Effects of the September 19, 2017 Earthquake on the Structural Pattern of Subsidence in the Southeastern Portion of the Basin of Mexico

Most of the urban area in the Basin of Mexico is composed mainly of clay and silt-sandy materials highly saturated with water, of fluvial-lacustrine origin intercalated by materials of volcanic origin (deposits of volcanic ashes, lava spills and pyroclastics), which are highly heterogeneous, in composition and structure. These offer poor foundation conditions for Low- and high-rise buildings. On the other hand, water supply requirements have led to over-exploitation of the aquifers, which increased the rate of subsidence affecting highly populated zones.

A geophysical study was carried out in 2009 in a residential zone located towards the southeastern of the Mexico Basin, employing electrical resistivity tomography (ERT-2D) methods to characterise the fracture and subsidence pattern at depth. Later on, local authorities refurbished the damaged buildings and urban infrastructure. Unfortunately, fractures showed up again after a while due to the continuous ground water over-exploitation.

As a consequence of the September-2017 earthquake with magnitude 7.2, new fractures appeared showing bigger displacements, affecting dramatically several apartment units, extending the original damaged area studied in 2009. Therefore a further study was carried out in the area, which included 15 ERT-2D profiles, a mapping of fractures by means of a DRON SISMEMA eBee Ag, with RGB system (high resolution camera G9x (20MP), and a regional geological investigation.

It was possible to define the main fracture system in the SW-NE, NS and NW-SE direction, showing a semicircular alignment in the E-SE direction, characterised by two main fractures that are almost parallels, whose separation varies between 5 and 25 m, depicting a sunken block between them. The September of 2017 earthquakes exacerbated the displacements between 5 cm and 30 cm more. The alignment of these structures may be due to the presence of a buried rocky horizon (remains of lava flows), coming from El Peñon del Marques volcano, located 3.8 km northeast of the studied area. Such a subsidence process was aggravated by the telluric movement and the over-exploitation of the aquifers in the zone.