The World’s distribution of Earthquakes coincides very closely with that of volcanoes. Earthquakes follow consistent patterns, occurring in three major zones on Earth, despite the potential to strike anywhere, anytime. Approx.~70% of Earthquakes occur in the Circum Pacific belt. ~20% occur in the Meditteranean– Himalayan belt including Asia Minor, Himalayas and parts of Northwest China. Elsewhere Earth’s crust is relatively stable and is less prone but not immune to earth tremors.
- Circum-Pacific Seismic Belt: The circum-Pacific seismic belt, along the Pacific Ocean rim, sees around 81% of the world’s largest earthquakes. The “Ring of Fire” nickname belongs to the belt formed by subduction of oceanic crust along tectonic plate boundaries. Earthquakes in these subduction zones are caused by slip between plates and rupture within plates.
- Mid Continental Belt: The Alpine earthquake belt stretches from Europe to Sumatra via the Himalayas, Mediterranean, and into the Atlantic. This belt accounts for about 17% of the world’s largest earthquakes, including some of the most destructive one.
- Mid-Atlantic Ridge: The third prominent belt follows the submerged mid-Atlantic Ridge. The mid-Atlantic Ridge, a divergent plate boundary, is mostly underwater and distant from human development.
- India lies in the Alpine earthquake belt which is one of the most destructive belt of earthquakes.
Causes of Earthquakes
- Fault Zones
- Plate tectonics
- Volcanic activity
- Human Induced Earthquakes
Fault Zones
- Shallow Earthquakes’ Immediate Cause: Most shallow earthquakes occur due to the abrupt release of stress along a fault rupture (crack) in the Earth’s crust.
- Rock Formations’ Sudden Slipping: Rock formations slip suddenly along fault ruptures because of variations in volume and density caused by extreme temperature and pressure within the Earth.
- Fault Length and Earthquake Magnitude: The earthquake’s magnitude is directly related to the fault’s length and width; longer and wider faulted areas result in larger magnitudes. The longest earthquake ruptures occur along thrust faults at approximately 1,000 km, whereas strike-slip faults are about half to one third as long, and normal faults have shorter ruptures.
Plate tectonics
Convergent Boundaries:
- Slipping of land along the fault line causes earthquakes.
- Reverse faults at convergent boundaries can lead to the most powerful earthquakes, known as megathrust earthquakes, typically with a magnitude of 8 or more.
- Megathrust earthquakes occur at subduction zones, where one tectonic plate is forced beneath another. For example, the 2004 Indian Ocean earthquake.
Divergent Boundaries:
- Slipping of land along the fault line also causes earthquakes.
- Earthquakes associated with normal faults at divergent boundaries are generally less severe, typically below magnitude 7.
Transform Boundaries:
- Slipping of land along the fault line can produce significant earthquakes, with magnitudes of up to about 8.
- The San Andreas Fault is an example of a transform fault where the Pacific plate and the North American plate move horizontally relative to each other, causing earthquakes along the fault lines.
Volcanic activity
- Volcanic activity can trigger earthquakes, but these volcanic-induced earthquakes are usually less severe and have more limited impact compared to those caused by crustal fracturing.
- Earthquakes in volcanic regions result from the release of elastic strain energy, both due to movement along tectonic faults and the movement of magma within volcanoes.
- These volcanic-induced earthquakes can sometimes serve as early warnings of impending volcanic eruptions, as observed during the 1980 eruption of Mount St. Helens.
- There is a notable correlation between the geographical distribution of volcanoes and major earthquakes, particularly in the Circum-Pacific Belt and along oceanic ridges.
- However, the epicenters of most major shallow earthquakes are often located several hundred kilometers away from volcanic vents, and many earthquake sources are unrelated to active volcanoes.
Human Induced Earthquakes
- Human-induced earthquakes, often characterized as minor seismic events or tremors, are triggered by various human activities, including mining, large-scale petroleum extraction, the creation of artificial lakes (reservoirs), and nuclear tests.
Shallow-Focus Earthquakes:
- Shallow-focus earthquakes are the most common type and are often referred to as “crustal earthquakes.”
- They typically have smaller magnitudes, ranging from 1 to 5, but some can be of higher magnitude and result in significant destruction.
- Shallow-focus earthquakes occur frequently and randomly, but many of them, particularly those along submarine ridges, may go unnoticed.
- Despite their lower magnitude, shallow-focus earthquakes can cause more surface damage because all their energy is concentrated in a small area.
Deep-Focus Earthquakes:
- Deep-focus earthquakes are generally those occurring deeper than 70 km beneath the Earth’s surface.
- They are associated with Benioff zones that dip into the Earth, indicating the presence of a subducting slab, and are sometimes known as intraplate earthquakes, triggered by plate collisions.
- Deep-focus earthquakes are typically of larger magnitude, ranging from 6 to 8, due to the release of substantial energy during plate collisions.
- However, these earthquakes may not cause as much destruction on the surface because their foci are at great depths, and the energy dissipates over a wider area.
- The most powerful deep-focus earthquake on record was the magnitude 8.3 Okhotsk Sea earthquake in 2013, which occurred at a depth of 609 km. The deepest earthquake ever recorded was a 4.2 earthquake in Vanuatu in 2004, reaching a depth of 735.8 km.
Read Also :
