A Broad-Band Study of Attenuation in Ocean Bottom Sediments

A standard ocean bottom seismometer (OBS) package of the U.S. OBS Instrument Pool (OBSIP) carries a seismometer and a pressure sensor. For broadband applications, the seismometer typically is a wide-band or broad-band three-components seismometer, and the pressure sensor is a differential pressure gauge (DPG). The purpose of the pressure sensor is manifold and includes the capture of pressure signals not picked up by a ground motion sensor (e.g. the passage of tsunami), but also for purposes of correcting the seismograms for unwanted signals generated in the water column (e.g. p-wave reverberations). Unfortunately, the instrument response of the widely used Cox-Webb DPG remains somewhat poorly known, and can vary by individual sensor, and even by deployment of the same sensor.Efforts have been under way to construct and test DPG responses in the laboratory. But the sensitivity and long&#8208;period response are difficult to calibrate as they &#160;vary with temperature and pressure, and perhaps by hardware of the same mechanical specifications. &#160;Here, we present a way to test the response for each individual sensor and deployment in situ in the ocean. This test requires a relatively minimal and inexpensive modification to the OBS instrument frame and a release mechanism that allows a drop of the DPG by 3 inches after the OBS package settled and the DPG equilibrated on the seafloor. The seismic signal generated by this drop is then analyzed in the laboratory upon retrieval of the data.&#160;The results compare favorably with calibrations estimated independently through post&#8208;deployment data analyses of other signals such as Earth tides and the signals from large teleseismic earthquakes. Our study demonstrates that observed response functions can deviate from the nominal response by a factor of two or greater with regards to both the sensitivity and the time constant. Given the fact that sensor calibrations of DPGs in the lab require very specific and stable environments and are time consuming, the use of in-situ DPG calibration frames pose a reliable and inexpensive alternative.&#160;

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Progressive change of microfabrics by compaction in superficial layers of ocean bottom sediments: examples from the Okinawa Trough

The Journal of the Geological Society of Japan ◽

10.5575/geosoc.114.97 ◽

2008 ◽

Vol 114 (3) ◽

pp. 97-108 ◽

Cited By ~ 2

Author(s):

Kawamura Kiichiro ◽

Kawamura Noriko

Keyword(s):

Bottom Sediments ◽

Okinawa Trough ◽

Ocean Bottom ◽

Progressive Change ◽

The Okinawa Trough

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Gas Hydrates in Ocean Bottom Sediments

AAPG Bulletin ◽

10.1306/03b5ac8c-16d1-11d7-8645000102c1865d ◽

1982 ◽

Vol 66 ◽

Author(s):

M. K. MacLeod (2)

Keyword(s):

Bottom Sediments ◽

Gas Hydrates ◽

Ocean Bottom

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Along-strike variation in slab geometry at the southern Mariana subduction zone revealed by seismicity through ocean bottom seismic experiments

Geophysical Journal International ◽

10.1093/gji/ggz272 ◽

2019 ◽

Vol 218 (3) ◽

pp. 2122-2135 ◽

Cited By ~ 5

Author(s):

Gaohua Zhu ◽

Hongfeng Yang ◽

Jian Lin ◽

Zhiyuan Zhou ◽

Min Xu ◽

...

Keyword(s):

High Resolution ◽

Subduction Zone ◽

Matched Filter ◽

Near Field ◽

Broad Band ◽

Velocity Model ◽

Ocean Bottom ◽

Slab Geometry ◽

Seismicity Distribution ◽

Geodynamic Processes

SUMMARYWe have conducted the first passive Ocean Bottom Seismograph (OBS) experiment near the Challenger Deep at the southernmost Mariana subduction zone by deploying and recovering an array of 6 broad-band OBSs during December 2016–June 2017. The obtained passive-source seismic records provide the first-ever near-field seismic observations in the southernmost Mariana subduction zone. We first correct clock errors of the OBS recordings based on both teleseismic waveforms and ambient noise cross-correlation. We then perform matched filter earthquake detection using 53 template events in the catalogue of the US Geological Survey and find >7000 local earthquakes during the 6-month OBS deployment period. Results of the two independent approaches show that the maximum clock drifting was ∼2 s on one instrument (OBS PA01), while the rest of OBS waveforms had negligible time drifting. After timing correction, we locate the detected earthquakes using a newly refined local velocity model that was derived from a companion active source experiment in the same region. In total, 2004 earthquakes are located with relatively high resolution. Furthermore, we calibrate the magnitudes of the detected earthquakes by measuring the relative amplitudes to their nearest relocated templates on all OBSs and acquire a high-resolution local earthquake catalogue. The magnitudes of earthquakes in our new catalogue range from 1.1 to 5.6. The earthquakes span over the Southwest Mariana rift, the megathrust interface, forearc and outer-rise regions. While most earthquakes are shallow, depths of the slab earthquakes increase from ∼100 to ∼240 km from west to east towards Guam. We also delineate the subducting interface from seismicity distribution and find an increasing trend in dip angles from west to east. The observed along-strike variation in slab dip angles and its downdip extents provide new constraints on geodynamic processes of the southernmost Mariana subduction zone.

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