Seismicity (Earth Shaking)

95% of all earthquakes occur on (or immediately next to) plate boundaries

Frequency MagnitudeAverage Annually 8 and higher ,000 (estimated) ,000 (estimated) ,300,000 (estimated)

As plates move, friction causes the ground to shake. This gives is the many small earthquakes or tremors. When the plates stick, pressure builds up. When this pressure is suddenly released parts of the surface experience an intense shaking motion that lasts for just a few seconds The point at which the plates jerk apart is the FOCUS Directly above this point is the EPICENTRE

Depth The depth of the focus is important as the deeper the focus, the further the energy must travel to reach the surface. The energy diminishes as it moves through the ground so shallow depth earthquakes have the potential to do more damage. However, due to the pressure exerted, deep focus are usually much more powerful, so despite the further distance to the surface, they are likely to do more damage Shallow = 0-70km deep (account for 70% of total quakes) Intermediate km deep Deep = km deep

Waves Energy released at the focus radiates out from this point to the surface. There are 4 types of waves

P Waves (Primary) P waves are the first to reach the surface. P waves pass through solid, such as rock, and they can also pass through water or air. When earthquake waves pass though the air, sometimes they produce a noise. They pass through mantle and core so can be measured on both sides of the world.

S Waves (Secondary) When S waves pass through rock they distort its shape. S waves cannot pass through liquid or air, and you would not feel them aboard a ship at sea. They can’t travel through the core so can’t be measured on the other side of the world wave_animation.gif wave_animation.gif

Which Wave? P and S waves take energy from the focus to the surface Once at the surface, we can see 2 more waves. These move more slowly that P or S waves, but cause much more damage.

L Waves (Love) The slowest waves and the ones that cause the most damage. The ground moves horizontally

R Waves (Raleigh) These radiate from the epicentre in a rolling motion up and down gif gif

Magnitude Richter Scale Based on the Energy of the event Modified Mercalli Scale Based on observations of the effects

THE RICHTER SCALE Invented by Charles Francis Richter in 1935 Measure of an earthquake’s magnitude Scale between 0 and 10(theoretically) Logarithmic scale

By the late 1800’s the first seismographs were developed, and were very simple instruments. Even as the entire world shakes around it, a suspended mass remains stationary due to its inertia. Seismic waves can then be recorded as wiggly lines by pen and ink on paper wrapped around a rotating drum. A typical recording of an earthquake records a typical “train” of seismic waves. First the P waves, followed in succession by the S and surface waves.

As we have seen, S waves are slower than P waves, and seismologists use this fact to tell how far away it is from the seismograph to the earthquake. The seismograph records the P wave first, then the S wave. If the seismologist knows the speed of each wave, then by knowing that both waves started at the same time it’s possible to work out how far the earthquake waves have traveled to reach the seismograph.

If we can determine the distance of the same earthquake from several different seismograph stations, we are able to locate the epicenter, the point on the Earth’s surface directly above the earthquake focus.

THE MODIFIED MERCALLISCALE Revision an old measure by Giuseppe Mercalli 1906 Measure of an earthquake’s impact Scale between 1 and 12

Effects of an Earthquake National Geographic 4 min clip – the effects of earthquakes. Link Link Use the textbook and following videos to make notes on the following Liqufaction Landslides/avalanches Effects on built environment Tsunamis Aftershocks

Remember Caused directly by the earthquake = Primary effect e.g. Liquefaction, damage to buildings Caused as a result of something else = Secondary effect e.g. Tsunami, fires, civil disorder

Liquefaction Explained

Mass Movement when masses of rock, earth, or debris move down a slope. Mass movement is movement of masses of bodies of soil, bed rock, rock debris, soil, or mud. They may be very small or very large, and can move at slow to very high speeds. Usually occur along steep-sided hills and mountains because of the pull of gravity. An avalanche is a sudden flow of a large mass of snow or ice down a slope, sometimes at speeds exceeding 160 km/hr. Mass movement and avalanches can be started when earthquakes destabilise rock and start a slide.

Tsunami - What is a Tsunami? Tsunami -a large wave, or series of waves, caused when an earthquake causes massive undersea crust movements and/or collapses which displace the water above. Earthquakes deform the ocean floor, pushing the overlying water up into a tsunami wave.

Earthquakes don’t kill people, buildings do.

BUILDINGS Side to side forces that cause collapse Damage caused by lateral forces

Aftershocks A tremor (or one of a series of tremors) occurring after the main shock of an earthquake. These are caused by the plates ‘settling’ :

Planning for an Earthquake Cities on fault lines know that at some point, they will experience an earthquake. Finances, population size, history of earthquakes will all play a part in determining how prepared a location is Earthquakes can not be predicted. Some scientists believe they can predict the area of the next earthquake based on historical events, but can’t give an approx. timescale Read side 1 of handout

Precursors? Animals going wild Argon gas in the soil Electrical charges in rocks Bulging of the ground Microquakes (perhaps the best indication) Sadly, so far there is no concrete evidence for any of the above. Some believe that micro quakes can be detected up to 60 seconds before the main quake. But even if this was the case, it is impossible to have a large scale evacuation at such short notice

Planning for an Earthquake Drill and train Drills at school. Earthquake education centre Tokyo

Planning for an Earthquake Shear walls Cross Bracing Reinforced stone wall

Cross bracing Construction of Pearl River Tower, China

LARGER BUILDINGS Base isolation Left: regular building Right: base isolated building

Deep Foundations

Ways of improving resistance to earthquakes

SKY SCRAPERS Tuned Mass Damper – Tapai 101 – 800 Tonnes The tuned mass damper at the top of the

Read side 2 of handout