Earthquakes

Mechanism of destruction

Vibrational energy transmitted through the earth's surface from depth. Vibration causes damage and collapse of structures, which in turn may kill and injure occupants. Vibration may also cause landslides, liquifaction, rockfalls and other ground failures, damaging settlements in the vicinity. Vibration may also trigger multiple fires, industrial or transportation accidents and may trigger floods through failure of dams and other flood retaining embankments.

Parameters of severity

Magnitude scales (Richter, Seismic Moment) indicate the amount of energy release at the epicenter - the size of an area affected by an earthquake is roughly related to the amount of energy released. Intensity scales (Modified Mercalli, MSK) indicate severity of ground shaking at a location - severity of shaking is also related to magnitude of energy release, distance away from epicenter of the earthquake and local soil conditions.

Causes

Energy release by geophysical adjustments deep in the earth along faults formed in the earth's crust. Tectonic processes of continental drift. Local geomorphology shifts. Volcanic activity.

Hazard assessment and mapping techniques

Past occurrence of earthquakes and accurate logging of their size and effects: tendency for earthquakes to recur in the same areas over the centuries. Identification of seismic fault systems and seismic source regions. In rare cases it may be possible to identify individual causative faults. Quantification of probability of experiencing various strengths of ground motion at a site in terms of return period (average time between events) for an intensity.

Potential for reducing hazard

None.

Onset and warning

Sudden. Not currently possible to predict short-term earthquake occurrence with any accuracy.

Elements most at risk

Dense collections of weak buildings with high occupancy. Non-engineered buildings constructed by the householder: earth, rubble stone and unreinforced masonry buildings. Buildings with heavy roofs. Older structures with little lateral strength, poor quality buildings or buildings with construction defects. Tall buildings from distant earthquakes, and buildings built on loose soils. Structures sited on weak slopes. Infrastructure above ground or buried in deformable soils. Industrial and chemical plants also present secondary risks.

Main mitigation strategies

Engineering of structures to withstand vibration forces. Seismic building codes. Enforcement of compliance with building code requirements and encouragement of higher standards of construction quality. Construction of important public sector buildings to high standards of engineering design. Strengthening of important existing buildings known to be vulnerable. Location planning to reduce urban densities on geological areas known to amplify ground vibrations. Insurance. Seismic zonation and land-use regulations.

Community participation

Construction of earthquake-resistant buildings and desire to live in houses safe from seismic forces. Awareness of earthquake risk. Activities and day-to-day arrangements of building contents carried out bearing in mind possibility of ground shaking. Sources of naked flames, dangerous appliances etc. made stable and safe. Knowledge of what to do in the event of an earthquake occurrence; participation in earthquake drills, practices, public awareness programs. Community action groups for civil protection: fire-fighting and first aid training. Preparation of fire extinguishers, excavation tools and other civil protection equipment. Contingency plans for training family members at the family level.