» Research Highlights
How will different parts of Bangalore respond to an earthquake?
- Prof. T.G. Sitharam
Bangalore has experienced several minor earthquakes in the 20th century. The damage caused by these earthquakes was not large. Bangalore has grown rapidly during the last 20 years. Many new buildings and colonies have been built on dry lake beds. What will happen to these buildings during a moderate earthquake? To ascertain these we need to identify sub-regions within Bangalore (essence of microzonation) that will then respond in a similar way to peak horizontal acceleration induced by an earthquake.
The Civil Engineering Department has embarked on microzonation of Bangalore city, to evaluate the future seismic hazard of Bangalore. We have installed 6 strong motion accelerographs and 2 borehole sensors (two of them in IISc) to measure ground acceleration in Bangalore city. The recent earthquake (of magnitude 3.4 on Richter scale) located near the borders of Tamilnadu, Andhra Pradesh and Karnataka on August 4th 2006 was recorded in our borehole sensor located on a rock at 15m depth. There were more than 700 events of 3 to 3.9 magnitude, about 150 events of 4 to 4.9 and about 25 events of 5 to 5.9 and 3 events around magnitude 6 reported in the study area. The generally low background seismicity and the long repetition interval in this region often cause a false sense of security. Recent studies (Sitharam and Anbazhagan, 2006) highlight the presence of potentially active geological structures in the vicinity of Bangalore; one of them passes right through the IISc campus.
We have generated amplification and peak ground acceleration (PGA) maps of Bangalore based on analysis of data collated from 950 boreholes drilled in various parts of the city and also measured shear wave velocity profiles at 55 locations in the city. PGA at ground level has been estimated from the borehole data and by carrying out one-dimensional site-specific ground response analysis. Acceleration time history at the ground surface and the response spectra have been generated and presented (Divya, 2006). This is further confirmed by recording the ambient noise for a selected period of duration at several locations in Bangalore city. The response spectrum is necessary to evaluate dynamic forces induced in structures. This will help us to design earthquake resistant structures. Based on the available data, ground motions have been simulated and expected Peak Ground Accelerations (PGA) at rock depths were calculated in a 220 square km area of Bangalore. For the Bangalore city, the maximum PGA at rock depth is estimated as about 0.15g. Maps showing variations in PGA and amplification factors at ground level considering site response, sediment thickness, and bedrock configuration have been generated. High value of surface accelerations estimated at some locations is on account of fundamental frequency of the soil columns coinciding with the frequency of earthquake motions.
Complete Details are available at:
- Divya .C., (2006) Site amplification studies for Bangalore city, ME Thesis, Department of Civil Engineering, Indian Institute of Science, Bangalore, pp 71.
- UPDATED INFO ON http://www.civil.iisc.ernet.in/~microzonation/.