October’s Bay Area Earthquake Was Many Times More Powerful

The magnitude of an earthquake is determined by measuring the ground motion that results from the quake, and a 5.5 quake like the one that struck Southern California Wednesday would be at least 60 times weaker than the 7.1 quake that devastated parts of the San Francisco Bay Area last October.

Earthquake magnitude is measured on a logarithmic scale, so a 7.1 quake is 10 times more powerful than a 6.1 quake and 100 times stronger than a 5.1 quake.

When an earthquake hits, it sends out two general types of shock waves: surface waves, which travel along the Earth’s surface and generally do the most damage, and body waves, which travel through the Earth. The fastest waves are body waves called compressional waves, or primary waves, and these are the waves felt first, usually as a sharp thud. They will be followed by secondary body waves, which arrive with a second jolt.

Then comes the rolling of the ground caused by the arrival of the surface waves.