Focal Mechanism from Very Low Frequency Earthquakes Associated with Tectonic Tremors in Guerrero, Mexico.

Abstract:

Tectonic Tremors (TT) have been found in Guerrero, in the flat part of the subduction interface. However, no extensive moment tensor analysis have been carried out. In this study we better constrain the location of TT and estimate their mechanisms. Very Low Frequency (VLF) earthquakes are thought to be part of the same process as tremors and are a good proxy for estimating the moment tensor of this deformation process. To do this, we locate TT by an envelope cross-correlation method [Ide, 2010; Obara, 2002]. Then, we stack the band-passed (0.02-0.05 Hz) waveform at the time of occurrence of tremors, for every tremor within 10 km of the point considered. The stacked waveforms are then inverted to determine the moment tensor and depth of these VLF events. Finally VLF signals are detected by network correlation with the stacked VLF events adding a better constraint on the magnitude of these events. Two time periods are considered. The first one, between 2005 and 2007, at the time of the MASE experiment, allows a good localization along dip of the tremors. The second time period, between 2009 and 2013, at the time of the GGAP experiment, gives better localization along strike of the main cluster. In addition, permanent broadband stations of the Servicio Seismológico Nacional (Mexico) are used for both time periods. The TT locations are found in two spots, the transient zone and “Sweet Spot” [Husker, 2012], as previously determined. The second time period reveals the sweet spot seems to be separated in two clusters in the strike direction. Our results show that the VLF sources are located at or close to the plate interface. These events have generally very low-angle thrust mechanisms in agreement with the geometry of the nearly horizontal subduction interface. The slip directions of VLF earthquakes are also consistent with the plates convergence vector. In addition, some clear VLF signals are detected without any stacking. Their focal mechanisms is consistent with the mechanisms of the stacked signal. These events occur at the same time and areas as the TT thus supporting a common origin of the two phenomena. The energy relationship between tremors and VLF earthquakes is then investigated with these coincident detections.