P K Champati Ray
The massive devastation caused by the Gujarat quake has gained world wide attention. Here is an attempt to compile the geological information thet exists in various Internet sites.
Internet in India for the first time provides a large amount of information on all aspects of the disaster. Here is an attempt to compile the geological information that exists in USGS site (URL:https://neic.usgs.gov/neis/eqhaz/010126.html) and its links for the benefit of the geoscientists in India and abroad.
The Bhuj earthquake has reconfirmed the high vulnerability of the region to earthquakes of higher magnitude causing substantial topographical changes and immense loss of life and property. The area around Bhuj on the western border of India is one of the known regions of high incidence of earthquakes in recent times and in the historical past. As per the hazard map of the region prepared under Global Seismic Hazard Assessment Programme (GSHAP), the area falls under moderate to high seismic hazard zone with high ground acceleration and as per the GSI-IMD map, it falls under highest seismically active zone-V, the only such zone outside Himalayan seismic belt. The Kachchh peninsula has undergone many stages of deformation in the geological past resulting in a number of east-west trending faults, folds and domal structure. This crustal deformation/re-adjustment is still continuing resulting in high seismic activity in the form of earthquakes of varying magnitudes. The most prominent ones include the Allah Bund earthquake of 1819, Anjar earthquake of 1956 and the most recent and devastating Bhuj earthquake of 2001.
|Perceived Shaking||Not felt||Weak||Light||Moderate||Strong||Very Strong||Severe||Violent||Extreme|
|Potential Damage||None||None||None||Very light||Light||Moderate||Moderate/Heavy||Heavy||Very Heavy|
Figure 1: Intensity map based on the effect of the event )
Brief Geology of Kachchh
This region lies within 400 km of the active plate boundary zone between the Indian subcontinent and the Asian plate along the India-Pakistan border. Tectonic geomorphology indicates the Kachchh region may lie within a transition zone between the stable continental interior of Peninsular India and the active plate margin. Major structural features of the Kachchh region include east-trending folds and faults that deform Mesozoic clastic deposits and Deccan Trap basalts, Tertiary sedimentary units, and Quaternary terrace and alluvial/intertidal sediments. The principal faults in the region are the east-west trending Katrol Hills fault, Kachchh Mainland fault, Island Belt fault and the Allah Bund fault, which was the source of the M 7.8 1819 Kachchh earthquake. The location of the epicentre and devastation indicate that the Kachchh Mainland fault, or a part of it, possibly got reactivated on 26th January 2001. The region has characteristics of both intraplate and plate margin environments. The presence of an active fold and thrust belt suggests that this region is part of the diffuse Indian/Asia plate boundary, or at least a transition zone between the stable portion of peninsular India and the plate boundary. With the presence of faults and rift zones, this area can be classified as a unique seismological setting of “Rifted Stable Continental Region (SCR) extended crust”.
Information from published literature indicate that the area had experienced several earthquakes, magnitude ranging from 4 to 8 and intensities between III and X+(MM) in last 200 years (Quittmeyer and Jacob, 1979; Johnston and Kanter, 1990; and Gowd et al. 1996). Most important damaging earthquakes occurred in 1819, 1844, 1845, 1856, 1869 and 1956 in the same vicinity as 2001 earthquake (https://cires.colorado.edu/~bilham). Amongst these, the 1819 earthquake of 19th century is well documented (Bilham, 1999). This event occurred on 16th June 1819 in the northwestern part of the Great Rann of Kachchh (Lat. 240 00’N; Long. 700 00’E), with a magnitude of 7.8 (Johnston and Kanter, 1990), and reached a maximum intensity of IX to X+(MM) (Quittmeyer and Jacob, 1979). It is considered to be one of the largest earthquake not only in this seismic belt but also in the entire Indian shield. According to Oldham (1883), Oldham (1926), Bilham (1998), and Malik et al.(2000) this earthquake resulted in the formation of 6 to 9 m high alluvial scarp trending approximately E-W for about 80 to 90 km, and killed over 1500 to 2000 peoples. This uplifted feature blocked the southeast flowing tributary of Indus river that was feeding fresh water in the Kachchh areas. Later, this uplifted feature was named by local peoples as “Allah Bund”- the Mound of God. Hence, this event in the seismological literature is well known as Allah Bund event of 1819. This is the only well documented evidence of recent deformation from Kachchh other than the 1956 Anjar earthquake of M 6.1.
Figure 2: Peak Ground Acceleration under Global Seismic Hazard Assessment Programme-GSHAP )
Massive surface deformations in the form of cracks are reported from many parts of the region. Uplift in the 1819 event created an 80-km-long natural dam (the Allah Bund or Dam of God) across the eastern most branch of the Indus River. The 1819 earthquake had resulted a surface flexure rather than a fault scarp near Allah Bund. A lake was formed south of the Allah Bund that remains a depression (Lake Sindri) that is flooded during the summer monsoons. Vertical deformation in 1819 reached 6m and in 1956, 1m. It is most likely that surface displacements will exceed 5m in 2001 earthquake (https://cires.colorado.edu/~bilham). The epicenter of the Bhuj earthquake is located on the north side of the Bhachau anticline along the Kachchh Mainland fault. RV Karanth of MS University, Badodora has reported that the ground deformation and rupture are more on the east of Bhuj along the Kachchh Mainland fault than on the western side. The surface deformation is more pronounced around Lodai, Rapar taluka and maximum around Amarsar, 8-10 Km north of Bhachau, which is considered by MS University team as epicentre due to maximum destruction and deformation. A zone of ground rupture occurred along the northern margin of the Bhachau anticline within the alluvial/sabka deposits. The ground ruptures are over 16 km long and 0.5 km wide, trend east-northeast, and are associated with extensive sand boils. The extensive ground failures in this area to be related to liquefaction and lateral spreading. The Tappar Dam area near Gandhidham was also reported to have some slumping and displacement perpendicular to its alignment (https://www.eeri.org/ Reconn/bhuj_India/ Fieldreport2.html).
Wide spread liquefaction and associated phenomenon were reported by various news agencies, hydrologists and by local villagers, with an indication that the flow was sufficient in some cases to activate desert rivers that have been dry for more than a century. Even it was reported in various leading newspapers (11.2.01) that sprouts of fresh water have emerged in the saline landscape. Satellite images have also shown sudden appearance of streams in barren Kachchh. In Ahmedabad and Gandhinagar, the automatic water level recorder (Peizometers) showed an increase in the water level to the tune of 2.5 cms, after the earthquake. Meanwhile, reports have come in from the Little Rann of Kachchh about saline water springing at few places, mainly between Maliya to Zinzuvada, where extremely salty water, even saltier than that found in Rann has been reported.
An explanation for wide spread fluid venting, consistent with fluid over pressuring at depth, is that the associated faults probably acted as conduit for fluids. The natural roughness of the fault planes that hold water locked in the interseismic periods, makes impermeable prior to rupture, but becomes very permeable during and following rupture, permitting the release of high fluid pressure that initiated rupture at depth.
Widespread liquefaction was also confirmed by SPOT imagery and by field observation by Colorado geodesy team. Many mud-volcanoes in the Rann of Kachchh have dimensions of hundreds of meters: one covers a 5 km diameter stretch of the southern Rann with dark sand and mud. Numerous ancient river channels have been illuminated by a pock mark pattern of sand vents and some have clearly flowed, and breached their old channels (https://cires.colorado.edu/~bilham/).The road near Bhachau has been ruptured by 2-3 m cracks and destruction to buildings here is apparently total. These may be the result of catastrophic lateral spreading related to liquefaction of the sediments that the road crosses, although the region is quite close to mapped surface faults. The port of Kandla is severely damaged by liquefaction processes. In Kandla port area it was reported that liquefaction sand boils/discharges randomly distributed throughout the port premises. The Port Signal Tower Building (pile supported) was tilting as a result of the liquefaction. The Port gate office structure was also tilting. At IOC, Kandla, 3 storage tanks that settled due to liquefaction (8-inches max), and 2 had damage to the rotation of floating covers. Settlement of one of the tanks resulted in cracking of the inlet line spraying out naptha.
The GWB reported that extensive damage has happened to pipelines, particularly those running north across the Rann of Kachchh where there was extensive liquefaction. In Tappar dam water is treated at a conventional treatment plant, and is transmitted 30 km into the Gandhidham city. During the earthquake, liquefaction occurred in the bottom of the reservoir, stirring up anaerobic sludge on the bottom resulting in a major water quality issue ).
Based on the surface damage, building damage and shock felt isoseismal intensities have been drawn for the 2001 event. The isoseismals of 1989 and 2001 earthquakes show overall similar pattern. The main characteristic of 1819 pattern of Mercalli intensity is the circular pattern of the affected areas in the zone IX. Where as in the 2001-intensity map, the high intensity region shows elongated pattern extending in to Pakistan. An important feature is that the south-western coastal areas of Kachchh show low intensity regions indicating less devastation.
Figure 3: Fig. 3: Seismic zoning of India as per GSI-IMD (https://www.gsi.gov.in/)
The preliminary results of rupture process for the present earthquake (Ms 7.7, revised) have been analysed by Tokyo university based on the teleseismic body waves (P-waves and S-waves) data recorded at IRIS. Two fault models: (strike, dip, rake) = (78, 58, 81), (276, 33, 105) which are inferred from teleseismic body wave analysis have been examined. As the difference of variances for two fault models is too small, it requires more field data to determine the actual fault plane (https://wwweic.eri.u-tokyo.ac.jp/yuji/southindia/index.html). However, USGS and Harvard solutions favour a N30W strike fault, parallel to the mapped faults in the southern Rann of Kachchh.
Seismological Hazard Zoning
Vulnerability of Kachchh is well demonstrated in the seismic hazard map prepared for countries in South and Central Asia under the Global Seismic Hazard Assessment Programme (GSHAP). The Kachchh region is the only zone in India apart from Himalayas and Koyana region exceeding PGA of 1.6 m/s2 indicating high seismic hazard. This part of the Indian shield (in the vicinity of 24oN and 72oE ) is characterised by N-S, NW-SE and E-W tectonic trends and show relatively higher level of seismicity. It is interesting to note that the intensity pattern of recent earthquake and the PGA show a good correlation validating such maps. In another seismic zonation map prepared by Geological Survey of India, the present epicentre region falls in the highest hazard category Zone-V (IX or more in MM scale), showing the vulnerability of the region.
This Internet based information presented here was very useful for analysing, interpreting and making maps as per the user defined parameters. This improves the understanding of the geological phenomena and saves a lot of time in collecting and managing data sets related to earthquake. The limited possibility of Internet GIS, what is experienced here tells the story ahead when a lot of socio-economic data along with geoscientific inputs will be made available for natural disasters studies and will go a long way in providing technological inputs to a sound disaster management programme.
Author expresses deep sense of gratitude to Dr. Roger Bilham, Dr. J. N. Malik, NGRI and many others who have generously put their observation/findings/data in the Internet for scientific usage.n