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.

A least squares method for earthquake mechanism determination using S-wave data

1964 ◽  
Vol 54 (6A) ◽  
pp. 2037-2047
Author(s):  
Agustin Udias
Keyword(s):  
Least Squares ◽  
Statistical Evaluation ◽  
Least Squares Method ◽  
Numerical Approach ◽  
Focal Mechanisms ◽  
Least Square ◽  
S Wave ◽  
Double Couple ◽  
Earthquake Mechanism

abstract In this paper a numerical approach to the determination of focal mechanisms based on the observation of the polarization of the S wave at N stations is presented. Least-square methods are developed for the determination of the orientation of the single and double couple sources. The methods allow a statistical evaluation of the data and of the accuracy of the solutions.

Polar radiation patterns of P and SV waves in a multi-layered medium

1973 ◽  
Vol 63 (2) ◽  
pp. 529-547
Author(s):  
Tien-Chang Lee ◽  
Ta-Liang Teng
Keyword(s):  
Displacement Field ◽  
Layered Medium ◽  
Focal Depth ◽  
Focal Mechanisms ◽  
P Wave ◽  
Wave Radiation ◽  
Radiation Patterns ◽  
Double Couple ◽  
Spatial Coordinates ◽  
P And Sv Waves

abstract The displacement field in a multi-layered medium due to incident plane P or SV waves is formulated in terms of Haskell's layer matrices. Based on the reciprocity theorem, the far-field polar radiation patterns of single force, double force, single couple, double couple, and dilatation in a multi-layered medium can be obtained from the displacement field and its first derivatives with respect to the spatial coordinates. Numerical results for models of one layer overlying a half-space indicate that (1) the radiation patterns are sensitive to the variation of focal depth, (2) the layering has a more pronounced effect on SV-wave radiation patterns than on P-wave radiation patterns, (3) the radiation patterns become simpler as the wavelength increases, (4) polarity may reverse abruptly somewhere beyond the critical angle in SV-wave radiation patterns, (5) radiation may be discontinuous across interfaces for some assumed focal mechanisms applied slightly above and below the interfaces, and (6) no clearcut distinction among the various radiation patterns can be used to single out one type of the assumed focal mechanisms from the rest.

The effect of tectonic stress release on explosion P-wave signatures

1976 ◽  
Vol 66 (5) ◽  
pp. 1441-1457
Author(s):  
Thomas C. Bache
Keyword(s):  
Computational Models ◽  
Body Wave ◽  
Tectonic Stress ◽  
P Wave ◽  
General Question ◽  
Appreciable Effect ◽  
Stress Release ◽  
Secondary Sources ◽  
Short Period ◽  
Double Couple

abstract The influence of induced tectonic stress release on the short-period teleseismic P-wave signature of underground nuclear explosions is studied. Primary attention is directed to the first few cycles of the record from which body-wave magnitude (mb) is determined. Computational models for both the explosion and the superimposed tectonic release double couple are employed and theoretical seismograms are computed. Interest is mainly in the largest tectonic release component that seems reasonable using surface-wave observations and independent estimates of the controlling parameters as constraints. It is concluded that for most, perhaps all, events, tectonic release has no appreciable effect on the amplitude of the short-period P waves. Even the frequency content of the early arriving P wave is little affected by tectonic release for most likely circumstances. The computations assume tectonic release due to stress relaxation around the fracture zone created by the explosion. However, the results are extended to apply to the alternate mechanism whereby stress is released along a pre-existing fault plane. Since a number of other mechanisms can cause superposition of a double couple on the explosion, the analysis is relevant to the general question of the size these secondary sources must attain before the short-period P-wave recording is significantly altered.

Inversion of regional surface-wave spectra for source parameters of aftershocks from the Loma Prieta earthquake

1991 ◽  
Vol 81 (5) ◽  
pp. 1726-1736
Author(s):  
Susan L. Beck ◽  
Howard J. Patton
Keyword(s):  
Surface Wave ◽  
Surface Waves ◽  
Focal Mechanism ◽  
Source Parameters ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
P Wave ◽  
Wave Spectra ◽  
Short Period ◽  
First Motion

Abstract Surface waves recorded at regional distances are used to study the source parameters for three of the larger aftershocks of the 18 October 1989, Loma Prieta, California, earthquake. The short-period P-wave first-motion focal mechanisms indicate a complex aftershock sequence with a wide variety of mechanisms. Many of these events are too small for teleseismic body-wave analysis; therefore, the regional surface-waves provide important long-period information on the source parameters. Intermediate-period Rayleigh- and Love-wave spectra are inverted for the seismic moment tensor elements at a fixed depth and repeated for different depths to find the source depth that gives the best fit to the observed spectra. For the aftershock on 19 October at 10:14:35 (md = 4.2), we find a strike-slip focal mechanism with right lateral motion on a NW-trending vertical fault consistent with the mapped trace of the local faults. For the aftershock on 18 October at 10:22:04 (md = 4.4), the surface waves indicate a pure reverse fault with the nodal planes striking WNW. For the aftershock on 19 October at 09:53:50 (md = 4.4), the surface waves indicate a strike-slip focal mechanism with a NW-trending vertical nodal plane consistent with the local strike of the San Andreas fault. Differences between the surface-wave focal mechanisms and the short-period P-wave first-motion mechanisms are observed for the aftershocks analyzed. This discrepancy may reflect the real variations due to differences in the band width of the two observations. However, the differences may also be due to (1) errors in the first-motion mechanism due to incorrect near-source velocity structure and (2) errors in the surface-wave mechanisms due to inadequate propagation path corrections.

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 DETERMINATION OF FAULT PLANE SOLUTIONS USING WAVEFORM AMPLITUDES AND RADIATION PATTERN

2004 ◽  
Vol 36 (3) ◽  
pp. 1529
Author(s):  
D. A. Vamvakaris ◽  
C. B. Papazachos ◽  
E. E. Karagianni ◽  
E. M. Scordilis ◽  
P. M. Chatzidimitriou
Keyword(s):  
Radiation Pattern ◽  
Multiple Solutions ◽  
Fault Plane ◽  
Synthetic Data ◽  
P Wave ◽  
Original Version ◽  
Fault Plane Solutions ◽  
S Waves ◽  
Short Period

In the present work a modified version of the program FPFIT (Reasenberg and Oppenheimer, 1985) is developed, in order to improve the calculation of the fault plane solutions. The method is applied on selected earthquakes from short period waveform data in the Mygdonia basin (N. Greece) as recorded by the permanent network of the Seismological Station of Aristotle University of Thessaloniki during the period 1989-1999. The proposed modification of the FPFIT program was developed in order to minimize the derivation of multiple solutions, as well as the uncertainties in the location of Ρ and Τ axis of the determined fault plane solutions. Compared to the original version of FPFIT the modified approach takes also into account the radiation pattern of SV and SH waves. For each earthquake horizontal and vertical components of each station were used and the first arrivals of Ρ and S waves were picked. Using the maximum peak-to-peak amplitude of Ρ and S waves the ratio Pmax/(S/\/2max+SE2max)1/2 was estimated, where S/Vmax and SEmax are the maximum amplitudes of the two horizontal components (N-S, E-W) for the S waves and Pmax is the maximum amplitude of the vertical one for the P- waves. This ratio for the observed data, as well as the corresponding ratio Prad/iS/Aad+SlAad)1'2 of the synthetic data was used as a weight for the determination of the observed and theoretical P-wave polarities, respectively. The method was tested using synthetic data. A significant improvement of the results was found, compared to the original version of FPFIT. In particular, an improved approximation of the input focal mechanism is found, without multiple solutions and the best-estimated Ρ and Τ axes exhibit much smaller uncertainties. The addition of noise in the synthetic data didn't significantly change the results concerning the fault plane solutions. Finally, we have applied the modified program on a real data set of earthquakes that occurred in the Mygdonia basin.

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.

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