Nepal earthquake: what we know

A powerful earthquake struck the Nepal region on Saturday, causing widespread casualties and triggering an avalanche on Mount Everest.

The key information known at this time:

HOW BAD IS THE DAMAGE?

It's too early to tell, but early indications suggest the early figure of more than 900 people killed in four countries is likely to rise substantially in the coming days. The magnitude-7.8 quake was the worst to hit Nepal in eight decades and caused damage and fatalities in neighbouring countries as well. In addition to the hundreds of deaths in Nepal, some 20 people were killed in India, six in Tibet, and two in Bangladesh. Two Chinese citizens died at the Nepal-China border.

• Nepal earthquake kills hundreds

The earthquake hit a heavily populated area of Nepal, including the capital, Kathmandu, and its impact spread far beyond the Kathmandu Valley. Strong aftershocks were felt an hour after the initial temblor.

Earthquake experts are preparing for high numbers in terms of deaths and damage.

"This is a very large earthquake in a significantly populated region with infrastructure that has been damaged in past earthquakes," U.S. Geological Survey seismologist Paul Earle said. "Significant fatalities are expected."

Local hospitals were already filling with injured residents, and Kathmandu's international airport was shut down, hampering initial relief efforts in the isolated mountainous country.

WHAT HAPPENED? AND WHAT HAPPENS NEXT?

Seismologist Earle said the quake happened on what is known as a "thrust fault." That describes the situation when one piece of the Earth's crust is moving beneath another piece.

In this case, it's the Indian plate that is moving north at 45 millimetres a year under the Eurasian plate to the north, Earle said. It's a different type of earthquake than the one that caused the 1906 San Francisco earthquake.

"This is what builds the Himalayan mountain range," Earle said.

The region and particular fault has a history of damaging earthquakes, including four temblors with magnitudes greater than 6.0 in the past 100 years, Earle said, warning that landslides are a particular worry now, given the steep slopes in the region.

WHAT DOES `RED ALERT' ISSUED BY U.S. OFFICIALS MEAN?

The U.S. Geological Survey said the earthquake was strong enough to merit a "red alert" for shaking-related fatalities and economic losses. It said that "high casualties and extensive damage are probable and the disaster is likely widespread. Past red alerts have required a national or international response."

Quick USGS calculations estimate a two-thirds likelihood of between 1,000 and 100,000 fatalities and damage between $100 million and $10 billion. Scientists estimate that more than 105 million people felt at least moderate shaking during the quake.

WHAT HAPPENED ON MOUNT EVEREST?

A devastating avalanche swept across Mount Everest after the earthquake, claiming at least eight lives with an unspecified number of people missing and injured.

The avalanche struck near one of the famed mountain's most dangerous spots. It swept down between the Khumbu Icefall, known for its harsh conditions, and the base camp used by international climbing expeditions.

There were unverified reports of avalanches on other parts of the mountain. Nepalese officials said some 30 people were injured at the base camp.

Facebook postings by climbers suggested that some people may have been buried in their tents when the avalanche hit. Climbers and their support teams were leaving the base camp Saturday looking for safer locations.

More than 4,000 climbers have scaled the 8,850-metre summit since 1953, when it was first conquered by New Zealand climber Edmund Hillary and Sherpa Tenzing Norgay.

HOW PREPARED IS NEPAL TO COPE WITH SUCH A CRISIS?

Nepal is a relatively poor country without extensive resources despite its rich cultural heritage and spectacular mountain scenery.

It has been plagued by instability in recent years, and general strikes have recently brought chaotic scenes to Kathmandu.

Nepal's constitution was supposed to have been written by the Constituent Assembly that was elected in 2008, following the end of a 10-year Maoist insurgency and the overthrow of the centuries-old monarchy, but the assembly was hampered by infighting and never finished its work.

The current assembly was chosen in 2013, but has faced the same problem.

Neighbouring Pakistan has offered help, and an international aid effort is likely to begin once Kathmandu's international airport can be reopened.