The influence of fluids in the unusually high-rate seismicity in the Ometepec segment of the Mexican subduction zone

The rate of earthquakes with magnitudes M w ≤ 7.5 in the Ometepec segment of the Mexican subduction zone is relatively high as compared to the neighbouring regions of Oaxaca and Guerrero. Although the reason is not well understood, it has been reported that these earthquakes give rise to a large number of aftershocks. Our study of the aftershock sequence of the 2012 M w 7.4 Ometepec thrust earthquake suggests that it is most likely due to two dominant factors: (1) the presence of an anomalously high quantity of overpressured fluids near the plate interface and (2) the roughness of the plate interface. More than 5400 aftershocks were manually detected during the first 10 d following the 2012 earthquake. Locations were obtained for 2419 events (with duration magnitudes M d ≥ 1.5). This is clearly an unusually high number of aftershocks for an earthquake of this magnitude. Furthermore, we generated a more complete catalogue, using an unsupervised fingerprint technique, to detect more smaller events (15 593 within 1 month following the main shock). For this catalogue, a high b-value of 1.50 ± 0.10 suggests the presence of fluid release during the aftershock sequence. A low p-value (0.37 ± 0.12) of the Omori law reveals a slow decaying aftershock sequence. The temporal distribution of aftershocks shows peaks of activity with two dominant periods of 12 and 24 hr that correlate with the Earth tides. To explain these observations, we suggest that the 2012 aftershock sequence is associated with the presence of overpressured fluids and/or a heterogeneous and irregular plate interface related to the subduction of the neighbouring seamounts. High fluid content has independently been inferred by magnetotelluric surveys and deduced from heat-flow measurements in the region. The presence of fluids in the region has also been proposed to explain the occurrence of slow-slip events, low-frequency earthquakes and tectonic tremors.