Description: The Mw = 9.0 earthquake of 11 March 2011 at the Japan Trench and its devastating tsunami underscore the importance of understanding seismogenic behavior of subduction faults and realistically estimating the potential size of future earthquakes and tsunamis. For the Cascadia subduction zone (Figure 1a), a critical knowledge gap is the level of microseismicity offshore, especially near the megathrust, needed to better understand the state of the locked zone. In 2010 the first detailed seafloor earthquake monitoring campaign along the northern Cascadia subduction zone recorded nearby earthquakes in the local magnitude (ML) range from possibly around zero to 3.8 (Figures 1b and 1c) and larger earthquakes from outside this region. Preliminary analyses indicate that the network appears to have yielded a fairly complete catalog for events with ML 〉 1.2. Only a few tens of these events occurred beneath the continental shelf and slope (Figure 1a). The majority of the earthquakes were located along the margin-perpendicular Nootka fault zone. The relatively low seismicity away from the Nootka fault is consistent with a fully locked megathrust. Land-based GPS measurements cannot resolve the question of whether the offshore part of the megathrust seismogenic zone is narrow and fully locked or wider and only partially locked (slowly creeping). If it were only partially locked, the seafloor seismometer data should show many more small earthquakes along the interface than were actually detected.

Description: Structure and growth of the Izu‐Bonin‐Mariana arc crust: 1. Seismic constraint on crust and mantle structure of the Mariana arc–back‐arc system Narumi Takahashi Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan Shuichi Kodaira Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan Yoshiyuki Tatsumi Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan Yoshiyuki Kaneda Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan Kiyoshi Suyehiro Institute for Research on Earth Evolution Japan Agency for Marine‐Earth Science and Technology Kanagawa Japan A high‐resolution seismic velocity model is presented for the crust and upper mantle of the Mariana arc–back‐arc system (MABS) based on active source seismic profiling. The major characteristics are (1) slow mantle velocity of 〈8 km s −1 in the uppermost mantle, especially, and deep reflectors under the Mariana arc (MA) and the West Mariana Ridge (WMR), (2) a deep reflector in the upper mantle beneath the relative thick crust of the Mariana Trough (MT) axis, (3) distribution of lower‐velocity lower crusts (6.7–6.9 km s −1 ) beneath the volcanic front and adjacent to the MT, and (4) high‐velocity lower crust (7.2–7.4 km s −1 ) beneath the boundary regions between the MA and MT, and between the WMR and the Parece Vela Basin (PVB), adding to structural characteristics of crust and upper mantle beneath the MABS. Of the characteristics described above, characteristic 1 suggests that the origins of the slow mantle velocity and the deep reflectors be explained by transfer of the lower crustal residues to the upper mantle across the Moho, considering that the WMR is extinct arc currently. On the other hand, characteristic 2 suggests that the origin of deep reflectors beneath the MT axis might be lower velocity materials due to the diffractive signals with strong amplitudes, characteristic 3 suggests that the lower‐velocity lower crust advanced crustal growth and characteristic 4 suggests that the high‐velocity lower crust beneath arc–back‐arc transition zone is composed of mafic/ultramafic materials created by extensive partial melting of mantle peridotites or last stage of the arc magmatism rather than serpentinized peridotite.

Description: Arc‐backarc systems are inherently shaped by subduction, representing an essential window into processes acting in the Earth's interior such as the recycling of subducted slabs. Furthermore, they are setting where new crust is formed and are believed to be sites where juvenile continental crust emerges. We present a seismic refraction and wide‐angle velocity model across the Izu arc‐backarc system, and use its characteristic features to constrain geochemically and petrologically different compartments, revealing processes governing crustal formation overlying subduction zones. Our result delineates the Izu arc with a maximum thickness of ∼20 km and the Shikoku Basin with thicknesses of ∼7 to 11 km. In the volcanic arc, the middle crust of the felsic to intermediate tonalitic layer (6.0–6.5 km/s) is remarkably thicker beneath the basalt‐dominated area than in the rhyolite‐dominated area, indicating that basaltic volcanism is indispensable in the transformation process from arc to continental crust. However, rhyolitic volcanism may relate to the juvenile stage of arc evolution or the remelting of middle crust due to the insufficient supply of basaltic magma from the mantle. The mafic restite and cumulates, which used to be part of the arc crustal material, are delaminated and foundered into the mantle, forming extremely low mantle velocities (〈7.5 km/s). In the Shikoku Basin, our result supports a fertile mantle source with passive upwelling and normal temperature during the opening process, but the lack of high velocity in the lower crust rules out hydrous melts entrained from the subducting slab or anomalous mantle trapped during subduction zone reconfiguration. Plain Language Summary As a vital factor in supporting the conditions for the evolution of life and ecosystems, the origin and evolution of the continents are still enigmatic. Volcanic arcs are generally seen as a place for creating continental crust while recycling the incoming subducting slab. In this study, we present a seismic velocity structure model across the Izu arc and Shikoku Basin, offshore south of Japan, to demonstrate the rules contained behind the transformation from arc to continental crust. Our results support that basaltic volcanism in the volcanic arc nurtures the generation of felsic to intermediate rocks, which provides the bulk of the continental crust. During this process, other anti‐continent materials, like mafic rocks, tend to be foundered into the mantle. Therefore, we propose that constant basaltic volcanism is critical in transferring arc crust to continental crust. Key Points A long seismic refraction and wide‐angle profile presents the seismic structure across the Izu arc and Shikoku Basin The transformation from arc to continental crust is closely associated with basaltic volcanism from the rear arc to volcanic front Passive melting of a fertile mantle source under normal temperature governs the opening of the Shikoku Basin