Seismic Hazard Assessment in Guwahati Area

Guwahati is situated on latitude of 26.18 degree North and longitude of 91.76 degree East and is at an average altitude of around 170 feet above mean sea level (MSL). The area of study is limited to Guwahati Municipals Region having area about 216 sq.km. It is the principal administrative, industrial, commercial, educational and cultural hub of northeast India. The area in and around Guwahati is subjected to high seismic risk because of its proximity to large and great earthquake source zones, and also due to rapid development and  construction of  buildings on filled up lands.
  
Seismic Microzonation for the Guwahati city area has already been done by the MoES, under the then DST Project where multidisciplinary and multi-organizational inputs were involved. Multidisciplinary inputs like geological, geophysical, geotechnical and geomorphological data were incorporated for the basement/ bedrock, landuse, landslide, soil stability, shear wave velocity, predominant frequency, peak ground acceleration and site response studies. All these maps are available with MoES which can be used for prevention planning and retrofitting measures and also to define safety zones for new reconstructions.

Data Source
In this study we are mainly concentrating on the available new data from all the concerned organizations for assessing the present seismic hazard scenario of the study area.

Seismic Data

The Guwahati city and its surrounding area is surrounded by the Himalayan thrusts ( MCT and MBT, HFT) zones  to the north, the long Kopili fault  and the Assam syntaxis zone to the east and northeast, the Dhubri fault to the west and the Shillong plateau source zone to the south. All these major earthquake source zones are almost within 100 km to the study area, except the syntaxis zone and the Indo-Burma subduction, which are far field (~ 500 km away) source  zones for the study area.   Out of these source zones, the Kopili fault is most active recently and is much near to the study area. Some recent earthquakes occurred in the Kapili fault make it prominent that the fault may be vulnerable for an impending large earthquake.

Borehole Data
We obtained some borehole litholog data from the Directorate of Geology and Mining (DGM), Guwahati.  These data are used to map the basement depth and to estimate Predominant Frequency (PF) for site amplifications.

Estimation of Seismic Potential:
A complete catalogue is very much important for the determination of seismic potential parameters such as Mmax, a-value, b-value and Lambda. So only the complete part of dataset is used to calculate the hazard parameters using ZMAP tool. The strong ground motion at the surface is estimated using the CRISIS program. The hazard parameters estimated as mentioned has been used to compute the strong ground motion in terms of Peak Ground Acceleration. The two attenuation relationships used for this study are- Abrahamson and Silva (1997) and Youngs et al (1997). The relationship by Abrahamson and Silva (1997) has been developed for shallow crustal earthquakes in active tectonic zones. The attenuation relationship by Youngs et al (1997) has been developed for subduction zone and therefore applied in the present case. Thus seismicity in and around the study area and estimated PGA value for 10% exceedance in 50 years.

The estimated PGA for our study area is within the range of 0.43 to 0.50g.