bluuebunny/crossref_metadata_2025 huggingface dataset:
Earthquake mechanisms from linear-programming inversion of seismic-wave amplitude ratios
Authors: Julian Bruce R. , Foulger G. R.
Published: September 2021

The amplitudes of radiated seismic waves contain far more information about earthquake source mechanisms than do first-motion polarities, but amplitudes are severely distorted by the effects of heterogeneity in the Earth. This distortion can be reduced greatly by using the ratios of amplitudes of appropriately chosen seismic phases, rather than simple amplitudes, but existing methods for inverting amplitude ratios are severely nonlinear and require computationally intensive searching methods to ensure that solutions are globally optimal. Searching methods are particularly costly if general (moment tensor) mechanisms are allowed. Efficient linear-programming methods, which do not suffer from these problems, have previously been applied to inverting polarities and wave amplitudes. We extend these methods to amplitude ratios, in which formulation on inequality constraint for an amplitude ratio takes the same mathematical form as a polarity observation. Three-component digital data for an earthquake at the Hengill-Grensdalur geothermal area in southwestern Iceland illustrate the power of the method. Polarities of P, SH, and SV waves, unusually well distributed on the focal sphere, cannot distinguish between diverse mechanisms, including a double couple. Amplitude ratios, on the other hand, clearly rule out the double-couple solution and require a large explosive isotropic component.
Numerical simulations of wave propagation in heterogeneous wave guides with implications for regional wave propagation and the nature of lithospheric heterogeneity
Authors: Wagner Gregory S.
Published: September 2021

I present results from elastic finite-difference simulations of regional wave propagation conducted in an effort to characterize, in a statistical sense, the nature of lithospheric heterogeneities required to generate scattered wave fields with characteristics consistent with those observed in regional array data. In particular, regional P, S, and Lg wave trains that are comprised not of the occasional coherent deterministic phase emersed in randomly scattered coda, but of a continuous succession of coherent forward-scattered arrivals. My modeling suggests that lithospheric heterogeneities should be parameterized using spatially anisotropic correlation functions. Models containing spatially isotropic heterogeneities inhibit the extent to which energy is forward scattered and trapped in the crustal wave guide and, consequently, produce regional wave fields whose characteristics are inconsistent with array observations. Models containing spatially anisotropic heterogeneities—which preferentially forward scatter energy that is subsequently trapped in the crustal wave guide—produce wave fields whose characteristics are consistent with regional array observations and provide intuitively appealing representations of subsurface structure.
Source characteristics of small to moderate earthquakes in the Kanto region, Japan: application of a new definition of theS-wave time window length
Authors: Watanabe Kazutoshi , Sato Haruo , Kinoshita Shigeo , Ohtake Masakazu
Published: September 2021

The S wave from a local earthquake generally consists of several pulses, in contrast to a simple pulse of P wave. This results in a large uncertainty in the estimation of seismic moment and wave energy when the window length for S-wave analysis is not chosen properly. In this study, we propose a new method to define objectively an appropriate time window length of the S wave for estimation of source characteristics based on the S-wave to P-wave energy ratio for a point shear dislocation source. Analyzing waveform data of 97 local earthquakes in the Kanto region, Japan, whose magnitudes M range from 3.3 to 6.0, we obtained simple equations for predicting the time window length of the S-wave TS as a function of earthquake magnitude or pulse width of P-wave TP; we got TS ≈ 1.8 TP for the relation between TP and TS. Applying the S-wave window thus defined, we estimated seismic energy E, seismic moment M0, and corner frequency fc for both P and S waves. Regression analysis of those parameters revealed (1) our method to define TS is confirmed by the fact that the seismic moment determined from P and S waves are consistent; (2) in the range of 1014 < M0 < 1018 (N·m), M0 is almost proportional to f−3c both for P and S waves; (3) the value of E/M0 is 2.7 ∼ 4.0 × 10−5; and (4) breakdown of the scaling relation is seen at M ≦ 4.
Rocking and overturning of precariously balanced rocks by earthquakes
Authors: Shi Baoping , Anooshehpoor Abdolrasool , Zeng Yuehua , Brune James N.
Published: September 2021

Precariously balanced rocks in seismically active regions may be used to provide constraints on probabilistic estimates of strong ground motion. A computer program is developed to study the rocking response of models of precariously balanced rocks when subjected to various levels of ground accelerations. The computer code, validated by comparing the numerical and experimental results for simple-shaped rigid blocks, is used to study the rocking response of rectangular blocks with various aspect ratios to the El Centro strong ground motion recording of the 18 May 1940 Imperial Valley earthquake. Results show that during earthquakes, the horizontal components of ground motion play a dominant role in overturning rocking objects. The rederived and corrected Housner's equation (1963) could be used to estimate the minimum overturning acceleration of precarious rocks at different frequencies by measuring the quasi-static toppling acceleration in the field.
Shallow velocity structure and poisson's ratio at the Tarzana, California, strong-motion accelerometer site
Authors: Catchings R.  D. , Lee W.  H. K.
Published: September 2021

The 17 January 1994, Northridge, California, earthquake produced strong ground shaking at the Cedar Hills Nursery (referred to here as the Tarzana site) within the city of Tarzana, California, approximately 6 km from the epicenter of the mainshock. Although the Tarzana site is on a hill and is a rock site, accelerations of approximately 1.78 g horizontally and 1.2 g vertically at the Tarzana site are among the highest ever instrumentally recorded for an earthquake. To investigate possible site effects at the Tarzana site, we used explosive-source seismic refraction data to determine the shallow (<70 m) P-and S-wave velocity structure. Our seismic velocity models for the Tarzana site indicate that the local velocity structure may have contributed significantly to the observed shaking. P-wave velocities range from 0.9 to 1.65 km/sec, and S-wave velocities range from 0.20 and 0.6 km/sec for the upper 70 m. We also found evidence for a local S-wave low-velocity zone (LVZ) beneath the top of the hill. The LVZ underlies a CDMG strong-motion recording site at depths between 25 and 60 m below ground surface (BGS). Our velocity model is consistent with the near-surface (<30 m) P- and S-wave velocities and Poisson's ratios measured in a nearby (<30 m) borehole. High Poisson's ratios (0.477 to 0.494) and S-wave attenuation within the LVZ suggest that the LVZ may be composed of highly saturated shales of the Modelo Formation. Because the lateral dimensions of the LVZ approximately correspond to the areas of strongest shaking, we suggest that the highly saturated zone may have contributed to localized strong shaking. Rock sites are generally considered to be ideal locations for site response in urban areas; however, localized, highly saturated rock sites may be a hazard in urban areas that requires further investigation.
Is the quiescence of major earthquakes (M ≧ 7.5) Since 1952 in the Himalaya and northeast India real?
Authors: Satyabala S.  P. , Gupta Harsh  K.
Published: September 2021

The earthquake catalog for the Himalaya and the northeast India regions (latitude 20°N to 38°N and longitude 75°E to 100°E, comprising of portions of the Himalaya and Arakan Yoma fold belt) for the period 1897 to May 1995 shows that there have been 14 major earthquakes (M ≧ 7.5) during the period 1897 to 1952 and no major earthquakes after 1952. To investigate the possibility that this quiescence is an artifact of nonuniform magnitude scales over the period of study, we examined three global catalogs and found that there is indeed an absence of major earthquakes after 1952.
Source complexity of the 1994 Northridge earthquake and its relation to aftershock mechanisms
Authors: Thio Hong Kie , Kanamori Hiroo
Published: September 2021

We determined the source process of the 1994 Northridge earthquake in relation to the aftershock mechanisms. To study the source complexity of the mainshock, we inverted the P and SH waveforms recorded by the IRIS and IDA/IRIS networks, using the method of Kikuchi and Kanamori (1991) in which the rupture is represented by a series of discrete subevents with varying mechanisms. The waveforms show that the rupture consisted of several subevents with about 2 sec in between. Our solution consists of three subevents with essentially the same mechanism, viz., strike, dip, and slip of 130°, 42°, and 116°, respectively. The first subevent occurred at a depth of about 19 km, followed after 2 sec by the second and largest subevent at a depth of 17 km and the third subevent again 2 sec after the second at a depth of about 13 km. The total moment from the body waves of this sequence is about 1.1 × 1026 dyne · cm (Mw = 6.6) with a source duration of 7 sec.

The large depths of these subevents explain the lack of any surface rupture. Furthermore, the upward propagation of the subevents is consistent with the depth of the hypocenter and the distribution of the aftershocks, which are shallower and more northerly than the mainshock hypocenter.

The aftershocks were analyzed using data from the TERRAscope network. We inverted short-period surface waves to determine the moment tensor for 70 events with Mw > 3.5. The aftershocks can be grouped into three regions based on the mechanisms: the eastern part of the aftershock zone, where we find thrust events with mechanisms very similar to the main event; a central area with predominantly strike-slip events; and an area to the west, where we find oblique thrust events but with more northerly P axes than in the eastern region. This distribution suggests that the fault system on which the Northridge earthquake occurred is segmented and that the extent of the Northridge rupture is controlled by a change in geometry of the fault. We find a high stress drop (270 bar) for the mainshock; we propose that the changes in the fault geometry prevented a slip pulse from propagating, thereby causing a high ratio of slip-to-rupture length.
Effects of the hanging wall and footwall on ground motions recorded during the Northridge earthquake
Authors: Abrahamson N. A. , Somerville P. G.
Published: September 2021

Systematic differences in ground motion on the hanging wall and footwall during the Northridge earthquake are evaluated using empirical data. An empirical model for the hanging-wall effect is developed for the Northridge earthquake. This empirical model results in up to a 50% increase in peak horizontal accelerations on the hanging wall over the distance range of 10 to 20 km relative to the median attenuation for the Northridge earthquake. In contrast, the peak accelerations on the footwall are not significantly different from the median attenuation over this distance range. Recordings from other reverse events show a similar trend of an increase in the peak accelerations on the hanging wall, indicating that this systematic difference in hanging-wall peak accelerations is likely to be observed in future reverse events.
Lifeline utilities performance in the 17 January 1994 Northridge, California, earthquake
Authors: Lund Le Val
Published: September 2021

Lifelines performed an important role in emergency response and restoring the community after the 17 January 1994 Northridge earthquake. Telecommunications, radio, and electronic media played a valuable role in directing emergency response for essential services, directing the repairing of damaged lifelines and informing the public of vital information. Water supply was necessary where available for public fire protection. Highways and roads were used to move rescue, repair, fire, and medical teams and their supplies and equipment to the damaged areas. Although the Northridge earthquake was severe, the lifeline disruption represented a relatively small percentage of inconvenience to the total population served within the total earthquake impact area. The 1994 Northridge event showed improvement in the seismic performance of lifeline facilities and equipment installed and built, under modern seismic codes, since the 1971 San Fernando earthquake.
Shallow crustal structure from short-period Rayleigh-wave dispersion data in southwestern Taiwan
Authors: Chung Jen-Kuang , Yeh Yeong Tein
Published: September 2021

Strong ground motions in the period range of 1 to 5 sec observed in Taiwan in a dense network during the Tapu earthquake (ML = 5.8) of 15 December 1993 were dominated by fundamental-mode Rayleigh waves. The shallow crustal structure beneath this network was determined from the group velocity dispersion data using standard inversion techniques. Results show that an alluvial layer with a thickness of only about 160 m exists over the sedimentary structure in the western coastal plain. A clear lateral variation in the shear-wave velocity along a cross section perpendicular to the western structural grain (Ho, 1982) was resolved.

For the purpose of retrieving the ground motions to confirm the proposed model, forward modeling with a two-dimensional finite-element method was used and agreed well with the observed seismograms when slightly lower velocities than those of the inversions were used. It is concluded that the velocity model estimated using group velocity data cannot definitively correspond to the real one at such short distances and in such a complex structure. On the basis of quantitative simulations, a very soft surface layer must be crucial in the interpretation of the slow wave trains with long duration and the rare prograde particle motions. The results suggest that these surface waves may be generated by the conversion of body waves at the boundary of the western coastal plain and foothills.
RegionalLgattenuation for the continental United States
Authors: Benz Harley M. , Frankel Arthur , Boore David M.
Published: September 2021

Measurements of the Fourier amplitude spectra of Lg phases recorded at high frequency (0.5 to 14.0 Hz) by broadband seismic stations are used to determine regional attenuation relationships for southern California, the Basin and Range Province, the central United States, and the northeastern United States and southeastern Canada. Fourier spectral amplitudes were measured every quarter octave from Lg phases windowed between 3.0 and 3.7 km sec−1 and recorded in the distance range of 150 to 1000 km. Attenuation at each frequency is determined by assuming a geometrical spreading exponent of 0.5 and inverting for Q and source and receiver terms. Both southern California and the Basin and Range Province are well described by low Lg Q and frequency-dependent attenuation. Lg spectral amplitudes in southern California are fit at low frequencies (0.625 to 0.875 Hz) by a constant Lg Q of 224 and by a frequency-dependent Lg Q function Q = 187−7+7f0.55(±0.03) in the frequency band 1.0 to 7.0 Hz. The Basin and Range Province is characterized by a constant Lg Q of 192 for frequencies of 0.5 to 0.875 Hz and by the frequency-dependent Lg Q function Q = 235−11+11f0.56(±0.04) in the frequency band 1.0 to 5.0 Hz. A change in frequency dependence above 5.0 Hz is possible due to contamination of the Lg window by Pn and Sn phases. Lg spectral amplitudes in the central United States are fit by a mean frequency-independent Lg Q of 1291 for frequencies of 1.5 to 7.0 Hz, while a frequency-dependent Lg Q of Q = 1052−83+91(f/1.5)0.22(±0.06) fits the Lg spectral amplitudes for the northeastern United States and southeastern Canada over the passband 1.5 to 14.0 Hz. Attenuation measurements for these areas were restricted to frequencies >1.5 Hz due to larger microseismic noise levels at the lower frequencies.
Bandpass filtering to reduce noise on seismograms: Is there a better way?
Authors: Douglas A.
Published: September 2021

Short-period (SP) P waves at low signal-to-noise ratios (S/Ns) usually appear as signals of about 1 Hz or higher frequencies riding on background noise with frequencies of 0.5 Hz or less. Often bandpass filtering is used to further attenuate the noise relative to the signal to improve the S/N. However, bandpass filtering introduces significant distortion into the signal. Here the use of optimum filters is described, such filters being constructed using autocorrelation functions of the signal and noise derived from noise and signal models. The autocorrelation of the impulse response of the recording system is used to describe the signal. For the noise, a tapered cosine wave is used that has the frequency of the main noise component. To compute the filter, the relative sizes of the signal and noise correlations should be proportional to the square of the S/N. Now, the noise can be estimated by subtracting the estimated signal from the observed seismogram. This property can be used to decide by trial and error on the S/N. The optimum S/N is found by gradually reducing the trial ratio starting from too high a value, until the noise has the amplitude expected and there is no evidence of significant signal in the noise estimate. Examples are given showing how the method can lift observed signals off the background noise with less distortion to the signal than usually occurs with bandpass filtering.
Inversion ofRgwaveforms recorded in southern Spain
Authors: Navarro Manuel , Corchete Victor , Badal José , Canas José A. , Pujades Luis , Vidal Francisco
Published: September 2021

Group velocity dispersion measurements of Rg waves generated either by blasts or by local earthquakes are used to investigate the shallow crustal structure of Almería (southern Spain). In principle, the usable frequency range of 250 to 2000 mHz allows determination of structures to depths of about 4 km. For this purpose, the main operations are a detailed dispersion analysis of high-frequency Rayleigh waves propagating along very short paths and the inversion of Rg-wave group velocities. A total of 21 seismic events were studied. These events had small magnitudes (2.0 to 2.5 approximately) and very shallow focal depths (about 100 m) and were taken from a set of 214 events that occurred in 1991 during a Spanish-Italian seismic experiment. The events were recorded at seven single-component stations belonging to the Regional Seismic Network of Andalucía at approximate distances of between 15 and 57 km from the source. These events were grouped into six seismic sources according to specific criteria. We used digital filtering techniques providing a significant improvement in signal-to-noise ratio to determine ray-path group velocities, and we inverted dispersion data via generalized inversion. In order to obtain refined dispersion data, we have carried out a further regionalization of group velocities, and thus six small subregions have been resolved in group velocities. The highest group velocity values, from 1.93 to 2.25 km sec−1, correspond to the Filabres mountain range, which is an area containing materials of the Nevado-Filábride complex of Paleozoic and Triassic age. On the other hand, low velocity values, between 1.39 and 1.56 km sec−1, correspond to the Alhamilla mountain range, which belongs to the Alpujárride complex and contains conglomerates of the Cambrian and Tortonian periods. The velocities obtained for the neogene-quaternary basin of the Andarax river, with materials of the Tortonian and Pliocene periods, are also very low, between 1.35 and 1.68 km sec−1. We inverted the regionalized group velocities in order to obtain the shear velocity structure of the region for depths down to 4 km. According to the regional Earth models that we obtained, we find clear variations in velocity both laterally and vertically for several zones with different composition. The Filabres mountain range shows high shear velocity values: 2.14 to 2.83 km sec−1. In the opposite end, we have the Andarax basin that presents the lowest shear velocity values, consistent with its sedimentary structure: 1.56 to 2.55 km sec−1. Intermediate shear-wave velocities characterize the remaining regions: the Tabernas-Sorbas basin, the Gádor mountain range, and the volcanic region of Nijar-Cape of Gata. Although the relationship between lateral changes in Rg dispersion and geologic structure may not be straightforward, in this study, we have observed a correlation between those changes and the sharply contrasting geology between adjacent geological formations.
Empirical observations regarding reverse earthquakes, blind thrust faults, and quaternary deformation: Are blind thrust faults truly blind?
Authors: Lettis William R. , Wells Donald L. , Baldwin John N.
Published: September 2021

Active thrust faults pose a significant seismic hazard worldwide. Many of these faults include “blind” thrusts, where the propagating fault tip does not reach the Earth's surface, and “buried” faults, where the geomorphic expression of the fault is obscured by subsequent sedimentation and/or erosion. This raises the issue of whether conventional geologic, geomorphic, and paleoseismic methods can be used to identify and characterize thrust faults for the assessment of seismic hazards or whether these faults sometimes are truly “blind.” We compiled a data base of 148 worldwide moderate- to large-magnitude thrust/reverse earthquakes to evaluate whether or not the event occurred on a fault that could have been identified prior to the earthquake on the basis of recognizable Quaternary surface deformation (i.e., a pre-existing fault or fold). Analysis of the data shows that interplate reverse earthquakes almost always are associated with pre-existing Quaternary deformation that was or could have been recognized prior to the earthquake. In particular, most interplate reverse earthquakes are associated with an active reverse fault at the surface and/or an active anticline. In contrast, intraplate reverse earthquakes seldom occur on faults associated with pre-existing recognizable surface deformation. We conclude that thrust faults can be detected in interplate regions with careful Quaternary geologic and geomorphic mapping; furthermore, the absence of Quaternary surface deformation can be used to infer the absence of an underlying active blind thrust fault in interplate tectonic settings. However, the data show that Quaternary geologic mapping techniques alone likely are insufficient to characterize blind thrusts in intraplate regions. In these areas, inclusion of a floating or random earthquake may be necessary to assess earthquake hazards.
Attenuation of coda waves at the Cerro Prieto geothermal field, Baja California, Mexico
Authors: Domínguez Tonatiuh , Rebollar Cecilio J. , Fabriol Hubert
Published: September 2021

We analyzed coda waves from 22 local events recorded by a temporary network of seismic stations deployed in the Cerro Prieto geothermal field of Baja California, Mexico, to estimate coda attenuation Qc in the frequency range 6 to 24 Hz. We used Sato's (1977) single scattering model for a coda window of 12.5 ± 2.5 sec beginning at twice the S-wave travel time. All events analyzed were located within the exploitation area with depths up to 6 km and source-to-receiver distances less than 8 km. At frequencies of 3, 4, and 5 Hz, rms amplitudes of coda to S waves showed no clear decay or even negative Qc values. This could be due to the inability of the theory to fit the data at low frequencies as it was suggested by Fehler et al. (1992). Within the range of variation, five of the seven stations showed similar Qc values that ranged from about 200 at 6 Hz to about 490 at 24 Hz. Assuming a power-law dependence of the form Qc (ƒ) = Q0ƒη, we estimated a value of Q0 = 111.5 ± 23 and a frequency dependence η of 0.41 ± 0.1. We found site effects at two stations that were located close to hot springs and mud pools.
Bias in surface-wave magnitudeMsdue to inadequate distance corrections
Authors: Rezapour Mehdi , Pearce Robert G.
Published: September 2021

We investigate bias in surface-wave magnitude using the complete ISC and NEIC datasets from 1978 to 1993. We conclude that although there are some small differences between the ISC and NEIC magnitudes, there is no major difference between these agencies for this presentation of the global dataset. The frequency-distance plot for reported surface-wave amplitude observations exhibits detailed structure of the body-wave amplitude-distance curve at all distances; the influence of the surface-wave amplitude decay with distance is much less apparent. This censoring via the body waves represents a large deficit in the number of potentially usable surface-wave amplitude observations, particularly in the P-wave shadow zone between Δ = 100° and 120°. We have obtained two new modified Ms formulas based upon analysis of all ISC data between 1978 and 1993. In the first, the conventional logarithmic dependence of the distance correction is retained, and we obtain

M s e = log ( A / T ) max + 1.155 log ( Δ ) + 4.269 .

In the second, we make allowance for the theoretically known contribution of dispersion and geometrical spreading, to obtain

M s t = log ( A / T ) max + 1 3 log ( Δ ) + 1 2 log ( sin Δ ) + 0.0046 Δ + 5.370.

Comparison of these formulas with other work confirms the inadequacy of the distance-dependence term in the Gutenberg and Prague formulas, and we show that our first formula, as well as that of Herak and Herak, gives less bias at all epicentral distances to within the scatter of the observed dataset. Our second formula provides an improved overall distance correction, especially beyond Δ = 145°. We show evidence that Airy-phase distance decay predominates at shorter distances (Δ≦30°), but for greater distances, we are unable to resolve whether this or non-Airy-phase decay predominates. Assuming 20-sec surface waves with U = 3.6 km/sec, we obtain a globally averaged apparent Q−1 of 0.00192 ± 0.00026 (Q ≈ 500). We argue that our second formula not only improves the distance correction for surface-wave magnitudes but also promotes the analysis of unexplained amplitude anomalies by formally allowing for those contributions that are theoretically predictable. We conclude that there remains systematic bias in station magnitudes and that this includes the effects of source depth, different path contributions, and differences in seismometer response. For intermediate magnitudes, Mts shows less scatter against log M0 than does Ms calculated using the Prague formula.
Estimates of regional and local strong motions during the great 1923 Kanto, Japan, earthquake (Ms8.2). Part 2: Forward simulation of seismograms using variable-slip rupture models and estimation of near-fault long-period ground motions
Authors: Sato Toshiaki , Graves Robert W. , Somerville Paul G. , Kataoka S.
Published: September 2021

This article addresses the forward simulation of regional and local strong motions from the 1923 Kanto, Japan, earthquake (Ms 8.2), using two variable-slip rupture models: the Wald and Somerville (1995) slip model (WS slip model) derived from geodetic and teleseismic data and a resampled version of the Takeo and Kanamori (1992) slip model (TK slip model) that was obtained from the forward simulation of the Ewing seismoscope record at Hongo (HNG, epicentral distance R = 60 km). Green's functions are calculated using flat-layered velocity models for specific source-receiver paths, which were developed in our first article (Sato et al., 1998). For regional stations Sendai (R = 350 km) and Gifu (R = 220 km), the WS slip model provides a much better match to the waveform data than the TK slip model. For local station HNG, the first 40 sec of the N77°E Imamura seismogram is successfully reproduced by both of the slip models, although the arrivals are delayed in the TK slip model. The large-amplitude long-duration, long-period (13 sec) later phases in the southwest-component Ewing seismogram are not reproduced by either of the slip models. Our results suggest that the WS slip model gives a better representation of the overall rupture process of the 1923 event than the TK slip model does. Near-fault long-period ground motions calculated at several stations using the WS slip model suggest that the motions at HNG were not the largest in the Tokyo metropolitan area during the 1923 event. In addition, we estimate that the ground motions near the southern margin of the fault plane were significantly larger than the recorded near-fault motions of recent magnitude 7 earthquakes, such as Northridge and Kobe, for periods longer than several seconds. This suggests that design codes based on the experience of these recent events may not adequately describe the long-period response expected during a magnitude 8 earthquake.
An intensity survey of households affected by the Northridge, California, earthquake of 17 January 1994
Authors: Dengler L. A. , Dewey J. W.
Published: September 2021

Following the Northridge, California, earthquake of 17 January 1994, the Humboldt Earthquake Education Center (HEEC) conducted a telephone survey of approximately 6000 adults from within the felt region to study the earthquake's effects on individual households. We tabulate human responses to the earthquake and observations of the earthquake's effects on inanimate objects from the HEEC survey as a function of independently assigned U.S. Geological Survey Modified Mercalli (USGS MM) intensity for the same communities. Human responses and subjective judgments about the severity of shaking, when averaged over a large number of samples, are useful discriminants of the levels of strong ground motion up to those associated with USGS MM intensity VII, notwithstanding that individual human responses and subjective judgments are notoriously unreliable. A threshold of strong ground motion corresponds to USGS MM intensity of about VII, above which over 40% of respondents described the earthquake as “violent” and most people reported difficulty standing, furniture displaced, and some damage to their homes. Even at intensities VIII and IX, however, relatively few people (about 15%) described their reaction as “panic” and only about 12% reported major damage to their homes. The HEEC phone-survey data show that, in communities of low to moderate shaking, USGS MM intensities estimated from a single postal questionnaire are quite robust. At USGS MM intensity of V and below, 88% of USGS MM intensities determined from postal questionnaires are within one intensity unit of intensities determined from the more numerous HEEC telephone survey data for the same community. We introduce the concept of a community decimal intensity scale (CDI) based on telephone-survey data and calibrated to agree on average with the USGS MM intensities. The CDIs are more regularly distributed than the USGS MM intensities and show much less scatter when plotted as a function of epicentral distance. CDIs show promise as a tool for comparing regional attenuation, for delineating variations in shaking strength within areas mapped as a single MM zone, and for rapid preliminary intensity estimates utilizing electronic media.
The effect of the earthquake radiation pattern on mb—A study using aftershocks in the 1976 Gazli sequence
Authors: Bowers David , Douglas Alan
Published: September 2021

We use published focal mechanisms to estimate radiation coefficients to four short-period arrays recording teleseismic P from 38 aftershocks in the 1976 Gazli, Uzbekistan, earthquake sequence. We divide the observed P-wave amplitude by its radiation coefficient to estimate the P-wave amplitude that would be observed if it was from the maximum of the double-couple radiation pattern. We use this new P-wave amplitude to calculate a P-wave magnitude, mCb, that is independent of the P-radiation pattern if the focal mechanisms are without error. Analysis of variance shows that the random error in mCb is reduced relative to that in the original P-wave magnitudes mCb and that this reduction is statistically significant at the 11% level. Further, analysis of variance demonstrates that the radiation coefficients calculated from the focal mechanisms contain error but that this error is probably not large enough to mask the detection of the radiation effect in mOb. Published averages of the logarithm of P-radiation coefficients allow an assessment of the differences in network-averaged mb due to the radiation pattern of point earthquake and explosion sources. Network-averaged mb from a vertical strike-slip earthquake can differ from an explosion of similar scalar moment by as much as 1.0 m.u. (magnitude units). However, this difference can be as little as 0.2 m.u. if the earthquake mechanism is 30° dip slip. We argue that, if mb is required to be independent of the earthquake mechanism, the most appropriate network average is mCb − 0.48.
The corner frequencies and stress drops of intraplate earthquakes in the northeastern United States
Authors: Shi Jinghua , Kim Won-Young , Richards Paul G.
Published: September 2021

This article presents the estimation of stress drops for small to middle-sized intraplate earthquakes in the northeastern United States. The vertical-component Sg and Lg waves of 49 earthquakes were analyzed, and their seismic corner frequencies and seismic moments were determined. For these events, both short-period and broadband records were obtained from stations in the region. There are eight events each of which has an aftershock good enough to be treated as its empirical Green's function, and their corner frequencies were estimated from empirical Green's function methods. For the other events, the corner frequencies were directly estimated by the spectral fitting of the vertical component of the Sg- or Lg-wave displacement spectrum with the ω-square source spectral model, using the available broadband and high-frequency short-period data and a frequency-dependent Q correction. The static stress drops, Δσ, were then calculated from the corner frequency and seismic moment.

From our study, the source corner frequencies estimated by fitting the Lg displacement spectrum with the assumed ω-square source model are more consistent with the corner frequencies measured from empirical Green's function deconvolution method than those estimated from the intersection of horizontal low-frequency spectral asymptote and a line indicating the ω−2 decay above the corner frequency. The source corner frequencies we estimated proved to be most appropriate for the small to middle-sized earthquakes. The static stress drops calculated from these corner-frequency estimates tend to be independent of seismic moment for events above a certain size. For earthquakes with size less than about 2 × 1020 dyne-cm, the stress drop tends to decrease with decreasing moment, suggesting a breakdown in self-similarity below a threshold magnitude. A characteristic rupture size of about 100 m is implied for these smaller earthquakes.
Properties and implications of dynamic rupture along a material interface
Authors: Ben-Zion Y. , Andrews D. J.
Published: September 2021

We perform two-dimensional plane-strain finite-difference calculations of dynamic rupture along an interface separating different elastic media. The calculations extend earlier results of Andrews and Ben-Zion (1997) who found a self-sustaining narrow slip pulse associated with dynamic reduction of normal stress along a material interface governed by constant friction, in agreement with Weertman (1980). The pulse propagates in a wrinklelike mode having remarkable dynamic properties that may be relevant to many geophysical phenomena. Here we examine the range of values of elastic parameters, friction coefficient, and strength heterogeneities allowing for the existence of the wrinklelike pulse. Rupture is initiated in the simulations by imposed slip in a limited space-time domain. Outside the region of the imposed slip, the pulse becomes narrower and higher with propagation distance along the interface. The strength of the wrinklelike pulse increases with S-wave velocity contrast up to a maximum at about 35% contrast. Beyond such a velocity contrast, there is no solution for a generalized Rayleigh wave along a material interface, and the strength of the pulse decreases. However, the wrinklelike pulse can still propagate in a self-sustaining manner for larger velocity contrasts. For a fixed S-wave velocity contrast, the strength has little dependence on density contrast or Poisson's ratio, but the pulse strength increases rapidly with increasing coefficient of friction. Stress and strength heterogeneities with small correlation length have little effect on the pulse, while long wavelength heterogeneities reduce the strength of the pulse. The high mechanical efficiency of the wrinklelike pulse suggests that earthquake ruptures may favor such mode of failure when possible.
Aftershock sequence parameters in New Zealand
Authors: Eberhart-Phillips Donna
Published: September 2021

Regional generic models describing the temporal and magnitude distribution of aftershocks are routinely used in California to assess aftershock hazard. This note applies the Reasenberg and Jones (1989) formulation of aftershock parameters to 17 New Zealand earthquake sequences of M ≧ 5.5, from 1987 through 1995. The median values of the aftershock parameters are similar to those obtained for California.
The 1995 off-Etorofu earthquake: Joint relocation of foreshocks, the mainshock, and aftershocks and implications for the earthquake nucleation process
Authors: Hurukawa Nobuo
Published: September 2021

Prominent foreshock activity preceded the Mw 7.9 off-Etorofu earthquake in the Kurile Islands on 3 December 1995. Using a modified joint hypocenter determination method (Hurukawa, 1995), we simultaneously relocated foreshocks, the mainshock, and aftershocks to study the foreshock activity in detail in view of the nucleation process of a large earthquake. The distribution of relocated earthquakes and their focal mechanisms suggest that the 1995 off-Etorofu earthquake was an interplate earthquake at the boundary between the North American and Pacific plates. Its source area overlaps with those of the 1958 off-Etorofu (Mw 8.3) and the 1963 off-Urup (Mw 8.5) earthquakes.

Nine days before the occurrence of the mainshock, a first immediate foreshock of magnitude 6.4 occurred at the deepest point of the foreshock area. This event was followed by many foreshocks, including three additional M ≧ 6 events, which occurred east and southeast of the first foreshock. The foreshock area expanded toward the trench axis with a velocity of several to several tens centimeters per second. The number of events per day increased daily, and the final size of the foreshock area was about 80 × 30 km. Eventually, the rupture of the mainshock started at the deepest point of the foreshock area. These observational facts are consistent with recent theoretical studies and laboratory experiments, in which foreshocks are regarded as the rupture of localized asperities in a broad weak zone where the nucleation of the large earthquake started.
3000-Year paleoseismological record from the region of the 1988 Saguenay, Quebec, earthquake
Authors: Doig Ronald
Published: September 2021

The 1988, magnitude mb 5.9 (mbLg 6.5) Saguenay, Quebec, earthquake occurred in a region considered to be aseismic, but the epicenter was less than 100 km northwest of the seismically active region of Charlevoix. Lake sediment cores representing some 3000 yr of sediment accumulation contain abnormal silt layers attributed to seismic shaking events prior to the 1988 earthquake. The layers are formed by the relatively rapid settling of the silt portion of the seismically resuspended organic-rich sediment. Cores were obtained over a distance of 120 km, perpendicular to the Saguenay graben structure. Several silt layers are much thicker and more widely distributed than the effect of the 1988 earthquake and are comparable to those observed at Charlevoix that were produced by magnitude 6 to 7 events. The silting events do not correlate between the sampled lakes nor does the pattern match that at Charlevoix, so that the shaking events are interpreted to be of local origin. The recurrence interval for magnitude ≧6 earthquakes ranges from 350 to 1000 yr, in contrast to the roughly 75-yr historical recurrence of earthquakes of magnitude greater than 6 at Charlevoix.
Propagation ofSHwaves in a layered half-space with a frictional contact interface
Authors: Wang Yue-Sheng , Yu Gui-Lan , Gai Bing-Zheng
Published: September 2021

The propagation of SH waves in a layered half-space with a frictional contact interface is considered. The incident wave is assumed to be sufficiently strong so that friction may be broken, and the local slip may take place at the interface. In the stick zones, both the displacements and stresses are continuous, while in the slip zones, the Coulomb friction model is adopted. The mixed boundary conditions lead to recurrence relations for the subcritical angle incidence or singular integral equations for the supercritical angle incidence. The extent and location of slip zones, which are unknown before the solution of the problem, are determined. The local slip velocities and the interface shearing tractions are calculated in detail for the subcritical angle incidence. The results show that the solution of the problem is dependent on the frequency of the incident wave due to the presence of the characteristic length—the thickness of the elastic layer. It is also found that, in some situations, there exist four slip zones instead of two over one representative period. All these features are quite different from those for infinite media.
Statistical analysis of irregular wave-guide influences on regional seismic discriminants in China: Additional results forPn/Sn,Pn/Lg, andPg/Sn
Authors: Fan Guangwei , Lay Thorne
Published: September 2021

Reducing scatter in measurements of regional-phase amplitude ratios is desirable for seismic discrimination applications and also provides insight into crustal structure controls on energy partitioning of regional seismic waves. Our previous analysis (Fan and Lay, 1998) of the regional seismic discriminant, Pg/Lg, indicated that variations in crustal structure cause path-specific fluctuations of those amplitude ratios for earthquakes recorded at broadband station WMQ in western China. In this study, we extend our multivariate regression analysis to Pn/Sn, Pn/Lg, and Pg/Sn amplitude ratios recorded at WMQ in frequency bands of 0.75 to 1.5 Hz, 1.5 to 3.0 Hz, and 3.0 to 6.0 Hz for the suite of path-specific parameters: path length, mean path elevation, variance of topography along the path, rms topographic slope variations, mean crustal thickness, and mean sediment thickness. Optimal three- and four-parameter models all achieve reductions in variance of the measurements relative to conventional distance corrections. At low frequency, the improvements for ratios involving Pg can be more than a factor of 2. For all short-period amplitude ratios, mean path elevation seems to play an important role. While crustal thickness and sediment thickness affect ratios involving Pg, topographic variance and surface slope variations have more influence on ratios involving Pn. Strong crustal variations associated with the structure of the Tibetan Plateau are responsible for much of the amplitude variations.
Aftershock decay of three recent strong earthquakes in the Levant
Authors: Rabinowitz Nitzan , Steinberg David M.
Published: September 2021

Aftershock sequences are studied from three recent strong earthquakes in the Levant: the MW = 7.1 earthquake of 22 November 1995 and the ML = 5.8 earthquake of 3 August 1993, both in the Gulf of Aqaba, and the MW = 6.8 earthquake of 9 October 1996 off the southwest coast of Cyprus. The modified Omori's law provides a good fit to the decay of aftershock activity for both events. The two sequences from the Gulf of Aqaba have rather low p values (0.90 in 1993 and 0.75 in 1995), reflecting slow decay of the aftershock activity. This may be attributed to low heat flow close to the epicenters. For the Cyprus sequence, the p value is 1.09, similar to that of many other sequences around the globe.
Assessing the scaling of a blockchain-based social contact tracking system
Authors: Shmelev Ilya
Published: September 2021

During a pandemic, one of the most important tasks is to track social contacts with those who are sick. This article categorizes projects that track these contacts. Projects are classified by architecture and the common components of such systems are highlighted. It is concluded that the hybrid architecture of such a solution based on an exclusive blockchain will have several advantages, and a conceptual model of such a system is described. However, an analysis of existing blockchain projects showed that their main problem is the unresolved issue of scaling such kinds of systems, which is becoming a key issue in the context of creating a global digital infrastructure of society. Further, the scaling of the system's conceptual model is assessed based on open-source information about the Moscow metro, and the main conclusions about the selected architectural solutions are confirmed.
LOCALIZATION OF EXPLOSION PULSES IN A CLOSED SPACE IN COAL MINES
Authors: Abdrakova F.Yu. , Tulepov M.I. , Baiseitov* D.A. , Beksultan Zh.B. , Chihradze M.
Published: September 2021

The data on the prospect of using an artificial high pressure water barrier as a method of localizing the explosion impulse in the confined space of tunnels and mines are presented. The explosion impulse and the process of its decay in interaction with water fog have been studied. In the course of field research, an explosion was simulated in the shock installation, and a method for its localization was developed using four water screens (barriers). The water screen was created using a system of ring-shaped water distribution headers with high pressure nozzles installed in a circle. Hexogen was used as an explosive. Experiments on localization of explosions were carried out on the base of the "Grigol Tsulukidze Mining Institute of Georgia" in Tbilisi, Georgia, together with the research group of the Faculty of Chemistry and Chemical Technology of  al Farabi Kazakh National University and the Institute of Combustion Problems. The influence of the water barrier on the process of shock wave attenuation at 3 points of overvoltage of the section is established. The test results showed that the average values of the overpressure in the three sections were reduced by 38.8%, 26.67% and 19.2%, respectively. The action of the shock wave occurs according to an exponential function, and all other wave changes along any other trajectory on the plane of h – t change are described by a single time dependence.
ХИМИЧЕСКИЕ ОСНОВЫ СНИЖЕНИЯ ПОЖАРНОЙ ОПАСНОСТИ МАТЕРИАЛОВ НА ОСНОВЕ ЦЕЛЛЮЛОЗЫ
Authors: Берлин Ал. Ал. , Сивенков А.Б. , Хасанова* Г.Ш.
Published: September 2021

Проведен анализ теоретических и экспериментальных работ по изучению особенностей процесса горения целлюлозы, а также химических подходов и технологий по снижению пожарной опасности материалов на основе целлюлозы. Показана взаимосвязь химической природы целлюлозы с ее горючестью, а также рассмотрены возможности эффективного снижения пожарной опасности целлюлозных материалов. Разобраны механизмы огнезащитного действия различных антипиренов. Обсуждаются проблемы и перспективы применения различных способов и видов огнезащиты для целлюлозных материалов.