bluuebunny/crossref_metadata_2025_split huggingface dataset:
State of stress from focal mechanisms before and after the 1992 landers earthquake sequence
Authors: Hauksson Egill 
Published: September 2021

AbstractThe state of stress in the Eastern California Shear Zone (ECSZ) changed significantly because of the occurrence of the 1992 Mw 6.1 Joshua Tree and the MW 7.3 Landers earthquakes. To quantify this change, focal mechanisms from the 1975 Galway Lake sequence, the 1979 Homestead Valley sequence, background seismicity from 1981 to 1991, and the 1992 Landers sequence are inverted for the state of stress. In all cases, the intermediate principal stress axis (S2) remained vertical, and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes (S1 and S3) and small variations in the value of the relative stress magnitudes (ϕ). In general, the stress state in the ECSZ has S1 trending east of north and ϕ = 0.43 to 0.65, suggesting that the ECSZ is a moderate stress refractor and the style of faulting is transtensional.South of the Pinto Mountain fault, in the region of the 1992 Joshua Tree earthquake, the stress state determined from the 1981 to 1991 background seismicity changed on 23 April and again on 28 June 1992. In the central zone, S1 rotated from N14° ± 5°E to N28° ± 5°E on 23 April and back again to N16° ± 5°E on 28 June. Thus, the Landers mainshock in effect recharged some of the shear stress in the region of the Mw 6.1 Joshua Tree earthquke.Comparison of the state of stress before and after 28 June 1992, along the Landers mainshock rupture zone, showed that the mainshock changed the stress orientation. The S1 trend rotated 7° to 20° clockwise and became progressively more fault normal from south to north. Along the Emerson-Camp Rock faults, the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor. The complex distribution of P and T axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock.The San Bernardino Mountains region of the Mw 6.2 Big Bear earthquake has a distinctively different state of stress, as compared to the Landers region, with S1 trending N3° ± 5°W. This region did not show any significant change in the state of stress following the 1992 Mw 6.2 Big Bear sequence.
The 1991 Sierra Madre earthquake sequence in southern California: Seismological and tectonic analysis
Authors: Hauksson Egill 
Published: September 2021

Abstract
The (ML 5.8) Sierra Madre earthquake of 28 June 1991 occurred at a depth of 12 km under the San Gabriel Mountains of the central Transverse Ranges. Since at least 1932 this region had been quiescent for M ≧ 3. The mainshock focal mechanism derived from first-motion polarities exhibited almost pure thrust faulting, with a rake of 82° on a plane striking N62°E and dipping 50° to the north. The event appears to have occurred on the Clamshell-Sawpit fault, a splay of the Sierra Madre fault zone. The aftershock sequence following the mainshock occurred at a depth of 9 to 14 km and was deficient in small earthquakes, having a b value of 0.6. Twenty nine single-event focal mechanisms were determined for aftershocks of M &gt; 1.5. The 4-km-long segment of the Clamshell-Sawpit fault that may have ruptured in the mainshock is outlined by several thrust focal mechanisms with an east-northeast-striking fault plane dipping to the north. To the west, several thrust aftershocks with east-striking nodal planes suggest some complexity in the aftershock faulting, such as a curved rupture surface. In addition, several strike-slip and normal faulting events occurred along the edges of the mainshock fault plane, indicating secondary tear faulting. The tectonic stress field driving the coexisting left-lateral strike-slip and thrust faults in the northern Los Angeles basin is north-south horizontal compression with vertical intermediate or minimum principal stress axis.
Initial investigation of site and topographic effects at Robinwood Ridge, California
Authors: Hartzell Stephen H. , Carver David L. , King Kenneth W. 
Published: September 2021

Abstract
Following the 1989 Loma Prieta earthquake, a dense array of seven digitally recorded, three-component seismograph stations was deployed on Robinwood Ridge 7.3 km northwest of the epicenter. The purpose of this array was to investigate the cause of high levels of structural damage and ground cracking observed on the ridge crest. Aftershocks recorded by the array allow a comparison of ground motion up the slope of the ridge from the base to the crest. The data present an extremely complicated pattern of ground motion that demonstrates the importance of the three-dimensionality of the problem. Slowness analysis of P wave trains show initial arrivals propagating away from the source with small angles of incidence and large apparent velocities, consistent with direct arrivals. After 0.5 sec, propagation azimuths become more random and apparent velocities drop, indicating nearly horizontal wave propagation and multiply reflected and diffracted phases within the ridge. Slowness analysis and particle motion diagrams of horizontal components of motion show dramatic variations in ground motion with changes in azimuth of the source and a complicated interaction between body waves and Rayleigh and Love waves. Results suggest that the larger amplitude, more coherent arrivals at the array stations favor a propagation direction parallel to the ridge axis. An amplification factor of from 1.5 to 4.5 is seen for frequencies from 1.0 to 3.0 Hz with wavelengths comparable to the base of the ridge, part of which may be caused by local site effects and part by topographic amplification. In addition, amplifications of up to a factor of 5 are seen at higher frequencies and are attributed to local site effects. These effects are most notable from 4 to 8 Hz on the vertical components, and from 6 to 9 Hz on the horizontal components. The entire Robinwood Ridge area may also have been situated in a region of heightened mainshock ground motion due to source directivity and radiation pattern effects.
Frequency-dependent attenuation of shear waves in the crust of the southern Kanto area, Japan
Authors: Kinoshita Shigeo 
Published: September 2021

Abstract
The attenuation characteristics of shear waves in the crust of the southern Kanto area, central Japan, were estimated using strong-motion data, including acceleration data recorded in the pre-Tertiary basement rocks by means of downhole observation. The quality factor Qs(f) was determined for a range of discrete frequencies from 0.5 to 16 Hz from the analysis of data from 13 local earthquakes with focal depths of less than about 50 km that occurred in the Philippine Sea plate and in the boundary zone between the lower part of the Eurasian plate and the upper part of the Philippine Sea plate. The estimated 1/Qs(f) shows a peaked structure in this frequency range on the assumption that the geometrical spreading exponent is -1 (body waves). The estimated peak 1/Qs(f) is of the order of 10-2 at 0.8 Hz.
A comparison of seismicity rates near Adak Island, Alaska, September 1988 through May 1990 with rates before the 1982 to 1986 apparent quiescence
Authors: Kisslinger Carl , Kindel Bruce 
Published: September 2021

Abstract
The rates of seismicity in the part of the Aleutian arc monitored by the Central Aleutians Seismic Network and in subregions of that zone during September 1988 through June 1990 have been compared with the rates during 1977 through 1981, 1981 through 1982, and 1983 through 1985. The purpose of the study was to determine whether the rate of earthquake occurrence had returned to the level observed before 1982, when a prolonged decrease in activity began. This decrease had been interpreted as seismic quiescence and was used as the basis of a prediction of a strong earthquake in this area. An MW = 8.0 earthquake did occur on 7 May 1986. The comparison with more recent activity is intended as one test of whether the observed decrease in activity was a real physical phenomenon or had been artificially produced by changes in network instrumentation or data-processing procedures. The only change in instrumentation since 1986 was the installation of a new digitizer, and tests were made to confirm that this new unit and the installation of new software for picking arrival times did not alter the completeness of detections or the magnitude values. The test cannot be completely definitive because aftershocks of the 1986 event were still in progress, according to the modified Omori fit to the data. The interval used is the latest possible because the network has been terminated. The 1988 to 1990 rates are similar to those in the prequiescence interval, but higher by an amount that is in qualitative agreement with the expected rate of continuing aftershocks. The rates in most of the zone are significantly higher than the rates during the period of decreased activity. Rates in the sites near the 1986 mainshock epicenter and the western termination of the 1986 rupture are higher than the pre-1982 rates, and indicate a concentration of the late aftershocks in these places. The results of the test support the conclusion that the 1982 to 1985 decrease in cataloged activity represents a real drop in the rate of seismicity prior to the 1986 earthquake. Examination of many more cases of seismicity changes before and following a strong earthquake is needed as the basis for judging the validity of quiescence as a true precursor. Another observation from recent seismicity is that an intense swarm in January 1989 and a magnitude mb = 6.1 event in March 1990 with a well-developed aftershock sequence have filled a spatial gap in the aftershocks of the 1986 event, near its epicenter.
Broadband waveform observation of the 28 June 1991 Sierra Madre earthquake sequence (ML = 5.8)
Authors: Ma Kuo-Fong , Kanamori Hiroo 
Published: September 2021

Abstract
The Sierra Madre earthquake (MI = 5.8) of 28 June 1991 occurred at a depth of about 12 km, on the Clamshell-Sawpit fault in the San Gabriel Mountains. High-quality seismograms were recorded with TERRAscope not only for the mainshock but also for the aftershocks at epicentral distances of about 16 km. We determined the focal mechanisms and seismic moments of the mainshock and 21 aftershocks by combining the waveform and first-motion data. We classified the events into five groups according to the location and waveforms recorded at PAS. Most events located within 5 km west of the mainshock are similar to the mainshock in waveform. The mechanisms thus determined are thrust mechanisms. A few events located east of the mainshock have waveforms different from the mainshock and have strike-slip mechanisms. The average Qβ values along the paths from the hypocenters of the Sierra Madre and the 3 December 1988 Pasadena earthquake (ML = 4.9) to PAS are about 130 and 80, respectively. The stress drop of the mainshock is about 500 bars. Most of the aftershocks have stress drops between 10 and 100 bars.
Seismic response of three-dimensional alluvial deposit with irregularities for incident wave motion from a point source
Authors: Kawano Masahiro , Matsuda Satoshi , Toyoda Kozo , Yamada Jun 
Published: September 2021

AbstractThe purpose of this article is to further investigate the influence of local site conditions of surface soil layers on seismic ground motion by studying the response of a semi-spherical alluvial deposit model under the more realistic type incident wave field. The incident wave field is a spherical wave radiated from a point source that is situated at the near focal distance and the far focal distance. A plane SH wavelet is also considered for comparison. A least-squares technique is applied to solve the boundary problem. The surface displacements on the alluvial deposit and in the elastic half-space are evaluated for various focal distances and source incidence angles. The numerical results indicate that there are significant differences concerning scattering and diffraction of wave motions between the two horizontal components when the semi-spherical alluvial deposit is subjected to incident wave motions from a point source. In the near field and for the component involving predominantly P and SV waves, the coupling of these waves induces a great variation in the spatial distributions, the amplitudes, and the time histories of the surface displacements compared to the tangential component, which involves mostly SH waves. The wave propagation characteristics of this component are very similar to those for a plane SH wavelet. In the far field and for the tangential component, there is not such strong coupling among waves. It is also shown that the first reflected waves at the curved boundary are deeply related to scattering and to diffraction of wave motion in a surface soil layer with irregularities and play an important role in the development of later arrivals.
Relationships between ocean bottom noise and the environment
Authors: Babcock Jeffrey M. , Kirkendall Barry A. , Orcutt John A. 
Published: September 2021

AbstractObservations of ocean bottom low-frequency noise and surface environmental data over a period of 27 days in the northern Atlantic during the SAMSON and SWADE experiments reveal how closely related the noise is to meteorological conditions. Double-frequency microseisms produced by nonlinear interactions of storm-induced surface gravity waves are especially evident in the frequency band 0.16 to 0.3 Hz and show a high variability in both amplitude and peak frequencies. Bifurcated at times, the peak that characterizes the microseism band contains local and distant or “teleseismic” components, which are generated at different locations. Weather and storm fetch appear to be the major contributions to the size and shape of microseism spectra. Storm development on the sea surface is associated with progressively lower microseism frequencies along with a concurrent increase in amplitude. The single-frequency microseism peak is a continuous feature and is observed to portray the same time-dependent spectral characteristics as the portion of the double-frequency peak associated with distant storms. Coherence studies confirm that both peaks (single and teleseismic double) originate at a distant source. These peaks are generated at roughly the same location with some storm component over the coastline.
The rupture process and aftershock distribution of the 6 April 1992 MS 6.8 earthquake, Offshore British Columbia
Authors: Cassidy John F. , Rogers Garry C. 
Published: September 2021

Abstract
On 6 April 1992, a magnitude 6.8 (MS) earthquake occurred in the triple-junction region at the northern end of the Cascadia subduction zone. This was the largest earthquake in at least 75 yr to occur along the 110-km-long Revere-Dellwood-Wilson (RDW) transform fault and the first large earthquake in this region recorded by modern broadband digital seismic networks. It thus provides an opportunity to examine the rupture process along a young (&lt;2 Ma) oceanic transform fault and to gain better insight into the tectonics of this triple-junction region. We have investigated the source parameters and the rupture process of this earthquake by modeling broadband body waves and long-period surface waves and by accurately locating the mainshock and the first 10 days of aftershocks using a well-located “calibration” event recorded during an ocean-bottom seismometer survey. Analysis of P and SH waveforms reveals that this was a complex rupture sequence consisting of three strike-slip subevents in 12 sec. The initial rupture occurred 5 to 6 km to the SW of the seafloor trace of the RDW fault at 50.55° N, 130.46° W. The dominant subevent occurred 2 to 3 sec later and 4.3 km beneath the seafloor trace of the RDW fault, and a third subevent occurred 5 sec later, 18 km to the NNW, suggesting a northwestward propagating rupture. The aftershock sequence extended along a 60- to 70-km-long segment of the RDW fault, with the bulk of the activity concentrated ∼30 to 40 km to the NNW of the epicenter, consistent with this interpretation. The well-constrained mechanism of the initial rupture (strike/dip/slip 339°/90°/−168°) and of the largest aftershock (165°/80°/170°) are rotated 15° to 20° clockwise relative to the seafloor trace of the RDW fault but are parallel to the Pacific/North America relative plate motion vector. In contrast, the mechanisms of the dominant subevent (326°/87°/−172°), and the long-period solution derived from surface waves aligns with the RDW fault. This suggests that small earthquakes (M &lt; 6) in this area occur along faults that are optimally aligned with respect to the regional stress field, whereas large earthquakes, involving tens of kilometers of rupture, activate the RDW fault. For the mainshock, we estimate a seismic moment (from surface waves) of 1.0 × 1026 dyne-cm, a stress drop of 60 bars, and an average slip of 1.2 m. This represents only 21 yr of strain accumulation, implying that there is either a significant amount of aseismic slip along the RDW fault or that much of the strain accumulation manifests itself as deformation within the Dellwood and Winona blocks or along the continental margin.
Earthquake “Quanta” as an explanation for observed magnitudes and stress drops
Authors: Selwyn Sacks I. , Rydelek Paul A. 
Published: September 2021

Abstract
The familiar linear relation (Gutenberg-Richter) between the logarithm of the number of earthquakes and their magnitude is commonly ascribed to the distribution (fractal) of fault sizes in a self-similar process. We show that a concept of earthquake quanta whose failure is governed by simple physics and suggested by observations explains not only the Gutenberg-Richter relation but also the relatively constant stress drop for larger magnitude events. Results from computer simulation are consistent with observations from detailed seismicity studies.
Relationship between Liao and Clayton-Engquist absorbing boundary conditions: Acoustic case
Authors: Cheng Ningya , Cheng Chuen Hon 
Published: September 2021

AbstractIn this short note, we derive the differential form of Liao's multi-transmitting formula. The reflection coefficient of the multi-transmitting formula in the acoustic case is obtained in closed form. These formulas are compared with Clayton-Engquist absorbing boundary condition and are shown to have a very close relationship.
Propagation of high-frequency (4- to 50-Hz) P waves in the northeastern United States
Authors: Chael Eric P. , Leahy Patrick J. , Carter Jerry A. , Barstow Noël , Pomeroy Paul W. 
Published: September 2021

Abstract
We have measured the decay rate of high-frequency (4- to 50-Hz) P waves in the northeastern United States. We analyzed signals from 28 explosions of a 1988 USGS/AFGL/GSC refraction survey recorded at distances between 30 and 400 km. Over this range, the decay rate steadily increases from Δ−2 at 10 Hz to Δ−4 at 45 Hz. If one assumes geometric spreading of Δ−1.3, then the remaining decay is consistent with a nearly frequency-independent Q of about 1000. The results provide a useful parameterization for predicting P-wave spectra at near-regional ranges.
Comment on “A hybrid method for the estimation of ground motion in sedimentary basins: Quantitative modeling for Mexico City” by D. Fäh, P. Suhadolc, St. Mueller, and G. F. Panza
Authors: Zahradník Jiří 
Published: September 2021

Abstract
Two possible extensions of the commented article are suggested, with the aim to more efficiently couple the source and path simulation on one side, and the site-effect simulation on the other: (1) New excitation lines, surrounding the sedimentary basin, reduce the necessary size of the 2D finite-difference model of the site. They also better represent the 3D nature of the excitation, including the focal-mechanism effect. (2) New (2.5D approximate) equations of motion improve the way the 3D excitation is propagated within the 2D site model. The two suggested improvements can help in solving problems when a purely 2D treatment is oversimplified, but a true 3D treatment is too expensive.
Lateral variation in crustal structure of the northern Tibetan Plateau inferred from teleseismic receiver functions
Authors: Zhu Lupei , Owens Thomas J. , Randall George E. 
Published: September 2021

AbstractWe investigate lateral variations in crustal structure across the northern boundary of the Tibetan Plateau using the receiver functions at three broadband stations deployed during the 1991-1992 Tibet PASSCAL experiment. The first 5 sec of the receiver functions vary systematically with backazimuth: the radial receiver functions are symmetric across the N-S axis while the tangential receiver functions are antisymmetric across this axis. This symmetry can be modeled by E-W striking dipping interfaces in the upper-middle crust. The strike direction is consistent with the E-W trend of surface geology. Modeling a P-to-S converted phase in the receiver functions at each station suggests that there is a mid-crustal low-velocity layer with its upper boundary dipping 20° to 30° to the south. In addition, a shallow northward-dipping interface is responsible for the “double-peaked” direct P arrivals in the radial receiver functions and large tangential motions at one of the stations. The low-velocity layer, together with other geological and seismological observations, suggests that there is a hot, possibly partial melt zone in the middle crust of northern Tibet. Alternately, dipping velocity interfaces might be associated with some buried thrust faults in the upper crust that accommodated crust shortening during the plateau formation.
Numerical modeling of fault-zone trapped waves: Acoustic case
Authors: Huang Bor-Shouh , Teng Ta-liang , Yeh Yeong Tein 
Published: September 2021

Abstract
We have obtained numerical results for an acoustic wave propagating in a 3D medium with mixed boundary conditions. The specific model involves a vertical low-velocity fault zone of varying thickness embedded in an otherwise homogeneous half-space. At the fault-zone boundary, the stress and displacement are continuous; on the free surface, the stress is zero. A pseudo-spectrum method is employed to achieve sufficient resolution with reasonable computation time on a CRAY. Results show the development of guided waves trapped in the fault zone. These guided waves display large amplitudes and lengthening waveforms; they propagate at lower velocity with amplitudes that drop off mainly due to energy leakage out of the fault zone. As the width of the fault zone varies, the wave energy tends to funnel into the new low-velocity wave guide. At large distance, these guided waves become the dominant arrivals on seismograms. The waveforms are useful to recover the geometry and properties of the fault zone.