Seismicity

Seismicity is the study of how often earthquakes occur in a given geographical location, which types of earthquakes occur there, why they occur, mechanisms and magnitude. Because almost all earthquakes originate from fault zones, it is essential to identify, map and study active faults in the region that is being studied (although most active faults are hidden by erosion, sedimentation, soil, vegetation and/or because they are blind faults). By considering the following factors, the seismicity of an area can be statistically quantified and forecasted:

Tectonic setting of the area (its proximity to a plate boundary and characteristics of the plate boundary)

Location and the number of active faults in the area

Earthquake history of the area (e.g. frequency)

Presence of geologic structures 

Amount of stress and strain on rocks 

TYPES OF EARTHQUAKES: Based on focus-depth, there are 3 types of earthquakes: 

1) shallow-focus: at depths <70 km,

 2) intermediate-focus: at depths of 70-300 km 

3) deep-focus at depths: >300 km. 

Of all earthquakes, 90% occur at depths <100 km and only 3% are deep-focus.

Earthquake Zone:  Earthquakes can strike any location at any time, but history has shown that they occur in the same general patterns year after year, principally in three large zones of the earth which coincides with continental plate boundaries known as earthquake belts:

 The Circum-Pacific Seismic Belt: This is the world's greatest earthquake belt, it is found along the rim of the Pacific Ocean, where about 81 percent of our planet's largest earthquakes occur. it has earned the nickname "ring of fire". why do so many earthquakes originate in this region? The belt exists along boundaries of tectonic plates, where plates of mostly oceanic crust are sinking (or subducting) beneath another plate. Earthquakes in these subduction zones are caused by slip between plates and rupture within plates. Earthquakes in the Circum-Pacific seismic belt include the M9.5 Chilean Earthquake [Valdivia Earthquake] (1960) and japan m9.0 Earthquake (tohoku earthquake, 2011).

 The Alpine earthquake belt: this belt extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. This belt accounts for about 17 percent of the world's largest earthquakes, including some of the most destructive, such as the 2005 M7.6 shock in Pakistan that killed over 80,000 and the 2004 M9.1 Indonesia earthquake, which generated a tsunami that killed over 230,000 people.  

The mid Atlantic ridge: The third prominent belt follows the submerged mid-Atlantic Ridge. The ridge marks where two tectonic plates are spreading apart (a divergent plate boundary). Most of the mid-Atlantic Ridge is deep underwater and far from human development, but Iceland, which sits directly over the mid-Atlantic Ridge, has experienced earthquakes as large as M6.9.

The remaining shocks are scattered in various areas of the world, therefore damaging shocks can occur outside these earthquakes pruned zones.  Follow next post to know how to measure earthquake and why Japan is highly seismic.