karageorgi E. , Clymer R. , McEvilly T. V.
Published: September 2021

For more than 3 years the propagation characteristics of shear waves have been monitored for paths near the 1966 hypocenter at Parkfield, the presumed nucleation site for the expected next M6 earthquake there. Data have been collected repeatedly (33 sets as of April 1991) from eight S-wave Vibroseis source positions into the 10 borehole-installed three-component seismometers of the local high-sensitivity digital network. Twenty-second correlated records from a 6- to 24-Hz sweep are acquired, and the entire seismogram is viewed for analysis as the elastic response of the local crustal structure, which includes the San Andreas fault zone. Amplitudes, travel times, spectra, and particle motions of the P and S waves are monitored for indications of any changes in these properties that may be attributed to processes associated with nucleation. The horizontal vibrator at each source point is positioned at three surface-orientations to study anisotropy. Unorthodox methods have been developed to display the waveform properties in time in order to visualize the resulting massive data sets. The first-order variations seen in some of the parameters are attributed to changes from dry to wet conditions in the shallow subsurface due to the seasonal rainfall, which affects the source function of the vibrator. Corrections have been devised for these source-specific variations. Secular variations not obviously coupled to seasonal near-surface changes are also seen in some localized time intervals within the 20-sec records. The most striking of such changes is a progressive travel-time decrease at rates of 3 to 7 msec / year seen for late arrivals (7 to 11 sec travel time) on at least five paths into station VCA, which sample the region southeast of the anticipated epicenter at Middle Mountain. This anomaly appears to be genuine and is now the subject of intensified study. In the same general area, along the fault in the southwestern block, the direct S wave is clearly split, with the faster of the split phases polarized parallel to the fault zone, a result in agreement with that from the VSP survey in the Varian well on the northeast side of the fault.
First two-station Rayleigh-wave velocity measurements for the northern Iberian region
Authors: Payo G. , Corchete V. , Badal J. , Seron F. , Canas J. A. , Pujades L.
Published: September 2021

Up to now, dispersion analysis of surface waves across the Iberian Peninsula and adjacent zones has been based on analog data recorded at the long-period Iberian stations. Also, the northern region of the peninsula has never been investigated due to the lack of seismological stations. With the ILIHA data set now available, it is possible to investigate the northern part of Iberia from quality digital records. To efficiently remove higher-mode interference and to improve isolation of the fundamental-mode Rayleigh wave from the seismograms, time-variable filtering is employed. Once the signal is filtered, multiple filtering is then used to compute group velocities at each station. The interstation Rayleigh-wave group velocity can thus be easily calculated. Frequency-domain Wiener deconvolution is used to determine the interstation phase velocity. We carried out inversion of velocity dispersion data containing both Rayleigh-wave phase velocities and group velocities according to the generalized inversion theory by means of the stochastic inverse operator. The theoretical 2-D Earth models determined by joint inversion allow us to obtain for the first time the distribution of the shear-wave velocity both laterally and with depth for the northern Iberian region, and to emphasize the main features of the crust-mantle structure of this area.
Earthquake site response estimation: A weak-motion case study
Authors: Field E. H. , Jacob K. H. , Hough S. E.
Published: September 2021

Using weak-motion recordings of aftershocks of the 1989 Loma Prieta earthquake recorded in Oakland, California, near the failed Nimitz Freeway, two methods have been applied to estimate the site response of an alluvium site and three mud-over-alluvium sites. The first estimate is the traditional spectral ratio, and the second utilizes the cross spectrum. Recordings obtained at a nearby bedrock site are used as estimates of the sediment site input motions. While the two site response estimates produce similar peaks and troughs, there is an approximate factor of 2 difference in amplitudes. This discrepancy is evidence that there is a much greater level of noise than would be expected from the pre-event ambient noise. We interpret this as signal-generated noise produced by scattering from heterogeneities, which causes the true sediment site input to differ significantly from the bedrock site recording. Given this level of noise, the cross-spectrum estimate suffers a severe downward bias (by a factor of 2 in this study) and should probably not be used when the input motion is estimated from a bedrock site recording. The spectral-ratio estimates are relatively unbiased, but the level of noise introduces a large degree of uncertainty. Therefore, inferences about site response from individual spectral ratios should probably be avoided. On the other hand, ensemble averages of the estimates significantly reduce the scatter to reveal resonances that agree quite well in frequency and overall shape with those of one-dimensional models whose parameters were determined independently. A discrepancy of higher observed amplitudes than predicted by theory remains unexplained but most likely results from the effects of boundary layer topography, which are not accounted for by the simple one-dimensional models.
Aftershock stress release along active fault planes of the 1984 Round Valley, California, earthquake sequence applying a time-domain stress drop method
Authors: Smith Kenneth D. , Priestley Keith F.
Published: September 2021

The 23 November 1984 ML 5.8 Round Valley earthquake is one in a series of moderate (ML ≈ 6) earthquakes to have occurred in the Bishop-Mammoth Lakes, California, area since 1978. This earthquake and its aftershock sequence occurred within a dense seismic network, and hypocentral location quality is excellent. In a previous study, we determined that the Round Valley sequence involved faulting on a conjugate set of fault planes; one, a near-vertical plane striking N30°E, the mainshock fault plane showing principally left-lateral strike-slip motion, and another subperpendicular to the mainshock fault plane striking N40°W and dipping 55°NE, exhibiting dominantly right-lateral strike slip. This conjugate fault plane conforms to a postulated extension of the Hilton Creek fault and is the only significant activity on this structure in the 12-year Bishop-Mammoth Lakes earthquake sequence. Source dimensions and stress drops for 87 aftershocks (ML 2.8 to 4.2) of the Round Valley sequence have been determined using an adaptation of the initial P-wave pulse width time-domain deconvolution technique of Frankel and Kanamori (1983). The aftershock sequence is confined to a limited volume of crust. We have shown that site and instrument effects and not whole-path attenuation control the minimum pulse widths for this limited region. The determination of a site minimum pulse width, rather than a minimum pulse width for each source receiver pair as in the Frankel and Kanamori study, makes the deconvolution procedure practical for processing the large numbers of events in an aftershock sequence. With the large data set available for the Round Valley aftershock sequence, patterns of the stress drop along the active fault planes can be seen in detail. Source radii systematically increase with magnitude from about 100 m for events near magnitude 3.0 to 500 m for events near magnitude 4.0. Static stress drops range from 10 to 200 bars and are not strongly correlated with magnitude or depth. The stress release pattern reveals a broad stress drop low (Δσ ≈ 10 bars) for aftershocks within the mainshock fault plane that is consistent with other evidence of the rupture surface of the Round Valley mainshock. Higher stress release occurs above and below the mainshock rupture surface and on the shallower, conjugate fault plane. Further distant from the rupture surface of the mainshock, stress drops decrease to average values. On the conjugate fault surface, stress drops are seen to be high in areas that may be interpreted as “off-fault” clusters with respect to the mainshock rupture surface.
Discrimination of explosions from simultaneous mining blasts
Authors: Smith Albert T.
Published: September 2021

Large mining blasts can complicate the identification and discrimination of small underground nuclear explosions and may offer evasion opportunities. Mining blasts typically show a unique spectral signature: spectral reinforcements associated with time-delayed detonations between adjacent shot holes or rows of shots. Discrimination of a nuclear detonation that is simultaneous with a mining blast must depend upon recognizing significant spectral or waveform abnormalities within seismic signals from the mining blasts. In this investigation, large, simultaneous detonations within mining blasts are simulated for observed explosions from the Mesabi Range in Minnesota and for a series of quarry blasts at the Kaiser Permanente Quarry in Cupertino, California, which included a simultaneous detonation conducted by Lee et al. (1989). The Mesabi explosions are examples of large, ripple-fired blasts with known blast patterns (Smith, 1989).

The models suggest that a large, single, deeply buried explosion dominates the waveform signature if it contains more than 5 to 15% of the total explosive in the mining blast. Spectral signatures of these combined explosions still show periodicities characteristic of ripple firing; however, their amplitude is greatly reduced. Inclusion of a deep simultaneous shot accentuates the high-frequency spectrum. If single explosions are sufficiently close to the combined quarry blast, their application as empirical Green's functions can isolate the simultaneous explosion within the blast. If empirical Green's functions are within 0.5 km of quarry blasts, individual explosions can be retrieved if delays are 100 msec between shot holes and signals extend to 40 Hz. Identification of large, simultaneous detonations within a blast may depend upon knowledge of the mine's blasting practices and its variability from blast to blast.
Tomography without rays
Authors: Ammon Charles J. , Vidale John E.
Published: September 2021

We present two new techniques for the inversion of first-arrival times to estimate velocity structure. These travel-time inversion techniques are unique in that they do not require the calculation of ray paths. First-arrival times are calculated using a finite-difference scheme that iteratively solves the eikonal equations for the position of the wavefront. The first inversion technique is a direct extension of linearized waveform inversion schemes. The nonlinear relationship between the observed first-arrival times and the model slowness is linearized using a Taylor series expansion and a solution is found by iteration. For a series of two-dimensional numerical tests, with and without random noise, this travel-time inversion procedure accurately reconstructed the synthetic test models. This iterative inversion procedure converges quite rapidly and remains stable with further iteration. The second inversion technique is an application of simulated annealing to travel-time topography. The annealing algorithm is a randomized search through model space that can be shown to converge to a global minimum in well-posed problems. Our tests of simulated annealing travel-time topography indicate that, in the presence of less than ideal ray coverage, significant artifacts may be introduced into the solution. The linearized inversion scheme outperforms the nonlinear simulated annealing approach and is our choice for travel-time inversion problems. Both techniques are applicable to a variety of seismic problems including earthquake travel-time tomography, reflection, refraction/wide-angle reflection, borehole, and surface-wave phase-velocity tomography.
Stratigraphic evidence for seven meters of dextral slip on the San Andreas fault during the 1857 earthquake in the Carrizo Plain
Authors: Grant Lisa B. , Sieh Kerry
Published: September 2021

The smallest geomorphic offsets along a 35 km section of the San Andreas fault in the Carrizo Plain vary from 7 to 10 m. Our three-dimensional excavation of alluvial deposits a few km southeast of Wallace Creek confirms that at least 6.6 to 6.9 m of dextral slip occurred there during the latest large earthquake, in 1857. Dates on detrital charcoal suggest that the last event prior to the 1857 earthquake occurred before a date within the range A.D. 1305 to 1623. The 3-m range in smallest offsets along this portion of the fault may reflect either a 3-m variation in slip along the San Andreas in 1857, or 2 to 3 m of slip during an event prior to 1857. Observations made after the recent Landers earthquake are compatible with the hypothesis of large, local variations in slip during a single earthquake, but do not explain the cause of such variations. Off-fault dextral rotations would be one plausible explanation. However, paleoseismic data in the Carrizo Plain are too sparse to allow rejection of the alternative hypothesis that slip in the event prior to 1857 was only 2 to 3 m, an amount of slip which would be several times too small to fit a time-predictable model.
Direct boundary element method for dynamics in a half-space
Authors: Nowak Paul S. , Hall John F.
Published: September 2021

An application of the direct boundary element method for solving the response of a linearly elastic half-space with a canyon cut into the surface is presented. This approach uses source solutions for an undamped half-space where the resulting singular integral equations are solved directly without adding any artificial damping. Solutions for the displacements on the canyon surface reveal an artificial resonance phenomenon when solving the exterior problem in the frequency domain. The use of an additional source loading in the boundary element method is shown to eliminate these resonances and yield accurate results. A method for solving the Rayleigh waves generated on the surface of the half-space caused by the canyon is shown.
Crustal anisotropy from local observations of shear-wave splitting in West Bohemia, Czech Republic
Authors: Vavryčuk Václav
Published: September 2021

The results of a systematic study of shear-wave splitting, observed in the three-component digital seismograms of about 400 local microearthquakes that occurred during the 1985/1986 West-Bohemian earthquake swarm are presented. Two shear phases, polarized nearly perpendicularly in horizontal projection, were observed at all five stations of the local seismic network for most of the events. The polarization of the faster shear wave is aligned in the WNW-ESE direction, regardless of the mechanism and hypocenter location of the individual events, and coincides with the direction of the maximum horizontal compressive tectonic stress in the region. These effects are interpreted in terms of effective anisotropy of the upper crust in the region. The maximum delay time between the split shear waves is 0.15 sec and corresponds to 6% anisotropy.

Directional variation of the delay time was observed at the nearest station (epicentral distance ≈ 5 km). This variation was compared with functions predicted by theoretical models of cracked media to resolve which model is the most appropriate for crustal anisotropy. Surprisingly, no agreement could be found for Hudson's model of dry or water-filled parallel cracks that is widely used by many authors. A successful fit was obtained for Schoenberg — Douma's model of a medium with parallel fractures. The fracture normal pointed N31°E, and the optimum fracture parameters were EN = 0.4 and ET = 0.02. The theoretically calculated fractures are aligned parallel to the tectonic stress direction and do not correspond to any real tectonic lines in the region.
Site effect evaluation using spectral ratios with only one station
Authors: Lermo Javier , Chávez-García Francisco J.
Published: September 2021

The spectral ratio technique is a common useful way to estimate empirical transfer function to evaluates site effects in regions of moderate to high seismicity. The purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station. The technique, originally proposed by Nakamura to analyze Rayleigh waves in the microtremor records, is presented briefly and it is discussed why it may be applicable to study the intense S-wave part in earthquake records. Results are presented for three different cities in Mexico: Oaxaca, Oax., Acapulco, Gro., and Mexico City. These cities are very different by their geological and tectonic contexts and also by the very different epicentral distances to the main seismogenic zones affecting each city. Each time we compare the results of Nakamura's technique with standard spectral ratios. In all three cases the results are very encouraging. We conclude that, if site effects are caused by simple geology, a first estimate of dominant period and local amplification level can be obtained using records of only one station.
Estimation of strong ground motions in meizoseismal region of the 1976 Tangshan, China, earthquake
Authors: Dan K. , Ishii T. , Ebihara M.
Published: September 2021

The 1976 Tangshan, China, earthquake of MS 7.8 killed 242,000 persons, seriously injured 164,000 persons, and caused direct property losses totaling 8 billion Yuan Ren Min Bi (US $4.3 billion). Few investigations have been performed to estimate the characteristics of the strong ground motions in the meizoseismal region of this earthquake using either seismological or earthquake engineering approaches.

In this paper, the observed far-field accelerograms of the mainshock are simulated by using the records of the second largest aftershock of MS 6.9 as Green's functions in order to obtain appropriate fault models for the mainshock. The strong ground motions in the meizoseismal region of the mainshock are then estimated by using these fault models and the records of several aftershocks with a magnitude of about 5 which were observed at temporary stations in the damaged area. The results indicate that large horizontal acceleration responses over 1000 cm/sec2 and vertical acceleration responses over 2000 cm/sec2 acted on the structures with natural periods shorter than 0.1 sec in the wide meizoseismal region having a length of 90 km.
Seismic wave-field observations at a dense, small-aperture array located on a landslide in the Santa Cruz Mountains, California
Authors: Xu Zhengyu , Schwartz Susan Y. , Lay Thorne
Published: September 2021

A rectangular (4 by 5) array of short-period three-component seismometers with 15-m spacing was deployed to record several U.S. Geological Survey calibration explosions detonated around the Santa Cruz Mountains. The array was located at a site where an earlier station had recorded frequency-dependent polarized site resonances for aftershocks of the 1989 Loma Prieta earthquake. The site is on a hillside believed to be a landslide structure, with the near surface consisting of poorly sorted sediments and weathered rocks with dipping subsurface layers. The primary objective was to explore the site effects in this complex three-dimensional soft-rock environment, characteristic of much of the Loma Prieta source region. The direct P waves from four nearby (15 to 20 km) explosions at easterly azimuths from the array show counterclockwise arrival azimuth anomalies of 30° to 50°. These deflections are attributed to the presence of more than one dipping velocity contrast beneath the array, with dips of from 10° to 50° and dip directions generally toward the south. One such boundary may correspond to the landslide slip surface, and the presence of dipping velocity contrasts underlying the site is probably responsible for some of the observed directional site resonance. A slowness vector analysis demonstrates that arrivals early in the P coda have similar azimuthal anomalies, while later scattered arrivals come from many azimuths. Particle motions indicate that the more coherent arrivals in the coda are comprised of scattered P waves and Rayleigh waves, probably associated with scattering from the rough topography in the region. The coda displays greater spatial coherency along the hill strike than down the slope, consistent with a wedge-shaped landslide. The overall wave-field spatial coherence, CCC(f, Δx), decreases with increasing frequency, f, and spatial offset, Δx, and on average can be well represented by CCC(f, Δx) = e−cfΔx, with c = 0.6 km−1 Hz−1 for the vertical P wave in the first 1-sec window. This behavior is comparable to that found for previously studied hard-rock locations.
The great Mexican earthquake of 19 June 1858: Expected ground motions and damage in Mexico City from a similar future event
Authors: Singh S.  K. , Ordaz M. , Pérez-Rocha L.  E.
Published: September 2021

The description of the great earthquake of 19 June 1858 is unusual: damage and high intensities were reported both in the state of Michoacan and in Mexico City. Although a coastal epicenter for this earthquake cannot be ruled out, the reports agree better with an intermediate-depth (about 50 km), normal-faulting event in the subducted Cocos plate. A careful examination of the reports of this event and other normal-faulting events below the Mexican altiplano suggests that a likely location is 18.0 °N, 100.8 °W, near the epicenter of the 6 June 1964 (M7.3, H = 55 km) event. This location is 220 km SW of the city. The magnitude of the earthquake is estimated to be about 7.7. We synthesize expected ground motions in CU, a hill-zone site in the city, from an event similar to that of 1858, using records from the 23 May 1994 earthquake (18.0 °N, 100.6 °W, H = 50 km, M5.7) as an empirical Green's function and stress parameter, Δσ, of 50, 160, and 300 bar. The expected peak horizontal acceleration in CU of Δσ = 160 bar is about 30 gals. Similar acceleration was recorded in CU during the 1985, Michoacan earthquake (M8.0). We compute expected ground motions at many sites in Mexico City using empirical transfer functions and random vibration theory and compare these motions and the expected damage in the city with those from the 1985 Michoacan earthquake. Results show that the overall expected damage during the postulated earthquake is ⅔ and 1⅓ of that during the Michoacan earthquake for Δσ = 160 and 300 bar, respectively. A greater percentage of low-rise construction, which constitute about 80% of the total in the city, will be damaged during the postulated earthquake than during the Michoacan earthquake. The expected ground motions for Δσ = 50 bar are smaller at all periods than those from the Michoacan earthquake. As the present building code for Mexico City contemplates coastal earthquakes of magnitude greater than 8.0, the case of Δσ = 50 bar is not of interest in this article. This preliminary study suggests a need for a more careful evaluation of expected ground motion in the Valley of Mexico from the postulated earthquake and its impact on the current design spectra of the city.
Northridge earthquake rupture models based on the global positioning system measurements
Authors: Shen Zheng-Kang , Ge Bob X. , Jackson David D. , Potter David , Cline Michael , Sung Li-yu
Published: September 2021

We use global positioning system (GPS) data to study the rupture mechanism of the 1994 Northridge earthquake in southern California. We include data from 62 observation sites, of which two (Palos Verdes and Jet Propulsion Lab) are permanent GPS geodetic array (PGGA) sites. We use a grid-search scheme to study the range of single- and dual-plane, uniform and varied slip models consistent with the data. We find that in order to fit the geodetic data with a fault model whose primary fault patch is confined to a plane through the aftershocks, a secondary fault plane is required above the primary fault plane. The moment release of the secondary fault can be as large as 1.9 × 1018 N-m, 14% of the moment release of the primary fault. This result implies significant deformation in the shallow crust associated with the mainshock. Our preferred model has a 14 × 14 array of dislocation patches on a plane through the main aftershock cluster and a 5 × 6 array of patches in the hanging wall west of the epicenter. We estimate the displacements on the patches by linear inversion with a first-order smoothness constraint. The estimated displacements on the main fault for this model are confined to a simple region between depths of 5 and 18 km, in the interior of the modeled fault surface. The mainshock lies at the bottom of the aftershock zone, near which about 1-m slip is shown on our modeled fault surface. The maximum slip on the fault surface is about 2.2 m, located at 34.28° N, 118.55° W, and 12.4 km at depth. The seismic moment release estimate of 1.34 ± 0.15 × 1019 N-m on the main fault at the 95% confidence is consistent with the estimate from strong-motion studies.
Contribution of seismic tomography in moment-tensor inversions using teleseismic surface-wave spectra
Authors: Dufumier Hugues , Trampert Jeannot
Published: September 2021

The knowledge of lateral heterogeneities is crucial for path corrections in moment tensor inversions using surface waves. After some attempts to use regionalized Earth models for very long-period surface-wave moment-tensor inversions, recent tomographic Earth models offer the possibility to make short-period path corrections and therefore retrieve more reliable moment tensors for teleseismic earthquakes. First we try to evaluate the precision required for path corrections in comparison with source effects. Some selected Earth models are tested to evaluate how their results compare to those using multiple-frequency filtering techniques. Some real cases illustrate the sensitivity of moment-tensor solutions to the different path corrections, and it appears clearly that regionalized Earth models and tomographic models deduced from long-period data alone (greater than 150 sec) cannot lead to trustworthy broadband moment-tensor inversions. Recent tomographic models using phase velocities at much shorter periods (40 to 200 sec) offer a precision comparable to that of the multiple-frequency filtering technique. Both methods lead to acceptable source mechanisms, using a small number of stations, in more than two cases out of three. The use of recent global tomographic models based upon shorter-period surface waves might thus be a useful alternative to heavy multiple-frequency filtering techniques to automate source studies, especially for rapid determinations using a small number of stations.
Water-level changes in response to the 20 December 1994 earthquake near Parkfield, California
Authors: Quilty Eddie G. , Roeloffs Evelyn A.
Published: September 2021

We analyze co-seismic changes of water level in nine wells near Parkfield, California, produced by an MD 4.7 earthquake on 20 December 1994 in order to test the hypothesis that co-seismic water-level changes are proportional to co-seismic volumetric strain. For each well, a quantitative relationship between water level and volumetric strain can be inferred from water-level fluctuations due to earth tides and barometric pressure. The observed co-seismic water-level changes, which ranged from −16 to +34 cm, can therefore be compared with volumetric strain recorded by borehole strainmeters or calculated using a dislocation model of the earthquake. We were able to find a dislocation model of the earthquake rupture that predicts volumetric expansion at five of the six wells where water level fell co-seismically, as well as volumetric contraction at one of the two sites where water level rose. Strain predicted by the dislocation model is in good quantitative agreement with the strain inferred from water-level changes observed at four of the well sites, as well as strain recorded by three borehole strainmeters. Water-level changes at two more well sites correspond to strain somewhat greater than predicted by the model but agree in sign with model-calculated strains. At three of the well sites, however, water-level changes took place that cannot be explained as responses to co-seismic volumetric strain for any plausible dislocation model of the earthquake rupture. At two of these sites, one in and one near the San Andreas fault, large water-level drops are probably influenced by co-seismic fault creep. The third site has a history of large water-level rises in response to earthquakes at distances up to several hundred kilometers. This data set shows that co-seismic water-level changes in many wells are proportional to volumetric strain but that other wells exist in which different mechanisms dominate co-seismic response.
High-frequency radiation process during earthquake faulting—envelope inversion of acceleration seismograms from the 1993 Hokkaido-Nansei-Oki, Japan, earthquake
Authors: Kakehi Yasumaro , Irikura Kojiro
Published: September 2021

We investigate the process of high-frequency (1 to 10 Hz) radiation on the fault plane of the 1993 Hokkaido-Nansei-Oki, Japan, earthquake (MW = 7.5) from the envelope inversion of strong-motion acceleration seismograms. For the analysis, empirical Green's functions are used because theoretical approach is not available for such high frequencies. The source is modeled with two fault planes with different strike angles. The rupture process of this earthquake is very complex in terms of high-frequency wave generation. The rupture, which started on the northern fault plane, had a delay of about 10 sec or propagated very slowly between the northern and southern fault planes. High-frequency wave radiation is large at the northern and southern edges of the source region. Deceleration of rupture is also observed there. This is interpreted to be associated with stopping of rupture. Another high-frequency wave radiation area is found at the center of the northern fault plane, where discontinuity in the depth distribution of aftershocks suggests an existence of a barrier. The areas of high- and low-frequency wave radiation are not correlated. This is considered to result from the complexity of rupture process. We cannot distinguish between westward and eastward dip of the southern fault plane because of one-sided station distribution.
Identification and picking ofSphase using an artificial neural network
Authors: Wang Jin , Teng Ta-liang
Published: September 2021

An artificial neural network (ANN) algorithm has been applied to the automatic picking of local and regional S phase. For a set of local three-component seismic data, a variety of features for signal detection and phase identification were analyzed in terms of sensitivity and efficiency. Comparing the performance of each feature in discriminating the local S phases, four features were selected as input attributes of the ANNS-phase picker: (1) the ratio between short-term average and long-term average, (2) the ratio between horizontal power and total power, (3) autoregressive model coefficients, and (4) the short-axis incidence angle of polarization ellipsoid. The four attributes were calculated in the frequency band of 2 to 8 Hz with a 2.56-sec moving window. This choice of frequency band and window length is appropriate for local microearthquake monitoring. The results of preliminary training and testing with a set of local earthquake recordings show that the ANNS-phase picker can achieve a good performance in identification and onset-time estimation for local S phases. In overall result, 86% correct rate of phase identification has been achieved by the trained ANNS-phase picker, 74% of them are precisely picked with less than 0.10-sec onset time error. We believe that the method presented here is a promising approach to automatic phase identification and onset-time estimation.
More onMmax
Authors: Ward Steven N.
Published: September 2021

M max the maximum magnitude earthquake that a fault is likely to suffer, plays an important role in earthquake hazard estimation. Although observational evidence summarized in plots of characteristic earthquake magnitude (Mchar) versus fault length indicate that smaller faults produce lower magnitude events, an argument has been made that any fault regardless of its length should have Mmax near magnitude 8. The rationale for this argument charges that the contrary observational evidence stems from historical catalogs of limited extent and that it largely excludes nonconventional earthquakes in which several short and apparently disconnected fault segments fail simultaneously. This article addresses Mmax using computer models of rupture on faults of various strengths and configurations. Computer models have advantages in that (a) Mmax earthquakes always can be generated by forcing complete stress drop on fully stressed faults, thus avoiding the limitations of short historical catalogs, and (b) the circumstances necessary for the failure of several segments to contribute to a large Mmax can be investigated quantitatively. I find that for a strikeslip California environment, it is physically unlikely for an M 8 event to break less than 300 to 400 km of fault. Were this M 8 rupture to occur on as few as five independent segments, shear strength of the participating faults would have to be raised to implausible levels. If the fault segments are not independent and their coseismic stress fields interact, then amplifications in slip are possible without drastic increase in strength. The range of fault geometries where strong interactions and amplifications of stress occur, however, is very restricted, and discontinuous faults separated by even 5% of their length act more or less independently. Mmax earthquakes breading realistic-looking distributions of discontinuous faults rarely are more than 0.1-magnitude unit bigger than would be predicted from a moment summation based on the characteristic magnitude Mchar of each of the individual faults. A prudent course in hazard analysis differentiates Mmax from Mchar allowing Mmax to be 0.2 to 0.3 units larger than Mchar but not automatically equal to 8.
Simulation-based comparison of four site-response estimation techniques
Authors: Coutel Fabien , Mora Peter
Published: September 2021

Recent earthquakes have triggered renewed interest to understand better earthquake site response. Most of the studies comparing various techniques for estimating site response were based on real data (from earthquakes, nuclear blasts, and seismic noise). A theoretical approach, using synthetic data generated with the pseudospectral method, is used to compare four site-response estimation techniques. The limits of applicability of each method were determined by modeling microtremors and incoming SV waves (with different incidence angles) and analyzing the site amplifications. The first two techniques investigated consist of dividing the spectrum of the horizontal motion at a site by that of a reference site using either incident S waves or microtremors. The latter was unable to reveal either the resonant frequencies or peak amplitudes in any cases. The two other techniques are based on the horizontal-to-vertical (H/V) spectral ratio using S waves or microtremors. These techniques were found to reveal at least the fundamental resonant frequency and amplitude (former method only) within 10% error, in the case of simple geology (flat layers). However, the results show that these techniques are unable to take into account 2D effects such as focusing effects and basin-edge effects and yield unreliable or incorrect results in such cases.
Three-dimensional finite-difference modeling of the San Andreas fault: Source parameterization and ground-motion levels
Authors: Graves Robert W.
Published: September 2021

Olsen et al. (1995) recently simulated an Mw 7.75 earthquake on the San Andreas fault, predicting long-period (T > 2.5 sec) ground velocities of 140 cm/sec in the Los Angeles basin, about 60 km from the fault. These motions are much larger than estimates derived from empirical relations or other numerical simulations. Standard area-magnitude relations predict that the 170 × 16 km fault used in the simulations would produce an Mw 7.5 earthquake, giving a moment of 2.0 × 1027 dyne-cm, which is 2.4 times smaller than the moment used by Olsen et al. (1995). Further, self-similar scaling predicts a rise time of 3 sec for an Mw 7.75 event and 2.2 sec for an Mw 7.5 event. The filtered impulse slip function used by Olsen et al. (1995) has an effective rise time of 1.6 sec, yielding a response that is about 2 times larger than expected for periods less than 5 sec. This combination of high seismic moment and short rise time, along with the use of a uniform slip distribution, leads to the extreme ground-motion levels predicted by Olsen et al. (1995).

To quantify the sensitivity of the long-period ground-motion response to source parameterization, we have performed 3D finite-difference (FD) simulations using various combinations of seismic moment, source rise time, and slip heterogeneity. These calculations incorporate the same grid dimensions, fault size, and bandwidth employed by Olsen et al. (1995). With a moment of 2.0 ՠ1027 dyne-cm, a rise time of 2 sec, and a smoothly heterogeneous slip distribution, we simulate peak long-period ground velocities of 155 cm/sec in the near-fault region and 40 cm/sec in the Los Angeles basin. These values are much closer to (although still higher than) empirical predictions. A uniform slip distribution produces the largest peak motions, both in the near-fault region and in the Los Angeles basin, whereas a rough asperity slip distribution noticeably reduces the maximum near-fault ground velocities. Our results indicate that the accurate simulation of long-period ground motions requires a realistic source parameterization, including appropriate choices of seismic moment and rise time, as well as the use of spatial and temporal variations in slip distribution.
Secession Tendencies of the Modern Process of State Formation
Authors:   Sociological Institute of the Russian Academy of Sciences, Korotkova Olga A. ,   Shevchenko Transnistrian State University, Osipova Svetlana A. ,   Shevchenko Transnistrian State University
Published: September 2021

An analysis of some of the events taking place in the modern socio-political environment shows that the processes of state formation, the preconditions for which were laid down in the 20th century, are acquiring new impulses and continuation in the modern world. At the same time, the political technologies used in the construction of new states are diverse, but the most common form of the state formation process in the modern world is secession.
Trending Models of Solid Municipal Waste Management in Russia
Authors:   Financial University Under the Government of the Russian Federation, Kozlov Vladislav M.
Published: September 2021

The world community is increasingly concerned about environmental issues. Disposal of municipal solid waste is one of the critical components of the system for improving and maintaining the current level of the environmental situation both at the national and international levels. Foreign countries have been developing technologies and models for organizing the disposal of solid utility costs for a long time; in Russia, this trend has become popular only after the beginning of the 21st century. The paper discusses a model for the disposal of municipal solid waste in the European Union, the United States, developing countries in Asia and Africa. The research methodology consists in comparing Russian and foreign advanced trends in the management of fixed utility costs.
Working Conditions of Moscow Teachers During a Pandemic
Authors:   Moscow City Organization of the Trade Union of Public Education and Science Workers of the Russian Federation, Ivanova Marina A. , Cherniakov Fedor A. ,   Russian State Social University
Published: September 2021

The article presents the results of a series of sociological surveys conducted by the Moscow Trade Union Organization using an online questionnaire in the period from September to December 2020. The purpose of this publication is to identify and analyze the main difficulties faced by Moscow teachers when switching to remote mode of work. The corresponding results were presented to the management of the Department of Education and Science of the City of Moscow for making decisions on reducing the workload of teachers. The novelty of the study is confirmed by the low level of knowledge at the time of publication of the phenomena associated with the working conditions of teachers during the pandemic.