The nature of seismicity: forms and causes 3.1.5.4 Hazards What you need to know The nature of seismicity and its relation to plate tectonics. Forms of seismic hazard including earthquakes, shockwaves, tsunamis, liquefaction, landslides. Spatial distribution, randomness, magnitude, frequency, regularity, predictability of hazard events. Introduction The crust of the Earth is made up of seven major plates and several minor ones. They are all on the move. At the plate margins, where plates are travelling in different directions, stress can build up. When the pressure - resulting from a build-up of friction - is released, a series of tremors or earthquakes can be felt. Spatial distribution of earthquakes The location of earthquakes is closely associated with plate margins. At destructive plate margins, earthquakes tend to occur at depth. They are associated with the subduction of one crust under another in a narrow area known as the Benioff zone, where compressional forces are greatest. Earthquakes occurring here can be very powerful and may take place under the sea close to heavily populated coastal zones; this makes the threat of dangerous tsunami much more likely. At constructive plate margins, earthquakes tend to be much shallower and less powerful. They are associated with tensional forces in the crust and occur along mid-ocean ridges away from land or large populations. At conservative plate margins, earthquakes tend to be shallow focused. Here the continental plates are dragging past each other and compressional forces are high. Earthquakes occurring here can be very powerful and damage can be severe if they occur in densely populated areas. Magnitude of earthquakes Most of the earthquakes that occur each year are too small to be felt. With increasing numbers of seismometers available to measure earthquakes and people living in more remote locations, we are reporting earthquakes more today than we have in the past. Earthquake magnitude can be measured on the Richter scale, a logarithmic scale 1-10, where 7 on the scale is ten times more powerful than a 6 and 100 times more powerful than a 5. Around 120 earthquakes of magnitude 6 and above occur each year, of these 20 measure 7 or above on the Richter scale (a major earthquake). Great earthquakes (measuring 8 or above) tend to occur once or twice a decade and for the people living at the epicentre it can completely destroy their community. Although the Richter scale is useful for measuring small-scale earthquakes, its accuracy decreases for larger earthquakes. The Moment Magnitude scale is a measure of the total distance a fault has moved and the force needed to generate it, known as the moment release of the earthquake, and is now used worldwide by seismologists. The Modified Mercalli Intensity (MMI) scale is more a measure of the effect on people and human infrastructure. The 12-point scale classifies the impact of the earthquake and indicates severity on humans rather than objective measurement of earthquake force. A moderate earthquake under a major city can register higher on the MMI than a strong earthquake in a largely unpopulated region.
The nature of seismicity: forms and causes 3.1.5.4 Hazards While 85 per cent of earthquakes occur near plate boundaries, some occur in intraplate locations. These earthquakes, internal to plates, can highlight undiscovered fault lines and areas of stress in crustal rocks, but they can also relate to other causes such as isostatic recoil, when the weight of glaciers is lifted as they melt and the crust lifts. Iceland s surface has lifted by 60m since the peak of the last ice age; with uplift comes stresses and strains in crustal rock and consequently earthquakes. Earthquakes can also be a result of human activity. These quasi-natural earthquakes are most commonly associated with the crustal deformation caused by the weight of water stored in reservoirs. Some researchers have attributed the 7.9-magnitude Sichuan earthquake in May 2008, which killed an estimated 80,000 people, to the construction of the Zipingpu Dam in China and the resulting pressures imposed on the rocks underlying its reservoir once at full capacity. Primary seismic hazards Earthquakes are the release of energy in the form of seismic waves from the point known as the focus where the pressure is released. Many earthquakes result from movements along fractures in rocks called faults. One of the most studied faults is the transform plate margin of California, USA. Years of research have found that this is not a single fault line, but a complex zone of faults. Once pressure is built up in a rock and released, seismic or shockwaves radiate out in all directions. The point on the surface of the Earth directly above the focus is known as the epicentre and can be the location of greatest damage. There are four types of seismic waves. P waves and S waves are body waves that travel through rock; P waves are the fastest and can also travel through liquids. When body waves arrive at the epicentre they cause people and property to rise and fall. This can sever water, gas and other infrastructure. Escaping gas frequently ignites and is difficult to extinguish, as ruptured water mains are unable to supply water to the emergency services. Surface waves (Rayleigh and Love Waves) travel out from the epicentre and they represent the most severe hazard to people and property. The rocking motion associated with these waves can shake and topple buildings like dominoes. Bedrock type can also affect the extent of damage to buildings. Soft clays and unconsolidated bedrock tend to wobble like jelly and amplify shaking. Solid bedrock tends to limit shaking. At Kobe, Japan, the port experienced severe damage as a result of the amplification of ground waves through the soft sands and muds it was built upon. Similarly, the Marina District of San Francisco, built on bay fill and muds was devastated as a result of the ground waves of the 1989 Loma Prieta earthquake. In general buildings constructed on solid bedrock are much less likely to suffer damage or collapse than those located on soft sediments. Secondary seismic hazards Soil liquefaction When seismic waves travel through soft sediments, they cause it to behave as if it were a liquid, due to an increase in pore water pressure. It affects unconsolidated sediments at depths of less than ten metres, which are saturated with water. As a
The nature of seismicity: forms and causes 3.1.5.4 Hazards result of soil liquefaction, the foundations of buildings become unsupported and consequently they sink or topple over. During the Christchurch earthquakes that struck New Zealand in 2011, liquefaction was largely responsible for the extensive damage to residential properties. Soil liquefaction is most dangerous when it causes the collapse of road and rail supporting structures, as happened to the San Francisco Oakland Bay Bridge and the Cypress Street Viaduct (Nimitz Freeway) during the Loma Prieta earthquake in 1989. Tsunami When a submarine earthquake occurs it can generate seismic waves that, if large enough, can generate a tsunami in the ocean. The rapid deformation of the sea bed can uplift a column of water as the oceanic crust is thrust upwards. The resulting collapsing column of water acts like ripples in a pond and radiates energy outwards from the focus. Some 90 per cent of damaging tsunami occur in the Pacific Basin as they are generated at subduction-convergent plate margins, particularly those bordering Japan, the Aleutians and South America. The magnitude 9.0 Tōhoku Earthquake which hit Japan in 2011, triggered a powerful tsunami with waves that reached up to 40 metres in height. The effects of the tsunami were felt around the world, but were most severe in north-east Japan where 130 000 homes collapsed, 4.4 million households were left without electricity and over 16,000 people died. Source NASA (public domain).
The nature of seismicity: forms and causes 3.1.5.4 Hazards Landslides Sudden ground shaking can cause slope failure on even gentle-slopes. In many earthquakes, particularly in mountainous zones, landslides can be more dangerous that the primary ground shaking. Between 200 and 300 million cubic metres of landslides occurred as a result of the Gorkha earthquake that struck Nepal in April 2015. They blocked some of the key routes to Kathmandu along the Langtang valley and obstructed the entire valley along the Trisuli River. An avalanche triggered by the quake swept through a Mount Everest base camp, killing 17 and injuring 61, making it the deadliest day in Everest history. The predictability of hazard events We know much about where earthquakes are likely to occur, due to their association with plate margins. However, predicting when earthquakes will occur is almost impossible. Seismologists are specialist scientists that study earthquakes. Hazard mapping can help to predict where the next earthquakes might occur. Each time an earthquake strikes, it is mapped. Places on plate margins that have not recently experienced an earthquake have a higher strain building up in the crustal rocks and a higher probability of experiencing an earthquake. This gap theory can be used to provide earthquake probabilities and help authorities plan for possible hazard events. The 2015 Gorkha earthquake in Nepal was predicted back in 2013, as measurements suggested that there was sufficient accumulated energy to produce a great earthquake (magnitude 8). The precise moment of occurrence was more difficult to define, however. If inhabitants don t have faith in earthquake predictions made by scientists, or forecasts have proved to be inaccurate in the past, they are less likely to heed warnings of impending seismic activity. Attempts to predict earthquakes involve monitoring pre-existing fault lines. This can be expensive as the precise time and location of earthquakes is unknown. The Hayward fault along the eastern side of San Francisco Bay is arguably one of the most hazardous faults in the world and also the most studied. Monitoring methods include the following: Tiltmeters and magnetometers are used to detect changes in the ground height and local magnetic field. Seismographs can detect foreshocks prior to the main seismic event. Water can be measured for radon gas and changes in the height of the water table. Strainmeters can monitor the increase in stress experienced in crustal rocks. Unusual animal behaviour can indicate an imminent earthquake. Horses, dogs, birds and other animals seem to sense electromagnetic disturbance as rock friction increases. Immediately prior to the devastating Boxing Day earthquake in the Indian Ocean and resultant tsunami affecting much of the coast of south east Asia, working elephants were seen to become extremely anxious and break their leg chains before running inland into the forest.
The Great Tōhoku earthquake, Japan Specification topic: seismicity forms and causes 3.1.5.4 Case study: Great Tōhoku earthquake Japan is located in a hazardous area at the junction between four major plates. In the north of the country the Pacific plate, moving westwards at a rate of 83mm a year, is being forced under the continental Eurasian plate at a subduction zone known as the Japan Trench. The fault uplifted by over 30 metres along a 300 km section of the Japan Trench. It was the most powerful earthquake ever recorded in Japan. The submarine earthquake occurred at a depth of 30 km; this is fairly shallow and the consequent ground shaking was violent and lasted over three minutes. Japan s coastal area is flat, low-lying and the unconsolidated soil amplified the shockwaves. Coastal bays are deep and amplify tsunami waves. The 11 March earthquake was preceded by a series of large foreshocks over the previous two days. The Japan Meteorological Agency sent out an earthquake warning on national television and radio, giving people minutes to evacuate. Primary effects 16,000 people were killed and 6,000 injured. The earthquake shifted the Earth on its axis by 10cm. Buildings in the city of Sendai (130 km from the epicentre) and Tokyo (370 km from the epicentre) swayed. Damage was limited due to earthquake-resistant design. Most casualties and damage occurred in Iwate, Miyagi and Fukushima where 127 000 buildings were completely destroyed and a million were damaged across northeast Japan One school in Ishinomaki, Miyagi lost 84 of 121 students and teachers. Around 4.4 million households in northeast Japan were left without electricity and 1.5 million without water. 1100 sections of train track were damaged. Only four trains were derailed because all high-speed trains had been automatically stopped by an early warning system. Secondary effects At least 1800 houses were destroyed in Fukushima when the Fujinuma dam failed. Oil refineries in Ichihara and Sendai set on fire. Tsunami waves only took 20 minutes to arrive and reached heights of over 30 metres at Miyako. Water travelled up to 10 km inland at Sendai. 56 bridges and 26 railways were washed away along the east coast of Honshu from Chiba to Aomori and 332 400 people left homeless The earthquake knocked out the power supply to the Fukushima Daiichi Nuclear Power Plant. The tsunami then flooded back-up generators and triggered nuclear meltdown in three reactors. Elsewhere, the tsunami caused massive slabs of ice to calve from the Sulzberger Ice Shelf, Antarctica and damage property in California, Hawaii and Ecuador.
The Great Tōhoku earthquake, Japan The cost of this disaster was in excess of $300 billion, the most expensive natural disaster in history. Responses Japan was overwhelmed by the magnitude of the disaster. Responses from 116 countries and 28 international organizations came quickly. The military were forced to bury the dead in mass graves to prevent the spread of disease. Japan declared a state of emergency following problems at six nuclear reactors. 150 000 residents were evacuated from a zone 20 km around the Fukushima nuclear power plant. All 55 nuclear reactors across Japan were taken offline for safety reasons; rolling power blackouts were common for weeks. Toyota and Sony stopped production, while other companies decided to relocate to safer countries. Some elderly people continued to live near Fukushima; otherwise it was all but abandoned. Japan s food exports were restricted due to radiation fears. Recovery and rebuilding to restore normality will take years due to the scale of the disaster. Exam style questions: 1. For a named area that experiences volcanic/and or earthquake hazards, explain how people perceive and are impacted by the hazard. (9 marks) 2. The extent to which earthquakes represent hazards depends on where they occur How far do you agree with this statement? (20 marks)
The Great Tōhoku earthquake, Japan 1. For a named area that experiences volcanic/and or earthquake hazards, explain how people perceive and are impacted by the hazard. (9 marks) Students need to justify why people perceive hazards in particular ways, this answer refers to the Great Tōhoku earthquake, but the focus could alternatively be a volcano such as Mount Ontake. In Japan, people perceive hazards in a variety of ways. Japan is a multi-hazardous environment and is affected by typhoons, volcanoes, earthquakes, tsunamis and geomorphological hazards such as landslides. The risk probability for each of these varies according to where people actually live. Coastal zones are less likely to suffer the effects of volcanoes or landslides and the inland mountainous region has a low probability of being affected by tsunamis or flooding. Hazard salience is the relative importance of the earthquake risk compared with other concerns. The Japanese economy could cause individuals greater concerns than the risk from natural hazards. Perception is often related to experience in living memory. As typhoons occur fairly regularly they are perceived to be risky, this is why most homes prior to 1995 were built with heavy tiled roofs to protect people from heavy rainfall, but after the Kobe earthquake in 1995, new homes were built to protect inhabitants from earthquakes. People were impacted by the Great Tōhoku earthquake in a number of ways: 16,000 people were killed. This caused psychological problems for many, especially when you consider death tolls in some coastal communities was massive. Oil refinery fires as well as ruptured water pipes and gas lines meant people were cut off from vital infrastructure (around 4.4 million households without electricity and 1.5 million without water), this made recovery difficult. 127,000 buildings were destroyed and a million were damaged, this left a huge amount of rubble to clear before reconstruction could occur. People had felt safe behind 6-8 metre harbour walls, but these were breached during the tsunami. Nuclear meltdown was a possibility. Some workers risked their lives to stop meltdown and the possibility of radiation reaching thousands of others. 2. The extent to which earthquakes represent hazards depends on where they occur How far do you agree with this statement? (20 marks) Students are required to use their knowledge and understanding in order to assess a range of factors that make earthquakes hazardous to humans and conclude as to whether location is the most significant factor.
The Great Tōhoku earthquake, Japan Locational factors Location in relation to the nearest plate margin and type of margin Location of epicentre if the epicentre is near urban areas more buildings and people are going to be affected Depth of focus shallow-focused earthquakes cause more shaking Geology - unconsolidated rocks such as clay and sand can amplify shockwaves and cause the collapse of buildings Non-locational factors Time of day more deaths can occur if people are asleep/inside, especially when buildings are poorly built Level of development rich countries can afford to mitigate the impact of earthquakes through prediction, protection and preparation Planning - building control and aseismic building designs can prevent building collapse and save lives Prediction forecasting when and where an earthquake might strike can provide planning time for people Argument for location as a significant factor Places on destructive plate margins such as Japan can suffer from earthquakes and tsunami, making them very hazardous. Japan has densely populated coastal areas meaning the population is very vulnerable e.g. Kobe 1995, Great Tōhoku 2011. Places on conservative plate margins can suffer strong earthquakes with no warning. These are hazardous and can affect densely populated areas such as California e.g. Loma Prieta 1989. Places on constructive plate margins can suffer weak shallow-focused earthquakes. These are less hazardous. The further away from a plate margin, the less hazardous earthquakes are; the British Isles has earthquakes, but they have a low magnitude and cause little more than light shaking, similar to a lorry passing by. The focus of the Great Tōhoku earthquake was shallow. Shaking was strong and lasted several minutes. This destroyed thousands of buildings, cut power supplies, ruptured gas mains and caused fires. Geology varies with location. Unconsolidated soft sediment amplifies shockwaves making earthquakes that strike coastal bays, old lakebeds and reclaimed land more hazardous. Topography varies with location. The very flat coastal area in the north-east of Japan allowed tsunami waves to travel far inland without impediment. It also flooded the back-up generators for the nuclear power plant resulting in this being a complex and very hazardous disaster. Argument against location as a significant factor The three Ps (planning, prediction and preparation) can be implemented anywhere and can reduce the hazard associated with earthquakes. Death tolls in modern earthquakes are much lower than in the past due to risk management. Although earthquakes are common in California, the population has a high capacity to cope and therefore death toll is low. The time of day can have a huge effect on the death toll, if people are in open spaces (usually during the day) or in bed in hazard-resistant buildings, their chance of surviving an earthquake are much higher.
Hazards: The nature of seismicity forms and causes 3.1.5.4 Q1 A B C D E Match the terms with their process description When the weight of glaciers is lifted and land subsequently uplifts When soft sediments behave like a liquid due to ground tremors When pressure built up between two plates is released When a denser tectonic plate is forced under a less dense one The resultant giant waves caused by submarine earthquakes F Slope failure and mass movement due to ground tremors Select from: landslides subduction earthquake liquefaction isostatic recoil tsunami Q2 Tick whether these are primary or secondary seismic hazards Primary Secondary A The resultant giant waves caused by submarine earthquakes B C D E Slope failure and landslides due to ground tremors When pressure built up between two plates is released When shaking causes loose snow to avalanche downslope When soft sediments behave like a liquid due to ground tremors Q3 Tick the 2 factors out of each trio that will be most influential in the following processes A Avalanches Depth of snow Strength of earthquake Rock type B Landslides Angle of slope Altitude of mountain Strength of earthquake C Liquefaction Water content Distance inland Depth of soft sediments D Earthquake intensity Depth of focus Population density Distance to epicentre E Tsunami Proximity to coast Depth of focus Strength of earthquake
Hazards: The nature of seismicity forms and causes 3.1.5.4 Q4 How would seismic processes be different along the Pacific Basin if these variables were changed? Strength of earthquakes Damage and death toll from earthquakes If the plates were moving apart If all the countries surrounding the Pacific were wealthy Height of tsunami waves The Pacific Basin landscape If the area was on a conservative plate If the plates were moving apart margin Q5 Compare and contrast the following characteristics of seismic processes on constructive and destructive plate margins. Magnitude of earthquakes: Frequency of seismic events: Probability tsunamis: of
Hazards: The nature of seismicity forms and causes 3.1.5.4 ANSWERS Q1 Match the terms with their process description A When the weight of glaciers is lifted and land subsequently uplifts isostatic recoil B When soft sediments behave like a liquid due to ground tremors liquefaction C When pressure built up between two plates is released earthquake D When a denser tectonic plate is forced under a less dense one subduction E The resultant giant waves caused by submarine earthquakes tsunami F Slope failure and mass movement due to ground tremors landslides Select from: earthquake landslides subduction liquefaction isostatic recoil tsunami Q2 Tick whether these are primary or secondary seismic hazards Primary Secondary A The resultant giant waves caused by submarine earthquakes B Slope failure and landslides due to ground tremors C When pressure built up between two plates is released D When shaking causes loose snow to avalanche downslope E When soft sediments behave like a liquid due to ground tremors Q3 Tick the 2 factors out of each trio that will be most influential in the following processes A Avalanches Depth of snow Strength of Rock type earthquake B Landslides Angle of slope Altitude of mountain Strength of earthquake C Liquefaction Water content Distance inland Depth of soft sediments D Earthquake intensity Depth of focus Population density Distance to epicentre E Tsunami Proximity to coast Depth of focus Strength of earthquake
Hazards: The nature of seismicity forms and causes 3.1.5.4 ANSWERS Q4 How would seismic processes be different along the Pacific Basin if these variables were changed? Strength of earthquakes Damage and death toll from earthquakes If the plates were moving apart If all the countries surrounding the Pacific were wealthy Earthquakes would have a lower magnitude as the plates would have tensional rather than compressional forces acting on them. The people living in the coastal zone would be better prepared and protected in the event of an earthquake. Less people would die. Height of tsunami waves If the area was on a conservative plate margin When plates move past each other they are less likely to generate tsunami waves as the earthquake will not occur directly under the ocean. The Pacific Basin landscape If the plates were moving apart If the plates were moving apart, magma would be basaltic and compressional forces would not be at work therefore large fold mountains would not form. Volcanoes would be shield-shaped rather than composite cones, so the land height would be much lower. Q5 Compare and contrast the following characteristics of seismic processes on constructive and destructive plate margins. Magnitude of earthquakes: The magnitude of earthquakes tends to be lower on constructive plate margins due to tensional forces and they tend to be higher on destructive plate margins where the plates experience stronger compressional forces. Frequency of seismic events: The frequency of seismic events is primarily related to the speed of plate travel rather than the type of plate margin where they occur. Greater movement is experienced where plates are moving in opposite directions. Constructive plate margins tend to experience more smaller frequency earthquakes than at destructive plate margins where greater pressure builds up to cause less frequent, but stronger earthquakes. Probability of tsunamis: Tsunamis are more likely to occur at destructive plate margins where denser oceanic plates are subducted under other plates and powerful earthquakes occur. At constructive plate margins, earthquakes are less powerful and rarely cause the uplift of water required to trigger a tsunami.