scholarly journals Modelling high-frequency seismograms at ocean bottom seismometers: effects of heterogeneous structures on source parameter estimation for small offshore earthquakes and shallow low-frequency tremors

2020 ◽  
Author(s):  
Shunsuke Takemura ◽  
Suguru Yabe ◽  
Kentaro Emoto
Keyword(s):  
Parameter Estimation ◽  
High Frequency ◽  
Low Frequency ◽  
Source Parameter ◽  
Ocean Bottom ◽  
Heterogeneous Structures ◽  
Ocean Bottom Seismometers

scholarly journals Modelling high-frequency seismograms at ocean bottom seismometers: effects of heterogeneous structures on source parameter estimation for small offshore earthquakes and shallow low-frequency tremors

2020 ◽  
Vol 223 (3) ◽  
pp. 1708-1723
Author(s):  
Shunsuke Takemura ◽  
Suguru Yabe ◽  
Kentaro Emoto
Keyword(s):  
Wave Propagation ◽  
Parameter Estimation ◽  
Seismic Wave ◽  
High Frequency ◽  
Source Parameters ◽  
Low Frequency ◽  
Accretionary Prism ◽  
Seismic Wave Propagation ◽  
Source Parameter ◽  
Seismic Events

SUMMARY The source characteristics of offshore seismic events, especially regular (or fast) and slow earthquakes, can provide key information on their source physics and frictional conditions at the plate boundary. Due to strong 3-D heterogeneities in offshore regions, such as those relating to sea water, accretionary prism and small-scale velocity heterogeneity, conventional methods using a 1-D earth model may mis-estimate source parameters such as the duration and radiation energy. Estimations could become severe inaccuracies for small offshore seismic events because high-frequency (>1 Hz) seismograms, which are strongly affected by 3-D heterogeneities, are only available for analysis because of their signal-to-noise ratio. To investigate the effects of offshore heterogeneities on source parameter estimation for small seismic events, we analysed both observed and simulated high-frequency seismograms southeast off the Kii Peninsula, Japan, in the Nankai subduction zone. Numerical simulations of seismic wave propagation using a 3-D velocity structure model clarified the effects of each heterogeneity. Comparisons between observations and model simulations demonstrated that the thick low-velocity accretionary prism has significant effects on high-frequency seismic wave propagation. Especially for shallow low-frequency tremors occurring at depths just below the accretionary prism toe, seismogram durations are significantly broader than an assumed source duration, even for stations with epicentral distances of approximately 10 km. Spindle-shape seismogram envelopes were observed even at such close stations. Our results suggest that incorporating 3-D heterogeneities is necessary for practical estimation of source parameters for small offshore events.

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

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.

Development of a Compact Broadband Ocean-Bottom Seismometer

2021 ◽  
Author(s):  
Masanao Shinohara ◽  
Tomoaki Yamada ◽  
Hajime Shiobara ◽  
Yusuke Yamashita
Keyword(s):  
Low Frequency ◽  
Plate Boundaries ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Broadband Seismometer ◽  
Seismic Sensors ◽  
Slow Earthquakes ◽  
Short Period ◽  
Ocean Bottom Seismometers

Abstract Studies of very-low-frequency earthquakes and low-frequency tremors (slow earthquakes) in the shallow region of plate boundaries need seafloor broadband seismic observations. Because it is expected that seafloor spatially high-density monitoring requires numerous broadband sensors for slow earthquakes near trenches, we have developed a long-term compact broadband ocean-bottom seismometer (CBBOBS) by upgrading the long-term short-period ocean-bottom seismometer that has seismic sensors with a natural frequency of 1 Hz and is being mainly used for observation of microearthquakes. Because many long-term ocean-bottom seismometers with short-period sensors are available, we can increase the number of broadband seafloor sensors at a low cost. A short-period seismometer is exchanged for a compact broadband seismometer with a period of 20 or 120 s. Because the ocean-bottom seismometers are installed by free fall, we have no attitude control during an installation. Therefore, we have developed a new leveling system for compact broadband seismic sensors. This new leveling system keeps the same dimensions as the conventional leveling system for 1 Hz seismometers so that the broadband seismic sensor can be installed conveniently. Tolerance for leveling is less than 1°. A tilt of up to 20° is allowed for the leveling operation. A microprocessor controls the leveling procedure. Some of the newly developed ocean-bottom seismometers were deployed in the western Nankai trough, where slow earthquakes frequently occur. The data from the ocean-bottom seismometers on the seafloor were evaluated, and we confirmed that the long-term CBBOBS is suitable for observation of slow earthquakes. The developed ocean-bottom seismometer is also available for submarine volcanic observation and broadband seafloor observation to estimate deep seismic structures.

Low-Frequency Tremors Immediately After the 2011 Tohoku-Oki Earthquake Detected by Near-Trench OBS Observations

2020 ◽  
Author(s):  
Hidenobu Takahashi ◽  
Ryota Hino ◽  
Naoki Uchida ◽  
Takanori Matsuzawa ◽  
Yusaku Ohta ◽  
...  
Keyword(s):  
Near Field ◽  
Low Frequency ◽  
Spectral Shape ◽  
Ocean Bottom ◽  
Aseismic Slip ◽  
Very Low Frequency ◽  
Repeating Earthquakes ◽  
Ocean Bottom Seismometers ◽  
Repeating Earthquake ◽  
Interplate Earthquakes

Abstract We used temporal seismic observation using pop-up type ocean-bottom seismometers to detect a number of low-frequency tremors (LFTs) immediately after the 2011 Tohoku-Oki earthquake in the northern periphery of its aftershock area. The near-field observation clearly distinguished LFTs from regular earthquakes based on their spectral shape in the frequency band of 1–4 Hz. In addition to the LFTs accompanied by known very low frequency earthquakes (VLFEs), more than 130 LFTs without known VLFE activity were detected during April–October, 2011. The newly detected LFTs were in the vicinity of a sequence of small repeating earthquakes indicating mixed distribution of LFTs and regular interplate earthquakes in the region. The LFTs and repeating earthquake activities show a periodicity of 60–100 days, which is similar to that of the LFT activity in the later period (2016–2018). This suggests that the LFT activity is modulated by sustained background aseismic slip events throughout the postseismic period of the 2011 mainshock.

scholarly journals Observations of Earth’s Normal Modes on Broadband Ocean Bottom Seismometers

2021 ◽  
Vol 9 ◽  
Author(s):  
Gabi Laske
Keyword(s):  
Normal Modes ◽  
Low Frequency ◽  
Free Fall ◽  
Ocean Bottom ◽  
High Quality ◽  
Broadband Seismometer ◽  
The Pacific ◽  
Ocean Bottom Seismometers ◽  
The Pacific Ocean ◽  
First Time

It is generally thought that high noise levels in the oceans inhibit the observation of long-period earthquake signals such as Earth’s normal modes on ocean bottom seismometers (OBSs). Here, we document the observation of Earth’s gravest modes at periods longer than 500 s (or frequencies below 2 mHz). We start with our own 2005–2007 Plume-Lithosphere-Undersea-Mantle Experiment (PLUME) near Hawaii that deployed a large number of broadband OBSs for the first time. We collected high-quality normal mode spectra for the great November 15, 2006 Kuril Islands earthquake on multiple OBSs. The random deployment of instruments from different OBS groups allows a direct comparison between different broadband seismometers. For this event, mode S06 (1.038 mHz) consistently rises above the background noise at all OBSs that had a Nanometrics Trillium T-240 broadband seismometer. We also report observations of other deployments in the Pacific ocean that involved instruments of the U.S. OBS Instrument Pool (OBSIP) where we observe even mode S04 (0.647 mHz). Earth’s normal modes were never the initial target of any OBS deployment, nor was any other ultra-low-frequency signal. However, given the high costs of an OBS campaign, the fact that data are openly available to future investigators not involved in the campaign, and the fact that seismology is evolving to investigate ever-new signals, this paper makes the case that the investment in a high-quality seismic sensor may be a wise one, even for a free-fall OBS.

Characteristics of Shallow Low‐Frequency Earthquakes off the Kii Peninsula, Japan, in 2004 Revealed by Ocean Bottom Seismometers

2019 ◽  
Vol 46 (23) ◽  
pp. 13737-13745 ◽  
Author(s):  
Koji Tamaribuchi ◽  
Akio Kobayashi ◽  
Takahito Nishimiya ◽  
Fuyuki Hirose ◽  
Satoshi Annoura
Keyword(s):  
Low Frequency ◽  
Ocean Bottom ◽  
Kii Peninsula ◽  
Ocean Bottom Seismometers

scholarly journals Sub-surface magma movement inferred from low-frequency seismic events in the off-Nicobar region, Andaman Sea

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
K. K. Aswini ◽  
Pawan Dewangan ◽  
K. A. Kamesh Raju ◽  
V. Yatheesh ◽  
Pabitra Singha ◽  
...  
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
Andaman Sea ◽  
Earthquake Swarms ◽  
Ocean Bottom ◽  
Upward Movement ◽  
Ocean Bottom Seismometer ◽  
Magma Pulses ◽  
Obs Data ◽  
Acoustic Phase

AbstractMonitoring volcanic activity along the submarine volcanoes that are usually induced by subsurface magmatism is a challenge. We present fresh set of Ocean Bottom Seismometer (OBS) data that shows geophysical evidence indicative of subsurface magmatism along the submarine volcanoes in the off Nicobar region, Andaman Sea. In this region, we observed for the first time, hybrid very long-period earthquakes documented by passive OBS experiment. These events were initiated by high-frequency (5–10 Hz) with a clear onset of P-phase followed by low-frequency (0.01–0.5 Hz) oscillations in the range of 300–600 s with a prominent high-frequency (10–40 Hz) hydro-acoustic phase. A total of 141 high-frequency events were detected on 21st and 22nd March 2014 out of which 71 were of low-frequency oscillations. These events are distributed in the northwest–southeast direction along the submarine volcanic arc and Seulimeum strand of Great Sumatra fault. Off Nicobar region has been witnessing frequent earthquake swarms since 26th December 2004 tsunamigenic Sumatra earthquake. These swarms occurred in January 2005, March and October 2014, November 2015 and March 2019. The occurrence of low-frequency earthquakes and prominent hydro-acoustic phase are suggestive of sub-surface tectonic and magmatic influence. We propose that upward movement of magma pulses from deeper magma reservoir to the shallow magma chamber activated the strike-slip movement of sliver faults and induced earthquake swarms in the off Nicobar region.

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