New constraints on the seismotectonics and crustal structure of the Eastern Trans-Mexican Volcanic Belt based on local seismicity, focal mechanisms and a preliminary state of stress evaluation

After the occurrence of unusual upper crustal seismic activity within the eastern sector of the Trans-Mexican Volcanic Belt (TMVB) in 2019, an area of generally low seismicity, an effort was made to interpret the origin. Concerns leading to the careful analysis presented here, were manifold: Shallow clustered seismicity can indicate active volcanism or related processes, as well as hydrothermalism. Additionally, the area was struck by a major earthquake (Mw 6.4) on January 4, 1920, which had devastating effects on the area's complex geological features and caused hundreds of deaths and significant material damage. As a result, several events of the 2019 sequence of unusual seismic activity have been relocated to precisely know the location. The 34 earthquakes of the 2019 seismic sequence occur in a relatively small area of less than 1600 km2. The seismic sequence can be further subdivided into 2 clusters, separated by the Pico de Orizaba – Cofre de Perote volcanic chain, which itself separates two tectonostratigraphic terranes, is a topographic divide and represents the limit to a considerable topographic drop. We also used the seismic sequence to constrain further the state of local stress and henceforth observe the tectonic regime of the area. For this purpose, focal mechanisms of the events with Mc > 4 (this sequence and previous data) were determined and the inversion of the stress tensor of this group was carried out. Similarly, the state of stress for the central zone of the TMVB was recalculated to establish its possible variations towards the eastern part of the TMVB. We show that the state of stress of the Eastern/Easternmost TMVB presents a transtensional regime, similar to the central TMVB, which has been suggested is mainly caused and controlled by the geometry of the subduction of the Cocos plate beneath the North American plate. A review of Rayleigh wave dispersion curves, calculated through interferometry of ambient seismic noise, a change is observed that suggests some variation in the structure of the eastern and easternmost areas. This contribution is relevant because little is known about the seismotectonics of the TMVB's eastern sector, in part because of the low cortical seismicity reported along the TMVB, the reduced coverage of seismic stations in this region and the distance from the trench Middle American Trench (MAT), where most of the important tectonic processes in Mexico take place. Recently, with the incorporation of a regional seismic network, the detection of small-magnitude events in the study area has significantly been improved.