Matteo Albano1, Marco Polcari1, Christian Bignami1, Marco Moro1, Michele Saroli2,1, Salvatore Stramondo1 1 - Istituto Nazionale di Geofisica a Vulcanologia, Via di Vigna Murata 605, 00143 Roma (Italy) 2 - Università degli Studi di Cassino e del Lazio meridionale, Via G. di Biasio 43, 03043 Cassino (Italy) Monitoring the change of soil seismic response through the InSAR-derived ground subsidence: application to the Mexico City subsidence

Outline Introduction Method The case study Results and validation Conclusions

Central Italy 2016 earthquakes Introduction SAR as a ground displacement measurement technique EARTHQUAKES Central Italy 2016 earthquakes (INGV, 2016) Campi Flegrei (Trasatti et al., 2015) VOLCANOS Subsidence of Bologna (Stramondo et al., 2007) ANTHROPOGENIC PHENOMENA

Introduction Urban subsidence caused by groundwater withdrawal Bologna (Stramondo et al., 2007) Morelia (Cigna et al., 2012) Guzman (Brunori et al., 2012) Thessaloniki (Svigkas et al., 2016) Effect on the dynamic behavior of soils

Introduction Effect on the dynamic behavior of soils: theory Soil Rock Hs vs a t a t vs DHs Rock Soil Uniform, damped soil on a rigid rock Hs = soil thickness Vs = shear wave velocity a t Amplification factor Frequency (w) (Kramer, 1996) 2 4 6 8 10 12 5 15 20 𝐴= 1 𝑐𝑜𝑠 2 𝜔𝐻 𝑣 𝑠 + [𝜉 𝜔𝐻 𝑣 𝑠 ] 2 𝜔 𝑛 = 𝑣 𝑠 𝐻 𝑠 𝜋 2 +𝑛𝜋 ; 𝑛=0, 1,…,∞ 𝜔 0 = 𝜋 𝑣 𝑠 2𝐻 𝑠 n = 0 𝑇 𝑠 = 2𝜋 𝜔 0 = 4 𝐻 𝑠 𝑣 𝑠 Hs Ts

Introduction Effect on the dynamic behavior of soils: experimental evidence Mexico city (Avilés and Pérez-Rocha, 2010; Arroyo et al., 2013) ALAMEDA PARK – CITY CENTER Fourier amplitude (m/s) Ts(s) Ts(s)

InSAR ground subsidence as a tool for local seismic hazard monitoring Objective Urban subsidence from InSAR Ts(s) Change of the soil resonant period InSAR ground subsidence as a tool for local seismic hazard monitoring

InSAR displacements t0  tn Method InSAR displacements t0  tn Soil resonant period Ts (tn) Soil thickness Hs (tn) (Gosar and Lenart, 2010) 𝐻 𝑠 =𝑎∙ 𝑇 𝑠 𝑏 𝐻 𝑠 =𝑎∙ 𝑇 𝑠 𝑏 Ts ??? Soil thickness (t0) 𝐻 𝑠 =𝑎∙ 𝑇 𝑠 𝑏

The case study MEXICO CITY Land area:1485 km2 8.84 million people Density: 6000 people/km2

The case study Seismic hazard http://gmo.gfz-potsdam.de/ Urban area Hard volcanic deposits Alluvial fans Clays and clayey silts Urban area http://gmo.gfz-potsdam.de/ The Global Seismic Hazard Map https://commons.wikimedia.org/w/index.php?curid=10821360 2280 2260 2240 2220 2200 2180 2160 2140 Height a.s.l. (m) Hs  60m Santoyo et al., 2005

The case study Ground subsidence SAR data Leveling GPS data Cabral-Cano et al., 2008 Leveling GPS data Cabral-Cano et al., 2008 Sowter et al., 2016 López-Quiroz et al., 2009 (Osmanoglu et al., 2011) Chaussard et al., 2014

The case study A-priori data Soil thickness (Hs) NORMAS TECNICAS COMPLEMENTARIAS DEL REGLAMENTO DE CONSTRUCCIONES DEL DISTRITO FEDERAL (November 2004) Soil thickness (Hs) Soil resonant period (Ts) Design response spectrum (Avilés & Pérez-Rocha, 2010)

Results and validation SAR data and results ENVISAT RADARSAT-2 Constellation ENVISAT RADARSAT-2 Time interval November,2004 February,2006 April,2013 September,2013 Image n° 13 10 Sensor C-band Mode Stripmap tilde IW-mode Resolution 100 m Orbit Descending Incidence angle 23° 35° SBAS (Berardino et al., 2002) IPTA (Werner et al., 2003)

Results and validation Soil thickness (Hs) 2004 2013 Variation between 2004-2013

Results and validation Soil resonant period (Ts) 2004 2013 Variation between 2004-2013

Design response spectrum Results and validation Soil resonant period (Ts) Variation between 2005-2013 Design response spectrum

Results and validation 27 accelerometric stations (Arroyo et al., 2013) RMSE = 0.2s AE02 BO39 PE10 CJ03

Conclusions We developed a procedure to evaluate the effect of ground subsidence on the spatial and temporal seismic response of soils by exploiting InSAR-derived ground displacements. The proposed procedure has been applied to estimate the resonant period change of the subsoil of Mexico City caused by the very fast subsidence which affects the city. The resonant period estimates have been validated with field measurements. The results can be improved by processing a complete SAR dataset from 2005 to 2017 The proposed procedure is suitable for the on-line implementation.