Subsidence and associated shallow faulting hazard assessment in central Mexico using InSAR and GPS. E. Cabral-Cano, Instituto de Geofísica, UNAM D. Solano-Rojas, Univ. of Miami-RSMAS T. Oliver-Cabrera Univ. of Miami-RSMAS S. Wdowinski, Univ. of Miami-RSMAS E. Chaussard, Univ. of California Berkeley L. Salazar-Tlaczani, Instituto de Geofísica, UNAM F. Cigna, British Geological Survey C. DeMets, Univ. Wisconsin-Madison J. Pacheco-Martínez, Univ. Aut. de Aguascalientes
Motivation While subsidence has affected Mexico City for over a century, other cities in central Mexico have been subjected to subsidence since the '80, as a result of their large urban expansion, population increase and aggressive groundwater extraction rates. The continuous subsidence results in severe damage to urban infrastructure and civil structures. The damage cost assessment and vulnerability are difficult to evaluate, because of the variable geographic extent and the continuous nature of the process, which have different characteristics than other phenomena characterized by localized, short duration events such as earthquakes or floodings with better media coverage. 1910 2010 Angel de la Independencia monument
Damage in Mexico City Main Cathedral
Damage in Celaya - Convento San Francisco (1683)
Damage in Celaya
Aguascalientes Pabellon de Hidalgo
In Mexico, subsidence-induced damage to houses or other urban infrastructure is not eligible for federal emergency relief funds, because it is not considered a naturally occurring hazard but rather an anthropogenically induced process. Furthermore, this phenomenon is localized and usually managed only at the local city or county-level administrations. Examples of damages to housing and other urban infrastructure in Aguascalientes and Celaya
InSAR analysis in central Mexico with ALOS 17 cities! Chaussard et al., 2014.
Subsidence rates vary from ~ 370 mm/yr in Mexico City to >100 mm/yr in the rest of cities in central Mexico Chaussard et al., 2014.
Goal Simple Detection of subsidence is not enough. This work is aimed towards a better recognition of subsidence as a major hazard in Mexico, assess the number of inhabitants affected and the spatial extent of the subsiding areas affected by this process.
Approach Our approach for hazard assessment of shallow faulting associated with subsidence is based on the determination of the horizontal subsidence gradient (e.g. Cabral-Cano et al., 2010). This analysis has proven to be an excellent proxy for detection of areas prone for faulting. Examples of the left show the velocity maps and corresponding subsidence horizontal gradient analysis in Mexico City, Aguascalientes and Morelia (Cigna et al., 2011).
How can we assess hazard from Subsidence and associated shallow faulting? Subsidence velocity Horizontal gradient Hazard zonation Mexico City hazard assessment; Solano-Rojas 2013 InSAR-derived ground subsidence velocity maps were merged with population, hydrology and meteorology data sets allow the creation of risk maps using a simple risk matrix.
Hazard matrix relating subsidence (top right) gradient and population density (bottom left). Morelia hazard assessment In Morelia, 7.0% (6.77 km 2 ) of the city's urban area and 14.0% of its population (84,900) is under high to very high risk of being affected by faulting associated with subsidence bottom (right panel) These products provide decision elements for water resource management agencies, or risk management purposes, such as vulnerability for shallow faulting, and land use zonations
Subsidence velocity Horizontal gradient Hazard zonation Mexico City hazard assessment; Solano-Rojas 2013 In Mexico City, 21.7% (351.0 km 2 ) of the city's urban area and 34.5% of its population (4,709,305) is under high to very high risk of being affected by faulting associated with subsidence (right panel)
InSAR + cgps time series However, different satellite sensors and sometimes widely spaced data availability make it difficult to derive long-term time series, or measure rapid changes or nonlinear variations of subsidence velocities. Higher temporal resolution subsidence observations of associated fault motion has been pursued using continuously operating GPS stations. We have developed a GPS network that covers 6 urban centers to detect short duration variations using different processing schemes that include both real-time solutions using RTNet as well as daily solution using Gipsy-Oasis.
Subsidence velocity map (ALOS) and cgps time series Aguascalientes INEG vert = -60.6 +/- 1.2 mm/yr UAGU vert =-31.5 +/- 1.0 mm/yr J
Subsidence velocity map (ALOS) and cgps time series Celaya CECM vert = -67.1 +/- 2.2 mm/yr CEFA vert = -15.1 +/- 2.9 mm/yr CEGA vert = -3.6 +/- 0.8 mm/yr J
Subsidence velocity map (ALOS) and cgps time series Irapuato UIRA vert = -82.7 +/- 2.8 mm/yr J
The continuous nature of the subsidence process makes necessary updates on velocity and timely detection of subsidence acceleration. New sensor availability creates new opportunities for velocity map updates Work in progress: update subsidence analysis using TSX, Sentinel-1, and ALOS-2. --> -->
Aguascalientes J TSX 2009/12/16-2012/03/31 (2+ years) White lines show field mapped faults. All displacements are completely constrained by mapped faults. Suburbs North of Aguascalientes (Jesus María) show largest displacements. Downtown Aguascaleintes is reducing subsidence velocity; consistent with GPS observations
Mexico City Sentinel-1A 2014/10/03-2014/12/02 White contours show geotechnical zonation. Subsidence is mostly constrained on the lacustrine zone (clay rich sediments) Subsidence patterns closely follow geologic structures (volcanic structures) High coherence areas closely follow up urbanized areas.
Mexico City 3.14-3.14 TSX 2011/05/18 2011/07/23 (66 days) J
Mexico City ALOS-2 2014/10/08 2015/02/25 (140 days) -π π J
ALOS-2 and Sentinel 1A comparison Mexico City ALOS-2 2014/10/08 2015/02/25 Mexico City Sentinel-1A 2014/10/03-2014/12/02 Both analyses yield a very similar subsidence pattern Good agreement with geotechnical zoning
Conclusions (1) Hazard assessment in central Mexico urban areas show that ground subsidence affects large areas and inhabitants. Ground subsidence origin (excessive groundwater extraction) and high pressure for new urban development as economic conditions improve, indicate that the situation will not be reversed in the short term. Some urban areas (e.g. Aguascalientes and Morelia) don t show linear behavior; thus hazard zonations need constant and periodic updates to evaluate changes in vulnerability.
Conclusions (2) Updates for hazard assessment are dependent on imagery availability. Municipalities and local administrations have been slow to adopt new satellite geodetic techniques such as InSAR due to cost but also because of data availability. Coverage in small scale cities is restricted and insufficient for analysis other than simple differential interferometric analysis. New sensors provide an excellent potential for solving some of the data availability issues.
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