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Mountain- Building To Earthquakes:
Most volcanoes have a habit of issuii ings before they erupt. These take vario In Hawaii, as has been mentioned, measurement of the tilt of the ground small local earthquakes supply some of t dence. At Krakatau the series of events' culminated in the greatest explosion May 20, 1883, and continued in the f< termittent explosions throughout the that year. At Sakura-jima, Japan, in part of 1913, smoke began to issue mountain, local earthquakes increased |_ number, and springs on the mountain aa their flow.The theory of plate tectonics was advanced in the late 1960s and has had a revolutionary effect on the earth sciences. It is a unifying, all-embracing theory, offering a plausible and logical explanation for many of the earth's varied structural and geophysical phenomena, ranging from mountain- building to earthquakesbuilding to earthquakes and continental drift.
A detectable earthquake occurs every 30 seconds. Fortunately, severe and damaging earthquakes are far less frequent. Since 1897 there has been an average of 16 severe earthquakes a year; that is, earthquakes with a rating. The year 1967 was unusual in that only six major earthquakes occurred, and up until May 1968 there had been no earthquakes of intensity greater than 8 on the Richter scale since the catastrophic Alaskan earthquake of March 27, 1964, which caused the deaths of 121 people.
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