Shallow water infrasonics: Propagation and noise studies with 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

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

Journal of Geophysical Research Atmospheres ◽

10.1029/2000jb000106 ◽

2001 ◽

Vol 106 (B12) ◽

pp. 30689-30699 ◽

Cited By ~ 8

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

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The Seismotectonics and Seismicity of the Laptev Sea Region: The Current Situation and a First Experience in a Year-Long Installation of Ocean Bottom Seismometers on the Shelf

Journal of Volcanology and Seismology ◽

10.1134/s0742046320060044 ◽

2020 ◽

Vol 14 (6) ◽

pp. 379-393

Author(s):

A. A. Krylov ◽

A. I. Ivashchenko ◽

S. A. Kovachev ◽

N. V. Tsukanov ◽

M. E. Kulikov ◽

...

Keyword(s):

Current Situation ◽

Laptev Sea ◽

Ocean Bottom ◽

Ocean Bottom Seismometers ◽

The Laptev Sea

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Comparison of the S/N ratios of low-frequency hydrophones and geophones as a function of ocean depth

Bulletin of the Seismological Society of America ◽

10.1785/bssa0710051649 ◽

1981 ◽

Vol 71 (5) ◽

pp. 1649-1659

Author(s):

Thomas M. Brocher ◽

Brian T. Iwatake ◽

Joseph F. Gettrust ◽

George H. Sutton ◽

L. Neil Frazer

Keyword(s):

Nova Scotia ◽

Continental Margin ◽

Low Frequency ◽

Weather Conditions ◽

Ocean Bottom ◽

Ocean Bottom Seismometer ◽

Scotian Shelf ◽

Ocean Bottom Seismometers ◽

Particle Velocities ◽

Refraction Data

abstract The pressures and particle velocities of sediment-borne signals were recorded over a 9-day period by an array of telemetered ocean-bottom seismometers positioned on the continental margin off Nova Scotia. The telemetered ocean-bottom seismometer packages, which appear to have been very well coupled to the sediments, contained three orthogonal geophones and a hydrophone. The bandwidth of all sensors was 1 to 30 Hz. Analysis of the refraction data shows that the vertical geophones have the best S/N ratio for the sediment-borne signals at all recording depths (67, 140, and 1301 m) and nearly all ranges. The S/N ratio increases with increasing sensor depth for equivalent weather conditions. Stoneley and Love waves detected on the Scotian shelf (67-m depth) are efficient modes for the propagation of noise.

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The GITEWS ocean bottom sensor packages

Natural Hazards and Earth System Science ◽

10.5194/nhess-10-1759-2010 ◽

2010 ◽

Vol 10 (8) ◽

pp. 1759-1780

Author(s):

O. Boebel ◽

M. Busack ◽

E. R. Flueh ◽

V. Gouretski ◽

H. Rohr ◽

...

Keyword(s):

Warning System ◽

Deep Ocean ◽

Indian Ocean Tsunami ◽

False Alarms ◽

Ocean Bottom ◽

Bottom Pressure ◽

Tsunami Early Warning ◽

Ocean Bottom Seismometers ◽

First Results ◽

Sensor Package

Abstract. The German-Indonesian Tsunami Early Warning System (GITEWS) aims at reducing the risks posed by events such as the 26 December 2004 Indian Ocean tsunami. To minimize the lead time for tsunami alerts, to avoid false alarms, and to accurately predict tsunami wave heights, real-time observations of ocean bottom pressure from the deep ocean are required. As part of the GITEWS infrastructure, the parallel development of two ocean bottom sensor packages, PACT (Pressure based Acoustically Coupled Tsunameter) and OBU (Ocean Bottom Unit), was initiated. The sensor package requirements included bidirectional acoustic links between the bottom sensor packages and the hosting surface buoys, which are moored nearby. Furthermore, compatibility between these sensor systems and the overall GITEWS data-flow structure and command hierarchy was mandatory. While PACT aims at providing highly reliable, long term bottom pressure data only, OBU is based on ocean bottom seismometers to concurrently record sea-floor motion, necessitating highest data rates. This paper presents the technical design of PACT, OBU and the HydroAcoustic Modem (HAM.node) which is used by both systems, along with first results from instrument deployments off Indonesia.

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