A powerful earthquake struck the east coast of Taiwan late Tuesday, killing two people and causing buildings to tilt at dangerous angles as aftershocks continued into the night.
As daybreak arrived Wednesday, authorities said 177 people remained missing or trapped, including 147 people who were believed to be in a residential building that was leaning so precipitously that rescue workers were unable to go in. Firefighters were evaluating whether to prop up the building with steel.
The U.S. Geological Survey reported that the quake measured magnitude 6.4, but that the shaking was felt as intensity level 7. That is considered a strong quake that can cause considerable damage in poorly built or badly designed structures.
The quake was centered 6.2 miles beneath the island's Pacific coast off Hualien County on the east coast. It was felt most strongly in Hualien and neighboring Yilan County, although buildings also shook in the capital, Taipei, about 35 miles away. The two deaths occurred in Hualien County; no details about the victims were immediately announced.
"We believe at this time we need to make sure we're making preparations for Hualien, providing enough medication and help for citizens so they can be safe as soon as possible," Taiwanese Premier William Lai said in a live YouTube video after the quake. "With such a large earthquake a lot of things will definitely happen."
A 10-story hotel partly collapsed, damaging the first floor and a basement level. At least 24 people were trapped inside as of 2 a.m. Wednesday, a local fire department representative in Hualien County said. A restaurant and a hospital were among other structures damaged, as was the residential building where the 147 people were believed to be stuck.
The National Fire Agency also reported two damaged bridges and cracks in a section of road.
As of 3 a.m. Wednesday, 150 people had been rescued, the county fire department representative said. Authorities said 214 people were hurt, and all hospitals in Hualien County had admitted people with minor injuries.
Taiwan's Ministry of National Defense sent more than 400 military personnel to help firefighters rescue people at the hotel. The government warned people in Hualien to leave any unsafe buildings, such as those with existing cracks, in case of aftershocks.
Taiwan is in a seismically active zone, part of a "ring of fire" that arcs across the Pacific Ocean and south along the West Coast of North America.
The island sits on the border of the Eurasian and Philippine plates. On Taiwan's west side, the Eurasian plate is moving underneath the Philippine plate; but off the northeastern coast where Tuesday's earthquake hit, the situation is reversed and the Philippine plate is moving underneath the Eurasian, according to U.S. Geological Survey geologist Devin McPhillips.
The city of Hualien is located just above a thrust fault, and also above a big pile of river sediment.
"These sorts of pretty loose sand, silt and gravel deposits" — not unlike the L.A. Basin — "can really interact in interesting and sometimes dangerous and unpredictable ways with seismic waves," McPhillips said. "Everywhere in Taiwan is really tectonically active. All of these plates are moving pretty fast."
A magnitude 7.6 earthquake in 1999 killed 2,400 people in Taiwan. A magnitude 6.4 quake in February 2016 toppled a building in the city of Tainan in southern Taiwan and killed 116 people.
Army Col. Huang Chung-chang, director of a military relief brigade at the disaster site, said he was stationed in Tainan in 2016, and immediately thought of that experience Tuesday. Based on that, he said, "when we knew about the collapsed building here, we mobilized people as fast as possible."
After looking at photos of the damage in Taiwan on Tuesday, California seismic safety commissioner Kit Miyamoto said it appears that buildings failed because of brittle concrete construction as well as flimsy ground stories. Similar failures were widely seen during the Mexico earthquake that struck in September, said Miyamoto, a structural engineer.
"Taiwan is a very advanced country as far as earthquakes are concerned," Miyamoto said. "The technology is as good as … California or Japan." However, he said, older buildings are still at risk — just as in California.
Brittle concrete buildings can collapse in an earthquake because they lack enough reinforcing steel to keep concrete from exploding out of the building's columns when shaken. Flimsy first floors are also a problem in what engineers call a "soft story building," in which the ground floor is weak.
The epicenter of the latest earthquake was around the scenic Taroko National Park, about 12 miles north of Hualien city. The temblor came during a period of seismic activity in the area:
The Central Weather Bureau had recorded 20 smaller earthquakes in the same Pacific coast zone off Taiwan since Sunday, when a magnitude 6.1 earthquake struck just less than two miles southeast of Tuesday's epicenter. The bureau reported at least 20 aftershocks after the Tuesday temblor, which were as strong as magnitude 5.4.
There is a risk that the earthquake could trigger more quakes to the south along the same fault system, which is about 90 miles long, said geologist David Jacobson, an analyst for Temblor, a seismic risk app based in Redwood City, Calif.
"Given the length of the fault, given the activity of it, people should be prepared," Jacobson said, adding: "Not to say something will happen, but that something could happen."
Tuesday's earthquake produced nearly triple the shaking energy as Sunday's. "A rule of thumb is for any given earthquake, within a week, there's about a 5% probability that an earthquake will trigger a bigger one," McPhillips said.
The government said it would ask fire agencies in Hualien County on Wednesday, weather permitting, to evaluate roads and bridges for any risk and immediately close off any that might buckle in another quake, Lai said.
Jennings is a special correspondent. Times staff writer Rong-Gong Lin II in San Francisco contributed to this report.
6:25 p.m.: This article has been updated with people trapped in residential building, comments from California geologist and army colonel; number of injured.
4 p.m.: This article has been updated with comments from California seismic safety commissioner.
2:45 p.m.: This article has been updated with scientific background, quotes. It also changes the spelling of the location from Hualian to Hualien, which is a more common English transliteration.
11:45 p.m.: This article has been updated throughout with two dead, additional details on damage, quotes.
9:30 a.m.: This article has been updated with additional damage reports.