Changes in stress revealed by seismic rate changes at El Chichón and Tacaná volcanoes, following the 8 September 2017, Mw8.2 Chiapas, Mexico, earthquake

In certain circumstances, such as the imminent eruption of a volcano, it has been suggested that "large" and "near-field" earthquakes have the capacity to "accelerate", "clock-advance", or "trigger" the eruption. In contrast, these earthquakes have no effect, or a minimal effect, on a volcano that is far from erupting. Changes in earthquake rates are frequently used as a stress-meter. In this study, we compared seismic activity beneath the two most active volcanoes in Chiapas, Mexico: El Chichón and Tacaná, before and after the 8 September 2017, M w 8.2 earthquake. This comparison can provide insight into volcanic stress changes and proximity (or not) to an eruption. At El Chichón, seismicity increased a few days after the 2017 earthquake and remained elevated for five weeks before returning to pre-earthquake levels. At Tacaná, seismicity increased 15 weeks after the 2017 earthquake and remained elevated for two weeks before returning to pre-earthquake levels. This timing difference may be related to the type of fluid involved in generating the swarms: water at El Chichón and magma at Tacaná. Although the presence of volcanic seismicity indicates that El Chichón and Tacaná are seismically active, minor changes in amplitude and duration following the 2017 earthquake suggest minimal stress changes at these volcanoes. This is consistent with the absence of eruptions in 2017-2018. Previous field observations identified zones of hydrothermal manifestations at both volcanoes. These zones coincide with the locations of hydrothermal seismicity that we identified. • Location of hydrothermal seismicity and relation to shallow geothermal manifestations • Identification of magma reservoir and dyke at Tacana volcano • Triggered seismicity after the 8 September 2017, Mw8.2 earthquake