Defining a seismicity rate requires several things. First off, you must define a region for which you wish to find a rate. That area can be any size you want it to be. You can even assign boundaries in depth, so that you're actually counting the rate of earthquakes within a particular volume. Whatever you choose, the boundaries should be definite, and fixed.
Naturally, to count earthquakes, you need a way to record and locate earthquakes, or access to a reliable source of data (already recorded for you). Working on this module, you will have access to data recorded by the Southern California Seismic Network, and stored at the SCEC Data Center. There are also many other seismic databases around the world that offer similar information to the public. That's good -- few people could afford to have their own seismic network!
For your rate measurement to be effective, there are two more things you will need to decide: (1) the interval, and (2) the duration. Are you going to count the number of earthquakes per hour, day, month, year, or some other period of time? And for how long will you continue your count?
Once you have determined all these parameters and have a source of data to work with, you are ready to begin calculating your seismicity rate. The exercise below will take you on a "tour" of seismicity rates in southern California, and then allow you to make your own seismicity rate calculations using a searchable earthquake database.
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Take a look at the varying rates of seismicity in southern California,
and use real seismic data to calculate a rate yourself! |
In the activity above, you saw how the seismicity rate can go up dramatically after a large earthquake, because of the occurrence of numerous aftershocks. You may have heard this term used before, but do you know, specifically, what kind of earthquake qualifies as an aftershock?
Seismicity Rates