## 1 Introduction  
The 2016 \(M\) 7.8 earthquake occurred offshore of the west coast of Northern Ecuador, where the plate boundary between the Nazca and the South America plates lies (Figure 1). During this earthquake, at least 668 people were killed, and 27,732 people were injured. More than 7000 buildings were destroyed, including most of the town of Pedernales and its surrounding urban areas [1]. The urban earthquake disaster is growing throughout the world and has become a serious problem in developing countries. Unless earthquake monitoring is improved, subsequent aftershocks will have more disastrous social and economic consequences [2]. Due to its unique geodynamic environment, seven \(M>7\) earthquakes have occurred within 250 km of this event since 1900.  
The \(M\) 7.8 mainshock was followed by six \(M\geq 6\) aftershocks, the largest being an \(M\) 6.9 aftershock on 18 May about one month after the mainshock. The epicenter locations and focal mechanisms of these aftershocks are consistent with the plate boundary subduction zone between the Nazca and the South America plates.  
In recent years, many seismology scientists worldwide have focused on coulomb stress triggering and the correlation between the mainshock and subsequent aftershocks in an earthquake sequence. Much research on large earthquake sequences has concluded that stress changes from the mainshock affect the locations of subsequent aftershocks [4; 5; 6]. The coulomb stress triggering theory has been proposed for evaluating aftershock hazards after great earthquakes. This theory implies that coulomb stress changes due to a mainshock can promote nearby faults closer to failure stress and trigger the occurrence of some aftershocks. It was thought that small coulomb stress changes can alter the likelihood of earthquakes on nearby faults [7; 8]. The occurrence of earthquake activity can be promoted when the coulomb stress increases as little as 0.1 bar on a seismogenic fault [9; 10]. Small positive changes in coulomb stress due to an earlier mainshock can trigger subsequent earthquakes.  
The coulomb stress change triggered by a mainshock has been widely studied to explore its interaction with aftershock activities. Coulomb stress triggering of moderate-to-large earthquakes has been observed in the US [4; 11; 12; 13], in Europe [14; 15; 16], on the Sunda Trench [17; 18; 19], and in Japan [20; 21; 22; 23; 24; 25]. However, in South America and Ecuador, the stress triggering theory has yet to be extensively studied [26; 27; 28]. The 2016 Ecuador earthquake sequence provides a unique opportunity to test the interaction between a large subduction earthquake that imparted coulomb stress changes and the subsequent seismicity. In this study, based on the Coulomb 3.3 software [29] (3.3, USGS, Menlo Park, CA, USA), which implements the elastic half space of Okada [30], we investigate the coulomb stress triggering theory with the 2016 Ecuador \(M\) 7.8 earthquake sequence. Firstly, we collect the focal mechanism solutions of six \(M\geq 6\) aftershocks from the USGS (US Geological Survey). Secondly, we compute the coulomb stress changes on both nodal planes of these events. We determine the regional stress direction from the principal axes. Thirdly, we compute the coulomb stress, normal stress, and shear stress changes in the focal source of each \(M\geq 6\) aftershock. Our primary goal is to find the correlation between the coulomb stress changes and the spatial distribution of the following  
Figure 1: The epicentral region of the 2016 Ecuador \(M\) 7.8 earthquake. The earthquake occurred as a result of shallow thrust faulting on the plate boundary subduction zone between the Nazca and South America plates, where the Nazca plate subducts eastward beneath the South America plate with a rate of 61 mm/year (modified from US Geological Survey (USCS) [3]).  
aftershocks. In addition, we want to determine whether the coulomb stress triggering theory can explain the Ecuador \(M\) 7.8 earthquake sequence.