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How will different parts of Bangalore respond to an earthquake?
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:
1. Sitharam T. G. and P.Anbazhagan (2006) Seismic Hazard Analysis for the Bangalore Region Journal of Natural Hazards (in press).
2. Sitharam. T.G, Anbazhagan. P and Ganesha Raj K. Use of Remote Sensing and Seismotectonic Parameters for Seismic Hazard Analysis of Bangalore , International Journal of Natural Hazard and Earth Science System, European Geophysical Union Journal, MS-NR: nhess-2006-0033 (accepted).
3. Divya .C., (2006) Site amplification studies for Bangalore city, ME Thesis, Department of Civil Engineering, Indian Institute of Science, Bangalore, pp 71.
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| Author: SajithkumarS 22 Feb 2008 | Member Level: Diamond Points : 3 |
You did scare me with this article. Hope you are not hinting at the possibilities of serious earthquake in Bangalore. It is the most unplanned and crowded cities in India today. So the likelihood of an earthquake could have devastating effect on the population.
Sajith
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| Author: Jonathan Phillip Obregon 30 Apr 2008 | Member Level: Gold Points : 2 |
Up until now nobody can detect the occurence of an earthquake. Even the most highly advanced countries like the United States and in Europian countries, there is no gadget that can predict when an earthquake will happen. The best thing to do is proper planning. In other words, conducting earthquake drills. As in this case, people will learn how to act accordingly when an earthquake occurs. We may not be able to predict it but planning in advance and training people how to react can help a lot. It is also important to have building assess from time to time with respect to its durability and foundation. This can help prevent collapse and obviously help save lives.
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