bluuebunny/crossref_metadata_2025_split huggingface dataset:
Trench investigations through the trace of the 1980 El Asnam thrust fault: Evidence for paleoseismicity
Authors: Meghraoui M. , Philip H. , Albarede F. , Cisternas A. 
Published: July 2021

Abstract
During the EI Asnam earthquake of 10 October 1980 (Ms = 7.3), a clear active thrust fault with left-lateral offset was observed. Three trenches have been excavated across this fault in order to determine slip rate and recurrence intervals between large earthquakes, and thus reconstruct its past activity.
Exposure I was excavated in the flood area created in 1980 by a pressure ridge across the Cheliff and Fodda Rivers. Six flood deposits (silty-sandy and muddy horizons) alternating with paleosoils appear in this exposure; they are affected by normal faults associated with the main thrust fault. Assuming that every flood deposit results from a tectonic event of magnitude greater than 7, we can correlate previous flood deposits with these events. Exposures II and III display thrust faults displacing different paleosoils. We propose a sequence of reconstructions based on the thickness of the various deposits and the dip-slip of each tectonic event.
The Late Holocene slip rate is 0.65 mm/yr for the dip-slip and 0.46 mm/yr for each of the horizontal and the vertical movements. Radiocarbon dates of coseismic movements indicate a rather irregular seismic activity during the past 7000 yr. Two sequences of large earthquakes around 4000 yr B.P. and around the modern age are separated with a period of quiescence. The average Late Holocene recurrence interval of large earthquakes is 1061 yr; however, during the active faulting episodes, the recurrence time varies from approximately 300 to 500 yr.
The argos seismic data message system
Authors: Derr John S. , Hunter Roger N. 
Published: July 2021

Abstract
A reliable, inexpensive method for sending limited daily seismic data messages from remote observatories to the National Earthquake Information Center has been developed for use with the Argos satellite system. Data messages are compressed on a microcomputer and passed automatically to a simple transmitter. About 4 hr later, the data are available at the National Earthquake Information Center, where they are decompressed and reformatted into standard telegrams for use in quick epicenter determinations. Epicenter data are available daily to the international scientific community.
Inversion for slip distribution using teleseismic P waveforms: North Palm Springs, Borah Peak, and Michoacan earthquakes
Authors: Mendoza Carlos , Hartzell Stephen H. 
Published: July 2021

Abstract
We have inverted the teleseismic P waveforms recorded by stations of the Global Digital Seismograph Network for the 8 July 1986 North Palm Springs, California, the 28 October 1983 Borah Peak, Idaho, and the 19 September 1985 Michoacan, Mexico, earthquakes to recover the distribution of slip on each of the faults using a point-by-point inversion method with smoothing and positivity constraints. In the inversion procedure, a fault plane with fixed strike and dip is placed in the region of the earthquake hypocenter and divided into a large number of subfaults. Rupture is assumed to propagate at a constant velocity away from the hypocenter, and synthetic ground motions for pure strike-slip and dip-slip dislocations are calculated at the teleseismic stations for each subfault. The observed seismograms are then inverted to obtain the distribution of strike-slip and dip-slip displacement for the earthquake.
Results of the inversion indicate that the Global Digital Seismograph Network data are useful for deriving fault dislocation models for moderate to large events. However, a wide range of frequencies, which includes periods shorter than those within the passband of the long-period Global Digital Seismograph Network instruments, is necessary to infer the distribution of slip on the earthquake fault. Although the long-period waveforms define the size (dimensions and seismic moment) of the earthquake, data at shorter periods provide additional constraints on the variation of slip on the fault. Dislocation models obtained for all three earthquakes are consistent with a heterogeneous rupture process where failure is controlled largely by the size and location of high-strength asperity regions.
Coda polarization properties of high-frequency microseismic events
Authors: Cichowicz A. , Green R. W. E. , van Zyl Brink A. 
Published: July 2021

AbstractEarthquake prediction research would benefit if a technique was available to continuously monitoring the physical characteristics of a volume of rock which immediately surrounds a seismic source. This study employs the polarization properties of microseismic events in order to estimate the volume containing the near-source scatterers associated with the coda arrivals.A three-component accelerometer installed for research purposes approximately 180 m ahead of an active stope in a gold mine was located at a depth of 3,200 m. Microseismic events that were analyzed occurred within 300 m of the accelerometers, and these events had an average seismic moment of 1012 dyne-cm, with an average P-wave corner frequency close to 1.5 kHz. In general, the seismograms showed a strong acceleration coda following the direct P and S arrivals. A polarization filter was utilized to investigate the linearity of these coda signals, and this in turn was used to estimate the location of the scattering sources for these P-coda waves. The small values for the deflection angles (deflection is defined relative to the backazimuth from the receiver to the source) that were recorded for the strong P-coda pulses suggested that the region of the scattering was close to the seismic source. The series of initial strong pulses associated with the P-coda waves (5 msec) resulted largely from forward scattering caused by velocity fluctuations; these variations occurred over a length greater than 1 m. After these initial pulses, a second partition of the P-coda wave consisted of backscattered energy, and these pulses were attributed to velocity and density fluctuations over a length much smaller than 1 m. Results of the analyses suggest that the volume containing the scatters can be approximated by a sphere of radius 35 m that encompasses the seismic source.Two parameters, F~2 and DP, were investigated, and these enabled a comparison to be made of the degree of polarization between different records. The degree of polarization F2(t) changes from having a value one, for linear polarization, to a value zero for the unpolarized signal. The second parameter DP was defined as the ratio of the integral of the square of the ground velocity multiplied by the polarization filter [sin2(2 arctan F2(t))] to the integral of the square of the ground velocity. This parameter facilitated the removal of the unpolarized signal from DP. The analyses of 25 events yielded an average value for F~2 of 0.57 ± 0.06 for the P coda and 0.47 ± 0.05 for the S coda. The corresponding values for DP were 0.66 ± 0.08 for the P coda and 0.42 ± 0.11 for the S coda. The P-coda was found to have a high degree of polarization over a significant portion of the coda; this was not the case for the S coda.
A proposed source model for the great Kau, Hawaii, earthquake of 1868
Authors: Wyss Max 
Published: July 2021

AbstractOn 2 April 1868, an earthquake occurred which destroyed all stone buildings in southern Hawaii. It was felt on Kauai Island at 600 km, and ground shaking of intensity VII was reported up to 130 km distance. Based on the magnitude versus felt-area relationship for Hawaii, it is estimated that the magnitude of the earthquake was about 8. The foreshock sequence lasted 5 days, and the aftershocks lasted for years to perhaps a decade. It appears that this earthquake was one of the very few largest events in historic time in the United States, excluding Alaska, but its return period is unknown.It is proposed that the source of this earthquake was slip of the upper crust towards the southeast along a near-horizontal plane at approximately 9 km depth. The rupture plane may have had dimensions of at least 50 km × 80 km. It is proposed that its eastern edge extended from near Mauna Loa's summit to the south along the volcano's southwest rift. In this model, magma intrusions into Mauna Loa and its southwest rift provide the stresses which act perpendicular to the rift and which push the volcano's southwest flank away from the edifice of the island of Hawaii. The oceanic sediment layer upon which this edifice is deposited acts as a layer of weakness containing the fault plane. This model explains the eruptive pattern of Mauna Loa and its southwest rift, as well as the growing separation between the southwest rift zones of the two volcanoes: Kilauea and Mauna Loa.Geodetic monitoring of southern Hawaii, particularly of the area between the two active volcano's southwest rifts, could test this hypothesis and lead to an estimate of the recurrence time.
Teleseismic spectral and temporalM0 and Ψ∞ estimates for four French explosions in southern Sahara
Authors: McLaughlin K. L. , Lees A. C. , Der Z. A. , Marshall M. E. 
Published: July 2021

AbstractEstimates for explosion moment M0, and reduced displacement potential, Ψ∞, are made for four French explosions at Taourirt Tan Afella Massif in southern French Sahara using data from the long-range seismic measurement network and the EKA and YKA arrays. Preparatory to determining moments, attenuation, t⋆, estimates are made for each station, and the source region t⋆ values of 0.30 to 0.35 sec are found for the southern Sahara test site. This source region attenuation level is consistent with the “hot spot” hypothesis for the Ahaggar plateau in northern Africa. Consequently, the attenuation bias between Ahaggar and the Nevada Test Site should be small.Both spectral estimation and broadband temporal deconvolution methods are used for estimation of the explosion moments. The deconvolution estimates of static moment are found to be consistent with the spectral estimation methods. Deconvolved seismograms for the explosions EMERAUDE, RUBIS, SAPHIR, and GRENAT show evidence of strong anisotropic free surface interaction that may be due to scattering or spall from the steep topography of the Taourirt Tan Affela Massif test site.
Second-order moment tensors of microearthquakes at The Geysers geothermal field, California
Authors: O'Connell Daniel R. H. , Johnson Lane R. 
Published: July 2021

AbstractThe Geysers geothermal field is the site of intense microseismicity which appears to be associated with steam production. It seems that focal mechanisms of earthquakes at The Geysers vary systematically with depth, but P-wave first-motion focal mechanism studies have been hampered by inadequate resolution. In this study an unconstrained frequency domain moment tensor inversion method is used to over come P-wave first-motion focal sphere distribution problems and to investigate microearthquake source properties. A goal was to investigate the feasibility of using waveforms to invert for the second-order moment tensor of microearthquakes in the complex setting of The Geysers. Derived frequency-domain moment tensors for two earthquakes were verified by mechanisms estimated from P-wave first motions and required far fewer stations. For one event, 19 P-wave first motions were insufficient to distinguish between normal-slip and strike-slip focal mechanisms, but a well-constrained strike-slip solution was obtained from the waveform principal moment inversion using data from six stations. Improved waveform focal mechanism resolution was a direct consequence of using P- and S-wave data together in a progressive velocity-hypocenter inversion to minimize Green function errors. The effects of hypocenter mislocation and velocity model Green function errors on moment tensor estimates were investigated. Synthetic tests indicate that these errors can introduce spurious isotropic and compensated linear vector dipole components as large as 26 per cent for these events, whereas principal moment orientations errors were &lt;8°. In spite of unfavorable recording geometries and large (0.6 km) station elevation differences, the results indicate that waveform moment tensor estimates for microearthquake sources can be robust and constrain source mechanisms using data from a relatively small number of stations.
Comparing strain events: A case study for the Homestead Valley earthquakes
Authors: King N. E. , Agnew D. C. , Wyatt F. 
Published: July 2021

Abstract
We present a procedure for comparing strain events at different sites, and apply it to observations for the Homestead Valley earthquake sequence of 15 March 1979. Coseismic strain steps occurred on two laser strainmeters at Piñon Flat Observatory (PFO) and on a creepmeter spanning the San Andreas fault at Wiebe Ranch (XWR); the creepmeter records were studied by Leary and Malin (1984). The PFO coseismic strains are consistent with a source model derived from near-field geodetic measurements, while the XWR coseismic extension is 1200 times too large and of the wrong sign. The XWR instrument showed an anomalous extension about 20 hours before the earthquake (Leary and Malin, 1984), but no such signals were detected on the PFO strainmeters at this time. To compare these preseismic observations we must make some assumption about the preseismic source, and we choose to assume that any precursory slip occurred on the fault plane that eventually ruptured. Under this assumption, the preseismic XWR and PFO records cannot be reconciled unless the preseismic dislocation is left-lateral or mostly thrust, and the fault zone at XWR magnifies strain by a factor of 2900 or more. This strain magnification implies that the fault-zone shear modulus at XWR is 107 Pa, three orders of magnitude smaller than the shear modulus of typical crustal rocks. However, earth-tide observations at XWR constrain the strain magnification at this site to be less than about 55. Thus the preseismic extension at XWR is probably not a precursor to the Homestead Valley earthquake.
Spectral evidence for source multiplicity in explosions: Application to regional discrimination of earthquakes and explosions
Authors: Baumgardt Douglas R. , Ziegler Kathleen A. 
Published: July 2021

AbstractSpectral and cepstral analysis were performed on regional-array NORESS recordings of mining explosions in Scandinavia and Russia in order to detect the effects of delayed explosions or “ripple firing” commonly used in mine-blasting practice. NORESS array-stacked spectra, corrected for instrument and noise, were computed in windows on Pn, Pg, Sn, and Lg waves from the mine blasts by averaging individual channel spectra, frequency by frequency, across the array. For comparison, spectra were also computed for six western Norway earthquakes located near the Blasjo and Titania Norwegian mines. Also, stack spectra of P coda, Sn, and Lg were computed using NORSAR seismograms for presumed peaceful nuclear explosions (PNEs) located at near-regional distances (Δ &gt; 10°) in western Russia. The NORSAR spectra were determined in the same manner as the NORESS spectra, except that an individual source correction was applied to the spectra. Comparison of western Norway mine-blast spectra and nearby earthquakes shows that the earthquake Pn spectra are peaked in the 8- to 16-Hz band, with little energy below 6 Hz, whereas the explosion spectra have flatter spectra from 3 to 16 Hz. This difference in the Pn spectra may be caused by the earthquakes occurring at greater depths than the explosions, differences in the near-source media, or source mechanism effects in the earthquake spectra. The main difference between earthquake and explosion spectra is that the explosion spectra exhibit marked scalloping or modulation patterns not observed in the earthquake spectra. The modulation patterns are identical in spectra of all phases, indicating that they are caused by multiple-shot sequences or ripple firing. Simulated multiple-source spectra, determined using the NORESS seismograms for one of the western Norway earthquakes, resulted in spectra that resembled many of the explosion modulation patterns. Based on the simulation studies and cepstral analysis of the spectra, the mine explosions appear to be composed of two to three separate explosions delayed by between 80 to 150 msec. Moreover, the NORSAR spectra of PNEs indicate that these signals are also produced by multiple explosions, although they have larger delay times of between 0.9 to 1.5 sec. These results show that spectral modulations in seismogram spectra of economic explosions, including mine explosions and nuclear explosions fired for peaceful, economic purposes, can be used to distinguish them from earthquakes and nuclear-explosion weapons tests.
Source spectral characteristics of Miramichi earthquakes: Results from 115P-wave observations
Authors: Chun Kin-Yip , Kokoski Richard J. , West Gordon F. 
Published: July 2021

AbstractSource scaling relation is studied over the magnitude (mN) range 2.6 to 5.4 using P waves generated by 31 tightly clustered earthquakes in New Brunswick, Canada. The recording stations, six in total, have a 60-points/sec sampling rate and a dynamic range of about 100 dB. They are located at regional distance (188 to 448 km), with a wide azimuthal coverage. The data interpreted consist of 115 spectral ratio curves (2-20 Hz), each obtained in a manner that allows effective cancellation of the effects caused by source radiation pattern, path attenuation, geometrical spreading, instrument error, and variability in site function. The data selected in this study differ from the single-station records used in a previous source-scaling study of Miramichi earthquakes (Chael, 1987) in having: 1) broader distance coverage; 2) greater recording dynamic range; 3) higher Nyquist frequency; and 4) larger data size. We conclude from the observed spectral ratios that source models having an ω−2 high-frequency fall-off (ω-square model) are strongly favored by the data over those having an ω−3 high-frequency fall-off (ω-cube model) and that stress drop increases with moment at a rate proposed earlier by Nuttli (1983a, b).
Source parameters of Montana earthquakes (1925-1964) and tectonic deformation in the northern Intermountain Seismic Belt
Authors: Doser Diane I. 
Published: July 2021

AbstractWaveform modeling and first motion analysis are used to determine the source parameters of six 5.8 ≦ M ≦ 6.8 earthquakes that occurred between 1925 and 1964 within the northern Intermountain Seismic Belt of Montana. Results of this study suggest that the 1925 Clarkston earthquake occurred along an oblique normal fault with a trend similar to the southern end of the Clarkston Valley fault. The two largest earthquakes of the 1935 Helena sequence occurred along right-lateral strike-slip faults with trends similar to the Bald Butte and Helena Valley faults. The 1947 Virginia City earthquake occurred along a northwest-southeast trending segment of the Madison fault. Movement at depth was along a fault with strike similar to that of the 1959 Hebgen Lake main shock. A reanalysis of a M = 6.0 aftershock of the 1959 Hebgen Lake sequence suggests the earthquake occurred at a depth of 8 km along a fault that is not seen at the surface. An M = 5.8 earthquake in 1964, located about 10 km from the 1959 aftershock, may have occurred along steeply dipping fault planes (48° to 80°) at depths of 8 to 14 km. Most events could be modeled as simple ruptures.
Network-averaged teleseismicP-wave spectra for underground explosions. Part II. Source characteristics of Pahute Mesa explosions
Authors: Murphy John R. 
Published: July 2021

AbstractA source scaling analysis is presented based on network-averaged, teleseismic P-wave spectra determined from short-period data recorded from a sample of 20 Pahute Mesa explosions. These explosions, which were all detonated below the water table in saturated tuff/rhyolite emplacement media, cover a range of announced yields from 155 to 1300 kt. The spectra were analyzed using a simple set of source and propagation models consisting of a Mueller/Murphy source coupling model, a conventional, frequency-independent t* model of anelastic attenuation and a “quasi-linear” description of the surface-reflected pP phase. It is demonstrated that these models can account for virtually all the observed spectral variability over the frequency band extending from 0.5 to 2.0 Hz, down to a level which is close to that associated with measurement uncertainty. In particular, the use of network averaged model parameters of t* = 0.75 sec, an average pP/P-amplitude ratio of about 0.4 and an average source medium velocity of 3.5 km/sec reduces the spectral amplitude data from these explosions to an essentially frequency-independent constant value with an associated standard error of estimate which averages to only about 20 per cent over this frequency band.
Slip on the Superstition Hills fault and on nearby faults associated with the 24 November 1987 Elmore Ranch and Superstition Hills earthquakes, southern California
Authors: McGill Sally F. , Allen Clarence R. , Hudnut Kenneth W. , Johnson David C. , Miller Wayne F. , Sieh Kerry E. 
Published: July 2021

AbstractAlignment arrays and creepmeters spanning several faults in southern California recorded slip associated with the 24 November 1987 Elmore Ranch and Superstition Hills earthquakes. No precursory slip had occurred on the Superstition Hills fault up to 27 October 1987, when the last measurement before the earthquakes was made. About 23 days before the earthquake, dextral creep events of about 13 mm and 0.5 mm may have occurred simultaneously on the Imperial and southern San Andreas faults, respectively, but the tectonic origin of the smaller event is questionable.Within 12 hr after the Superstition Hills earthquake, 20.9 cm of dextral slip occurred on the main fault trace at the Superstition Hills alignment array, and 39.8 cm of dextral slip was recorded over the entire 110-m width of the array. Despite this initial wide distribution of slip, nearly all of the postseismic slip is occurring on the main fault trace. As of 3 August 1988, the alignment array had recorded a total of 80.2 cm of dextral slip. As of 5 days after the earthquakes, 65 to 80 per cent of the total slip measured by the alignment array had occurred on discrete, mappable fractures.In addition, the two earthquakes triggered slip on the Coyote Creek fault, the southern San Andreas fault, and on the Imperial fault. Telemetered data from creepmeters on the southern San Andreas and Imperial faults indicate that triggered slip began there within 3 min or less of each of the two earthquakes. Additional triggered slip occurred on the Imperial fault beginning 3.5 hr after the second earthquake.
The 1987 Superstition Hills earthquake sequence: Strains and tilts at Piñon Flat Observatory
Authors: Agnew Duncan Carr , Wyatt Frank K. 
Published: July 2021

AbstractWe describe observations at Piñon Flat Observatory of preseismic, co-seismic, and postseismic strain and tilt changes associated with the Superstition Hills earthquake sequence of 24 November 1987. The data come from two long-base strainmeters, one long-base tiltmeter, and a borehole dilatometer. Co-seismic offsets were seen for the two largest earthquakes; these offsets roughly agree with those from a dislocation in a half-space, though with disagreements at the 30 per cent level, suggesting some error in one or more of the co-seismic observations. For the main shock, the moment deduced from the offsets is 5 to 6 × 1025 dyne-cm, less than that found from the seismic data. No precursive strain changes were seen. For the 8 hr before the main shock, any strain changes were less than the instrument noise level (1 per cent of the co-seismic strain); during the last 1000 sec before this earthquake, any precursory strains must be less than 0.5 per cent of the co-seismic amount. Postseismic strain during the 2 hr after the main shock was about 10 per cent of the co-seismic strain.