Earth’s crust: Faults

Faults

Earthquakes generally originate on a plane of weakness in the Earth’s crust called a ‘fault’. Faults are formed when two crustal rock beds slip relative to each other. Faults are classified according to the directions of relative slippage.

1  Strikeâ€'slip fault

If the movement or slippage is primarily horizontal it is a strikeâ€'slip fault. They can either be left lateral or right lateral (see Fig. 16.3).

2  Dipâ€'slip fault

If the slippage is vertical it is called a dipâ€'slip fault. Such faults may be normal faults or reverse faults. A fault that reveals itself on the Earth’s surface due to past earthquake activity is known as an earthquake fault. The well-noted ‘San Andreas’ fault, for example, emerges to the surface between Point Arena and the Gulf of California and reveals its presence by a linear trough in the Earth’s surface approximately 250 km long. The San Andreas fault occurs along the intersection of the Pacific Plate and the North American

Plate and has a total length of approximately 960 km, extending almost vertically into the Earth to a depth of 30 km.

Earthquakes often occur at ‘active faults’. Active faults are faults for which there is a past history and movements or deformation. The San Andreas fault system has been active during the past 200 years and other faults have been active for thousands of years.

Most earthquakes occur in the world along the boundaries of the tectonic plates which are called inter-plate boundaries (e.g., 1897 Assam earthquake). A number of earthquakes can also occur within the plate itself away from the plate boundaries (e.g., 1993 Lattur earthquake). These are called intra-plate boundaries. In both types of earthquake, the slip generated at the fault during earthquake is along both vertical and horizontal directions (called dipâ€'slip), and lateral directions (called strikeâ€'slip), with one of them dominating at some times.

In the Indian subcontinent, part of the Indo-Australian Plate is pushing against the Eurasian Plate along the Himalayan belt. Therefore the Himalayan belt is highly seismic (see Table 16.3), whereas peninsular India, which is not traversed by any plate boundary, is relatively less seismic. Earthquakes became frequent after the construction of Koyna Dam and these are regarded as reservoir-induced (or artificial). However, the Latur earthquake of 1993, which occurred in what was previously considered to be the most stable region on the Earth, implies that no region is entirely safe from devasting earthquakes.

When considering the regional distribution of earthquakes in the Indian subcontinent, the whole area can be divided in to the following seismic regions:

Kashmir and western Himalayas.

•         Central Himalayas (including Nepal).

•         North-east India.

•         Indo-Gangetic basin (Rajasthan).

•         Cambag and the Rann of Kutch.

•         Peninsular India.

•         Andaman and Nicobar Islands.

The seismic activities of these regions are summarized in Table 16.4.

Earthquake belts in the world

Circum-Pacific belt: This follows the region around the Pacific Ocean from Japan, China, Alaska Aleutians, California (San Andreas fault system),

Mexico, Equador, Peru, Chile and New Zealand. These are the areas characterized by plate subduction, active volcanoes, strain build-up and periodic release, causing earthquakes.

•         Mid-Atlantic Ridge: This follows the lines of Mid-Oceanic Ridges from near the North Pole to the equator, turning around South Africa and ends up to Rift Valley region of East Africa. Seismicity is low in this region.

•         The Alpine-Himalayan Trans-Asiatic Belt: The Alpine mountain areas of Europe, North Africa, Asia Minor, Caucasus, Turkey, Iraq, Iran, Himalayan region such as Kashmir to Assam, Myanmar and Philippines. This zone passes through boundaries of continental crustal plate with high mountain ranges where intense compression takes place.