scholarly journals Crosscorrelation of Earthquake Data Using Stationary Phase Evaluation: Insight into Reflection Structures of Oceanic Crust Surface in the Nankai Trough

2012 ◽  
Vol 2012 ◽  
pp. 1-8 ◽  
Author(s):  
Shohei Minato ◽  
Takeshi Tsuji ◽  
Toshifumi Matsuoka ◽  
Koichiro Obana
Keyword(s):  
Stationary Phase ◽  
Oceanic Crust ◽  
Wave Reflection ◽  
Nankai Trough ◽  
Plane Waves ◽  
Sea Surface ◽  
Source Distribution ◽  
Ocean Bottom ◽  
Phase Evaluation ◽  
Crust Surface

Seismic interferometry (SI) has been recently employed to retrieve the reflection response from natural earthquakes. We perform experimental study to apply SI to Ocean Bottom Seismogram (OBS) records in the Nankai Trough, southwest Japan in order to reveal the relatively shallow geological boundaries including surface of oceanic crust. Although the local earthquakes with short raypath we use to retrieve reflection response are expected to contain the higher-frequency components to detect fine-scale structures by SI, they cannot be assumed as plane waves and are inhomogeneously distributed. Since the condition of inhomogeneous source distribution violates the assumption of SI, the conventional processing yields to the deteriorated subsurface images. Here we adopt the raypath calculation for stationary phase evaluation of SI in order to overcome this problem. To find stationary phase, we estimate the raypaths of two reflections: (1) sea-surfaceP-wave reflection and (2) sea-surface multipleP-wave reflection. From the estimated raypath, we choose the crosscorrelation traces which are expected to produce objective reflections considering the stationary phase points. We use the numerical-modeling data and field data with 6 localized earthquakes and show that choosing the crosscorrelation traces by stationary phase evaluation improves the quality of the reflections of the oceanic crust surface.

scholarly journals The application to real data of a technique for measuring the plane‐wave reflection coefficient of the ocean bottom

1982 ◽  
Vol 72 (S1) ◽  
pp. S97-S97
Author(s):  
George V. Frisk ◽  
Douglas R. Mook ◽  
James A. Doutt ◽  
Earl E. Hays ◽  
Alan V. Oppenheim
Keyword(s):  
Reflection Coefficient ◽  
Plane Wave ◽  
Wave Reflection ◽  
Real Data ◽  
Ocean Bottom

Ocean-Bottom Strong-Motion Observations in the Nankai Trough by the DONET Real-Time Monitoring System

2018 ◽  
Vol 52 (3) ◽  
pp. 100-108 ◽  
Author(s):  
Takeshi Nakamura ◽  
Narumi Takahashi ◽  
Kensuke Suzuki
Keyword(s):  
Real Time ◽  
Nankai Trough ◽  
Strong Motion ◽  
Deep Ocean ◽  
Disaster Mitigation ◽  
Ocean Bottom ◽  
Source Modeling ◽  
Origin Time ◽  
Subduction Earthquakes ◽  
Sediment Layers

AbstractThe deployment of real-time permanent ocean-bottom seismic and tsunami observatories is significant for disaster mitigation and prevention during the occurrence of large subduction earthquakes near trough areas. On April 1, 2016, a moderate-sized suboceanic earthquake occurred beneath Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) stations that were recently deployed in deep ocean-bottom areas near the Nankai Trough in southwest Japan. P-waves arrived at the ocean-bottom station within 4 s of the origin time, which was 6 and 13 s earlier than the arrival of P- and S-waves at a land station in the coastal area, respectively; this implies earlier detection of strong motion than at land stations. However, the waveforms are amplified by sediment layers and even contaminated with acceleration offsets at some stations, which would lead to overestimations during source investigations. Such amplification and offset did not occur at a borehole station connected to DONET. The amplifications caused by the sediment layers and the offset were found to have a considerable spatial variation, not only between the DONET stations and land and borehole stations but also among the DONET stations, implying that the amplitude evaluation could be unstable. Therefore, procedures for correcting or suppressing the amplification and offset problem are required for conducting waveform analyses, such as magnitude estimations and source modeling, during large subduction earthquakes.

scholarly journals Western Nankai Trough seismogenic zone: Results from a wide-angle ocean bottom seismic survey

2000 ◽  
Vol 105 (B3) ◽  
pp. 5887-5905 ◽  
Author(s):  
Shuichi Kodaira ◽  
Narumi Takahashi ◽  
Jin-Oh Park ◽  
Kimihiro Mochizuki ◽  
Masanao Shinohara ◽  
...  
Keyword(s):  
Nankai Trough ◽  
Seismic Survey ◽  
Seismogenic Zone ◽  
Ocean Bottom ◽  
Wide Angle

scholarly journals Synthesizing sea surface height change including seismic waves and tsunami using a dynamic rupture scenario of anticipated Nankai trough earthquakes

2019 ◽  
Vol 769 ◽  
pp. 228166 ◽  
Author(s):  
Tatsuhiko Saito ◽  
Toshitaka Baba ◽  
Daisuke Inazu ◽  
Shunsuke Takemura ◽  
Eiichi Fukuyama
Keyword(s):  
Seismic Waves ◽  
Nankai Trough ◽  
Sea Surface Height ◽  
Sea Surface ◽  
Dynamic Rupture ◽  
Height Change

Shear attenuation and anelastic mechanisms in the central Pacific upper mantle

2020 ◽  
Author(s):  
Zhitu Ma ◽  
Colleen Dalton ◽  
Joshua Russell ◽  
James Gaherty ◽  
Greg Hirth ◽  
...  
Keyword(s):  
Grain Boundary ◽  
Rayleigh Wave ◽  
Rayleigh Waves ◽  
Plane Waves ◽  
Shear Velocity ◽  
Grain Boundary Sliding ◽  
Absorption Peak ◽  
Ocean Bottom ◽  
Central Pacific ◽  
Transition Depth

<p>We determine the mantle attenuation (1/Q) structure beneath 70 Myr seafloor in the central Pacific. We use long-period (33-100 sec) Rayleigh waves recorded by the NoMelt array of broadband ocean-bottom seismometers. After the removal of tilt and compliance noise, we are able to measure Rayleigh wave phase and amplitude for 125 earthquakes. The compliance correction for ocean wave pressure on the seafloor is particularly important for improving signal-to-noise at periods longer than 55 sec. Attenuation and azimuthally anisotropic phase velocity in the study area are determined by approximating the wavefield as the interference of two plane waves. We find that the amplitude decay of Rayleigh waves across the NoMelt array can be adequately explained using a two-layer model: in the shallow layer, in the deeper layer, and a transition depth at 70 km, although the sharpness of the transition is not well resolved by the Rayleigh wave data. Notably, observed in the NoMelt lithosphere is significantly higher than values in this area from global attenuation models. When compared with lithospheric measured at higher frequency (~3 Hz), the frequency dependence of attenuation is very slight, revising previous interpretations. The effect of anelasticity on shear velocity (V<sub>S</sub>) is estimated from the ratio of observed velocity to the predicted anharmonic value. We use laboratory-based parameters to predict attenuation and velocity-dispersion spectra that result from the superposition of a weakly frequency dependent high-temperature background and an absorption peak. We test a large range of frequencies for the position of the absorption peak (<em>f</em><sub>e</sub>) and determine, at each depth, which values of <em>f</em><sub>e</sub> predict and V<sub>S</sub> that can fit the NoMelt and V<sub>S </sub>values simultaneously. We show that between depths of 60 and 80 km the seismic models require an increase in <em>f</em><sub>e</sub> by at least 3-4 orders of magnitude. Under the assumption that the absorption peak is caused by elastically accommodated grain-boundary sliding, this increase in <em>f</em><sub>e</sub> reflects a decrease in grain-boundary viscosity of 3-4 orders of magnitude. A likely explanation is an increase in the water content of the mantle, with the base of the dehydrated lid located at ~70-km depth.   </p>

Geophysical measurements in the region of the Explorer ridge off western Canada

1978 ◽  
Vol 15 (9) ◽  
pp. 1508-1525 ◽  
Author(s):  
R. D. Hyndman ◽  
G. C. Rogers ◽  
M. N. Bone ◽  
C. R. B. Lister ◽  
U. S. Wade ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Seismic Refraction ◽  
Ocean Bottom ◽  
Low Velocity ◽  
Geothermal Heat ◽  
Juan De Fuca Plate ◽  
The North ◽  
The Pacific ◽  
Ocean Bottom Seismometers ◽  
Seismic Reflection Profiles

The region of the Explorer spreading centre off Vancouver Island, British Columbia, has been studied through a marine geophysical survey. Earthquake epicentres located by three ocean bottom seismometers confirm that the boundary between the Pacific plate and the Explorer plate (the northern extension of the Juan de Fuca plate) at present lies along the Sovanco fracture zone, the Explorer ridge, and the Dellwood Knolls. The epicentres of earthquakes in this area as determined by the onshore seismic network are found to be subject to significant errors. The ocean bottom seismometers also have been used for a detailed seismic refraction line just to the north of the Explorer spreading centre employing explosives and a large airgun as sources. A preliminary analysis of the data indicates a fairly typical crustal structure but a shallow and low velocity mantle near the ridge crest, and illustrates the value of ocean bottom seismometers in oceanic refraction studies. A new geothermal heat flux probe was employed in this study that permitted repeated 'pogostick' penetrations without raising the instrument to the surface. Six profiles with a total of 112 penetrations provided valuable data on the nature of hydrothermal circulation in the oceanic crust. Eleven standard heat probe stations provided some restraints on the poorly known age of the oceanic crust along the margin. Seismic reflection profiles using a 3.5 kHz system, a high resolution pulser profiler, and a large airgun were used as aids in the interpretation of the seismic and heat flow data.

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