Research Paper THEMED ISSUE: Subduction Top to Bottom 2 GEOSPHERE Geologic controls on up-dip and along-strike propagation of slip during subduction zone earthquakes from a high-resolution GEOSPHERE, v. 15, no. 6 seismic reflection survey across the northern limit of slip during https://doi.org/10.1130/GES02099.1 18 figures; 1 supplemental file the 2010 Mw 8.8 Maule earthquake, offshore Chile Anne M. Tréhu1, Bridget Hass1,*, Alexander de Moor1,†, Andrei Maksymowicz2, Eduardo Contreras-Reyes2, Emilio Vera2, and Michael D. Tryon3,§ CORRESPONDENCE: [email protected] 1College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331-5503, USA 2Departamento de Geofísica, Facultad de Ciencias Físicas y Mathemáticas, Universidad de Chile, Santiago, Chile CITATION: Tréhu, A.M., Hass, B., de Moor, A., Maksy- 3Scripps Institution of Oceanography, La Jolla, California 92093, USA mowicz, A., Contreras-Reyes, E., Vera, E., and Tryon, M.D., 2019, Geologic controls on up-dip and along- strike propagation of slip during subduction zone earthquakes from a high-resolution seismic reflec- tion survey across the northern limit of slip during ABSTRACT ■ INTRODUCTION the 2010 Mw 8.8 Maule earthquake, offshore Chile: Geosphere, v. 15, no. 6, p. 1751–1773, https://doi.org A grid of closely spaced, high-resolution multichannel seismic (MCS) reflection profiles was The Tohoku earthquake (offshore Japan) in 2011 /10.1130 /GES02099.1. acquired in May 2012 over the outer accretionary prism up dip from the patch of greatest slip generated a devastating tsunami as slip extended to during the 2010 M 8.8 Maule earthquake (offshore Chile) to complement a natural-source seismic the subduction trench, and the structural signature Science Editor: Shanaka de Silva w Guest Associate Editor: Laura M. Wallace experiment designed to monitor the post-earthquake response of the outer accretionary prism. of this process was captured in a remarkable pair We describe the MCS data and discuss the implications for the response of the accretionary prism of “before” and “after” seismic reflection images Received 1 December 2018 during the earthquake and for the long-term evolution of the margin. The most notable observa- (Kodaira et al., 2012). Since then, a number of inves- Revision received 17 June 2019 tion from the seismic reflection survey is a rapid north-to-south shift over a short distance from tigators have studied the structural characteristics of Accepted 23 August 2019 nearly total frontal accretion of the trench sediments to nearly total underthrusting of undeformed other subduction zones to try to infer tsunamigenic Published online 7 November 2019 trench sediments that occurs near the northern edge of slip in the 2010 earthquake. Integrating potential from the seismic reflection signature of the our structural observations with other geological and geophysical observations, we conclude that deformation front (e.g., Dean et al., 2010; Gulick et al., sediment subduction beneath a shallow décollement is associated with propagation of slip to the 2011; Cubas et al., 2016; Bécel et al., 2017; Han et al., trench during great earthquakes in this region. The lack of resolvable compressive deformation in 2017). As part of an experiment to monitor potential the trench sediment along this segment of the margin indicates that the plate boundary here is very post-seismic deformation of the outer accretionary weak, which allowed the outer prism to shift seaward during the earthquake, driven by large slip prism up dip from the patch of greatest slip during down dip. The abrupt shift from sediment subduction to frontal accretion indicates a stepdown in the 2010 Mw 8.8 Maule earthquake (offshore Chile) the plate boundary fault, similar to the stepovers that commonly arrest slip propagation in strike- (Tréhu and Tryon, 2012), we acquired 1500 km of slip faults. We do not detect any variation along strike in the thickness or reflective character of the high-resolution multichannel seismic reflection trench sediments adjacent to the change in deformation front structure. This change, however, is data using a 600-m-long, 48-channel hydrophone correlated with variations in the morphology and structure of the accretionary prism that extend streamer and two Generator-Injector (GI) guns in as far as 40 km landward of the deformation front. We speculate that forearc structural heterogene- 45/105 mode (Fig. 1), referring to the volume (in cubic ity is the result of subduction of an anomalously shallow or rough portion of plate that interacted inches) of the generator and injector airguns. We also with and deformed the overlying plate and is now deeply buried. This study highlights need for acquired coincident swath bathymetric, 3.5 kHz sub- three-dimensional structural images to understand the interaction between geology and slip during bottom profiling, and gravity data. Although many subduction zone earthquakes. of the early slip models for the Maule earthquake indicated that slip did not extend to the trench (e.g., Moreno et al., 2010; Delouis et al., 2010; Tong et al., 2010; Lorito et al., 2011; Vigny et al., 2011), the up-dip *Now at National Ecological Observatory Network, Boulder, Colorado 80301, USA This paper is published under the terms of the †Now at Canyonlands Field Institute, Moab, Utah 84532, USA extent of slip is poorly constrained in most models, CC-BY-NC license. §Retired and recent studies suggest that slip may have locally © 2019 The Authors GEOSPHERE | Volume 15 | Number 6 Tréhu et al. | Geologic controls on up-dip and along-strike propagation of slip during 2010 Maule earthquake Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/6/1751/4876538/1751.pdf 1751 by guest on 30 September 2021 Research Paper reached the trench (Yue et al., 2014; Maksymowicz 74˚W 73˚W 72˚W LEGEND et al., 2017; Wang et al., 2017). Water depth In this paper, we examine the seismic reflec- (meters) Epicenter of 2010 Maule earthquake N tion data collected over the trench and accretionary 400 32˚S Centroid moment tensor, Maule prism north of the epicenter and near the north- 1300 earthquake ern boundary of slip from the Maule earthquake 2200 Slip model of Moreno et al. (2012), contours labeled in meters to evaluate whether along-strike variations in the 3100 structure of the deformation front can be related to 4100 Multichannel seismic (MCS) data VFB from R/V Melville cruise MV1206 variations in the response of the outer prism to plate Juan Fernández Ridge boundary slip on both short and long time scales. Absolute pressure gauge deployed during MV1206 Most models indicate that the largest slip during the 33˚S Valparaiso Ocean-bottom seismometer earthquake occurred in this region. We document deployed during MV1206 SAC the presence of an along-strike transition over a SPOC seismic lines (Geersen et al., short distance from nearly total sediment accretion 66 mm/yr 2011) to dominantly sediment subduction at the deforma- VG02 seismic lines (Contardo et al., 2008) tion front, and look both seaward and landward of the deformation front for possible mechanisms to aipo MGL1701 seismic lines (Bangs io M R et al., 2017) explain this variability. We do not detect any sys- ODP Leg 203 drill sites tematic changes in the reflectivity structure or total 34˚S 2 VG02-02 thickness of the trench sediments that can explain 4 Axial channel this observation. Landward of the deformation front, Pichilemu Large-aperture seismic proles we provide evidence for a transpressive boundary (Contreras-Reyes et al., 2017) VG02-03 6 that separates the active accretionary prism from Continental backstop 8 the metamorphic rocks of the paleo–accretionary 6 (Contreras-Reyes et al., 2017) 16 prism, which acts as a backstop to subduction here 10 VFB Valparaiso forearc basin 12 (Moscoso et al., 2011; Contreras-Reyes et al., 2017) 35˚S 14 SAC San Antonio Canyon Rio and note variations in prism and backstop mor- Mataquito VG02-06 phology and structure that are correlated with the Figure 4 ConstitucionConstitución along-strike change in deformation front structure. Rio Maule Our ultimate objectives are to explore whether 10 2 Geersen et al. (2011) segment boundaries (2011) segment Geersen et al. structural patterns in the outer prism preserve 4 4 a signature of consistent, long-term patterns of strain release and to investigate the implications 36˚S of such observations for seismic and tsunami haz- 8 ard evaluation. Integrating our observations with Site 1235 Site 1234 prior results from bathymetric, seismic, potential field, geodetic, and coastal uplift studies (e.g., Riet- Rio Itata brock et al., 2012; Lange et al., 2012; Métois et al., 2012; Hayes et al., 2013; Cubas et al., 2013; Lieser et ConcepcionConcepción al., 2014; de Moor, 2015; Maksymowicz et al., 2015, 6 37˚S 6 2017; Bassett and Watts, 2015a, 2015b; Saillard et al., 2017), we conclude that sediment subduction is Rio Bio Bio 10 associated with up-dip propagation of slip during 8 earthquakes in this region and speculate that the Figure 1. Bathymetry and topography of the central Chile subduction zone. Elevations are from Global Multi-Resolution Topography correlation between the along-strike transition in (Ryan et al., 2009), version 3.6.6. SPOC—Subduction Processes Off Chile; ODP—Ocean Drilling Program. deformation front structure and the accretionary prism morphology results from subduction of a GEOSPHERE | Volume 15 | Number 6 Tréhu et al. | Geologic controls on up-dip and along-strike propagation of slip during 2010 Maule earthquake Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/6/1751/4876538/1751.pdf 1752 by guest on 30 September 2021 Research Paper particularly shallow or rough, and now completely is linked to glaciation-deglaciation and rapid denu- trench is anomalously high in the region of our buried, part of the oceanic Nazca plate.