Earthquakes and Seismic Waves

• An earthquake is a phenomenon caused by the sudden and rapid movement of large volumes of rock along fractures on the surface of the earth called faults.
• The outermost shell of the earth consists of tectonic plates, which include the crust and the uppermost mantle.
• Tectonic plates are massive and irregularly shaped slabs of rock.
• The edges of tectonic plates get stuck as the plates continue to move due to their roughness.
• Seismic waves radiate energy in all directions through rocks beneath the earth, creating the shaking felt on the surface.
• The two main types of seismic waves are body waves and surface waves.
• Body waves travel through the interior of the earth.

Primary Waves

• Primary (P) waves are the first to reach the surface, causing the ground to move back and forth in the direction of travel.
• The shaking from P waves is light, or sometimes not felt.

Secondary Waves

• Secondary (S) waves are body waves that vibrate perpendicular to the direction of travel, resulting in an up-and-down motion.
• S waves move slower but shake the ground more strongly than P waves.
• Surface waves are trapped near the surface.

Love Waves

• Love waves are surface waves with horizontal motion perpendicular to their direction of travel.

Rayleigh Waves

• Rayleigh waves shake the ground in a rotational manner without transverse motion.
• The severity of an earthquake is measured by its magnitude (energy released) and intensity (effects on people and structures).
• Frequency is the number of times a movement such as ground shaking is repeated within a certain amount of time.
• High frequency earthquakes affect small buildings more than tall buildings.
• Ground shaking is the primary cause of earthquake damage, leading to the collapse of structures and potential casualties.
• Flash floods can occur if ground shaking breaks water dams.
• Fire is a secondary hazard triggered by damage to electric and gas lines.
• Ground shaking can also cause landslides and liquefaction.
• Building codes set standards for building construction to ensure structures are built properly.
• Ground rupture is the visible breaking and displacement of the Earth's surface along the trace of a fault.
• Ground rupture can involve vertical, lateral, or combined movements, depending on the fault type.

Strike-Slip Faults

• Strike-slip faults are vertical or near-vertical faults that displace rock horizontally.
• If the block opposite you moves left, it is a sinistral (left-lateral) fault.
• If the block moves right, it is a dextral (right-lateral) fault.
• Normal faults involve downward movement of the hanging wall relative to the footwall.
• Reverse or thrust faults move the hanging wall up.
• Dip-slip faults cause vertical displacement of the ground.
• Liquefaction occurs when the ground loses stiffness and behaves like a liquid during earthquakes.
• During shaking, grains vibrate, and undergo compaction where sediments are compressed, squeezing out fluids.
• Pore water pressure increases as spaces between sediment grains decrease.
• Liquefaction happens when the pore water pressure equals the weight of the overlying material.
• Sand boils involve geysers of sand and water shooting from the ground due to liquefaction at depth.
• Mass wasting, also known as a landslide, is the downslope movement of rocks or sediments due to gravity.
• Landslides follow weathering and removal of rock or debris.
• Landslides are triggered by factors such as rain, oversteepened slopes, removal of vegetation, and earthquakes.
• Creep is the gradual, almost imperceptible downslope displacement of rock or soil due to strain buildup.
• Factors affecting earthquake-induced landslides include earthquake strength, distance from the fault, topography, climate, rock/soil characteristics, and human modifications.
• A tsunami is a series of waves caused by water displacement, often from earthquakes or landslides.
• The Japanese word "tsunami" translates to "harbor wave."
• Earthquakes beneath the sea cause the water above to move upwards and produce tsunamis.
• Tsunami waves grow in height as they move into shallower depths, a process called wave shoaling, and approach the shore.
• Tsunamis often originate from earthquakes in subduction zones.
• Subduction zones are convergent boundaries where tectonic plates collide, and one plate descends beneath another due to temperature differences.
• Tsunamis can be local or regional.
• Local tsunamis originate nearby, affecting coasts within 100 km or less than an hour's travel time.
• Regional tsunamis affect a wide geographical area, typically within 1,000 km or within 1-3 hours of wave travel.
• Tsunamis can be predicted, but their exact damage cannot be foreseen.