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How many earthquakes are there?
In the refrain of a famous German lullaby children are asked: "Do you know how many stars are twinkling in the sky?" The answer, of course, depends on how you look for them and on the brightness of the heavenly objects. Venus, Jupiter and Sirius can be spotted even under bright city lights. If you go out into the country, on a moonless night you can see hundreds of stars with the naked eye and thousands through binoculars. Using the Hubble Space Telecope astronomers are able to spot millions.
The situation is very similar, when you ask how many earthquakes occur during a year. The answer depends on how strong the temblors are, how far away you are from their focus, and how you try to detect them. The shaking of most moderate and all strong earthquakes is so obvious, they they are felt by everybody, sometimes even hundreds of miles from their focus. You may not notice smaller rattlings, say an earthquake of magnitude 4, when you are busily running around. Sitting down quietly at home in Orinda, the blogger has felt even microearthquakes of magnitude 2 occuring almost five miles away on the Hayward Fault under Kensington. However, each year in the Bay Area alone seismologists detect hundreds of earthquakes which are never felt. They use seismometers which are so sensitive, that they pick up the small rumblings of a car driving by hundreds of yards away.
Using networks of such seismometers, scientists have gained a pretty complete picture of how many large earthquake occur worldwide per year (Figure 1). On the average, for every really big shaker of magnitude 8 or larger, there are 17 quakes with magnitudes between 7.0 and 7.9 and 134 temblors with magnitudes in the "sixes". Simply said: With every step down on the magnitude scale, the number of earthquakes worldwide increase by a factor of ten.
Looking at California the earthquake statistic gets somewhat murkier. During the last century we had not a single temblor of magnitude 8 or greater. In the same interval 16 earthquakes occurred with magnitudes in the "sevens" and 39 quakes had magnitudes between 6.0 ad 6.9. For magnitude 5's, the number is in the low hundreds and it reaches just about one thousand for temblors with magnitudes between 4.0 and 4.9. The uncertainty about the number of earthquakes which occur rises significantly as we look for smaller quakes.
As of this writing, more than 400 earthquakes had occured in California during the last week alone (Clickable list of current earthquakes). But only eight of them had magnitudes over 3. The rest were all microearthquakes, which would have passed mostly unnoticed, if it weren't for the more than 600 seismometers, which the California Integrated Seismic Network (CISN) operates in our State. (hra003)
|Figure1: Worldwide statistics for large earthquakes in the 1990s (Courtesy of USGS)|