Probabilistic Focal Mechanism Estimation Based on Body‐Wave Waveforms through Source‐Scanning Algorithm

2018 ◽  
Vol 108 (4) ◽  
pp. 1962-1971
Author(s):  
Frédérick Massin ◽  
Alison Malcolm
Keyword(s):  
Focal Mechanism ◽  
Body Wave ◽  
Focal Mechanism Estimation ◽  
Scanning Algorithm

scholarly journals Focal mechanism and depth of the 1956 Amorgos twin earthquakes from waveform matching of analogue seismograms

2013 ◽  
Vol 5 (2) ◽  
pp. 1901-1940
Author(s):  
A. Brüstle ◽  
W. Friederich ◽  
T. Meier ◽  
C. Gross
Keyword(s):  
Focal Mechanism ◽  
Transfer Functions ◽  
Body Wave ◽  
Normal Fault ◽  
Benioff Zone ◽  
Grid Search ◽  
African Plate ◽  
Technical Problems ◽  
Time Frequency ◽  
Best Fitting

Abstract. Historic analogue seismograms of the large 1956 Amorgos twin earthquakes which occurred in the volcanic arc of the Hellenic Subduction Zone (HSZ) were collected, digitized and reanalyzed to obtain refined estimates of their depth and focal mechanism. In total, 80 records of the events from 29 European stations were collected and, if possible, digitized. In addition, bulletins were searched for instrument parameters required to calculate transfer functions for instrument correction. A grid search based on matching the digitized historic waveforms to complete synthetic seismograms was then carried out to infer optimal estimates for depth and focal mechanism. Owing to incomplete or unreliable information on instrument parameters and frequently occurring technical problems during recording such as writing needles jumping off mechanical recording systems, much less seismograms than collected proved suitable for waveform matching. For the first earthquake, only 7 seismograms from three different stations (STU, GTT, COP) could be used. Nevertheless, the grid search produces stable optimal values for both source depth and focal mechanism. Our results indicate a shallow hypocenter at about 25 km depth. The best-fitting focal mechanism is a SW–NE-trending normal fault dipping either by 30° towards SE or 60° towards NW. This finding is consistent with the local structure of the Santorini–Amorgos graben. For the second earthquake, 4 seismograms from three different stations (JEN, GTT, COP) proved suitable for waveform matching. Whereas it was impossible to obtain meaningful results for the focal mechanism owing to surface wave coda of the first event overlapping body wave phases of the second event, waveform matching and time-frequency analysis point to a considerably deeper hypocenter located within the Wadati–Benioff-zone of the subducting African plate at about 120–160 km depth.

scholarly journals Focal mechanism and depth of the 1956 Amorgos twin earthquakes from waveform matching of analogue seismograms

Solid Earth ◽  
2014 ◽  
Vol 5 (2) ◽  
pp. 1027-1044 ◽  
Author(s):  
A. Brüstle ◽  
W. Friederich ◽  
T. Meier ◽  
C. Gross
Keyword(s):  
Surface Wave ◽  
Focal Mechanism ◽  
Wave Amplitude ◽  
Body Wave ◽  
The Body ◽  
Grid Search ◽  
Source Depth ◽  
Normal Faulting ◽  
Intermediate Depth ◽  
First Motion

Abstract. Historic analogue seismograms of the large 1956 Amorgos twin earthquakes which occurred in the volcanic arc of the Hellenic subduction zone (HSZ) were collected, digitized and reanalyzed to obtain refined estimates of their depth and focal mechanism. In total, 80 records of the events from 29 European stations were collected and, if possible, digitized. In addition, bulletins were searched for instrument parameters required to calculate transfer functions for instrument correction. A grid search based on matching the digitized historic waveforms to complete synthetic seismograms was then carried out to infer optimal estimates for depth and focal mechanism. Owing to incomplete or unreliable information on instrument parameters and frequently occurring technical problems during recording, such as writing needles jumping off mechanical recording systems, much less seismograms than collected proved suitable for waveform matching. For the first earthquake, only seven seismograms from three different stations at Stuttgart (STU), Göttingen (GTT) and Copenhagen (COP) could be used. Nevertheless, the waveform matching grid search yields two stable misfit minima for source depths of 25 and 50 km. Compatible fault plane solutions are either of normal faulting or thrusting type. A separate analysis of 42 impulsive first-motion polarities taken from the International Seismological Summary (ISS bulletin) excludes the thrusting mechanism and clearly favors a normal faulting solution with at least one of the potential fault planes striking in SW–NE direction. This finding is consistent with the local structure and microseismic activity of the Santorini–Amorgos graben. Since crustal thickness in the Amorgos area is generally less than 30 km, a source depth of 25 km appears to be more realistic. The second earthquake exhibits a conspicuously high ratio of body wave to surface wave amplitudes suggesting an intermediate-depth event located in the Hellenic Wadati–Benioff zone. This hypothesis is supported by a focal mechanism analysis based on first-motion polarities, which indicates a mechanism very different from that of the first event. A waveform matching grid search done to support the intermediate-depth hypothesis proved not to be fruitful because the body wave phases are overlain by strong surface wave coda of the first event inhibiting a waveform match. However, body to surface wave amplitude ratios of a modern intermediate-depth event with an epicenter close to the island of Milos observed at stations of the German Regional Seismic Network (GRSN) exhibit a pattern similar to the one observed for the second event with high values in a frequency band between 0.05 Hz and 0.3 Hz. In contrast, a shallow event with an epicenter in western Crete and nearly identical source mechanism and magnitude, shows very low ratios of body and surface wave amplitude up to 0.17 Hz and higher ratios only beyond that frequency. Based on this comparison with a modern event, we estimate the source depth of the second event to be greater than 100 km. The proximity in time and space of the two events suggests a triggering of the second, potentially deep event by the shallow first one.

A seismic discriminant based on focal mechanism

1971 ◽  
Vol 61 (6) ◽  
pp. 1827-1830
Author(s):  
Atiq A. Syed ◽  
Carl Kisslinger ◽  
Otto W. Nuttli
Keyword(s):  
Focal Mechanism ◽  
Plate Tectonics ◽  
Body Wave ◽  
Body Wave Magnitude

abstract Utilizing the observation that a predominant focal mechanism exists for a given hypocentral region, a seismic discriminant based on body-wave magnitude has been developed. This discriminant enables one to identify earthquakes that do not fit mechanisms expected from plate tectonics. It also sorts out explosions as anomalies, even for those regions in which the focal mechanism results in compressional first motions at most or all available seismograph stations.

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.

Joint inversion of source location and focal mechanism of microseismicity

Geophysics ◽  
2016 ◽  
Vol 81 (2) ◽  
pp. KS41-KS49 ◽  
Author(s):  
Chuntao Liang ◽  
Yangyang Yu ◽  
Yihai Yang ◽  
Liang Kang ◽  
Chen Yin ◽  
...  
Keyword(s):  
Hydraulic Fracturing ◽  
Focal Mechanism ◽  
Velocity Structure ◽  
Joint Inversion ◽  
Optimal Solution ◽  
Effective Property ◽  
Stress And Strain ◽  
Data Set ◽  
As Stress ◽  
Scanning Algorithm

The seismic focal mechanism (FM) is an effective property to indicate source physics, as well as stress and strain distribution in regional, local, and microscales. We have developed an algorithm to jointly invert for the FM and source locations. For a given velocity structure, all possible combinations of source locations ([Formula: see text], [Formula: see text], and [Formula: see text]) and FM (strike, dip, and rake) were used to compute traveltimes and polarities of waveforms. Correcting normal moveout times and polarities and stacking all waveforms, the ([Formula: see text], [Formula: see text], [Formula: see text], strike, dip, and rake) combination that gave the strongest stacking power was identified as the optimal solution. Compared with the traditional source scanning algorithm (SSA) that only scanned source locations, this algorithm was thereby called the joint source scanning algorithm (jSSA). The jSSA method was tested rigorously, and it was applied to a hydraulic fracturing data set. Our work determined several advantages against the SSA method: (1) The jSSA method could identify many shear sources that could not be detected by the SSA method due to polarity variation; (2) the jSSA almost always yielded more events than the SSA method, and the added events could often provide much better characterization of the hydraulic fracturing; (3) the statistics of source mechanisms could provide additional knowledge on the orientation of fractures, as well as the local and regional stress and strain field; and (4) for those events that were detected by both methods, the stacking power of jSSA was always higher than that obtained in SSA.

Waveform Characteristics and Focal Mechanism Solutions of Five Aftershocks of the 1983 Goodnow, New York, Earthquake by Polarization Analysis and Waveform Modeling

1991 ◽  
Vol 62 (2) ◽  
pp. 123-133 ◽  
Author(s):  
Zuyuan Liu ◽  
Robert. B. Herrmarnn ◽  
Jiakang Xie ◽  
E. Cranswick
Keyword(s):  
New York ◽  
Focal Mechanism ◽  
Fault Plane ◽  
Body Wave ◽  
Moment Tensor ◽  
Moment Tensor Inversion ◽  
Systematic Search ◽  
P Wave ◽  
S Wave ◽  
Focal Mechanism Solutions

Abstract Waveforms of the direct P-, SV- and SH-waves of five 1983 Goodnow, New York, aftershocks (mb = 1.4–3.1), locally recorded at four hard-rock sites (epicentral distances=1.9–8.0 km) with GEOS systems, were studied to obtain their focal mechanism solutions by waveform fit using both systematic search and moment tensor inversion. Both synthetic and observed data were low-pass filtered at 10 Hz to reduce sensitivity to shallow earth structure. It was discovered that only the first cycle of P-wave and S-wave appear to have pure direct body wave characteristics. The strong P- and S-coda have no stable polarization. The five aftershocks have similar locations, identical P-first motions, but varying direct S-waveforms. A layered velocity model with a P-wave velocity of 4.4 km/s in the surface layer was derived. Fault plane solutions of four events indicate reverse faulting mechanisms that have a near horizontal P-axis with a strike of ENE. This is similar to the fault plane solution of the mainshock (October 7, 1983, mb = 5.1) and the composite focal mechanism of the aftershocks. Four aftershocks occurred on the fault planes with the strike NW-N and dip of 52°–64° toward NE-E. The fifth event studied has significant strike-slip motion with the P axis is also nearly horizontal and oriented NE. The results of systematic search technique agree well with those of moment tensor inversion. The first motion directions, pulse widths, amplitudes, amplitude ratios and arrival times of the direct P-, SV- and SH-phases of the synthetic seismograms are consistent with those of the observed seismograms. The results of the research demonstrated that the S-wave amplitude can provide important constraints on the focal mechanism.

A method of correcting P-wave magnitudes for the effect of earthquake focal mechanism

1971 ◽  
Vol 61 (6) ◽  
pp. 1811-1826 ◽  
Author(s):  
Atiq A. Syed ◽  
Otto W. Nuttli
Keyword(s):  
Atlantic Ocean ◽  
Focal Mechanism ◽  
Present Method ◽  
Body Wave ◽  
Geographic Region ◽  
P Wave ◽  
Aleutian Island ◽  
Transform Fault ◽  
Routine Manner ◽  
Earthquake Focal Mechanism

abstract This paper presents a methodology for correcting body-wave magnitudes for the effect of focal mechanism in a routine manner. The method requires a knowledge of the prevailing or dominant mechanism for a geographic region, from which tables are constructed which enable one to make the necessary correction. Included in the paper are tables for Aleutian Island, Kamchatka and mid-Atlantic Ocean earthquakes. From a study of seven earthquakes, it is concluded that the present method gives essentially the same average magnitude with the same standard deviation as a more exact method of correcting for the focal mechanism. The latter method uses the focal-mechanism parameters of the earthquake, which must be determined independently for each earthquake. The existing distribution of seismograph stations is such that transform-fault earthquakes of the mid-Atlantic Ocean will consistently have their P-wave magnitudes underestimated by about 0.2 magnitude units, if no correction is made for the focal mechanism. On the other hand, P-wave magnitudes of earthquakes in Kamchatka and south of the axis of the Aleutian Trench will be overestimated by about 0.2 units.

scholarly journals Directional Statistics, Bayesian Methods of Earthquake Focal Mechanism Estimation, and Their Application to New Zealand Seismicity Data

2021 ◽  
Author(s):  
◽  
David Walsh
Keyword(s):  
New Zealand ◽  
Focal Mechanism ◽  
Seismic Velocity ◽  
Region Of Interest ◽  
Velocity Model ◽  
Tectonic Stress ◽  
Focal Mechanism Solutions ◽  
Earthquake Focal Mechanism ◽  
Leibler Divergence ◽  
Focal Mechanism Estimation

<p>A focal mechanism is a geometrical representation of fault slip during an earthquake. Reliable earthquake focal mechanism solutions are used to assess the tectonic characteristics of a region, and are required as inputs to the problem of estimating tectonic stress. We develop a new probabilistic (Bayesian) method for estimating the distribution of focal mechanism parameters based on seismic wave polarity data. Our approach has the advantage of enabling us to incorporate observational errors, particularly those arising from imperfectly known earthquake locations, allowing exploration of the entire parameter space, and leads to natural point estimates of focal mechanism parameters. We investigate the use of generalised Matrix Fisher distributions for parameterising focal mechanism uncertainties by minimising the Kullback-Leibler divergence. We present here the results of our method in two situations. We first consider the case in which the seismic velocity of the region of interest (described by a velocity model) is presumed to be precisely known, with application to seismic data from the Raukumara Peninsula, New Zealand. We then consider the case in which the velocity model is imperfectly known, with application to data from the Kawerau region, New Zealand. We find that our estimated focal mechanism solutions for the most part are consistent with all available polarity data, and correspond closely to solutions obtained using established methods. Further, the generalised Matrix Fisher distributions we examine provide a good fit to our Bayesian posterior PDF of the focal mechanism parameters, enabling the posterior PDF to be succinctly summarised by reporting the estimated parameters of the fitted distribution.</p>

Sign in / Sign up

Export Citation Format

Share Document