Temporal and spatial variations of travel-time residuals in central California for Novaya Zemlya events

1976 ◽  
Vol 66 (5) ◽  
pp. 1733-1747 ◽  
Author(s):  
Russell Robinson ◽  
H. M. Iyer
Keyword(s):  
Travel Time ◽  
Seismic Velocity ◽  
Temporal Variations ◽  
P Wave ◽  
Central California ◽  
Nuclear Explosions ◽  
Novaya Zemlya ◽  
San Andreas ◽  
Temporal And Spatial

abstract Eight large nuclear explosions in Novaya Zemlya, from October 1969, through November 1974, were used to monitor long-term variations in crustal seismic velocity near the San Andreas fault in central California. Relative P-wave travel-time residuals appear to be accurate to approximately ±0.1 sec. Of the over 100 stations used, none show clearly significant temporal variations in residual greater than this amount, corresponding to about a 4 per cent change in velocity in the upper crust. Average relative residuals at individual stations show a large spatial variation of about 1.5 sec. These variations reflect both a complex crustal geology and changes in crustal thickness and provide a potentially powerful tool for studying crustal structure.

P-wave travel-time variations before the August 1, 1975 Oroville, California earthquake

1977 ◽  
Vol 67 (1) ◽  
pp. 9-26
Author(s):  
Chris H. Cramer ◽  
Charles G. Bufe ◽  
Paul W. Morrison
Keyword(s):  
Travel Time ◽  
P Wave ◽  
Fault System ◽  
Mean Values ◽  
Central California ◽  
San Andreas ◽  
Regional Sources ◽  
San Andreas Fault System ◽  
Future Earthquake ◽  
Time Variations

abstract On August 1, 1975, a magnitude 5.9 (mb) (BRK, M = 5.7), normal dip-slip earthquake occurred 10 km south of Oroville, California. P arrivals for teleseismic and regional sources at the few seismographs in the area have been carefully timed to an accuracy of ±0.02 sec and the relative residual technique has been applied to these data. The data cover the period from August 1968 through March 1976. A significant delay of about 0.1 sec in travel-time residuals for Russian nuclear blasts was observed over a 3-year period preceding the Oroville earthquake at station ORV 10 km north of the epicenter. A 0.1-sec delay in travel-time residuals for U.S. nuclear blasts occurred after the Oroville event at station MGL, 40 km north of the main shock's epicenter. P arrivals from deep Tonga-Fiji earthquakes have also been analyzed but reveal no systematic time variations beyond ±0.05 sec from their mean values. P arrivals from moderate-size earthquakes along the San Andreas fault system in central California proved to be an unsatisfactory source of data because of ambiguities created by multiple P-phase arrivals and the emergent nature of the arrivals. The sparse station coverage does not allow adequate delineation of the extent and character of the anomalous P velocity zones, but the data do provide some limitations. The postearthquake travel-time delay at MGL may be precursory to a future earthquake or may only be related to the redistribution of stress in the Oroville area.

Temporal stability of P-velocity anisotropy before earthquakes in central California

1977 ◽  
Vol 67 (4) ◽  
pp. 1075-1090 ◽  
Author(s):  
J. Alan Steppe ◽  
William H. Bakun ◽  
Charles G. Bufe
Keyword(s):  
Travel Time ◽  
Source Region ◽  
Temporal Stability ◽  
Focal Depth ◽  
Temporal Variations ◽  
P Wave ◽  
Depth Range ◽  
Arrival Times ◽  
Central California ◽  
Velocity Anisotropy

Abstract An examination of P-wave travel-time residuals from small earthquakes (source events) located near three larger earthquakes (4 ≦ M ≦ 5) that occurred on the San Andreas fault, near Bear Valley in central California, shows no temporal variations in the residuals extending over broad azimuth ranges (Δφ > ∼40°). Such variations could have resulted from changes in horizontal velocity anisotropy precursory to the larger events. The examination also shows (1) numerous azimuthal variations in the residuals within narrow azimuth bands (Δφ < ∼30°), apparently due to spatial heterogeneity of crustal velocity, (2) a dependence of residual on magnitude for a few stations but not for most stations, and (3) trends of residual versus source event focal depth for about one-third of the 75 source region-station pairs examined. Residuals in these cases typically change by 0.1 sec, and occasionally by 0.2 to 0.3 sec, over the 2- to 12-km focal depth range sampled. The trends vary from station to station in a complex manner. The assumption that traveltime changes are reflected in the residuals is tested by modifying arrival times from some source events according to assumed forms for the travel-time change, relocating those events, and comparing the resulting residuals with those from the unmodified data.

scholarly journals Constraints on temporal variations in velocity near Anza, California, from analysis of similar event pairs

1995 ◽  
Vol 85 (1) ◽  
pp. 194-206 ◽  
Author(s):  
Jennifer S. Haase ◽  
Peter M. Shearer ◽  
Rick C. Aster
Keyword(s):  
Travel Time ◽  
Seismic Velocity ◽  
Correlation Method ◽  
Temporal Variations ◽  
P Wave ◽  
Seismic Gap ◽  
S Wave ◽  
Arrival Times ◽  
Time Separation ◽  
Time Changes

Abstract Similar earthquake pairs recorded by the Anza Seismic Network in southern California are used as repeatable sources to place an upper limit on temporal changes in seismic velocity which occurred in the vicinity of the Anza seismic gap in the last 9 yr. Relative arrival times for each pair of events are found using a cross-correlation method and relative locations are calculated to verify that the pairs have nearly identical hypocenters. The time separation between events in these pairs varies from less than a day to almost 7 yr. The long-term changes in seismic travel times, as measured from the pairs with the longest time separation, are not significantly greater than the noise level estimated from the short-time-separation event pairs. Almost all P-wave paths show less than 0.06% (0.007 sec) change in travel time and all S-wave paths have less than 0.03% (0.004 sec) change. Sensitivity tests place an upper bound on travel-time changes that could be compensated by hypocenter mislocation at 0.2%. There is no evidence that localized stress accumulation causes measurable changes in seismic velocity in the Anza region.

Study of low-magnitude seismic events near the Novaya Zemlya nuclear test site

1997 ◽  
Vol 87 (6) ◽  
pp. 1563-1575
Author(s):  
Frode Ringdal
Keyword(s):  
Monitoring Network ◽  
Test Site ◽  
Nuclear Test ◽  
P Wave ◽  
Scaling Effects ◽  
Underground Nuclear Explosions ◽  
Nuclear Explosions ◽  
Novaya Zemlya ◽  
Source Scaling ◽  
Low Magnitude

Abstract A study of available seismic data shows that all but one of the 42 known underground nuclear explosions at Novaya Zemlya have been detected and located by stations in the global seismic network. During the past 30 years, only one seismic event in this area has been unambiguously classified as an earthquake (1 August 1986, mb = 4.3). Several other small events, most of which are thought to be either chemical explosions or aftereffects of nuclear explosions, have also been detected. Since 1990, a network of sensitive regional arrays has been installed in northern Europe in preparation for the global seismic monitoring network under a comprehensive nuclear test ban treaty (CTBT). This regional network has provided a detection capability for Novaya Zemlya that is shown to be close to mb = 2.5. Three low-magnitude events have been detected and located during this period, as discussed in this article: 31 December 1992 (mb = 2.7), 13 June 1995 (mb = 3.5), and 13 January 1996 (mb = 2.4). To classify the source types of these events has proved very difficult. Thus, even for the mb = 3.5 event in 1995, we have been unable to provide a confident classification of the source as either an earthquake or explosion using the available discriminants. A study of mb magnitude in different frequency bands shows, as expected, that the calculation of mb at regional distances needs to take into account source-scaling effects at high frequencies. Thus, when comparing a 4 to 8 or 8 to 16 Hz filter band to a “teleseismic” 2 to 4 Hz band, the smaller events have, relatively speaking, significantly more high-frequency energy (up to 0.5 mb units) than the larger events. This suggests that a P-wave spectral magnitude scale might be appropriate. The problem of accurately locating small events using a sparse array network is addressed using the 13 January 1996 event, which was detected by only two arrays, as an illustrative example. Our analysis demonstrates the importance of using accurately calibrated regional travel-time curves and, at the same time, illustrates how array processing can be used to identify an interfering phase from a local disturbance, thereby avoiding location errors due to erroneous phase readings.

Seismic wave travel times from nuclear explosions

1958 ◽  
Vol 48 (4) ◽  
pp. 377-398
Author(s):  
Dean S. Carder ◽  
Leslie F. Bailey
Keyword(s):  
Travel Time ◽  
Travel Times ◽  
Strong Variation ◽  
Time Data ◽  
Owens Valley ◽  
Central California ◽  
Nuclear Explosions ◽  
Travel Time Data ◽  
The Pacific ◽  
Wave Travel

Abstract A large number of seismograph records from nuclear explosions in the Nevada and Pacific Island proving grounds have been collected and analyzed. The Nevada explosions were well recorded to distances of 6°.5 (450 mi.) and weakly recorded as far as 17°.5, and under favorable circumstances as far as 34°. The Pacific explosions had world-wide recording except that regional data were necessarily meager. The Nevada data confirm that the crustal thickness in the area is about 35 km., with associations of 6.1 km/sec. speeds in the crust and 8.0 to 8.2 km/sec. speeds beneath it. They indicate that there is no uniform layering in the crust, and that if higher-speed media do exist, they are not consistent; also, that the crust between the proving grounds and central California shows a thickening probably as high as 70 or 75 km., and that this thickened portion may extend beneath the Owens Valley. The data also point to a discontinuity at postulated depths of 160 to 185 km. Pacific travel times out to 14° are from 4 to 8 sec. earlier than similar continental data partly because of a thinner crust, 17 km. or less, under the atolls and partly because speeds in the top of the mantle are more nearly 8.15 km/sec. than 8.0 km/sec. More distant points, at 17°.5 and 18°.5, indicate slower travel times—about 8.1 km/sec. A fairly sharp discontinuity at 19° in the travel-time data is indicated. Travel times from Pacific sources to North America follow closely Jeffreys-Bullen 1948 and Gutenberg 1953 travel-time curves for surface foci except they are about 2 sec. earlier on the continent, and Arctic and Pacific basin data are about 2 sec. still earlier. The core reflection PcP shows a strong variation in amplitude with slight changes in distance at two points where sufficient data were available.

Seismic magnitudes of underground nuclear explosions

1973 ◽  
Vol 63 (1) ◽  
pp. 105-131 ◽  
Author(s):  
P. W. Basham ◽  
R. B. Horner
Keyword(s):  
Computational Procedure ◽  
Test Site ◽  
P Wave ◽  
Nevada Test Site ◽  
Underground Nuclear Explosions ◽  
Nuclear Explosions ◽  
Novaya Zemlya ◽  
Yield Information ◽  
Short Period ◽  
Propagation Effects

abstract Using an Ms computational procedure that minimizes path-propagation effects, and with Ms values found to be empirically independent of test site and detonation medium among consolidated rock explosions, available yield information is employed to illustrate that the seismic scaling of explosions in realistic detonation environments produces teleseismic Rayleigh-wave displacements proportional to the 1.2-power of yield over the range from low yields to greater than three megatons. Ms values independent of network, path, and site can be employed to estimate unknown yields at uncalibrated test sites to within average errors judged to be about 20 per cent. P-wave magnitudes, in the form of a calibrated teleseismic measure of short-period P-wave displacements, show a theoretically supported dependence of displacement on the 1.1-power of yield over the range from 6 kt to 1 mt. Studied explosions separate into two categories: the Nevada Test Site granite explosions, LONG SHOT, the Sahara February 1965 explosion and (by empirical inference) Novaya Zemlya and Eastern Kazakh explosions exhibit P-wave displacements about a factor of 3 greater than explosions of the same yield in tuff, rhyolite, and shale. P-wave magnitudes of explosions are subject to such a diversity of source, propagation, and measurement phenomena that any estimation of unknown yields without a closely controlled site and network calibration can be subject to large errors.

scholarly journals 3-D seismic travel-time tomography validation of a detailed subsurface model: a case study of the Záncara river basin (Cuenca, Spain)

Solid Earth ◽  
2019 ◽  
Vol 10 (1) ◽  
pp. 177-192
Author(s):  
David Marti ◽  
Ignacio Marzan ◽  
Jana Sachsenhausen ◽  
Joaquina Alvarez-Marrón ◽  
Mario Ruiz ◽  
...  
Keyword(s):  
High Resolution ◽  
Travel Time ◽  
River Basin ◽  
Seismic Velocity ◽  
Underground Structure ◽  
High Capacity ◽  
Seismic Source ◽  
P Wave ◽  
Shallow Subsurface ◽  
Travel Time Tomography

Abstract. A high-resolution seismic tomography survey was acquired to obtain a full 3-D P-wave seismic velocity image in the Záncara river basin (eastern Spain). The study area consists of lutites and gypsum from a Neogene sedimentary sequence. A regular and dense grid of 676 shots and 1200 receivers was used to image a 500 m×500 m area of the shallow subsurface. A 240-channel system and a seismic source, consisting of an accelerated weight drop, were used in the acquisition. Half a million travel-time picks were inverted to provide the 3-D seismic velocity distribution up to 120 m depth. The project also targeted the geometry of the underground structure with emphasis on defining the lithological contacts but also the presence of cavities and fault or fractures. An extensive drilling campaign provided uniquely tight constraints on the lithology; these included core samples and wireline geophysical measurements. The analysis of the well log data enabled the accurate definition of the lithological boundaries and provided an estimate of the seismic velocity ranges associated with each lithology. The final joint interpreted image reveals a wedge-shaped structure consisting of four different lithological units. This study features the necessary key elements to test the travel time tomographic inversion approach for the high-resolution characterization of the shallow subsurface. In this methodological validation test, travel-time tomography demonstrated to be a powerful tool with a relatively high capacity for imaging in detail the lithological contrasts of evaporitic sequences located at very shallow depths, when integrated with additional geological and geophysical data.

Decay rate of P-wave amplitudes from nuclear explosions and the magnitude relations in the epicentral distance range 1° to 98°

1970 ◽  
Vol 60 (2) ◽  
pp. 447-460
Author(s):  
K. L. Kaila
Keyword(s):  
Decay Rate ◽  
Travel Time ◽  
Epicentral Distance ◽  
P Wave ◽  
Amplitude Curve ◽  
Distance Range ◽  
Nuclear Explosions ◽  
The Earth

abstract Study of the decay rate with epicentral distance of P-wave amplitudes from nuclear explosions in the distance range 1° to 98° has revealed that instead of the continuous amplitude curve of Gutenberg, the amplitudes are most appropriately represented by six discontinuous curves. Corresponding to these six amplitude decay curves, magnitude relations were worked out. The new magnitude relations yield consistent magnitudes for nuclear explosions as well as earthquakes independent of epicentral distance over the entire distance range of 1° to 98°. The magnitude values are however slightly shifted towards the higher side by 0.22 magnitude unit (at M = 4.5) as compared to those predicted by the Gutenberg-Richter method. Magnitudes are evaluated for Logan, Blanca and Salmon explosions using the new magnitude relations and they agree fairly well with the published magnitudes for these events determined by other workers using the Gutenberg-Richter method. These amplitude decay curves when compared with the travel-time curves of Carder (1964) do show a reasonable correspondence between the starting points of amplitude curves and the distances where the deeply refracted phases from plausible velocity discontinuities in the mantle start appearing as first arrivals on the surface of the Earth.

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