Hundreds of people were killed in an earthquake in Indonesia on November 21, with rescuers searching for survivors trapped under the rubble amid a series of aftershocks.

The shallow 5.6-magnitude quake struck in mountains in Indonesia’s most populous province of West Java.

What is shallow earthquake?

Earthquakes can occur anywhere between the Earth’s surface and about 700 kilometers below the surface. For scientific purposes, this earthquake depth range of 0-700 km is divided into three zones: shallow, intermediate and deep.

i) Shallow earthquakes are between 0 and 70 km deep.

ii) Intermediate earthquakes 70-300 km deep

iii) Deep earthquakes 300-700 km deep. 

In general, the term “deep-focus earthquakes” is applied to earthquakes deeper than 70 km.

Earthquake in Indonesia

• The US Geological Survey said the quake measured 5.6 magnitude and struck at a depth of 10 kilometres.

• While the magnitude would typically be expected to cause light damage to buildings and other structures, experts say proximity to fault lines, the shallowness of the quake and inadequate infrastructure that cannot withstand earthquakes all contributed to the damage.

• Buildings not being constructed using earthquake-proof methods was a factor in the devastation.

• The worst-affected area is close to several known faults. A fault is a place with a long break in the rock that forms the surface of the earth. When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other.

Earthquake-prone country

• The country of more than 270 million people is frequently struck by earthquakes, volcanic eruptions and tsunamis because of its location on the arc of volcanoes and fault lines in the Pacific Basin known as the “Ring of Fire”.

• The area spans some 40,000 kilometres and is where a majority of the world’s earthquakes occur.

• Many of Indonesia’s earthquakes are minor and cause little to no damage. But there have also been deadly earthquakes.

• In February, a magnitude 6.2 earthquake killed at least 25 people and injured more than 460 in West Sumatra province. In January 2021, a magnitude 6.2 earthquake killed more than 100 people and injured nearly 6,500 in West Sulawesi province.

• A powerful Indian Ocean quake and tsunami in 2004 killed 230,000 people in a dozen countries, most of them in Indonesia. 

Some terms related to earthquake

Earthquake: Earthquake is the manifestation of sudden release of strain energy accumulated in the rocks over extensive periods of time in the upper part of the Earth. Earthquakes are classified as, Slight (M<5.0), Moderate (5.0<M<6.9) and Great (M>7.0) depending upon the magnitude on the Richter scale. An earthquake having a magnitude, M<2.0 is termed as a microearthquake. 

Seismograph: A seismograph, or seismometer, is an instrument used to detect and record earthquakes. Generally, it consists of a mass attached to a fixed base. During an earthquake, the base moves and the mass does not. The motion of the base with respect to the mass is commonly transformed into an electrical voltage. The electrical voltage is recorded on paper, magnetic tape, or another recording medium. This record is proportional to the motion of the seismometer mass relative to the earth, but it can be mathematically converted to a record of the absolute motion of the ground. Seismograph generally refers to the seismometer and its recording device as a single unit.

Richter scale: 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 for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake. 

Epicenter: It is the point on the surface of the Earth, vertically above the place of origin (hypocenter or focus) of an earthquake.

Aftershock: An earthquake that follows a large magnitude earthquake called, “main shock” and originates in or around the rupture zone of the main shock. Generally, major earthquakes are followed by a number of aftershocks, which show a decreasing trend in magnitude and frequency with time.

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