Nature, magnitude and frequency of seismic activity (earthquakes)

Nature of earthquakes

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

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

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

S waves

Waves are refracted towards the normal as they enter denser layers of the earth and speed up Waves are refracted away the normal as they enter less dense layers of the earth and slow down

Path of P and S waves through the earth

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

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

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

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)

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)

Mercalli Scale

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.

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

Seismicity by numbers 9.5 9.0 24km 40.4m 10km 1 minute 20,000 $300 billion 3.8 8km

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