Subduction Systems Revealed: Studies of the Hikurangi Margin

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Article in Eos Transactions American Geophysical Union · November 2010 DOI: 10.1029/2010EO450001

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Rebecca E Bell Stephen Bannister Imperial College London GNS Science

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Structure and Stress from Depth to Surface in the Te Mihi Geothermal Field, New Zealand View project Investigating landscape response to active faulting, Southern Gulf of Corinth, Central Greece View project

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Volume 91 number 45 9 November 2010 EOS, Transactions, American Geophysical Union pages 417–428

edge of the overriding plate and a low taper angle for the accretionary wedge (4°–6°) Subduction Systems Revealed: are also observed in this region of stick-slip behavior (Figure 1). Studies of the Hikurangi Margin In distinct contrast, plate motion at the northern Hikurangi subduction thrust is PAGES 417–418 To address these questions, the scien- accommodated by both steady and episodic tific community in New Zealand is endeav- aseismic creep at shallow depths (<15 kilo- Convergence between tectonic plates at oring to develop international interest in meters). This part of the margin also exhibits subduction boundaries is mostly accommo- New Zealand’s Hikurangi subduction mar- subduction erosion and seamount subduc- dated as slip along the subduction interface gin as a natural laboratory for studying sub- tion, a thinner (~1-kilometer-thick) package thrust fault. The portion of the fault where duction megathrust processes. The north- of sediment on the incoming plate, and a earthquakes nucleate is known as the seis- ern Hikurangi margin is one of the few steep taper angle for the accretionary wedge mogenic zone. In the seismogenic zone the places on Earth where a subduction thrust (7°–10°). The northern Hikurangi margin has fault is thought to be locked (due to friction) fault dominated by steady, aseismic creep produced moderate subduction thrust earth- during the time between earthquakes, and occurs close to land and in relatively shal- quakes in historical times, including two most of the motion between the subducting low (<3000-meter) water depths. In contrast, Mw 6.9–7.1 tsunami-generating earthquakes and overriding plates occurs instantaneously the southern Hikurangi subduction inter- in 1947 (Downes et al. [2000] and Doser and in major megathrust earthquakes, which face appears to undergo stick-slip behavior Webb [2003]; see Figure 2 for locations). include the largest earthquakes recorded over large regions and is capable of produc- on the planet. However, in some cases sub- ing great megathrust earthquakes [Wallace A Natural Laboratory for Understanding SSEs duction thrust slip occurs as steady aseismic et al., 2009]. creep (without a major earthquake), and it Over the past decade, targeted studies The recent confirmation of episodic SSEs has been recently discovered that portions at Hikurangi have revealed that these pro- as a new class of shear slip at subduction of many subduction thrusts slip episodically nounced changes in subduction interface margins worldwide has profoundly influ- in slow slip events (SSEs) that take weeks behavior from north to south also corre- enced scientific understanding of subduc- to years to occur [see, e.g., Schwartz and spond to observed changes in other fun- tion interface behavior. Most well-known Rokosky, 2007]. In most cases, these SSEs damental subduction margin characteris- SSEs on subduction interfaces occur occur along the deeper edges of the seismo- tics (see reviews by Wallace et al. [2009] between depths of 25 and 60 kilometers; genic zone. and Barnes et al. [2010]) (Figure 1). Thus, highly accurate imaging and characteriza- Much of the world’s population lives close the Hikurangi margin may hold the key to tion of the interface at these depths is diffi- to subduction zone plate boundaries. The understanding the physical mechanisms cult with current technological capabilities. increased risk posed to these people pro- that control stick-slip versus aseismic creep However, the shallow depth of the northern vides an important motivation to better behavior on subduction thrusts. Hikurangi SSE source area (<15 kilometers) understand subduction fault zone dynam- makes it amenable to more detailed stud- ics. Indeed, several subduction zones have A Closer Look at the Southern ies and is an ideal location for testing and been intensely studied, including the Nankai and Northern Hikurangi Margins developing ideas regarding the genesis of subduction margin in southwestern Japan, slow slip and the nature of the transition the Middle America Trench offshore Costa Westward subduction of the Pacific plate from stick-slip to aseismic creep behavior. Rica, and the Cascadia subduction margin beneath eastern North Island, New Zea- Recognition of episodic SSEs on the in the northwestern United States and west- land, is accommodated at the Hikurangi Hikurangi subduction interface is made pos- ern Canada. Trench. Global Positioning System (GPS) sible by GeoNet, a New Zealand–wide geo- Despite these efforts to understand sub- data show that the southern portion of the physical monitoring network currently con- duction systems worldwide, some key ques- subduction interface is currently locked to sisting of 151 continuous GPS (cGPS) stations about subduction fault behavior remain depths of 25–40 kilometers, while episodic tions, 49 broadband and 108 short- period unanswered [e.g., Hyndman, 2007]. For exam- SSEs occur on the subduction interface seismometers, and 271 strong motion accel- ple, why do some subduction interface faults (25–60 kilometers deep) below the region erograph stations (http://www.geonet.org undergo stick-slip behavior and produce of locking. Although there is a lack of great .nz/). GeoNet is designed, built, and oper- great (magnitude > 8.0) megathrust earth- (Mw > 8.0) megathrust earthquakes in New ated by GNS Science and funded prin- quakes, while others appear to be dominated Zealand’s short historical record (~170 cipally by the New Zealand Earthquake by aseismic slip and rupture only in moder- years), modern-day crustal strain accumula- Commission. ate thrust events? What physical mechanisms tion measured by GPS techniques suggests Since the start of GeoNet in 2002, scien- control the occurrence and character of epi- that the southern Hikurangi interface fault tists have detected 15 distinct SSEs on the sodic slow slip? will eventually undergo slip in Mw 8.0–8.5 Hikurangi subduction thrust fault [Wallace earthquakes [Wallace et al., 2009]. Well- and Beavan, 2010]. As observed at subduc- By L. M. Wallace, R. Bell, J. Townend, developed tectonic accretion of a thick tion margins elsewhere, the SSEs are located S. Ellis, S. Bannister, S. Henrys, R. Sutherland, (3- to 6-kilometer) package of sediment on the subduction interface at the transi- and P. Barnes from the incoming plate onto the leading tion from stick-slip to steady, aseismic creep Eos, Vol. 91, No. 45, 9 November 2010

Fig. 1. Perspective view of the Hikurangi margin illustrating the portions of the subduction interface that undergo stick slip versus aseismic slip, in terms of a coupling coefficient (red and blue shading); coupling coefficients of 1 indicate areas that are currently locked and likely to rupture in future subduction thrust earthquakes, while coupling coefficients near zero indicate regions of steady, aseismic creep. Green contours show areas of slip in slow slip events since 2002 from Wallace and Beavan [2010]. Convergence rates accommodated at the trench are labeled as red numbers (in millimeters per year (mm/yr)) near the trench. Motion of the Pacific plate rela- tive to the Australian plate (PAC/AUS) is shown by the black arrow. The black dotted line marks the approximate location of the seismic profile depicted in Figure 2. Along-strike variations in various subduction margin properties are summarized beneath the perspective plot. The inset shows New Zealand’s tectonic setting. HT is Hikurangi Trench, KT is Kermadec Trench, TT is Tonga Trench, NI is North Island, and SI is South Island.

behavior (Figure 1). The deepest (30–60 images of their source areas using seismic Cascadia. Seismic imaging of the southern kilometers), longest duration (1–2 years), reflection techniques. Recently acquired Hikurangi margin is being undertaken as and largest (Mw 7.0–7.2) SSEs occur at the seismic data reveal a zone of high-amplitude part of a new, collaborative project involv- southern Hikurangi margin. Conversely, reflectivity near the subduction interface, ing researchers from New Zealand, the the shallowest (<15 kilometers), shortest 6–15 kilometers below the seafloor, that Earthquake Research Institute of Tokyo,

(1–2 weeks), and smallest (Mw 6.3–6.8) SSEs coincides with geodetically determined SSE and the University of Southern California. occur at the northern Hikurangi margin source areas [Bell et al., 2010] (Figure 2). During the first phase of this experiment, [Wallace and Beavan, 2010]. The duration These high-amplitude reflective zones are in 2009–2010, researchers acquired 480 and magnitude characteristics of Hikurangi interpreted to be fluid-rich sediments, sug- kilometers of multichannel marine seis- SSEs appear to be related to the depths at gesting that high fluid pressures play an mic data and used the associated marine which they occur. Nonvolcanic tremor asso- important role in the occurrence of SSEs at air gun shots and natural earthquakes ciated with slow slip has yet to be identified the northern Hikurangi margin. recorded more than 50 onshore and at the Hikurangi margin, but Delahaye et al. 20 ocean bottom seismometers to image [2009] documented reverse-faulting micro- Imaging Where the Subduction Interface the crust and faults beneath the south- earthquakes triggered by SSEs at the north- Undergoes Stick-Slip Behavior ern Hikurangi margin. The experiment ern Hikurangi margin using the GeoNet seis- will be completed in 2011 when scientists mic data set. Overall, the southern Hikurangi mar- will deploy 900 short-period seismometers The proximity of SSEs to the Earth’s sur- gin has many characteristics in common across the southern North Island and use face at the northern Hikurangi margin with subduction margins that rupture in onshore explosive shots to complete the makes it possible to obtain high-quality great earthquakes, such as Nankai and image. Eos, Vol. 91, No. 45, 9 November 2010

Future Directions in Subduction Margin Science

It has so far not been possible to explain the changes observed between the steadily creeping northern and stick-slip southern portions of the Hikurangi margin in terms of one or two simple parameters. Instead, some suggest that the variable geometry of the Hikurangi seismogenic zone is caused by interplay between upper and lower plate structure, subducting sediment, subduction interface temperatures, regional tectonic stress regime, and fluid pressures [see Wal- lace et al., 2009]. How each of these processes influences the seismogenic zone geometry is currently unknown and is a focus of ongoing research efforts. A broad international group of researchers recently submitted an Integrated Ocean Drill- ing Program (IODP) preproposal to drill into the northern Hikurangi SSE source area and associated high-amplitude reflectivity zone on Fig. 2. (a) Uninterpreted and (b) interpreted seismic profile from the northern Hikurangi margin the subduction interface at about 5 kilometers (see Figure 1 for location). The solid red line represents the subduction interface, dotted red lines are splay faults within the upper plate, the yellow line is the base of well-stratified sediments, below the seafloor (Figure 2). This target is and the green line is an unconformity within the stratified sediments. The yellow star denotes unique because it is the only well-documented the position of the March 1947 tsunamigenic earthquake [Doser and Webb, 2003]. (c) Depth- subduction SSE source area on Earth at depths converted interpretation based on velocity model described by Barker et al. [2009]. Areas of within range of modern offshore drilling meth- high-amplitude reflectivity [Bell et al., 2010] are shaded in blue. The vertical white line shows ods. The potential for drilling, downhole mea- a potential position for offshore drilling to access the slow slip source area and high-amplitude surements, sampling, and monitoring of the reflectivity (HRZ) zone by intersecting the interface about 5.1 kilometers below the seafloor. northern Hikurangi margin SSE source area The color bar at top of figure schematically denotes the regions of the subduction interface that provides an excellent opportunity to defini- undergo stick slip (green) and slow slip (red). BSR is bottom simulating reflector, VE is vertical tively test hypotheses regarding the physical exaggeration, CDP is common depth point, bsf is below seafloor, and TWTT is two-way travel conditions leading to SSE occurrence and, ulti- time. mately, to unlock the secrets of slow slip. Through these and other emerging research efforts in the Hikurangi subduction system, Geophys. J. Int., 180(1), 34–48, doi:10.1111/j.1365 Schwartz, S. Y., and J. M. Rokosky (2007), Slow major headway can be made in understand- -246X.2009.04401.x. slip events and seismic tremor at circum- Pacific ing what controls seismogenic zone geometry Delahaye, E. J., J. Townend, M. E. Reyners, and subduction zones, Rev. Geophys., 45, RG3004, and the physics behind both subduction thrust G. Rodgers (2009), Microseismicity but no tremor doi:10.1029/2006RG000208. earthquakes and SSEs. Moreover, an increased accompanying slow slip in the Hikurangi subduc- Wallace, L. M., and J. Beavan (2010), Diverse slow slip behavior at the Hikurangi subduc- focus on contrasting processes at the north- tion zone, New Zealand, Earth Planet. Sci. Lett., 277(1-2), 21–28, doi:10.1016/j.epsl.2008.09.038. tion margin, New Zealand, J. Geophys. Res., ern and southern Hikurangi margin may help Doser, D. I., and T. H. Webb (2003), Source doi:10.1029/2010JB007717, in press. to solve the mystery of why some subduction parameters of large historical (1917–1961) Wallace, L. M., et al. (2009), Characterizing the margins rupture in megathrust earthquakes earthquakes, North Island, New Zealand, Geo- seismogenic zone of a major plate boundary and others do not. phys. J. Int., 152(3), 795–832, doi:10.1046/j.1365 subduction thrust: Hikurangi Margin, New Zea- -246X.2003.01895.x. land, Geochem. Geophys. Geosyst., 10, Q10006, References Downes, G., T. H. Webb, M. J. McSaveney, doi:10.1029/2009GC002610. C. Chague-Goff, D. J. Darby, and A. Barnett Barnes, P. M., G. Lamarche, J. Bialas, S. Henrys, (2000), The March 25 and May 17 1947 Gisborne Author Information I. Pecher, G. L. Netzeband, J. Greinert, J. J. Mount- earthquakes and tsunami: Implication for tsuna- joy, K. Pedley, and G. Crutchley (2010), Tectonic mi hazard for East Coast, North Island, New Zea- Laura M. Wallace, GNS Science, Lower Hutt, New and geological framework for gas hydrates and land, in Tsunami Risk Assessment Beyond 2000: Zealand; E-mail: l.wallace@ gns .cri .nz; Rebecca cold seeps on the Hikurangi subduction mar- Theory, Practice and Plans, edited by V. K. Gusia- Bell, Imperial College London, London, UK; John gin, New Zealand, Mar. Geol., 272(1-4), 26–48, kov, B. W. Levin, and O. I. Yakovenko, pp. 55–67, Townend, School of Geography, Environment and doi:10.1016/j.margeo.2009.03.012. Intergov. Oceanogr. Comm., Moscow. Earth Sciences, Victoria University of Wellington, Bell, R., R. Sutherland, D. H. N. Barker, S. Henrys, Hyndman, R. (2007), The seismogenic zone of sub- Wellington, New Zealand; Susan Ellis, Stephen Ban- S. Bannister, L. Wallace, and J. Beavan (2010), duction thrust faults: What we know and don’t nister, Stuart Henrys, and Rupert Sutherland, GNS Seismic reflection character of the Hikurangi know, in The Seismogenic Zone of Subduction Science; and Philip Barnes, National Institute of subduction interface, New Zealand, in the Thrust Faults, edited by J. C. Moore and T. Dixon, Water and Atmospheric Research, Wellington, New region of repeated Gisborne slow slip events, pp. 15–40, Columbia Univ. Press, New York. Zealand