CHARACTERIZING SHALLOW TREMORS SOURCE IN THE GUERRERO SEISMIC GAP, MEXICO
The Guerrero seismic gap, located along the Mexican subduction zone, has not experienced a major earthquake in over 110 years, making it a key region for studying seismic processes. GNSS data indicate that this segment exhibits relatively low interplate coupling compared to neighboring areas, raising questions about the mechanical behavior of the plate interface. Additionally, the Guerrero gap is known for hosting a variety of slow earthquakes, including tectonic tremor and slow slip events, many of which occur offshore.
Thanks to a recent deployment of ocean-bottom seismometers (OBSs) and ocean-bottom pressure sensors (OBPs), new shallow slow earthquakes have been detected offshore, shedding light on previously unknown sources. In this study, we focus on tectonic tremors recorded by the OBS array, which had been identified but not yet characterized in detail. Estimating the focal mechanisms of these tremors is particularly challenging due to sparse and non-ideal offshore data. However, after correcting for horizontal sensor orientation, we inverted the S-wave polarization angles to determine their focal mechanisms. The resulting slip azimuth directions align with the convergence between the Cocos and North American plates, providing evidence that these shallow tremors are occurring along the subduction plate interface.
To assess the energy release, we corrected the observed tremor spectra for attenuation and site effects using recent offshore estimates in a frequency band between 2-8 Hz. Our results show a heterogeneous distribution of radiated seismic energy along the strike of the seismic gap, with scaled energies ranging from 10⁻⁸ to 10⁻⁷. This variability may reflect structural and frictional contrasts across the shallow interface, providing new insights into the mechanics of slow slip and tremor in a key seismic region.