Earthquakes

Occur when built-up tension, compression or shear forces in rocks are suddenly release when the rocks break and move. The ground shakes as a series of seismic shock waves spread out form the earth move through it.
Causes: the release of seismic waves happens when rock that is under strain breaks. The point underground where the breakage occurs is call the focus of the earthquake.
Earthquake and volcanoes tend to occur at plate boundaries. As tectonic plates move relative to each other, the energy is stored in the rock is released as earthquakes.
Earthquakes can also be caused by magma moving through the crust.
Earthquake locations generally cluster around lithospheric plate boundaries. Differences in size, magnitude, frequency and depth of foci of earthquakes relate to geographical location and plate movement.

Seismic Waves/ Earthquake Waves

There are two fundamental types of seismic waves; surface waves (Love and Raleigh), which travel along the outer plane through complex particle motions and arrive last at distant recording; and body waves, which travel through the interior of the Earth. Which are further divided into P-waves and S-waves. Huge amounts of stored up elastic potential energy is released as a variety of wave types (body waves).
- - P-waves are the fastest seismic waves and are the first to be recorded by seismographs during an earthquake. They are also known as compressional waves or primary waves. P-waves travel through solid, liquid, and gaseous materials. These waves cause particles in the Earth's crust to move back and forth in the same direction that the wave is traveling, similar to the way sound waves travel through air. P-waves can travel through both solids and liquids, and they can travel through the Earth's interior.
  - S-waves: are the second type of seismic waves to be recorded by seismographs. They are also known as shear waves or secondary waves. S-waves travel more slowly than P-waves and can only move through solid materials. These are transverse shear waves, which means they move like a rope that is fixed at one end and shaken either horizontally (left and right) or vertically (up and down) at the other end . Unlike P-waves, S-waves cannot travel through liquids, which is why they do not propagate through the Earth's outer core, which is molten.
  - L-waves: are a type of surface waves that travels along the Earth's surface and are the slowest among the seismic waves. Surface waves are actually a combination of two distinct wave types Rayleigh waves and Love waves.
    - - Rayleigh waves cause vertical motion of particles and produce movement similar to ocean waves.
      - Love waves produce the horizontal shaking that is most noticed by people who experience strong earthquakes.
        L-waves result from the interaction between P-waves and S-waves as they reach the Earth's surface. L-waves move in a horizontal, side-to-side motion and cause the ground to ripple in a snake-like pattern. They can cause significant damage to buildings and structures due to their large amplitudes and long wavelengths.

Elastic Rebound Theory

Is a fundamental concept in the field of seismology that helps explain the occurrence of earthquakes and the release of energy along faults. It was developed by American geologist Harry Fielding Reid in the early 1900s.
According to the Elastic Rebound Theory, the Earth's crust is not stationary but constantly undergoing slow deformation due to tectonic forces. This deformation leads to the accumulation of stress along faults, which are fractures in the Earth's crust where movement occurs. Eventually, the stress becomes too great, causing the rocks on either side of the fault to exceed their elastic limit.
When the stress exceeds the elastic limit, the rocks rupture and experience rapid movement along the fault. This sudden release of accumulated stress results in an earthquake. The rocks snap back to their original, stress-free shape, just like a stretched rubber band rebounds when released.
During an earthquake, the energy that was stored in the deformed rocks is released in the form of seismic waves. These waves radiate outward from the earthquake's focus, which is the point of initial fault rupture, and are detected by seismographs.

Methods Used to Monitor to Predict Earthquakes

Earthquakes are monitored in a number of ways that aim to measure changes in the stresses around a fault.

Seismometers detect small movements along faults, and this information allows us to determine how the fault moves.
Strain gauges measure bending within rocks.
Tilt meters measure flexing of the surface and measures of ground water levels.
Temperature provides information about changing stresses within the rocks.
When rocks are compressed their electrical conductivity changes and their resistance decreases.
Crystals within the rocks begin to fracture and release a gas called radon, which these fluctuations are monitored with Geiger counters.

There are no obvious patterns that occurs days out from earthquakes. This means that these methods warn scientists that an earthquake is due but do not allow them to know the exact time it will happen.

Rock displacement within the Earth occurs at the focus of the earthquake.
The epicenter is the point on Earth's surface directly above the focus.
Isoseismal lines join places of equal earthquake intensity.
The differential travel rates of P and S waves determines the latitude, longitude and focal depth of an earthquake, calculated using the arrival times of seismic waves at multiple seismographs around the world.
Earthquakes worldwide are reported in real time by the permanent digital network of seismological and other geophysical sensors of the Global Seismographic Network. This information is continually recording the location, depth and magnitude of earthquake and automatically sent to appropriate authorities so they may issue any necessary guidance to the population of affected regions.

Seismographs and Seismograms

Seismographs are instruments are used to measures Earth's local vibrational movements and are located on bedrock in urban areas, mining activities, military installations, or other sources of vibrational background noise.
Recordings occur through the use of suspended mass with a connected pen that remains fixed in space as a rotating drum moves up and down or back and forth with ground motions (figure A).
These recordings of passing earthquakes waves made on the rotating drum is called a seismogram.
The vertical axis records show wave amplitude as a function of arrival time (figure B below).
Seismographs have three separate components to digitally measure tiny displacements in X, Y and Z directions.
P-waves arrive first followed by S-waves, and then surface waves which have the largest amplitude and cause the most damage.

How Big is an Earthquake

Two details of an earthquake's severity are required to compare events. Magnitude or the amount of energy released at its point of origin and the earthquakes' intensity.
Earthquakes with a high magnitude may not be felt if they have a very deep focus or occur under oceans or in other remote locations.

Earthquakes Australia

Earthquakes are common though in Australia. There is an estimated that there are more than 100 earthquakes each year with a magnitude above 3, which is the lower limit that humans can sense. While it is true that Australia is located in the middle of the Australian tectonic plate, earthquakes can still occur in the region. There are a few reasons for this:

Intraplate Earthquakes: Although most earthquakes occur at plate boundaries, where tectonic plates interact, there can still be seismic activity within the interior of a plate. These are called intraplate earthquakes. In the case of Australia, there are old and relatively inactive fault lines within the continent that can occasionally experience seismic activity.
Plate Boundary Effects: While Australia is not located at a plate boundary, its proximity to active plate boundaries can still influence its seismic activity. For example, the northern part of Australia is close to the boundary between the Australian plate and the Eurasian plate, which is a region of ongoing tectonic activity. This can result in some seismic activity spilling over into the Australian continent.
Stress Buildup and Release: Even in the absence of direct plate boundary interactions, stress can accumulate within the Earth's crust over time. This stress can be released as earthquakes, even in areas located within a tectonic plate. Geological processes, such as the slow deformation and stretching of the Earth's crust, can lead to the build-up of stress that eventually causes earthquakes.
Incomplete Plate Boundaries: Plate boundaries are not always well-defined or confined to a single line. There can be diffuse boundaries where the transition between two plates is more gradual. In such cases, seismic activity may occur over a wider area, including within the interior of a plate.

It's important to note that earthquakes in Australia are generally less frequent and less intense compared to regions near active plate boundaries. However, the presence of earthquakes in seemingly stable regions like Australia serves as a reminder that the Earth's crust is dynamic, and seismic activity can occur even away from plate boundaries.

Click the investigation below to go to the intergrated webpage

Secondary Effects of Earthquakes

Ground movement: on the land surface both vertical and horizontal crustal displacement generally follows pre-existing fractures. Ground movement extent depends on a range of factors including seismic energy released, distance from the focus, duration and number of aftershocks, and charateristics of the substrate below the location and through which waves travel. Solid bedrock beneath a thin layer of soil behaves differently than a thick pile of loosely consolidated sediments.
Liquification: the propagation of waves through loose, saturated silty and sandy soils shakes them and causes a collapse of their granular structure, which throws the load from a structure directly on the incompressible water between the particles. The soil then has no shear strength and is unable to support the structure, which sinks into it. The structure is sitting on a dense fluid and promptly settles, usually unevenly.
- - Can only occur in waterlogged soils.
Landslides: when earthquakes hit steeply sloping terrain the soil and bedrock move and can collapse under the pull of gravity.
- - The main reason landslide is caused by the pressure of ground water in the slope increasing due to percolation by rain, causing the water table to rise to a level at which the contact stress between the soil particles is reduced sufficiently by bring the shear strength of the soil below what can support the slope. With the continual lateral stress is enough to overcome the remaining soil strength and cause a slip.
  - The potential for landslides is much greater if a slope has been destablised by deforestation or high rainfall. Roots act as a binding agent but with high rainfall they lose their rigidity and behave like a fluid.
Tsunamis: occur when displacement of the sea floor (uplift and subsidence) displaces the water above it, forming large waves that propagate from the source of the ground shake.
- - The damage caused by tsunami is due to the ocean rising the flooding across the land. Any lose material can be pushed along by the water and act as battering rams on structures.
  - When a plate edge releases and springs back, the water is uplifted, causing a wave that can reach several stories high.
  - They can travel at speeds up to 800 Km h-1 = as fast as a jet plane.
Fires: electrical wires and gas mains ruptured by seismic waves feed flames already lit when the quake hits. The blocking of many roads combined with ruptured water mains mean that fire departments are unable to combat the many small fires which soon grow into big fires.
Floods: excessive ground shaking can damage dams, reservoirs and levees. The flow of water cannot be halted and resulting in flooding of urban areas damages structures and endangers life. Delayed flooding may occur when natural or artificial dams are damaged by an earthquake and fail sometime after the event.
Diseases: in the aftermath of an earthquake, diseases can spread among survivors due to lack of clean water, sanitation and medical facilities. Diarrheal diseases such as cholera are a threat in particular and are the cause of 40% fatalities of fatalities in refugee camps. This is spread via contamination of drinking water with fecal matter. This is a problem after any natural disaster if the area affected had poor nutrition and/or sanitation standards beforehand.
Economic Effects: in the modern world, the economies of most countries are connected globally to the economies of their trading partners. Share markets reinforce these economic connections. This means that if on country is heavily impacted by a natural hazard, the economic impacts are likely to spread to other countries that were not physically affected.
- - The economic impacts or earthquakes can be enormous:
    - - Loss of infrastructure.
      - Destroy tourist areas and the money that they bring, and they may be to be scared to come.
      - Earthquakes feature prominently on any list of the most expensive natural disasters e.g., Kobe, Japan 1995, was an industrial center and the rebuild cost $150 Billion US dollars.

Most at Risk

The three main factors that greatly increase the potential rick posed by all tectonic hazards are the 3 Ps;

Proximity: to a tectonic boundary, places you in the impact zone of serious effects.
Poverty: equals poor building quality and a lack of infrastructure to warn of, or deal with such events.
Population: density paces you much more at risk from secondary effects of these events.

The combination of these factors of these factors further magnifies the potential risk associated with tectonic hazards. To illustrate point, of the top seven natural disasters in terms of fatalities, four were China, two were in India and the other was Bangladesh - all countries with enormous population densities and widespread poverty.

Mapping Potentially Hazardous Zones for Earthquakes for town planning

Previous frequency data is mapped.
Previous magnitude data is mapped.
Areas of particular concern (plate boundaries, hotspots) are located.
Particularly those with less competent rock or in highly populated areas.

Monitoring known faults also helps us to build a risk profile. Faults that are experiencing regular small earthquakes are having the interna strain released regularly. This makes it less likely to produce a large earthquake than a fault that has been locked in place for years, accumulating strain and potential energy.

Japan has invested in earthquake early warning systemms for its major cities. Given its wealth, small size and densely populated urban areas, the government felt that the costs of this system would be returned as saved lives.

Japan's Response to Earthquakes and Tsunamis

Japan is a country that is highly prone to earthquakes due to its location along several tectonic plate boundaries. Over the years, Japan has developed extensive measures to prepare for future earthquake events. Here are some key aspects of Japan's earthquake preparedness:

Building Codes and Regulations: Japan has stringent building codes and regulations in place to ensure that structures can withstand seismic activity. These codes dictate design and construction standards, including the use of advanced engineering techniques, flexible building materials, Structural reinforcement (horizontal and vertical) and seismic isolation systems. Building codes are regularly updated based on the latest scientific knowledge and lessons learned from past earthquakes.
Earthquake-resistant Infrastructure: Japan invests heavily in constructing earthquake-resistant infrastructure, including bridges, tunnels, highways, and railways. These structures are designed to withstand strong seismic forces and minimize damage during an earthquake. Advanced engineering techniques, such as rollers, base isolation and damping systems, are employed to absorb and dissipate seismic energy. Stabilisation of particularly hazardous areas (like road cuttings with friable rocks).
Early Warning Systems: Japan has one of the most advanced earthquake early warning systems in the world. The Japan Meteorological Agency (JMA) operates a network of seismometers that detect earthquake waves and issue warnings to the public before the arrival of strong shaking. These warnings provide valuable seconds to minutes of advance notice, allowing people to take immediate protective actions and automated systems to activate safety measures. A 2-day warning is needed for an orderly evacuation but is only possible for volcanic eruptions. Short-term earthquake warnings can still be very helpful. A warning of up to 30 seconds allows trains to slow, airplanes to stop taxiing, lifts to open and people to move away from dangerous machinery or take cover under a desk. Utilities can be isolated to reduce the number of fires and emergency vehicles can be moved out of their garages.
Public Education and Awareness: Japan places great emphasis on public education and awareness about earthquake preparedness. From a young age, Japanese students are taught about earthquakes, safety procedures, and evacuation drills. The government and various organisations conduct regular campaigns to educate the public on earthquake risks, preparedness measures, and self-help techniques. Information is disseminated through brochures, websites, public service announcements, and community events. Disaster training is not limited to schools in Japan. Since 1960, Japan has held Disaster Prevention Day on 1st September each year.
Emergency Response and Training: Japan has well-established emergency response systems to handle earthquake disasters. Local governments, emergency services, and volunteer organisations collaborate in disaster response efforts. Regular drills and training exercises are conducted to enhance coordination, communication, and response capabilities. These exercises involve simulations of large-scale earthquakes to test the effectiveness of emergency plans and improve the readiness of response teams.
Tsunami Countermeasures: Given Japan's vulnerability to tsunamis triggered by undersea earthquakes, extensive efforts are made to develop and maintain tsunami countermeasures. These include the construction of coastal defense structures, such as seawalls and breakwaters. Evacuation routes, tsunami evacuation maps, and education campaigns are also in place to ensure the public knows how to respond when a tsunami warning is issued.
Research and Technology Development: Japan invests in research and technology development to advance earthquake prediction, early warning systems, and structural engineering. Collaborations between government agencies, academic institutions, and private organizations aim to improve scientific understanding of earthquakes and develop innovative solutions for mitigation and response. Continuous research and development efforts contribute to the ongoing improvement of earthquake preparedness measures.

It's important to note that while Japan has made significant progress in earthquake preparedness, earthquakes can still cause damage and loss of life. The country's comprehensive approach to preparedness serves as a model for other earthquake-prone regions around the world.

Page updated

Report abuse