bluuebunny/crossref_metadata_2025_split huggingface dataset:
Advancement in source estimation techniques using broadband regional seismograms
Authors: Zhu Lupei , Helmberger Donald V. 
Published: September 2021

Abstract
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

Abstract
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

AbstractTen 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

Abstract
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

Abstract
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

AbstractWe 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

AbstractThe 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

AbstractThe 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

AbstractDuring 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

Abstract
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 &gt; 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

AbstractThe 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

Abstract
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 &lt; 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

AbstractHigh-frequency regional records from small earthquakes (magnitude &lt;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.