Good afternoon! Today I thought I would talk about some basic seismological terms. Many of these terms are freely used by seismologists or in the news, but most are poorly understood by the public - very little of this is taught in schools 1/n
When people talk about an earthquake, they talk about a sudden movement of rock beneath the surface of the Earth. This movement can reach the surface, but starts beneath. It is always the result of an outside force, such as pressure from water in rock or two plates colliding 2/n
Earthquakes can be both natural or induced. Natural earthquakes are those caused by tectonic stresses - the rock beneath our feet being contorted, stretched or compressed - as well as by processes related to volcanoes - such as fluid movement, be that water, steam or magma 3/n
Induced earthquakes are produced in part or whole by man-made activities. These include mine collapses, or events associated with the extraction (water or petroleum) or the pumping of fluids into the Earth (geothermal power, hydraulic fracturing, wastewater injection) 4/n
Natural earthquakes produce the largest events - the largest recorded quake was a magnitude 9.5 in Chile in 1960 - but induced earthquakes can also be large, with the 2016 Pawnee Earthquake in Oklahoma, USA, having a magnitude of 5.8. Almost all quakes tend to be harmless 5/n
Earthquakes are the result of sudden movement on weaknesses within the Earth's interior (the lithosphere, which is composed of the crust & the uppermost mantle). These weaknesses are called faults, and they are the result of stresses within the Earth's lithosphere 6/n
Plate tectonics - the movement of large slabs or jigsaw pieces of the Earth's lithosphere across the surface of the planet - is the primary driver of all stresses in lithosphere. Other sources include fluid movement - groundwater for example - and human activities like mining 7/n
As stress is increased by the motion of the plates, faults creak under the pressure exerted upon them. When the fault can take no more stress it relaxes in a sudden motion - this is an earthquake. Imagine stretching a slinky to the furthest you can reach, then release it 8/n
An earthquake generates waves - with a slinky those waves are sound waves detected as a noise in our ears. In an earthquake those waves transmit through the rock beneath our feet, and if the waves have enough energy left they produce a noticeable shaking at the surface 9/n
To pop back a couple of steps: plates are thought to move like a conveyor belt due to convection currents. These are the result of differences in temperature causing motion up & down within the semi-liquid mantle the word rheid describes this state): 10/n
Back to the waves: two classes of waves are produced. The fastest are the body-waves: The P-waves (compressional wave) compresses the ground back & forth in the direction of travel, the S-waves (secondary wave) oscillate the ground at 90 degrees to direction of travel 11/n
The surface waves are the slowest waves: Rayleigh waves ripple the surface of the Earth in the direction of travel (like rippling a carpet), whereas Love waves move the ground in ripples both parallel and perpendicular to the direction of travel of the waves 12/n
If you wish to see a visualisation of these four wave types & two classes of waves, I highly recommend these animations produced by @IRIS_EPO: iris.edu/hq/inclass/ani… 13/n
These waves do not originate from a single point. When a fault ruptures in an earthquake, it is an area between two blocks of rock which slide past one another - like all the hoops in our slinky move a volume of the air to produce the sound we hear 14/n
These faults can be linear cracks in the ground or they can be curved, but they all move in different ways depending on whether the rock is being stretched, compressed or contorted by the large tectonic stresses produced by the plates moving 15/n
If a volume of rock is being stretched or compressed (pushing my slinky in or stretching it out), the fault produced is called a dip-slip fault. These faults are inclined - they dip - and because they produce earthquakes they slip (geologists can be very unimaginative!) 16/n
A dip-slip fault has a block beneath the fault and a block above it. The block beneath is known as the footwall, whereas the block above it is known as a hanging wall (I believe that these terms got borrowed from mining terminology) 17/n 
If the rock is being stretched then the hanging wall is pushed down relative to the footwall. This creates space above the hanging wall, thus stretching the surface of the Earth. This is called a normal fault! These occur at mid ocean ridges & in volcanic areas 18/n
If the rock is being compressed then the hanging is pushed up and over the footwall. This reduces space at the surface of the Earth. This is called a reverse fault (further lack of imagination in terminology!). These create mountain ranges and occur where plates collide 19/n
If the stress exerted on the rock shears it (pushing two pieces of wood past one another) then the fault type is called strike-slip - the alignment of a feature in geology is known as it's strike. The most famous example is the San Andreas Fault in California 20/n
If there is a combination of these stress exerted on a rock - the rock is being contorted - the fault may have a components of both dip-slip and strike-slip motion. These are called oblique faults and are very commonplace in complex tectonic environments 21/n
The images above are sourced via @IRIS_EPO from @AKearthquake - animations of these faults undergoing slip can also be found on this awesome IRIS webpage: iris.washington.edu/gifs/animation… 22/n
There are only two more terms to introduce. These are the most commonly misused - epicentre and focus/hypocentre. Where an earthquake begins at depth is called the focus (or hypocentre in the US). The point on the surface of the Earth directly above is called the epicentre 23/n
I'll be back in a short while to explain something a bit more technical & instrumental to what I do day-in, day-out to do with earthquakes. I'll leave with two loosely-linked pictures - I will be very impressed if anybody will figure out the link! 😃 24/24
