Measuring Earthquakes Magnitude, Intensity, Predicting

How are Earthquakes Measured? Magnitude is a measure o the amount of energy released at the source of the earthquake. Determined from measurements on seismographs. Intensity measures the strength of shaking produced by the earthquake at a certain location. Describes how much people felt and the damage it caused. An earthquake can have many different intensity numbers, even though it has only one magnitude.

Richter Scale measures Magnitude The Richter Scale is a standardized scale of earthquake magnitude (amount of shaking). The higher the number, the stronger the earthquake When the Richter magnitude of an earthquake goes up by one unit, the amount of ground shaking caused by the earthquake goes up 10 times. For example, an earthquake with a magnitude of 5.0 is 10 times stronger than an earthquake with a magnitude of 4.0.

Richter Scale

Modified Mercalli Intensity Scale Modified Mercalli Intensity Scale is used to compare the intensity of different earthquakes. Rather arbitrary set of definitions based upon what people in the area feel, and their observations of damage to buildings around them.  The effects of an earthquake can be very different from place to place.

Mercalli Intensity Scale Click Link for Interactive Demo http://elearning.niu.edu/simulations/images/S_portfolio/Mercalli/Mercalli_Scale.swf

Locating the Epicenter

SDF

ADF

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Predicting Earthquakes Scientists can’t predict earthquakes. However, by looking at how often earthquakes have happened in the past, they can estimate where and when an earthquake is likely to happen. Table shows the number of earthquakes of different sizes that happen every year. There are many more weak earthquakes than strong earthquakes every year.

Predicting Earthquakes Scientists can guess when an earthquake will happen by looking at how many have happened in the past. For example, if only a few strong earthquakes have happened recently in an earthquake zone, scientists can guess that a strong earthquake will happen there soon.

Gap Hypothesis Some faults are very active. They have a lot of earthquakes every year. These faults sometimes have very strong earthquakes. A part of an active fault that hasn’t had a strong earthquake in a long time is called a seismic gap. The gap hypothesis says that if an active fault hasn’t had a strong earthquake in a long time, it is likely to have one soon. In other words, it says that strong earthquakes are more likely to happen in seismic gaps.

Vancouver’s Next “Big Earthquake” The Juan de Fuca Plate, Explorer Plate, and Gorda Plate are all subducting under the North American Plate Each of the three plates have transform fault lines Will most likely be produced by stress and strain generated at a transform fault or along the subduction zone under the North American Plate.

Cascadia Subduction Zone (CSZ) a 1,000 km long fault that stretches from Northern Vancouver Island to northern California. It separates the Juan de Fuca and North America plates