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

scholarly journals Microearthquake seismicity and focal mechanisms at the Rodriguez Triple Junction in the Indian Ocean using ocean bottom seismometers

2001 ◽  
Vol 106 (B12) ◽  
pp. 30689-30699 ◽  
Author(s):  
Kei Katsumata ◽  
Toshinori Sato ◽  
Junzo Kasahara ◽  
Naoshi Hirata ◽  
Ryota Hino ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Triple Junction ◽  
Focal Mechanisms ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometers ◽  
The Indian Ocean

Travel-time residuals of teleseismic P-waves at the Rodriguez Triple Junction in the Indian Ocean using ocean-bottom seismometers

1996 ◽  
Vol 23 (7) ◽  
pp. 713-716 ◽  
Author(s):  
Toshinori Sato ◽  
Kei Katsumata ◽  
Junzo Kasahara ◽  
Naoshi Hirata ◽  
Ryota Hino ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Travel Time ◽  
Triple Junction ◽  
Ocean Bottom ◽  
P Waves ◽  
Ocean Bottom Seismometers ◽  
The Indian Ocean

Ceramic Pressure-Tight Housings for Ocean-Bottom Seismometers Applicable to 11-km Water Depth

2012 ◽  
Vol 37 (4) ◽  
pp. 756-763 ◽  
Author(s):  
K. Asakawa ◽  
T. Hyakudome ◽  
M. Yoshida ◽  
N. Okubo ◽  
M. Ito ◽  
...  
Keyword(s):  
Water Depth ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometers

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.

scholarly journals Basin inversion: Reactivated rift structures in the Ligurian Sea revealed by OBS

2021 ◽  
Author(s):  
Martin Thorwart ◽  
Anke Dannowski ◽  
Ingo Grevemeyer ◽  
Dietrich Lange ◽  
Heidrun Kopp ◽  
...  
Keyword(s):  
Focal Mechanisms ◽  
Ocean Bottom ◽  
Uppermost Mantle ◽  
Ligurian Sea ◽  
Northern Margin ◽  
Basin Inversion ◽  
Plate Convergence ◽  
Ocean Bottom Seismometers ◽  
Analyse Data ◽  
Ligurian Basin

Abstract. The northern margin of the Ligurian Basin shows notable seismicity at the Alpine front, including frequent magnitude 4 events. Seismicity decreases offshore towards the Basin centre and Corsica, revealing a diffuse distribution of low magnitude earthquakes. We analyse data of the amphibious AlpArray seismic network with focus on the offshore component, the AlpArray OBS network, consisting of 24 broadband ocean bottom seismometers deployed for eight months, to reveal the seismicity and depth distribution of micro-earthquakes beneath the Ligurian Sea. Two clusters occurred between ~10 km to ~16 km depth below sea surface, within the lower crust and uppermost mantle. Thrust faulting focal mechanisms indicate compression and an inversion of the Ligurian Basin, which is an abandoned Oligocene rift basin. The Basin inversion is suggested to be related to the Africa-Europe plate convergence. The locations and focal mechanisms of seismicity suggest reactivation of pre-existing rift structures. Slightly different striking directions of faults in the basin centre compared to faults further east and hence away from the abandoned rift may mimic the counter-clockwise rotation of the Corsica-Sardinia block during ~20–16 Ma. The observed cluster events support the hypothesis of strengthening of crust and uppermost mantle during rifting related extension and thinning of continental crust.

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.

A study of microearthquake seismicity and focal mechanisms within the Sea of Marmara (NW Turkey) using ocean bottom seismometers (OBSs)

2004 ◽  
Vol 391 (1-4) ◽  
pp. 303-314 ◽  
Author(s):  
Toshinori Sato ◽  
Junzo Kasahara ◽  
Tuncay Taymaz ◽  
Masakazu Ito ◽  
Aya Kamimura ◽  
...  
Keyword(s):  
Focal Mechanisms ◽  
Ocean Bottom ◽  
Sea Of Marmara ◽  
Nw Turkey ◽  
Ocean Bottom Seismometers

Resistance Tests of an Articulated Pusher Barge System in Deep and Shallow Water

2021 ◽  
Author(s):  
Li Zhang ◽  
Lei Xing ◽  
Mingyu Dong ◽  
Weimin Chen
Keyword(s):  
Shallow Water ◽  
Water Depth ◽  
Deep Water ◽  
Water Resistance ◽  
Model Tests ◽  
System 2 ◽  
Changing Trend ◽  
The Difference ◽  
Transport Vessel ◽  
Resistance Performance

Abstract Articulated pusher barge vessel is a short-distance transport vessel with good economic performance and practicability, which is widely used in the Yangtze River of China. In this present work, the resistance performance of articulated pusher barge vessel in deep water and shallow water was studied by model tests in the towing tank and basin of Shanghai Ship and Shipping Research Institute. During the experimental investigation, the articulated pusher barge vessel was divided into three parts: the pusher, the barge and the articulated pusher barge system. Firstly, the deep water resistance performance of the articulated pusher barge system, barge and the pusher at design draught T was studied, then the water depth h was adjusted, and the shallow water resistance at h/T = 2.0, 1.5 and 1.2 was tested and studied respectively, and the difference between deep water resistance and shallow water resistance at design draught were compared. The results of model tests and analysis show that: 1) in the study of deep water resistance, the total resistance of the barge was larger than that of the articulated pusher barge system. 2) for the barge, the shallow water resistance increases about 0.4–0.7 times at h/T = 2.0, 0.5–1.1 times at h/T = 1.5, and 0.7–2.3 times at h/T = 1.2. 3) for the pusher, the shallow water resistance increases about 1.0–0.4 times at h/T = 2.7, 1.2–0.9 times at h/T = 2.0, and 1.7–2.4 times at h/T = 1.6. 4) for the articulated pusher barge system, the shallow water resistance increases about 0.2–0.3 times at h/T = 2.0, 0.5–1.3 times at h/T = 1.5, and 1.0–3.5 times at h/T = 1.2. Furthermore, the water depth Froude number Frh in shallow water was compared with the changing trend of resistance in shallow water.

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