Forms of seismic hazard SEISMIC HAZARDS Forms of seismic hazard

Forms of seismic hazard Earthquakes primary hazard Shock waves (seismic waves) primary hazard Tsunamis secondary hazard Liquefaction secondary hazard Landslides secondary hazard

Earthquakes An earthquake is a shock or series of shocks caused by a sudden movement of rocks at or beneath the Earth’s surface Shallow earthquakes 0-70km depth Intermediate earthquakes 70-300km depth Deep earthquakes over 300km depth Earthquakes make the ground shake - usually for less than a minute The point at which the earthquake is generated inside the earth’s crust or upper mantle is called the focus or hypocentre The point on the Earth’s surface directly above the focus is called the epicentre Seismic intensity decreases with distance from the earthquake focus Isoseismal lines are imaginary lines which join locations with equal degrees of seismic intensity - they form concentric circles around the earthquake epicentre

Earthquakes The largest earthquakes are caused by the movement of tectonic plates As plates move their rough edges rub against each other and may get stuck, stress then builds up until the rocks slip or break and suddenly move Shallow focus earthquakes usually cause the most damage Earthquakes are also caused by movement of rocks along existing fault lines a long way from plate boundaries Landslides, avalanches and volcanic eruptions may also trigger earthquakes Human activity may also cause earthquakes: underground explosions mining subsidence creation of water reservoirs (very heavy) behind dams fracking for natural gas by injecting high pressure fluids into sub-surface rocks

Locating the earthquake epicentre Record arrival time of the P waves Record arrival time of the S waves Calculate the interval between the arrival times of the P and S waves Use the S-P time to calculate the distance of each location from the epicentre (using P wave velocity and S wave velocity data) Needs three seismometers to fix the epicentre

An earthquake was recorded in Oakland, California by three seismometers. Calculate the time and location for this earthquake by using the seismogram information and the seismic travel time graph.   Seismometer 1 P-wave arrival time 13:19:58.9 S-wave arrival time 13:20:04.7 Seismometer 2 P-wave arrival time 13:20:02.6 S-wave arrival time 13:20:10.8 Seismometer 3 P-wave arrival time 13:19:54.7 S-wave arrival time 13:19:57.4

An earthquake was recorded in Oakland, California by three seismometers. Calculate the time and location for this earthquake by using the seismogram information and the seismic travel time graph.   Seismometer 1 ~ 48 kilometres P-wave arrival time 13:19:58.9 S-wave arrival time 13:20:04.7 Seismometer 2 ~ 67 kilometres P-wave arrival time 13:20:02.6 S-wave arrival time 13:20:10.8 Seismometer 3 ~ 22 kilometres P-wave arrival time 13:19:54.7 S-wave arrival time 13:19:57.4

epicentre

Shock waves (seismic waves) Seismic waves are earthquake shock waves generated from the focus within the Earth’s crust or upper mantle There are three main types of seismic wave: Primary waves travel deep underground through solid and liquid material Secondary waves travel deep underground through solid material only Surface waves travel at ground level and include Rayleigh and Love waves Each type of seismic wave has a distinctive pattern of movement Primary waves move very quickly Surface waves are slow and powerful and cause most earthquake damage

BODY WAVES SURFACE WAVES Type of wave Speed Motion Pathway through the Earth Potential for causing damage Travel through the body of the earth BODY WAVES Primary waves (P) Secondary waves (S) Travel near the surface of the earth SURFACE WAVES Rayleigh waves (R) Love waves (L)

Body waves

Primary or pressure waves P waves are the fastest seismic waves typically 5km per second Compressional waves - alternately push and pull vibrating forwards and backwards Travel through all types of material (solid/liquid/gas) so they travel through crust, mantle and core to other side of the earth

Secondary or shear waves Slower than P waves - typically 3 km per second Transverse waves - vibrate at right angles to the direction of travel either side to side or up and down Only travel through solid materials so do not travel through the liquid outer core to the opposite side of the earth Shearing motion of S waves causes much more damage than P waves

Surface waves

Rayleigh waves Slower than P waves and S waves Move in an elliptical rolling motion like sea waves - ground moves up and down Travel near the ground surface - can travel through solids and liquids

Love waves Slower than P waves and S waves Transverse waves - travel at right angles to the direction of travel so ground moves side to side Travel near ground surface - only through solids Destructive due to their shearing action

Shock waves (seismic waves) Foreshocks are shock waves recorded a short time before the main shock and are caused by the initial slip or fracture of the stressed rock Aftershocks are minor shock waves which occur hours, days or even months after the main series of earthquake shock waves and result from the readjustment of the overstressed rocks as they settle down following the main slip or fracture

Tsunamis Tsunamis are large seismically generated waves caused by submarine earthquakes, submarine volcanic eruptions and submarine landslides (caused by seismic activity) Earthquakes are a significant cause of tsunamis particularly where there is vertical slipping at a subduction zone or mid-ocean ridge Shaking of the sea bed suddenly displaces the entire water column above and waves are formed as the displaced water mass, under the influence of gravity, attempts to regain its equilibrium - when large areas of the sea floor move vertically a tsunami may be generated Tsunamis are not always seismically generated: may occur in large lakes following a landslide or glacier breaking away into the water body may also be caused by falling meteorites and asteroids

Size of circles is proportional to the earthquake magnitude for tectonic tsunamis Colour represents the tsunami intensity on the Soloviev-Imamura scale

Liquefaction Soils and sediments with a high water content start to behave like a fluid when they are vibrated by earthquake shock waves Individual particles lose their frictional contact as they are shaken apart and as a result the ground loses its mechanical strength Buildings and other built structures collapse as their foundations subside or break in the liquefied ground

Marina District, San Francisco (Loma Prieta earthquake 1989)

Landslides Earthquake shock waves can trigger slope failure causing a landslide or an avalanche Earthquake shock waves vibrate the ground briefly disrupting the stability of the slope material so that gravity can pull it downwards