AFTERSHOCKS

• Although aftershocks tend to be weaker than the main seismic event, they can cause significant damage
•  In Turkey, reports suggest that significant damage that has followed the original quake has been a result of aftershocks
• With the main earthquake already weakening structures, aftershocks have sent many buildings tumbling
• The massive 7.5 magnitude aftershock in Turkey can actually be more devastating, say experts
• This aftershock was extremely shallow, only 10 km deep, which worsens the shaking felt
•  The main quake was 17.9 km deep. Shallower quakes are generally felt more intense than deeper quakes due to their proximity to the surface
•  According to the ShakeMap, which calculates the intensity of quakes, this earthquake has been classified to have an intensity of VIII, indicating severe shaking and moderate to heavy damage
• The original quake reported an intensity of IX
• Crucially, aftershocks throw a spanner in ongoing relief and rescue operations, often hurting rescuers themselves

• Earthquakes occur in the crust or upper mantle, which ranges from the earth's surface to about 800 kilometers deep (about 500 miles)
• The strength of shaking from an earthquake diminishes with increasing distance from the earthquake's source, so the strength of shaking at the surface from an earthquake that occurs at 500 km deep is considerably less than if the same earthquake had occurred at 20 km depth
• Also, the depths of earthquakes gives us important information about the Earth's structure and the tectonic setting where the earthquakes are occurring
• The most prominent example of this is in subduction zones, where plates are colliding and one plate is being subducted beneath another
• By carefully plotting the location and depth of earthquakes associated with a subduction zone, we can see details of the zone's structure, such as how steeply it is dipping, and if the down-going plate is planar or is bending
• These details are important because they give us insight into the mechanics and characteristics of the deformation in the subduction zone
• The deepest earthquakes occur within the core of subducting slabs - oceanic plates that descend into the Earth's mantle from convergent plate boundaries, where a dense oceanic plate collides with a less dense continental plate and the former sinks beneath the latter
• The plate boundary contact between two such plates generate very large, shallow subduction zone earthquakes such as the Sumatra 2004 M9.1 event, and the 2011 M9.0 Japan earthquake, and is only active to relatively shallow depths - approximately 60 km
• However, because oceanic slabs are relatively cold with respect to the surrounding mantle in deeper subduction zone environments, faults within the core of these slab remain brittle and can generate earthquakes to depths of as much as 700 km (e.g., Pacific Plate beneath Japan and Kamchatka, and beneath Tonga)
• As the slab descends into the mantle, rheology changes (viscosity characteristics) cause the plate to bend and deform, and generates these earthquakes. The trend of such events can be seen in cross-sections of subduction zones, and are known as "Wadati-Benioff Zones"