VARIATIONS IN SHORT-PERIOD RECORDS FROM CANADIAN SEISMOGRAPH STATIONS

1965 ◽  
Vol 2 (5) ◽  
pp. 510-542 ◽  
Author(s):  
M. Ichikawa ◽  
P. W. Basham
Keyword(s):  
Regional Variation ◽  
Wave Amplitude ◽  
Signal Amplitude ◽  
Theoretical Models ◽  
P Wave ◽  
Spectral Studies ◽  
Spectral Amplitude ◽  
Low Velocity ◽  
Short Period ◽  
Wave Trains

A study of the relative recording ability of some of the Canadian seismograph stations has indicated a pattern of P-wave amplitude anomalies varying from station to station and, at any one station, showing a significant regional variation which does not seem to be entirely produced by source mechanism effects. Rather local crustal effects appear important, and spectral studies indicate that significant effects can be produced in low velocity upper crustal layers. The initial investigation of early P-wave trains and theoretical models suggests that these crustal effects can best be examined by the use of apparent incident angles, and that spectral amplitude decrements change significantly from station to station, although this is unexplained.It appears that the different recording ability of stations can be explained by a combination of shallow crustal effects operating on the signal amplitude and the local noise properties.

scholarly journals The April 29, 1965, Puget Sound earthquake and the crustal and upper mantle structure of western Washington

1977 ◽  
Vol 67 (3) ◽  
pp. 693-711 ◽  
Author(s):  
Charles A. Langston ◽  
David E. Blum
Keyword(s):  
Upper Mantle ◽  
Source Parameters ◽  
Puget Sound ◽  
P Wave ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Low Velocity Zone ◽  
Crustal Model ◽  
Short Period ◽  
Velocity Zone

abstract Simultaneous modeling of source parameters and local layered earth structure for the April 29, 1965, Puget Sound earthquake was done using both ray and layer matrix formulations for point dislocations imbedded in layered media. The source parameters obtained are: dip 70° to the east, strike 344°, rake −75°, 63 km depth, average moment of 1.4 ± 0.6 × 1026 dyne-cm, and a triangular time function with a rise time of 0.5 sec and falloff of 2.5 sec. An upper mantle and crustal model for southern Puget Sound was determined from inferred reflections from interfaces above the source. The main features of the model include a distinct 15-km-thick low-velocity zone with a 2.5-km/sec P-wave-velocity contrast lower boundary situated at approximately 56-km depth. Ray calculations which allow for sources in dipping structure indicate that the inferred high contrast value can trade off significantly with interface dip provided the structure dips eastward. The effective crustal model is less than 15 km thick with a substantial sediment section near the surface. A stacking technique using the instantaneous amplitude of the analytic signal is developed for interpreting short-period teleseismic observations. The inferred reflection from the base of the low-velocity zone is recovered from short-period P and S waves. An apparent attenuation is also observed for pP from comparisons between the short- and long-period data sets. This correlates with the local surface structure of Puget Sound and yields an effective Q of approximately 65 for the crust and upper mantle.

A discrimination analysis of short-period regional seismic data recorded at Tonto Forest Observatory

1982 ◽  
Vol 72 (4) ◽  
pp. 1351-1366
Author(s):  
J. R. Murphy ◽  
T. J. Bennett
Keyword(s):  
Epicentral Distance ◽  
Test Site ◽  
P Wave ◽  
Set Cover ◽  
Spectral Amplitude ◽  
Nevada Test Site ◽  
Data Set ◽  
Short Period ◽  
Single Station ◽  
Regional Phases

abstract A new seismic discriminant based on spectral differences of regional phases from earthquakes and explosions recorded at a single station has been tested and found to work remarkably well. The test data consisted of a well-constrained set of 30 Nevada Test Site (NTS) explosions and 21 earthquakes located within about 100 km of NTS which were recorded on short-period seismographs at the Tonto Forest Observatory in central Arizona at an epicentral distance averaging 530 km. The events in the data set cover a magnitude range from 3.3 to 4.8 (mb) for which Pn, Pg, and Lg phases have been analyzed. We found that, although Lg phases from earthquakes are typically more prominent than for explosions with comparable P-wave amplitude levels, simple time-domain Lg/P amplitude ratios do not result in a separation of the earthquake and explosion samples consistent enough to provide reliable discrimination. However, spectral analyses of the data over the frequency band from 0.5 to 5.0 Hz revealed significant differences in the spectra of certain regional phases which proved to be a quite reliable discriminant. In particular, both the Pg and Lg spectra from earthquakes have been found to be richer in high-frequency content than corresponding explosion spectra. A discriminant measure, defined as the ratio of average Lg spectral amplitude level in the 0.5- to 1.0-Hz passband to that in the 2.0- to 4.0-Hz passband, provides good separation of earthquake and explosion populations.

Network-averaged teleseismic P-wave spectra for underground explosions. Part II. Source characteristics of Pahute Mesa explosions

1989 ◽  
Vol 79 (1) ◽  
pp. 156-171
Author(s):  
John R. Murphy
Keyword(s):  
Frequency Band ◽  
P Wave ◽  
Spectral Amplitude ◽  
Model Parameters ◽  
Scaling Analysis ◽  
Wave Spectra ◽  
Amplitude Data ◽  
Short Period ◽  
Frequency Independent ◽  
Averaged Model

Abstract A 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.

Seismic detection of the low-velocity anomaly at the crust and uppermost mantle beneath the central Tien Shan

2020 ◽  
Author(s):  
Ran Cui ◽  
Yuanze Zhou
Keyword(s):  
Tarim Basin ◽  
Velocity Structure ◽  
Orogenic Belt ◽  
Tien Shan ◽  
P Wave ◽  
Uppermost Mantle ◽  
Low Velocity ◽  
Velocity Anomaly ◽  
Short Period ◽  
Seismic Detection

<p>As one of the most active intracontinental orogenic belts in the world, the Tien Shan orogenic belt originated in the Paleozoic and then experienced tectonic activities such as plate subduction and closure of the Paleo-Asian Ocean. Previous seismological and geodynamic studies have shown the observed the low-velocity anomaly (LVA) beneath the central Tien Shan at the uppermost mantle, which has a significant influence on the formation and modification of the crust and mantle lithosphere ( Lei et al, 2007). However, the distribution, morphology and physical property of the LVA are highly debatable.</p><p>We conduct 2-D forward waveform modeling based on spectral-element method (SEM) to investigate waveform distortions that were generated by the velocity contrast boundary of the LAV. The broadband P- and S- waves from three intermediate-depth earthquakes at Hindu Kush-Pamir were recorded by the Chinese Digital Seismograph Network (Zheng et al., 2010). We use these records to confirm the location, shape and velocity decrement of the LVA by fitting the observed records with the synthetics through SEM based on the 1D velocity structures (TSTB-B) of the central Tien Shan and northern Tarim basin (Gao et al., 2017). We find the LVA at 10~100 km beneath the eastern part of the central Tien Shan. And the northward under-thrusting of the Tarim Basin may trigger some mantle upwelling, contributing to the observed LVA.</p><p>Lei, J., Zhao, D. (2007). Teleseismic P-wave tomography and the upper mantle structure of the central Tien Shan orogenic belt.<em> Physics of the Earth and Planetary Interiors</em>, 162, 165-185, doi: 10.1016/j.pepi.200704010.</p><p>Zheng, X., Jiao, W., Zhang, C., et al. (2010). Short-Period Rayleigh-Wave Group Velocity Tomography through Ambient Noise Cross-Correlation in Xinjiang, Northwest China.<em> Bulletin of the Seismological Society of America</em>, 100(3): 1350-1355, doi: 10.1785/0120090225.</p><p>Gao, Y., Cui, Q., Zhou, Y. (2017). Seismic detection of P-wave velocity structure atop MTZ beneath the Central Tian Shan and Tarim Basin. <em>Chinese Journal of Geophysics ( in Chinese with English Abstract )</em>, 60 (1) : 98-111, doi: 10.6038 /cjg20170109.</p>

Seismic studies in central and eastern Tennessee

1978 ◽  
Vol 68 (4) ◽  
pp. 1081-1094
Author(s):  
E. S. Sodbinow ◽  
G. A. Bollinger
Keyword(s):  
Surface Wave ◽  
Theoretical Models ◽  
Wave Dispersion ◽  
Time Frame ◽  
Low Noise ◽  
Cumberland Plateau ◽  
Surface Wave Dispersion ◽  
Mode Dispersion ◽  
Short Period ◽  
Wave Trains

abstract Some of the seismic characteristics of Tennessee were investigated by means of a short-period surface-wave dispersion study in central Tennessee and a microearthquake survey of the eastern portion of the state. The tripartite method of phase velocity determination was applied to data from a 4-element SPZ array at the Cumberland Plateau Observatory (CPO). Seven short-period (0.5 to 1.4 sec.) surface-wave trains were analyzed. These wave trains exhibited both fundamental and first higher mode dispersion. Theoretical models, consisting of 2 or 3 layers over a half-space were developed that explain the observed dispersion. The layers, which model the surficial sediments of the region, range in total thickness from 1.6 to 2.1 km and have shear velocities from 1.70 to 3.10 km/sec. A 5-station array of portable seismographs was deployed in eastern Tennessee and 2 months of operation yielded 3000 low-noise hours of data. Eleven microearthquakes −1.3 ≦ M ≦ 1.1 were recorded during that time frame indicating that, at least for periods of several weeks, the microseismicity of eastern Tennessee can be very low.

Network-averaged teleseismic P-wave spectra for underground explosions. Part I. Definitions and examples

1989 ◽  
Vol 79 (1) ◽  
pp. 141-155
Author(s):  
J. R. Murphy ◽  
B. W. Barker ◽  
A. O'Donnell
Keyword(s):  
Test Site ◽  
P Wave ◽  
Spectral Amplitude ◽  
Scaling Analysis ◽  
Wave Spectra ◽  
Correction Factors ◽  
Station Correction ◽  
Wave Source ◽  
Amplitude Data ◽  
Short Period

Abstract A new procedure is described for estimating network-averaged P-wave spectra from short-period, teleseismic recordings of explosions. This procedure is then applied to a large sample of approximately 1000 digitized Nevada Test Site (NTS) explosion seismograms to simultaneously derive estimates of frequency-dependent station correction factors and network-averaged P-wave spectra for each of the 50 selected explosions. An analysis of the station correction factors is then presented which demonstrates that the Veith-Clawson (1972) B(Δ) curve for time domain mb adequately describes the teleseismic distance dependence of the spectral amplitude data over the entire short-period band extending from 0.50 to 2.25 Hz. A simplified scaling analysis of the network-averaged spectra is described and used to confirm the fact that the P-wave source coupling characteristics below the water table at the Yucca Flat and Pahute Mesa testing areas of NTS are essentially identical. We conclude that this new spectral measure can provide robust means for comparing different explosion testing areas.

Synthesis of teleseismic P waves from sources near sinking lithospheric slabs

1975 ◽  
Vol 65 (6) ◽  
pp. 1667-1680
Author(s):  
Ronald W. Ward ◽  
Keiiti Aki
Keyword(s):  
Wave Amplitude ◽  
Wave Theory ◽  
P Wave ◽  
Wave Form ◽  
Shadow Zone ◽  
P Waves ◽  
Wave Source ◽  
Long Period ◽  
Short Period ◽  
Dominant Period

abstract A wave theory method is used to determine the effect of a sinking lithospheric slab on short-period and long-period waves. We consider a simplified model of the lithospheric slab with a 10 per cent velocity contrast and compute both short-period and long-period theoretical seismograms from a P-wave source located in or near the slab. For this model, the ray-theoretical amplitude agrees quite well with the short-period amplitude. In the ray-theoretical shadow zone the long-period seismograms (15- to 25-sec dominant period) typically have amplitudes 50 per cent (or greater) of the direct P-wave amplitude and exhibit wave-form broadening. Similar wave-form broadening has been attributed to the dynamics of earthquake faulting. The effect of the lithosphere on long-period waves from nearby sources must be taken into account in studies which utilize the observed variation in wave-form broadening to infer earthquake source dynamics.

The effect of the earthquake radiation pattern on mb—A study using aftershocks in the 1976 Gazli sequence

1998 ◽  
Vol 88 (2) ◽  
pp. 523-530 ◽  
Author(s):  
David Bowers ◽  
Alan Douglas
Keyword(s):  
Radiation Pattern ◽  
Analysis Of Variance ◽  
Wave Amplitude ◽  
Focal Mechanisms ◽  
P Wave ◽  
Radiation Coefficient ◽  
Short Period ◽  
Double Couple ◽  
Earthquake Mechanism ◽  
Scalar Moment

Abstract We use published focal mechanisms to estimate radiation coefficients to four short-period arrays recording teleseismic P from 38 aftershocks in the 1976 Gazli, Uzbekistan, earthquake sequence. We divide the observed P-wave amplitude by its radiation coefficient to estimate the P-wave amplitude that would be observed if it was from the maximum of the double-couple radiation pattern. We use this new P-wave amplitude to calculate a P-wave magnitude, mCb, that is independent of the P-radiation pattern if the focal mechanisms are without error. Analysis of variance shows that the random error in mCb is reduced relative to that in the original P-wave magnitudes mCb and that this reduction is statistically significant at the 11% level. Further, analysis of variance demonstrates that the radiation coefficients calculated from the focal mechanisms contain error but that this error is probably not large enough to mask the detection of the radiation effect in mOb. Published averages of the logarithm of P-radiation coefficients allow an assessment of the differences in network-averaged mb due to the radiation pattern of point earthquake and explosion sources. Network-averaged mb from a vertical strike-slip earthquake can differ from an explosion of similar scalar moment by as much as 1.0 m.u. (magnitude units). However, this difference can be as little as 0.2 m.u. if the earthquake mechanism is 30° dip slip. We argue that, if mb is required to be independent of the earthquake mechanism, the most appropriate network average is mCb − 0.48.

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