We've been seeing a lot of earthquakes lately. There have been earthquakes in Turkey, Taiwan, Mexico, and California in summer and fall 1999 - and they've been large earthquakes, too.

So does that mean we're having more earthquakes? Since 1900, the average number of major earthquakes annually has been 18. This year, the major earthquake count stands at 13.

According to the U.S. Geological Survey, the annual number of major earthquakes does not vary widely, because most major earthquakes are the result of tectonic plate interactions, and these interactions occur at a steady rate. Plates slide past, collide with, and subduct beneath one another.

Each plate moves at a constant speed - some as fast as the growth of your fingernail - but the movement is usually impeded at the boundaries where two plates are in contact. Stresses accumulate along the boundary until the impediment can be overcome, and the locked section of the plate moves to catch up with the rest of the plate.

The deadly earthquake in Turkey on August 17 was along the North Anatolian fault, where the Turkish microplate is sliding past the Eurasian plate. The Turkish microplate is moving about 24 mm (1 inch) per year to the west relative to the Eurasian plate. The magnitude 7.4 earthquake produced 16 ft of sideways slip between the plates. There were more than 15,000 fatalities, mostly caused by the collapse of nearly 76,000 buildings.

The magnitude 7.6 Taiwan earthquake occurred on September 20 along the Shundong fault, where the westward-moving Philippine Sea plate collides with and overrides the Eurasian plate. Up to 26 ft of uplift was measured from the thrust motion of the fault. The earthquake caused the collapse of more than 38,000 structures and killed more than 2,300 people.

On September 30, a magnitude 7.5 earthquake shook the Oxaca region of Mexico and caused 18 deaths. The earthquake was deep within the northeastward-moving Cocos plate, which subducts beneath the North American plate. The depth of the earthquake and the sparse population of the region kept the death toll from being higher.

On Sunday, October 16, a magnitude 7.1 earthquake undulated through the Mojave desert in southern California. The earthquake was centered on the Lavic Lake fault, a strike-slip fault that takes up some of the slip between the northwestward-moving Pacific plate and the westward-moving North American plate. Up to 15 ft of right-lateral offset was measured along a 24-mile-long surface rupture. Except for a cracked highway overpass and broken items that fell, damage was minimal in the lightly populated area.