Lateral variation of the uppermost oceanic plate in the outer-rise region of the Northwest Pacific Ocean inferred from Po-to-s converted waves

Abstract

The uppermost structures of incoming oceanic plates have been investigated by seismic exploration surveys, primarily based on two-dimensional profiles. However, their regional-scale lateral variations and shear wave velocity structures with higher-frequency components remain elusive. This study, using passive seismic records, attempted to retrieve Po-to-s converted waves (Pos) by cross-correlating Po wave coda from the radial and vertical components in the 2-6 Hz frequency band and to show lateral variations of the shear wave velocity within the oceanic crust, including crust-related seismic interfaces and layer-dependent anisotropic structures. Po waves were collected from continuous records acquired from active seismic surveys performed across wide areas of the Northwestern Pacific over shorter observation periods and passive seismic surveys performed near the Japan Trench over longer observation periods. As a result, this study obtained clear Pos waves converted at the basement and oceanic Moho of the seaward region, whereas weakened or no Pos waves were observed at ocean bottom seismometers near the trench. The primary reasons for Pos wave weakening or absence were considered to be structural changes, including normal faults due to the plate flexures in the outer-rise region, hydration at the uppermost oceanic plate, and fractures associated with volcanic activities that can be seen on the oceanic plate (petit spots). Furthermore, layer-dependent shear wave anisotropies were estimated for the sediment and crust. Fast polarization directions were oriented in trench-parallel directions near the trench and in the NNW-SSE directions in the seaward region. The pattern change to near-trench polarization directions would correspond to stress field-induced aligned fractures, including cracks and normal faults, created by stress fields induced by plate bending in the outer-rise region.

Azimuthal Variation of Lithospheric Heterogeneity in the Northwest Pacific Inferred From Po/So Propagation Characteristics and Anomalously Large Ground Motion of Deep In‐Slab Earthquakes

“…(2014). Based on a 2‐D FDM simulation with a finer (0.02 km) grid, we confirmed that such a thin layer has only a minor effect on Po/So propagation, especially for Z component, though it may cause large Po ‐to‐ S conversion at the seafloor for R motion (Tonegawa et al., 2018, see, Figure S2 in Supporting Information).…”

Seismic Structure of the Oceanic Crust Around Petit‐Spot Volcanoes in the Outer‐Rise Region of the Japan Trench

“…Although we could not constrain the mantle velocity structure well, seismic velocities in the uppermost mantle could be low (V p < 8.0 km/s) near petit-spot volcanoes ( Figure S4), and we imaged an unclear or absent Moho reflection in the distance range from 55 to 80 km on the MCS reflection section (Figure 2). Furthermore, Tonegawa et al (2018) also reported a weakened or absent interface at the crust-mantle boundary near the same petit-spot. Therefore, ascending magma associated with petit-spot volcanic activity may modify not only the shallower part of the oceanic crust but also the crust-mantle boundary.…”

“…Specifically, the reduced crustal velocity below the basement can be explained by fractures and hydration associated with petit-spot volcanic activity. In their analysis of passive seismic records, Tonegawa et al (2018) used Pos waves to image the basement and oceanic Moho in the outer-rise region of the Japan and Kuril trenches. They imaged a clear basement and oceanic Moho in seaward regions, whereas they obtained weakened or no Pos waves near the trench.…”

“…Fujiwara et al (2007) collected seismic reflection data in an area of petit-spot volcanism with a single-channel streamer and imaged opaque and disrupted structures at the top of the oceanic crust. Tonegawa et al (2018), who analyzed passive seismic records in the outer-rise region of the northwestern Pacific Plate, observed that Po-to-s converted (Pos) waves were weakened or absent at the basement and Moho under petit-spot volcanoes. These studies suggest that the crustal structure of the old oceanic plate can be modified by intraplate volcanism.…”

Configuration and structure of the Philippine Sea Plate off Boso, Japan: constraints on the shallow subduction kinematics, seismicity, and slow slip events

“…The RFs were calculated through a spectral division of the radial components by the vertical components with a water level of 0.001 (e.g., Langston 1977Langston , 1979. We used a time window from − 2 to 3 s from the picked P arrival times in the vertical component to avoid contamination of water reverberation in the deconvolution (Tonegawa et al 2018) and applied a cosine taper of 10% of the time window. The time window of the radial component was from − 2 s to 14 s from the picked P arrival times.…”