Strong tectonic jolts continue to torment southwest China in the wake of its deadly May 12 earthquake, injuring dozens and leveling some 420,000 houses. These tremors—some as powerful as magnitude 6.4—persist as 600 emergency workers and soldiers Tuesday desperately tried to evacuate 80,000 residents and drain swelling lakes to keep them from flooding already devastated areas. More will have to be evacuated if the drainage plan fails. The official death toll from the quake and aftershocks now stands at more than 67,000.

If the 4.6-billion-cubic-foot (130-million-cubic-meter) Tangjiashan lake, formed when post-quake landslides choked a portion of the Jianjiang River, crests over its mud banks, workers will have to evacuate as many as 1.3 million people, according to China's Xinhua News Agency. Workers are digging an artificial channel through the mud and debris to divert some of the lake's water from local communities damaged during the earthquake. The news agency says that crews hope to finish the channel by June 5. Tangjiashan, the largest of 35 backed-up lakes formed after the initial earthquake registering 7.9 on the Richter scale, is inaccessible by road and can only be reached by foot or air.

A magnitude 5.4 aftershock hit Qingchuan County in Sichuan Province at about 4:03 P.M. local time Tuesday, injuring 63 people. And a tremor at magnitude 5.7 hit neighboring Ningqiang County in Shaanxi Province a short time later at 4:35 P.M. local time, according to the China National Seismological Network. Qingchuan was also the epicenter of a magnitude 6.4 jolt on May 25, the strongest aftershock since the original earthquake struck Sichuan's Wenchuan County on May 12. Officials have confirmed at least eight dead and hundreds of others injured in that aftershock.

Although the Richter scale has no upper limit, the most devastating earthquakes have magnitudes of 8.0 or higher. More moderate quakes register at 4.5 or greater in magnitude, whereas those of 2.0 or less are typically referred to as "microearthquakes," according to the U.S. Geological Survey (USGS). American seismologist Charles Richter (1891 to 1989) developed the eponymous scale in 1935 to quantify earthquake magnitude, or strength.

Earthquakes are triggered by the release of energy stored in rocks clustered around a fault, which separates masses of Earth's crust known as tectonic plates. These rocks are held in place by friction and, as time goes by, the movement of the plates causes the rocks around the fault to bend and stretch. This causes the rocks to behave like compressed springs, storing energy until the friction across the fault is not able to hold the rocks back, thereby forming a crack across the fault as it begins to slip. This releases part of the built-up energy, some of which creates the seismic waves that travel to the surface and cause damage. Earthquakes stop when there is not enough energy to maintain them. The energy released by the sliding fault needs to be substantial enough to break the friction holding rocks in place.