The 1 April 2014 Iquique, Chile, Mw 8.1 earthquake rupture sequence

PUBLICATIONS Geophysical Research Letters RESEARCH LETTER 10.1002/2014GL060238 Key Points: • The 1 April 2014 Mw 8.1 earthquake ruptured about 20% of the 1877 seismic gap • The rupture was very localized and did not rupture to the trench • The northern and southern ends of the 1877 gap have now had similar ruptures Supporting Information: • Readme • Figure S1 • Figure S2 • Figure S3 • Figure S4 • Figure S5 • Figure S6 • Figure S7 The 1 April 2014 Iquique, Chile, M w 8.1 earthquake rupture sequence Thorne Lay 1 , Han Yue 1 , Emily E. Brodsky 1 , and Chao An 2 Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, USA, 2 School of Civil and Environmental Engineering, Cornell University, Ithaca, New York, USA Abstract On 1 April 2014, a great (M w 8.1) interplate thrust earthquake ruptured in the northern portion of the 1877 earthquake seismic gap in northern Chile. The sequence commenced on 16 March 2014 with a magnitude 6.7 thrust event, followed by thrust-faulting aftershocks that migrated northward ~40 km over 2 weeks to near the main shock hypocenter. Guided by short-period teleseismic P wave backprojections and inversion of deepwater tsunami wave recordings, a finite-fault inversion of teleseismic P and SH waves using a geometry consistent with long-period seismic waves resolves a spatially compact large-slip (~2–6.7 m) zone located ~30 km downdip and ~30 km along-strike south of the hypocenter, downdip of the foreshock sequence. The main shock seismic moment is 1.7 × 10 21 N m with a fault dip of 18°, radiated seismic energy of 4.5–8.4 × 10 16 J, and static stress drop of ~2.5 MPa. Most of the 1877 gap remains unbroken and hazardous. Correspondence to: T. Lay, tlay@ucsc.edu 1. Introduction Citation: Lay, T., H. Yue, E. E. Brodsky, and C. An (2014), The 1 April 2014 Iquique, Chile, M w 8.1 earthquake rupture sequence, Geophys. Res. Lett., 41, 3818–3825, doi:10.1002/2014GL060238. Received 15 APR 2014 Accepted 21 MAY 2014 Accepted article online 24 MAY 2014 Published online 6 JUN 2014 Northern Chile experienced a great subduction zone megathrust earthquake on 9 May 1877 with an estimated seismic magnitude of 8.7–8.9 [Comte and Pardo, 1991] and a tsunami magnitude M t of 9.0. Recent geodetic measurements of eastward deformation of the upper plate indicate that most of the 1877 rupture zone (Figure 1) from 19°S to 23°S has a high coupling coefficient, albeit with some patchiness along strike and along dip [e.g., Bejar-Pizarro et al., 2013; Metois et al., 2013]. This region has been identified as the north Chilean seismic gap [Kelleher, 1972; Nishenko, 1985] based on the lack of large earthquakes for the 137 years over which the Nazca plate has been underthrusting South America at about 65 mm/yr [DeMets et al., 2010]. On the order of 6 to 9 m of slip deficit may have accumulated since 1877. The earthquake history prior to 1877 is uncertain [Nishenko, 1985; Comte and Pardo, 1991], so it is unclear whether the region regularly fails in huge single ruptures or intermittently in larger ruptures then sequences of smaller ruptures, as is the case along the Ecuador-Colombia coastline [Kanamori and McNally, 1982]. The rupture zone of the 1868 Peru earthquake, with an estimated seismic magnitude of 8.5–8.8, partly reruptured in the 23 June 2011 M w 8.4 Peru earthquake (Figure 1), leaving an ~100 km long region offshore of southeastern Peru just north of the 1877 gap that may also have large slip deficit [e.g., Loveless et al., 2010]. On 16 March 2014, a M w 6.7 thrust event occurred on or near the megathrust about 60 km north- northwest of Iquique, Chile, and was followed by two weeks of thrust aftershocks that slowly migrated (~20 km/week) northward along the megathrust from 20.2°S to 19.6°S. The location of this sequence in the northern portion of the 1877 seismic gap focused attention on the region, and on 1 April 2014, a M w 8.1 interplate thrust earthquake initiated at the northern end of the foreshock sequence (19.642°S, 70.817°W, 23:46:46 UTC [U.S. Geological Survey (USGS) National Earthquake Information Center (NEIC): http://earthquake.usgs.gov/regional/neic/]). The global centroid moment tensor (gCMT) solution for this event [http://www.globalcmt.org/CMTsearch.html] indicates an almost purely double-couple faulting geometry with strike 357°, dip 18°, and rake 109° at a centroid depth of 21.9 km and centroid location south of the hypocenter (19.77°S, 70.98°W), with a centroid time shift of 42.5 s and seismic moment of 1.69 × 10 21 N m (M w 8.1) (Figure 1). A substantial aftershock sequence ensued, the largest of which occurred on 3 April 2014 with M w 7.7 (02:43:14 UTC, 20.518°S, 70.498°W, centroid depth 31.3 km), 49 km southwest of Iquique. The region extending from updip of the 3 April event southward to ~23°S, updip of the 2007 M w 7.7 Tocopilla earthquake rupture zone [e.g., Bejar-Pizarro et al., 2010; Schurr et al., 2012], remains strained and has potential for either an ~M w 8.5 event or several smaller great events (Figure 2). LAY ET AL. ©2014. American Geophysical Union. All Rights Reserved.