Revision:Seismographs

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Seismographic waves are waves from vibrations in the Earth (found in the core, mantle, oceans, etc), which also occur on other planets, for example the moon. They can be natural - due to earthquakes, volcanic eruptions or meteor strikes - or man-made - due to explosions or anything that thumps the earth hard. They are used in prospecting and research with the same principle as echo sounding.

The science of seismology is also used in the following areas

• to locate distant earthquakes and major explosions.
• to research the internal structure of the Earth: S-wave 'shadows' provide evidence that the core of the Earth is molten.

Scientists have used seismographs in the past to conclude that the Mantle is significantly larger than the rest of the earth in proportion.

Earthquake prediction is a popular pastime for psychics and pseudo-scientists, and extravagant claims of past success are common. Predictions claimed as "successes" may rely on a restatement of well-understood long-term geologic earthquake hazards, or be so broad and vague that they are fulfilled by typical background seismic activity. Neither tidal forces nor unusual animal behavior have been useful for predicting earthquakes. If an unscientific prediction is made, scientists can not state that the predicted earthquake will not occur, because an event could possibly occur by chance on the predicted date, though there is no reason to think that the predicted date is more likely than any other day. Scientific earthquake predictions should state where, when, how big, and how probable the predicted event is, and why the prediction is made. The National Earthquake Prediction Evaluation Council reviews such predictions, but no generally useful method of predicting earthquakes has yet been found.

It may never be possible to predict the exact time when a damaging earthquake will occur, because when enough strain has built up, a fault may become inherently unstable, and any small background earthquake may or may not continue rupturing and turn into a large earthquake. While it may eventually be possible to accurately diagnose the strain state of faults, the precise timing of large events may continue to elude us. In the Pacific Northwest, earthquake hazards are well known and future earthquake damage can be greatly reduced by identifying and improving or removing our most vulnerable and dangerous structures.

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