! ! ! ! "#$%&'!&'#!()*+')*#!&*!(*)),!&*! ! -'.$*#!/0,)'#1!-21!&*!34536! ! ! ! ! ! ! ! ! ! ! ! ! 7.89*)#8&,&*!&*!:;'!<,%0'!=7:<>!!*!!7.89*)#8&,&*!&*!?),#@08,!=7.?>! ! ! ! ! Centro de Sismologia da USP IAG / IEE !!!!/*.$)'!&*!:8#+'0'A8,!&,!7:<!=BC2DB"">! ! ! :B:D7.?E!FG#*)9,$H)8'!:8#+'0HA8I'!&,!7.?! ! ! A lacunaJ*0,$H)8'!!&*!! sísmica55K4LK345L do! Norte do Chile: estado da arte ! "! Hans Agurto-Detzel São Paulo, 15-Setembro-2014 B05310 SCHOLZ AND CAMPOS: SEISMIC COUPLING B05310 Figure 4. The earthquake history and coupling of Chile. See text for sources of couplingScholz and Campos, 2012. data. JGR Ward [1990] as far as 400 km downdip. Because we study concluded it was fully coupled on a 20 dipping surface only the seismogenic portion of the interface, for our cal- down to 35 km depth, below which the coupling linearly culations we use the geodetic estimates of the moment. tapered to zero at 55 km. By the same reasoning as discussed Lorenzo-Martin et al. [2006] found, from GPS measure- in the Cascadia case, we take the fully locked region to be ments, that c = 0.96 for A = 110 109m2 and an assumed the seismogenic area. This boundary seems to have been G c  subduction velocity of 69 mm/yr, yielding P_ G = 7.3 F. defined by the Mw 7.7 Tocopilla earthquake of 2007, in [65] The only known preceding earthquake of size similar which its region of maximum slip was concentrated around to the 1960 event was one that occurred in 1575 [Cisternas 35 km, with some slip, possibly post-seismic, extending to 50 km depth [Bejar-Pizarro et al., 2010]. Hence with this et al., 2005]. Using that 385 yr recurrence time yields P_ S = assumption c = 1 and with v = 79 mm/yr on a 102 450 6.2 F, for a cS = 0.8. If we include in that period the 1837 G p  earthquake, estimated to have been about half the size of km surface, P_ G = 3.6 F. Vargas et al. [2005] identified sec- tor-rupturing earthquakes prior to 1877 in 1768, 1543, and 1960 [Abe, 1979], we get P_ S = 9.3 F, larger than P_ G (this is an overestimate because a goodly portion of the 1837 earth- 1430, which indicate an average T of 149 years [Comte and quake occurred in the Central Chile segment). The tsunami Pardo, 1991; Nishenko, 1985]. Assuming Mw 8.8 for these record of this region indicates great tsunami-generating earthquakes and this value of T yields P_ S = 3.4 F and cS = events every 285 years on average [Cisternas et al., 2005]. 0.94. This consists of the less frequent 1960-type events that rup- ture the entire 850 km length of this segment and smaller 3.16. Central and Southern Peru earthquakes like that of 1837 that rupture portions of it. [67] The earthquake history and coupling of this region are [66] Northern Chile: This region, from 18 to 23S, is a shown in Figure 5 (modified after Dorbath et al. [1990]). 450 km long seismic gap that last ruptured in the Iquique The coupling of the northern two-thirds of this region was earthquake of 1877, with a magnitude estimated at Mw determined by Perfettini et al. [2010]. They found three 8.8–9.0 [Abe, 1979; Bejar-Pizarro et al., 2010; Comte and zones of high coupling, shown in Figure 5. A narrow zone of Pardo, 1991; Kausel and Campos, 1992]. Chlieh et al. low coupling separates the northern two. The southern two [2004] studied this area with GPS and INSAR and are separated by a much wider zone of low coupling that 12 of 22 LETTER doi:10.1038/nature13681 Gradual unlocking of plate boundary controlled initiation of the 2014 Iquique earthquake Bernd Schurr1, Gu¨nter Asch1, Sebastian Hainzl1, Jonathan Bedford1, Andreas Hoechner1, Mauro Palo1, Rongjiang Wang1, Marcos Moreno1, Mitja Bartsch1, Yong Zhang2, Onno Oncken1, Frederik Tilmann1, Torsten Dahm1, Pia Victor1, Sergio Barrientos3 & Jean-Pierre Vilotte4 On 1 April 2014,Northern Chile wasstruck by a magnitude 8.1earth- rupture area was less than 2.6 m (refs 6, 7) leaving most of the past slip quake following a protracted series of foreshocks. The Integrated Plate deficit of ,8–9 m accumulated since 1877 unaffected8. On 1 April 2014, Boundary Observatory Chile monitored the entire sequence of events, the Mw 8.1 Iquique earthquake north of Iquique struck the central por- providing unprecedented resolution of the build-up to the main event tion of the gap. Using seismological and geodetic observations we here and its rupture evolution. Here we show that the Iquique earthquake analyse the rupture, its relationship toprevious locking of the plate inter- broke a central fraction! of the so-called northern Chile seismic gap, face, and the pre-seismic transients leading up to the earthquake. the last major segment of the South American plate boundary that 1,2 2 M density had not ruptured in the past century . Since July 2013 three seismic 0 0 M 0 1 1868 2001 8 clusters, each lasting a few weeks, hit this part of the plate boundary .4 1906 with earthquakes of increasing peak magnitudes. Starting with the –18º 1948 second cluster, geodetic observations show surface displacements that Arica can be associated with slip on the plate interface. These seismic clus- 1956 tersand their slip transients occupied a part of the plate interface that 1987 –19º was transitional between a fully locked and a creeping portion. Leading 1945 up to this earthquake, the b value of the foreshocks gradually decreased 1877 2014 04 01 M 8.1 Pisagua 1869 1911 during the years before the earthquake, reversing its trend a few days 1933 –20º before the Iquique earthquake. The mainshock finally nucleated at Iquique the northern end of the foreshock area, which skirted a locked patch, 1940 2014 04 03 M 7.6 Se 1871 and ruptured mainly downdip towards higher locking. Peak slip was ismic gap –21° 1906 attained immediately downdip of theforeshockregionandatthemar- 1905 gin of the locked patch. We conclude that gradual weakening of the –1 1967 central part of the seismic gap accentuated by the foreshock activity Latitude 6.7 cm yr in a zone of intermediate seismic coupling was instrumental in caus- 2007 –22° 1928 ing final failure, distinguishing the Iquique earthquake from most 2007 great earthquakes. Finally, only one-third of the gap was broken and 5 M 7.7 –23° 4 Mejillones the remaining locked segments now pose a significant, increased seis- 1947 peninsula 1995 3 slip mic hazard with the potentialto host anearthquake with a magnitude (m) of .8.5. 1998 2 Antofagasta 1987 –24° 1995 The northern Chile–southern Peru seismic gap last broke in 1877 in 1 M 8.1 1 1942 BB+SM a great earthquake (Mw ,8.8) that ruptured from south of Arica to the 100 km SM 1936 1910 1947 Mejillones peninsula (see Fig. 1). The reported historical recurrence inter- 1960 GPS 50 cm 1988 val for the past 500 years in this region has been estimated at 111 6 33 1900 2000 –73° –72° –71° –70° –69° 1 years , making it probably the most mature seismic gap along the South Time (years) Longitude American plate boundary. In the past two decades the two adjoining segments south and north broke in the M 8.1 Antofagasta earthquake Figure 1 | Map of Northern Chile and Southern Peru showing historical w earthquakes and instrumentally recorded megathrust ruptures. IPOC of 1995 (refs 3, 4) and the Mw 8.4 Arequipa earthquake of 2001 in south- 5 instruments used in the present study (BB, broadband; SM, strong motion) are ern Peru . In the previous cycle the southern Peru and northern Chile shown as blue symbols. Left: historical1,2 and instrumental earthquake record. segments broke within few years (Fig. 1), suggesting that they might be Centre: rupture length was calculated using the regression suggested in ref. 28, 1 coupled in time . The imminence of a large megathrust event in this with grey lines for earthquakes M .7 and red lines for Mw .8. The slip region motivated the setting up of an international monitoring effort distribution of the 2014 Iquique event and its largest aftershock derived in this with the Integrated Plate Boundary Observatory Chile (IPOC). Having study are colour coded, with contour intervals of 0.5 m. The green and black started in 2007, there now exists an exceptional database that monitors vectors are the observed and modelled horizontal surface displacements of the 4,5,7 the gradual plate boundary failure with various geophysical techniques. mainshock. The slip areas of the most recent other large ruptures are also . plotted. Right: moment deficit per kilometre along strike left along the plate Several major earthquakes (Mw 7) have occurred in this gap since boundary after the Iquique event for moment accumulated since 1877, 1850 (Fig. 1); the largest until now was the Mw 7.7 Tocopilla earthquake assuming current locking (Fig. 3a). The total accumulated moment since 1877 in 2007, which broke the southern rim of this segment beneath and north from 17u Sto25u S (red solid line) is 8.97; the remaining moment after of Mejillones peninsula along a total length of 150 km. Only the down- subtracting all earthquake events with Mw .7 (grey dotted line) is 8.91 for the dip end of the locked zone slipped in this event, and the total slip in the entire northern Chile–southern Peru seismic gap. Lay et al. (2014) GRL 1GFZ Helmholtz Centre Potsdam, German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany. 2School of Earth andSchurr Space Sciences, et al. (2014) Nature Peking University, Beijing 100871, China.