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or collective redistirbution other or collective or means this reposting, of machine, is by photocopy article only anypermitted of with portion the approval The O of This article has been published in published been This has article in the

Seamount , Volume 1, a quarterly 23, Number The O journal of and

By Anthony B. Watts, Anthony A.P. Koppers, and David P. Robinson ceanography S ociety. Daiichi-Kashima ©

Seamount 2010 by The O Japan EURASIAN

PLATE ceanography S A ceanography S

B ociety. A Mw = 7.0 1982 Permission is reserved. ll rights granted to in teaching copy this and research. for use article R ociety. S PACIFIC Figure 1. Composite figure illustrating

the morphology of Daiichi-Kashima end all correspondence to: [email protected] Th e O or 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 that splits the seamount vertically and offsets its once- 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 ceanography S 5.0 5.0 The red-filled star shows the approximate 6.0 location of the 1982 Mw = 7.0 c) 10 (Mochizuki et al., 2008). (c) Deep seismic km structure (Nishizawa, et al. 2009). The light ociety, P a) 6.0 brown contour lines show the P-wave

7.0 1931, R O Box velocity in km s-1. The estimated extent of the surface volcanic load and underlying A 15 epublication, systemmatic reproduction, flexed oceanic are shown as brown

and grey shaded regions, respectively. M D 20849-1931, USA ockville, B 6.0

100 km 7.0 .

166 Oceanography Vol.23, No.1 Abstract. are ubiquitous features of the seafloor that form part of however, if the resulting increased the fabric of . When a seamount enters a subduction zone, it has a major surface roughness acts to locally increase affect on morphology, the uplift history of the arc, and the structure of friction in the vicinity of a subducted the downgoing . 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 . 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 . 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 the surrounding seafloor (Wessel et al., especially the size and frequency of large Shipboard 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., (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 , 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 , 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 and Java- or low friction. In the case of low fric- Tonga- (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 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- (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 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 (the Ridge and Cascadia seamounts are swath bathymetry data. These data at the so-called ) 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 (Figure 2). sediment, and the depth to the crystal- Ocean Drilling Program core samples line is relatively shallow. show that these seamounts comprise Anthony B. Watts (tony@. A good example of a subducted serpentinites (see Spotlight 9 on page 174 ox.ac.uk) is Professor of seamount is Muroto in the Nankai accre- of this issue [Wheat et al., 2010]) and and , Department of Earth tionary wedge offshore Southwest Japan. other 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 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 N

SCARS SCAR

TRENCH Fisher Seamount SEA- Quepos MOUNTS 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 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. (2000)

168 Oceanography Vol.23, No.1

Figure 2 Watts, Koppers and Robinson 2009 In addition to modifying the upper shallow depths (Figure 3a), whereas oceanic plate generally increases with age, shallow part of the forearc, seamount if it is low relative to thick- the long-term rheological properties of subduction may modify its lower deep ness, it may be carried some distance the overriding plate are unknown. Many part (Dominguez et al., 1998). For and decapitated at greater depths forearcs (e.g., ) comprise an example, it may cause tectonic , when it jams up against the roof of the accretionary wedge that is assumed to be steepening any overlying thrusts and channel (Figure 3b). an intrinsically weak material. Subducted folds and releasing and fluids seamounts might therefore plow through into the underlying subduction zone Relative Strength of the the forearc, experiencing little or no (Bangs et al., 2006), and it may modify Subducting and Overriding Plates deformation. Others (e.g., Cascadia), the and uplift history of a It has long been accepted that the nega- however, reveal a forearc with a distinct forearc (Lallemand and Le Pichon, 1987; tive buoyancy (i.e., sinking) of oceanic crystalline basement structure, which Lallemand et al., 1989; Dominguez et al., initiates subduction (see would be much stronger and offer more 1998; Oakley et al., 2008). A seamount Koppers and Watts, 2010). Although it is resistance, causing a subducted seamount chain, rather than an isolated seamount, known that the strength of a subducting to jam and possibly break up. might induce a “deformation wave” that alternately inflates and deflates a forearc as the seamounts are progressively a) SHALLOW carried into the subduction zone by plate DECAPITATION motion (Laursen et al., 2002). Forearc Thin Layer Sediment The Mechanics of Thickening Seamount Subduction Subduction But what actually happens to a seamount Channel when it is subducted? In particular, what determines whether a seamount Subducting Oceanic Crust Trench is broken up and accreted to the Axis forearc or is carried into a subduc- TRUNCATED SEAMOUNT tion zone more or less intact? Several factors maybe involved. b)

Thickness of the Forearc Thick Layer Subduction Channel Sediment Seismic images of the subduction channel off Ecuador (Sage et al., 2006) Thinning Subduction show a layer of poorly consolidated and Channel intensively sheared sediment that has Subducting been dragged down by the subducting Oceanic Crust Trench plate beneath the overriding plate. Cloos Axis DEEP and Shreve (1996) suggest that it is the DECAPITATION ratio of the relative thickness of the sedi- Figure 3. Schematic diagram illustrating the Cloos and Shreve (1996) model for seamount subduc- ments within the channel to the height tion. (a) Mariana-type case where a relatively large seamount is carried into a subduction zone with a of a seamount that determines its fate. If relatively thin subduction channel. The seamount is truncated at relatively shallow depths and, hence, low confining pressures, so there is little or no seismicity. (b) Chilean-type case where a relatively small the seamount is high relative to channel seamount is carried into a subduction zone with a relatively thick subduction channel. The seamount, in thickness, it might be decapitated at this case, is truncated at relatively deep depths and high confining pressures, which leads to seismicity.

Oceanography March 2010 169 Internal Structure of between the edifice and oceanic crust intrusion. These structures would be Subducted Seamounts may already have acted as a décolle- much more difficult to accommodate in Seismic studies reveal that the volcanic ment surface during growth a subduction channel. edifice of a seamount builds up and out (Got et al., 2008). Some seamounts on a gently flexed, uniformly layered, and oceanic , however, have Buoyancy of oceanic crust. It is easy to envisage how complex internal structures with Subducted Seamounts such a structure would deform when it dense volcanic cores (e.g., Tenerife; Oceanic flexure studies show that reaches a trench because the flexed layers Canales et al., 2000) and evidence of seamounts are compensated differently of the edifice and underlying crust will both upper (e.g., Daiichi-Kashima at depth, depending on whether they be rotated and sheared as they enter a Seamount; Nishizawa et al., 2009) and formed on a weak plate near a mid- subduction zone. Deformation may be lower (e.g., Louisville Ridge; Contreras- ocean ridge or on a strong plate in a plate facilitated by the fact that the interface Reyes et al., 2010) oceanic crustal interior (see Koppers and Watts, 2010). As a result, seamounts will be in different Trench states of crustal buoyancy as they enter Axis trenches (Figure 4). For example, a seamount on a strong plate would be 0 Outer Rise regionally supported and, hence, have had much of its compensation removed Subducting before its arrival at a trench. Because it Overriding Oceanic Plate 10 Plate is less buoyant, such a seamount would m) be less likely to lift up the forearc and, Seamount Formed a) thus, more weakly coupled to the over- on a Strong Plate

Depth (k riding plate. A seamount formed on 20 Te = 25 km a weak plate, however, is more locally km compensated and so retains more of its buoyancy and is more likely to jam 30 0 100 200 a subduction zone. Outer 0 Rise Seamounts and the Subducting Rupture Histories of Overriding Oceanic Plate Large Subduction Zone 10 Plate

m) Earthquakes b) Seamount Formed It has been more 35 years since on a Weak Plate

Depth (k Kanamori (1971) first suggested that 20 Te = 5 km large earthquakes (i.e., Mw > 8.0) at convergent plate boundaries reflect the degree of coupling between the over- 30 riding and subducting plates. In his Figure 4. Simple model for the crustal structure of two seamounts—one formed off- view, some arc systems were strongly ridge and the other on-ridge—that are about to enter a subduction zone. (a) Off-ridge coupled (e.g., Aleutian) and generated seamounts are more regionally compensated so that much of their support would have already been removed by subduction. These seamounts are therefore less buoyant large earthquakes when they finally (small orange arrow) and thus less well coupled to the forearc when they enter a slipped, while others (e.g., Izu-Bonin and subduction zone. (b) On-ridge seamounts are locally more compensated so that little Mariana) were weakly coupled and large of their support would have been removed. These seamounts are therefore more buoyant (large orange arrow) and thus better coupled to the forearc. earthquakes were either rare or absent.

170 Oceanography Vol.23, No.1 Kelleher and McCann (1976) noted, sediment in the subduction channel that aftershocks, bathymetric features on the however, that many arc-trench systems is intersected at 26°S by the Louisville subducting plate acted as an asperity were highly segmented with regard to Ridge seamount chain. The intersection during the rupture. their seismicity. Some segments were is characterized by a shoaling of the The degree of coupling between the active on historical time scales, while trench and a ~ 170-km-long seismic overriding and subducting plates may others were not (so-called “seismic gap”). gap. The gap has been interpreted as a not always be strong over a seamount, Because the seafloor varies spatially locally well-coupled zone that is seismi- even if it is locally compensated and in roughness, Kelleher and McCann cally locked and is of sufficient length to buoyant. Mochizuki et al. (2008) studied

(1976) suggested that there might be generate an Mw > 8.0 earthquake in the a large earthquake (1982, Mw = 7) in the a link between large earthquakes and future when it slips (Scholz and Small, Japan forearc where an active-source the morphology of the subducting 1997). Indeed, several great earthquakes seismic experiment revealed a subducted oceanic plate. In particular, they noted have already occurred along other parts seamount at a depth of ~ 10 km. that the size and frequency of large of the Tonga-Kermadec arc. Mochizuki et al. (2008) found, however, earthquakes were reduced in regions Since 2000, there has been a rapid only a few aftershocks at depths corre- of thick oceanic crust where aseismic increase in our knowledge of the sponding to the top of the seamount, ridges and other “bathymetric highs” rupture history of large subduction suggesting weak coupling between the (i.e., seamount chains) were about to zone earthquakes, the deep seismic subducting and overriding plates. They enter a subduction zone. structure of forearc overriding plates, attributed the weak coupling in what is Cloos (1993) argued that only the and the morphology of subducting otherwise a strongly coupled arc system thickest continental “blocks” (30 km oceanic plates. Kodaira et al. (2000) to tectonic erosion that resulted in a and thicker) would be buoyant enough used an integrated data set of earth- transfer of fluids from the forearc into to resist subduction. However, Cloos quake aftershock relocations, seismic the underlying subduction channel, and Shreve (1996) pointed out that large refraction, and swath bathymetry thereby lubricating and weakening the seamounts could be subducted to great data to suggest that seamounts acted contact surface between the seamount enough depths to cause an earthquake if as a barrier to the rupture zone that and the overriding plate. the sediment in the subduction channel generated the 1946 Mw = 8.2 Nankai Recently, Das and Watts (2009) is sufficiently thick (Figure 4). In such earthquake, while Husen et al. (2002) reviewed the rupture histories of the 1986 a case, subducted seamounts act as a suggested that seamounts acted as Mw = 8.0 Andreanof Island, , and strong asperity (or sticking point), such an asperity during the 1990 Mw = 7 the 2001 Mw = 8.4 Arequipa, southern that the fault ruptures sporadically when of Nicoya earthquake. Peru, earthquakes. These earthquakes frictional resistance is overcome. Bilek et al. (2003) studied the rupture ruptured 28,000 and 60,000 km2 areas of Scholz and Small (1997) reconsidered history of three large subduction zone the forearc from shallow depths to depths the role played by subducted seamounts earthquakes at the Costa Rica margin of ~ 45 km and 35–40 km, respectively in coupled and uncoupled and well and where both seismic and swath bathym- (Das and Kostrov, 1990; Robinson et al., poorly sedimented arc-trench systems. etry surveys have been carried out. They 2006). In the Andreanof Island earth- For coupled arcs (e.g., Aleutian), they showed that the epicenters of the 1990 quake, high-slip areas within the rupture argue that a large seamount would Mw = 7.0 Gulf of Nicoya and the 1999 surface line up with relative plate motions increase the coupling between the Mw = 6.9 Quepos earthquakes line and with a cluster of small seamounts subducting and overriding plates even up with the Fisher Seamount and the on the subducting Pacific oceanic plate more due its buoyancy. For decoupled Quepos Plateau on the subducting Cocos (Figure 5). In southern Peru, however, arcs (e.g., Izu-Bonin and Mariana), a oceanic plate (Figure 2). Both earth- seamounts on the subducting large seamount would only increase the quakes had a relatively simple rupture appear to have acted as a barrier that held coupling locally. The Tonga-Kermadec history and Bilek et al. (2003) suggest up the earthquake for some 18 seconds. trench is a decoupled arc with very little that because of the limited extent of the

Oceanography March 2010 171 54 be subducted at the ends of their EURASIAN cycles. The hundreds of thousands of PLATE seamounts still riding the plates today will eventually enter subduction zones, where they will be deformed and may be Aleutian destroyed. The small number of studies Islands that have been carried out thus far 52 suggest that seamount subduction is an Andreanof Earthquake important geological process that affects Mw = 8.0, 1986 the morphology, structure, and vertical- H motion history of subduction zones ENC 800 TR and, most notably, the rupture history of large earthquakes. 0

Future work should focus on more Trench Parallel Trench -800

Topography (m) Topography swath bathymetry mapping and better 50 Seamounts seismic imaging of the deep structure of seamounts on the subducting plate and PACIFIC in the forearc region. Field mapping, PLATE not only of subducted, but obducted Seismic Slip Intensity seamounts (e.g., central Oman; Searle, 1983) in belts, would also be Amelia 0.0 0.8 1.6 2.4 3.2 4.0 4.8 6.0 useful in order to better understand the 48 internal structure of seamounts. We also -178 -176 -174 -172 -170 need more accurate slip measurements,

Figure 5. The final slip distribution of the 1986M w = 8.0 Andreanof Islands, Alaska, earthquake better aftershock locations, and rupture superimposed on a shaded relief GEBCO bathymetry map. Based on Das and Kostrov (1990) histories of large earthquakes. Finally, and Das and Watts (2009). The red-filled star shows the region where the rupture initiated. The arrow shows the convergence direction between the Pacific and Eurasian plates. The solid black we will require more analogue and lines show the bathymetry along a trench-parallel profile constructed from the GEBCO grid. The numerical modeling studies of seamount figure shows that the region of highest slip during the earthquake aligns with a number of small subduction using realistic rheologies. seamounts that rise up to ~ 800 m above the depth of the surrounding seafloor.

Acknowledgements We thank Shamita Das for providing the total slip data for the Andreanof Conclusions and seamounts, but it is interesting to specu- Island earthquake. We also thank Roland Future Work late on the effect of a seamount chain. von Huene and Hubert Staudigel for This review suggests that seamount A chain could potentially decouple one detailed and very useful reviews of subduction is linked in some way segment of a forearc (on one side of the Figurean early 5 version of this paper. AAPK Watts, Koppers and Robinson 2009 with the seismicity of convergent plate chain) from another. Also, seamount was supported by the National Science boundaries. It is not presently clear, chains that form a continuous feature Foundation through the SBN Research however, whether subducted seamounts across the have the Coordination Network (EF-0443337). cause or promote large earthquakes potential to severely restrict the spatial DPR is supported by NERC award or simply hold them up until the next extent of the slip plane, preventing any NE/C518806/1. one occurs. Here, we have mainly been rupture across them. considering the subduction of isolated It is inevitable that seamounts will

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