or collective redistirbution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The approval portionthe ofwith any articlepermitted only photocopy by is of machine, reposting, this means or collective or other redistirbution This article has This been published in MOUNTAINS IN THE SEA Oceanography Seamount Subduction journal of The 23, Number 1, a quarterly , Volume and Earthquakes BY AnTHONY B. WATTS, AnTHONY A.P. KOppERS, AND DAVID P. ROBINSON O ceanography ceanography S ociety. ociety. Daiichi-Kashima © Seamount The 2010 by Japan EURASIAN Trench O PLATE ceanography A O ceanography ceanography S B ociety. Mw = 7.0 A ll rights reserved. Permission is granted to copy this article for use in teaching and research. article for use and research. this copy in teaching to granted ll rights reserved. is Permission 1982 S ociety. ociety. PACIFIC Figure 1. Composite figure illustrating S the morphology of Daiichi-Kashima or Th e [email protected] to: correspondence all end PLATE Seamount, which is about to enter the b) Japan subduction zone. (a) Schematic model. Based on Figure 2 in Mogi and Nishizawa (1980) (b) Perspective view 4 km showing the trench-parallel fault that splits the seamount vertically and offsets its once-flat top into two gently tilted 2.0 surfaces, A and B. Based on swath bathym- etry data in Lallemand, et al. (1989), 3.0 5 8 km 2.0 Dominguez et al. (1998) and http://www. 4.0 utdallas.edu/~rjstern/pdfs/TrenchProof.pdf O ceanography ceanography 5.0 5.0 The red-filled star shows the approximate 6.0 location of the 1982 Mw = 7.0 earthquake c) 10 (Mochizuki et al., 2008). (c) Deep seismic S km structure (Nishizawa, et al. 2009). The light P ociety, a) 6.0 brown contour lines show the P-wave 7.0 -1 O velocity in km s . The estimated extent of Box 1931, the surface volcanic load and underlying R A 15 reproduction, systemmatic epublication, flexed oceanic crust are shown as brown R and grey shaded regions, respectively. ockville, M B 6.0 D 100 km 20849-1931, 7.0 USA . 166 Oceanography Vol.23, No.1 AbSTRACT. Seamounts are ubiquitous features of the seafloor that form part of however, if the resulting increased the fabric of oceanic crust. When a seamount enters a subduction zone, it has a major surface roughness acts to locally increase affect on forearc morphology, the uplift history of the island arc, and the structure of friction in the vicinity of a subducted the downgoing slab. It is not known, however, what controls whether a seamount is seamount or if the increased presence of accreted to the forearc or carried down into the subduction zone and recycled into subducted fluids acts to locally reduce it. the deep mantle. Of societal interest is the role seamounts play in geohazards, in In this paper, we briefly discuss particular, the generation of large earthquakes. the shape and structure of subducted seamounts, what controls whether they InTRODUCTION affect the morphology, structure, and are accreted or subducted, and the links The seafloor is littered with seamounts, vertical motion history of the forearc that might exist between seamounts and most of which are small (Hillier and region between the trench axis and the the rupture history of large subduction Watts, 2007). Satellite-derived gravity volcanic arc. Seamount subduction may zone earthquakes. anomaly data, however, suggest that there also influence the degree of coupling maybe as many as 12,000 large seamounts between the overriding and subducting SHAPE AND STRUCTURE OF that rise > 1.5 km above the depth of plates and may affect the seismicity, SEAMOUNTS NEAR TREncHES the surrounding seafloor (Wessel et al., especially the size and frequency of large Shipboard bathymetry profiles show 2010). Irrespective of whether these earthquakes (Kelleher and McCann, that seamounts about to enter Pacific seamounts are growing up during their 1976). Recently, Nishizawa et al. (2009) subduction zones range in height from volcanic construction or are sinking, and Das and Watts (2009) demonstrated 1–5 km, have widths from 20–100 km, once they become inactive, they are being that seamounts play a major role in and have both pointed and gently curved carried along by plate motions and will controlling the rupture history of large or flat tops. Surveys of Daiichi-Kashima eventually be subducted (Staudigel and earthquakes, in particular, acting as Seamount at the southern end of the Clague, 2010). Indeed, there are many either barriers (e.g., Kodaira et al., Japan trench (Mogi and Nishizawa, present-day examples of seamounts that 2000) or as asperities (e.g., Husen et al., 1980) show that it is bisected by a steep, are in the throes of being subducted. 2002) to earthquakes. A subducted trench-parallel normal fault that has Niue Seamount (DuBois et al., 1975) seamount may prevent earthquakes from vertically offset its flat top by ~ 1.5 km in the Southwest Pacific Ocean, and proceeding or propagating through an (Figure 1). Similar-trending faults have Christmas Seamount (Woodroffe et al., area. Such a barrier could have either been reported from other seamounts in 1990) in the Indian Ocean, for example, very high friction, high enough to the vicinity of the Mariana-Izu Bonin are being uplifted as they ride the flexural prevent the earthquake proceeding, trenches (Fryer and Smoot, 1985) and bulge seaward of the Tonga and Java- or low friction. In the case of low fric- Tonga-Kermadec trench (Coulbourn Sumatra trenches. Osbourn (Lonsdale, tion, the area does not store significant et al., 1989). Only a few deep seismic 1986) and Bourgainville (DuBois et al., stress during the interseismic period, as studies of trench seamounts exist, but 1988) in the Southwest Pacific Ocean are stress is released aseismically or in small recent surveys over Daiichi-Kashima seamounts that have been tilted a few earthquakes. The absence of stress results (Nishizawa et al., 2009) and the degrees as they move downslope toward in prevention of earthquake propaga- Louisville Ridge (Contreras-Reyes the Tonga-Kermadec and New Hebrides tion. An asperity is an area with locally et al., 2010) show these seamounts are trench axes. Finally, Erimo and Daiichi- increased friction. Typically, due to the underlain by thickened crust. Moreover, Kashima (Nishizawa et al., 2009) in the increased friction, an asperity exhibits a P-wave velocity contours are elevated, West Pacific Ocean are presently located reduced amount of interseismic aseismic suggesting that these seamounts have in the Japan trench axis. slip relative to the surrounding regions relatively dense cores and that the Once a seamount enters a subduction and hence will slip an increased amount underlying flexed crust may have been zone, it would be expected to profoundly during an earthquake. It is unknown, intruded by magmatic material. Oceanography March 2010 167 FOREARCS AND BURIED Ridge (von Huene et al., 1997), Cascadia prior to entering the subduction zone. SEAMOUNTS (Wells et al., 2009), and Mariana (Oakley Perhaps the most striking manifesta- Many forearcs comprise a thick sequence et al., 2007, 2008). The Mediterranean tion of seamount subduction is seen in of highly deformed sediment (the Ridge and Cascadia seamounts are swath bathymetry data. These data at the so-called accretionary wedge) that has marked by gravity and magnetic anomaly Costa Rica margin show mass-wasting been scraped off the subducting plate highs. However, similar data over the features such as head scars, slumps, and and structurally deformed by folding seamounts in the Mariana forearc are slides that align with seamounts on the and thrusting. Others, however, have less associated with lows. Dredge haul and subducting Cocos Plate (Figure 2). sediment, and the depth to the crystal- Ocean Drilling Program core samples line basement is relatively shallow. show that these seamounts comprise Anthony B. Watts (tony@earth. A good example of a subducted serpentinites (see Spotlight 9 on page 174 ox.ac.uk) is Professor of Marine Geology seamount is Muroto in the Nankai accre- of this issue [Wheat et al., 2010]) and and Geophysics, Department of Earth tionary wedge offshore Southwest Japan. other rock types, including muds, that Sciences, University of Oxford, Oxford, Seismic data show that this seamount is have flowed out onto the forearc sedi- UK. Anthony A.P. Koppers is Associate ~ 50-km wide at its base, ~ 25-km wide ments. Geochemical sampling of these Professor, College of Oceanic & Atmospheric at its top and 4-km high, and is underlain muds suggest that the serpentinite proto- Sciences, Oregon State University, Corvallis, by flexed oceanic crust (Kodaira, et al., liths might be basalts that formed on or OR, USA. David P. Robinson is Postdoctoral 2000). Other forearcs where seamounts near a mid-ocean ridge and therefore that Researcher, Department of Earth Sciences, have been imaged include Mediterranean these seamounts may have been altered University of Oxford, Oxford, UK. Quepos Earthquake M = 6.9, 1999 Nicoya Earthquake w Mw = 7.0, 1990 CONTINENTAL SHELF 30 km NORTH AMERICAN PLATE N SCARS SCAR TRENCH Fisher Seamount SEA- Quepos MOUNTS Plateau COCOS PLATE Figure 2. Perspective view of the Costa Rica margin showing the morphology of the subducting Cocos oceanic plate and the overriding North American Plate, constructed using swath bathymetry data acquired onboard MV Sonne. Seamounts on the subducting plate align with landslide scars on the overriding plate, suggesting that subducting seamounts are able to “plow through” the forearc and generate large-scale slumps and slides. The red-filled stars show the projected locations of 1990M w = 7.0 and 1999 Mw = 6.9 earthquakes (Bilek et al., 2003). Based on von Huene et al.