Bulletin of the Seismological Society of America, 1982
Seismic waves from nuclear explosions and from well-located local earthquakes have been recorded ... more Seismic waves from nuclear explosions and from well-located local earthquakes have been recorded along several profiles through western Nevada, southeast Oregon, and eastern California. Pn delay times from these data and from additional travel times of NTS explosions recorded across the western Nevada seismic network have been interpreted in terms of varying crustal thickness of the western Great Basin. Our interpretation of these data implies that the crust thins from greater than 32 km thick in the vicinity of Mono Lake to 20 to 22 km thick over a broad region in northwest Nevada. The Pn headwave propagates to a distance of about 550 km with a velocity of 7.8 km/sec. At distances greater than 600 km a low velocity (7.44 km/sec), larger amplitude phase is observed. This phase may result from long-period energy diffracting into a shadow zone. A prominent, high-velocity second arrival following Pn by 1 to 2 sec in the distance range 340 to 475 km, is interpreted as a reflection from ...
Bulletin of the Seismological Society of America, 1973
P-wave residuals at stations of the Nevada seismic network are analyzed for 80 teleseisms distrib... more P-wave residuals at stations of the Nevada seismic network are analyzed for 80 teleseisms distributed over a range of azimuth and epicentral distance. For South American earthquakes (azimuth 120° to 140° from Nevada), teleseismic P arrivals at northern Nevada stations Lovelock, Battle Mountain, and Elko are early by as much as 1.4 sec relative to the Tonopah station to the south. The North Reno station has early arrivals, relative to Tonopah, for Leeward Islands earthquakes, at an azimuth of 100° from the station. Interpretation of these residuals indicates the presence of a high-velocity lithospheric plate, striking northeast and dipping southeast, under northern Nevada. The high-velocity plate is interpreted as a paleosubduction zone.
Bulletin of the Seismological Society of America, 1980
Precise epicentral determinations based on local network recordings are compared with mapped faul... more Precise epicentral determinations based on local network recordings are compared with mapped faults and volcanic features in the western Great Basin. This region is structurally and seismically complex, and seismogenic processes vary within it. In the area north of the rupture zone of the 1872 Owens Valley earthquake, dispersed clusters of epicenters agree with a shatter zone of faults that extend the 1872 breaks to the north and northwest. An area of frequent earthquake swarms east of Mono Lake is characterized by northeast-striking faults and a crustal low-velocity zone; seismicity in this area appears to be related to volcanic processes that produced thick Pliocene basalt flows in the Adobe Hills and minor historic activity in Mono Lake. In the Garfield Hills between Walker Lake and the Excelsior Mountains, there is some clustering of epicenters along a north-trending zone that does not correlate with major Cenozoic structures. In an area west of Walker Lake, low seismicity suppo...
Abstract Several thousand aftershocks of the August 1, 1975 Oroville, California, earthquake ( M ... more Abstract Several thousand aftershocks of the August 1, 1975 Oroville, California, earthquake ( M L = 5.7 ) were recorded by an 8-station field-seismic network. Focal coordinates of 104 of these events were fitted by least-squares to a plane striking N07°W and dipping 59°W; the strike (but not the dip) of this plane is in good agreement with that (N09°W) obtained from a fault-plane solution for a large foreshock 8 sec before the main shock, and it agrees fairly well with the trend (N15°W) of structural lineaments in the vicinity of Lake Oroville. The surface trace of the plane of foci passes through the Oroville Dam, as well as through surface cracking 12 km south of the dam. The main shock occurred 7 years after the filling of Lake Oroville, but only a month after the most rapid filling since 1968. The rate of aftershock occurrence during the first month decayed approximately as 1/t . Event duration was measured for more than 2,000 aftershocks during August and September; average log-duration, taken over samples of 100 events, decreased gradually during this period. Close-in spectra obtained from strong-motion recordings of several of the larger aftershocks have corner frequencies that are quite high compared to other western U.S. earthquakes of similar magnitude. The Oroville earthquakes had several features in common with another Sierra Nevada earthquake sequence, near Truckee, California, in September, 1966.
: Evaluation of the Group of Scientific Experts Technical Test (GSETT), and the exploitation of t... more : Evaluation of the Group of Scientific Experts Technical Test (GSETT), and the exploitation of the GSETT database for more general investigations of network assessment continued during this period. These analyses provide material for subsequent United States reports to the GSE in addition to increasing our understanding of seismic network design and capabilities. Research with the associated objectives of providing analysis tools for the Center while improving understanding of discrimination between events at regional distances was initiated, together with work on yield estimation, primarily in support of the DARPA-managed DoD Seismic Review Panel. The report also covers work related to on-line data acquisition and other programming efforts, and on database development. Keywords: Seismology, Seismic data exchange, Monitoring nuclear tests. (JD)
Bulletin of the Seismological Society of America, 1964
This paper contains a brief review of the operations that are involved in digital processing of a... more This paper contains a brief review of the operations that are involved in digital processing of array seismic recordings by the methods of velocity filtering, summation, cross-multiplication and integration, and by combinations of these methods (the UK method and multiple correlation). Analyses by the several techniques were made on array recordings that were obtained by the U. S. Geological Survey of seismic waves from chemical and nuclear explosions in the western United States. Seismograms were synthesized, using recordings of seismic noise and a Pn signal, such that the signal-to-noise ratio, onset time and apparent velocity of the signal were predetermined for the synthetic records. These records were then analyzed by summation, cross-multiplication and the UK method, and the results were compared. From the examples presented, it appears that for recordings with initial peak-signal-to-rms-noise ratio less than about 0.8, the best improvement can be obtained by the process of ve...
: This report covers results from an international seismic data exchange test sponsored by the Gr... more : This report covers results from an international seismic data exchange test sponsored by the Group of Scientific Experts, U.N. Conference on Disarmament. It also reports on computer program development at the Center for Seismic Studies, on m sub b bias as deduced from Soviet seismograms, and on noise levels and detection thresholds of seismic stations that participated in the data exchange experiment. Keywords: Seismology; Data Center; Seismic Data Exchange; Monitoring Nuclear Tests; Magnitude Bias.
The seismic cycle in Nevada, corresponding to the re-rupture time for major faults, is of the ord... more The seismic cycle in Nevada, corresponding to the re-rupture time for major faults, is of the order of thousands of years long. Following the occurrence of a large (M > 7) earthquake, aftershock activity in the rupture zone continues for about a century. From earthquakes reported in the meizoseismal zones of the 1915 Pleasant Valley and 1954 Dixie Valley earthquakes, it appears that great Nevada earthquakes are also preceded by a moderate increase in seismicity, lasting several decades. Faults considered to be “active” on geologic grounds are distributed fairly evenly over most of the Nevada region, so that fault mapping alone is not enough for determination of seismic potential. In addition, because of the length of the seismic cycle, the distribution of large historic earthquakes is by itself inadequate for seismic regionalization. A better method for this region may be to spatially correlate epicenter lineups for microearthquakes and small earthquakes with mapped geologic faul...
Anomalous temporal variations in the southern part of the Sierra Nevada-Great Basin boundary zone... more Anomalous temporal variations in the southern part of the Sierra Nevada-Great Basin boundary zone (SNGBZ) from 1977 to 1980 followed a pattern that several authors have identified as precursory to strong earthquakes. From January 1977 to September 1978, a general decrease in seismicity was observed in the southern part of the zone, followed by a burst of moderate earthquakes from Doyle in the northern part of the SNGBZ to Bishop in the south. The 1977 to 1979 variations are particularly noteworthy because they occurred over the entire SNGBZ, indicating a regional, rather than local cause. In the southern part of the zone, a magnitude 5.7 shock 25 km northwest of Bishop on 4 October 1978 was followed by an earthquake swarm in the Mammoth Lakes area, with peaks in activity during late 1979 and the spring of 1980. The Mammoth Lakes activity spread gradually to the west during the first year of this sequence, apparently involving several northerly-trending fracture zones in an area of a...
A method is described for determining expected acceleration return periods, based on calculations... more A method is described for determining expected acceleration return periods, based on calculations involving magnitude, fault length and distance to the causative fault. The method permits earthquake magnitude and duration of strong motion to be associated with these return periods. In addition, because attenuation equations are in terms of distance to the causative fault, instead of focal distance, sites can be considered which are in the immediate vicinity of potential faults. Results of calculations indicate that for an average site in the western Nevada region maximum-amplitude, maximum-duration ground motion has a recurrence time of the order of thousands of years. This result, based on a relatively brief sample of instrumental data, is entirely consistent with geological field data representing time periods two to three orders of magnitude longer. Smaller ground motions have correspondingly smaller return periods, down to about a decade for accelerations greater than 0.1 g, whe...
The instrumental epicenter of the Hebgen Lake earthquake is found to lie within the region of sur... more The instrumental epicenter of the Hebgen Lake earthquake is found to lie within the region of surface faulting. The depth of focus had a maximum value of 25 kilometers. Times of P are studied in detail for epicentral distances less than 13 degrees. The apparent scatter of arrival times from 700 to 1400 kilometers can be explained by variations of the velocity of Pn between the physiographic provinces of the western United States. A comparison of observations for the Hebgen Lake earthquake with published times for blasts in Nevada and Utah indicates that the velocity of Pn in the central and eastern Basin and Range is about 7.5 km/sec, and that the crust in that region thickens toward the east and thins toward the south. A comparison of apparent velocities in northern California, in directions parallel and transverse to the structure, indicates that the crust thins by about 19 kilometers, from the edge of the Sierra Nevada to the Pacific Ocean. A discontinuity is observed in the trav...
This paper presents a detailed analysis of S-wave data for two areas in western Nevada. Some of t... more This paper presents a detailed analysis of S-wave data for two areas in western Nevada. Some of these data were used by Gupta (1973b, c) as the basis for his claim that large premonitory changes in the extent of stress-induced S-wave splitting are observed for moderate-sized earthquakes in Nevada. Analysis of particle motion for an earthquake sequence near Slate Mountain indicates that changes in S-wave splitting did not occur during that sequence. For the Mina area, comparison of S-wave signatures for 158 events occurring over a 3-year period resulted in the identification of numerous events that would be considered anomalous by Gupta's criteria, but these were not followed by larger earthquakes. The present state of knowledge on crustal structure and seismic source parameters in the western Basin and Range province is not sufficient to discriminate between stress-induced velocity anisotropy and the many other factors that contribute to the complexity of S-wave signatures.
A method is described for determining recurrence times as a function of distance to the causative... more A method is described for determining recurrence times as a function of distance to the causative fault and magnitude, for earthquakes distributed along a linear source zone. The method takes into account rupture length, which is scaled to magnitude, and permits direct calculation of approximate return periods for peak ground-motion parameters for large earthquakes, when the appropriate attenuation functions are known. Several examples are presented using instrumentally determined seismicity along the San Andreas fault zone. Results illustrate the necessity of incorporating rupture length in calculations related to seismic risk; for large earthquakes, it is also necessary to use a source region large enough to contain the rupture zones of all such events. For a site in the San Andreas fault zone we find that the recurrence time to be within 10 km of the causative fault of an earthquake with M ≧ 8 is 200 to 300 years, depending on the choice of maximum magnitude (8.6 or 8.4). For a s...
The increase in frequency of occurrence of earthquakes with decreasing magnitude is well known. I... more The increase in frequency of occurrence of earthquakes with decreasing magnitude is well known. In a few cases observations have shown that this relation holds for extremely small events, including those with magnitudes well below zero, and that the energy of the smaller shocks is confined largely to the higher seismic frequencies. These facts suggest that portable seismographs with ultra-high sensitivity might record a sufficient number of nearby microearthquakes in a short interval of time, say one day, so that some measure of the seismic activity of a very local area might be obtained very quickly. This idea was tested in west central Nevada where ten sites were occupied for short intervals of time. Microearthquakes were recorded at rates ranging from several per day to over two hundred per day. Generally, consistently high microseismicity was observed in areas of recent faulting. A lower level of activity, well above that of aseismic areas however, was observed at other sites in...
Fourier amplitude spectra were computed for 40 central Nevada microearthquakes, selected to consi... more Fourier amplitude spectra were computed for 40 central Nevada microearthquakes, selected to consider, independently, effects of azimuth and distance from known sources. Spectra were averaged for groups of events to eliminate peculiarities of individual records and emphasize group characteristics. Spectral characteristics did not behave systematically as a function of azimuth from the recording site to the source, but peak spectral frequency was found to correlate strongly with event magnitude and to some degree also with focal distance. These preliminary results suggest that recordings of small earthquakes and microearthquakes can be used to provide detailed information on the character of seismic signals related to properties of the source and propagation path.
Bulletin of the Seismological Society of America, 1982
Seismic waves from nuclear explosions and from well-located local earthquakes have been recorded ... more Seismic waves from nuclear explosions and from well-located local earthquakes have been recorded along several profiles through western Nevada, southeast Oregon, and eastern California. Pn delay times from these data and from additional travel times of NTS explosions recorded across the western Nevada seismic network have been interpreted in terms of varying crustal thickness of the western Great Basin. Our interpretation of these data implies that the crust thins from greater than 32 km thick in the vicinity of Mono Lake to 20 to 22 km thick over a broad region in northwest Nevada. The Pn headwave propagates to a distance of about 550 km with a velocity of 7.8 km/sec. At distances greater than 600 km a low velocity (7.44 km/sec), larger amplitude phase is observed. This phase may result from long-period energy diffracting into a shadow zone. A prominent, high-velocity second arrival following Pn by 1 to 2 sec in the distance range 340 to 475 km, is interpreted as a reflection from ...
Bulletin of the Seismological Society of America, 1973
P-wave residuals at stations of the Nevada seismic network are analyzed for 80 teleseisms distrib... more P-wave residuals at stations of the Nevada seismic network are analyzed for 80 teleseisms distributed over a range of azimuth and epicentral distance. For South American earthquakes (azimuth 120° to 140° from Nevada), teleseismic P arrivals at northern Nevada stations Lovelock, Battle Mountain, and Elko are early by as much as 1.4 sec relative to the Tonopah station to the south. The North Reno station has early arrivals, relative to Tonopah, for Leeward Islands earthquakes, at an azimuth of 100° from the station. Interpretation of these residuals indicates the presence of a high-velocity lithospheric plate, striking northeast and dipping southeast, under northern Nevada. The high-velocity plate is interpreted as a paleosubduction zone.
Bulletin of the Seismological Society of America, 1980
Precise epicentral determinations based on local network recordings are compared with mapped faul... more Precise epicentral determinations based on local network recordings are compared with mapped faults and volcanic features in the western Great Basin. This region is structurally and seismically complex, and seismogenic processes vary within it. In the area north of the rupture zone of the 1872 Owens Valley earthquake, dispersed clusters of epicenters agree with a shatter zone of faults that extend the 1872 breaks to the north and northwest. An area of frequent earthquake swarms east of Mono Lake is characterized by northeast-striking faults and a crustal low-velocity zone; seismicity in this area appears to be related to volcanic processes that produced thick Pliocene basalt flows in the Adobe Hills and minor historic activity in Mono Lake. In the Garfield Hills between Walker Lake and the Excelsior Mountains, there is some clustering of epicenters along a north-trending zone that does not correlate with major Cenozoic structures. In an area west of Walker Lake, low seismicity suppo...
Abstract Several thousand aftershocks of the August 1, 1975 Oroville, California, earthquake ( M ... more Abstract Several thousand aftershocks of the August 1, 1975 Oroville, California, earthquake ( M L = 5.7 ) were recorded by an 8-station field-seismic network. Focal coordinates of 104 of these events were fitted by least-squares to a plane striking N07°W and dipping 59°W; the strike (but not the dip) of this plane is in good agreement with that (N09°W) obtained from a fault-plane solution for a large foreshock 8 sec before the main shock, and it agrees fairly well with the trend (N15°W) of structural lineaments in the vicinity of Lake Oroville. The surface trace of the plane of foci passes through the Oroville Dam, as well as through surface cracking 12 km south of the dam. The main shock occurred 7 years after the filling of Lake Oroville, but only a month after the most rapid filling since 1968. The rate of aftershock occurrence during the first month decayed approximately as 1/t . Event duration was measured for more than 2,000 aftershocks during August and September; average log-duration, taken over samples of 100 events, decreased gradually during this period. Close-in spectra obtained from strong-motion recordings of several of the larger aftershocks have corner frequencies that are quite high compared to other western U.S. earthquakes of similar magnitude. The Oroville earthquakes had several features in common with another Sierra Nevada earthquake sequence, near Truckee, California, in September, 1966.
: Evaluation of the Group of Scientific Experts Technical Test (GSETT), and the exploitation of t... more : Evaluation of the Group of Scientific Experts Technical Test (GSETT), and the exploitation of the GSETT database for more general investigations of network assessment continued during this period. These analyses provide material for subsequent United States reports to the GSE in addition to increasing our understanding of seismic network design and capabilities. Research with the associated objectives of providing analysis tools for the Center while improving understanding of discrimination between events at regional distances was initiated, together with work on yield estimation, primarily in support of the DARPA-managed DoD Seismic Review Panel. The report also covers work related to on-line data acquisition and other programming efforts, and on database development. Keywords: Seismology, Seismic data exchange, Monitoring nuclear tests. (JD)
Bulletin of the Seismological Society of America, 1964
This paper contains a brief review of the operations that are involved in digital processing of a... more This paper contains a brief review of the operations that are involved in digital processing of array seismic recordings by the methods of velocity filtering, summation, cross-multiplication and integration, and by combinations of these methods (the UK method and multiple correlation). Analyses by the several techniques were made on array recordings that were obtained by the U. S. Geological Survey of seismic waves from chemical and nuclear explosions in the western United States. Seismograms were synthesized, using recordings of seismic noise and a Pn signal, such that the signal-to-noise ratio, onset time and apparent velocity of the signal were predetermined for the synthetic records. These records were then analyzed by summation, cross-multiplication and the UK method, and the results were compared. From the examples presented, it appears that for recordings with initial peak-signal-to-rms-noise ratio less than about 0.8, the best improvement can be obtained by the process of ve...
: This report covers results from an international seismic data exchange test sponsored by the Gr... more : This report covers results from an international seismic data exchange test sponsored by the Group of Scientific Experts, U.N. Conference on Disarmament. It also reports on computer program development at the Center for Seismic Studies, on m sub b bias as deduced from Soviet seismograms, and on noise levels and detection thresholds of seismic stations that participated in the data exchange experiment. Keywords: Seismology; Data Center; Seismic Data Exchange; Monitoring Nuclear Tests; Magnitude Bias.
The seismic cycle in Nevada, corresponding to the re-rupture time for major faults, is of the ord... more The seismic cycle in Nevada, corresponding to the re-rupture time for major faults, is of the order of thousands of years long. Following the occurrence of a large (M > 7) earthquake, aftershock activity in the rupture zone continues for about a century. From earthquakes reported in the meizoseismal zones of the 1915 Pleasant Valley and 1954 Dixie Valley earthquakes, it appears that great Nevada earthquakes are also preceded by a moderate increase in seismicity, lasting several decades. Faults considered to be “active” on geologic grounds are distributed fairly evenly over most of the Nevada region, so that fault mapping alone is not enough for determination of seismic potential. In addition, because of the length of the seismic cycle, the distribution of large historic earthquakes is by itself inadequate for seismic regionalization. A better method for this region may be to spatially correlate epicenter lineups for microearthquakes and small earthquakes with mapped geologic faul...
Anomalous temporal variations in the southern part of the Sierra Nevada-Great Basin boundary zone... more Anomalous temporal variations in the southern part of the Sierra Nevada-Great Basin boundary zone (SNGBZ) from 1977 to 1980 followed a pattern that several authors have identified as precursory to strong earthquakes. From January 1977 to September 1978, a general decrease in seismicity was observed in the southern part of the zone, followed by a burst of moderate earthquakes from Doyle in the northern part of the SNGBZ to Bishop in the south. The 1977 to 1979 variations are particularly noteworthy because they occurred over the entire SNGBZ, indicating a regional, rather than local cause. In the southern part of the zone, a magnitude 5.7 shock 25 km northwest of Bishop on 4 October 1978 was followed by an earthquake swarm in the Mammoth Lakes area, with peaks in activity during late 1979 and the spring of 1980. The Mammoth Lakes activity spread gradually to the west during the first year of this sequence, apparently involving several northerly-trending fracture zones in an area of a...
A method is described for determining expected acceleration return periods, based on calculations... more A method is described for determining expected acceleration return periods, based on calculations involving magnitude, fault length and distance to the causative fault. The method permits earthquake magnitude and duration of strong motion to be associated with these return periods. In addition, because attenuation equations are in terms of distance to the causative fault, instead of focal distance, sites can be considered which are in the immediate vicinity of potential faults. Results of calculations indicate that for an average site in the western Nevada region maximum-amplitude, maximum-duration ground motion has a recurrence time of the order of thousands of years. This result, based on a relatively brief sample of instrumental data, is entirely consistent with geological field data representing time periods two to three orders of magnitude longer. Smaller ground motions have correspondingly smaller return periods, down to about a decade for accelerations greater than 0.1 g, whe...
The instrumental epicenter of the Hebgen Lake earthquake is found to lie within the region of sur... more The instrumental epicenter of the Hebgen Lake earthquake is found to lie within the region of surface faulting. The depth of focus had a maximum value of 25 kilometers. Times of P are studied in detail for epicentral distances less than 13 degrees. The apparent scatter of arrival times from 700 to 1400 kilometers can be explained by variations of the velocity of Pn between the physiographic provinces of the western United States. A comparison of observations for the Hebgen Lake earthquake with published times for blasts in Nevada and Utah indicates that the velocity of Pn in the central and eastern Basin and Range is about 7.5 km/sec, and that the crust in that region thickens toward the east and thins toward the south. A comparison of apparent velocities in northern California, in directions parallel and transverse to the structure, indicates that the crust thins by about 19 kilometers, from the edge of the Sierra Nevada to the Pacific Ocean. A discontinuity is observed in the trav...
This paper presents a detailed analysis of S-wave data for two areas in western Nevada. Some of t... more This paper presents a detailed analysis of S-wave data for two areas in western Nevada. Some of these data were used by Gupta (1973b, c) as the basis for his claim that large premonitory changes in the extent of stress-induced S-wave splitting are observed for moderate-sized earthquakes in Nevada. Analysis of particle motion for an earthquake sequence near Slate Mountain indicates that changes in S-wave splitting did not occur during that sequence. For the Mina area, comparison of S-wave signatures for 158 events occurring over a 3-year period resulted in the identification of numerous events that would be considered anomalous by Gupta's criteria, but these were not followed by larger earthquakes. The present state of knowledge on crustal structure and seismic source parameters in the western Basin and Range province is not sufficient to discriminate between stress-induced velocity anisotropy and the many other factors that contribute to the complexity of S-wave signatures.
A method is described for determining recurrence times as a function of distance to the causative... more A method is described for determining recurrence times as a function of distance to the causative fault and magnitude, for earthquakes distributed along a linear source zone. The method takes into account rupture length, which is scaled to magnitude, and permits direct calculation of approximate return periods for peak ground-motion parameters for large earthquakes, when the appropriate attenuation functions are known. Several examples are presented using instrumentally determined seismicity along the San Andreas fault zone. Results illustrate the necessity of incorporating rupture length in calculations related to seismic risk; for large earthquakes, it is also necessary to use a source region large enough to contain the rupture zones of all such events. For a site in the San Andreas fault zone we find that the recurrence time to be within 10 km of the causative fault of an earthquake with M ≧ 8 is 200 to 300 years, depending on the choice of maximum magnitude (8.6 or 8.4). For a s...
The increase in frequency of occurrence of earthquakes with decreasing magnitude is well known. I... more The increase in frequency of occurrence of earthquakes with decreasing magnitude is well known. In a few cases observations have shown that this relation holds for extremely small events, including those with magnitudes well below zero, and that the energy of the smaller shocks is confined largely to the higher seismic frequencies. These facts suggest that portable seismographs with ultra-high sensitivity might record a sufficient number of nearby microearthquakes in a short interval of time, say one day, so that some measure of the seismic activity of a very local area might be obtained very quickly. This idea was tested in west central Nevada where ten sites were occupied for short intervals of time. Microearthquakes were recorded at rates ranging from several per day to over two hundred per day. Generally, consistently high microseismicity was observed in areas of recent faulting. A lower level of activity, well above that of aseismic areas however, was observed at other sites in...
Fourier amplitude spectra were computed for 40 central Nevada microearthquakes, selected to consi... more Fourier amplitude spectra were computed for 40 central Nevada microearthquakes, selected to consider, independently, effects of azimuth and distance from known sources. Spectra were averaged for groups of events to eliminate peculiarities of individual records and emphasize group characteristics. Spectral characteristics did not behave systematically as a function of azimuth from the recording site to the source, but peak spectral frequency was found to correlate strongly with event magnitude and to some degree also with focal distance. These preliminary results suggest that recordings of small earthquakes and microearthquakes can be used to provide detailed information on the character of seismic signals related to properties of the source and propagation path.
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