Slip segmentation and slow rupture to the trench during the 2015, M_w8.3 Illapel, Chile earthquake

Abstract

The 2015 Mw8.3 Illapel, Chile earthquake is the latest megathrust event on the central segment of that subduction zone. It generated strong ground motions and a large (up to 11 m runup) tsunami which prompted the evacuation of more than 1 million people in the first hours following the event. Observations during recent earthquakes suggest that these phenomena can be associated with rupture on different parts of the megathrust. The deep portion generates strong shaking while slow, large slip on the shallow fault is responsible for the tsunami. It is unclear whether all megathrusts can have shallow slip during coseismic rupture and what physical properties regulate this. Here we show that the Illapel event ruptured both deep and shallow segments with substantial slip. We resolve a kinematic slip model using regional geophysical observations and analyze it jointly with teleseismic backprojection. We find that the shallow and deep portions of the megathrust are segmented and have fundamentally different behavior. We forward calculate local tsunami propagation from the resolved slip and find good agreement with field measurements, independently validating the slip model. These results show that the central portion of the Chilean subduction zone has accumulated a significant shallow slip deficit and indicates that, given enough time, shallow slip might be possible everywhere along the subduction zone.

Subduction zone megathrust earthquakes

“…Efforts are under way to synthesize the accumulated data sets (e.g., Kanamori, 2014;Lay, 2015;Ye et al, 2016bYe et al, , 2016cDenolle and Shearer, 2016;Meier et al, 2017;Melgar and Hayes, 2017;Hayes, 2017), and we will not attempt to summarize the multitude of studies. Large shallow coseismic slip occurred in the 2015 Illapel (e.g., Li et al, 2016;Melgar et al, 2016), 2011 Tohoku (e.g., Lay et al, 2011b;Iinuma et al, 2012;Ozawa et al, 2012;Satake et al, 2013;Romano et al, 2014;Bletery et al, 2014;Melgar and Bock, 2015;Lay, 2017), 2010 Maule (e.g., Vigny et al, 2011;Yue et al, 2014b;Yoshimoto et al, 2016;Maksymowicz, et al, 2017), and 2004 Sumatra (e.g., Ammon et al, 2005;Rhie et al, 2007;Fujii and Satake, 2007) events, accompanying slip on the downdip portions of the megathrusts. In other cases, such as the 2014 Iquique, Chile (e.g., Lay et al, 2014;Hayes et al, 2014b), 2012 Nicoya, Costa Rica (e.g., Yue et al, 2013;Liu et al, 2015), 2003 Tokachi-oki, Japan (e.g., Miyazaki and Larson, 2008;Romano et al, 2010), and 2007 Pisco, Peru, ruptures (e.g., Lay et al, 2010a;Sladen et al, 2010), slip was concentrated on the central or deeper portion of the rupture zone, with no shallow coseismic slip.…”

“…The deeper parts of the megathrust produce significantly more high-frequency energy than the shallower megathrust for most of the largest earthquakes in recent decades, such as the 2010 Maule Chile M w 8.8 (e.g., Kiser and Ishii, 2011;Koper et al, 2012), 2011 Tohoku M w 9.1 (e.g., Koper et al, 2011;Ide et al, 2011;Ishii, 2011), and the 2015 M w 8.3 Illapel, Chile, event (e.g., Melgar et al, 2016;Yin et al, 2016). Ye et al (2016c) documented modest depth dependence in highfrequency radiation for the 100+ large events, with lower spectral decay rates for deeper events on subduction zone megathrusts.…”

Rupture features of the 2010 Mw 8.8 Chile earthquake extracted from surface waves

“…Furumoto and Nakanishi 1983;Christensen and Ruff 1986). Yin et al (2016) used compressive sensing (CS) method by using P-waves with frequencies of 0.05-0.5 Hz to derive an average rupture velocity of 1.4 km/s, which also low as also reported by Melgar et al (2016) in 2.0 km/s. In other words, these earthquakes occurring in the Chile region all show low rupture velocity, as analyzed from low-frequency signals (Fig.…”

“…Hwang et al 2001Hwang et al , 2011Chang et al 2010;Hwang 2014). Under the assumption that the rupture process is uniform and has a constant rupture velocity, the increasing travel time depends on the source duration and is half the source duration (e.g., Ben-Menahem 1961;Hwang 2014 Furumoto and Nakanishi 1983;Christensen and Ruff 1986;Melgar et al 2016;Yin et al 2016). Circles denote the aftershocks of the 2010 Chile earthquake.…”

Backprojection to image slip

“…M W 9.0 2011 Tohoku-oki, Japan and M W 8.3 2015 Illapel, Chile, earthquakes;Meng et al 2011;Melgar et al 2016;Meng et al 2018), it is plausible that there exist kinematic features around the rupture tip or the healing front (e.g. Madariaga et al 2006) even at shallow.…”