Remotely triggered subsidence acceleration in Mexico City induced by the September 2017 M_w 7.1 Puebla and the M_w 8.2 Tehuantepec September 2017 earthquakes

Mexico City, a large megacity with over 21 million inhabitants, is exposed to several hazards, including land subsidence, earthquakes, and flooding. Hazard assessments for each hazard type is typically treated separately and usually do not include considerations for any relations among the hazards. Our data makes it plausible for an earthquake triggering case that temporarily accelerated the subsidence rate in the metropolitan area as a result of the M_w 8.2 Tehuantepec and the M_w 7.1 Puebla, September 2017 earthquakes that affected Mexico City. Furthermore, the triggering effect induced rapid slip along previously developed shallow faults associated with subsidence. These results indicate that any future scenario of land subsidence should consider a potential triggering effect by large earthquakes. Similarly, earthquake hazard assessments should also consider potential impact on shallow faulting and fracturing associated with land subsidence.

1 Land subsidence in Mexico City

Mexico City is one of the fastest-subsiding metropolises in the world, where subsidence rates exceed 360 mm yr⁻¹ (Fig. 1). The subsidence occurs mainly as the response to aggressive groundwater extraction over the past century, causing progressive damage to the city's buildings and critical infrastructure. The subsidence process has been documented for almost a century (Gayol, 1925; Carrillo, 1948; Figueroa-Vega, 1984; Ortega et al., 1993). Subsidence rates vary spatially, mainly due to highly heterogeneous shallow stratigraphic sequence beneath the city (Santoyo-Villa et al., 2005; Auvinet et al., 2017), which responds differentially to groundwater extraction. Highest subsidence rates occur above thick layers of lacustrine sediments (Cabral-Cano et al., 2008; Solano-Rojas et al., 2015), whereas non-subsiding areas correspond to volcanic rocks. Subsidence occurs differentially, particularly between stable volcanic rocks and highly subsiding sediments, producing significant topographic elevation changes and causing shallow faulting and fracturing along well-defined areas of the city (Fig. 2).

Figure 2 Horizontal subsidence gradient map for Mexico City derived from the Sentinel-1 velocity map (Fig. 1). Black lines show Mexico City's municipalities. Blue lines show the field mapped subsidence associated faults after CENAPRED (2017). The shaded topographic base layer is taken from Continuo de Elevaciones Mexicano 3.0 © Instituto Nacional de Estadística Geografía e Informática (INEGI).

Figure 3 Sentinel-1 based wrapped differential interferograms for the Mexico City Metropolitan Area covering (a) 19 July–12 August 2017, (b) 12 August–5 September 2017, and (c) 5–29 September 2017. Color lookup table for all interferograms depicts 1 cycle = 6.6 cm.

Mexico City subsidence has been studied in the past with conventional topographic methods (e.g., Auvinet et al., 2017). However, in the past decade subsidence has monitored by advanced satellite geodetic techniques, mainly satellite Interferometric Synthetic Aperture Radar (InSAR) and GPS (e.g. Cabral et al., 2008; López-Quiroz et al., 2009; Osmanoğlu et al., 2011; Du et al., 2019). These studies have routinely detected the highest subsidence rates in the eastern sector of the city within two areas that correspond to the former Texcoco and Chalco-Xochimilco lakes (Fig. 1). Moreover, most of the previous studies focused on the city-wide subsidence signal, which is important for understanding the city's aquifer mechanics in response to groundwater extraction. Both GPS and InSAR measurements indicated limited temporal changes in the subsidence rate throughout the past two decades with limited seasonal variability.

2 September 2017 earthquakes

Mexico City recently experienced two large earthquakes that took place only 11 d apart: the M_w 8.2 8 September 2017 with its epicenter offshore Chiapas, and the M_w 7.1 19 September 2017 with its epicenter in Puebla (Singh et al., 2018). The epicenter of the M_w 8.2 earthquake was located over 700 km away from Mexico City causing only minor impact to the city. However, the epicenter of the M_w 7.1 earthquake occurred only ∼ 100 km away from the city, which resulted in severe damaged in the city. Mexico City is very susceptible to seismic-induced damage, because part of the city is built over a sedimentary basin with clay rich lacustrine sediments up to 400 m thick (Santoyo-Villa et al., 2006; Auvinet et al., 2017). This sedimentary unit amplifies seismic energy causing site effects and making buildings and infrastructures more vulnerable, depending on their locations within the city.

Local seismic acceleration and site effects have been previously studied as part of the city building code developed after the M_w 8.2 1985 earthquake. However, not much attention has been paid to the shallow faulting associated with land subsidence that has developed on the transitional zones between the lacustrine deposits and the volcanic structures outcropping and its role as a potential risk during large earthquakes. This situation creates a strong horizontal gradient of subsidence where faulting and fracturing of the surface is common and is and addition element when considering the vulnerability for civil structural damage.

3 Remotely triggered subsidence acceleration

We use satellite-acquired SAR data from the Sentinel-1 satellites provided by the European Space Agency (ESA) through the Alaska Satellite Facility SAR Data Center repository. The InSAR process using ISCE software includes multilooking, topography removal, flattening, filtering, unwrapping, georeferencing and re-wrapping steps to obtain a longer-term baseline for the regional subsidence velocity (Fig. 1).

We examined the effects of both earthquakes on the up to 360 mm yr⁻¹ subsidence process in Mexico City using Sentinel-1 derived subsidence. As a baseline for pre-seismic deformation, we generated two 24 d SAR interferograms of data acquired between 19 July–12 August (Fig. 3a) and 12 August–5 September (Fig. 3b). These interferograms, covering the Mexico City Metropolitan area, show roughly 0.5–1 π phase change (light blue to purple), which is equivalent to 0.7–1.4 cm surface change in the highly subsiding regions within the city (Fig. 1). As the observed subsidence occurred over a period of 24 d, the two interferograms reflect subsidence rate of roughly 100–200 mm yr⁻¹. We also processed a third differential interferogram of data acquired on 5 and 29 September, which encompasses the integrated deformation for both the M_w 8.2 and M_w 7.1 earthquakes (Fig. 3c). This interferogram shows that the previously mentioned areas underwent an increase subsidence with much larger spatial coverage in comparison to the previous interferograms (Fig. 3a and b) which are 24 and 48 d older. The change in subsidence velocities (up to ∼ 33 mm total vertical displacement, equivalent to ∼ 38 % rate increase over a 24 d time window) and areal extent integrated during both seismic events (Fig. 4) indicates that most of the seismically triggered subsidence centered on these rapidly subsiding areas.

It is indeed possible that the energy released during these earthquakes is responsible for a distinctive deformation velocity pattern that is not shared by other sectors of the city where the underlying lacustrine sediment package is either thinner or absent.

Figure 4 Subsidence velocity difference obtained by subtracting the 24 d subsidence vertical velocity field calculated from Fig. 3b prior to the M_w 8.2 Tehuantepec and the M_w 7.1 Puebla, September 2017 earthquakes vertical velocity field calculated from Fig. 3c.

A closer analysis of this circumstance shows that a large portion of the reported subsidence associated damage correlates with the presence of preexistent, subsidence-related faults (CENAPRED, 2017). Moreover, we find evidence of phase discontinuities in the 5–29 September interferogram, which also correspond to these areas around the lower slopes of the Sierra de Santa Catarina.

We conclude that the seismic energy from both earthquakes induced a fast soil consolidation and triggered the coseismic faulting of the preexistent subsidence related faults. This circumstance not previously observed 32 years ago during the M_w 8.1 19 September 1985 earthquake creates a new variable that needs to be addressed in future updates to the building codes and urban zoning considerations in Mexico City.

4 Conclusions

Our data makes it plausible that the seismic energy released by the M_w 8.2 8 September 2017, and the M_w 7.1 19 September 2017 earthquakes induced fast soil consolidation within a short time span, and triggered slip on the preexisting subsidence associated faults. This effect has not been previously documented but may be triggered again during another strong earthquake. Future hazard assessments for Mexico City should consider this triggering mechanism for the assessment and the inclusion shallow faulting for future urban building code and land use.