Shallow Nonvolcanic Tremor Activity and Potential Repeating Earthquakes in the Chile Triple Junction: Seismic Evidence of the Subduction of the Active Nazca–Antarctic Spreading Center

ABSTRACT In southern Chile, at ∼46.2°S and ∼75.2°W, the active spreading center between the Nazca and Antarctic plates is colliding with the South American plate, forming the Chile triple junction (CTJ). For 1 yr, from March 2009 to February 2010, five ocean‐bottom seismometers (OBSs) were deployed over the CTJ. We used a portion of the OBS data to study the seismic signatures of the subduction of the active Nazca–Antarctic spreading center. Using the envelope technique, we detected long episodes of shallow nonvolcanic tremor (NVT) activity. To improve the identified location of the NVT activity, we cross‐correlated the vertical and horizontal components of all located NVTs. In different months, we measured the local maximum of the lag‐time correlation near 2 s, which is associated with the lag between the S and P waves (S−Ptime). Furthermore, we observed that in the days with intense tremor activity, the maxima corresponding to S−Ptime emerged in windows without observable NVTs. We suggest that days with intense tremor activity correspond to an almost continuous slow slip, which may accelerate and decelerates nearly randomly, with spatial and temporal heterogeneity. In addition, we detected some potential repeating earthquakes with an S−Ptime near 2 s, as well as NVTs. The detected NVT activity and potential repeating earthquakes suggest the existence of a shallow region close to the CTJ that is able to generate brittle (earthquakes) and brittle–ductile (potential repeating earthquakes and NVTs) ruptures.