Lg wave propagation in the area around Japan: observations and simulations

Abstract

Regional wavefields are strongly influenced by crustal structure heterogeneity variations along their propagation paths. Observations of such effects between the Asian continent and the Japanese subduction zone across the Sea of Japan (East Sea) have been strongly assisted recently by the development of high-density seismic networks in Japan and Korea, as well as the supercomputer-based three-dimensional finite difference method seismic wave propagation simulations using detailed heterogeneous crustal models. In this study, an Lg propagation map derived from 289,000 ray paths connecting sources to observation stations reveals efficient Lg wave propagation from continental Asia to Kyushu through the Korean Peninsula, and to Hokkaido, thus indicating a laterally consistent crustal structure extending from continental Asia to Japan. However, the Lg wave propagation to the Japanese main island of Honshu is totally blocked as it crosses the continental-oceanic boundary surrounding the Sea of Japan. Three-dimensional (3-D) finite-difference method seismic wave propagation simulations performed using a detailed crustal structural model allow us to clearly visualize the way in which an Lg wave develops from a shallow source in the crust and its propagation in the crustal waveguide by means of multiple post-critical S wave reflections in the continental structure. The sudden thinning of the continental crust at the edge of the Asian continent adjacent to the oceanic crust in the Sea of Japan, which involves a thickness change from 30 to 10 km within a 100-km distance (with the thinner crust extending over 600 km), decreases the Lg wave energy by 10%. It has been confirmed that 50% of this Lg wave energy loss occurs during wave passage along the thinner crust and that the other 50% results from conversion into P energy in the overlying seawater.

Evidence of strong long-period ground motions of engineering importance for Nankai Trough plate boundary earthquakes: comparison of ground motions of two moderate-magnitude earthquakes

“…The larger Mj values for crustal events away from subduction zones in western Japan are posited to be related to the efficient propagation of Lg-type waves in the region (Furumura and Kennett 2001). Seismic wave propagations for the 2016 event may also have been compounded by wave-guide effects due to the subducting Philippine Sea Plate in the offshore region, and Lg-type wave conversion and transmission through the inland crustal structure in southwest Japan (Furumura et al 2014). Together, all these effects contributed to the JMA magnitude being larger by 0.7 magnitude units than its F-net catalog Mw value of 5.8.…”

Average QLg, QSn, and observation of Lg blockage in the Continental Margin of Nova Scotia

“…Using a normal‐mode Lg representation they showed that the overall thickness of oceanic crustal waveguide is too low to allow the Lg to develop as a significant phase in the frequency band 0.3–2.0 Hz. Other researchers [e.g., Maupin , ; Shapiro et al ., ; Mendi et al ., ; Furumura et al ., ] believe simple crustal thinning at the continent‐ocean boundary alone cannot explain the observed extinction of the Lg. However, McNamara et al .…”

“…Press and Ewing [] revealed that even if just a small portion of the Lg path as little as 100 km in length passes through a thin oceanic structure, Lg arrivals would be extinct on the seismogram. “Lg blockage” has been reported in many regions around the world, such as the North Sea [ Gregersen , ; Kennett and Mykkeltveit , ; Kennett et al ., ], the Caspian Sea [ Maupin , ], China [ McNamara et al ., ; Rapine et al ., ], the Himalayan Belt [ Ruzaikin et al ., ], Tibetan Plateau [ Xie , ; Fan and Lay , ], Central Asia [ Baumgardt , ], the Alpine range [ Campillo et al ., ], the oceanic region in the Sea of Japan [ Furumura and Kennett , ; Furumura et al ., ], Pyrenees [ Chazalon et al ., ], Korean Peninsula [ Hong et al ., ], and the Ligurian Sea [ Shapiro et al ., ].…”

“…This was the general explanation of Lg propagation mechanism at the oceanic‐to‐continental transition zone. Numerous researchers have tried to explain the Lg blockage and the complete elimination of the Lg arrival on regional seismograms at the blocking zones by drastic variations in crustal thickness and topography [ Kennett et al ., ], scattering from tectonic boundaries [ Regan and Harkrider , ], strong anelastic attenuation in upper crustal or deep sediments [ Mitcheel and Hwang , 1987; Baumgardt , ], energy delayed or converted in the overlying thick sediments [ Gregersen and Vaccari , ; Mendi et al ., ; Furumura and Kennett , ], mode conversions leakage of Lg energy downward into the mantle [ Kennett , ; Mendi et al ., ; Hong et al ., ; Furumura et al ., ], the presence of the overlying water column and topographical variations [ Cao and Muirhead , ], a thinning crust [ Zhang and Lay , ; Furumura et al ., ], strong topographical variations of the Moho [e.g., Gregersen , ; Kennett and Mykkeltveit , ; Bouchon and Coutant , ; Mendi et al ., ], the dissipation of the Lg energy into seawater by conversion to P wave [ Furumura et al ., ], and/or strong attenuation by scattering [ Campillo et al ., ].…”

“…Moho depth at the Shelf is around 37 km, but it shallows rapidly to 27 km between 45 and 100 km along the profile in Figure coinciding with the thickening of the sediments. The elevated Moho by energy coupling between Lg modes and Sn modes can be a major contributor to the attenuation of Lg wave by leaking the energy out of the crustal waveguide and into the upper mantle [ Gregersen , ; Kennett and Mykkeltveit , ; Mendi et al ., ; Hong et al ., ; Furumura et al ., ]. The change in Moho depth does induce the diffraction of a part of the incident Lg wave train into mantle waves, giving rise to the Sn phase on the other side of the basin.…”