First analysis of shallow tremors in the Guerrero seismic gap.

The Guerrero seismic gap, at the Mexican subduction zone, has been a region of great seismological interest because of the absence of a large earthquake in more than 110 years. If an earthquake were to rupture the entire Guerrero seismic gap the resulting earthquake could be disastrous to major Mexican cities. Additionally, the Guerrero subduction zone has plenty of slow earthquake activity with large slow slip events and tectonic tremors, located at the deep plate interface. To obtain a new and unique observation point of seismicity in the Guerrero seismic gap and continue evaluating its seismic risk, we deployed an array of ocean bottom seismometers (OBS) offshore the Guerrero seismic gap. We were able to detect and locate shallow tremors near the trench and deduce that a portion of the shallow plate interface undergoes stable slip. We used data from the OBS to analyse the new catalogue of shallow tremors and describe their source. Focal mechanisms of shallow tremors were estimated using S wave polarisation. We found that slip azimuth tends to follow the subduction plate motion, suggesting that tremors rupture at the plate interface. We also estimated shallow tremor radiated seismic energy. We found a heterogeneous energy release of shallow tremors along strike. Our observations of a heterogeneous shallow tremor energy release can be explained with the different mechanical properties, inside and outside the Guerrero seismic gap, and help to characterise the seismogenic zone at the shallow plate interface.