Seismological zones are generally classified according to their vulnerability to earthquakes and other seismic events. These categories are determined by the degree of seismic risk and the geological and tectonic features present in the zones. Although the classification can differ from region to region or country to country, the following are common categories for seismic zones:
High Seismic Risk Zones:
These areas have a high chance of experiencing powerful earthquakes and are usually found near plate boundaries, subduction zones, or places with active fault lines. Strict building codes and earthquake-resistant infrastructure are needed in these zones. Areas in this zone include the Pacific Ring of Fire (Japan, Indonesia, New Zealand, Chile, North and South America's west coast), the Himalayan region (Nepal, India), and the Mediterranean region (Italy, Greece, Turkey).
Moderate Seismic Risk Zones:
These regions have a lower likelihood of significant earthquakes but still face risks, and normally situated further away from plate boundaries, and have fewer active faults. Seismic hazard assessments inform construction practices in these areas. Areas in this zone include the Central United States (Oklahoma, Missouri), Eastern Europe (Romania, Bulgaria), and parts of China away from high-risk areas like the Sichuan Basin.
Low Seismic Risk Zones:
There is minimal chance of major earthquakes in these areas, and are located far from tectonic plate boundaries, such as within continents' interiors. Despite the low risk, building codes might still incorporate some seismic provisions. Areas in this zone include the Central United States far from fault lines, central Australia away from northern plate boundaries, and much of Canada except for the western part.
Intraplate Seismic Zones:
Situated within tectonic plates' interiors and distant from plate boundaries, Intraplate earthquakes can happen due to old faults or other geological structures. These zones typically feature a lower yet not negligible level of seismic risk. Areas in this zone include the Eastern United States (New Madrid Seismic Zone), central Australia experiencing intraplate earthquakes, and central Europe (Germany, France) with intraplate seismic activities.
Volcanic Seismic Regions:
In regions with volcanic seismic activity, the risk commonly comes from volcanic occurrences, potentially including eruptions. The earthquakes that happen in these areas are known as volcanic earthquakes and result from magma displacement. Places within this zone feature countries with active volcanoes such as Japan, Indonesia, Iceland, Italy, and volcanic island nations in the Pacific Ocean like the Philippines, Vanuatu, and Hawaii.
Subduction Zones:
These zones are notorious for producing some of the most disastrous earthquakes and tsunamis. Subduction zones are located at convergent plate boundaries where one tectonic plate sinks beneath another. Relevant areas include significant plate boundaries like the west coast of South America, North America, Japan, Indonesia, the U.S. Pacific Northwest's Cascadia Subduction Zone, and the Andes Mountains.
Transform Fault Zones:
In these areas, earthquakes occur when tectonic plates slide past each other and are characterized by horizontal plate movement along fault lines. Notable regions in this zone consist of California's San Andreas Fault, Turkey's North Anatolian Fault, and New Zealand's Alpine Fault.
Mid-Ocean Ridge Regions:
Mid-ocean ridges represent divergent plate boundaries where plates gradually separate. Earthquake activity in these areas can involve volcanic events and underwater quakes. Examples of regions belonging to this zone include the Mid-Atlantic Ridge within the Atlantic Ocean, the East Pacific Rise found in the eastern Pacific Ocean, and the East African Rift System (Ethiopia, Kenya).