Page Content (OCR)
In 1860, the U.S. Geological Survey and numerous international agencies began cataloging earthquakes worldwide. Between 1890 and 1899 there was one earthquake noted world-wide which was greater than 6.0 on the Richter scale. This frequency remained unchallenged until the decade between 1950 and 1960 when nine earthquakes occurred that were greater than 6.0 on the Richter scale. However, between 1980 and 1989 an unprecedented increase occurred with a total of 86 killer earthquakes greater than 6.0 on the Richter scale occurring worldwide. And in the first three and a-half years of the 1990s there have already been more than 100 killer earthquakes greater than 6.0, according to the U.S. Geological Survey. Skeptics argue that the increase in frequency of earthquakes is simply due to our improved ability to measure earthquakes In the first 3.5 years of the 1990s, there were already more than 100 killer earthquakes greater than 6.0 (source: U.S. Geological Survey: http://www.usgs.gov/). worldwide. However, when this objection is carefully examined, it does not stand up. Since the 1960s the number of seismographs has been sufficient to detect any earthquake greater than 5.0 on the Richter scale occurring worldwide. In the last 30 years our ability to detect earthquakes has been so sophisticated that an earthquake of 5.0 or greater will be picked up by a minimum of ten seismographs, usually within a several-thousand-mile radius. Consequently, our ability to detect large earthquakes has been essentially unchanged in the last 30 years. During that period killer earthquakes have increased from 13 events between 1960 and 1970 to more than 100 events between 1990 and 1993. In California alone from 1980 to 1995 there were 18 earth-quakes greater than 5.0 on the Richter scale. That is more than the entire state experienced in the previous 80 years. Since that time, there have been four additional 5.0 or greater aftershocks to the October 1989 Bay area quake which measured 7.1 and the 1994 Northridge quake which 101 increasing to unprecedented levels since 1980. KILLER EARTHQUAKES GREATER THAN 6.0 ON THE RICHTER SCALE