Age of the Subducting Philippine Sea Slab and Mechanism of Low‐Frequency Earthquakes

Abstract

Nonvolcanic low‐frequency earthquakes (LFEs) usually occur in young and warm subduction zones under condition of near‐lithostatic pore fluid pressure. However, the relation between the LFEs and the subducting slab age has never been documented so far. Here we estimate the lithospheric age of the subducting Philippine Sea (PHS) slab beneath the Nankai arc by linking seismic tomography and a plate reconstruction model. Our results show that the LFEs in SW Japan take place in young parts (~17–26 Myr) of the PHS slab. However, no LFE occurs beneath the Kii channel where the PHS slab is very young (~15 Myr) and thin (~29 km), forming an LFE gap there. According to the present results and previous works, we think that the LFE gap at the Kii channel is caused by joint effects of several factors, including the youngest slab age, high temperature, low fluid content, high permeability of the overlying plate, a slab tear, and hot upwelling flow below the PHS slab.

Tomography of the 2016 Kumamoto earthquake area and the Beppu-Shimabara graben

“…Figure 2a shows the geometry of the subducting PHS slab compiled from models of local earthquake tomography 3 and teleseismic tomography 4 . The lithospheric age of the subducting PHS slab 5 , 6 ranges from 24 Myr in NE Kyushu to 38 Myr in SW Kyushu (Fig. 2b ).…”

“…2b ). The lateral variations of the slab age may affect the arc magmatism and the generation of large crustal earthquakes 5 and low-frequency microearthquakes 6 . Because of the active plate subduction, seismic and volcanic activities are very intense in Kyushu.…”

Seismic Anisotropy Within the Subducting Northern Philippine Sea Plate, SW Japan, Using DONET Seafloor Observation Network

“…This is different with the over 45 Ma Nazca slab (Hua et al., 2018; Müller et al., 2008), where the observed fast splitting directions from deeper 200–400 km within the Nazca slab are trench‐parallel, perpendicular to the fossil fast axis formed during plate spreading, and are interpreted as a result of along‐strike extension induced by flattening of the slab during subduction (Eakin et al., 2015). This reflects the relatively older Nazca slab at depths more 300 km must be sufficiently weak to undergo internal deformation and overprint the fossil fabrics during subduction (Agrawal et al., 2020; Eakin et al., 2015).…”

“…It then ceased spreading, moved along with nPSP toward NW, and subducted beneath the EP (Akuhara & Mochizuki, 2015;Akuhara et al, 2013;Asamori & Zhao, 2015;Hirose et al, 2008;Nakajima & Hasegawa, 2007b;Niu et al, 2020;Okino et al, 1999;Sdrolias et al, 2004). The fossil spreading center has been partly subducting beneath the Kinki Channel (Hua et al, 2018). The dip of the subducted nPSP varies along the margin, caused by the drastic deformation under the Kinki Peninsula associated with the subduction initiation when the young and weak plate was forced to subduct against its own buoyancy (Hirose et al, 2008;Nakajima & Hasegawa, 2007a;2007b;Wang et al, 2004).…”

Complexity of Deep Low‐Frequency Earthquake Activity in Shikoku (Japan) Imaged From the Analysis of Continuous Seismic Data

“…This also implies that material in the vicinity of the subduction interface in the eastern part is mechanically weaker. One important aspect influencing this variation should be related to the observed along‐strike west to east change of the subducting PHS slab age and, implicitly, temperature from older and colder in the west to younger and warmer in the east (Hua et al., 2018). This variation in the temperature affects slab dehydration reactions which are related to the fluid content of the overriding plate observed by tomographic studies (e.g., Nakajima & Hasegawa, 2016).…”