The large earthquake that jolted Ecuador on Saturday, April 16, reached magnitude 7.8. The shaking occured in a fault region that has unleashed much bigger quakes in the past, including the largest earthquake ever recorded by scientific instruments, according to the U.S. Geological Survey.

Saturday's quake near the Pacific coast killed at least 270 people, many crushed in collapsed buildings. The quake occured in a very seismically active area on the Pacific coast of South America. At the earthquake site, a piece of Earth's crust called the Nazca plate is diving beneath South America, moving eastward at 61 milimeters per year, according to the geological survey. The red line in the map above shows the boundary of the two plates; the other lines show the extent of the shaking on Saturday. To the south, the forces driving these plates together are so powerful that, over millions of years, they have created a zone of crumpled rocks that has thrust the Andes mountains high into the sky.

The grinding of huge slabs of rock at this boundary creates thrust faults, when one chunks juts out over another. As the plates push on these chunks, they slip in jolts, creating large earthquakes. The largest earthquake detected by a quake sensor happened in southern Chile in 1960. That quake was an almost unimaginable magnitude 9.5.

It is not the only large jolt that has shaken the region in the past. The geological survey, on its web site, notes that seven magnitude 7 or greater earthquakes have occurred within 250 km of Saturday's quake since 1900. On January 31st, 1906 an 8.3 shaker rumbled through this Pacific-South American collision zone 90 km to the northeast of Saturday's quake. It ripped the seam between the rocks open for about  400-500 km, and the undersea movements produced a tsunami that killed between 500 and 1,500 people.  In 1987, about 240 km east of Saturday's event, a shallow quake--a rupture high up the crust--reached magnitude 7.2, and killed about 1,000 people.