bluuebunny/crossref_metadata_2025_split huggingface dataset:
A perturbation scheme for obtaining partial derivatives of Love-wave group-velocity dispersion
Authors: Kosloff Dan 
Published: September 2021

abstract
A method is derived for obtaining partial derivatives of Love-wave group-velocity spectra for a layered medium using a second-order perturbation theory. These partials are a prerequisite for systematic inversion of group-velocity spectra but they are helpful as well in trial and error methods. Mathematically the equation of motion and boundary conditions for Love waves are a singular Sturm Liouville type eigenvalue problem. In the case of a fixed wave number, the eigenvalues are the negative of the square of the frequencies. Thus, by expressing the first- and second-order perturbations of the eigenvalues in terms of partial derivatives of the frequency with respect to the wave number and material parameters of the medium, one can relate these perturbations to group-velocity partials. The scheme should be relatively economical and easy to incorporate in Love-wave dispersion codes.
Accuracy in phase determination of LP electromagnetic seismographs based on transient calibration pulse
Authors: Mitronovas W. 
Published: September 2021

abstract
A modification of the Jarosch and Curtis (1973) formulation for the calibration pulse inversion is suggested in an effort to improve its accuracy. The modification involves the introduction of the origin time of the pulse (time of the step in the calibration current) as an additional parameter to be solved in the least-squares inversion. This approach is found to be not completely satisfactory in general, but does improve the solution when a large error in origin time (&gt; ±0.5 sec) is introduced in digitizing the pulse. A typical solution, when the origin time of the pulse has to be determined by least-squares, is accurate to within ± 1 sec in the phase response of a system for periods up to 500 sec. A more satisfactory approach is to record the origin time of calibration pulses on seismograms. Only a minor modification of the existing calibration circuit is necessary to provide a simultaneous signal to a time relay and calibration coil through a double pole switch. For pulses, where the origin time is known accurately, the existing techniques give results accurate to within ± 0.3 sec in phase response.
The identification of building structural systems
Authors: Udwadia Firdaus E. , Marmarelis Panos Z. 
Published: September 2021

abstract
This paper investigates the response of structural systems to strong earthquake ground shaking by utilizing some concepts of system identification. After setting up a suitable system model, the Weiner technique of nonparametric identification has been introduced and its experimental applicability studied. The sources of error have been looked into and several new results have been presented on accuracy calculations stemming from the various assumptions in the Wiener technique.
The method has been applied in studying the response of a 9-story reinforced concrete structure to earthquake excitation as well as ambient vibration testing. The linear contribution to the total roof response during strong ground shaking has been identified, and it is shown that a marked nonlinear behavior is exhibited by the structure during the strong-motion portion of the excitation.
A physical basis for earthquakes based on the elastic rebound model
Authors: Ohnaka Mitiyasu 
Published: September 2021

abstract
The elastic rebound model explaining seismological data quantitatively is derived by developing the original elastic rebound theory proposed by H. F. Reid. Assuming that the dislocation front propagates in one direction along the long axis of the fault plane, the shear strain drop Δɛ, the earthquake volume V, the stiffness of the fault, the mass of inertia, and the seismic energy radiated Es are evaluated in terms of the fault-plane dimensions, the dislocation D, the propagating velocity of dislocation v, and the Shear-wave velocity. The elastic strain energy released is evaluated in terms of V, Δɛ, and the initial shear strain. It is shown that the order of magnitude of Es is virtually given by μWD2, where μ is the rigidity and W is the fault width. The order of magnitude of the initial slip acceleration is estimated by making use of the formula derived in a previous paper. The moment of the elastic rebound force is calculated. The maximum amplitude of the far-field wave motion is in proportion to vM0/L, where M0 is the seismic moment and L is the fault length: this predicts that log (M0/L) is linearly related to the magnitude M, if v is assumed to be almost constant for actual earthquakes. The good linear relation, log (M0/L) = 1.2M + 11.7 (M0/L in dynes), is found empirically over a wide range of M (2 ≦ M ≦ 8.5). The directly proportional relationship between the logarithm of seismic moment per unit area and the magnitude seems to hold empirically.
Three-dimensional seismic structure of the lithosphere under Montana Lasa
Authors: Aki Keiiti , Christoffersson Anders , Husebye Eystein S. 
Published: September 2021

abstract
Using P-wave residuals for teleseismic events observed at the Montana Large Aperture Seismic Array (LASA), we have determined the three-dimensional seismic structure of the lithosphere under the array to a depth of 140 km. The root-mean-square velocity fluctuation was found to be at least 3.2 per cent which may be compared to estimate of ca. 2 per cent based on the Chernov random medium theory. The solutions are given by both the generalized inverse and stochastic inverse methods in order to demonstrate the relative merit of different inversion techniques.
The most conspicuous feature of the lithosphere under LASA is a low-velocity anomaly in the central and northeast part of the array siting area with the N60°E trend and persisting from the upper crust to depths greater than 100 km. We interpret this low-velocity anomaly as a zone of weakness caused by faulting and shearing associated with the building of the Rocky Mountains.
A permanent seismograph array around the Gulf of California
Authors: Brune James N. , Lomnitz Cinna , Allen Clarence , Mooser Federico , Lehner Francis , Reyes Alfonso 
Published: September 2021

abstractA permanent seismographic array has been established around the Gulf of California. Solar-powered stations are operating at Caborca, Bahia de los Angeles, Guaymas, Topolobampo and La Paz. Trailer stations using local 60-cycle power are operating at San Felipe, Rio Hardy, Rancho Meling, and Ensenada. The array is unique in the following aspects: (1) It is the only array operating in close proximity to an active region of sea-floor spreading; (2) its sophisticated low-power design and use of solar energy made siting choices simpler so that local noise sources could be minimized; (3) it is the first accurately timed array to operate in northwest Mexico.The array has already provided important information on seismicity in the Gulf of California and has aided in several sonobuoy studies of swarms and aftershock sequences. As more data accumulate, many of the important questions concerning the seismicity, tectonics, and structure of the Gulf of California will be answered.
The August 1975 Oroville earthquakes
Authors: Lahr K. M. , Lahr J. C. , Lindh A.G. , Bufe C. G. , Lester F. W. 
Published: September 2021

abstract
On August 1, 1975, a magnitude 5.9 (mb) earthquake occurred approximately 8 km SSE of the town of Oroville, California. This earthquake and its associated foreshock-aftershock sequence are of particular interest because of their possible relation to the impounding of the 4.3 billion m3 Lake Oroville. Hypocenter locations for 336 aftershocks that occurred during August define a fault plane striking N3°E and dipping 60° to the west to a depth of 10 km. Dimensions of the epicentral area are approximately 7 km in an east-west direction by 15 km in a north-south direction. The fault plane passes beneath Oroville Dam at 5-km depth, and if projected up dip, would crop out beneath the reservoir to the east. The distribution in space and time of foreshocks and aftershocks suggests that rupture began at depth and progressed up dip and to the north and south.
Strong-motion earthquake accelerograms at Brawley, California: January 25, 1975
Authors: Johnson Dennis A. , Hanks Thomas C. 
Published: September 2021

abstract
During the Brawley earthquake swarm of January–February, 1975, a single strong-motion accelerograph installed after the swarm began recorded 27 positively identified earthquakes in 14 hr, at hypocentral distances of approximately 5 km. Eight of these had magnitudes (ML) ≧ 3.5, and several had ML &lt; 2.5; the instrument independently triggered 18 times. Peak accelerations in excess of 10 per cent g were obtained for five of the eight earthquakes with ML ≧ 3.5, and a peak acceleration of 25 per cent g was recorded for the largest event, ML = 4.3.
The far-field spectrum of the displacement generated by a sudden SH crack
Authors: Husseini Moujahed I. 
Published: September 2021

abstract
The far-field spectrum of the displacement caused by a two-dimensional SH crack (antiplane shear crack) that appears suddenly over a strip of width 2R is obtained in terms of the elliptic cylinder Mathieu functions. The amplitude spectrum of the displacement exhibits the following four features in terms of the normalized frequency h (h = Rω/β where R is the half-width of the crack, ω is the circular frequency and β is the shear-wave velocity): (1) for h &lt; 1 the amplitude spectrum behaves as h−1,2; (2) at h = 1 the amplitude spectrum has a corner frequency; (3) for h &gt; 1 the amplitude spectrum behaves as h−2; (4) the azimuthal dependence of the amplitude is like sin θ.
The first result is a consequence of the infinite area occupied by the crack. The corner frequency at h = 1 is smaller than the value proposed in other studies (e.g., Brune, 1970; Randall, 1973), and this is attributed to the two-dimensional nature of the problem. The high-frequency falloff of h−2 is in agreement with the model of Brune (1970). The radiation pattern of sin θ is identical to that obtained by Burridge and Halliday (1971) for the same problem.
The effect of tectonic stress release on explosionP-wave signatures
Authors: Bache Thomas C. 
Published: September 2021

abstractThe influence of induced tectonic stress release on the short-period teleseismic P-wave signature of underground nuclear explosions is studied. Primary attention is directed to the first few cycles of the record from which body-wave magnitude (mb) is determined. Computational models for both the explosion and the superimposed tectonic release double couple are employed and theoretical seismograms are computed. Interest is mainly in the largest tectonic release component that seems reasonable using surface-wave observations and independent estimates of the controlling parameters as constraints. It is concluded that for most, perhaps all, events, tectonic release has no appreciable effect on the amplitude of the short-period P waves. Even the frequency content of the early arriving P wave is little affected by tectonic release for most likely circumstances.The computations assume tectonic release due to stress relaxation around the fracture zone created by the explosion. However, the results are extended to apply to the alternate mechanism whereby stress is released along a pre-existing fault plane. Since a number of other mechanisms can cause superposition of a double couple on the explosion, the analysis is relevant to the general question of the size these secondary sources must attain before the short-period P-wave recording is significantly altered.
Seismicity, gravity, and tectonics of northeast India and northern Burma
Authors: Verma R. K. , Mukhopadhyay Manoj , Ahluwalia M. S. 
Published: September 2021

abstract
Practically the whole of northeastern India and northern Burma is characterized as an anomalous gravity field as well as an area of high seismicity. The Bouguer anomaly in the region varies from +44 mgals over Shillong Plateau to −255 mgals near North Lakhimpur in Assam Valley. Isostatic anomaly (Hayford) varies from +100 to −130 mgals in these areas. Over Arakan-Yoma and the Burmese plains, the isostatic anomalies vary from −20 mgals to −100 mgals.
Regions of high seismicity in the area include the eastern Himalaya (including Assam syntaxis), Arakan-Yoma including the folded belt of Tripura, Irrawaddy basin, Shillong Plateau, Dauki fault and the northern part of Bengal basin. The abnormal gravity and seismicity are related to large scale tectonic movements that have taken place in the area mostly during the Cretaceous and Cenozoic times, due to interaction of the Indian, Tibetan, and Burmese plates. The high seismicity indicates that the movements are continuing. The seismic zone underlying Burma is approximately V shaped and dips toward the east underneath Arakan-Yoma. Most of the intermediate-focus earthquakes in Burma underlie the area characterized by negative isostatic anomalies, indicating the probable existence of a subduction zone underneath the Arakan-Yoma and the Burmese plains.
The Shillong Plateau has a history of vertical uplift since Cretaceous times. Provided this statement is true, the uplift of the plateau preceded Himalayan tectonics starting 20 to 30 m.y. before continental India made solid contact with the Eurasian plate. The plateau is characterized by large positive isostatic anomalies as well as high seismicity. The positive isostatic anomalies may be due to intrusion or incorporation of basic material from the mantle into the crust underlying the Plateau. These intrusions may have taken place through deep seated faults such as the Dauki and could be responsible for its uplift as well.
Microearthquake survey of Tehran region, Iran
Authors: Hedayati A. , Brander J. L. , Berberian M. 
Published: September 2021

abstract
A short microearthquake survey of part of north central Iran around the city of Tehran made late in 1974, showed that earthquakes occurred at the rate of 21.37 events per day.
Epicentral locations for 37 events, using a geometric ray path technique, show the observed activity to be concentrated in three principal groups: two associated with the known major fault on the south flank of the Alborz mountain, while the third constitutes a previously unrecognized lineation passing very close to the city of Tehran.
The cumulative number versus magnitude relation was obtained from the study of 198 well-recorded events. The slope of the linear portion of the relation (b value), −0.79, agrees well with the value of −0.86 determined from the USCGS records of larger earthquakes between 1961 and 1973.
Composite fault-plane solutions were made for each of the three principal groups, showing predominantly strike-slip movement with a thrust component on the south of the Alborz and thrusting on the new lineation.
The seismic intensity risk curve calculated for the area shows that the city of Tehran can expect ground movement of MM intensity VII every 12 years, and intensity IX every 350 years.
Intensities and travel times of seismic rays in a sphere with a linear velocity function
Authors: Odegard Mark E. , Fryer Gerard J. 
Published: September 2021

Abstract
Equations are presented which permit the calculation of distances, travel times and intensity ratios of seismic rays propagating through a spherical body with concentric layers having velocities which vary linearly with radius. In addition, a method is described which removes the infinite singularities in amplitude generated by second-order discontinuities in the velocity profile. Numerical calculations involving a reasonable upper mantle model show that the standard deviations of the errors for distance, travel time and intensity ratio are 0.0046°, 0.057 sec, and 0.04 dB, respectively. Computation time is short.
Time and magnitude relations for three microaftershock series near Hollister, California
Authors: Udías Agustín 
Published: September 2021

Abstract
Three series of aftershocks occurred in the region near Hollister, less than 15 km from the San Andreas Geophysical Observatory in 1968, 1970, and 1971 following shocks of magnitudes 2.3, 3.2, and 4.0, respectively. The high-gain system operating at SAO permitted a full study of these series for events as small as magnitude -0.7. The distribution of time intervals between consecutive shocks and magnitudes show certain definite differences in the time, space, and magnitude structure of the three sequences. The 1968 and 1970 series are highly concentrated in time and originate from quite small source volumes. Both correspond to the mainshock-after-shock type with high values of b (1.1 and 0.91). The 1971 series located in the Bear Valley area about 15 km south of the other two belongs to the swarm type with a low value of b (0.41). The situation suggests different physical conditions in two neighboring areas of the same fault with less homogeneous material and higher stresses acting at the area of the 1971 series. Variations in the b value with time for an area of 2500 km2 around SAO can be interpreted as due to migration of seismic sources from one area to another, or definite changes in rock properties with time.
A study of the strong ground motion of the Borrego Mountain, California, earthquake
Authors: Heaton Thomas H. , Helmberger Donald V. 
Published: September 2021

Abstract
Several synthetic models are constructed to fit the first 40 sec of the transversely polarized displacement, as recorded at El Centro, of the April 9, 1968 Borrego Mountain earthquake. The modeling is done in the time domain using the response computed for a distributed set of point shear dislocations embedded in a layered half-space. The beginning 10 sec of the observed record is used to model the spatial and temporal distribution of faulting whereas the remaining portion is used to determine the upper crustal structure based on surface-wave periodicity. A natural depth criterion was provided by comparing the amplitude of the direct arrival with the surface-wave excitations. Trade-offs are found to exist between source models and velocity structure models. Within the framework of a layer over a half-space model, faulting of finite vertical extent is required, whereas the horizontal dimensions of faulting are not resolvable. A model which is also consistent with the teleseismic results of Burdick and Mellman indicates massive faulting near a depth of 9 km with a fast rise time producing a 10-cm displacement pulse of 1 sec duration at El Centro. The faulting appears to slow down approaching the surface. The moment is calculated to be approximately 7 ×1025 dyne-cm which is somewhat smaller than the moment found by Burdick and Mellman (1976).
Amplitude-distance relation for 20-second Rayleigh waves
Authors: von Seggern David 
Published: September 2021

Abstract
Amplitude measurements on Rayleigh waves near a period of 20 sec were extracted from an 8-month data base for nine stations of the High-Gain Long-Period network. Application of an appropriate statistical model resulted in station, event, and distance terms; the distance terms defined a relation significantly different from the standard distance-dependent correction for Ms estimation. The apparent Q for 20-sec Rayleigh waves implied by this new data is roughly 700, in contrast to the 350 implied by the standard correction.
Seismic magnitude from Fourier analysis
Authors: Grant John A. , Mansinha L. 
Published: September 2021

Abstract
The inherent frequency dependence of the maximum peak-to-peak amplitude of seismograms suggests the use of Fourier analysis to calculate magnitudes. Since most magnitude scales use logarithms of amplitudes and amplitude/period ratios, the stability of the equivalent frequency-domain quantities was examined in a study of the spectra of Rayleigh waves from five earthquakes, a total of 73 records. The use of spectral amplitudes in formulas which were empirically developed for use with time-domain measurements resulted in smaller values of magnitude. Nevertheless, the two quantities were found to be linearly related. It was found that the use of the maximum Fourier amplitude resulted in the most stable spectral magnitudes with respect to epicentral distance and the time-domain magnitudes. This indicates that the maximum peak-to-peak amplitude of the time-series is not directly related to the Fourier amplitude at the same period, but rather to the maximum Fourier amplitude. Based on these 73 records for events with MS = 5.9 to MS = 6.7, we conclude that magnitudes may be determined from spectral measurements, with a marginal reduction in standard deviation over the time-domain formulas.
Spectral and magnitude characteristics of anomalous Eurasian earthquakes
Authors: Kim So Gu , Nuttli Otto W. 
Published: September 2021

AbstractA number of main shock-aftershock sequences in the Eurasian interior contain some aftershocks whose mb:MS values are close to those of underground explosions. This paper is concerned with a study of the amplitude spectra of the P waves and Rayleigh waves for earthquakes of those main shock-aftershock sequences. It is found that for any given sequence studied, there is little if any variation in focal depth or focal mechanism. This rules out variations in these quantities as being the cause of anomalous mb:MS values. A study of the P-wave spectra establishes that one or both of the corner periods of anomalous earthquakes are smaller than those of non-anomalous earthquakes of the same moment. Thus the cause of anomalous mb:MS values of the earthquakes studied is a relative enrichment of the short-period portion of the spectrum of the anomalous events, which cannot be attributed to focal depth or focal mechanism.
Noise level variation at norsar and its effect on detectability
Authors: Ringdal F. , Bungum H. 
Published: September 2021

AbstractFor a 3-year period, noise level measurements of short- and long-period data at NORSAR have been sampled at hourly intervals. Significant seasonal fluctuations in noise level have been found, in particular for long-period data. The noise amplitude distribution is approximately lognormal for band-pass filtered short-period data in the P-wave detection band, while the long-period noise data show a skewness that cannot be represented by a lognormal distribution. Diurnal fluctuations in noise level are quite small, but definitely present both for short and horizontal component long-period data. Cultural sources are found to account for the short-period variability, while the long-period fluctuations are attributed to atmospheric pressure variation. Event detection performance generally follows the noise level trends, with an increase in the number of reported events during summer of about 50 per cent relative to winter.