Forearc crustal structure and slip during subduction zone earthquakes: results from recent OBS experiments offshore Chile

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1 Forearc crustal structure and slip during subduction zone earthquakes: results from recent OBS experiments offshore Chile Anne Tréhu, Oregon State University OBS symposium, Sept , 2017, Portland Maine Collaborators: Mike Tryon (SIO); Emilio Vera & Eduardo Contreras-Reyes (Un. Chile); Dietrich Lange, Heidrun Kopp & Michael Riedel (GEOMAR); Nathan Bangs (UTIG); Alex de Moor & Kathy Davenport (OSU) and many other students and post-docs at OSU, UTIG, Un. Chile, GEOMAR and elsewhere.

2 Overarching objective: To understand the impact of crustal structure on the distribution of slip during large earthquakes in order to read the geologic record of past earthquake history and anticipate future behavior. Why: A growing number of case studies of earthquakes for which co-seismic slip models as well as well as constraints on pre- and post-earthquake deformation with resolution on a similar scale to geologic structure. Recently several observations have hinted at possible precursors to large megathrust earthquakes. A better understanding of these phenomena, including the geologic setting, is needed so that they can be used in forecasting and early warning. At present, earthquake hazard estimates treat earthquake histories as a stochastic process. If histories, which are in all cases too short and possibly incomplete, can be conditioned to account for geologic factors, then more robust statistical estimates should be possible.

3 A geophysicsist s view of the megathrust: a patchy surface with heterogeneous frictional properties. A geologist s view of the megathrust: a volume with structural, hydrologic and chemical complexity. What causes the patchiness is a geologic & hydrologic question. upper plate erosion and underplating lower plate hydration and then dehydration 2.5D Lay et al., D Wang, 2009

4 Much of Kirk s work had similar objectives with complementary field sites. Seismicity of the Earth (Mw > 5.5) Tarr et al.

5 The Nicaragua convergent margin : seismic reflection imaging of the source of a tsunami earthquake Kirk McIntosh, Eli Silver, Imtiaz Ahmed, Arnim Berhorst, Cesar Ranero, Robyn Kelly and Ernst Flueh in The seismogenic zone of subduction thrust faults, Dixon and Moore, eds., conclusions: the thin wedge below the lower slope tends to conform with the subducting plate structures, so it must be relatively weak. the midslope morphological change reflects a transition in structural integrity of the margin wedge subducted seamounts are zones of high moment release and may allow rupture at shallower depths, generating tsunami earthquakes.

6 Highlight 3 conclusions: the thin wedge below the lower slope tends to conform with the subducting plate structures, so it must be relatively weak. the midslope morphological change reflects a transition in structural integrity of the margin wedge. Speculated that subducted seamounts are zones of high moment release and may allow rupture at shallower depths, generating tsunami earthquakes.

7 PICTURES: 2016 (phase 1) Pisagua/Iquique Crustal Tomography to Understand the Region of the Earthquake Source 60 SIO OBS; 19 GEOMAR OBS; R/V Marcus Langseth with km streamer and 6600 cubic inch source ChilePEPPER: Project Exploring Prism Post-Earthquake Response 10 LDEO OBSs with integrated fluid flow meters; high-res MCS CEVICHE (with UTIG) : 2017 Seismic reflection with 15 km streamer. No OBSs see session S033 at fall AGU

8 Maule 2010 M8.8 Observations that led to ChilePEPPER: Red: 5, 10, 15 m slip contours Lorito et al. (2011) Blue: 10, 15 m slip contours Vigny et al. (2011) White: 10, 15 m slip contours Yue et al. (2014) Most slip models agreed on the position of the patch of greatest slip north of the epicenter. Few aftershocks in the outer wedge. Plate bending seismicity updip from the patch of greatest slip. Aftershock map from Rietbrock et al. 2012

9 ChilePEPPER: AGU 2010: Most discussions concluded that slip did not extend to the trench. Feb 2011 proposal to MG&G for 10 BB OBS to determine post-seismic response of outer accretionary complex to slip down-dip during the Feb earthquake. (Mike Tryon) If slip did not extend to the trench and the earthquake generated by slab pull, the outer wedge should be squeezed, leading to fluid flow and LFE and/or tremor. Into the water on the R/V Melville in May Recovered in March 2013 from R/V Point Sur. OBS were modified to include Tryon/Brown CAT flow meter in the anchor (Mike Tryon, SIO co-pi) 1200 km of high-resolution multichannel seismic data acquired along with swath bathymetry and potential field data. One bathymetric swath was coincident with a swath acquired in 2008 and in 2011.

10 Cruise MV1206 Science Party Oregon State University Anne Trehu, chief scientist Chris Kenyon, student Yi Lou, visiting student Mark Williams, student Universidad de Chile Santiago: Emilio Vera Sommer, professor Eduardo Contreras Reyes, professor Emilio Bravo, student Natalia Cornejo, student Felipe Gonzalez, student Andrei Maksymowicz, student Chilean Observer Patricio Opaza Arriagada Scripps Institution of Oceanography Mike Tryon, co-chief scientist Lee Ellett, seismic technician Jon Meyer, computer technician Keith Shadle, technician Jay Turnbull, seismic technician Lamont Doherty Earth Observatory David Gassier, OBS engineer Ted Koczynski, OBS engineer Vincent Oletu, OBS technician Drew Stolzman, OBS technician Marine Mammal Observers Chris Cutler Patti Haase Paula Olson OSU students Bridgit Hass and Alex de Moor contributed to data analysis.

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12 What we did NOT find: No detectable fluid flow. No low frequency earthquakes (but we did see the Haida Gwaii tsunami, verifying the sensitivity of the APGs at long period. All other coherent long-period signals could be associated with a teleseismic earthquakes. [note: only vertical/pressure signal available for long periods vertically propagating shear waves would not be detected]

13 ChilePEPPER results: seismicity (above) Lieser et al., (2014): aftershocks from 20 Sept. to 25 Dec., 2010 overlain by critically tapered wedge from Cubas et al. (2013) (center, right) aftershocks from May 2012-March 2013 from de Moor MS thesis (2015) (right) Velocity model from Moscoso et al. (2011) used for locating earthquakes.

14 Alex de Moor MS thesis (2015) ChilePEPPER results: seismicity (cont.) 4 events had been independently located by the Chilean National Network - depths decreased when relocated with the OBS array. A cluster of events near the plate boundary and the transition from the paleo-accretionary complex (Vp>4.5 km/s) to the younger accretionary wedge (Vp<4.5 km/s)

15 ChilePEPPER results: repeated bathymetry (2008 vs 2011/2012) Evidence for uplift of the outer wedge. (Maksymowicz et al., 2017) Swath coincident with MC06

16 ChilePEPPER seismic reflection: MC06 shows a roof thrust, with sediment thrust under the edge of the wedge and very little deformation of trench sediment. MC06 MC06 (all data time-migrated)

17 ChilePEPPER seismic reflection: ~10 km north sediment accretion: Wedge building outward. Deformation broadly distributed. Frontal thrust from seafloor almost to basement MC19 MC19 (all data time-migrated)

18 Strong variations along strike in the structure at the deformation front and in the trench. Slope along 290, illuminated from SW MC06 MC19 Bathymetric slope azimuth 290 degrees

19 ChilePEPPER lessons and questions: Seismicity at the triple junction between the subducting plate and the paleo and active acretionary wedge and within paleo-wedge and subducting plate. Uplift of the wedge between 2008 and Rapid variation along strike in deformation front structure and structure throughout the active wedge. Can we read a signature of slip to the trench in the deformation front structure? Not yet, but Yue et al, and 15 m slip contours

20 Collaboration between OSU, GEOMAR and Un. of Chile: chief-scientist Anne Trehu Co-chiefs: Emilio Vera, Michael Riedel. Post-doc: Kathy Davenport. Students from Un. Washington, GeoAzur, Geomar, Tech. Un. Berlin, Un. Munich., Un. Chile. 60 SIO SP OBSs; 19 GEOMAR OBSs (including 5 new deepwater instruments to 8000 m); 2000 km MCS data with km streamer). All OBSs recovered; Data were recorded on all SIO OBSs; We came off ship the sgy files cut with Langseth shot instants (as well as continuous miniseed).

21 Motivation: Pisagua/Iquique April 1, 2014, earthquake partially filled a seismic gap. Seismicity and geodetic deformation well recorded since 2007 because of IPOC (Integrated Plate Boundary Observatory Chile Chile, Germany, France, CalTech) (Schurr et al., Nature, 2014)

22 Significant foreshocks, well recorded by IPOC. -19 January 1, (Ruiz et al., 2014; Hayes et al., 2014)

23 Correlation between Pisagua earthquake sequence, pre-earthquake locking, and residual gravity anomalies. Slip model from Duputel et al., 2015; preearthquake locking from Métois et al., 2013.

24 (PICTURES bathymetry over GMRT. Pink lines are GEOMAR waveglider track for seafloor geodetic experiment) (Onshore piggyback: Diana Comte and Andreas Rietbrock and students.)

25 OBS data examples from PICTURES ~0.1% of OBS data available for constructing velocity cube. 12 s 2 s 12 s 65 km 42 km 2 s 54 km 10 s 85 km 0 s 121 km 15 km

26 MC19 3 of 11 deformation front crossings: Outer ~10 km of wedge reflects the subducted topography, consistent with McIntosh et al. (2007) conclusion that it is very weak. MC25 MC19 MC25 MC10 MC10

27 0 MC04: the plate boundary reflection from the deformation front to the coast CDP number x Time (s) MC01: Moho MC04: 8-14 s twtt plate boundary reflection from trench to the coast. MC04 Migrated stack Near-surface images processed by Felipe Gonzalez, Uchile; Deep structure images by Anne Trehu and Emilio Vera

28 Preliminary conclusions from PICTURES: the thin wedge below the lower slope tends to conform with the subducting plate structures, so it must be relatively weak. the midslope morphological change reflects a transition in structural integrity of the margin wedge, where velocities are very high. Plate boundary can be continuously imaged, on at least 1 transect, from the trench to the the coast. Stay posted for more 3D structure of the upper plate and the plate boundary (This is Nicaragua from McIntosh et al. (2017), but MC04 will look similar.)

29 Thank you!