Earth-Science Reviews 141 (2015) 4555

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Earth-Science Reviews

j ourna l homepage: www.e lsev ie r .com/ locate /earsc i revCoseismic slip on shallow dcollement megathrusts: implications forseismic and tsunami hazardJudith Hubbard , Sylvain Barbot, Emma M. Hill, Paul TapponnierEarth Observatory of Singapore, 50 Nanyang Avenue, Nanyang Technological University, 639798, Singapore Corresponding author. Tel.: +65 9855 2730, +65E-mail addresses: judith.a.hubbard@gmail.com (J. H

http://dx.doi.org/10.1016/j.earscirev.2014.11.0030012-8252/ 2014 Elsevier B.V. All rights reserved.a b s t r a c ta r t i c l e i n f oArticle history:Received 24 July 2014Accepted 8 November 2014Available online 15 November 2014

Keywords:EarthquakesDcollementSubduction zoneFold-and-thrust beltSeismogenic zoneFaultsFor years,many studies of subduction zones and on-land fold-and-thrust belts have assumed that the frontal por-tions of accretionary prisms are tooweak to rupture coseismically andmust therefore be fully creeping.We pres-ent a series of examples, both on-land and offshore, demonstrating that inmany cases, shallow dcollements arecapable of large, coseismic slip events that rupture to the toes of the fault systems. Some of these events are as-sociatedwith ruptures that initiate down-dip, while others appear to be limited to the frontal, shallow portion ofthe wedge.We suggest that this behavior is not limited to the examples described here, but rather is common tomany (per-haps most) accretionary wedges and fold-and-thrust belts around the world. Indeed, there may be many otherexamples of similar earthquakes, where existing data cannot constrain slip at the toe. We do not characterizethe regions and events described here as unusual, as they encompass a wide range of settings. This study indi-cates that there is an urgent need to reevaluate seismic and tsunami hazard in fold-and-thrust belts and subduc-tion zones around the world, allowing for the possibility of shallow dcollement rupture.

2014 Elsevier B.V. All rights reserved.Contents1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462. Evidence for shallow slip in megathrust earthquakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

2.1. The Himalaya . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472.2. Bolivia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482.3. Western Taiwan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492.4. Santa Barbara Channel, Southern California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492.5. Sumatra subduction zone, Mentawai segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502.6. Sumatra subduction zone, northern segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502.7. Java subduction zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502.8. Japan Trench . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502.9. Kuril Trench . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502.10. Japan, Nankai Trough . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502.11. Solomon Islands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512.12. Middle America megathrust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512.13. Alaskan Trench . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512.14. Peru . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512.15. Chile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512.16. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3. How can weak decollements behave seismically? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513.1. The state of stress within the wedge, and preferred slip planes at geological time scales . . . . . . . . . . . . . . . . . . . . . . . . . 523.2. Variations in strength through the earthquake cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

4. Consequences of seismogenic decollements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536592 7537.ubbard), sylbar.vainbot@gmail.com (S. Barbot), ehill@ntu.edu.sg (E.M. Hill), tappon@ntu.edu.sg (P. Tapponnier).

http://crossmark.crossref.org/dialog/?doi=10.1016/j.earscirev.2014.11.003&domain=pdfhttp://dx.doi.org/10.1016/j.earscirev.2014.11.003mailto:judith.a.hubbard@gmail.commailto:sylbar.vainbot@gmail.commailto:ehill@ntu.edu.sgmailto:tappon@ntu.edu.sghttp://dx.doi.org/10.1016/j.earscirev.2014.11.003http://www.sciencedirect.com/science/journal/00128252www.elsevier.com/locate/earscirev

46 J. Hubbard et al. / Earth-Science Reviews 141 (2015) 45555. Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 531. Introduction

Many regions of plate convergence are underlain by dcollementmegathrusts. They form the base of both accretionary wedges and fold-and-thrust belts. These faults may extend laterally for hundreds or thou-sands of kilometers, and downdip for tens to hundreds of kilometers. Tra-ditionally, estimates of seismic hazard have assumed that these faults slipaseismically, without radiating significant seismic energy (e.g., Pachecoet al., 1993; Hyndman et al., 1997; Oleskevich et al., 1999). However, inseveral recent cases, shallow dcollements have been shown to slip inlarge, discrete events (e.g., 2011 Mw 9.0 Tohoku-Oki earthquake, Japan;2010 Mw 7.8 Mentawai earthquake, Indonesia; 1999 Mw 7.6 Chi-Chiearthquake, Taiwan). On land, dcollements frequently lie along the bor-ders of large, populated basins, and therefore pose an important seismichazard, threatening such large cities as Dhaka (Bangladesh), Chengdu(China), Baghdad (Iraq), and Delhi (India). Offshore, they form thelower boundaries of accretionary prisms in subduction zones, and shouldbe considered in seismic and tsunami hazard assessment in subductionzones around the world.

Unlike reverse faults, which form at dips of ~2060 according toboth observation and theory, dcollements dip gently, at angles of b110 (Davis et al., 1983). This is possible because these faults take advan-tage of preexisting weaknesses in the rock, forming along stratigraphichorizons with weak materials like salt or shale (Suppe, 2007; Hubbardet al., 2010), in some cases with high pore pressures (Behrmann et al.,1988; Bilotti and Shaw, 2005; Cubas et al., 2013). The existence andlong-term deformation associated with dcollements is understood notonly through observation (e.g., Ye et al., 1997; Adam et al., 2004;Moore et al., 2009; Morley et al., 2011), but also through laboratory(e.g., Malaveille, 2010; Graveleau et al., 2012), computer (e.g., Strayeret al., 2001; Burbidge and Braun, 2002), and theoretical modeling(Davis et al., 1983; Dahlen et al., 1984; Dahlen, 1990). Large, activedcollements are known to exist in regions both onshore (Himalayas;Taiwan; Bolivia; Bangladesh; Sichuan, China;) and offshore (Sumatra,Java, Japan, Peru, Cascadia, Antilles, Makran, Guatemala) (Davis et al.,1983; Bilotti and Shaw, 2005; Hubbard et al., 2010; Morley et al., 2011).

Many studies of seismic hazard have assumed that becausedcollements are weak, they are unable to support large stresses andstore sufficient elastic energy to produce hazardous earthquakes, butmust rather be fully creeping, generating only small and micro-earthquakes (e.g., Byrne et al., 1988; Hyndman et al., 1997). Studies ofsubduction zones have generally observed an updip limit to interplateseismicity that persists for several decades (the seismic front, Byrneet al., 1988; Fig. 1),which has led to the application of the term aseismicto the portion of the dcollement underlying the accretionary prism. Al-though this term is correctly applied in that we observe little seismicityin this region, it has also been taken to mean that this part of the wedgenever slips in association with moderate to large earthquakes an as-sumption rather than an observation.

Experimental studies of clay and gouge materials show that barerock surfaces and thin gouge layers exhibit potentially unstablevelocity-weakening behavior, while slip within thick gouge showsvelocity-strengthening behavior (Marone and Scholz, 1988). Theupdip limit has been inferred to be associated with a zone of thick,velocity-strengthening material in the accretionary prism that resistsrapid rupture (Marone and Scholz, 1988). Thus, the absence of observedmoderate to large earthquakes in this regionhas been used to infer a slipbehavior that in turn has been used to justify a stratigraphic model thatis consistent with creeping behavior.As a consequence, until recently, manymodels of both coseismic slipand interseismic coupling on subduction zones started with the as-sumption that there was no coseismic slip at the tip of the wedge, andthat the region updip of the seismic front was fully creeping (Byrneet al., 1988; Hyndman et al., 1997; Chlieh et al., 2007, 2008; LovelessandMeade, 2009). Inversions for coseismic slip or interseismic couplingoften appear to support the idea that the shallow region is creeping.However, these inversions often force the near-trench area of the faultto creep during the interseismic period or prevent it from slipping dur-ing earthquakes. Inferences about the kinematic behavior of the toe ofthe prism are therefore not usually directly supported by data, asdiscussed by Rhie et al. (2007) and Loveless and Meade (2011).

We agree that dcollements appear to have long-term weak behav-ior (Suppe, 2007), and that there is often a strong drop-off in recordedseismicity updip of the seismic front. However, neither of these observa-tions is a compelling reason to infer creeping behavior. Seismic behavioris possible for a weak fault: for example with a low effective confiningpressure of 50 MPa and a low effective coefficient of friction of 0.1,there is enough frictional resistance for a complete coseismic stressdrop of 5 MPa, a value larger than that for typical inter-plate earth-quakes (Venkataraman and Kanamori, 2004). The opposite can also betrue: a creeping fault may not be weak, but rather be creeping at ahigher shear stress than a stick-slip fault, averaged over the seismiccycle. And indeed, contrary to the suggestion that low seismicity indi-cates creeping behavior, current understanding of fault zones rathersuggests the opposite: we often expect to see seismicity where faultsare creeping, and little seismicity where they are locked (e.g., Rubinet al., 1999; Barbot et al., 2013). Further, these observations of limitedseismicity may be dependent on the earthquake cycle: in Sumatra, theseismic front is clear prior to the year 2000, but following the Mw 7.9earthquake in Southern Sumatra in 2000 (and the great earthquakesin 2004 and 2005), many earthquakes were recorded in the frontalpart of the system (Fig. 2: Aceh, Nias/Simeulue, andMentawai 2010 seg-ments). In addition to the temporal change in seismicity in Sumatra, wealso observe tremendous spatial variability, with portions of themegathrust exhibiting minimal seismicity everywhere, other showinga clear seismic front, and yet others generating earthquakes everywhereup to 300 km from the trench (Fig. 2). Thus, a single dcollement mayhave extremely variable seismic patterns both spatially and temporally.As described below, we can find evidence inmany dcollement systemsfor large, episodic slip events at the tips of wedge systems, often associ-ated with recorded earthquakes. This indicates that current models offully creeping behavior on these systems are flawed (Fig. 1).

Several recent earthquakes, including the 2010 Mw 7.8 Mentawaitsunami earthquake, Indonesia, and the 2011Mw 9.0 Tohoku-Oki earth-quake, Japan have prompted a reevaluation of the assumption of creep-ing behavior near the trench (Lay and Bilek, 2007; McCaffrey, 2008;Avouac, 2011; Faulkner et al., 2011; Loveless and Meade, 2011; Hillet al., 2012; Kozdon and Dunham, 2013) and the development of newmodels where creep and coseismic slip occur at different stages of theearthquake cycle (Noda and Lapusta, 2013). Here, we present a reviewof additional data from these and other dcollement systems aroundthe world, both on- and offshore, that support the inference that theycan rupture in earthquakes associated with large slip at their tips.

Although many studies implicitly treat subduction zones and conti-nental fold-and-thrust belts as different classes, we show throughoutthis paper that they are structurally similar (Fig. 3) and exhibit muchthe same behavior. This paper brings together perspectives from struc-tural geology, geodesy, and earthquake dynamics, both on-land and

Fig. 1. Schematic model of a subduction zone, showing the locations of different types of earthquakes. (A) Redrawn after Byrne et al. (1988). (B) Our model. We suggest that thedcollement at the base of the accretionary prism is seismogenic and/or capable of participating in large thrust events on the subduction interface. We suggest that the faults risingfrom the shallow dcollement are also capable of participating in earthquakes, either in addition to or instead of rupture all the way to the toe.

47J. Hubbard et al. / Earth-Science Reviews 141 (2015) 4555offshore.We suggest that exposed fold-and-thrust belts can provide im-portant constraints and observations that can be used to understand thebehavior of subduction zones, where observations are much morelimited.2. Evidence for shallow slip in megathrust earthquakes

Below, we describe a set of regions with large dcollements showingevidence for earthquakes that rupture to their tips in large slip events(Fig. 3; Shaw and Suppe, 1994; Ye et al., 1997; Lav and Avouac, 2000;Ranero et al., 2000; Yue et al., 2005; Singh et al., 2008; Moore et al.,2009; Uba et al., 2009; Shulgin et al., 2011; Singh et al., 2011; Hubbardet al, 2014). This evidence falls primarily into four categories: (1) lockingoccurs along the dcollement, with geodetic observations of strain accu-mulating downdip; (2) slip propagating to the toe of the system overtimescales of hundreds of years to tens of thousands of years, implyingthe toe is active; (3) slip that occurs episodically (i.e. not as interseismiccreep), and (4) slip associated with earthquakes, as opposed to justslow slip events (e.g., Meade and Loveless, 2009).2.1. The Himalaya

One of the most compelling cases for large earthquakes ondcollements is in the Himalaya. This ongoing continent-continentcollision is now primarily accommodated along a 2000-km-widesubhorizontal thrust that dips gently beneath the lesser Himalaya.In the Nepal Himalaya, the dcollement extends about 90 kmdowndip, and then steepens beneath the high Himalaya on a blindthrust ramp (Schelling and Arita, 1991; Jackson and Bilham, 1994;Pandey et al., 1995). Much like subduction zones, the Himalaya ex-hibits a seismic front, with intense microseismicity and frequentsmall earthquakes along the ramp, and limited seismicity along thedcollement itself (Pandey et al., 1995).

Geodetic measurements in the Himalaya demonstrate that thedcollement is currently locked from the surface to about 100 kmdowndip (Bilham et al., 1997; Larson et al., 1999; Jouanne et al., 2004;Betinelli et al., 2006; Ader et al., 2012). Over the ten thousand year time-scale, the intraplatemotion is accommodated bymigrating slip onto thepresently locked zone, andmost of this slip propagates to the toe, basedon uplifted terraces on the frontal fault of the system (Lav and Avouac,

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Fig. 2. (Left)Mapof Sumatra showing seismicity,filtered for thrust events (data extracted from theGlobal CMT catalog). Contours for the subduction zone are shownas thin black lines. Dashedlines show boundaries of regions for seismicity histograms (a-f; shown on the right). Focal mechanisms for earthquakes M N =7.0 are shown. (Right) Histograms of seismicity along the Su-matran megathrust seismicity (red) prior to the 2000 Mw 7.9 earthquake in Southern Sumatra and the later great earthquakes, and (black) from 20002014, for six different portions of themegathrust. Seismicity is binned into 10-km stripes down-dip. The megathrust demonstrates strong variability in seismicity, both temporally and spatially (along strike and down dip).

48 J. Hubbard et al. / Earth-Science Reviews 141 (2015) 45552000). This happens in discrete events with large amounts of slip at thetoe that can be tied to historical large earthquakes (Sapkota et al., 2013;Bollinger et al., 2014). Thus, in Nepal it appears not only that thedcollement is slipping in large events that reach the surface, but alsothat these events are radiating significant seismic energy.2.2. Bolivia

The eastern side of the Central Andes forms a backarc fold-and-thrust belt underlain by a shallowly dipping dcollement. Shorteningacross the region is estimated at ~7-13 mm/yr from both geological

Fig. 3. Schematic diagrams of continental fold-and-thrust belts and subduction zones from around the world that show evidence of shallow dcollement earthquakes, shown at a fixedscale. Most of the regions discussed in the text are represented here.

49J. Hubbard et al. / Earth-Science Reviews 141 (2015) 4555and geodetic data (Uba et al., 2009; Brooks et al., 2011b). Like in theHimalaya, geodeticmeasurements suggest that the dcollement is lockedfrom the toe to about 100 kmdowndip (Brooks et al., 2011b). In addition,Brooks et al. (2011a) have found geological evidence for a surface-rupturing event with at least 7 m of slip at the range-front fault at thetip of the system.2.3. Western Taiwan

The 1999 Mw 7.6 Chi-Chi earthquake represents a clear example inwhich a shallow, bedding-parallel dcollement ruptured with large sur-face slip (3-10 m) (Lee and Ma, 1999; Johnson et al., 2001; Yue et al.,2005). Although the rupture involved a complex geometry and faultproperties (Ma et al., 2000, 2003), it is clear that the earthquake producedsignificant coseismic slip (and afterslip) on the bedding-parallelChelungpu-Sanyi thrust system, which extends as a shallow dcollement(5-8 km deep) beneath western Taiwan for tens of kilometers (Yue et al.,2005; Rousset et al., 2012).

2.4. Santa Barbara Channel, Southern California

The southwesternmargin of the Transverse Ranges extends offshoreinto the Santa Barbara Channel along a set of bedding-paralleldcollements at 36 km depth, imaged by seismic reflection data(Shaw and Suppe, 1994). Syntectonic sediments deposited on thesestructures produce distinctive growth triangles that record active slipin the Quaternary accommodating ~45% of the geodetically measuredshortening rate across the channel. North of the toe of the dcollement,the Ventura-Pitas Point fault splays upward, producing the Ventura Av-enue anticline (Hubbard et al., 2014); recent terrace measurementsdemonstrate that this fault produces large, episodic uplift events(Rockwell, 2011). These large uplift events would require rupture ofnot only the Ventura-Pitas Point fault, but also the dcollement and

50 J. Hubbard et al. / Earth-Science Reviews 141 (2015) 4555other fault systems along strike (Hubbard et al., 2014). We thereforeconclude that shortening is at least in part accommodated by large, ep-isodic events on the dcollement system.

2.5. Sumatra subduction zone, Mentawai segment

The 2010 Mw 7.8 Mentawai earthquake produced a tsunami thatwas much larger than expected based on the seismic magnitude, lead-ing to its classification as a tsunami earthquake (Kanamori, 1972).Hill et al. (2012) used GPS data and a tsunami field survey to demon-strate that the earthquake must have been associated with high faultslip at shallow depths close to the oceanic trench, with most of theslip at depths shallower than 6 km. Yue et al. (2014) used a joint inver-sion of high-rate GPS and teleseismic data to confirm this result, placingeven higher slip (up to 23 m) in the shallowest section (above 5 km).Such high slip at the tip of an accretionary prism must be the result ofslip along the dcollement at its base. This event provides a possible ex-ample of a large earthquake that initiated updip of the seismic front,indicating that the dcollement here may be not only seismic, butseismogenic. Alternatively, it is possible that the rupture initiateddeeper, but that the deeper slip was low in magnitude compared tothe shallower slip, and is therefore not well constrained. Coralmicroatolls in this region also record a slightly deeper, but still shallowslip event in ~A.D. 1314, suggesting that the 2010 earthquake was notan isolated event (Philibosian et al., 2012).

2.6. Sumatra subduction zone, northern segment

The greatMw9.2 Sumatra earthquake of 26December 2004 propagat-ed ~1300 km along strike and produced a devastating tsunami. Singhet al. (2008) imaged large, active thrust faults at the front of the accretion-arywedgewith seismic reflection data. They also note the presence of af-tershocks with thrust mechanisms consistent with the imaged faults.These observations suggest that the 2004earthquakemayhave propagat-ed updip to the tip of the accretionary prism. This is compatiblewith geo-detic data (Rhie et al., 2007), and supported by backprojections ofradiated energy,which indicate significant energy release near the trenchfor hundreds of kilometers along strike (Ishii et al., 2005). However, theresolution of the slip distribution based on seismic energy release andgeodetic deformation is poor, and several alternative slip models fit thedata reasonably well with primarily deeper slip patches (Ammon et al.,2005; Chlieh et al., 2007; Pietrzak et al., 2007; Shearer and Brgmann,2010). This is a common problem for offshore earthquakes far fromland. If significant slip near the trench did occur, it could have producedlarge uplift of the seafloor and contributed to the height of the tsunami.

Nearly a century prior to the 2004 Sumatran earthquake, a ~ M 7.8earthquake occurred near the southern extent of the 2004 event in1907 (Kanamori et al., 2010). This earthquake produced an extensivetsunami that affected nearly 950 km of the Sumatran coast. Detailed in-vestigations of historical seismograms has led to the conclusion that thisearthquake probably initiated on the subduction interface, and propa-gated up-dip into the shallow sediments, causing the large tsunami(Kanamori et al., 2010).

2.7. Java subduction zone

The 17 July 2006 Mw 7.8 Java earthquake produced a tsunami withlocal runup heights over 20 m and average tsunami heights of 5 to7 m along 200 m of coastline (Ammon et al., 2006; Fritz et al., 2007).Like the 2010 Mentawai earthquake, this qualifies as a tsunami earth-quake, due to the size of the tsunami relative to the earthquakemagni-tude (Kanamori, 1972). Also like the Mentawai earthquake, a rupturetowards the toe of the accretionarywedge could produce significant up-lift, and generate a tsunami of larger size than would be predicted by adeeper rupture. This is consistentwith the observation of low energy re-lease and slow rupture, features that are inferred to be associated withrupture within the low-rigidity materials of an accretionary prism(Newman and Okal, 1998; Polet and Kanamori, 2000). Thus, we inferthat the Java portion of the Sunda subduction zone, like Sumatra tothe north, may be capable of large ruptures in the frontal part of thewedge.

2.8. Japan Trench

The 2011 Mw 9.0 Tohoku-Oki earthquake ruptured the plate inter-face between the Pacific and Okhotsk plates, east of the island of Hon-shu. Tremendous slip vertically displaced the sea bottom by up to50 m, as measured by differential bathymetry, GPS measurements, un-derwater acoustic sounding, and sea-bottom pressure records of thetsunami waves, demonstrating that rupture reached close to the Japantrench (Avouac, 2011; Fujiwara et al., 2011; Sato et al., 2011). Indeed,seismic reflection profiles pre- and post-earthquake directly imageslip at the trench axis during the earthquake (Kodaira et al., 2012),and demonstrate that the dcollement underlying this prism slippedin this event, with large coseismic slip reaching the toe of the wedge.

Shouldwe have known prior to the Tohoku-Oki earthquake that thissubduction zone was capable of trench ruptures? Indeed, we shouldhave. The Mw 8.5 1896 Sanriku earthquake ruptured a nearby portionof the trench and generated a tsunami with a maximum run-up of25 m, despite only weak shaking being felt onshore (Tanioka andSatake, 1996). Tsunami modeling suggests that slip was concentratednear the trench (Tanioka and Satake, 1996). Repeated greater-than-expected tsunamis with weak shaking support the hypothesis of multi-ple toe ruptures.

2.9. Kuril Trench

In addition to the two earthquakes in the southern part of the Japan-Kuril trench, two tsunami earthquakes occurred further north: a Ms 7.2event in 1963, and a Ms 7.0 event in 1975 (Fukao, 1979; Pelayo andWiens, 1992). The Oct. 20 1963 earthquake occurred as an aftershockof a larger, more down-dip earthquake on Oct. 13, and has beeninterpreted as having a shallow origin (~5 km depth, Fukao, 1979;~9 km depth, Pelayo and Wiens, 1992). The June 10, 1979 tsunamiearthquake occurred about 300 km southwest of the 1963 earthquake;body wave inversion provides a best-fit source depth of 5 km, which isconsistent with the location of the epicenter relative to the trench(Pelayo and Wiens, 1992). Both of these events exhibited anomalouslyslow rupture speeds, suggesting that they occurred within the weaksediments of the accretionary prism (Pelayo and Wiens, 1992).

In 2006, a Mw 8.3 event ruptured just northeast of the 1963 earth-quake region (Ammon et al., 2008; Lay et al., 2009). Although the tsuna-mi was relatively limited in this event (mostly b1.2 m), this is in partbecause of the lack of local tide gauge recordings. A finite sourcemodel for the rupture suggests that at shallow depths (b25 km), a250 km long segment of the subduction zone slipped an average of4.3-6.5 m (Lay et al., 2009). This calculation constrains slip at the trenchto zero. It seems likely that this rupture extended to the trench.

2.10. Japan, Nankai Trough

High-resolution seismic reflection profiles across the Nankai Troughimage both a series of shallowly dipping thrust faults in the frontal partof the wedge and a large thrust to the hinterland termed a megasplaythatmergeswith the subduction interface at ~89 kmdepth (Park et al.,2002; Moore et al., 2009). Cores were drilled through both the frontalthrust at the toe of the accretionary prism and across the megasplay,and analyzed for vitrinite reflectance geothermometry (Sakaguchiet al., 2011). It was determined that both fault zones underwent local-ized temperatures of more than 380 C, implying that frictional heatingoccurred, likely due to coseismic slip. This evidence strongly suggeststhat the frontal dcollement slipped coseismically at least once.

51J. Hubbard et al. / Earth-Science Reviews 141 (2015) 4555Although it is not possible to link this recorded heating event to a partic-ular earthquake, both the Mw 8.1 1944 Tonankai and the Mw 8.1 1946Nankaido earthquakes are potential candidates, as they producedground shaking and damaging tsunamis in this region (Kato andAndo, 1997).2.11. Solomon Islands

The 2007 Mw 8.1 Solomon Islands earthquake produced a large tsu-nami, with up to 12m of runup (Chen et al., 2009; Furlong et al., 2009).Inversion of geodetic data and surveys of uplifted coral reefs and othercoastal features suggests that there was up to 30 m of slip at the trench(Chen et al., 2009). A few years later, the 2010 Mw 7.1 Solomon Islandstsunami earthquake producedwidespread coseismic subsidencewithin20 kmof the San Cristobal trench (Newman et al., 2011). Analysis of thenear-trench deformation, tsunami run-up, open-ocean wave heightdata, and seismic records indicates that the earthquake occurred onthe shallow part of the low-angle dcollement in this region. The best-fit slip model suggests that the maximum slip was close to the trenchand reached over 7 m (Newman et al., 2011).2.12. Middle America megathrust

The September 2 1992 M 7.6 Nicaragua earthquake generated up to10mof tsunami runup, but only exhibitedweak ground shaking. Exten-sive studies of this earthquake and its associated tsunami demonstratethat it ruptured a 40-50 kmwide fault plane extending from the trenchto depths of ~10 km (Satake, 1994; Lay and Bilek, 2007). Finite-fault in-version based on teleseismic P wave inversion shows slip down to~20 km depth, but with large amounts within the upper 10 km (Yeet al., 2013). In 2012, another earthquake ruptured the MiddleAmerica megathrust, this time a Mw 7.3 in El Salvador. Both this eventand the 1992 earthquake exhibit low radiated energy for their seismicmoment and slow rupture velocities, characteristics of tsunami earth-quakes (Kanamori and Kikuchi, 1993; Ye et al., 2013). Although the2012 earthquake did not produce a tsunami due to its relatively lowmagnitude, finite-fault inversion based on teleseismic P-wave inversionindicates that most of the slip was within the upper 15 km. The inver-sion model of Ye et al. (2013) imposes zero slip at the trench, butexhibits a steep slip gradient in the upper 5 km.

Kanamori and Kikuchi (1993) suggest that the shallow rupture in1992 may have been made possible by the lack of a sedimentarywedge offshore, resulting in different frictional properties than inother wedges. Heesemann et al. (2009) propose that seamounts onthe subducting Cocos plate might alter the stress state and promote dy-namic slip on a normally aseismic dcollement. However, our discussionhere demonstrates that such ruptures have occurred inmany other sub-duction zones without these characteristics.2.13. Alaskan Trench

The 1964Mw 9.2 Great Alaskan earthquake ruptured the eastern seg-ment of the Alaska-Aleutian Trench. Joint inversion of tsunami and geo-detic data indicates that a large area of high slip concentrated over PrinceWilliam Sound extended to the trench, with an average of ~11 m of slipin the region above 15 km depth (Johnson et al., 1996). However,the resolution of this slip model is limited. Slip certainly extendedas far out as Middleton Island, which lies about 8 km above themegathrust: uplift in the earthquake produced the most recent ofsix beach terraces visible on the island (Savage et al., 2014). Wheth-er the slip continued to the trench or instead was diverted onto thesplay fault that is presumably responsible for the growth of this is-land (or both) is not known.2.14. Peru

The Mw 7.6 Peru tsunami earthquake of Nov. 20, 1960, had a muchlonger rupture duration than would have been otherwise expected foran earthquake of this magnitude (Pelayo and Wiens, 1990), suggestingthat it occurred within the low-rigidity accretionary prism. Body wave-form inversion indicates that it ruptured a fault plane dipping ~6 closeto the trench axis (Pelayo and Wiens, 1992). This earthquake occurredin a portion of the Peru trench that previously had beenmostly aseismic,leading to the suggestion that itmight not be capable of generating largeearthquakes (Nishenko, 1991). However, the 1960 event demonstratedthat shallow, tsunamigenic earthquakes can occur in this region, andmay occur in other, apparently aseismic regions as well.

The 1960 earthquake was followed by two other events with evi-dence of shallow slip on the Peruvian subduction zone: a Mw 7.4 tsuna-mi earthquake in February 1996, which generated a large tsunami thanexpected from its surfacemagnitude and exhibited a long rupture dura-tion (Heinrich et al., 1998), and a Mw 8.5 earthquake in June 2001, forwhich joint inversion of seismic and geodetic data suggest significantslip above 15 km depth, reaching close to the trench (Pritchard et al.,2007).

2.15. Chile

The great 1960 Mw 9.5 Chile earthquake was the largest earthquakeever recorded. Inversions for the slip distribution from ground deforma-tion using a 3D fault model suggest that there was at least 1 m of slip atthe trench for a distance of 200 km, with 50 km of that distanceexperiencing over 5 m of slip (Barrientos and Ward, 1990). In addition,they note that substantial undetected offshore slip is likely, given thelarge tsunami generated by the earthquake. Moreno et al. (2009) usedthe same geodetic dataset combined with a 3D fault model to estimatethat over 200 km of the trench along strike slipped N10 m, with over50 km of that distance experiencing N20 m of slip. A joint inversion ofgeodetic and tsunami data for the 1960 earthquake estimates 13-21 mof slip near the trench in the southern part of the source (Fujii andSatake, 2013). These studies demonstrate the importance of consideringtsunami data in slip inversions, since geodetic data onshore cannot ef-fectively constrain slip occurring at the trench.

2.16. Summary

The fifteen regional examples that we describe include continent-continent, continent-ocean, and ocean-ocean convergent settings, andeven oblique portions of transform margins; subduction zones withold and young downgoing plates, with and without sediment deposi-tion at the trench. They include fold-and-thrust belts that extend for1000s of km and b100 km along strike, and systems that deform pri-marily through forethrusts, backthrusts, and combinations of the two.They include very large and moderate-sized earthquake ruptures, andruptures that initiate at deep and shallow levels. Thus, it is not possibleto point to one common feature that makes rupture possible on theshallow dcollement, implying that these faults may always have thepotential to be seismic and/or seismogenic.

3. How can weak decollements behave seismically?

Given that there is evidence for large, episodic slip at the tips ofdcollements, wemust reevaluate the assumption that dcollementsare too weak to support stresses and store large strain energy. How-ever, critical taper wedge mechanics implies that these are weak fea-tures, significantly weaker than the surrounding rocks (Suppe,2007). Reconciling these apparently conflicting views requires usto examine our assumptions about fault strength, earthquake dy-namics and rock properties.

52 J. Hubbard et al. / Earth-Science Reviews 141 (2015) 45553.1. The state of stress within the wedge, and preferred slip planes atgeological time scales

We know that dcollements are weak because they form and slip atunfavorable angles. However, we could turn this around and say insteadthat these faults are oriented at unfavorable angles, and therefore the dif-ferential stress in the ambient rocksmust be high in order for these faultsto slip. In fact, because we see thrust faults rising from dcollements, weknow that sometimes it is easier to break a more steeply dipping thrustfault and slip on it than to slip on the dcollement surface. At othertimes, however, it must be easier to slip on the dcollement; we knowthis because we see slip at the toes.

How is it possible that both of these options are favorable at differenttimes? We can use a Mohr diagram, which describes the normal andshear stresses acting on a plane of any orientation, and comparesthem to failure criteria, to try to understand this problem (Fig. 4). Wepropose a setting where minor changes in stress or rock propertiescan cause slip on either the dcollement or on a thrust fault to be pref-erable at a given time.Fig. 4.Mohr circle analysis of failure in a critical taperwedge. (A) Basic concept of aMohr circle.at an orientation that forms an anglewith respect to the primary stress orientation1. The sizeThe black line represents the failure criterion of the material, with slope = the angle of interhesion (here considered cohesionless). If the circle intersects the failure criterion, thematerial wAlaskanwedge at the toe of thewedge andwithin the interior. Herewe consider the potential foangles of internal friction for the ramp and the detachment, respectively. A value of r=0.64wafor rupture). d can have a value as low as 0.035 without triggering slip on the dcollement atshaded blue area on theMohr circles represents this range of possible d values. Note that theseremain the same. At the toe of the wedge, 1 is horizontal, and the ramp reaches failure, even ttowards the toe, rotating the planes represented on theMohr circle clockwise. This causes the dfor both the unrotated and rotated 1 orientation, to better illustrate the effect of the rotation.However, we do not see randomvariations in slip on the dcollementvs. thrusting. Instead, we typically observe in-sequence break-forwardpropagation: the thrust faults towards the toe are more active, andthose in the hinterland have been abandoned (Morley, 1988). Thus, rup-tures today are typically choosing to continue along the dcollement allthe way to the toe rather than to break upward along preexisting thrustfaults, even though they previously chose those branches. This pattern ofbreak-forward propagation can be understood by the fact that the prin-cipal stresses are rotatedwithin thewedge due to the topographic gradi-ent (Dahlen et al., 1984; Savage et al., 1985; Cubas et al., 2008;Mary et al.,2013). The maximum stress direction dips gently in the direction of thewedge toe (see 1 orientation on the right side of Fig. 4). This increasesthe angle between the maximum stress orientation, 1, and thedcollement. There is a lateral transition in the amount of stress rotation,with 1 horizontal just beyond the toe of the wedge to tilted in the inte-rior of the wedge. A larger angle between the 1 direction and thedcollement dip is equivalent to a smaller in the Mohr diagram(Fig. 4a). This makes slip on the dcollement more favorable. Thus, weexpect ruptures to follow the dcollement until they reach a pointEvery point on the circle represents the normal () and shear () stresses acting on a planeand location of the circle is defined by the largest (1) and smallest (3) principal stresses.

nal friction, and the y-axis intercept for the failure criterion (here zero) represents the co-ill fail on the plane represented by the point of intersection. (B)Mohr circle analysis of ther failure of the dcollement and a ramp rising from the dcollement. r and d represent thes chosen tomatch the observed dips of the fault ramps (assumed to be the ideal orientationthe toe, and must have a value below 0.187 to allow slip in the interior of the wedge. Thevalueswould change in the presence of pore pressures or cohesion, but the principlewouldhough the rampmaterial has a higher coefficient of friction. In the interior, 1 dips slightlycollement, rather than the ramp, to reach failure. TheMohr circle on the right shows lines

53J. Hubbard et al. / Earth-Science Reviews 141 (2015) 4555(usually near the toe)where the stressfield is less tilted, and at that pointto break upward along a thrust fault. In other words, the slip on theramps builds a topographic gradient that causes those ramps to shutoff, and the dcollement to propagate forward.

3.2. Variations in strength through the earthquake cycle

There are additional mechanisms that could allow stress to accumu-late on dcollements. In particular, the interseismic and coseismicstrengths of the dcollement may differ. In this case, the long-term re-cord visible as the total deformation of the systemmay reflect theweak-er of the two. Low-temperaturemechanical processes ofweakening andhealing (abrasion, pulverization, and fusion) are not likely to be able todramatically affect effective friction (Biegel et al., 1992; Wang andScholz, 1994). However, at high slip velocities, frictional heating mayallow strong-weakening phenomena, such as melt-welts (Brown andFialko, 2012) and other flash-weakening effects (Rice, 2006) and porepressurization through thermal expansion (Ghabezloo and Sulem,2009; Ferri et al., 2010) or mineral decomposition (Han et al., 2007).Strong-weakening mechanisms are activated under specific conditions,so we can expect different frictional resistance at subseismic and seis-mic slip speeds. For example, reaching sufficiently high temperaturefor melting or for substantial expansion of pore fluids requires largeslip (Rice, 2006), so these strong-weakening effects are expected tooccur mostly during large earthquakes. But the complexity of fault rhe-ology may also affect the interseismic period. For example, slow slipevents occurred near the hypocenter of the 2011 Mw 9.0 Tohoku-Okiearthquake and the 2014 Mw 8.1 Iquique, Chile earthquake with signif-icant overlap with the seismic rupture (Ito et al., 2013; Kato andNakagawa, 2014; Ruiz et al., 2014), and models of fault slip evolutionover many earthquake cycles suggest the possibility that the same seg-ment may experience creep, slow slip, and seismic ruptures during dif-ferent periods (Noda and Lapusta, 2013; Noda and Hori, 2014). Thisindicates that the seismogenic potential of faults may not be fullyassessed during a short period of observation.

4. Consequences of seismogenic decollements

Dcollements are some of the largest faults on Earth. However,existing scaling relationships between fault area and earthquakemagni-tude (Wells and Coppersmith, 1994; Hanks and Bakun, 2008) may beinappropriate for dcollements, as they are structurally different fromtypical dip-slip or strike-slip faults. In addition, weak dcollementsand accretionarywedges exhibit much lower rigidity thanmore consol-idated rock (Kanamori, 1972). Because earthquake moment is definedas the product of the rigidity, slip, and area of a fault, lower rigiditywill result in lower earthquake magnitudes for a given amount of slipand area (Fukao, 1979).

Such an alteration to the typical slipmagnitude relationship wouldreduce expected seismicmoments in regionswhere hazard is estimatedfrom slip, uplift amounts, or GPS shortening. Initially, one would expectthis alteration to reduce seismic hazard. However, because the slipwould be expected to occur directly beneath large regions at shallowdepths, the resulting ground shaking levelsmight be as ormore hazard-ous than that predicted from higher seismic moments on more distantfaults.

In contrast, if seismic hazard is estimated from earthquake moment,we may see earthquakes with higher slip than expected based on seis-mic records. This would help explain tsunami earthquakes, as evenallowing slip to propagate to the tips of accretionary prisms is not al-ways sufficient to explain resulting tsunami heights (e.g., Hill et al.,2012).

This discrepancy between earthquake magnitude and slip can alsobe partially solved by structural analysis. Seismic reflection imaging ofaccretionary prisms demonstrates that dcollement tips frequently ter-minate into more steeply dipping thrust faults that break towards thesurface (Fig. 3). If slip propagates to the tip of the dcollement, we canexpect that it will then extend upward onto such a thrust fault and pro-duce a band of high uplift at the toe of the wedge, resulting in largertsunamis.

5. Discussion and conclusions

For many years, studies of subduction zones have assumed that thefrontal portions of accretionary prisms were too weak to rupturecoseismically (Byrne et al., 1988; Hyndman et al., 1997). Hazard studiesof fold-and-thrust belts on land have often suffered from the same as-sumption, based on the assumption that the shallow dcollement istoo weak to rupture in large earthquakes. We present a series of exam-ples, both on-land and offshore, demonstrating that inmany cases, shal-low dcollements are capable of producing large, coseismic slip eventsthat rupture to the toes of the systems. Some of these events are associ-ated with ruptures that initiate down-dip of the seismic front, whileothers are limited to the frontal, shallow portion of the wedge

We suggest that this behavior is not limited to the examples de-scribed here, but rather is common to many (perhaps most) accretion-ary wedges and fold-and-thrust belts. Although many earthquakes insubduction zones have been interpreted to have no slip at the tip ofthe accretionary prism, this interpretation is typically driven by modelassumptions, rather than the data. In addition to the examples providedhere, there may bemany other examples of similar earthquakes, whereknown slip downdip obscures the need for slip at the toe (hidden tsu-nami earthquakes).

We do not characterize the regions and events described here as un-usual, as they encompass a wide range of settings. However, we recog-nize that as far as reliably documented events go, these events are stillfairly limited.Whether this fact is due to a lack of data or a lack of eventsis not clear, and accurately assessing this distinctionmay require the ac-quisition of new forms of data, including pre- and post- bathymetry sur-veys and seafloor geodesy (as has been used to observe deformation inthe 2011 Tohoku-Oki earthquake). The fraction of slip at the trench thatoccurs in large seismogenic events is unknown, and likely varies be-tween dcollements, and thus assessing the recurrence rates of suchearthquakes will in many cases be difficult, especially for subductionzones, where paleoseismic evidence is lacking. As a result, it would besafest to assume that trench-rupturing events can occur until we haveenough evidence to show that a particular fault system is behaving dif-ferently. Many attempts to assess seismic hazard for particular regionshave relied on extrapolating the likelihood of large earthquakes fromthe occurrence of small ones, resulting in unexpected large earth-quakes with striking underestimations (by two to three orders of mag-nitude) of expected fatalities (Kossobokov and Nekrasova, 2012; Wysset al., 2012). This study indicates that there is an urgent need for studiesthat evaluate seismic and tsunami hazard in fold-and-thrust belts andsubduction zones around theworld and allow for the possibility of shal-low dcollement rupture.

Acknowledgements

We thank our two reviewers, Jeff Freymueller and Thorne Lay, fortheir constructive comments. This research was supported by the Na-tional Research Foundation of Singapore under the NRF Fellowshipscheme (National Research Fellow Award No. NRF-NRFF2013-06) andby the EOS, the National Research Foundation of Singapore and theSingapore Ministry of Education under the Research Centres of Excel-lence initiative. This is EOS paper number 76.

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Coseismic slip on shallow dcollement megathrusts: implications for seismic and tsunami hazard1. Introduction2. Evidence for shallow slip in megathrust earthquakes2.1. The Himalaya2.2. Bolivia2.3. Western Taiwan2.4. Santa Barbara Channel, Southern California2.5. Sumatra subduction zone, Mentawai segment2.6. Sumatra subduction zone, northern segment2.7. Java subduction zone2.8. Japan Trench2.9. Kuril Trench2.10. Japan, Nankai Trough2.11. Solomon Islands2.12. Middle America megathrust2.13. Alaskan Trench2.14. Peru2.15. Chile2.16. Summary

3. How can weak decollements behave seismically?3.1. The state of stress within the wedge, and preferred slip planes at geological time scales3.2. Variations in strength through the earthquake cycle

4. Consequences of seismogenic decollementsAcknowledgementsReferences