bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurements of intrinsic and scattering seismic attenuation in the crust
Authors: Del Pezzo Edoardo , Ibanez Jesus , Morales José , Akinci Aybige , Maresca Rosalba
Published: September 2021

Intrinsic and scattering attenuation parameters, Qi and QS, have been measured in three different tectonic areas for local and shallow earthquakes located close to the receiver. The approach developed by Wennerberg (1993), which takes into account the numerical correction of the coda-Q parameter for the multiple scattering formulation of Zeng, was used to infer from the estimates of coda Q and direct S-wave Q the intrinsic (Qi) and scattering (QS) Q values. Results for 1 to 12 Hz range show that Qi is comparable to QS for the Etna volcano and for the Campi Flegrei area, while Qi for the tectonically active area of Granada is lower than QS. Coda Q is close to intrinsic Q, suggesting that, at least in the crust, coda Q is a good estimate of the intrinsic Q. Volcanic areas show a reasonable higher degree of heterogeneity, if compared with the nonvolcanic area of Granada.
Correlation of seismicity and water level in the Açu reservoir—an example from Northeast Brazil
Authors: Ferreira Joaquim M. , Oliveira Ronaldo T. De , Assumpção Marcelo , Moreira José A. M. , Pearce Robert G. , Takeya Mário K.
Published: September 2021

Seismic monitoring of the Açu reservoir (31-m depth, 2.4 × 109 m3 in volume), in Rio Grande do Norte State, Northeastern Brazil, started in 1987 about 2 years after impoundment. The largest earthquake so far (magnitude 2.8) occurred in August 1994. From 1987 to 1989, the monthly number of induced events had a clear correlation with the water level, with a 3-month delay, and the activity occurred mainly inside the reservoir. Although no preimpoundment monitoring had been carried out, the correlation of the water level and the seismicity strongly suggests the activity was induced by pore pressure diffusion. Since 1990, the activity migrated toward the border of the reservoir, and the number of events no longer correlated with the water level. A seismographic network was deployed around the reservoir during two field campaigns (1989 and 1990/91), showing that the activity occurred preferentially with strike-slip mechanisms on NE oriented faults. The E-W orientation of the P axes, parallel to the regional maximum horizontal stress, and the presence of many NE trending faults and fractures in the Precambrian basement near the reservoir, suggest that the probably induced seismicity was a typical case of water pore pressure facilitating earthquake occurrence in pre-existing zones of weakness under high regional stresses.
Search of a worldwide catalog for earthquakes triggered at intermediate distances
Authors: Lomnitz Cinna
Published: September 2021

An anomalous increase of seismicity was recorded at intermediate distances from the Landers, California, earthquake of 28 June 1992 (M = 7.3). Several authors have suggested that enhanced activity was triggered at distances of up to 17 fault lengths. We test a 90-year catalog of shallow world earthquakes (M > 7) for evidence of this effect. It is found that triggering is present even at these high magnitude levels. The pattern of enhanced activity comprises two concentric regions: (a) the aftershock region and (b) the region of triggered seismicity at distances of 300 to 1000 km. The two regions are activated together; they are separated by a ring-shaped gap at a distance of around 300 km. This “Mexican Hat” pattern is attributed to recovery flows generated by the stress drop of the earthquake in a steady-state convecting system.
Horizontal-to-vertical spectral ratio and geological conditions: The case of Garner Valley Downhole Array in southern California
Authors: Theodulidis N. , Bard P.-Y. , Archuleta R. , Bouchon M.
Published: September 2021

The aim of the present article is to further check the use of the horizontal-to-vertical (h/v) spectral ratio, which has been recently suggested as an indicator of site effects. The data set consists of 110, three-component, high sensitivity accelerograms, recorded at five different depths by the Garner Valley Downhole Array (GVDA), in southern California, with peak ground accelerations 0.0002 g ≦ ag ≦ 0.04 g, magnitudes 3.0 ≦ ML ≦ 4.6, and hypocentral distances 16 km ≦ R ≦ 107 km. First, the stability of the (h/v) spectral ratio is investigated by computing the mean for the whole data set in different depths. The (h/v) spectral ratio on the surface is compared with the surface-to-depth standard spectral ratio, with theoretical S-wave transfer functions derived from the vertical geotechnical profile, as well as with the (h/v) spectral ratio of synthetic accelerograms generated by the discrete wavenumber method. Both theoretical and experimental data show a good stability of the (h/v) spectral ratio shape, which is in good agreement with the local geological structure and is insensitive to the source location and mechanism. However, the absolute level of the (h/v) spectral ratio depends on the wave field and is different from the surface-to-depth spectral ratio. Consequently the (h/v) spectral ratio technique provides only partially the information that can be obtained from a downhole array. But surface-to-depth ratios may also be misleading because they combine effects at surface and at depth.
RMS response of a one-dimensional half-space toSH
Authors: Day Steven M.
Published: September 2021

We examine the extent to which the response of a perfectly elastic half-space to an SH-wave incident from below can be characterized when knowledge about the elastic structure is limited to the near surface. Elastic properties are modeled as piecewise continuous functions of the depth coordinate. It is found that the site amplification function can be determined with a frequency resolution that depends inversely on the depth to which the elastic structure is known. Specifically, certain spectral averages of the site amplification function, concentrated over bandwidth Δƒ, depend only on the elastic structure down to a two-way travel-time depth of 1/Δƒ. These spectral averages are entirely independent of the elastic properties at greater depth. Equivalently, when the incident motion has a bandlimited white power spectrum of bandwidth Δƒ, the site amplification of the root mean square (rms) ground motion depends only on the elastic structure down to a two-way travel-time depth of 1/Δƒ. When the bandwidth is sufficiently large, the following corollary applies: the rms surface ground motion equals the rms incident motion multiplied by 2√Ib/I0, where I0 and Ib are shear impedances at the ground surface and basement depth, respectively. This result provides justification for a procedure conventionally used to correct stochastic estimates of earthquake ground motion to account for local site effects. The analysis also clarifies the limitations of that conventional procedure.

The results define specific site-response parameters that can be computed from knowledge of shallow structure alone and may thereby contribute to improved understanding of the physical basis for, and limitations of, site classification schemes that are based on average S-wave velocity at shallow depth. While the analytical results are rigorous only for infinite Q, numerical experiments indicate that similar results apply to models with finite, frequency-independent Q. The practical utility of the results is likely to be limited primarily by the degree of lateral heterogeneity present near sites of interest and the degree to which the sites respond nonlinearly to incident ground motion.
Seismogram synthesis for multi-layered media with irregular interfaces by global generalized reflection/transmission matrices method. III. Theory of 2DP-SVcase
Authors: Chen Xiaofei
Published: September 2021

In this third article of a series study attempting to present a new method of seismogram synthesis for irregular multi-layered media problem, I shall present the theoretical development for the 2D P-SV case. Following the similar procedures developed for solving the SH problem presented in an earlier article (Chen, 1990), I first set up the basic matrix equations starting from the basic boundary integral equations, then introduce and determine the global generalized reflection/transmission (R/T) matrices, and finally solve the basic matrix equations by using the global generalized R/T matrices. It can be demonstrated that for the case of horizontally layered media (a special case of the general model), the general solution derived in this study naturally becomes identical to that of the generalized reflectivity method, which is directly developed for horizontally layered media. Therefore, this new method can be viewed as an extension of the generalized reflectivity method to the case of irregular multi-layered media. For the problems of seismic ground motion and irregular topography, I derive the simplified formulas. This new method can provide an efficient, accurate, and unified algorithm to calculate the synthetic seismograms for a variety of the laterally heterogeneous media.
Propagation characteristics ofLgacross the Tibetan Plateau
Authors: McNamara D. E. , Owens T. J. , Walter W. R.
Published: September 2021

We present the results of a study on the propagation of Lg within the Tibetan Plateau. We analyze over 1200 seismograms for Lg propagation from 185 local and regional earthquakes recorded at distances from 150 to 2000 km, 52 of which are located within the Tibetan Plateau. Although the propagation of Lg has been observed across most of Asia, Lg has not been observed for paths crossing the Tibetan Plateau. We analyze paths that cross the plateau boundaries and paths contained entirely within the Tibetan Plateau. Lg is generated within the Tibetan Plateau and can propagate to epicentral distances of at least 600 km. For events located outside of the Tibetan Plateau, Lg is either absent or severely diminished for paths that cross both the Himalayan and Kunlun ranges, confirming that the margins of the plateau weaken Lg transmission.

Based on the observations from our qualitative analysis, we invert Lg amplitudes for event/station paths that are confined to the Tibetan Plateau to estimate the quality factor, Q. The inversion yields a frequency-dependent Q function:

Q ( f ) = ( 366 ± 37 ) f ( 0.45 ± 0.06 ) ( 0.5 ≦ f ≦ 16 H z ) .

Similar observations in other areas indicate that frequency-dependent apparent Q within the Tibetan Plateau is well below that expected for a typical continental interior. Instead, it has a frequency dependence that is characteristic of a tectonically active region.
Use of empirical Green's functions, spectral ratios, and kinematic source models for simulating strong ground motion
Authors: Haddon R. A. W.
Published: September 2021

Ground motions for large and moderately large earthquakes at short and moderate distances are particularly important for seismic hazard estimation in eastern North America (ENA). Very few direct observations of such ground motions have been obtained, however, because of the sparsity of recording sites and the relatively low rates of occurrence of large earthquakes inside the region. Estimation of strong ground motion must therefore rely heavily on theoretical models to extend empirical results obtained from small earthquakes and from the few larger ones for which reliable data are available. Because of the generally large distances between recording stations, the main source of useful data comes from Lg wave trains observed at relatively large distances. For the two largest earthquakes to have occurred near populated regions of southeastern Canada during the past decade, spectral ratios of the Lg wave trains of the mainshocks, with respect to those of their aftershocks, are found to depend almost entirely upon the source radiation characteristics of the sources alone. This result is utilized to derive elastodynamically-based kinematic rupture models that are consistent with the empirical spectral ratio data. Such models provide a firm physical basis from which to infer the most probable spectral characteristics for future large earthquakes in the region. In converse application, it is shown that spectral ratios obtained from such models, along with empirical seismograms from small earthquakes, can be used to accurately simulate strong ground motions at short and moderate (as well as large) distances. As such small-event seismograms are relatively plentiful, the problem of reliable strong ground motion estimation is therefore reduced to that of obtaining reliable representative source spectral models. The solution of this latter problem must continue to depend upon whatever empirical data are available and upon appropriately detailed theoretical modeling.
Analysis of spectral ratios for estimating ground motion in deep basins
Authors: Dravinski M. , Ding G. , Wen K.-L.
Published: September 2021

Use of Nakamura's spectral ratio (horizontal versus vertical components) is investigated theoretically for deep sedimentary basins by considering semi-circular and semi-spherical valleys. The ratio is evaluated from the steady-state surface response for different incident waves. Based on this ratio, both the resonant frequencies and ground motion amplification are determined. The results based on Nakamura's ratio are compared with those based on the sediment-to-bedrock spectral ratios (Kagami's ratio).

The results show that for both two- and three-dimensional models, Nakamura's technique predicts well the fundamental resonant frequency, but it could not determine higher resonant frequencies of the basins. The error in Nakamura's estimate of the fundamental resonant frequency increases for stations near the valley center. For alluvial valleys considered in this article, Nakamura's ratio failed to predict accurately surface ground-motion amplification.
Improving teleseismic event locations using a three-dimensional Earth model
Authors: Smith Gideon P. , Ekström Göran
Published: September 2021

A comparison is made between seismic event locations derived from standard spherically symmetric Earth models (JB, PREM, IASP91) and a recent Earth model (S&P12/WM13) that incorporates large-scale lateral heterogeneity of P- and S-wave velocities in the mantle. Events with known hypocentral coordinates are located in the different Earth models using standard methods. Two sets of events are considered: a data set of 26 explosions, including primarily nuclear weapons test explosions and peaceful nuclear explosions in the United States and former USSR; and a published data set of 82 well-located earthquakes with a more even global distribution. IASP91 and PREM are shown to offer similar errors in event location and origin time estimates with respect to the JB model. The three-dimensional (3D) model S&P12/WM13 offers improvement in event locations over all three one-dimensional (1D) models with, or without, station corrections. For the explosion events, the average mislocation distance is reduced by approximately 40%; for the earthquakes, the improvements are smaller. Corrections for crustal thickness beneath source and receiver are found to be of similar magnitude to the mantle corrections, but use of station corrections together with the three-dimensional mantle model provide the best locations.
A new regional hypocenter location method
Authors: Xie Zhi , Spencer Terry W. , Rabinowitz Philip D. , Fahlquist Davis A.
Published: September 2021

A new regional hypocenter location method is presented in this article to address location problems associated with sparse station distributions and lack of velocity information. For an arbitrary slowness structure, an unknown parameter, slowness deviation, is introduced to construct a more precise slowness model than a homogeneous slowness model. By incorporating a genetic algorithm, the new method yields reasonable solutions for epicenters and origin times both inside and outside the seismic network. The synthetic data tests indicate that outside the network the new method gives excellent results for epicenter locations compared with Geiger's method, even when Geiger's method is used with the true velocity structure. The relocation of the Loma Prieta mainshock and 26 aftershocks occurring within the first 24 h after the mainshock by using the new hypocenter location method places the aftershocks an average of 2.29 km to the southwest of the locations published by the Northern California Earthquake Data Center. The new locations are in better agreement with the geodetic measurements.
A comparison of the high-frequency (>1 Hz) surface and subsurface noise environment at three sites in the United States
Authors: Young Christopher J. , Chael Eric P. , Withers Mitchell M. , Aster Richard C.
Published: September 2021

Surface and subsurface high-frequency (>1 Hz) noise data were recorded using nearly identical instrumentation at three widely separated sites in the United States (Amarillo, Texas; Datil, New Mexico; and Pinedale, Wyoming) for extended periods of time under varying wind conditions. While the sites are geologically distinct, the near-surface noise environments have many common features that we believe may be due in large part to the presence of a surficial layer of highly attenuative unconsolidated material at each site. Noise levels seen at or near the surface (5 m or less) are much higher (up to 30 dB) and much more variable (power range up to 44 dB) than those seen at depth (the smallest range was 9 dB for 1951 m at Amarillo). The greatest gains in noise level reduction are realized within the first 100 m and probably much shallower (< ∼ 10 m). Regardless of the wind profile or local lithology, all sites show an excellent correlation between increased noise levels and higher wind speed, even at significant depths (367 m at Amarillo). Wind-generated noise is broadband (at least 15 to 60 Hz) and apparently nonlinear, increasing dramatically when a wind speed threshold is exceeded (3 to 4 m/sec within a few meters of the surface; as high as 8 m/sec at a depth of a few hundred meters). It is possible to be essentially completely shielded from the wind-generated component of seismic noise by deploying instruments at sufficient depth, but we observed this only for the two deepest deployments (1219 and 1951 m, both at Amarillo). Reducing the wind profile at the surface, however, can yield similar reductions for a much smaller cost. Cultural or “workday” noise, if present (depending on the remoteness of the site), is typically much weaker (10 dB or less) than wind noise but may propagate very effectively to great depths and therefore could be of concern for very deep deployments where wind is not a factor.
Advancement in source estimation techniques using broadband regional seismograms
Authors: Zhu Lupei , Helmberger Donald V.
Published: September 2021

One important constraint on source retrieval from regional seismograms comes from the amplitude difference between various phases (such as Pnl/surface wave, SV/SH). Because the misfit errors used in some waveform inversions are normalized by the data and synthetics, the amplitude information in the data has not been fully utilized. In this article, we modify the “cut and paste” source estimation technique (Zhao and Helmberger, 1994) by removing this type of normalization. It is shown that the modified method increases the stability and resolution of inversion. When multiple stations at different distance ranges are used, a distance scaling factor is introduced to compensate for the amplitude decay with distance. By applying the technique to the TERRAscope data, we have determined source mechanisms and depths of 335 southern Californian events with ML ≧ 3.5. The amplitude decays with distance are r1.13 for Pnl, r0.55 for Love waves, and r0.74 for Rayleigh waves. In contrast to generally shallow source depths reported by the southern California short-period network, the depth distribution from waveform inversion shows a strong peak around 12 km with few earthquakes occurring above 5 km and below 20 km.
Convenient formulas for determining dip-slip fault parameters from geophysical observables
Authors: Cohen Steven C.
Published: September 2021

Simple equations are presented for estimating slip magnitude, fault dip angle, width of the zone of compression, and other co-seismic parameters from geologic, seismic, and geodetic observations made in subduction zone and continental dip-slip environments. These equations are easy to memorize and use without computational aids.
Site effects and microzonation in the city of Thessaloniki (Greece) comparison of different approaches
Authors: Lachet Corinne , Hatzfeld Denis , Bard Pierre-Yves , Theodulidis Nikos , Papaioannou Christos , Savvaidis Alekos
Published: September 2021

Ten seismological stations were installed for 3 months in the city of Thessaloniki (Greece) to record earthquakes and ambient noise over areas of different geotechnical characteristics. About 40 local earthquakes were recorded with a good signal-to-noise ratio, in more than two stations. We analyze the records with three different techniques: (a) the classical spectral ratios to a reference station, (b) the receiver functions, and (c) the horizontal-to-vertical (H/V) spectral ratios on noise recordings. The results are compared in terms of predominant frequencies and amplification levels. The three methods are found equally able to reveal the fundamental frequency. Regarding local site amplification, they provide significantly different results with a general trend of the two H/V ratio methods (b and c) to underestimate the amplification levels compared to that inferred from the classical spectral ratio technique (a). The results are then used in relation with the microzonation of the city of Thessaloniki. A good correlation is found between the local site amplification and the type and age of the local geological formation as well as with the depth of bedrock. Finally, we compare our observations with the damage distribution in the city, which was hit by a magnitude 6.5 earthquake in 1978. The amplification levels from spectral ratios obtained at each site exhibit a good agreement with the map of damage intensity.
The anomalous seismic response of the ground at the Tarzana hill site during the Northridge 1994 southern California earthquake: A resonant, sliding block?
Authors: Rial J.  A.
Published: September 2021

The Northridge, California, earthquake of 17 January 1994 (Mw = 6.7) caused widespread damage in the San Fernando valley and adjacent areas. In many places, the earthquake motion was amplified or strongly modified by the local ground response, but the most localized and extreme site effect occurred at Tarzana, a small hill of gentle topography at the northern edge of the Santa Monica mountains where strong amplification (horizontal peak acceleration of 1.82g) and other unusual site effects were recorded at a CDMG accelerograph. The anomalous site effect was confined to a small area 50 m in radius around the station, beyond which the ground shaking was down to its theoretically expected level. If strong ground motion can vary so much in such a small scale, microzonation of urban areas may be an impossible goal at worst or, at best, only reliable at those sites where ground response has previously been measured. Hence, to clearly understand the dynamic response of the ground at the Tarzana site has become an important problem in earthquake hazard assessment and microzonation.

This article reports results on the characteristics of the seismic response of the Tarzana site based on the investigation of aftershock data recorded on a local array and recorded accelerograms. Preliminary results strongly suggest that the Tarzana hill is formed by an old landslide of the Modelo formation. Such a structure is consistent with the anomalous resonant behavior at C00, the observed resonant frequencies, and the complex body wave recordings observed throughout the local array. The results also suggest that the 1.82g peak acceleration at C00 could have been caused by a small (Mw = 1.3) earthquake, located some 60 m from the station, generated by a downslope sliding of the hill with a maximum displacement of 4 cm over a circular area of nearly 50 m in radius. The slide was triggered by the Northridge mainshock.
The effects of the atmospheric pressure changes on seismic signals or how to improve the quality of a station
Authors: Beauduin R. , Lognonné P. , Montagner J.  P. , Cacho S. , Karczewski J.  F. , Morand M.
Published: September 2021

Seismic investigations are mainly limited by seismic noise. Two microbarometers have been installed in the seismic vault of two different GEOSCOPE stations, one at SSB and the other at TAM. All vertical components and most of the horizontal components show a significant correlation with pressure. In order to correct the seismic signals from the atmospheric pressure noise, a transfer function between the pressure data and the seismic data is inverted. Results show that, after correction, the noise levels reached on the horizontal components are similar between the two stations, and the vertical components display noise levels below the low-noise model as defined by Peterson (1993). This technique reduces part of the noise and allows detection of small earthquakes and a better extraction of normal modes. The analysis of the lowest normal modes of the Earth excited by the MS = 8.2 Macquarie Island earthquake is given to illustrate the perspectives of the method.
The slip history of the 1994 Northridge, California, earthquake determined from strong-motion, teleseismic,GPS, and leveling data
Authors: Wald David J. , Heaton Thomas H. , Hudnut K. W.
Published: September 2021

We present a rupture model of the Northridge earthquake, determined from the joint inversion of near-source strong ground motion recordings, P and SH teleseismic body waves, Global Positioning System (GPS) displacement vectors, and permanent uplift measured along leveling lines. The fault is defined to strike 122° and dip 40° to the south-southwest. The average rake vector is determined to be 101°, and average slip is 1.3 m; the peak slip reaches about 3 m. Our estimate of the seismic moment is 1.3 ± 0.2 × 1026 dyne-cm (potency of 0.4 km3). The rupture area is small relative to the overall aftershock dimensions and is approximately 15 km along strike, nearly 20 km in the dip direction, and there is no indication of slip shallower than about 5 to 6 km. The up-dip, strong-motion velocity waveforms are dominated by large S-wave pulses attributed to source directivity and are comprised of at least 2 to 3 distinct arrivals (a few seconds apart). Stations at southern azimuths indicate two main S-wave arrivals separated longer in time (about 4 to 5 sec). These observations are best modeled with a complex distribution of subevents: The initial S-wave arrival comes from an asperity that begins at the hypocenter and extends up-dip and to the north where a second, larger subevent is centered (about 12 km away). The secondary S arrivals at southern azimuths are best fit with additional energy radiation from another high slip region at a depth of 19 km, 8 km west of the hypocenter. The resolving power of the individual data sets is examined by predicting the geodetic (GPS and leveling) displacements with the dislocation model determined from the waveform data, and vice versa, and also by analyzing how well the teleseismic solution predicts the recorded strong motions. The general features of the geodetic displacements are not well predicted from the model determined independently from the strong-motion data; likewise, the slip model determined from geodetic data does not adequately reproduce the strong-motion characteristics. Whereas a particularly smooth slip pattern is sufficient to satisfy the geodetic data, the strong-motion and teleseismic data require a more heterogeneous slip distribution in order to reproduce the velocity amplitudes and frequency content. Although the teleseismic model can adequately reproduce the overall amplitude and frequency content of the strong-motion velocity recordings, it does a poor job of predicting the geodetic data. Consequently, a robust representation of the slip history and heterogeneity requires a combined analysis of these data sets.
Directional topographic site response at Tarzana observed in aftershocks of the 1994 Northridge, California, earthquake: Implications for mainshock motions
Authors: Spudich Paul , Hellweg Margaret , Lee W. H. K.
Published: September 2021

The Northridge earthquake caused 1.78 g acceleration in the east-west direction at a site in Tarzana, California, located about 6 km south of the mainshock epicenter. The accelerograph was located atop a hill about 15-m high, 500-m long, and 130-m wide, striking about N78°E. During the aftershock sequence, a temporary array of 21 three-component geophones was deployed in six radial lines centered on the accelerograph, with an average sensor spacing of 35 m. Station C00 was located about 2 m from the accelerograph. We inverted aftershock spectra to obtain average relative site response at each station as a function of direction of ground motion. We identified a 3.2-Hz resonance that is a transverse oscillation of the hill (a directional topographic effect). The top/base amplification ratio at 3.2 Hz is about 4.5 for horizontal ground motions oriented approximately perpendicular to the long axis of the hill and about 2 for motions parallel to the hill. This resonance is seen most strongly within 50 m of C00. Other resonant frequencies were also observed. A strong lateral variation in attenuation, probably associated with a fault, caused substantially lower motion at frequencies above 6 Hz at the east end of the hill. There may be some additional scattered waves associated with the fault zone and seen at both the base and top of the hill, causing particle motions (not spectral ratios) at the top of the hill to be rotated about 20° away from the direction transverse to the hill. The resonant frequency, but not the amplitude, of our observed topographic resonance agrees well with theory, even for such a low hill. Comparisons of our observations with theoretical results indicate that the 3D shape of the hill and its internal structure are important factors affecting its response. The strong transverse resonance of the hill does not account for the large east-west mainshock motions. Assuming linear soil response, mainshock east-west motions at the Tarzana accelerograph were amplified by a factor of about 2 or less compared with sites at the base of the hill. Probable variations in surficial shear-wave velocity do not account for the observed differences among mainshock acceleration observed at Tarzana and at two different sites within 2 km of Tarzana.
Engineering implications of ground motions from the Northridge earthquake
Authors: Chang Susan W. , Bray Jonathan D. , Seed Raymond B.
Published: September 2021

The magnitude, duration, and frequency content of ground motions from the Northridge earthquake are analyzed and compared to predictive relationships typically used in engineering design and to the 1994 Uniform Building Code (UBC). A relationship between maximum horizontal acceleration on soil versus maximum horizontal acceleration on rock is presented based on strong-motion recordings at free-field sites. The effect of geologic conditions on localized damage patterns is shown to be important for this earthquake, although many of the sites within the affected region are stiff soil sites classified as S1 or S2 sites by the UBC. The results of preliminary seismic site response analyses performed at two deep alluvial sites indicate that much of the observed site amplification can be captured by one-dimensional wave propagation analyses.
Engineering observations on ground motion at the Van Norman Complex after the 1994 Northridge earthquake
Authors: Bardet J. P. , Davis C.
Published: September 2021

During the 1994 Northridge earthquake, the Van Norman Complex yielded an unprecedented number of recordings with high acceleration, in the close proximity of the fault rupture. These strong-motion recordings exhibited the pulses of the main event. One station recorded the largest velocity ever instrumentally recorded (177 cm/sec), resulting from a 0.86 g peak acceleration with a low frequency. Throughout the complex, the horizontal accelerations reached peak values ranging from 0.56 to 1.0 g, except for the complex center, where the peak acceleration did not exceed 0.43 g. The vertical acceleration reached maximum peak values comparable with those of the horizontal acceleration. The acceleration response spectra in the longitudinal and transverse directions were significantly different. Such a difference, which is not yet well documented in the field of geotechnical earthquake engineering, indicates that the amplitude and frequency content of the ground motion was directionally dependent in the Van Norman Complex.
Seismicity rate change before the Irpinia (M = 6.9) 1980 earthquake
Authors: Wyss M. , Console R. , Murru M.
Published: September 2021

The stability of seismicity rate in central and southern Italy was examined in two data sets: from 1975.0 to 1995.0 with M ≧ 3.4 and from 1987.5 to 1996.0 with M ≧ 2.5. These are the approximate minimum magnitudes of complete reporting for the respective periods. The first set was used to evaluate the possibility that the 23 November 1980 Irpinia (M 6.9) earthquake was preceded by precursory seismic quiescence; the second was used to evaluate the conditions under which a current seismic quiescence could be detected in central or southern Italy. During the years before the Irpinia earthquake, the seismicity rate in the northern half of the rupture area and north of it was low. Whether this was a case of precursory quiescence or not is subject to interpretation because the background rate cannot be established for the years before 1975. If we accept the relatively constant seismicity rate in the Irpinia volume during the decade after the mainshock as representative for the background rate, we have a clear case of precursory quiescence lasting at least 1.3 yr up to the mainshock. Alternatively, it can be postulated that the seismicity rate during the decade following this shock was elevated regionally because of the stress redistribution and that the low rates seen before it represent the normal background rate. Even if this reasoning is accepted, the fact remains that a volume including the northern part of the rupture produced no M ≧ 3.4 earthquakes during 1.3 yr before the Irpinia earthquake but produced 10 earthquakes during the 4.7 previous yr. Given these facts, we favor the interpretation that the Irpinia 1980 earthquake was preceded by precursory seismic quiescence. In the entire data set, there are three other cases of quiescence of higher significance without a mainshock following. Since no other mainshock with M > 6 exists in the data set, no missed event exists. We propose that in Italy precursory seismic quiescence may precede mainshocks and that it may be detected in the future by the improved catalog of the modern data set beginning in 1987.5. Major magnitude scale changes give the mistaken appearance that fewer large-magnitude earthquakes occur in Italy now than in years before 1987. We postulate that the rate of earthquakes has not changed and that the magnitude scale should be adjusted.
Distribution of ground-motion intensity inferred from questionnaire survey, earthquake recordings, and microtremor measurements—A case study in Christchurch, New Zealand, during the 1994 arthurs pass earthquake
Authors: Toshinawa Takumi , Taber J. John , Berrill John B.
Published: September 2021

The areal distribution of seismic ground-motion intensity in the city of Christchurch, New Zealand, during the 1994 Arthurs Pass Earthquake (ML 6.6) was evaluated using an intensity questionnaire together with local site amplifications inferred from seismic recordings and microtremors. In order to estimate the intensity in parts of the city where no intensity data were available, intensity data were compared to relative levels of shaking determined from both weak-motion and microtremor recordings. Weak ground-motion amplification factors were determined using ratios of ground accelerations at five sediment sites with respect to a rock site. Microtremor amplification factors were determined from horizontal-to-vertical spectral ratios at a 1-km spacing throughout the city. A positive correlation between weak-motion and microtremor amplification factors allowed extrapolation of microtremor amplification to estimated MM intensity (EMMI). EMMI ranged from 3 to 6 and was consistent with the questionnaire intensity and geological conditions and showed detailed information on the areal distribution of ground-motion intensity in the city.
Statistical spectral model of earthquakes in the eastern Tohoku district, Japan, based on the surface and borehole records observed in Sendai
Authors: Satoh Toshimi , Kawase Hiroshi , Sato Toshiaki
Published: September 2021

Strong motions of 18 earthquakes (3.4 ≦ MJ ≦ 7.1, MJ: JMA magnitude) in the eastern Tohoku district, Japan, have been observed at 12 borehole sites within a 20- × 20-km region in Sendai. In our previous study, we defined a Pliocene layer, whose S-wave velocity VS is greater than 500 m/sec and whose N value of the standard penetration test is greater than 50, as engineering bedrock in Sendai and calculated 304 engineering bedrock waves (hypocentral distance X = 15 to 300 km) by removing the site response between the engineering bedrock and the surface. As the second stage of our study toward a quantitative prediction of strong ground motions of horizontal components, we propose here a statistical spectral model of the engineering bedrock waves by introducing the site response between the pre-Tertiary bedrock (VS ≈ 3000 m/sec) and the engineering bedrock (VS ≈ 500 to 700 m/sec) and a frequency-dependent Q into Boore's spectral model. We separate the site response between the pre-Tertiary bedrock and the engineering bedrock, an attenuation function, and source spectra from the engineering bedrock spectra by assuming an ω−2 model with the seismic moment M0 from the Harvard CMT solution estimated for earthquakes of MJ ≧ 5.3. Initial values of corner frequency f0 are determined based on a previous empirical M0 − f0 relationship in this district. Using six moderate-sized earthquakes (5.3 ≦ MJ ≦ 6.0), we first estimate frequency-dependent Q to be Q = 110f0.69 (f: frequency) by minimizing the standard deviation of the site response, which is defined as the ratio of observed engineering bedrock spectra with respect to the estimated attenuation and the assumed source spectra. The averaged site response is simultaneously estimated to be 1 at 0.1 Hz, 5 at 1 Hz, and 3 at 20 Hz. We then invert f0 and cutoff frequencies fmax for all 18 earthquakes and M0 for seven small-sized earthquakes of MJ < 5.3 by minimizing the difference between the model and observed spectra. The average Brune stress drop obtained from an M0 − f0 relationship estimated from 17 earthquakes except for the smallest earthquake with M0 less than 1021 dyne·cm is 200 bars. The estimated M0 − f0 relationship is identical to the one used to calculate initial values of f0 so that we do not need to perform the inversion iteratively. The obtained stress drop for subduction zone earthquakes in the eastern Tohoku district is consistent with other previous studies. The dependence of the inverted fmax on M0 is not significant, and the logarithmic average of fmax is found to be 13.5 Hz. By using these controlling parameters and the M0 − f0 relationship obtained from a regression analysis, acceleration spectra on the engineering bedrock in Sendai can be predicted statistically from X and M0 or MJ by considering standard deviations of the site response, f0, and fmax.
Discrimination of earthquakes and explosions in southern Russia using regional high-frequency three-component data from the IRIS/JSP Caucasus network
Authors: Kim W.-Y. , Aharonian V. , Lerner-Lam A. L. , Richards P. G.
Published: September 2021

High-frequency regional records from small earthquakes (magnitude <4.5) and comparable magnitude chemical explosions are analyzed to find a reliable seismic discriminant in southern Russia near Kislovodsk. The digital, three-component seismograms recorded during 1992 by the Caucasus Network operated by Lamont-Doherty Earth Observatory since 1991 in the distance ranges 15 to 233 km are used. Mean vertical-component Pg/Lg spectral amplitude ratios in the band 8 to 18 Hz are about 1.3 and 3.2 for earthquakes and explosions, respectively, in this region. We find that the vertical-component Pg/Lg spectral ratio in the frequency band 8 to 18 Hz serves quite well for classifying these events. A linear discriminant function analysis indicates that the Pg/Lg spectral ratio method provides discrimination power with a total misclassification probability of about 7%. The Pg/Lg spectral ratios of rotated, three-component regional records improve the discrimination power of the spectral ratio method over the vertical-component Pg/Lg ratios. Preliminary analysis indicates that distance-corrected vertical-component Pg/Lg ratios improve the discrimination power by about 4% over uncorrected ratios. But we find that an even better discriminant is the Pg/Lg spectral ratio of the three-component regional records corrected for the free-surface effect. In the frequency band 8 to 18 Hz, the free-surface-corrected three-component Pg/Lg spectral ratio provides discrimination power with a total misclassification probability of only 2.6%. Free-surface-corrected and network-averaged Pg/Lg spectral ratios provide transportability of the spectral ratio method to various regions worldwide.
Site amplification in the San Fernando Valley, California: Variability of site-effect estimation using theS-wave, coda, andH/Vmethods
Authors: Bonilla Luis Fabián , Steidl Jamison H. , Lindley Grant T. , Tumarkin Alexei G. , Archuleta Ralph J.
Published: September 2021

During the months that followed the 17 January 1994 M 6.7 Northridge, California, earthquake, portable digital seismic stations were deployed in the San Fernando basin to record aftershock data and estimate site-amplification factors. This study analyzes data, recorded on 31 three-component stations, from 38 aftershocks ranging from M 3.0 to M 5.1, and depths from 0.2 to 19 km. Site responses from the 31 stations are estimated from coda waves, S waves, and ratios of horizontal to vertical (H/V) recordings. For the coda and the S waves, site response is estimated using both direct spectral ratios and a generalized inversion scheme. Results from the inversions indicate that the effect of Qs can be significant, especially at high frequencies. Site amplifications estimated from the coda of the vertical and horizontal components can be significantly different from each other, depending on the choice of the reference site. The difference is reduced when an average of six rock sites is used as a reference site. In addition, when using this multi-reference site, the coda amplification from rock sites is usually within a factor of 2 of the amplification determined from the direct spectral ratios and the inversion of the S waves. However, for nonrock sites, the coda amplification can be larger by a factor of 2 or more when compared with the amplification estimated from the direct spectral ratios and the inversion of the S waves. The H/V method for estimating site response is found to extract the same predominant peaks as the direct spectral ratio and the inversion methods. The amplifications determined from the H/V method are, however, different from the amplifications determined from the other methods. Finally, the stations were grouped into classes based on two different classifications, general geology and a more detailed classification using a quaternary geology map for the Los Angeles and San Fernando areas. Average site-response estimates using the site characterization based on the detailed geology show better correlation between amplification and surface geology than the general geology classification.
Moho dip and crustal anisotropy in northwestern Nevada from teleseismic receiver functions
Authors: Peng Xiaohua , Humphreys Eugene D.
Published: September 2021

Receiver functions are derived from teleseismic waves recorded during the 1988 to 1989 PASSCAL Basin and Range passive-source seismic experiment in northwestern Nevada. A velocity model involving both a planar dipping Moho and crustal anisotropy is needed to explain the radial and tangential motions of the observed PS conversions. An arrival-time difference often observed between radial and tangential Moho PS conversions suggests an anisotropic crust. The PS conversions are large and indicate a major discontinuity under the area. The particle motion directions for most of the tangential components change sign between South American events and events from the other two source areas (Japan and Tonga regions), providing good evidence for a Moho dipping approximately to the north. Also, the PS conversions from the Moho follow direct P arrivals by about 3.2 sec under the southern part of the array, 3.4 sec under the southern central part of the array, and 3.7 sec under the northern part of the array, indicating a Moho that varies in depth from about 26 to 31 km. A velocity model with the Moho dipping ∼9° in a nearly northerly direction and an anisotropic crust with a split time of ∼0.25 sec and a fast axis of ∼130° best explain these observations.
Stable baseline correction of digital strong-motion data
Authors: Chiu Hung-Chie
Published: September 2021

Most baseline errors of analog strong-motion data still exist in highresolution data. In this study, we identify the major baseline errors of digital strong-motion data and propose a three-step algorithm to correct these errors. The major baseline errors found in these digital data consist of constant drift in the acceleration, low-frequency instrument noise, low-frequency background noise, the small initial values for acceleration and velocity, and manipulation errors. This threestep algorithm includes fitting the baseline of acceleration by the least squares, applying a high-pass filter in acceleration, and subtracting the initial values in velocity. A least-squares fit of a straight line before filtering can effectively remove the baseline drift in acceleration. Then, the filtering removes the linear trend and other low-frequency errors that exist in the acceleration. Finally, the subtracting of the initial velocity removes the linear trend of displacement. Among these three steps, only the filtering in the second step may introduce a side effect. Compared to the Volume II routine developed by Trifunac and Lee (1973), this three-step processing significantly reduces computational efforts and side effects resulting from unnecessary manipulation of data. This algorithm has been successfully tested on several types of digital strong-motion data. Several independent validations show that the proposed algorithm is stable.