Interaction between historical earthquakes in the seismic gap of central Chile and the Marga-Marga crustal Fault: The seismic potential of the Valparaiso region.

The characterization of the spatial distribution of historical earthquake ruptures in a seismic segment plays a fundamental role in our understanding of the seismic cycle of significant earthquakes and in assessing the potential hazards associated with future events of this nature.

Due to its tectonic behavior, Chile has been impacted by megathrust earthquakes of considerable magnitude, such as the Valdivia 1960, Maule 2010, and more recently, the Illapel 2015 events. However, there are certain areas where no large earthquakes have occurred and are thus considered to be in a seismic gap. Despite experiencing some significant events, they do not manifest the required energy release properties and depth to compensate the accumulated friction. All these earthquakes, which represent varying stages of the seismic cycle, interact with different geological characteristics of the segment. This is evident in the central zone of Chile, specifically in the Valparaíso region, which has been in a seismic gap since the last major surface-rupturing earthquake of 1730.

During the Maule 2010 and Illapel 2015 earthquakes, rupture occurred only in the southern and northern segments in the mentioned area. Despite seismic activity in 1822, 1906, 1985, and 2017, and the presence of the Marga-Marga crustal fault in Viña del Mar, the energy release has not been sufficient to trigger the expected seismic sequence. It is worth noting that the fault is dangerously located in the most densely populated and frequented area of the city of Viña del Mar, presenting a threat to the surrounding population greater than what could be expected from a subduction earthquake itself.

This research aims to identify and quantify the interaction between the subducting and the Marga-Marga faults in order to assess the potential seismic activity in the area, considering that the crustal earthquakes caused by faults such as Marga-Marga are potentially more destructive than subduction earthquakes of equal magnitude. A relevant precedent is the interaction between the rupture of the Maule 2010 earthquake and the active fault segment of Pichilemu, which triggered a seismic swarm in 2011.

To achieve this, a study was conducted to characterize the slip associated with tsunamigenic events that occurred in the Central Chile segment in 1730, 1906, and 1985. The study revealed deformation patterns, indicating that the last shallow movement occurred in 1730, followed by deep patterns along the coast for subsequent events. Historical data was collected, and a stochastic modeling methodology was applied to comprehensively reconstruct the events. The Coulomb stress transmission between the Marga-Marga fault and subduction events, such as the one in 1906, was then characterized using the newly acquired information from historical deformation to identify potential activation zones of the crustal fault. Currently, efforts are underway to implement a methodology that uses computational simulation tools to visualize the impact of a coseismic event, such as the one in 1730, on the crustal fault and the surrounding region. The aim is to understand the past behavior of the region to be prepared for potential future activations.