Earthquakes in India
Mainshock,foreshocks and aftershocks A large earthquake is generally preceded and followed by many smaller shocks. The largest earthquake is called the main shock. The smaller ones that precede the main shock are called foreshocks and the subsequent shocks are called aftershocks. Earthquake swarmsThe earthquake swarms are groups of earthquakes which are concentrated in a certain region, but none of them is significantly larger than the others. SeismographSeismograph is the instrument for recording motions of the earth’s surface caused by seismic waves, as a function of time. The simplest earthquake recording system consists of a sensor and an analog or digital recorder. The record is known as a seismogram. Location and magnitude of an earthquake are calculated from seismograms.
Hypocentre, epicentre and hypocentral region of a earthquake (Earthquake in Japan, 1996)
Seismic wavesIn earthquake studies, we generally deal with P, S and surface waves. P wave is the primary or the fastest wave travelling away from an earthquake source, consisting of a train of compressions and dilatations parallel to the direction of travel of the wave. S wave is the secondary wave, travelling more slowly than the P wave and consisting of elastic vibrations transverse to the direction of travel. Earthquakes also generate surface waves that follow the Earth’s surface only, with a speed less than the S waves. Propagation of surface waves also causes considerable damage during an earthquake. The difference in arrival time between the P wave and the S wave can suggest the approximate distance from the epicentre. From records of earthquakes at several places, the epicentre can be determined by reading the time differences.IntensityIntensity is description of the effects of an earthquake at a particular place, based on observations of damage, using a descriptive scale like the Modified Mercalli Scale. A map showing intensities at individual locations may be contoured based on isoseismals, which are lines of equal intensity. An isoseismal map provides a representation of broad variations of shaking over the region surrounding the earthquake.MagnitudeMagnitude is a measure of the size of the earthquake, calculated from the amplitude of the seismic waves and is closely related to the energy released by the earthquake. If the magnitude increases by 1, then the energy is about 30 times larger; if it increases by 2, then the energy is about 900 times. Richter magnitude, surface-wave and body-wave magnitudes are commonly used to indicate this measure. Duration or coda- magnitude based on the duration of the seismic signal is also in use.Hypocentre and epicentreThe earthquake occurs as a result of the motion of a fault, that is, by rupture of rocks. The point where the rupture originates is called the hypocentre or the focus and the point directly above this on the ground is called the epicentre. Depth to the hypocentre is known as the focal depth.
The Latur earthquake generated a surface rupture that was traceable for about 2 km. The maximum height of the scarp observed near Killari was about one metre. Several geological studies conducted in this deformation zone by various agencies provided valuable information about the seismogenic characyeristics of this important fault zone in the Stable Continental Region (Photo: Kusala Rajendran, CESS )
The Latur (Killari), Maharashtra, earthquake of September 30, 1993 is the most devastating SCR earthquake in the world.Its epicentre was located in a region considered to be aseismic
This earthquake occurred in the typical rural setting of India. The severity of the destruction was compounded by the nature of village settlements. While most of the engineered structures survived the earthquake, the non-engineered ones were totally damaged.
Over 10,000 lives were lost in this earthquake and several villages were destroyed. With a magnitude 6.3 and focal depth less than 10 km, this earthquake is similar to other moderate events in the Australian and Canadian shields.
It is suggested that the repeat time of moderate SCR earthquakes are of the order of hundreds of thousands of years. The recurrence interval at Latur may also be of the same order, and the recorded human history may not document any previous earthquakes. Thus, we have very little information about the earthquake history of such regions and the earthquakes occur as a total surprise.
This event led to several studies, giving a new perspective to seismic hazard assessment in the peninsular India. It also led to strengthening of the seismic network, upgrading several existing facilities
The 1991 Uttarkashi earthquake (M 6.5) which occurred in the Tehri region in Gharwal Himalaya shook the north-central India
This earthquake, a moderate one, attracted a lot of attention due to its proximity to the high Tehri dam
This is among the recent earthquakes that have been instrumentally recorded and studied
This event killed many people and damaged completely or partially about 48,000 houses
Geologists believe that this earthquake occurred because of a slippage along the Main Central Thrust (MCT), a major tectonic boundary, which also divides lesser Himalayan terrain in the south from the snow clad mountains in the north
Induced seismicity at Koyna
Seismicity associated with the Shivaji Sagar lake formed by the Koyna dam is considered to be a classic example of earthquake activity triggered by reservoirs.
Over hundred cases of reservoir induced seismicity have been reported from all over the world.
Koyna is among the four cases that have generated earthquakes of magnitude >6.0. An earthquake of magnitude 6.3 (1967) and many of magnitude >5.0, have occurred at Koyna.
Seismicity at Koyna shows remarkable correlations with the filling cycles in the reservoir. It is believed that the pore pressure changes induced by the reservoir reduces the strength of the rocks leading to failure along a major fault zone in the vicinity of the dam.
An earthquake of magnitude 6.0 occurred on May 22,1997 in the Jabalpur area, Madhya Pradesh
The first major earthquake in peninsular India to be recorded by the newly installed broadband digital stations in the shield region
This earthquake which caused widespread devastation in and around Jabalpur left 48 people dead and many injured and homeless
The earthquake generated a great deal of data that helped to understand the response of various types of structures, offering valuable guidelines in the design and construction of earthquake resistant structures
The spatial association of the Jabalpur earthquake with the Narmada-Son lineament has triggered a lot of interest from the seismotectonic point of view
CESS STUDIES QUAKE HIT BHUJ
The Bhuj earthquake (Mw 7.7) ofthe State of Gujarat, is the most January 26, 2001, that occurred in disastrous earthquake in India’s history. While the actual figures of death and injury remain uncertain, going by the official figures, at least 20,000 people were dead and more than 200,000 were injured. Nearly 400,000 houses were destroyed and twice as much damaged. Although damages of such proportion were astonishing, the occurrence of the event itself was not surprising, considering the geologic and seismic history of the region. Seismologists, Drs. Kusala Rajendran and C.P. Rajendran from CESS, visited Bhuj during February 12 to March 5, 2001, for post seismic field observations and to assess the damage pattern and study the response of structures, field effects and aftershock activity. Many aspects about this earthquake make itan exceptional event. Most significantly, this earthquake occurred in a region considered to be part of a Stable Continental Region (SCR)that has already generated a large earthquake in the historic past. The Kutch region forms part of a Mesozoic rift system and has been noted for occurrence of large earthquakes since historic times.
The 1819 and 1956 earthquakes have caused significant damages in the area. Since 1997 CESS had beenstudying the epicentral region around the 1819 earthquake with a view to understand the morphological characteristics of previousdeformation associated with that earthquake and to understand its paleoseismic history. Occurrence of the present earthquake in the region has given an opportunity to study the coseismic deformation and also to compare the effects of the two earthquakes. The CESS team conducted a detailed field investigation in the epicentral area of the Bhuj earthquake and documented major surface effects and liquefaction features generated by the earthquake.
India Meteorological Department (IMD) has located this earthquake at 23.40°N, 70.28°E, using 53 stations forming part of a national grid. The U. S. Geological Survey has used teleseismic data and located it at 23.36°N and 70.34°E. Focus of this earthquake is placed at 24 km by the IMD and 22 km by the USGS. Both these are located north of Bachau, a village that was totally destroyed in this earthquake.
The Bhuj earthquake has also destroyed several historical monuments in the region, dating from 9th century A.D. The Sun Temple at Kotai (11th century A.D.) and the Punvareshwar Temple at Manjal (9th century A.D.) are among the oldest temples that were destroyed. While the Lakhpatji’s Chhatardi (A.D. 1752-61) collapsed completely, Aina Mahal Palace(~ 200 years old) is still standing, with extensive damage. These structures seem to have survived the 1819 and 1956 earthquakes, but the ground shaking was apparently too severe during the recent earthquake. Although the damage was not severe, it is conceivable that the Bhuj earthquake was the largest to have occurred in the close proximity of these structures since 9th century A.D.This earthquake also provided an opportunity to compare the liquefaction features induced by the current earthquake with that of otherearthquakes. Craters observed in the area exposed a major liquefaction event prior to 2001 and we consider this to be due to the 1819 event, based on our previous observationsin several trenches in this area. This event is more dramatic in regions close to the Allah Bund, but small-scale venting episodes prior to 2001 are observed also in the Banni Plains. At this time, most of these craters are wet and the conditions are not ideal for trenching excavations. We plan to take up these at a later stage
The 2001 Kutch (Bhuj) earthquake: Coseismic surface features and their significance
Kusala Rajendran* , C. P. Rajendran*, Mahesh Thakkar † and Martitia P. Tuttle ‡ *Centre for Earth Science Studies, P.B. No. 7250, Akkulam, Thiruvananthapuram 695 031, India † Department of Geology, R.R.Lalan College, Bhuj, 370 001, India ‡ M. Tuttle & Associates, Tibbetts Lane, Georgetown, ME 04548, USA
CURRENT SCIENCE, VOL. 80, NO. 11, 10 JUNE 2001
Seismic zonation map of India
Seismic zonation map of a country is a guide to the seismic status of a region and its susceptibility to earthquakes
India has been divided into five zones with respect to severity of earthquakes. Of these, Zone V is seismically the most active where earthquakes of magnitude 8 or more could occur
Recent strong motion observations around the world have revolutionized thinking on the design of engineering structures, placing emphasis also on the characteristics of the structures themselves
It should be realized that in the case of shield type earthquakes, historic data are insufficient to define zones because recurrence intervals are much longer than the recorded human history
This may often give a false sense of security. Occurrence of the damaging earthquake at Latur, falling in Zone I is a typical example of this situation