Forming a Mogi Doughnut in the Years Prior to and Immediately Before the 2014 M8.1 Iquique, Northern Chile, Earthquake

Abstract

Asperities are patches where the fault surfaces stick until they break in earthquakes. Locating asperities and understanding their causes in subduction zones is challenging because they are generally located offshore. We use seismicity, interseismic and coseismic slip, and the residual gravity field to map the asperity responsible for the 2014 M8.1 Iquique, Chile, earthquake. For several years prior to the mainshock, seismicity occurred exclusively downdip of the asperity. Two weeks before the mainshock, a series of foreshocks first broke the upper plate then the updip rim of the asperity. This seismicity formed a ring around the slip patch (asperity) that later ruptured in the mainshock. The asperity correlated both with high interseismic locking and a circular gravity low, suggesting that it is controlled by geologic structure. Most features of the spatiotemporal seismicity pattern can be explained by a mechanical model in which a single asperity is stressed by relative plate motion.

Microseismicity appears to outline highly coupled regions on the Central Chile megathrust

“…Feng et al, 2012;Ader et al, 2012;Yarce et al, 2019). The along-strike separators, in contrast, are only active in episodically occurring bursts (Figure 2c), most prominently when activated by nearby events (similar to observations of Schurr et al, 2020). This means that studies of only a few years of seismicity (like ours) may or may not observe the signature of such separators.…”

“…With a simple mechanical model setup using realistic geometries and plate velocity conditions ( Figures 6, 8), we were able to retrieve distributions of shear stress accumulation where concentrations of stress correspond to where seismicity is observed on the Central Chile megathrust ( Figure 6), and predicted upper plate deformation from these models fits GPS observations reasonably well (Figure 9). The physical process demonstrated by our models has already been shown and discussed in Dmowska and Li (1982) and Schurr et al (2020) for the case of a single asperity surrounded by weaker material capable of releasing part of the interseismic loading through creep. Our present results extend this to the case of several (here: three) asperities in a line setup, two of them in a region that is considered to be in the late part of the interseismic stage.…”

“…From the analysis of 4.5 years of seismic data, we obtain a high-resolution earthquake catalog that contains >8,500 events located on the Central Chile plate interface. These events are not homogeneously distributed, but describe geometries resembling half-ellipses, similar to what has recently been observed for the period before the 2014 Iquique earthquake in Northern Chile (Schurr et al, 2020). In order to check whether the seismicity geometries could outline areas of elevated interplate coupling and thus frictional strength on the megathrust, we use the obtained geometries to constrain GPS inversions for interplate locking and mechanical models of the megathrust.…”

“…Dmowska and Li, 1982) and observed in rock mechanics experiments (Goebel et al, 2012), only very few clear observations of Mogi doughnuts have been made to date (e.g. Schurr et al, 2020). In contrast, observations of aftershock seismicity surrounding the main shock slip areas are well established (Das and Henry, 2003).…”

Seismic images of the Northern Chilean subduction zone at 19°40′S, prior to the 2014 Iquique earthquake

“…Here, interface reflections are traceable until 45 km downdip of the subduction front. Based on the imaging results, we provide a characterization of the subduction interface in the rupture area prior to the Iquique earthquake and compare it with the location of both the main slip zone, as well as fore-and aftershocks (Sippl et al 2018;Schurr et al 2020), and with results from other geophysical studies, such as refraction interfaces from wide-angle analysis of this profile (Patzwahl et al 1999) and density-depth-modelling from Maksymowicz et al (2018). Furthermore, we discuss the tectonic implications of the imaged structures in the frontal unit and the sediment layers, the structures in the continental crust, as well as the large splay faults.…”