1. Forms of seismic hazard
SEISMIC HAZARDS
Forms of seismic hazard

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

3. 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 km 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

6. 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

8. 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

9. 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

12. 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

16. epicentre

17. 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

18. 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)

19. Body waves

20. 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

21. 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

22. Surface waves

23. 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

24. 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

25. 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

26. 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

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

29. 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

31. Marina District, San Francisco (Loma Prieta earthquake 1989)

32. 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