Hidden Structures and Magmatic Pathway Evidenced by Small Earthquakes on Piton de la Fournaise Volcano (La Réunion)

Since the 1998 earthquake migration under Piton de la Fournaise Volcano, from 7 km depth to the surface, the volcano-tectonic activity below sea level have been relatively quiet despite a high-rate of eruptions (2.5 eruptions per year in average). A noticeable deep activity was a downward earthquake migration that followed the collapse of the summit crater on April 2, 2007. Exploiting 17 broadband stations installed in the past few years on the volcano, we applied a template matching method to detect and localize small earthquakes. The template database is built using all located events listed in the bulletin from 2012 to 2016. Shallow (a.s.l) and deep earthquakes being not distinguishable on the same set of stations, we detect theses two classes of events separately and use two different template databases. Using a set of about 150 and 1000 events for deep and shallow templates, we recovered several thousands detections of small seismic events. For each new detections, time-delays for P and S waves are measured by cross-correlation with respect to other detected events and the template database. The entire earthquake catalog (templates and new detected events) is then relocated using the double-difference technique. This approach evidences shallow and deep persistent seismic structures through years of eruptive activity. A deep upward migration of seismicity is identified in April 2015, unveiling a magma transfer that follows a pathway similar to the one identified in 1998. This deep activity is immediately followed by the occurrence of shallow events suggesting a hydraulic connection between the lower part of the volcanic system and magma reservoir located around sea level. Above sea level and below the summit crater, we identify a ring shaped structure dipping toward a decollement plane under the east flank of the volcano.