Dr. Jayant Sinha, Sri Sahendra Singh, Sri Indrajeet, Sri Tarun Kumar
*. Dept. of Geology, St.Xavier’s College, Ranchi, Jharkhand, India
**. Vighneshwar E. BIZ Pvt. Ltd., Mumbai, India
***. Software Consultant, Bhubaneshwar, India
Earthquakes are the most destructive among all the natural hazards. Most of the time, they occur without any warning, which makes them most feared and unpredictable natural phenomena. Globally, on an average two earthquakes of magnitude 8 are known to occur every year. Some of the countries like Japan, China and United States have suffered several damaging earthquakes in the past. India has also experienced a number of high intensity earthquakes in the recent past and more than 650 earthquakes of magnitude > 5.0 have been reported in India since 1890.
The recently occurred Bhuj earthquake of Gujrat State (India) has once again exposed our limitations in earthquake science and also our preparedness against such natural disasters. Earthquakes are known to occur at frequent intervals causing loss both in terms of economy and human life, the effect of which is not only felt at the local level but also at the national level. Still we have not been able to make enough inroads into the areas of their identifications, prevention and control, earthquake can still strike without any warning and there is no way to control them prior to its occurrence, except than some post earthquake rehabilitation measures. Off late there has been considerable advance in the delineation of earthquake prone areas and the whole world has been delineated into different seismic regions based upon their seismic characteristics. India has also been divided into five seismic zones from zone one to zone five, with the fifth one being of highest seismicity.
The true nature of the causes of an earthquake must be fairly well understood before adopting any control measure. Two models are being tested to justify these control measures.
- Dilatancy-diffusion theory developed in the U.S.
- Dilatancy-instability theory USSR
The first stage of both models is an increase of elastic strain in a rock that causes them to undergo a dilatency state; which is an inelastic increase in volume that starts after the stress on a rock reaches one half its breaking strength. During Dilatancy State, open fracture developing the rocks. So it is in this state the first physical change takes place indicating future earthquake. Here the two models diverge. The U.S. model suggest that the dilatancy and fracture of the rocks are first associated with a low water containing dilated rock, which helps in producing lower seismic velocity, lower electrical resistivity and fewer minor seismic event.
The pore water pressure then increases due to influx of water into the open fracture, weakening the rock and facilitating movement along the fracture, which is recorded as an earthquake.
In contrast the Russian model state that the first phases is accompanied by an avalanches of fracture that release some stress but produce an unstable situation that eventually cause a large movement along a fracture.
Seismic gaps are defined as an area along active fault zones, capable of producing large earthquake but that have not recently produced an earthquake. These areas are thought to store tectonic strain and thus are candidate for future large earthquake.
Any fault that has moved during quaternary can be called as active fault. It is generally assumed that these faults can get displaced at any time. Faults that have been inactive for the last three million years are generally classified as inactive fault. Active faults are basically responsible for seismic shaking and surface rupture
Effects of earthquake:
Like all other natural hazards earthquakes also produce primary and secondary effects. Primary effects include surface vibration, which may be associated with surface rupture and displacement along fault plane. These vibrations may sometimes lead to the total collapse of large buildings, dams, tunnels, pipelines and other rigid structures. Deterministic ground motion analysis is one of the tools to determine the spatial distribution of surface vibration. Secondary effects of earthquake include a variety of short-range events; such as liquefaction, landslides, fires, tsunamis and floods. Long range effects include regional phenomena such as regional subsidence or emergence of landmasses, river shifting and regional changes in ground water level.
Liquefaction results from transformation of water-saturated granular material from solid to liquid state as a result of increase in pore water pressure. It may result in three types of failure.
- Landslide on moderate slope.
- Landslide occurring on gentle or nearly flat slopes.
- Quick condition failure.
Landslide of all varieties in addition to those associated with liquefaction, are triggered or directly caused by earthquake. Earthquakes also cause catastrophic destruction from fires due to disruption of electrical power lines and broken gas lines. Coastal or submarine earthquake also generate tsunamis or seismic sea waves. Other secondary effects of large earthquake are regional changes in land elevation. The destruction of critical facilities may cause catastrophic losses of life, property damage or disruption of society, e.g. large dams, nuclear power plant and liquid natural gas.
Earthquake hazard reduction:
For any modern nation public safety concern are of prime importance and India is of no exception. In view of this concern there should be adequate thrust towards understanding earthquakes the science of which is known as seismology. These studies may be helpful in seismic hazard assessment and mitigation
A comprehensive earthquake hazard reduction programme should include
- Control measure.
- Post earthquake rehabilitation measure.
First successful prediction of a major earthquake was made in 1975. The earthquake took place in China ( Haichung) on Feb 4 1975. The intensity of the earthquake was 7.3 on ritcher scale and about ninety percent of the structure was destroyed in a city of 90,000 people. In this case thousand of people were saved by the massive evacuation from unsafe housing just before the earthquake. The short-term prediction was possible primarily on a series of foreshock that began four days prior to the main shock. Unfortunately these types of short-range prediction on the basis of foreshock are not always reliable.
Earthquake prediction by any geoscientist is far from success, however a detailed and systematic investigation may lift haze in its prediction. Earthquake prediction in an area may be carried out under following heads:
- Lithological characterization and structural setting of the region
- Crustal deformational studies
- Frequency of fore shock
- Repetitive land level survey
- Water tube tiltmeters
- Geomagnetic observation.
- Geothermal gradient.
- Gravity survey
- Hazard mapping
Lithological Characterization and Structural Setting of the region
Geological mapping of an area is the first step towards the surface and subsurface investigation of a region. The accuracy of these investigations decides the prediction accuracy before an earthquake and also the post earthquake control and reduction measures.
The advent of Geoinformatics has brought revolutionary change in these investigations. Now a day number of geological softwares are available in the market for the geological mapping of the area. These softwares are highly useful in the speedy and accurate execution of mapping work. The arrival of GPS has the capability of recording spatial co-ordinates with accuracy level up to millimeter.
Remote sensing and air photogrammetry is of immense potential at the reconnaissance stage of the mapping. Now it is possible to map the inaccessible regions through the satellites.
Structural setting indicates the future earthquake by giving enough information regarding the palaeoseismology of the area. It is also helpful in the hazard mapping of the area to take preventive and control measures.
“Earthquake don’t kill people, but the unsafe building”. Carrying out systematic lithological mapping of the terrain can very well minimize the magnitude of seismic destruction. The Bhuj quake aftermath is an ideal example since such a high magnitude of destruction was possible mainly because of unconsolidated basement of the structures. It has been observed that the structures on a consolidated foundation, e.g. igneous and metamorphic rocks, are more safe than those on unconsolidated basements, viz. Alluvial, sand and loamy soil. Different surface materials behave differentially in response to seismic shaking of various frequencies. Unconsolidated earth materials (mud alluvium and bedrock) vibrate more in compare to hard bedrock. Therefore an area sensitive to earthquake hazards must be mapped for it to be available to land-use decision maker.
So a highly accurate geological map can be prepared with the help of recent geomatic tools and they can be analysed through GIS to use it for the prediction or the preparation of action plan during the post earthquake rehabilitation measures.
Crustal Deformation Studies through GPS
Throughout the world most of the earthquake activity are confined to plate margin associated with crustal deformation. Now a day’s crustal movement can be recorded with high degree precision using GPS.
Crustal deformation through GPS is one of the fast emerging area and most probably the only area where the real potential of GPS lies as far as effective earthquake prediction are concerned. GPS is now being used effectively for monitoring of crustal movement. In view of its prediction potential, Indian Meteorological Department (IMD) has planned a national GPS programme. In the long term an extensive GPS network consisting of permanent station (for continuos observation) and semi permanent station (at least for a year) and other field station are planned. Twenty-seven permanent station-covering regions from Himalayas to the south India are planned. Twelve of these are for the Northwestern Himalayas and ten stations are planned for the northeastern region of India. The continuos and repeated data recording over a fixed time interval from these stations can be of great help in preparation of crustal deformation model of Indian subcontinent. Such a model will give accurate detail about crustal movement of Indian plate. Recent advancements in the space geodetic techniques have been accepted worldwide for the understanding of crustal dynamics and earthquake mechanism.
Himalayan frontal arc of the Indian subcontinent is one of the seismically active regions of the world and shows many unique features as far as earthquakes are concerned. This region has shown evidences of frequent seismic activities starting from the first phase of Himalayan orogeny. The peninsular shield of India is not left far behind and has experienced some remarkable earthquakes. The earthquakes of Latur, Maharastra(1993), Koyna, Maharastra(1967); Jabalpur, Madhya Pradesh(1997), and Bhuj, Gujrat(2001) are some of the examples of earthquakes of peninsular shield, which have caused considerable destruction. The 1819 Rann of Kutch earthquake is one of the largest intraplate events that produced a surface scarp about 100 Km Long.
The Seismotectonic Zones in the Himalayan arc bears direct bearing with the Crustal deformational history of India. Following the breakup of the Gondwana supercontinent, India first moved southwards along with Antarctica and Australia. It then moved eastwards with Madagascar and later drifted northwards as part of the African plate before rapidly moving northward by itself to collide with Asia. Throughout most of its history subduction was taking place to the north of India.
Repeated GPS measurement at different GPS stations have revealed that out of 54 mm/yr convergence rate of Indian shield, a shortening of nearly 4mm/yr is taking place between Bangalore and Delhi, within Indian shield. Whereas there is no apparent surface contraction taking place between the northern tip of the Indian shield(Delhi) and the lesser Himalayan sites. It has been observed that a stretch of nearly 100 km between Main boundary thrust ( MBT ) and Main central thrust
( MCT ) is getting shortened with the rate of nearly 10-14mm/yr while no crustal movement was detected across any of the major Himalayan frontal thrust including MBT and MCT. Modeling of the GPS data have shown that at 50 km across the strike width of the decollement is locked, to the north of which free movement is taking place. As a result strain is developed which is likely to be released through earthquake along the northern tip of the locked fault system lying the MCT (Main central thrust ). This narrow zone is seismically highly active.
In modern time, earthquakes are studied with more authenticity, as high quality seismic and geodetic data are available globally. India has also made substantial
Progress in this field with the establishment of Broad band digital seismograph and geodetic network. Data accumulated through the seismological, geological and geodetic observations can be of great help in the delineation of the earthquakes prone areas. This will have direct impact on the hazard assessment and public safety measures.
Frequency of Foreshock
There are cases where minor foreshocks have indicated the major coming shock. So it is highly desirable to establish the seismological observatory for the continuous monitoring of seismic activity in an area. In modern time, earthquakes are studied with more authenticity, as high quality seismic and geodetic data are available globally. India has also made substantial progress in this field with the establishment of Broad band digital seismograph and geodetic network. Data accumulated through the seismological, geological and geodetic observations can be of great help in the delineation of the earthquakes prone areas. This will have direct impact on the hazard assessment and public safety measures.
For this purpose in India a network of observations have been set up by Indian meteorological department, which include 45 national observatories and 13 special purpose observatories. The seismological laboratories help us in demarcating an area with varying seismicity. On that basis India has been divided into 5 zones with respect to intensity of earthquake. Of these, zone v is seismically the most active where earthquake of magnitude 8 or more could occur. Zone I is the least active region.
There are various parameters, which can be analyzed collectively for the purpose of an earthquake prediction. Surface parameter include topographical changes and subsurface parameter include subsurface geomagnetic, geothermal and gravity variation. Rates of uplift and subsidence, specially rapid or anomalous change may be significant in predicting earthquake. For example for more than ten years before the 1964 earthquake near Nigata, Japan there was anomalous uplift of the earth crust. It has also been observed that speed of primary waves may decrease for a month, and then increase to normal just before an earthquake. With recent advancement in science & technology, a proper sensor developed to measure these variations can be put on satellite to get regional idea by periodic and continuous monitoring of these variations and their quantification may helps us in forecasting of such hazardous events. Changes in the electrical resistivity of an area have also been reported before earthquake. Increase in the amount of radioactive radon gas that is dissolved in deep well water has also been reported.
b) Earthquake preventive measures
The main objective of earthquake preventive measures should be to develop and promote knowledge, practices and policies that reduce fatalities, injuries and other economic losses from earthquake. Providing Geoscientific information to the masses can well minimize these losses. Formulation of preventive measures includes:
- Compiling digital surfacial geological maps to find out area more prone to crustal movement.
- Preparation of ground shaking amplification maps to demarcate area susceptible to high amount of destruction.
- Preparation of liquefaction and lateral spreading susceptibility maps
- Preparation of landslide susceptibility maps mainly in high relief region to delineate area highly susceptible to landslide
- Compilation of GIS databases of existing data on active earthquake source zones and make these databases easily accessible to user groups
- Modification of palaeoseismological maps
- Geoscientific modeling of the shallow crust using seismic, geodetic and geological data for earthquake hazard evaluation.
Earthquake hazard prevention depends on proper understanding of the destructive effect of earthquake. The areas of interest in earthquake studies include the effect of the earthquake source, local and regional geological structure and near surface geological deposits on strong ground shaping. Seismic and crustal deformation monitoring network can provide real time information for emergency response. GIS database of active earthquake source zone with up to date information on slip-rate and recurrence intervals will improve earthquake hazard identification and risk assessment.
A variety of methods have been used for earthquake prediction, ranging from planetary movement to odd behavior of animal but earthquake prediction continue to be an elusive goal. There have been isolated cases of success, most of which are of long range and intermediate range prediction based on seismic zone delineation. Short- term earthquake prediction is still the biggest scientific challenge today. In the absence of a reliable tool for short- term prediction the only thing we are able to do is to get prepared for post-earthquake rehabilitation measure and adopt an action plan with the objective of
- The improvement in the understanding of earthquake occurrence, their effect and consequence.
- Improvement in the area of earthquake hazards identification and risk assessment method and their use.
- Maintenance and improvement of comprehensive earthquake monitoring with focus on real time system in urban areas.