Earthquake Hazards: Geology and Seismology

• Geology is the science that deals with the dynamics and physical history of the Earth and rocks, including physical, chemical, and biological changes.
• A geologist is a scientist that studies the solid, liquid, and gaseous matter that constitutes the Earth and other terrestrial planets, studying the processes that shape them.
• A geologic hazard is an adverse geologic condition capable of causing damage or loss of property and life.

Earthquake Hazards

• An earthquake (quake, tremor, or temblor) is the perceptible shaking of the Earth's surface resulting from the sudden release of energy in the Earth's crust that creates seismic waves.
• Seismology is the study of earthquakes and seismic waves and their movement.
• Seismography is the scientific measuring and recording of the shock and vibrations of earthquakes.
• Seismologists study earthquakes and planetary activities, including their effects, like tsunamis.
• An earthquake is a type of hazard that depends on the strength of seismic activity, as well as local topographic and built features, subsurface geology, and groundwater.
• Earthquake hazards can be classified with primary and secondary effect.

Primary Effects

• Primary effects are permanent features of an earthquake, such as surface ruptures and the offset of natural or human-constructed objects.

Secondary Effects

• Secondary effects happen when ground movement results in other types of destruction.
• Examples of secondary effects: landslides, tsunamis, liquefaction, and fire.

Ground Shaking

• The Earth shakes as earthquake waves pass through, radiating energy that was stored in stressed rocks and released when a fault broke.
• The strength of ground shaking is measured in the velocity and acceleration of the ground motion, the frequency content of the shaking, and the duration.

Ground/Surface Rupture

• Surface rupture occurs when fault rupture extends to the Earth's surface, causing an offset of the ground surface.
• Structures built across the fault are at particular risk of being torn apart when the two sides of the fault slip.

Liquefaction

• Soil liquefaction is a phenomenon in which the strength and stiffness of soil decreases as it is reduced by earthquake shaking or other rapid loading.
• It normally occurs in saturated soils, in which the spaces between individual soil particles are filled with water.

Ground Subsidence

• Earthquake-induced ground subsidence, or lowering of the ground surface, often occurs during earthquakes.
• This can result from downward vertical displacement on one side of the fault, sometimes affecting a huge land area.

Lateral Spreading

• Earthquake-induced lateral spreading occurs where sloping ground moves downhill, causing cracks to open up, often seen near hill crests and river banks.

Landslide

• Earthquake-induced landslides are triggered by strong ground motions and classified as an important secondary earthquake hazard.
• The term "landslide" includes a wide range of ground movement, such as rock falls, deep failure of slopes, and shallow debris flow.

Tsunami

• A tsunami, also known as a seismic wave, is a series of waves in a water body caused by displacement of a large volume of water, generally in an ocean or a large lake.

Tsunami Natural Signs

• Animal behavior changes. Some species like elephants can sense subsonic Rayleigh waves from an earthquake or tsunami.
• Drawback, which is an observable natural sign of an impending tsunami.

Methods of Reducing Risks

• Accurately record and interpret ground motion.
• Construct seismic hazard maps.
• Develop resistant structures.