The epicenter is the point on the earth’s surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins.

A seismograph is the primary earthquake measuring instrument. The seismograph produces a digital graphic recording of the ground motion caused by the seismic waves. The digital recording is called a seismogram. A network of worldwide seismographs detects and measures the strength and duration of the earthquake’s waves.

There are two primary scales used to measure earthquakes: the Richter scale and the Mercalli scale. The Richter scale is most common in the United States, while worldwide, scientists rely on the Mercalli scale. The moment magnitude scale is another earthquake measurement scale used by some seismologists.

Seismographs measure ground motion, including the energy released by an earthquake. Scientists prefer the moment magnitude scale over the Richter scale because it can more accurately compare various types of earthquakes—big or small, near or far—at the same scale.

Geologists use seismic waves to locate an earthquake’s epicenter, measuring the difference between the arrival times of the P waves and S waves.

The intensity is a number (written as a Roman numeral) describing the severity of an earthquake in terms of its effects on the earth’s surface and on humans and their structures. There are many intensities for an earthquake, depending on where you are, unlike the magnitude, which is one number for each earthquake.

The rupture speed of most earthquakes tops out around 5,600 to 6,700 mph (9,000 to 10,800 kilometers per hour) which is slower than the speed at which seismic shear waves (one type of seismic wave generated by an earthquake) emanate from the epicenter of the quake.

Propagation velocity of the seismic waves through solid rock ranges from approx. 3 km/s (1.9 mi/s) up to 13 km/s (8.1 mi/s), depending on the density and elasticity of the medium. In the Earth’s interior, the shock- or P-waves travel much faster than the S-waves (approx. relation 1.7:1).

Last September, an earthquake triggered a deadly tsunami in Indonesia. Scientists now have clocked the speed of rupture at a blistering 9,600 miles per hour.

A tsunami with a record run-up height of 1720 feet occurred in Lituya Bay, Alaska. On the night of July 9, 1958, an earthquake along the Fairweather Fault in the Alaska Panhandle loosened about 40 million cubic yards (30.6 million cubic meters) of rock high above the northeastern shore of Lituya Bay.

No, earthquakes of magnitude 10 or larger cannot happen. The magnitude of an earthquake is related to the length of the fault on which it occurs. The largest earthquake ever recorded was a magnitude 9.5 on May 22, 1960 in Chile on a fault that is almost 1,000 miles long…a “megaquake” in its own right.

Of the ten places with populations over 50,000 and the highest violent crime rates, only two had populations under 100,000: they are Santa Cruz (8.26) and Compton (11.49). The other places include (in descending order of violent crime rate) Oakland, Stockton, San Bernardino, Vallejo, and San Francisco.