Non-Volcanic Tremor activity in the Chile Triple Junction detected by OBS stations

Five ocean bottom seismometers (OBS) were deployed over the Chile Triple Junction (CTJ). The maximum distance between stations was 60 km and the minimum distance was 23 km. Four stations were deployed on the East of the trench and one on the West zone. For six months, from March to August 2010, the five OBS stations recorded continuously. Using the envelope technique, we were able to detect and localize episodes of non-volcanic tremors (NVT) that lasted from hours to days. Unlike ordinary local earthquakes, the NVT in the CTJ are characterized by a low frequency content (2-8 Hz) and durations greater than 100 [s]. In order to improve the location of NVT, we cross-correlated the vertical and horizontal components of all NVT located by the envelope technique. In different months, we measured a local maximum of the correlation at a lagged time near to 2 [s] and associated with the lag between the S and P waves (S-P time). Furthermore, we observed that in the days with intense tremor activity the maxima corresponding to the S-P time emerges in windows without observable NVT. We suggest that days with intense tremor activity correspond to a continuous slow slip, which accelerates and decelerates nearly randomly. The changes of moment rate of slow slip are observed as intense NVT activity. When the velocity of the slow slip is nearly constant, visible tremors are lost in the ambient seismic noise, however they are detectable by S-P time cross-correlate technique.