Geomorphic Hazard- Earthquake: Concept, Causes And Measurement

Natural hazards are threatening events that can cause widespread damage to life and property. They have long-term consequences, and their continuing impact can change or modify both physical and social space.

Earthquakes are one of the most devastating and destructive natural disasters experienced by an individual and society. They occur without warning in different areas throughout the world. Earthquakes can cause maximum damage and deaths in densely populated areas.

Earthquakes result from vibrations generated by sudden movements and ruptures in the rocks that are being stressed beyond its elastic limits. These stresses can be natural or man-made.

The intensity of an earthquake can vary from a faint tremor to wild shaking of the ground. The point at which an earthquake is generated is called the focus or hypocenter. The point located just above the focus is called an epicentre.

Near the epicentre of the quake, the impact is direct, resulting in immediate damage such as collapse and destruction of buildings and other infrastructure. The area near the epicentre then experiences secondary or indirect impact such as fires and landslides. Usually, the depth of the focus has been traced between 10-700 Km from the surface.

Earthquakes occur in a sequence of shocks. The largest earthquake in a sequence is called a Mainshock. It is preceded by one or more Foreshocks and followed by Aftershocks. The smaller earthquake that occurs after the large one is designated as Aftershocks. They are the result of the adjustment of the ‘fault plane’ formed by the displacement of the part of the earth’s crust after the occurrence of the Main shock. Aftershocks are very dangerous because they are usually unpredictable and can be very intense. It can raze buildings to the ground already damaged by the Mainshock.

The scientific study of earthquakes is known as Seismology (Seismos in Greek means earthquakes). Therefore, the waves generated by earthquakes are also called seismic waves. Mainly there are three kinds of seismic waves, Primary, Secondary and Surface waves.

Causes of Earthquakes

Earthquakes are mainly caused due to the disturbance of equilibrium in any part of the earth’s crust. Such happenings may be the result of diastrophic forces or sudden movements. It may sometimes be caused by the expansion and contraction of surface area due to hydrostatic pressure produced by human-made reservoirs and water bodies. The following are the major causes:

Plate Tectonics:

The theory of plate tectonics provides the best possible explanation for the occurrences of earthquakes. Earth’s crust is made up of solid and moving plates. These plates can be oceanic or continental. There are seven major and twenty minor plates that are constantly moving under the influence of thermal convective currents originating deep within the earth.

The interaction at the margins or edges of the earth’s tectonic plates generates the World’s most earthquakes and volcanoes. There are mainly three types of boundaries where the plates interact – convergent, divergent and transform fault.

Convergent Boundaries:

The area where two tectonic plates collide with each other. In case of a collision between two oceanic plates, one of the plates, which is older, larger and heavier, is forced to be subducted under the lighter plate. The deepest trench, Mariana, lies above the place where the Pacific plate is being subducted under the Philippines plate. When one oceanic plate collides with a continental plate, the denser plate is usually subducted under the lighter plate.

Nazca plate is subducted under the South American plate near the Peru-Chile trench. When the edges of colliding boundaries are continental crust, it forms huge Mountains such as the Himalayas. All these plate interactions may generate shallow to deep-focus earthquakes. Just beneath volcanic arcs and continental margins lies a subduction zone, also known as Wadati- Benioff zone. The Benioff zone is a major zone of earthquake activity because of the thrust faulting of two plates over a wider area.

Divergent Boundaries:

This margin is characterised by moderate earthquakes, where two plates are moving in opposite directions. It results in the rupture of the crust and the formation of mid-oceanic ridges. Shallow-focus earthquakes (0-70 km)are mostly produced near the constructive plate boundaries.

Transform Fault:

The area where plates slide past each other in opposite directions. The most famous transform fault is the San Andreas Fault in California, USA. Here, the southeast-moving North American Plate meets the northwest-moving Pacific Plate. Earthquakes frequently occur in the surrounding regions as the two plates brush each other, stick together, and slip along transform faults.

Elastic Rebound Theory and Faulting:

Sudden displacement and slippage of rocks take place due to tensile and compressive forces leading to the creation of faulting. It can also trigger tremors due to the re-adjustment of rock blocks.

According to H.F. Reid (1960) Earthquakes are linked to the elastic rebound of previously stored elastic stress. If the rubber is stretched or broken, the elastic energy that was stored during the stretching process is released suddenly. Likewise, the earth’s crust can also store elastic stress that can be released during an earthquake.

Vulcanicity:

Volcanic activity and earthquakes are intimately related to each other. They have cause and effect relationship with each other. The eruption of the Krakatoa volcano in Sumatra produced a severe earthquake that was experienced 12000 km away.

Hydrostatic Causes and Anthropogenic Activities:

Most prominent is Reservoir Induced Seismicity due to the disequilibrium in already isostatically adjusted rocks below the reservoir. For example, the earthquake of Koyna (Maharashtra, India) in 1967, Hoover Dam (USA) in 1936 and Marathon Dam (Greece), in 1931.

Magnitude and Measurement of Earthquakes

It is commonly measured on the Richter scale. The scale is based on the energy released at the earthquake centre. Therefore, it measures the severity of the particular earthquake. The intensity of the earthquake is measured on a scale of 1 to 10. On the logarithmic scale, each whole number represents 32 times more energy.

On the other hand, Mercalli Scale is based on the power of destruction and the severity of earthquakes remembered and felt by humans. It measures the damage caused to buildings, dams, bridges and other infrastructures.

Global Distribution of Earthquakes

The location of the World’s earthquakes presents a striking pattern. Occurrences of earthquakes are concentrated in certain areas and stretch across the World. An area with high volcanic activity and earthquakes is called as “Pacific Ring of Fire” or the Circum-Pacific Belt.

It is the zone of plate collision, subduction and young fold mountains. It includes North America, South America, East Asia coastal areas and Island arcs. The 2004 Sumatra, 1964 Alaska and 1960 Chile earthquakes occurred in the subduction zone connected to the Pacific Ring of Fire.

Mid-Continental or Alpine-Himalayan belt is also a major earthquake-prone area of the World. It is particularly characterised by the collision and subduction of continental plates (or part of plates). It comprises Alpine Europe, the Mediterranean Sea, North Africa, the Himalayas and Burma. The belt contains weaker zones of folded mountains, where fault-generated earthquakes are very common.

For example, Chamoli, the earthquake of 1991, and the massive earthquake in Pakistan in 2005 due to the convergence between the Indian and Eurasian plates. Earthquakes also occur in areas of transform fault such as California, East African Rift Valley and Mid-oceanic ridges. The 1906 San Francisco earthquake is linked with the 1820 km long San Andreas Fault.

Earthquake Hazard

Earthquakes are associated with a variety of specific hazards. Some are characterised as primary hazards, such as:

Ground Motion: when seismic waves travel through populated areas, ground motion is felt as shaking. The destruction linked to ground motion depends upon the design and construction of buildings.
Ground Breaking: It includes a wide opening in the ground due to earthquakes. These ground breaks may have vertical, horizontal or combined displacements.
Mass Wasting: It may trigger the downhill movement of material lying on the slope. It can range from gradual creep to rolling large blocks of rocks. Earthquakes may induce landslides and avalanches on steep slopes.
Liquefaction: It is a process where sudden and intense vibrations and shaking converts certain types of sand and mud into a slurry or a substance with a consistency of the liquid.
Changes in Ground Level: Due to earthquakes, sometimes blocks of earth shift relative to one another. It may lead to changes in ground level, base level and water table.

Other secondary and tertiary hazards associated with earthquakes are:

Tsunamis
Seiche waves
Fires and Explosion
Displacement of People
Loss of jobs and livelihood

Earthquake Prediction and Risk Reduction

It is very difficult to predict earthquakes; no definite means of predicting earthquakes are available. However, it can be predicted indirectly by analysing unusual animal behaviours, studying hydro-chemical pressures and increases in turbidity. Seismic in many parts of the World are monitored using sensors, global positioning systems and satellite technology.

Earthquakes cannot be stopped, but preventive measures can reduce the risk. These measures include building earthquake-proof or resistant structures and safer homes. Spreading awareness, conducting earthquake preparedness drills, and capacity building for emergencies are some of the significant solutions.

Geomorphic Hazard- Earthquake: Concept, Causes and Measurement