Probability of a major earthquake (magnitude 7.0+) in the next 3 decades in the San Francisco area, they said a month later, was now raised from 67 to 90%.

No part of the world is totally immune from great earthquakes, as can be seen from a global map of earthquake occurrences. Most temblors are concentrated on the continental plate boundaries---the ring of fire around the Pacific Rim or the African Rift zone running from the heart of Africa through the Red Sea, Dead Sea and Jordan River Valley for example. At least a million earthquakes per year, large and small, release a total energy equivalent to nearly one billion tons of TNT.

Although this may seem to be a large amount of energy, heat flow from radioactive decay in the earth's crust is 1000 times greater. Energy released by earthquakes per year is only 10% of all energy used by man; heat from the sun which reaches the earth amounts to 5 million times more energy than is released in earthquakes.

There is no scientific evidence (as far as I know) that the number of earthquakes per year is increasing, contrary to the claims of some Bible prophecy teachers who seem to be misinterpreting passages in the Olivet Discourse of Jesus such as Matthew 24:7. There is more instrumentation to record earthquakes around the world in place today compared to former times, of course.

Increases in world population mean that the impact of earthquakes on people has become greater. (See Addendum).

Earthquakes are confined to the earth's crust. Most occur within 40 miles of the surface though a few extend ten times deeper. Stress accumulates gradually as the continental plates move and mountains shift and this is relieved periodically through gradual creep---or sudden jolts. Although some wish it were otherwise, California is not likely to drop off into the sea from a major earthquakes---the main faults move mostly sideways or at shallow angles, rather than vertically.

. Earthquake magnitudes are usually quote using the Richter Magnitude scale.* One unit on the Richter scale is a ten-fold amplitude increase in the earthquake vibrations, however this translates into a hundred-fold increase in energy released. A magnitude 4.5 quake releases typically the energy equivalent of 100 tons of TNT, a 5.5 quake, 10 kilotons, a 6.5 event, one megaton, and a 7.5 quake, of the order of 100 Mt.

Of course the degree of damage from an earthquake depends to a large degree on geological conditions, building construction standards, traffic conditions, duration of the shaking, and distance to the epicenter.