2:45 PM - 3:00 PM
[SCG54-17] Mantle Structure Beneath the Japan Sea and its Surrounding Areas Revealed by Multiscale Global Tomography
Keywords:the Sea of Japan, seismic tomography, uppermost mantle, subduction zone, big mantle wedge (BMW)
The expansion of the Japan Sea is a phenomenon induced by northwestward deep subduction of the Pacific plate and can be explained as part of the big mantle wedge (BMW) activity above the Pacific slab under the eastern margin of East Asia (e.g. Zhao et al., 2004) . According to the BMW model, hot and wet mantle upwelling triggered by dehydration of the subducting Pacific slab and corner flows in the BMW induce intraplate volcanism and earthquakes in the East Asian interior far from the Japan trench. However, the subsurface structure of the Japan Sea and its surrounding areas is still not very clear due to the lack of sufficient seismic stations.
To date, seismic tomography studies focusing on the Japan Sea have been conducted mainly by regional tomography based on local and teleseismic events. Analyses using teleseismic events employ relative travel times, assuming that the effects of structural heterogeneities outside the target region on seismic data from an event are common for all seismic stations in the study region and can thus be subtracted. On the other hand, global tomography, which uses absolute travel times, does not require such assumptions. However, due to its larger spatial coverage, global tomography generally has lower resolution. To resolve this problem, Zhao et al. (2017) developed a new global tomography method (the so-called multiscale global tomography) which allows for setting finer 3-D grid under the target region so as to achieve high resolution comparable to that of regional tomography.
In this study, we apply the method of multiscale global tomography to the Japan Sea and its surrounding areas to determine a detailed 3-D P-wave velocity (Vp) model of the mantle. We collect two datasets for this purpose. The first dataset consists of 6,408,268 P, pP, and PP arrival times from 14,286 seismic events and recorded at 14,145 stations, which are downloaded from the ISC-EHB database (Dataset 1). The second dataset was recorded by the portable NECESSArray used by Chen et al. (2017), containing 15,769 P-wave arrival times from 228 seismic events and recorded at 683 stations distributed across China, Korea, and Japan (Dataset 2). The target region has a latitude range of 30°N - 50°N, a longitude range of 120°E - 145°E, and a depth range of 0 - 950 km. The initial velocity model used is the IASP91 model (Kennett & Engdahl, 1991).
Our 3-D Vp model derived from Dataset 1 exhibits the following features. (1) The velocity structure at a depth of 100 km below the Japan Sea changes from the north to south with a boundary at ~40°N. In particular, a distinct low-velocity zone appears in the southern part of the region, which is thought to reflect the BMW. (2) A low-velocity anomaly is visible at a depth of ~500 km below Vladivostok, which might be due to dehydration of the subducting Pacific slab. (3) A spot-like
high-velocity zone exists in the upper mantle beneath eastern Eurasia, which may reflect delamination of the densified continental lithosphere.
We plan to present an improved 3-D Vp model of the study region obtained with both the data sets and with additional arrival-time data of PcP and Pdiff phases.
References
Chen, C., Zhao, D., Tian, Y., Wu, S., Hasegawa, A., Lei, J., Park, J.-H., & Kang, I.-B. (2017). Mantle transition zone, stagnant slab and intraplate volcanism in Northeast Asia. Geophysical Journal International, 209, 68-85. https ://doi.org/10.1093/gji/ggw491
Kennett, B., & Engdahl, E. R. (1991). Traveltimes for global earthquake location and phase identification. Geophysical Journal International, 105, 429-465. https://doi. org/10.1111/j.1365-246X.1991.tb06724.x
Zhao, D., Fujisawa, M., & Toyokuni, G. (2017). Tomography of the subducting Pacific slab and the 2015 Bonin deepest earthquake (Mw 7.9). Scientific Reports, 7, 44487. https://doi.org/10.1038/srep44487
Zhao, D., Lei, J., & Tang, R. (2004). Origin of the Changbai intraplate volcanism in Northeast China: Evidence from seismic tomography. Chinese Science Bulletin, 49, 1401- 1408. https://doi.org/10.1360/04wd0125
To date, seismic tomography studies focusing on the Japan Sea have been conducted mainly by regional tomography based on local and teleseismic events. Analyses using teleseismic events employ relative travel times, assuming that the effects of structural heterogeneities outside the target region on seismic data from an event are common for all seismic stations in the study region and can thus be subtracted. On the other hand, global tomography, which uses absolute travel times, does not require such assumptions. However, due to its larger spatial coverage, global tomography generally has lower resolution. To resolve this problem, Zhao et al. (2017) developed a new global tomography method (the so-called multiscale global tomography) which allows for setting finer 3-D grid under the target region so as to achieve high resolution comparable to that of regional tomography.
In this study, we apply the method of multiscale global tomography to the Japan Sea and its surrounding areas to determine a detailed 3-D P-wave velocity (Vp) model of the mantle. We collect two datasets for this purpose. The first dataset consists of 6,408,268 P, pP, and PP arrival times from 14,286 seismic events and recorded at 14,145 stations, which are downloaded from the ISC-EHB database (Dataset 1). The second dataset was recorded by the portable NECESSArray used by Chen et al. (2017), containing 15,769 P-wave arrival times from 228 seismic events and recorded at 683 stations distributed across China, Korea, and Japan (Dataset 2). The target region has a latitude range of 30°N - 50°N, a longitude range of 120°E - 145°E, and a depth range of 0 - 950 km. The initial velocity model used is the IASP91 model (Kennett & Engdahl, 1991).
Our 3-D Vp model derived from Dataset 1 exhibits the following features. (1) The velocity structure at a depth of 100 km below the Japan Sea changes from the north to south with a boundary at ~40°N. In particular, a distinct low-velocity zone appears in the southern part of the region, which is thought to reflect the BMW. (2) A low-velocity anomaly is visible at a depth of ~500 km below Vladivostok, which might be due to dehydration of the subducting Pacific slab. (3) A spot-like
high-velocity zone exists in the upper mantle beneath eastern Eurasia, which may reflect delamination of the densified continental lithosphere.
We plan to present an improved 3-D Vp model of the study region obtained with both the data sets and with additional arrival-time data of PcP and Pdiff phases.
References
Chen, C., Zhao, D., Tian, Y., Wu, S., Hasegawa, A., Lei, J., Park, J.-H., & Kang, I.-B. (2017). Mantle transition zone, stagnant slab and intraplate volcanism in Northeast Asia. Geophysical Journal International, 209, 68-85. https ://doi.org/10.1093/gji/ggw491
Kennett, B., & Engdahl, E. R. (1991). Traveltimes for global earthquake location and phase identification. Geophysical Journal International, 105, 429-465. https://doi. org/10.1111/j.1365-246X.1991.tb06724.x
Zhao, D., Fujisawa, M., & Toyokuni, G. (2017). Tomography of the subducting Pacific slab and the 2015 Bonin deepest earthquake (Mw 7.9). Scientific Reports, 7, 44487. https://doi.org/10.1038/srep44487
Zhao, D., Lei, J., & Tang, R. (2004). Origin of the Changbai intraplate volcanism in Northeast China: Evidence from seismic tomography. Chinese Science Bulletin, 49, 1401- 1408. https://doi.org/10.1360/04wd0125