1. Nature, magnitude and frequency of seismic activity (earthquakes)

3. Nature of earthquakes

4. Nature of earthquakes
An earthquake is a shock or series of shocks caused by a sudden movement of rocks at or beneath the Earth’s surface.
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

5. 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

6. 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 - Primary waves move very quickly
Secondary waves travel deep underground through solid material only
Surface waves travel at ground level and include Rayleigh and Love waves - Surface waves are slow and powerful and cause most earthquake damage
Each type of seismic wave has a distinctive pattern of movement

8. S waves

9. Path of P and S waves through the earth

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
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

15. 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

19. epicentre

20. Nature, magnitude and frequency of seismic activity (earthquakes)

21. 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

22. Magnitude and Frequency
The magnitude of an earthquake can be measured in a variety of ways…

23. Richter Scale
Developed in 1930s by American seismologist Charles Richter
Magnitude detected from the logarithm of the amplitude of seismic waves on seismogram with a correction for the distance between the seismometer and earthquake focus
Method used to measure magnitude of medium size earthquakes in California (shallow focus earthquakes < 16km depth and close to seismograph stations) so no need for correction for depth of earthquake focus
Each whole number on the scale = 10 x (log 10) increase in earthquake size (as measured by seismic wave amplitude) and 30 x (log 1.5) increase in energy released

26. Measuring earthquake magnitude
Richter magnitude measured by amplitude of seismic waves becomes unreliable for earthquakes > 600km distance
Richter scale cannot accurately measure the magnitude of very large earthquakes - tend to record magnitudes around 7 to 8
Richter scale not commonly used now - replaced by the moment magnitude scale by many geologists
Richter magnitude is still used by the UK’s British Geological Survey for ‘local’ UK earthquakes (up to 600km away)

27. Moment magnitude scale (MMS)
Devised by the USGS (US Geological Survey) in the 1970s
Best scale to use for large earthquakes and now used by the USGS to estimate the magnitude of all modern large earthquakes
Each whole number on the scale = 10 x (log 10) increase in energy released
Magnitude calculation includes measurement of the size of the rupture surface - larger earthquakes have larger rupture surfaces
Seismogram measure seismic waves over a range of frequencies rather than one single frequency graph (like Richter Scale)

28. Mercalli Scale
The Modified Mercalli Intensity Scale. The effect of an earthquake on the Earth's surface is called the intensity. The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally - total destruction.

29. Mercalli Scale

30. Larger earthquakes occur less frequently than smaller ones
Larger earthquakes occur less frequently than smaller ones. This relationship is exponential, i.e. there are ten times as many magnitude 6 or larger earthquakes in a given time period than magnitude 7 or larger earthquakes.

32. There is no evidence that earthquakes are becoming more frequent but…
There are in certain places due to human activity – in this case fracking.

33. Seismicity by numbers
9.5 biggest earthquake recorded in 20th century Chile 1960
9.0
24km
40.4m
10km
1 minute
20,000
$300 billion
3.8 magnitude of Oakham earthquake Oakham Rutland, UK January 2015
8km depth of Oakham earthquake Oakham Rutland, UK January 2015

34. Seismicity by numbers
9.5 biggest earthquake recorded in 20th century Chile 1960
9.0 magnitude of earthquake Tohoku Japan 2011
24km depth of earthquake focus Tohoku Japan 2011
40.4m maximum height of tsunami Tohoku Japan 2011
10km furthest distance inland reached by tsunami Tohoku Japan 2011
1 minute warning time for Tokyo from earthquake warning system Tohoku Japan 2011
20,000 estimated people lost (dead and missing) Tohoku Japan 2011
$300 billion estimated cost of damage earthquake Tohoku Japan 2011
3.8 magnitude of Oakham earthquake Oakham Rutland, UK January 2015
8km depth of Oakham earthquake Oakham Rutland, UK January 2015