Saturday, February 10, 2001
M A I N   F E A T U R E

What are the chances of a quake in the region?

IN the aftermath of the devastating earthquake that rocked Gujarat on January 26, a lot of interest has been generated in the technical details of the tremors.

Prof. C. S. DubeyWhy do earthquakes occur and can they be predicted? How does one measure the magnitude of tremors? Such questions preoccupy the minds of one and all even as the country struggles to come to grips with the Gujarat disaster. Seismology or the study of earthquakes, by its very nature, is not as exact a science as physics and it is inter-disciplinary in nature with scientific simulation of data being one of its primary elements. In the wake of the recent earthquake, Prof. C. S. Dubey of the Department of Geology, University of Delhi, explains to Gaurav Choudhury some of the finer nuances of earthquakes.


Excerpts from the interview:

How do earthquakes occur and is it possible to predict them?

One of the most frightening and destructive phenomena of nature is a severe earthquake and its terrible after-effects. An earthquake is a sudden movement of the earth, caused by the abrupt release of strain that has accumulated over a long time. For hundreds of millions of years, the forces of plate tectonics have shaped the earth as the huge plates that form the earth’s surface slowly move over, under and past each other. Sometimes the movement is gradual. At other times the plates are locked together and are unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free. If the earthquake occurs in a populated area, it may cause many deaths and injuries and extensive damage to property. The layer of the earth we live on is broken into a dozen or so rigid slabs (called tectonic plates by geologists) that are moving relative to one another. The earthquake in Kutch is believed to be due to a reverse fault along the Allah-Bund Nullah Scarp. In a reverse fault, the block above the fault moves up relative to the block below the fault. This fault motion is caused by compressional forces and results in shortening. A reverse fault is called a thrust fault if the dip of the fault plane is small. The same phenomena is also known as thrust fault, reverse-slip fault and compressional fault.

How is the epicentre located? How is the intensity of earthquakes measured?

Earthquakes are the result of forces deep within the earth’s interior that continuously affect the surface of the earth. The energy from these forces is stored in a variety of ways within the rocks. When this energy is released suddenly, for example by shearing movements along faults in the crust of the earth, an earthquake occurs. The area of the fault where the sudden rupture takes place is called the focus or hypocentre of the earthquake. The point on the earth’s surface directly above the focus is called the epicentre of the earthquake.

The effect of an earthquake on the earth’s surface is called the ‘intensity’.

The intensity scale consists of a series of certain key responses such as the number of people who woke up (by the tremors), movement of furniture, damage to chimneys, and finally — total destruction.

Various scales are used to measure the intensity of earthquakes, including the Richter Magnitude Scale (see box) and the Modified Mercalli Intensity Scale:

Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the USA is the Modified Mercalli (MM) Intensity Scale. It was developed in 1931 by American seismologists Harry Wood and Frank Neumann. This scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is marked by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects. The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the non-scientist than the magnitude because intensity refers to the effects actually experienced at that place. After the occurrence of widely-felt earthquakes, the Geological Survey mails questionnaires to postmasters in the disturbed area, requesting the information so that the intensity value can be assigned. The results of this postal canvass and information furnished by other sources are used to assign an intensity value, and to compile isoseismal maps that show the extent of various levels of intensity within the felt area. The maximum observed intensity generally occurs near the epicentre.

Another measure of the relative strength of an earthquake is the size of the area over which the shaking is noticed. This has been particularly useful in estimating the relative severity of historic shocks that were not recorded by seismographs or did not occur in populated areas. The extent of the associated felt areas indicates that some comparatively large earthquakes have occurred in the past in places not considered by the general public to be regions of major earthquake activity. For example, the three shocks in 1811 and 1812 near New Madrid were each felt over the entire eastern USA. Because there were so few people in the area west of New Madrid, it is not known how far it was felt in that direction.

Why are there so many earthquake magnitude scales?

Earthquake size, as measured by the Richter Scale, is a well known but not a properly understood concept. What is even less well understood is the proliferation of magnitude scales and their relation to Richter’s original magnitude scale? Charles Richter first developed the idea of a logarithmic earthquake magnitude scale in the 1930s for measuring the size of earthquakes occurring in southern California, using relatively high-frequency data from nearby seismograph stations. This magnitude scale was referred to as ML, with the L standing for local. This eventually came to be known as the Richter magnitude.

As more seismograph stations were installed around the world, it became apparent that the method developed by Richter was strictly valid only for certain frequency and distance ranges. In order to take advantage of the growing number of globally distributed seismograph stations; new magnitude scales that are an extension of Richter’s original idea were developed. These include body-wave magnitude, MB, and surface-wave magnitude, MS. Each is valid for a particular frequency range and type of seismic signal. In its range of validity each is equivalent to the Richter magnitude.

Because of the limitations of all three magnitude scales, ML, MB, and MS, a new, more uniformly applicable extension of the magnitude scale, known as moment magnitude, or MW, was developed. For very large earthquakes, moment magnitude gives the most reliable estimate of earthquake size. New techniques that take advantage of modern telecommunications have recently been implemented, allowing reporting agencies to obtain rapid estimates of moment magnitude for significant earthquakes.

Is it natural for aftershocks to occur after a major quake? For how long can one expect aftershocks?

After major earthquakes the after shocks continue as the faulted blocks and plates continuously readjust to bring the system to re-equilibrium i.e. adjustment of the faulted blocks. They may continue for weeks or months depending on the magnitude of the earthquake and the state of stress on the blocks.

How large is the possibility of earthquakes occurring in India?

On the basis of the seismic studies, India has been divided into five seismic zones and the areas falling under zones 5 and 4 are prone to high-magnitude earthquakes. But certain areas in parts of Maharashtra and Gujarat, earlier thought to be in low seismic zones, are also thought to be vulnerable by the geologists.

There have been some discussions about the possibility of earthquakes occurring in northern India. How prone are the states of Himachal Pradesh, Punjab, J&K, Haryana, Rajasthan and Delhi to quakes?

Most of the areas fall in seismic zone 4 and 5. Already Delhi has witnessed several shocks of 3 and 4 on Richter scale and the stress accumulation is observed by Geoscientists, thus chances of a major earthquake in future are quite high.

Japan is believed to be one of the most earthquake-prone zones in the world. But the magnitude of the destruction of life and property has never been so large as has been witnessed in the Gujarat quake. What measures would you suggest to prevent devastation of such order?

The United Nations declared 1990 to 2000 as International Decade for Natural Disaster Reductions by its resolution 44/236 suggests the formation a Disaster Management Fund and Plan especially for developing countries. The Union Government has no such funds and plans. Japan caters for such management funds, and plans at regional and local levels to impart specialised skills in handling earthquake situations. It gives training and spreads awareness among the common people in the earthquake-prone areas.

In terms of sheer magnitude, can the intensity of earthquakes be compared to those of hydrogen bombs? If so, how would the magnitude of the Bhuj earthquake compare to the impact of a hydrogen bomb?

In some newspapers the Gujarat earthquake was compared to a hydrogen bomb of 5.3 mega tons i.e., it was related to the amount of devastation, which would occur, on the explosion of such a bomb.

Can any periodicity be attached to the occurrence of quakes in a particular region?

Yes, it is believed that a high magnitude earthquake of 7 and above on Richter scale over a fault of more than 16 km in length will recur in 50-100 years of time. But in case of Gujarat, the recurrence of the high magnitude earthquakes is decreasing (885-1034 A.D., 16th June 1819, 1956, 1970 and 2001).

How does the Richter Scale measure the intensity of quakes?

Seismic waves are the vibrations from earthquakes that travel through the earth; they are recorded on instruments called seismographs. Seismographs record a zigzag trace that shows the varying amplitude of ground oscillations beneath the instrument.

Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, location and magnitude of an earthquake can be determined from the data recorded by seismograph stations. The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included in the magnitude formula to compensate for the variation in the distance between the various seismographs and the epicentre of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude of 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a 10-fold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value.

Earthquakes with a magnitude of about 2.0 or less are usually called micro-earthquakes; they are not commonly felt by people and are generally recorded only on local seismographs. Events with a magnitude of about 4.5 or greater — there are several thousand such shocks annually — are strong enough to be recorded by sensitive seismographs all over the world. Great earthquakes, such as the one in Gujarat on the January 26, have a magnitude of 7.1 or higher. On an average, one earthquake of such size occurs somewhere in the world each year. Although the Richter Scale has no upper limit, the largest known shocks have had magnitudes in the 8.8 to 8.9 range. Recently, another scale called the moment magnitude scale has been devised for more precise study of great earthquakes. The Richter Scale is not used to express damage. An earthquake in a densely populated area which results in many deaths may have the same magnitude as a shock in a remote area that does nothing more than frighten the wildlife. Large-magnitude earthquakes that occur beneath the oceans may not even be felt by humans.


Levels of Intensity

The lower numbers of the Modified Mercalli Intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on the observed structural damage. Structural engineers usually contribute information for assigning intensity values of VIII or above.

The following is an abbreviated description of the dozen levels of Modified Mercalli Intensity.

I. Not felt except by a very few under especially favourable conditions.

II. Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.

III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognise it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck.

IV .Felt indoors by many, outdoors by few during the day. At night, some are awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.

Vl. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

Vll. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.

Vlll. Damage slight in specially designed structures; considerable damage in ordinary buildings. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, and walls. Heavy furniture overturned.

IX. Damage considerable in specially designed structures. Damage great in substantial buildings. Buildings shifted off foundations.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

Xl. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

Xll. Damage total. Lines of sight and level are distorted. Objects thrown into the air.