The southwestern edge of the Ryukyu subduction zone: A high Q mantle wedge

Seismic attenuation tomography of the Northeast Japan arc: Insight into the 2011 Tohoku earthquake (Mw9.0) and subduction dynamics

“…Many low‐frequency microearthquakes occurred in or around the low‐ Q anomalies in the lower crust and uppermost mantle (Figures and ) due to the arc magma rising from the mantle wedge to the crust [ Hasegawa and Zhao , ; Zhao et al ., ]. Such low‐ Q anomalies in the mantle wedge have been also revealed in many other subduction zones, e.g., Tonga [ Roth et al ., ], New Zealand [ Eberhart‐Phillips and Chadwick , ; Eberhart‐Phillips et al ., ], southwest Japan [ Shito and Shibutan , ], Alaska [ Stachnik et al ., ], Central America [ Rychert et al ., ], Mariana [ Pozgay et al ., ], south Apennines [ Monna and Dahm , ], Taiwan [ Ko et al ., ], and Central Java [ Bohm et al ., ]. We think that such a feature is a common seismological characteristic of subduction zones, which reflects the source of arc magmatism caused by a combination of slab dehydration and corner flow in the mantle wedge [ Zhao et al ., , ; Hasegawa and Zhao , ; Wiens et al ., ; Long , ].…”

“…For the ω 2 source model [ Brune , , ], the source spectrum S i ( f ) is given by where Ω 0 i and f ci are long‐period plateau value and corner frequency for event i , respectively. Assuming whole‐path attenuation, the attenuation spectrum B ij ( f ) can be written as where α is frequency‐dependent factor [e.g., Scherbaum , ; Eberhart‐Phillips and Chadwick , ; Eberhart‐Phillips et al ., ; Ko et al ., ]. Using equations and , equation can be rewritten as if we can remove or minimize the effects of the site response T j ( f ) and the instrumental response I j ( f ).…”

Subduction Zone Tomography

“…3.15b) has a much lower resolution, but low-Q anomalies are revealed down to a depth of ~ 250 km beneath the Tonga arc and the Lau back-arc spreading center, which is generally consistent with the velocity image. Ko et al (2012) has revealed high-Q anomalies in the mantle wedge beneath the SW edge of the Ryukyu arc, suggesting the presence there of a cold and dynamically sluggish edge environment due to a low temperature and probably low water content. These features may result from the coupling of the slab laterally with the thick Eurasian lithosphere, which inhibits back-arc rifting, retards subduction, and reduces the water supply to the mantle wedge.…”

“…These features may result from the coupling of the slab laterally with the thick Eurasian lithosphere, which inhibits back-arc rifting, retards subduction, and reduces the water supply to the mantle wedge. The SW Ryukyu arc represents a subduction-zone edge type distinct from more commonly documented free or warm edges (Ko et al 2012). …”

“…In the earlier stages of seismic tomography, up until the late 1990s, only a few 3-D attenuation (Q) models were determined for subduction zones, most of which focused on the Japan Islands (e.g., Umino and Hasegawa 1984;Furumura and Moriya 1990;Sekiguchi 1991). Since the end of the 1990s, however, there have been rapid advances in Q tomography, and the 3-D Q structure of many subduction zones has been studied, including Tonga (Roth et al 1999(Roth et al , 2000, New Zealand (Eberhart-Phillips et al 2008), Japan (Tsumura et al 2000;Salah and Zhao 2003b;Nakamura et al 2006;Nakajima et al 2013;Liu et al 2014), Alaska (Stachnik et al 2004;Abers et al 2006), Izu-Bonin (Shito et al 2009), Mariana (Pozgay et al 2009), Taiwan Ko et al 2012;Cheng 2013), Central America (Rychert et al 2008;Chen and Clayton 2012;Dominguez and Davis 2013), South America (Myers et al 1998;Haberland and Rietbrock 2001), and Himalaya (Sheehan et al 2014). According to these studies, the subducting oceanic slabs generally show a very weak attenuation with Q values of up to 1000-1500, whereas the mantle wedge and the crust beneath active arc volcanoes show a very strong attenuation with Q values of 100 or smaller.…”