5. What are the consequences of an earthquake?

Direct consequences of earthquakes are related to:

  • Ground shaking: The main danger in an earthquake is the effect of ground shaking on buildings as large ground shaking (or the deformation of the ground beneath them), can seriously damage buildings or bring them down. Moderate earthquakes can also produce liquefaction of loose rocks; water saturated sands or soils lose their bearing capacity and structural foundations “sink”. Strong surface waves can also produce landslides, mudslides, and avalanches on steeper hills or mountains due to heavy sollicitations;
  • Ground Displacement: Where subsidence or horizontal displacements take place, large damage is produced on structures and infrastructure built across the fault. If the fault zone reaches the surface, there is a great probability that damage will be wide and important.
The clock tower of Finale Emilia (Modena Province, Italy) half destroyed after the 20 May 2012 Mw5.9 earthquake that stroke the Emilia area. Source: http://www.thepostinternazionale.it/
Other important indirect consequences of earthquakes are:
  • Flooding: In the case of an earthquake, dam and river levee structures can suffer damages. If such damage are strong enough, water can be released and induced floods can damage structures and sweeping away or drowning people;
  • Tsunamis: Large seismic shaking of the sea bottom transfers the motion to the water; large waves are then propagating into the sea. Coastlines hit by Tsunamis can result in large damage for people and goods;
  • Fire: If gas and electric lines are damaged by an earthquake, fires can spread and add danger and destruction; examples of towns destroyed by earthquakes induced fires are San Francisco (1906) and Lisbon (1755).
  • Landslides: it is well documented that seismic shocks can trigger landslides, since seismic waves act to strongly increase the pore water pressure inside dormant and active landslide bodies, causing their sudden activation.
  • Ground cracks and liquefaction: the pore water pressure increase may cause ground cracks and liquefaction phenomena in loose sandy deposits, at shallow depths, if below a water table. These phenomena may induce, among others, structural instability to buildings due to sand spills and the consequent creation of voids beneath the foundations.

Earthquakes do not produce direct danger to people in open spaces, where no objects fall and there is no building collapse: in this case, people are not exposed to a large impact by an earthquake.

Ground cracks and liquefaction phenomena at San Carlo (Ferrara Province, Italy) induced by the 20 May 2012 Mw5.9 earthquake that struck the Emilia area (photo by D. Castaldini)