scholarly journals Focal Mechanisms and the Stress Field in the Aftershock Area of the 2018 Hokkaido Eastern Iburi Earthquake (MJMA = 6.7)

2020 ◽  
Author(s):  
Yuki Susukida ◽  
Kei Katsumata ◽  
Masayoshi Ichiyanagi ◽  
Mako Ohzono ◽  
Hiroshi Aoyama ◽  
...  
Keyword(s):  
Stress Field ◽  
Principal Stress ◽  
Focal Mechanism ◽  
P Wave ◽  
Japan Meteorological Agency ◽  
Inversion Method ◽  
Reverse Fault ◽  
Focal Mechanism Solutions ◽  
Fault Type ◽  
Aftershock Area

Abstract The tectonic stress field was investigated in and around the aftershock area of the Hokkaido Eastern Iburi earthquake (MJMA = 6.7) occurred on 6 September 2018. We deployed 26 temporary seismic stations in the aftershock area for approximately 2 months and located 1785 aftershocks precisely. Among these aftershocks 818 focal mechanism solutions were determined using the first motion polarity of P wave from the temporary observation and the permanent seismic networks of Hokkaido University, Japan Meteorological Agency (JMA), and High Sensitivity Seismograph Network Japan (Hi-net). We found that (1) the reverse faulting and the strike-slip faulting are dominant in the aftershock area, (2) the average azimuths of P- and T-axes are N78° ± 33°E and N3° ± 52°W, respectively, and (3) the average dips of P- and T-axes are 25° ± 16° and 46° ± 20°, respectively: the P-axis is close to be horizontal and the T-axis is close to be vertical. We applied a stress inversion method to the focal mechanism solutions to estimate a stress field in the aftershock area. As a result, we found that the reverse fault type stress field is dominant in the aftershock area. An axis of the maximum principal stress (σ1) has the azimuth of N73° ± 8°E and the dipping eastward of 17° ± 6° and an axis of the medium principal stress (σ2) has the azimuth of N126° ± 91°E and the dipping southward of 16° ± 13°, indicating that both of σ1- and σ2-axes are close to be horizontal. An axis of the minimum principal stress (σ3) has the dipping westward of 64° ± 9° that is close to be vertical. The results strongly suggest that the reverse-fault-type stress field is predominant as an average over the aftershock area which is in the western boundary of the Hidaka Collision Zone. Although the average of the stress ratio is R = 0.6 ± 0.2 in the whole aftershock area, R decreases systematically as the depth is getting deep, which is modeled by a quadratic polynomial of depth.

scholarly journals Focal mechanisms and the stress field in the aftershock area of the 2018 Hokkaido Eastern Iburi earthquake (MJMA = 6.7)

2021 ◽  
Vol 73 (1) ◽  
Author(s):  
Yuki Susukida ◽  
◽  
Kei Katsumata ◽  
Masayoshi Ichiyanagi ◽  
Mako Ohzono ◽  
...  
Keyword(s):  
Stress Field ◽  
Principal Stress ◽  
Focal Mechanism ◽  
P Wave ◽  
Japan Meteorological Agency ◽  
Inversion Method ◽  
Reverse Fault ◽  
Focal Mechanism Solutions ◽  
Fault Type ◽  
Aftershock Area

AbstractThe tectonic stress field was investigated in and around the aftershock area of the Hokkaido Eastern Iburi earthquake (MJMA = 6.7) occurred on 6 September 2018. We deployed 26 temporary seismic stations in the aftershock area for approximately 2 months and located 1785 aftershocks precisely. Among these aftershocks, 894 focal mechanism solutions were determined using the first-motion polarity of P wave from the temporary observation and the permanent seismic networks of Hokkaido University, Japan Meteorological Agency (JMA), and High Sensitivity Seismograph Network Japan (Hi-net). We found that (1) the reverse faulting and the strike-slip faulting are dominant in the aftershock area, (2) the average trend of P- and T-axes is 78° ± 33° and 352° ± 51°, respectively, and (3) the average plunge of P- and T-axes is 25° ± 16° and 44° ± 20°, respectively: the P-axis is close to be horizontal and the T-axis is more vertical than the average of the P-axes. We applied a stress inversion method to the focal mechanism solutions to estimate a stress field in the aftershock area. As a result, we found that the reverse fault type stress field is dominant in the aftershock area. An axis of the maximum principal stress (σ1) has the trend of 72° ± 7° and the dipping eastward of 19° ± 4° and an axis of the intermediate principal stress (σ2) has the trend of 131° ± 73° and the dipping southward of 10° ± 9°, indicating that both of σ1- and σ2-axes are close to be horizontal. An axis of the minimum principal stress (σ3) has the dipping westward of 67° ± 6° that is close to be vertical. The results strongly suggest that the reverse-fault-type stress field is predominant as an average over the aftershock area which is in the western boundary of the Hidaka Collision Zone. The average of the stress ratio R = (σ1 − σ2)/(σ1 − σ3) is 0.61 ± 0.13 in the whole aftershock area. Although not statistically significant, we suggest that R decreases systematically as the depth is getting deep, which is modeled by a quadratic polynomial of depth.

Focal mechanism and stress field in the northeastern Tibetan Plateau: insight into layered crustal deformations

2019 ◽  
Vol 218 (3) ◽  
pp. 2066-2078 ◽  
Author(s):  
Cunrui Han ◽  
Zhouchuan Huang ◽  
Mingjie Xu ◽  
Liangshu Wang ◽  
Ning Mi ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Stress Field ◽  
Focal Mechanism ◽  
Horizontal Stress ◽  
P Wave ◽  
The Tibetan Plateau ◽  
Linear Inversion ◽  
Focal Mechanism Solutions ◽  
Stress Orientations ◽  
Ne Tibetan Plateau

SUMMARY Focal mechanism solutions (FMSs) reflect the stress field underground directly. They provide essential clue for crustal deformations and therefore improve our understanding of tectonic uplift and expansion of the Tibetan Plateau. In this study, we applied generalized Cut and Paste and P-wave first-motion methods to determine 334 FMSs (2.0 ≤ Mw ≤ 6.4) with the data recorded by a new temporary network deployed in the NE Tibetan Plateau by ChinArray project. We then used 1015 FMSs (including 681 published FMSs) to calculate the regional stress field with a damped linear inversion. The results suggest dominant thrust and strike-slip faulting environments in the NE Tibetan Plateau. From the Qilian thrust belt to the Qinling orogen, the maximum horizontal stress orientations (${S_\mathrm{ H}}$) rotate clockwise from NNE to NE, and further to EW, showing a fan-shaped pattern. The derived minimum horizontal stress orientations (${S_\mathrm{ h}}$) are parallel to the aligned fabrics in the mantle lithosphere indicated by shear wave splitting measurements, suggesting vertically coherent deformation in the NE Tibetan Plateau. Beneath the SW Qinling adjacent to the plateau, however, the stress orientations in the shallow and deep crust are different, whereas the deep crustal stress field indicates possible ductile crustal flow or shear.

Direct stress field estimation through waveform matching

2020 ◽  
Vol 221 (2) ◽  
pp. 843-856
Author(s):  
Wenhuan Kuang ◽  
Jie Zhang
Keyword(s):  
Stress Field ◽  
Focal Mechanism ◽  
Stress Pattern ◽  
P Wave ◽  
Local Source ◽  
Stress Inversion ◽  
Inversion Process ◽  
Physical Relationship ◽  
New Approach ◽  
Field Estimation

SUMMARY Conventionally, the routine workflow of stress field estimation from seismic data consists of two steps: focal mechanism inversion and stress inversion. This two-step workflow suffers from the cumulative uncertainties of both the focal mechanism inversion process and the stress inversion process. To mitigate the cumulative errors, a few previous studies have put efforts to directly estimate the stress field using P-wave polarities. In this study, we develop a new approach to directly estimate tectonic stress fields with better accuracy through waveform matching. This new approach combines the two steps into a one-step workflow to mitigate the cumulative uncertainties through the physical relationship between a stress field and the recorded waveforms. This method assumes a homogeneous stress field in space in the local source region and that the fault slip occurs in the direction of the resolved shear stress acting on the fault plane. Under these assumptions, the stress pattern that generates the theoretical waveforms that best fit the waveforms observed is directly retrieved as the true stress field. The merits of the new approach include that this approach can mitigate the cumulative uncertainties suffered from the conventional two-step workflow and does not require determination of the focal mechanisms of each event; thus, this method is applicable to data sets with few stations. Synthetic tests with and without noise are conducted to demonstrate the performance and merits of this method. Then, the new approach is applied to a real data set from central Oklahoma between March 2013 and March 2016. The resulting stress pattern is consistent with that estimated from previous studies examining the same region. These applications show the benefits and validity of the new approach.

Characterising induced acoustic emission activity observed during a mine-scale hydraulic-fracturing experiment in anisotropic crystalline rock

2020 ◽  
Author(s):  
Carolin Boese ◽  
Grzegorz Kwiatek ◽  
Georg Dresen ◽  
Joerg Renner ◽  
Thomas Fischer ◽  
...  
Keyword(s):  
Acoustic Emission ◽  
Focal Mechanism ◽  
Crystalline Rock ◽  
P Wave ◽  
Principal Stresses ◽  
Injection Point ◽  
Hydraulic Stimulation ◽  
Focal Mechanism Solutions ◽  
In Situ Conditions ◽  
Vertical Borehole

<p><span>Between early 2018 and late 2019 the STIMTEC hydraulic stimulation experiment was performed at ca.~130 m below surface at the Reiche Zeche research mine in Freiberg, Saxony/Germany. The project aims at gaining insight into the creation and growth of fractures in anisotropic and heterogeneous crystalline rock units, to develop and optimise hydraulic stimulation techniques </span><span>for EGS applications</span><span> and to control the associated induced seismicity under in situ conditions. A series of ten hydro-frac experiments w</span><span>ere</span><span> performed in a 63 m-long, 15°-inclined injection borehole and five mini-fracs for stress measurements in a sub-vertical borehole. </span><span>These were monitored using a </span><span>seismic monitoring system of twelve high-sensitivity Acoustic emission </span><span>(AE) </span><span>sensors, three accelerometers and one broadband sensor.</span> <span>More than 11,000 high-frequency AE events with source sizes on the cm-to-dm scale accompanied the hydraulic stimulation in five of ten stimulat</span><span>ed</span><span> intervals in the injection borehole. Several hundred AE events were recorded during the mini-fracs in the vertical borehole. We investigate the characteristics of induced AE events by combining information obtained from high-accuracy</span> <span>event locations using a transversely isotropic P-wave velocity model per station with station corrections, relative hypocentre locations, and focal mechanism solutions of selected events. The </span><span>AE </span><span>event clouds extend ca. 5 m radially from the injection point</span><span>s and show</span> <span>vari</span><span>ying </span><span>orientations and dips. The </span><span>ca. </span><span>150 focal mechanism s</span><span>olutions</span><span> obtained using P-wave polarisation</span><span>s</span><span> display mixed-mode failure with a significant portion of them showing compaction. </span><span>The orientation </span><span>of the </span><span>maximum principal stress inferred from the hydro-fracs in the injection and vertical boreholes </span><span>has a trend </span><span>of </span><span>N</span><span>348°</span><span>E</span><span> and </span><span>a </span><span>plunge </span><span>of</span><span> 20°, as typical for southeast Germany. However, discrepancies in the magnitudes of the principal stresses were measured between these boreholes ca. 15 m apart, resulting in different faulting regimes. We present stress orientations obtained from inverting focal mechanism solutions to provide additional information for interpreting stress-characterisation measurements.</span></p>

scholarly journals Precise aftershock distribution of the 2019 Yamagata-oki earthquake using newly developed simple anchored-buoy ocean bottom seismometers and land seismic stations

2022 ◽  
Vol 74 (1) ◽  
Author(s):  
Masanao Shinohara ◽  
Shin’ichi Sakai ◽  
Tomomi Okada ◽  
Hiroshi Sato ◽  
Yusuke Yamashita ◽  
...  
Keyword(s):  
Shallow Water ◽  
Travel Time ◽  
Focal Mechanism ◽  
Water Depth ◽  
Source Area ◽  
Focal Mechanisms ◽  
Reverse Fault ◽  
Ocean Bottom ◽  
Fault Type ◽  
Ocean Bottom Seismometers

AbstractAn earthquake with a magnitude of 6.7 occurred in the Japan Sea off Yamagata on June 18, 2019. The mainshock had a source mechanism of reverse-fault type with a compression axis of WNW–ESE direction. Since the source area is positioned in a marine area, seafloor seismic observation is indispensable for obtaining the precise distribution of the aftershocks. The source area has a water depth of less than 100 m, and fishing activity is high. It is difficult to perform aftershock observation using ordinary free-fall pop-up type ocean bottom seismometers (OBSs). We developed a simple anchored-buoy type OBS for shallow water depths and performed the seafloor observation using this. The seafloor seismic unit had three-component seismometers and a hydrophone. Two orthogonal tiltmeters and an azimuth meter monitored the attitude of the package. For seismic observation at shallow water depth, we concluded that an anchored-buoy system would have the advantage of avoiding accidents. Our anchored-buoy OBS was based on a system used in fisheries. We deployed three anchored-buoy OBSs in the source region where the water depth was approximately 80 m on July 5, 2019, and two of the OBSs were recovered on July 13, 2019. Temporary land seismic stations with a three-component seismometer were also installed. The arrival times of P- and S-waves were read from the records of the OBSs and land stations, and we located hypocenters with correction for travel time. A preliminary location was performed using absolute travel time and final hypocenters were obtained using the double-difference method. The aftershocks were distributed at a depth range of 2.5 km to 10 km and along a plane dipping to the southeast. The plane formed by the aftershocks is consistent with the focal mechanism of the mainshock. The activity region of the aftershocks was positioned in the upper part of the upper crust. Focal mechanisms were estimated using the polarity of the first arrivals. Although many aftershocks had a reverse-fault focal mechanism similar to the focal solution of the mainshock, normal-fault type and strike–slip fault type focal mechanisms were also estimated. Graphical Abstract

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.

scholarly journals Inversion Method of Initial In Situ Stress Field Based on BP Neural Network and Applying Loads to Unit Body

2020 ◽  
Vol 2020 ◽  
pp. 1-15
Author(s):  
Xiaopeng Li ◽  
Xuejun Zhou ◽  
Zhengxuan Xu ◽  
Tao Feng ◽  
Dong Wang ◽  
...  
Keyword(s):  
Neural Network ◽  
Stress Field ◽  
Principal Stress ◽  
Bp Neural Network ◽  
In Situ Stress ◽  
Inversion Method ◽  
Linear Regression Method ◽  
Underground Engineering ◽  
In Situ Stress Field

The initial in situ stress field is the fundamental factor causing the deformation and failure of underground engineering and is an important basis for the feasibility analysis, design, and construction of underground engineering. However, it is difficult to obtain the whole in situ stress field of large-scale underground engineering in difficult and dangerous areas by field measurement. In view of the fact that the measured in situ stress components (σxx, σyy, σzz, τxy, τxz, τyz) of Sichuan-Tibet Railway in China are linear with the buried depth, a method is proposed to solve the in situ stress by applying corresponding loads to all unit bodies in the calculation area based on BP neural network and FLAC3D. Through this method, the in situ stress of the tunnel is inverted. The results show that both the maximum principal stress and minimum principal stress increase with the increase of buried depth, and when the tunnel passes through faults or anticlines, the main stress will suddenly drop. Furthermore, compared with the results of the multiple linear regression method, it is found that the proposed method has higher accuracy; especially for the simulation of the maximum horizontal principal stress and vertical stress, the average relative error is reduced by 26.44% and 77.27%, respectively. The research in this paper can provide a new idea for the initial in situ stress inversion of engineering.

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