Migration-Based Local Event-Location Workflow for Ocean-Bottom Seismometer (OBS) Records in Subduction Zones: A Practical Approach for Addressing a Large Number of Events

2021 ◽  
Author(s):  
Shinji Yoneshima ◽  
Kimihiro Mochizuki
Keyword(s):  
Subduction Zone ◽  
Subduction Zones ◽  
Real Data ◽  
Japan Trench ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
S Waves ◽  
Local Earthquakes ◽  
Ocean Bottom Seismometers ◽  
Event Location

ABSTRACT An efficient event-location workflow is highly desired to analyze large numbers of local earthquakes recorded by ocean-bottom seismometers (OBSs) in subduction zones. The present study proposes a migration-based event-location approach for evaluating OBS records to examine local subduction-zone earthquakes. This approach can significantly reduce the amount of manual time picks compared with conventional methods. The event-location workflow was designed to detect arrival onsets of both P and S phases. Synthetic tests have shown that the proposed migration-based event-location method is robust against different types of noise, such as environmental noise and local spike noise. This workflow was then applied to real OBS data in the off-Ibaraki region at the southern end of the Japan trench. The results show that this approach is applicable to real data from subduction-zone events: It gives reasonable agreement with manual time picks for both P and S waves and reasonable error bars, and it demonstrates a clear down-dip trend of seismicity. The results also show fair agreement with event distributions from previous studies of the off-Ibaraki region. This proposed workflow can be used to examine the seismicity of local earthquakes around the subduction zone using OBSs. This approach is especially effective when the seismicity is high and/or in cases in which long-term OBS monitoring has recorded a large number of events.

scholarly journals Along‐Arc Heterogeneity in Local Seismicity across the Lesser Antilles Subduction Zone from a Dense Ocean‐Bottom Seismometer Network

2019 ◽  
Vol 91 (1) ◽  
pp. 237-247 ◽  
Author(s):  
Lidong Bie ◽  
Andreas Rietbrock ◽  
Stephen Hicks ◽  
Robert Allen ◽  
Jon Blundy ◽  
...  
Keyword(s):  
Subduction Zone ◽  
Subduction Zones ◽  
Lesser Antilles ◽  
Seismogenic Zone ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Plate Interface ◽  
Local Seismicity ◽  
Slow Spreading ◽  
Lesser Antilles Arc

Abstract The Lesser Antilles arc is only one of two subduction zones where slow‐spreading Atlantic lithosphere is consumed. Slow‐spreading may result in the Atlantic lithosphere being more pervasively and heterogeneously hydrated than fast‐spreading Pacific lithosphere, thus affecting the flux of fluids into the deep mantle. Understanding the distribution of seismicity can help unravel the effect of fluids on geodynamic and seismogenic processes. However, a detailed view of local seismicity across the whole Lesser Antilles subduction zone is lacking. Using a temporary ocean‐bottom seismic network we invert for hypocenters and 1D velocity model. A systematic search yields a 27 km thick crust, reflecting average arc and back‐arc structures. We find abundant intraslab seismicity beneath Martinique and Dominica, which may relate to the subducted Marathon and/or Mercurius Fracture Zones. Pervasive seismicity in the cold mantle wedge corner and thrust seismicity deep on the subducting plate interface suggest an unusually wide megathrust seismogenic zone reaching ∼65  km depth. Our results provide an excellent framework for future understanding of regional seismic hazard in eastern Caribbean and the volatile cycling beneath the Lesser Antilles arc.

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.

Subduction zone heterogeneity observed across the magnitude spectrum for megathrust earthquakes

2021 ◽  
Author(s):  
Susan Bilek ◽  
Emily Morton
Keyword(s):  
Spatial Heterogeneity ◽  
Subduction Zone ◽  
Stress Drop ◽  
Subduction Zones ◽  
Cascadia Subduction Zone ◽  
Ocean Bottom ◽  
Small Earthquake ◽  
Megathrust Earthquakes ◽  
Seismic Slip ◽  
Subduction Zone Earthquakes

<p>Observations from recent great subduction zone earthquakes highlight the influence of spatial geologic heterogeneity on overall rupture characteristics, such as areas of high co-seismic slip, and resulting tsunami generation.  Defining the relevant spatial heterogeneity is thus important to understanding potential hazards associated with the megathrust. The more frequent, smaller magnitude earthquakes that commonly occur in subduction zones are often used to help delineate the spatial heterogeneity.  Here we provide an overview of several subduction zones, including Costa Rica, Mexico, and Cascadia, highlighting connections between the small earthquake source characteristics and rupture behavior of larger earthquakes.  Estimates of small earthquake locations and stress drop are presented in each location, utilizing data from coastal and/or ocean bottom seismic stations.  These seismicity characteristics are then compared with other geologic and geophysical parameters, such as upper and lower plate characteristics, geodetic locking, and asperity locations from past large earthquakes.  For example, in the Cascadia subduction zone, we find clusters of small earthquakes located in regions of previous seamount subduction, with variations in earthquake stress drop reflecting potentially disrupted upper plate material deformed as a seamount passed.  Other variations in earthquake location and stress drop can be correlated with observed geodetic locking variations. </p>

Evolution of Caribbean subduction from P-wave tomography and plate reconstruction

2020 ◽  
Author(s):  
Robert Allen ◽  
Benedikt Braszus ◽  
Saskia Goes ◽  
Andreas Rietbrock ◽  
Jenny Collier ◽  
...  
Keyword(s):  
Subduction Zones ◽  
Plate Boundary ◽  
P Wave ◽  
Lesser Antilles ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Equatorial Atlantic ◽  
The North ◽  
The Caribbean ◽  
Plate Reconstruction

<p>The Caribbean plate has a complex tectonic history, which makes it  particularly challenging to establish the evolution of the subduction zones at its margins. Here we present a new teleseismic P-wave tomographic model under the Antillean arc that benefits from ocean-bottom seismometer data collected in our recent VoiLA (Volatile Recycling in the Lesser Antilles) project. We combine this imagery with a new plate reconstruction that we use to predict possible slab positions in the mantle today. We find that upper mantle anomalies below the eastern Caribbean correspond to a stack of material that was subducted at different trenches at different times, but ended up in a similar part of the mantle due to the large northwestward motion of the Americas. This stack comprises: in the mantle transition zone, slab fragments that were subducted between 70 and 55 Ma below the Cuban and Aves segments of the Greater Arc of the Caribbean; at 450-250 km depth, material subducted between 55 and 35 Ma below the older Lesser Antilles (including the Limestone Caribees and Virgin Islands);  and above 250 km, slab from subduction between 30 and 0 Ma below the present Lesser Antilles to Hispaniola Arc. Subdued high velocity anomalies in the slab above 200 km depth coincide with where the boundary between the equatorial Atlantic and proto-Caribbean subducted, rather than as previously proposed, with the North-South American plate boundary. The different phases of subduction can be linked to changes in the age, and hence buoyancy structure, of the subducting plate.</p>

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.

Converted shear waves as seen by ocean bottom seismometers and surface buoys

1977 ◽  
Vol 67 (5) ◽  
pp. 1291-1302 ◽  
Author(s):  
Brian T. R. Lewis ◽  
James McClain
Keyword(s):  
Shear Waves ◽  
Shear Velocity ◽  
Ocean Bottom ◽  
Sediment Layer ◽  
S Waves ◽  
High Attenuation ◽  
Ocean Bottom Seismometers ◽  
Sedimentary Layer ◽  
Low Shear

abstract It is found that ocean bottom seismometers (O.B.S.) deployed in sedimented areas produce markedly different seismograms from surface hydrophones. These differences are found to be due to ringing on the O.B.S. records produced by converted shear waves trapped in the sediment layer. These shear waves do not propagate into the water and hence the hydrophone record is much “cleaner” than the O.B.S. record. It is also shown that the presence of refracted shear waves like P-S-P and P-S-S may be related to the presence of a sedimentary layer in some areas. It is suggested that the disappearance of refracted S waves in some areas without sediment is related to high attenuation and/or very low shear velocities caused by cracks and inhomogeneities in the crust. Under sedimented areas the cracks may be sufficiently filled so as to substantially reduce the attenuation and/or increase the bulk shear velocity.

Stability of ambient noise H/V spectra obtained from OBS near the Japan Trench

2020 ◽  
Author(s):  
Atikul Haque Farazi ◽  
Emmanuel Soliman M. Garcia ◽  
Yoshihiro Ito
Keyword(s):  
Ambient Noise ◽  
Temporal Stability ◽  
Spectral Ratio ◽  
Japan Trench ◽  
Future Application ◽  
Ocean Bottom ◽  
S Wave ◽  
Slow Earthquakes ◽  
Short Period ◽  
Ocean Bottom Seismometers

<p>Ocean bottom seismometers (OBS) are widely in use since recent past to monitor seismicity of slow earthquakes as well as that of ordinary earthquakes. Seismic velocity structures, especially of S-wave are essential to estimate hypocenters of them with accuracy. Here we focus on spatial and temporal stability of ambient noise horizontal to vertical spectral ratio (H/V) spectra calculated from ocean bottom seismometers, as the first step toward future application of ambient noise H/V to estimate S-wave velocity structure. We aim to use the Nakamura’s method (1989) for ambient noise H/V spectra using a 3-component OBS array in the Japan Trench, to image deep structure above the plate interface near the trench. To achieve the imaging, it is necessary to examine spatial and temporal stability of the derived H/V spectra from these seismometers. First, we split each 24-hours record into 1-hour windows after removing the instrumental response, Then, Fourier amplitude spectra of each component is taken and smoothed using Konno and Ohmachi (1998) method, with applying downsampling, mean and trend removal, and tapering to each window. Finally, a 1-hour H/V spectral ratio is calculated with taking quadratic mean of two horizontal components. However, a total of 21 OBS, 3 broadband and 18 short-period, stations have been used in this study. A daily variation and stability of the H/V spectra are examined along with comparing them spatially from one station to another. Stability of the H/V spectra from OBS is promising for carrying out our future endevour of deeper observation using the ambient noise H/V method.</p>

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