How can you tell an earthquake is coming?

One of the yet unconquered fears of mankind is that the best of seismologists can't tell when an earthquake is coming- only when an earthquake is likely.

Retired geologist Jim Berkland of Santa Clara County, California, proposed in the late 1980s that household pets are sensitive to impending earthquakes. He tested his theories by monitoring the local newspaper want ads for announcements of missing dogs and cats and reported a fairly high correlation between such reports and earthquakes within the next couple of days. Chinese geologists have been doing similar work by observing barnyard animals. It isn't known what the animals are sensing, but some theories suggest changes in the Earth's magnetic field, electric fields, or small infrasonic tremors that precede the main quake. Building electronic detectors of such phenomena that can automatically warn of earthquakes has been elusive. Mr. Berkland also combines animal observations with positions of the Sun and Moon. During a new moon, when the Moon is on the same side of the Earth as the Sun and is simultaneously at perigee (closest distance), gravitational pull on the Earth's crust is maximized, increasing the chances that an earthquake will be triggered. It gives clues about when a quake will occur, but not where.