Probing deformation at depth using passive seismology: case of the Guerrero Mexico slow slip events

Over the last 6 years an important effort has been done in geodetic and seismic instrumentation of the Guerrero subduction zone e.g. the MASE seismic experiment (from 2005 to 2007) and the GGAP project, both a seismic and a geodetic experiments (from 2009 to 2011). Analyzing these continuous geodetic and seismic recordings, we observe seismic velocity changes and non-volcanic tremors (NVT) activities during two large slow slip events (SSE) sequences in 2006 and 2009-2010. Here we propose to compare the occurrence in time and space of SSEs, NVTs, seismic velocity changes over the two last SSEs in order to better understand the complex mechanical behavior of the subduction zone in this region. A perturbation of the mechanical properties of rocks within the Earth’s crust has been recently observed from a velocity perturbation associated with quasi-static deformation at depth produced by the 2006 SSE (Rivet et al. 2011). This velocity perturbation maximizes in the middle crust at 20km depth. Similarly we observe a velocity decrease during the 2009-2010 SSE. This last SSE presents two distinctive slipping patches (Walpersdorf et al, 2011). The velocity drop is greater at the time of the first subevent, which occurred below the seismic array, than at the time of the second subevent. We observe in both 2006 and 2009-2010 SSE that the velocity perturbation within the crust is closely related to the strain rate produced by the slip at the plate interface. This suggests that the deformation of the overlying crust shows significant nonlinear elastic behavior. During the 2006 SSE, Kostoglodov et al. (2010) and Husker et al. (in prep) observe an increase in NVT activity. Using the slip model that describes the slip evolution during the SSE (Radiguet et al., 2010) we observe that the NVTs follow the slip front where the shear stress maximizes. However other NVTs burst are observed outside the SSEs period and are not associated with SSEs or velocity changes. Only one of these bursts in 2005 is clearly associated to a small SSE detected on geodetic measurement and a velocity drop. These observations suggest a rather complex relation between slow slip and NVTs activity.