字幕表 動画を再生する 英語字幕をプリント Earthquakes strike all over the world, and once they're over a certain threshold, they become BIG news. But what do those numbers they tell you on the news really mean? Nepal was recently hit by a 7.3 on the Richter Scale, less than 3 weeks after being hit by a 7.8. The 7.8 destroyed buildings, killed thousands and caused an avalanche; the 7.3 caused a panic, and killed only a few dozen. Does Zero Point Five on the scale make that big a difference? Firstly, let me step back for a bit, when you describe a great dessert to a friend, you probably give it a rating. We humans love to rate things -- but to truly rate it fairly, you'd have to understand the ingredients, the preparation, the plating, and the ideal dessert comparison, what you've just eaten prior, what you're drinking, where you're sitting, the temperature in the room you're in and your mood at the time; not to mention whether you had a fight with your friend earlier that day!! Science needs history, context, and tons of points of data, and that's just for a piece of cake! How can you take all that data and turn it into one simple rating? Most people don't. They measure how they FEEL about it, right? Like a layer cake, earthquakes are all different. When rocks slide, knock and buffet against one another; they can slip, crack or jerk and cause an earthquake, but the friction, the fault, the pressure, depth, type of rock and varied stressors of its history; all matter. To turn that into one number is pretty crazy, if you think about it. The Richter scale was invented in the 1930s to give some understanding to the magnitude of a quake as measured by seismographs; ground monitoring equipment. It's logarithmic, so every 1.0 point on the scale is a TEN FOLD increase. So a 3 to a 4 is a ten-fold increase, and 3 to a 5 is 100 fold; or, put another way, a 7 is a 10,000 times more powerful than a 3. The U.S. Geological Survey says there are 1.4 million quakes that register above 2.0 every year, and only 16 that get a 7 or higher, but even after I've explained it to you, do you know what it means? I don't! Richter is only concerned with the amount of energy released by a single quake, and doesn't tell us much about what it DID. Maybe it was deep, or away from a populated area, maybe the quake was under soft ground, or in the middle of firm bedrock -- all of that would change how it was experienced; or how humans FEEL it. Mercalli Scale or Modified Mercalli Scale, measures intensity. This is used by the U.S. Geological survey to understand how the earthquake FELT on the surface! The Mercalli scale was developed in the 30s too and it uses Roman numerals to determine how intense a quake is from I to XII. It is subjective, so, for example, a II is weak and is quote "Felt only by a few persons at rest,especially on upper floors of buildings," but a VI is strong and quote "Felt by all, many are frightened. Some heavy furniture is moved; a few instances of fallen plaster. Damage slight." This measure of how it FEELS adds more data to what happened in an earthquake, moving us closer to understanding what really happened! There's also the Moment Magnitude scale, which is a new measure for quakes. Moment Magnitude allows scientists to use computer models to create synthetic seismograms and determine exactly how the energy released relates to the magnitude of a quake. Essentially how far a fault moved, and the force required to move it. It gets seismologists really excited because it's WAY more accurate than the antiquated Richter measurements, works all over the world as a standard measure, and can go above M8, which Richter cannot. Okay, so now that you know the basics… According to the Incorporated Research Institutions for Seismology, the April 25th earthquake in Nepal was a M7.8 with MMI ranging from IV to IX. Most of the reports were in V to VI -- so it felt strong, the ground moved and for some people it was severe and violent,. This complicated cocktail of factors that have to be considered help determine how earthquakes are experienced, and categorized; just like anything we can always learn more.