The Basics Seismology - The Study Of Seismic Waves and Earthquakes
Geoscience Weekly: Exploring Earth's Phenomena VI
This weekly newsletter delves into a different captivating topic from the world of geoscience, exploring Earth’s fascinating phenomena in bite-sized, easy-to-understand segments each week.
Monday 18th of March, Seismology is simply the study of seismic waves as mentioned in the title. In this newsletter we will dive into the world of seismology looking into detail at propagation of elastic waves through the Earth in the form of Earthquakes. Seismology includes studies of earthquake environmental effects such as tsunamis alongside other seismic sources such as glacial, fluvial and tectonic activities. Firstly, it’s important to understand that a recording of Earth motion is created by a seismograph called a seismogram which measure, amplify and record the motion of the ground and this work is usually done by seismologists. This information is often used to determine earthquake locations. Seismic waves are elastic waves and we will get into a lot more detail about what exactly a seismic wave is alongside the types of waves which are important. We will not be going into detail about paleoseismology which is a related field that uses geology to work out information about past earthquakes. So let’s get into some basics.
The Basics and Elastic Waves
A wave is a disturbance produced in a medium that carries energy without a net movement of particles. All waves are oscillating. In physics, a medium is defined as a substance that transfers energy from one place to another place in which the medium can transfer any form of energy such as sound waves for example — air is the medium of sound waves or the Earth is the medium of seismic waves. Seismic waves are elastic waves meaning in any medium an elastic disturbance propagates. Elastic wave propagation is when an elastic wave travels through a medium without any permanent structural or physical changes however the properties of elastic waves are influenced by the elastic properties of the propagation medium. The elastic wave is produced in a medium called a disturbance and due to the inertia of the medium, the disturbance in the medium transfers energy to neighbouring particles allowing the disturbance to spread until it comes back to equilibrium (a state in which opposing forces or influences are balanced) due to the elasticity of the medium.
Elastic waves can be divided into two types: transversal and longitudinal. The difference between these two waves is the direction of propagation however both types of waves need a medium to propagate. Transversal waves include vibrations at right angles to direction of travel. An example of a transversal wave is a seismic S-wave which we will get into more detail later. On the diagram below, the amplitude is the measurement of the distance between a peak (crest) which is the top of the wave and the position of the medium at rest (equilibrium position). The wave length is the distance between two adjacent points on a wave e.g. the distance between two successive crests. The bottom of a wave is known as a trough. Longitudinal waves include vibrations which are parallel to the direction of wave travel. Longitudinal waves have areas of compression and rarefaction which correspond to the crests and troughs of the wave. The areas of compression on the diagram below are the lines close together whereas as the areas of rarefaction are the lines with larger gaps between them. The length between two points of compression/rarefaction is the wavelength. The period of a wave is the time it takes for two successive waves to pass a fixed point or the motion of the wave to complete one cycle. An example of a longitudinal wave is a seismic P-wave which we will look at next.
P/S-waves
Body waves are waves that travel through the interior of the Earth. The two types are pressure/primary waves known as P-waves and shear/secondary waves known as S-waves. As mentioned previously, P-waves are longitudinal waves and they’re the fastest moving waves through solids. S-waves are transverse waves which are slower than P-waves meaning on a seismogram they appear later than P-waves. When these P and S-waves interact on the surface of the Earth they create surface waves which are dispersive (waves with different frequencies have different velocities). The two types of surface waves are Rayleigh waves and Love waves. Rayleigh waves have compressional and shear motions. The Rayleigh wave rolls along the ground appearing like waves on an ocean however the particle motion is opposite to waves on an ocean and because Rayleigh waves roll the ground, the ground is moved up and down, forward and backwards in the direction of propagation. Earthquake shaking is due to Rayleigh waves.
Love waves are the fastest surface waves which cause a horizontal shifting of the earth, moving the ground from side to side in a horizontal motion. Love waves can be difficult to understand but if you imagine somebody doing a ‘Mexican wave’, the principles are similar. There are variations in Love waves which are beyond the scope of this newsletter known as polarised and unpolarised which you can read more about in wave physics.
Earthquakes
Earthquakes occur at plate margins called the focus or hypocentre. This is due to large plates of crust moving and as they move they stick together which causes friction and pressure to rise. When the plates slip there is a sudden release of energy causing the seismic waves to make the ground shake. At ground surface, the epicentre is the right above the focus of the earthquake. Seismic waves also can be caused by geological processes like the Menominee Crack where magma moves towards the surface producing seismic waves. Often earthquakes occur along faults and there are many types such as Normal faults, Reverse faults and Strike-slip faults each with their own unique properties defined by their motion.
Normal faults exhibit fractures where one block of rock is sliding downward and away from another block of rock. These faults typically form in regions where the Earth’s crust is gradually stretching or where two tectonic plates are diverging.
Reverse faults originate where the Earth’s crust experiences compression. They arise in areas where the crust is folding upward due to pressure from another tectonic plate. In these faults, one block of rock moves beneath another block, or one block is pushed upward over the other.
Strike-slip faults are situated between two sections of the Earth’s crust that slide horizontally past each other. They are prevalent in regions such as California, where the Pacific Plate moves north-westward in relation to the North American Plate. In a pure strike-slip fault, there is no vertical motion along the fault line.
The types of faults correspond to the types of plate margins in which the Earth’s crust is made up of several tectonic plates divided into oceanic and continental plates. These plates move on the mantle known as continental drift and the three types of plate margins are constructive/convergent (where the plates are diverging), destructive/divergent (where the plates are converging) and transform (where the plates slide past each other). Most earthquakes occur along plate margins for example the pacific ring of fire where 90% of all earthquakes occur along with 75% of all active volcanoes. With a rough circular ring shape stretching 25,000 miles it inhibits over 1,500 earthquakes yearly. Earthquakes can also cause tsunamis causing a sudden uplift or fall causing a vertical displacement in the ocean floors causing the water to rise. This rise is often small however its going at high velocity and gains energy as it moves towards the shore gradually increasing in size. Most tsunamis are generated by earthquakes with a magnitude of over 7.0 and a shallow focus often less than 100 km below the ground.
Conclusion
Seismology is a fascinating topic which is very important in understanding the complex dynamics of the Earth and helps in detecting and locating earthquake epicentres which enable governments and aid institutions to prepare and defend against the potential devastating effects of earthquakes. In summary it’s important to remember that a wave is an energy-carrying disturbance in a medium without displacing particles. Seismic waves, which are elastic, propagate through a medium without causing permanent changes, influenced by the medium’s elasticity. The disturbance transfers energy until equilibrium is reached and now you know all about seismic waves.
Thank You
Thank you for reading, all information is taken from reputable sources and are linked below. All images are free to use under copyright laws.
Michigan Tech ‘ National Oceanic and Atmospheric Administration ‘ Study Smarter ‘ University of Southampton ‘ CoastalWiki