Source Variations of Small Magnitude Events in the Downdip Region of the Mexican Subduction Zone.

Slip in subduction zones produce a range of events including typical seismicity, slow slip events, and non-volcanic tremor. Understanding the nature of the transition between these different types of slip is an important area of study in order to advance our understanding of the conditions required for the spectrum of slip. The portion of the Middle America subduction zone underneath Oaxaca, Mexico is an ideal place to study this relationship because a local land based seismic and geodetic network lies above a large portion of the seismogenic zone to capture large and small magnitude earthquakes. This is also an area with slow slip events, non-volcanic tremor, and evidence for an ultra slow velocity layer near the downdip transitional region. In order to study the earthquakes at the downdip transition, we use a catalog of small magnitude (M < 5.5) locally recorded events from 2006 to 2012, prior to the 2012 M_w 7.4 Ometepec, Mexico earthquake. For each downdip event in the catalog, we compute first motion focal mechanisms using a standard community algorithm, FocMec. We see significant heterogeneity in our focal mechanism solutions. Our results show a lack of thrust events in an area with a known megathrust seismic gamp updid and prominent slow slip events observed downdip. We also find an abundance of thrust events in an area with an ultra slow layer and east of the slow slip events. Further analysis of stress drop will be used to study temporal patterns relative to the slow slip events as well as the Ometepec event.