SHANGHAI, China—Rescue workers are using microphones and fiber-optic cable to probe rubble, searching for people trapped in Monday's devastating earthquake in Sichuan Province. Specially trained dogs are being used to lead crews to areas where humans may be buried beneath slabs of concrete, and workers gingerly sift through the wreckage, hoping for survivors. The death toll has reached nearly 15,000 and at least 25,000 are missing, with time running out to rescue them.

In addition to trying to save injured survivors, physicians here are preparing for another challenge: possible viral outbreaks or other epidemics. The pipes that carry water in many of the cities of Sichuan have been destroyed or damaged, including the famous World Heritage Site irrigation waterworks of Dujiangyan, exposing them and drinking water to contamination. Many survivors are sleeping outdoors for fear of aftershocks—a powerful one ripped through the province on Tuesday that measured 6.1 on the Richter scale—exacerbating the poor sanitation situation.

The magnitude 7.9 earthquake damaged schools, hospitals, roads, railway lines, factories, chemical plants, power lines and dams. The potential looms for a disaster, on the scale of that in 1975 when 62 dams collapsed in Henan Province during Typhoon Nina and nearly 100,000 people died, according to officials here. As a result, Vice Minister of Water Resources Jiao Yong is personally heading up the team that will assess dams in the four provinces affected by the quake.

At least six power plants, including Dongfang Electric in the city of Deyang where building collapses killed at least 100 workers and trapped hundreds more, have been damaged by the quake, according to the State Grid Corporation of China. Two chemical plants also collapsed in Deyang, leaking noxious liquid ammonia into the surrounding countryside.

The Ministry of Environmental Protection has assembled a special team to assess the risk of water pollution and to investigate potential problems at the undisclosed number of nuclear reactors in Sichuan , where uranium fuel is produced. The city of Mianyang, also in the province, is home to China's nuclear weapons program.

This is the worst earthquake to hit China since 1976 when a magnitude 7.5 temblor struck the northern Chinese city of Tangshan, leaving 240,000 dead. Despite decades of effort since then, seismologists still have not developed an early warning system for such deadly quakes, though systems to spread warnings as soon as one strikes exist in Japan and Europe. ( Nevertheless, anecdotal reports in Chinese media speak of unusual swarms of toads in the region immediately prior to the devastating quake.

The epicenter of the quake was in Wenchuan County in Sichuan near the famed Wolong Nature Reserve—home of the panda breeding program—but was felt as far away as Bangkok, Thailand.

The earth shook at 2:28 p.m. local time Monday and caused major damage in the provincial capital of Chengdu as well as the city of Chongqing. Minutes later, Chengdu experienced its own smaller quake—magnitude 3.9—that triggered the evacuation of office buildings.

Aftershocks rattled the western province causing yet more damage, including the collapse of a middle school in the Sichuan city of Dujiangyan that trapped 900 children. Just east of the epicenter, 1,000 students and teachers were reportedly killed or missing at a collapsed high school in Beichuan County. China's Premier Wen Jiabao flew to the province to direct disaster relief efforts and troops were dispatched to the area to help.

Earthquakes are initiated 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. As time passes, the movement of the plates causes the rocks around the fault to bend and stretch. This causes the rocks to act like compressed springs, storing energy until the friction across the fault is no longer able to hold the rocks back, and a crack forms 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 keep them going. The energy released by the sliding fault needs to be substantial enough to break the friction holding rocks in place. In some cases, rather than slipping, the two sides along a fault rub together, which can cause a destructive, high-speed quake.

Although the Richter scale has no upper limit, the most devastating earthquakes it measures 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. American seismologist Charles Richter (1900 to 1985) developed the eponymous scale in 1935 to quantify earthquake magnitude, or strength.

—From David Biello in Shanghai and wire reports