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Geomatics in Earthquake Mitigation

Role of Geomatics in Tectonic Study
Lineaments and faults are the sources of neotectonic activities, which may often lead to an increase in seismic hazards of the region. LISS-III and panchromatic (PAN) merged images can be used to delineate vertical to high angle faults or suspected faults. All lineaments mapped from enhanced False Color Composite (FCC) and edge enhancement images can be merged to bring out the total lineament map of the area. Based on Mohr's theory, failure criteria, and statistical analysis of remotely sensed lineament data, horizontal compressive stress values (SHmax) can be estimated (Sahoo et.al., 2000, Saraf et.al., 2002). Such compressive stress determination has been done for the Kachchh area after the Bhuj earthquake and they are found to match roughly with those provided by USGS sources (Maiti, 2001).

The drainage pattern is an excellent indicator of not only the surficial lithology and the geological structures but also the ongoing morphotectonic processes of the planet Earth. Among the various drainage patterns, the 'eyed drainage' pattern is considered to be one of the most significant anomalies and such mega-eyed drainage are found to signify ongoing tectonic movements (Ramasamy et.al., 2000). IRS-1A and IRS-1C FCC imagery (using blue, green and red colours in spectral ranges) can be interpreted with a specific look to bring out the eyed drainages.

Synthetic Aperture Radar (SAR) i.e. Space-borne interferometry, which gives high-resolution imagery of earthquake prone areas, accompanied by GPS survey provides an ability to identify subtle changes on Earth's crust (Lunetta and Elvidge, 1999) and identification of non-homogenous surface deformation (co-seismic deformation). The displacement field of Landers earthquake, California (magnitude 7.3, 28th June 1992) has been computed using ERS-1 SAR data (Massonnet et al., 1993). Land subsidence case studies using D-InSAR techniques over parts of Germany, Italy and Mexico have also been reported. Similar use of D-InSAR technique has also been used to monitor Latur earthquake, 1993 wherein surface deformation of the order of 2.8cms could be mapped (Majumdar et al., 2002).

Geomatics in Seismic Zoning
Earthquake induced damage can occur due to liquefaction and or related phenomena, landslides, ground motion, tsunamis and seiches, ground rupture and tectonic subsidence or uplift. Seismic Zoning can be defined as delineation of geographic areas with varied potentials for surface faulting, ground shaking, liquefaction and landsliding during future earthquakes of specific size and location (Berlin, 1980). While seismic zoning takes into account the distribution of seismic hazard over the entire country or region, seismic micro-zonation takes into account the effect of local site conditions i.e. the detailed distribution of earthquake risk within each seismic zone. For seismic micro-zoning all data related to geology, ground acceleration, historical earthquake and remote sensing derived parameters are incorporated into a common spatial database and then analyzed to get the Hazard map. Such a study has been attempted for the Kachchh area (Gupta, 2002).

Liquefaction is one important aspect of seismic micro-zoning and refers to the loss of shearing resistance (when the effective stress reduces to zero) or the development of excessive strains resulting from transient or repeated disturbances of saturated unconsolidated fine cohesionless soils, leading to dramatic examples of damage. Liquefaction sensitivity index (LSeI) is useful for determining the relative liquefaction hazard potential and provide an index of possible maximum ground displacement. LSeI can be mapped using pre and post earthquake LISS-III and WiFS data sets (Ramakrishna et.al., 2003). Figure 1 shows a typical ground signature of liquefaction vis-a-vis that in remote sensing image.

Fig 1. (i) Ground evidence of liquefaction
(ii) IRS-WiFS Difference FCC of Kachchh showing spatial extent of liquefaction
(iii) LSeI in and around Bhuj
High revisit capabilities of IRS-WiFS images have been helpful in studying the persistence of the released water (water surges) i.e. liquefaction in near real time (Mohanty et.al., 2001). There was relative increase in the volume of water in the WiFS images of 29th January (Bhuj earthquake), which completely disappeared on 4th February 2001. Even liquefaction zone boundaries can be delineated or identified on remote sensing images like Landsat TM and MSS as has been done by Gupta et al., 1995 in the Bihar-Nepal region for the 1934 earthquake.

Micro-zonation study has been carried out in various earthquake-prone parts of the world including Memphis, Mexico, British Columbia, Puerto-Rico, City of Basel etc. It has also been observed that at many sites surface motions are influenced primarily by top 20-30 m of soil, so sediment cover (subsurface geology) has a role to play in earthquake and so 3-D subsurface map must be prepared. This can be done with help from GIS as has been attempted for the city of Delhi.