Good morning from a UK with slightly less dull weather! I want to discuss two topics which are often confused & misunderstood, but first I wanted to clarify a few things from yesterday's threads #earthquake #Stress #faults 1/n
Referring back to the block diagram for dip-slip faults from yesterday, @CriticalStress_ noted that the stress arrows at the top were wrong. All faults have a confining (inward) compressional stress, but stresses from other directions can be larger 2/n
This diagram from @SEELeeds shows that in the example block diagram I gave the smallest stress (sigma-3, min) is inward perpendicular to the fault. The primary stress - the largest amount of force acting on the rock to produce a normal fault - is vertical: GRAVITY! 3/n 
This diagram also helps explain another dimension I forgot to explain. Normal faults have an inclination (dip) of 45-60 degrees; reverse faults tend to be shallow & if the dip is below 45 degrees are called thrust faults; and strike-slip faults are vertical (90 degrees dip) 4/n
An update on global seismicity from overnight is very simple as not a great deal happened. Nonetheless there have been some earthquakes: a small Hindu Kush quake is a common occurrence, a less usual quake in Germany & aftershocks to recent quakes in the western Pacific 5/n
On this day (17 November) in the past 5 years (2013-2017) there have been four quakes larger than magnitude 6.0. The least interesting were a M6.1 southeast of Africa (Andrew Bain Fracture Zone) in 2014 & a M6.4 in Tibet, north of Assam province, in 2017 6/n
Of greater interest are the quakes in 2013 & 2015. In 2013 a massive magnitude 7.7 earthquake struck near the South Orkney Islands, between South America & Antarctica. This quake was the result of the rupture of a ~280 kilometre long strike-slip fault: earthquake.usgs.gov/earthquakes/ev… 7/n
A damaging magnitude 6.5 quake occurred near the island of Lefkada in western Greece #OTD in 2015. This strike-slip quake killed 2 people, damaged 120 buildings & destroyed 42, as well as triggering landslides and rockfalls: earthquake.usgs.gov/earthquakes/ev… 8/n
By mentioning these earthquakes I have introduced MAGNITUDE as a term used liberally by seismologists. You may have heard of quakes being given a magnitude when reported on the evening news bulletin, the radio or in your newspaper. But what does magnitude mean? 9/n
Magnitude is a term used to quantify the size of an earthquake. Put simply the smaller a magnitude the smaller the earthquake was & the less impact a quake will in theory have (there are a many caveats to this - I will explain those later on when I introduce the second term) 10/n
The magnitude is calculated using the recordings of an earthquake made on an instrument called a seismograph (or seismometer). This instrument records the shaking of a quake, using a pendulum or spring which moves as seismic waves arrive to record the motion of the ground 11/n
The first magnitude system was developed in the 1930s by a man called Charles Richter. Today journalists often refer to the magnitude of an earthquake as a Richter magnitude - though this is often incorrect, all of the magnitude systems developed are based on his original 12/n
Richter decided that the smallest earthquakes the seismographs of the day could detect would be a magnitude 0. The magnitude was linked to the maximum amplitude of the seismic signal recorded, and each time the amplitude became 10 times larger, the magnitude increased by 1 13/n
Therefore the magnitude system is a logarithmic scale - every increase of 1 magnitude equals an increase in the signal's amplitude by 10. If a signal is 1000 times larger than that from a magnitude 2.0 earthquake, it is magnitude 5.0; if 10 times larger it is M3.0 & so on... 14/n
Richter realised that his magnitude system had no real correlation to the physical size of the earthquake - it only related to the "size" of the seismic signals detected, not the amount of energy released or dimensions of the fault responsible for the earthquake 15/n
There were other problems with Richter's local magnitude (ML=local magnitude). The instruments used for his system could not easily record quakes which occurred large distances away - only the long-period, surface waves (Rayleigh & Love waves) would be recorded by them 16/n
At closer distances the maximum amplitude would be found in the recordable body-waves (P- & S-waves), so the lack of these waves at large distances meant that the magnitudes calculated there would be highly inaccurate. A method to compensate for this needed to be devised 17/n
Two of these were indeed devised. The first used the largest amplitude in a period of surface-waves - this was called the surface-wave magnitude (Ms) & is rarely used today. The second used the largest amplitude in a period of body-waves - the body-wave magnitude (Mb) 18/n
These magnitude systems were more stable as they allowed for accurate magnitude calculation over large distances on the Earth's surface - a station in the UK could now have a better magnitude estimate for a quake in California. However, for large quakes this did not work 19/n
Three major quakes in the 20th century were the 1906 San Francisco Earthquake, the 1960 Valdivia (Chile) Earthquake & the 1964 Great Alaskan Earthquake. These all had different sized energy releases, but the surface-wave magnitude made them all equal as Ms 8.3 20/n
This problem of differentiating the size of large quakes is known as magnitude saturation - the size of a quake cannot be differentiated by the amplitude of a particular seismic wave. This occurs at about Mb = 6.0 and at Ms = 7.3 respectively 21/n
The result was the devising of a more accurate magnitude system which better represented the physical size and energy release involved during an earthquake. This magnitude system - what is commonly referred to in the news wrongly as Richter magnitude - is moment magnitude 22/n
Moment magnitude is based on the principle of a moment. A moment measures how much energy is needed to push an object which pivots on a point - the amount of force (N) exerted multiplied by the distance (m) from the point you push to the pivot point. This is measured in Nm 23/n
N = newtons; m = metres, so the unit of moment is Newton-metres. Imagine two people rotating a table. If they push towards each other from opposite sides the table will not move; if they push away from one another from opposite sides the table will rotate 24/n
The principle for a quake is similar as opposing forces produce an earthquake ((). In this case, rather than force & distance, the three components of seismic moment are the ability of the rock to move, the average slip on the fault & the fault's area 25/n
This idea was first used for an earthquake in 1966, but a magnitude system (moment magnitude - Mw) utilising this theory was first devised by Hiroo Kanamori & Thomas Hanks (NOT the actor!) in 1979. Ever since this magnitude has become the preferred magnitude system 26/n
This system works very well for the three 20th century earthquakes; rather than all being M8.3, the 1906 San Francisco quake had a moment magnitude (Mw) of 7.7; 1960 Valdivia was Mw 9.5 - the largest quake ever recorded - and 1964 Alaska was Mw 9.2 27/n
These different magnitude systems are often calculated when an earthquake occurs. For moderate or large (>M4.5) quakes body-wave magnitude (Mb) is often first calculated as body-waves are the first to arrive & thus easy to measure for an initial magnitude estimate 28/n
As more seismograph stations detect the earthquake the accuracy of the body-wave magnitude increases. In the same time the waves required to calculate a moment magnitude have also arrived, and as more data comes in the moment magnitude is calculated to greater accuracy 29/n
This is why an initially recorded magnitude is reported by the news, but in reality is out-of-date & therefore wrong as this was a very early estimate of the magnitude. A great explanation of this process by @IRIS_EPO can be found here: 30/n
I hope this explains what a magnitude is & also explains why it is confusing for people - there is not just one type of magnitude but several, one of which (surface-wave) is rarely used & another (original Richter local magnitude) is no longer used at all 31/31
