Taiwan, which sits on the edge of two plates on the Earth's crust, gets shaken by dozens of earthquakes each year. Most are centered in the Pacific Ocean and rarely cause damage.
But the magnitude 7.6 quake that hit Taiwan early Tuesday was very unusual because it occurred beneath the island's central mountains.
The quake killed more than 1,800 people. It was Taiwan's worst since a 7.4 magnitude temblor hit the island in 1935, killing 3,276 people. In 1986, a magnitude 7.8 quake off Taiwan's east coast killed 15.
Researchers also say the earthquake is not related to recent ones in Greece and Turkey.
This quake was centered about 120 miles south of the capital, Taipei, in the Changyung Mountains. The mountains, which soar as high as 13,000 feet, are riddled with faults, but not much is known about their instability because the area is not as seismically active as the fault zone off the island's east coast.
Several geologists and engineers from the United States were preparing to fly to Taiwan for a closer look at the physical changes that may have occurred to the island, as well as the damage to buildings and roadways.
"This earthquake is the only major one to occur in 40 years within this mountain range," said seismologist Jeff Barker of the State University of New York at Binghamton. "This could be the piece of information to explain what's happening geologically in Taiwan."
The combination of the earthquake's magnitude, location and damage caught scientists off guard.
"In my experience, this is largest occurring in Taiwan, and one of the very large ones in history," said Wai-Ying Chung, a University of Memphis seismologist who studied in Taiwan. "It is comparable to the 1906 earthquake in San Francisco, and the 1976 earthquake in China."
Scientists said Tuesday's quake was as powerful as the one that struck Turkey a month ago, but it was caused by different seismic motions.
The Turkey earthquake occurred along a strike-slip fault, similar to the San Andreas fault that runs beneath California. Quakes along those faults move horizontally.
The Taiwan quake occurred along a reverse-strike fault. It occurs where two plates, the Eurasian and the Philippine, press against each other. Pressure from below pushes one plate up, while the other plate drops at a 45-degree angle.
"The energy is focused upward with a reverse-slip event," Barker said. "They tend to be more dangerous because that's where the people are."
Although the plates of the Earth's crust are loosely connected, researchers said it is unlikely that earthquakes even in the same part of the world -- Turkey and Greece, for example -- are related events.
Scores of earthquakes occur every day around the world. The National Earthquake Information Center in Golden, Colo., has registered more than 200 earthquakes with a magnitude above 3.5 in the past 2 weeks.