The effect of an initial hypocentral stress upon the radiation patterns of P and S waves

1972 ◽  
Vol 62 (5) ◽  
pp. 1173-1182 ◽  
Author(s):  
F. A. Dahlen
Keyword(s):  
Radiation Pattern ◽  
Fault Plane ◽  
Deviatoric Stress ◽  
P Wave ◽  
Stress Deviator ◽  
Radiation Patterns ◽  
Fault Plane Solutions ◽  
S Waves ◽  
Quadrupole Component ◽  
Sufficient Magnitude

Abstract The effect of an initial hypocentral deviatoric stress upon the radiation patterns of radiated P and S waves is explicitly described for the case of an infinitesimal, nonpropagating seismic dislocation. A nonzero hypocentral stress deviator produces two small changes in the familiar quadrupole radiation pattern; it gives rise to a small additional explosion-like component, and it acts to skew slightly the quadrupole component relative to the fault plane and auxiliary plane. The latter phenomenon is not of sufficient magnitude to give rise to any serious uncertainties in the interpretation of fault-plane solutions; in fact, both phenomena are so small that they will be exceedingly difficult ever to detect. The recent measurements of P-wave amplitudes on the focal sphere by Randall and Knopoff (1970) cannot be explained by these results.

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.

A comparison of some S-wave studies of earthquake mechanisms

1961 ◽  
Vol 51 (2) ◽  
pp. 277-292
Author(s):  
William Stauder ◽  
Adams W. M.
Keyword(s):  
Fault Plane ◽  
Analytical Techniques ◽  
P Wave ◽  
Graphical Methods ◽  
S Wave ◽  
Fault Plane Solutions ◽  
P Waves ◽  
S Waves ◽  
Analytical Technique ◽  
Direction Of Polarization

Abstract Graphical and analytical techniques for using S-waves in focal mechanism studies are compared. In previous applications the analytical technique has shown little or no agreement with the results of fault-plane solutions from P-waves, whereas for other groups of earthquakes the graphical methods have shown good agreement between the S-waves and the P-wave solutions. It is shown that the graphical and analytical techniques are identical in principle and that when the graphical methods are applied to the same three earthquakes to which the analytical technique had been applied the identical results are obtained. Closer examination of the graphical presentation of the data, however, shows that the disagreement between the S-waves and the fault plane solutions from P is largely apparent. The discrepancy follows upon the peculiar scatter in the S-wave data and the chance occurrence of observations of S at stations located along closely parallel planes of polarization of S. Once this is understood, it is seen that the direction of polarization of S-waves is in substantial agreement with the methods of analysis of focal mechanisms from P-waves, and that the data are consistent with a simple dipole as the point model of the earthquake focus.

Aftershocks of the February 21, 1973 Point Mugu, California earthquake

1976 ◽  
Vol 66 (6) ◽  
pp. 1931-1952
Author(s):  
Donald J. Stierman ◽  
William L. Ellsworth
Keyword(s):  
Fault Zone ◽  
Main Shock ◽  
Fault Plane ◽  
Body Wave ◽  
P Wave ◽  
Fault System ◽  
Wave Radiation ◽  
Fault Plane Solutions ◽  
Complex Deformation ◽  
Transverse Ranges

abstract The ML 6.0 Point Mugu, California earthquake of February 21, 1973 and its aftershocks occurred within the complex fault system that bounds the southern front of the Transverse Ranges province of southern California. P-wave fault plane solutions for 51 events include reverse, strike slip and normal faulting mechanisms, indicating complex deformation within the 10-km broad fault zone. Hypocenters of 141 aftershocks fail to delineate any single fault plane clearly associated with the main shock rupture. Most aftershocks cluster in a region 5 km in diameter centered 5 km from the main shock hypocenter and well beyond the extent of fault rupture estimated from analysis of body-wave radiation. Strain release within the imbricate fault zone was controlled by slip on preexisting planes of weakness under the influence of a NE-SW compressive stress.

Visualizing effects of anisotropy on seismic moments and their potency-tensor isotropic equivalent

Geophysics ◽  
2018 ◽  
Vol 83 (3) ◽  
pp. C85-C97 ◽  
Author(s):  
Nepomuk Boitz ◽  
Anton Reshetnikov ◽  
Serge A. Shapiro
Keyword(s):  
Radiation Pattern ◽  
Elastic Anisotropy ◽  
Moment Tensor ◽  
Matrix Multiplication ◽  
Tensor Decomposition ◽  
P Wave ◽  
Propagation Path ◽  
Tectonic Deformation ◽  
Radiation Patterns ◽  
Rock Anisotropy

Radiation patterns of earthquakes contain important information on tectonic strain responsible for seismic events. However, elastic anisotropy may significantly impact these patterns. We systematically investigate and visualize the effect of anisotropy on the radiation patterns of microseismic events. For visualization, we use a vertical-transverse-isotropic (VTI) medium. We distinguish between two different effects: the anisotropy in the source and the anisotropy on the propagation path. Source anisotropy mathematically comes from the matrix multiplication of the anisotropic stiffness tensor with the source strain expressed by the potency tensor. We analyze this effect using the corresponding radiation pattern and the moment tensor decomposition. Propagation anisotropy mathematically comes from the deviation between the polarization and the propagation direction of a quasi P-wave in an anisotropic medium. We investigate both effects separately by either assuming the source to be anisotropic and the propagation to be isotropic or vice versa. We find that both effects have a significant impact on the radiation pattern of a pure-slip source. Finally, we develop an alternative visualization of source mechanisms by plotting beach balls proportional to their potency tensors. For this, we multiply the potency tensor with an isotropic elasticity tensor having the equivalent shear modulus [Formula: see text] and [Formula: see text]. In this way, we visualize the tectonic deformation in the source, independently of the rock anisotropy.

scholarly journals Present-day stress state in northwestern Syria

2020 ◽  
Vol 59 (4) ◽  
pp. 299-316
Author(s):  
Mohamad Khir Abdul-Wahed ◽  
Mohammed ALISSA
Keyword(s):  
Fault Plane ◽  
Eastern Mediterranean ◽  
Active Faults ◽  
Plate Boundary ◽  
Shear Zones ◽  
Stress Pattern ◽  
P Wave ◽  
Fault System ◽  
Fault Plane Solutions ◽  
Stress Regime

Northwestern Syria is a key area in the eastern Mediterranean to study the active tectonics and stress pattern across the Arabia-Eurasia convergent plate boundary. This study aims to outline the present-day stress regime in this region of Syria using the fault plane solutions of the largest events recorded by the Syrian National Seismological Network from 1995 to 2011. A dataset of fault-plane solutions was obtained for 48 events having at least 5 P-wave polarities. The tectonic regime for most of these events is extensional and produces normal mechanisms in agreement with the local configurations of the seismogenic faults in the region. Strike-slip mechanisms are more scarce and restricted to certain areas, such as the northern extension of the Dead Sea fault system. The results of the current study reveal the spatial variations of SHmax orientation across the northwestern Syria region. This spatial variation of the present-day stress field highlights the role of main geometrically complex shear zones in the present-day stress pattern of northwestern Syria. However, these results show, regardless of the relatively small magnitudes of the studied events, they provide a picture of the local stress deviations that have currently been taking place along the local active faults.

scholarly journals Microearthquake Source Locations and Mechanisms: Ice Stream B, West Antarctica (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 199-199
Author(s):  
S. Anandakrishnan ◽  
D. D. Blankenship ◽  
C. R. Bentley
Keyword(s):  
Depth Profile ◽  
Fault Plane ◽  
P Wave ◽  
West Antarctica ◽  
Precise Location ◽  
Fault Plane Solutions ◽  
Ice Flow ◽  
Ice Stream ◽  
Ice Stream B

An array of nine seismographic stations, each sensitive to all three components of motion, was deployed on Ice Stream B, West Antarctica, during the austral summer of 1985–86. The network was sensitive to high-frequency (=400 Hz) seismic activity within a 350 km2 area of the ice stream, and the deployment geometry allows the precise determination of depths for events beneath the 10 km2 array. Microearthquakes from both beneath and beside the ice stream were detected and recorded (Blankenship and others 1987). Inversion of P-wave and S-wave travel times and radiation patterns allows the determination of locations and fault-plane solutions for many of these events.We find that bottom events involve low-angle thrusting, in the down-stream direction, of ice or till; displacement is ∼½ cm per event over a (15 m)2 area. Such faulting is rare and releases an insignificant part of the total energy dissipated by ice flow. However, this is a possible mechanism for plucking of the ice-stream bed.Fault-plane solutions for most major surface events are consistent with the opening of tensional fractures oriented transverse to ice flow. Precise location of these events shows that they correspond to open crevasses, mapped by Vornberger and Whillans (1986), that are oriented transverse to ice flow.In addition, shear-wave splitting observed on some of the microearthquakes shows that the c-axes in the ice stream are slightly, but not strongly, anisotropic. Precise location of the sources requires the use of a detailed velocity-depth profile in the firn, which was obtained by seismic short-refraction studies (Anandakrishnan and others 1988, this volume). A density-depth profile calculated from these velocities agrees well with direct density measurements on a single core nearby (Alley and Bentley 1988, this volume).

S waves: Alaska and other earthquakes

1960 ◽  
Vol 50 (4) ◽  
pp. 581-597 ◽  
Author(s):  
William Stauder
Keyword(s):  
Focal Mechanism ◽  
Fault Plane ◽  
P Wave ◽  
Wave Analysis ◽  
Point Model ◽  
S Wave ◽  
S Waves ◽  
Wave Solutions ◽  
Single Force

ABSTRACT Techniques of S wave analysis are used to investigate the focal mechanism of four earthquakes. In all cases the results of the S wave analysis agree with previously determined P wave solutions and conform to a dipole with moment or single couple as the point model of the focus. Further, the data from S waves select one of the two nodal planes of P as the fault plane. Small errors in the determination of the angle of polarization of S are shown to result in scatter in the data of a peculiar character which might lead to misinterpretation. The same methods of analysis which in the present instances show excellent agreement with a dipole with moment source are the methods which in a previous paper required a single force type mechanism for a different group of earthquakes.

Fault plane solutions for northeastern United States earthquakes

1981 ◽  
Vol 71 (6) ◽  
pp. 1875-1882
Author(s):  
Jay J. Pulli ◽  
M. Nafi Toksöz
Keyword(s):  
United States ◽  
New York ◽  
New England ◽  
Fault Plane ◽  
Local Stress ◽  
P Wave ◽  
Northeastern United States ◽  
Stress Concentrations ◽  
Fault Plane Solutions ◽  
Entire Area

Abstract Fault plane solutions for eight earthquakes occurring in the northeastern United States have been determined using P-wave first motions and a computer algorithm for picking all valid solutions. The predominant mechanism in the area is thrust faulting, however the direction of the P axis is not consistent throughout the entire area. In central New England (Maine-New Hampshire), the P axis trends nearly E-W. In southeastern New England, the P axis trends N-S to NE-SW. In the Adirondacks region of New York, the P axis trends NE-SW as previously reported by Yang and Aggarwal (1981). Although the stress distribution appears to be complicated, as in the Central United States (Street et al., 1974), an underlying E-W compressive stress may exist in the New England area. These small earthquakes may represent the response to local stress concentrations.

Model-based radiation pattern correction for interferometric redatuming in 4D seismic

Geophysics ◽  
2018 ◽  
Vol 83 (4) ◽  
pp. Q25-Q35 ◽  
Author(s):  
Yang Zhao ◽  
Weichang Li
Keyword(s):  
Image Quality ◽  
Radiation Pattern ◽  
Matched Filter ◽  
Time Lapse ◽  
Wave Mode ◽  
P Wave ◽  
Radiation Patterns ◽  
Near Surface ◽  
Model Based ◽  
Isotropic Radiation

Interferometric virtual source (VS) redatuming converts surface-triggered source records into the equivalent records as if they originated from buried receiver locations by crosscorrelating downgoing waves with the corresponding upgoing waves. The theory suggests that when the receivers are surrounded by an enclosing boundary of sources, then the VS has an isotropic radiation pattern and yields an accurate response. The resultant records should determine improvement in the seismic repeatability and image quality compared with non-VS. However, in the presence of a complex near surface, an intricate shallow structure and highly variable weathering layers can severely distort the raypath, such that it produces uneven angle coverage to the buried VS. In addition, near-surface reverberations, surface multiples, and other mode-converted waves may leak into the time-gated early arrivals and further corrupt the direct wavefields. The above-mentioned issues can result in distorted radiation patterns and contaminated responses of the VS. We address these issues explicitly by spatially filtering the potentially contaminated direct wavefields using a zero-phase matched filter, such that the filtered wavefield is consistent with a model-based ideal direct P-wavefield observed at common receiver locations. This ideal reference response is computed from a homogeneous approximation to the local near-surface overburden on top of each VS. The phases of the original direct arrivals are preserved. Components associated with the reverberations and other noises can be effectively suppressed as their spatial radiation patterns deviate from that of the ideal single P-wave mode. Toward an isotropic radiation pattern by the iterative matched filter, we reduce the unbalanced illumination arising from imperfect source coverage and near-surface complexity. Compared with previous methods, the new VS approach provides significantly improved image quality and repeatability based on a pilot field of 13 time-lapse surveys, which solved a significant repeatability problem across a 17 month survey gap.

scholarly journals Microearthquake Source Locations and Mechanisms: Ice Stream B, West Antarctica (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 199
Author(s):  
S. Anandakrishnan ◽  
D. D. Blankenship ◽  
C. R. Bentley
Keyword(s):  
Depth Profile ◽  
Fault Plane ◽  
P Wave ◽  
West Antarctica ◽  
Precise Location ◽  
Fault Plane Solutions ◽  
Ice Flow ◽  
Ice Stream ◽  
Ice Stream B

An array of nine seismographic stations, each sensitive to all three components of motion, was deployed on Ice Stream B, West Antarctica, during the austral summer of 1985–86. The network was sensitive to high-frequency (=400 Hz) seismic activity within a 350 km2 area of the ice stream, and the deployment geometry allows the precise determination of depths for events beneath the 10 km2 array. Microearthquakes from both beneath and beside the ice stream were detected and recorded (Blankenship and others 1987). Inversion of P-wave and S-wave travel times and radiation patterns allows the determination of locations and fault-plane solutions for many of these events. We find that bottom events involve low-angle thrusting, in the down-stream direction, of ice or till; displacement is ∼½ cm per event over a (15 m)2 area. Such faulting is rare and releases an insignificant part of the total energy dissipated by ice flow. However, this is a possible mechanism for plucking of the ice-stream bed. Fault-plane solutions for most major surface events are consistent with the opening of tensional fractures oriented transverse to ice flow. Precise location of these events shows that they correspond to open crevasses, mapped by Vornberger and Whillans (1986), that are oriented transverse to ice flow. In addition, shear-wave splitting observed on some of the microearthquakes shows that the c-axes in the ice stream are slightly, but not strongly, anisotropic. Precise location of the sources requires the use of a detailed velocity-depth profile in the firn, which was obtained by seismic short-refraction studies (Anandakrishnan and others 1988, this volume). A density-depth profile calculated from these velocities agrees well with direct density measurements on a single core nearby (Alley and Bentley 1988, this volume).

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