howdy everyone and thank you for continuing on with module 4 in this video we're going to explore how scientists can measure earthquakes there are two fundamentally different measurements that are used to describe the size of an earthquake intensity scales use observed property damage to estimate the amount of ground shaking and magnitude scales use data from seismographs which we explored in the last video to estimate the amount of energy released remember earthquake magnitude measures the energy released where earthquake intensity measures the degree of damage to measure intensity scientists use the modified Mali intensity scale which was developed using California buildings as its standard it was developed in 1902 by GSE Meri it is based on damage and what people actually feel the values change based on the distance from the epicenter and it's useful for studying historic earthquakes that happened when there were no seismoggrams present but it is also useful for talking about the earthquake damage because that doesn't always increase with an increase in magnitude believe it or not this scale goes from Roman numeral 1 which we call instrumental where it is not felt except by a very few under especially favorable circumstances all the way up to Roman numeral 12 which we call catastrophic and results in total destruction of everything being destroyed waves are actually seen on ground surfaces with level 12 and objects are thrown upwards into the air here is a seismic intensity map from the USGS after a magnitude 5.8 earthquake in central Virginia you can see the map on the right shows varying colors with yellows indicating strong shaking and light damage closer to the epicenter with some greens representing moderate shaking to very light damage as you move farther away and then blues representing weak to light shaking with no damage all right so how to determine the magnitude or the actual size of the earthquake well there are two scales one called the RTOR scale and another more accurate one we will get into next called the moment magnitude scale let's start with the RTOR scale which was introduced by Charles RTOR in 1935 it calculated the magnitude by measuring the amplitude of the largest seismic wave recorded on a seismog it's a logarithmic scale that accounts for the decrease in wave amplitude with an increase in distance so this means each unit on the scale represents a tfold difference in wave amplitude and a 32fold difference in energy released what does that mean exactly it means that each step on the scale represents 32 times more energy released so that means a 6.0 magnitude earthquake has 32 times more energy released than a 5.0 magnitude earthquake and a 7.0 earthquake has 1,024 times more energy released than a 5.0 magnitude earthquake because that is two steps up so 32 * 32 is 1,024 so you can only imagine the energy released in a large magnitude earthquake so I mentioned seismologists study seismoggrams and we use them to determine the location of epicenters but we also use them to calculate magnitude on the rter scale you don't really need to memorize these steps but how we determine the magnitude is we first measure the amplitude an amplitude is the height of the largest wave on the seismog for example here is 23 mm high then we determine the distance to the earthquake using that time travel graph we used before when trying to find the epicenter the time interval here is 24 seconds which corresponds to about 210 km then we connect the two plots that measure amplitude and distance with the rter scale in the center where the line intersects the rter scale that's our magnitude so in this case we have a magnitude 5.0 it's pretty cool stuff in my opinion again you will not be asked to do this so please please do not panic if you're confused and no you don't need to memorize those steps i just thought it was really cool to share now this is a really cool chart that shows those differences in ground motion and energy release between the differences in magnitude by changing magnitude by only 0.1 we see an an increase in 1.3 times more ground motion and 1.4 four times more energy released that's just 0.1 difference in magnitude one full step on the magnitude scale remember is 10 times the difference in ground motion and 32 times um the difference in energy released and a difference of 2.0 is 100 times more ground motion and over a,000 times more energy released remember 32 * 32 a 3.0 0 difference is 1,000 times more ground motion and 32,000 times more energy released that's 32 * 32 * 32 then a difference between a 4.0 is 10,000 times more ground motion and 1 million more times more energy released insane okay so today we actually use a more accurate scale that takes the amount of slippage on a fault and the strength of the faulted rock into consideration when calculating the total energy released and we call this scale the moment magnitude scale here is our moment magnitude scale i have it shown here going from 1 to 10 but technically it has no end the largest earthquake ever recorded was a 9.5 in Chile in 1960 this is a really nice figure because it shows how the magnitude corresponds with frequency a description of their usual impacts some notable examples and the amount of energy release so take a moment to take a look at this chart in your notes okay let's now talk about where we see earthquakes occurring on Earth in relation to plate tectonics and those not related to plate tectonics i'll see you there