Earthquakes and Associated Hazards
This revision and study page provides a good insight into the characteristics of earthquakes and their causes. It then develops the associated primary and secondary hazards associated with them.
Earthquakes take place at all types of plate boundaries but most frequently at conservative plate boundaries where two plates slide past each other rather than away or towards each other. Earthquakes with the highest magnitude occur at destructive plate boundaries where subduction takes places but these tend to be less frequent.
An earthquake occurs when the fault suddenly releases stored stress. Stress is the force per unit area acting on a plane within a body. There are three types of stress. Tensional stress builds up in faults at divergent plate boundaries where plates are moving away from each other. Compressional stress builds up in faults where the plates are converging and shear stress builds up in faults where the plates move side by side. When stress is released from the focus of the fault seismic waves transfer the energy in all directions. This can be seen in the diagram bottom left, which shows the focus of the earthquake. This marks the point in the fault, where stress was released. The epicenter marks the point directly above the focus and is where energy on the surface will be greatest. The seismic waves can be seen moving energy away from the focus. There are three types of fault shown in the diagram bottom right. These are called normal, reverse and strike-slip faults. Earthquakes can be found at varying depths along these three faults. The following video reveals the complex pattern around the strike-slip faulting in California.
Earthquakes are also caused by rising magma in chambers and conduits within volcanoes. Increased frequency of earthquakes in volcanoes are known to be important warnings prior to eruption. The rising magma in the conduit can be seen to the right. Earthquake frequency can be seen to increase with magma height within the conduit.
Human activity can also cause earthquakes. Mining activities disturb rock structure and the use of dynamite and heavy machinery have been known to cause earthquakes. Dam construction for reservoirs adds significant weight and load to faults and can also cause earthquakes. Fracking whereby shale gas is extracted from rock seams by injecting high pressure fluids has increased the frequency of earthquakes across the USA.
Types of Seismic Wave
Seismic waves can be seen in the following seismograph. There are three distinct types of seismic wave. Primary or Body waves, Secondary or Shear waves and Surface waves of which their ar types Love waves and Rayleigh waves
Alternating compressions (“pushes”) and dilations (“pulls”) move in the same direction as the wave is propagating. P motion travels fastest in materials, 5-7km/s in the crust and 8km/s in the Earth's mantle and core. Therefore the P-wave is the first-arriving energy on a seismograph. P-waves are generally smaller and higher frequency than the S and Surface-waves.
Alternating transverse motions (perpendicular to the direction of propagation. The particle motion is in both vertical and horizontal planes. S-waves do not travel through fluids, so do not exist in Earth’s outer core or in air or water or molten rock (magma). S waves travel slower than P waves in a solid, 3-4km/s in the Earth's crust, 4.5km/s in the mantle, 2.5km/s in the inner core. They therefore, arrive after the P wave.
Transverse horizontal motion, perpendicular to the direction of propagation and generally parallel to the Earth’s surface. Love waves exist because of the Earth’s surface. They are largest at the surface and decrease in amplitude with depth. Love waves are dispersive, that is, the wave velocity and depth is dependent on frequency, generally with low frequencies propagating at higher velocity and greater depth. Their velocity varies from 2-4.4km/s.
Motion is both in the direction of propagation and perpendicular (in a vertical plane), and “phased” so that the motion is generally elliptical. Rayleigh waves are also dispersive and the amplitudes generally decrease with depth in the Earth. Appearance and particle motion are similar to water waves. Depth of penetration of the Rayleigh waves is also dependent on frequency, with lower frequencies penetrating to greater depth. Their velocity varies from 2-4.2 km/s.
Earthquakes and Associated Hazards
The primary hazards of the ground are linked directly to seismic waves. Earthquakes can causes movement of the surface, including rolling and shaking. Brittle surfaces may fracture and infrastructure may collapse. Collapsing buildings and bridges is considered a primary hazard of earthquakes.
Secondary hazards result from the moving surface and collapsing infrastructure. These might include fires set off by broken gas lines, tsunamis caused by fault movement displacing water, and liquefaction where by the water and soil particles in the ground become separated leading to failure in the soil structure.
A tsunami is classified as a shallow water wave because its wave is bigger than the depth of ocean it moves over. Earthquakes generally cause tsunamis as a result of fault movement in a subduction zone. As the oceanic plate is subducted, it typically locks on
to part of the continental plate and drags it with it. However at some point the connection becomes broken and the crust spring back in the direction of the ocean trench in a process called elastic rebound. This sudden movement of ocean floor around the fault leads to ocean displacement and a series of tsunami waves that permeate away from the displacement in all directions.
In some instances such as the 2011 Japan tsunami the fault movement can be as much 10 meters in one single event. So this means than 10 meters of water is pushed upwards. The Gif to the left also shows how the displaced water creates a series of waves known as a tsunami train.
Deep Ocean versus Coastal Tsunamis
Tsunamis behave very different in the open ocean compared to the coast where the water is shallow.
A tsunami in the deep ocean has very long wavelengths and very low amplitude, meaning the wave crests are far apart. In fact as the diagram shows the wavelength is so great that in deep ocean a ship won’t even notice the wave has past. It also travelling at a tremendous speed.
Approaching the shore the tsunami will slow down in speed and its amplitude increases dramatically. The base of the wave on its approach to the shore is slowed down by friction but the surface water is less effected and so the amplitude of wave increases. Consequently, as the tsunami’s speed
diminishes as it travels into shallower water, its height grows. Waves that are unnoticeable in deep ocean can reach 10-50meters at the shore and although they slow down dramatically they can still be travelling at speeds up to 200km/hr. Tsunamis are hugely powerful and have the ability to destroy everything in their path. They arrive as a series of wave trains with alternating deep waves and shallower troughs. There have been a number of devastating tsunamis in recent times. The Indonesian earthquake and tsunami in 2004 was 9.0 on the Richter Scale and had major global impact, directly impacting 18 countries and cost the lives of 250,000 people. The Japanese earthquake and resulting tsunami of 2011, which was 9.1 on the Richter Scale, cost the lives of nearly 20,000 people and caused a nuclear power station in Fukushima to leak radioactive waste. Just in the summer of 2019 Indonesia was hit by another tsunami in Sulawesi. This earthquake which was just 7.5 on the Richter Scale cost the lives nearly 4,500 people and also produced dramatic liquefaction.
Soil liquefaction, also called earthquake liquefaction, refers to ground failure or loss of strength which leads to otherwise solid soil to behave temporarily as a viscous liquid. The phenomenon occurs in water-saturated unconsolidated soils affected by seismic S waves (secondary waves), which cause ground vibrations during earthquakes. Areas with poorly drained fine-grained soils such as sandy, silty, and gravelly soils are the most susceptible to liquefaction.
The following map and images show the scale of liquefaction following the 2010 earthquake in Christchurch, New Zealand. The maps shows how large parts of the center liquefied close to the river banks, where softer sediments were found. The photographs show how the ground sunk leading to roads and buildings to subside.
The image to the right shows how liquefaction impacted an entire section of Palu in Sulewesi, Indonesia. Part of the town just slipped downslope in a similar way to a landslide. The images were captured by seaming together satellite imagery of the town before, during and after the disaster. In this way liquefaction has a similar impact as a landslide.
Earthquakes are often causes of secondary hazards such as landslides and avalanches. The stress exerted on unstable slopes leads to mass movements. The photos below show a mass landslide event on the island of Hokaido, in Japan, following a 6.7 earthquake in 2018.
The landslides that numbered in the hundreds were thought to have been caused when the slopes were inundated during heavy rains. The wet pumice and the soil on top of it slid away due to the shear force of the earthquake rupturing the strata of soils and pumice.