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.

scholarly journals The effects of tectonic release on short-period P waves from NTS explosions

1984 ◽  
Vol 74 (3) ◽  
pp. 819-842
Author(s):  
Thorne Lay ◽  
Terry C. Wallace ◽  
Don V. Helmberger
Keyword(s):  
Frequency Component ◽  
Temporal Variations ◽  
P Wave ◽  
High Frequency Component ◽  
Stress Regime ◽  
P Waves ◽  
Underground Nuclear Explosions ◽  
Long Period ◽  
Short Period ◽  
Amplitude Pattern

Abstract The first cycle (ab amplitude) of teleseismic short-period P waves from underground nuclear explosions at Pahute Mesa (NTS) show a systematic azimuthal amplitude pattern that can possibly be explained by tectonic release. The amplitudes vary by a factor of three, with diminished amplitudes being recorded at azimuths around N25°E. This azimuthal pattern has a strong sin(2φ) component and is observed, to varying degrees, for 25 Pahute Mesa events, but not for events at other sites within the NTS. Events that are known to have large tectonic release have more pronounced sin(2φ) amplitude variations. A synthesis of long-period body and surface wave investigations of tectonic release for Pahute Mesa events shows that, in general, the nonisotropic radiation is equivalent to nearly vertical, right-lateral strike-slip faulting trending from N20°W to due north. Long-period P waves at upper mantle distances demonstrate that there is a significant high-frequency component to the tectonic release. Using the long-period constraints on orientation, moment, and frequency content of the tectonic release, the expected short-period P wave effects are predicted. For models in which the downgoing P wave from the explosion triggers tectonic release within a few kilometers below the shot point, a factor of 2.5 amplitude variation with azimuth is predicted for the short-period ab amplitudes, with the lowest amplitudes expected near N25°E. Rather subtle azimuthal variations in the waveforms are expected, particulary for downward propagating ruptures, which is consistent with the absence of strong variations in the data. The occurrence of the azimuthal pattern, albeit with varying strength, for all of the Pahute Mesa events suggests a tectonic release model in which the shatterzone surrounding the explosion cavity is extended preferentially downward by driving a distributed network of faults and joints underlying the Mesa several kilometers beneath the surface. In this model, all events could have a component of tectonic release which would reflect the regional stress regime, although there may be slight spatial and temporal variations in the tectonic release contribution. Some events may trigger slip on larger throughgoing faults as well. While it is shown that tectonic release can affect teleseismic short-period signals significantly, and may contribute to the Pahute Mesa amplitude pattern, other possible explanations are considered.

scholarly journals P-wave complexity and fault asperities: The Borrego Mountain, California, earthquake of 1968

1982 ◽  
Vol 72 (2) ◽  
pp. 413-437 ◽  
Author(s):  
John E. Ebel ◽  
Donald V. Helmberger
Keyword(s):  
Strong Motion ◽  
Source Model ◽  
P Wave ◽  
Aftershock Activity ◽  
P Waves ◽  
Deconvolution Analysis ◽  
Long Period ◽  
Short Period ◽  
Forward And Inverse Modeling ◽  
Stress Drops

Abstract Results from a synthetic seismogram analysis of the short-period P waves from the Borrego Mountain earthquake of 9 April 1968 (ML = 6.4) are used to model the strong-motion recording at El Centro. A short-period-long-period deconvolution analysis of the teleseismic P waves suggested that a two-source model would fit the data much better than the single-source model presented by Burdick and Mellman (1976). Forward and inverse modeling of the data demonstrated that two sources, each of less than 2-sec duration, the second occurring 2.2 sec after the first and both being at about 8-km depth, best fit the short-period waveforms. From this model, long-period synthetics were generated which were found to be quite compatible with the data. This source model was also used to synthesize the strong-motion SH displacement, velocity, and acceleration records from El Centro, California. The close match of synthetics and data is used to argue that short-period waveforms contain much information about asperities which play a crucial role in the near-source strong motions from an earthquake. The Borrego Mountain event probably began with the failure of a fault asperity. The evidence for this is the several-hundred-bars stress drops of the two short-period sources and the probable location of these sources in a place where there was almost no aftershock activity or postseismic creep on the fault.

Effects of centroid location on determination of earthquake mechanisms using long-period surface waves

1990 ◽  
Vol 80 (5) ◽  
pp. 1205-1231
Author(s):  
Jiajun Zhang ◽  
Thorne Lay
Keyword(s):  
Surface Wave ◽  
Rayleigh Waves ◽  
Body Wave ◽  
Moment Tensor ◽  
Source Location ◽  
P Wave ◽  
Nodal Plane ◽  
Wave Source ◽  
Long Period

Abstract Determination of shallow earthquake source mechanisms by inversion of long-period (150 to 300 sec) Rayleigh waves requires epicentral locations with greater accuracy than that provided by routine source locations of the National Earthquake Information Center (NEIC) and International Seismological Centre (ISC). The effects of epicentral mislocation on such inversions are examined using synthetic calculations as well as actual data for three large Mexican earthquakes. For Rayleigh waves of 150-sec period, an epicentral mislocation of 30 km introduces observed source spectra phase errors of 0.6 radian for stations at opposing azimuths along the source mislocation vector. This is larger than the 0.5-radian azimuthal variation of the phase spectra at the same period for a thrust fault with 15° dip and 24-km depth. The typical landward mislocation of routinely determined epicenters of shallow subduction zone earthquakes causes source moment tensor inversions of long-period Rayleigh waves to predict larger fault dip than indicated by teleseismic P-wave first-motion data. For dip-slip earthquakes, inversions of long-period Rayleigh waves that use an erroneous source location in the down-dip or along-strike directions of a nodal plane, overestimate the strike, dip, and slip of that nodal plane. Inversions of strike-slip earthquakes that utilize an erroneous location along the strike of a nodal plane overestimate the slip of that nodal plane, causing the second nodal plane to dip incorrectly in the direction opposite to the mislocation vector. The effects of epicentral mislocation for earthquakes with 45° dip-slip fault mechanisms are more severe than for events with other fault mechanisms. Existing earth model propagation corrections do not appear to be sufficiently accurate to routinely determine the optimal surface-wave source location without constraints from body-wave information, unless extensive direct path (R1) data are available or empirical path calibrations are performed. However, independent surface-wave and body-wave solutions can be remarkably consistent when the effects of epicentral mislocation are accounted for. This will allow simultaneous unconstrained body-wave and surface-wave inversions to be performed despite the well known difficulties of extracting the complete moment tensor of shallow sources from fundamental modes.

WIDE‐BAND EXTRACTION OF MANTLE P WAVES FROM AMBIENT NOISE

Geophysics ◽  
1964 ◽  
Vol 29 (5) ◽  
pp. 672-692 ◽  
Author(s):  
Milo Backus ◽  
John Burg ◽  
Dick Baldwin ◽  
Ed Bryan
Keyword(s):  
Ambient Noise ◽  
Wide Band ◽  
P Wave ◽  
Cumberland Plateau ◽  
P Waves ◽  
Single Output ◽  
Short Period ◽  
Multichannel Filtering ◽  
On Line ◽  
Element Array

The spatial correlation characteristics of ambient short‐period (0.5 to 5 cps) noise at Ft. Sill, Oklahoma, and on the Cumberland Plateau in Tennessee were investigated on “permanent” arrays with 3–4 kilometer diameter. Dominant ambient noise at the two locations is spatially organized, and to first order may be treated as a combination of seismic propagating wave trains. At the Tennessee location noise energy above one cps is dominantly propagating with velocities from 3.5 to 4.5 km/sec, and must be carried in deeply trapped, high‐order modes. Generalized multichannel filtering (Burg) can be used to preserve a large class of mantle P‐wave signals, wide‐band, in a single output trace, while at the same time specifically rejecting ambient noise on the basis of its organization. Results of generalized multichannel filtering applied on‐line at the nineteen‐element array in Tennessee and applied off‐line are discussed.

scholarly journals The February 9, 1971 San Fernando earthquake: A study of source finiteness in teleseismic body waves

1978 ◽  
Vol 68 (1) ◽  
pp. 1-29 ◽  
Author(s):  
Charles A. Langston
Keyword(s):  
Near Field ◽  
Dislocation Line ◽  
Strong Motion ◽  
Body Waves ◽  
P Waves ◽  
Long Period ◽  
Short Period ◽  
Wave Forms ◽  
San Fernando ◽  
The Time Domain

abstract Teleseismic P, SV, and SH waves recorded by the WWSS and Canadian networks from the 1971 San Fernando, California earthquake (ML = 6.6) are modeled in the time domain to determine detailed features of the source as a prelude to studying the near and local field strong-motion observations. Synthetic seismograms are computed from the model of a propagating finite dislocation line source embedded in layered elastic media. The effects of source geometry and directivity are shown to be important features of the long-period observations. The most dramatic feature of the model is the requirement that the fault, which initially ruptured at a depth of 13 km as determined from pP-P times, continuously propagated toward the free surface, first on a plane dipping 53°NE, then broke over to a 29°NE dipping fault segment. This effect is clearly shown in the azimuthal variation of both long period P- and SH-wave forms. Although attenuation and interference with radiation from the remainder of the fault are possible complications, comparison of long- and short-period P and short-period pP and P waves suggest that rupture was initially bilateral, or, possibly, strongly unilateral downward, propagating to about 15 km depth. The average rupture velocity of 1.8 km/sec is well constrained from the shape of the long-period wave forms. Total seismic moment is 0.86 × 1026 dyne-cm. Implications for near-field modeling are drawn from these results.

Teleseismic p-wave transmission through slabs

1973 ◽  
Vol 63 (4) ◽  
pp. 1349-1373
Author(s):  
Norman H. Sleep
Keyword(s):  
P Wave ◽  
P Waves ◽  
Nuclear Explosions ◽  
Velocity Models ◽  
Shallow Earthquakes ◽  
Short Period ◽  
Systematic Effects ◽  
First Motion ◽  
Low Amplitude ◽  
Good Agreement

abstract Theoretical ray paths through velocity models constructed from numerically calculated thermal models of slabs were computed. The results were in good agreement with observed travel times. First motion amplitudes of P waves at teleseismic distances were measured from long- and short-period WWSSN records of intermediate focus earthquakes in the Tonga, Kermadec, and Kurile regions and of nuclear explosions and shallow earthquakes in the Aleutian region. These amplitudes were corrected for source mechanism. The Aleutian data were sufficient to show that intermediate focus earthquakes in that region occur in the colder regions of the slab. At short periods, for regions other than the Aleutians, shadowing effects which could be associated with the slab were not very marked, less than a factor of 2 reduction for epicentral distances between 30° and 50°. No systematic effects due to plates were found in the long-period data. Some stations in the predicted shadow zone of a Tonga earthquake recorded low amplitude precursors which probably were greatly defocused waves which ran the full length of the slab. Simple diffraction is incapable of explaining the short-period results.

Noise level variation at norsar and its effect on detectability

1977 ◽  
Vol 67 (2) ◽  
pp. 479-492 ◽  
Author(s):  
F. Ringdal ◽  
H. Bungum
Keyword(s):  
Atmospheric Pressure ◽  
Noise Level ◽  
Amplitude Distribution ◽  
P Wave ◽  
Seasonal Fluctuations ◽  
Long Period ◽  
Short Period ◽  
Noise Data ◽  
Band Pass ◽  
Period Data

Abstract For 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.

The El Asnam, Algeria, earthquake of 10 October 1980: Multiple-source mechanism determined from long-period records

1982 ◽  
Vol 72 (4) ◽  
pp. 1111-1128
Author(s):  
A. Deschamps ◽  
Y. Gaudemer ◽  
A. Cisternas
Keyword(s):  
Main Shock ◽  
Body Wave ◽  
P Wave ◽  
P Waves ◽  
Small Component ◽  
Long Period ◽  
The North ◽  
Complex Source ◽  
Dip Angle ◽  
Good Agreement

abstract We present a study of the El Asnam, Algeria, earthquake of 10 October 1980 from a large collection of long-period surface and body wave records. The focal mechanism of the main shock is well constrained by the P-wave first motions at teleseismic distances and field observations: it was a thrust event on a plane striking N45°E, and a dip angle of 54° to the north. It had a small component of left-lateral motion (λ = 83°) (Ouyed et al., 1981; Gaudemer et al., 1981). This earthquake was very well recorded on WWSSN stations and on GDSN and IDA digital stations, with a good azimuthal distribution. From these records, we confirm the focal solution and obtain a seismic moment Mo = 5 × 1026 dyne-cm. The P-wave seismograms indicate a complex source. We show that it is not sufficient to model the source by a multiple event, but it is also necessary to include a propagation effect in order to explain accurately the waveform. With assumptions based on field observation of the surface breaks, we model the P waves, including two discontinuities of the propagation, along the fault plane and obtain a good agreement of the waveform.

Surface-wave magnitudes of Eurasian earthquakes and explosions

1975 ◽  
Vol 65 (3) ◽  
pp. 693-709 ◽  
Author(s):  
Otto W. Nuttli ◽  
So Gu Kim
Keyword(s):  
Surface Wave ◽  
Focal Mechanism ◽  
Rayleigh Waves ◽  
Body Wave ◽  
Focal Depth ◽  
Vertical Component ◽  
P Waves ◽  
Long Period ◽  
Short Period ◽  
Continental Interior

abstract Body-wave magnitudes, mb, and surface-wave magnitudes, MS, were determined for approximately 100 Eurasian events which occurred during the interval August through December 1971. Body-wave magnitudes were determined from 1-sec P waves recorded by WWSSN short-period, vertical-component seismographs at epicentral distances greater than 25°. Surface-wave magnitudes were determined from 20-sec Rayleigh waves recorded by long-period, vertical-component WWSSN and VLPE seismographs. The earthquakes had mb values ranging from 3.6 to 5.7. Of 96 presumed earthquakes studied, 6 lie in or near the explosion portion of an mb:MS plot. The explosion mb:MS curve was obtained from seven Eurasian events which had mb values ranging from 5.0 to 6.2 and MS values from 3.2 to 5.1. All six anomalous earthquakes were located in the interior of Asia, in Tibet, and in Szechwan and Sinkiang provinces of China. In general, oceanmargin earthquakes were found to have more earthquake-like mb:MS values than those occurring in the continental interior. Neither focal depth nor focal mechanism can explain the anomalous events.

Spectral and magnitude characteristics of anomalous Eurasian earthquakes

1977 ◽  
Vol 67 (2) ◽  
pp. 463-478
Author(s):  
So Gu Kim ◽  
Otto W. Nuttli
Keyword(s):  
Focal Mechanism ◽  
Rayleigh Waves ◽  
Main Shock ◽  
Focal Depth ◽  
P Wave ◽  
Wave Spectra ◽  
P Waves ◽  
Aftershock Sequences ◽  
Short Period ◽  
Amplitude Spectra

Abstract A 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.

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