The tsunami was triggered by a magnitude 7.7 earthquake that hit at 9:42 p.m., local time, on Monday near the western island of Sumatra. The resulting waves killed more than 300 people. (See an Indonesia map.)
A few hours later the 9,700-foot (3,000-meter) volcano Mount Merapi, on the eastern island of Java, blew a pillar of hot ash and debris into the sky, killing at least 30 people living on its slopes. (See pictures of Mount Merapi erupting.)
Mount Merapi, Indonesia's most active volcano, had been building up steam for several days. But the timing of the main burst so soon after the earthquake raises the question of whether the shaking ground set off the eruption—even though the epicenter of the quake was 800 miles (1,300 kilometers) away from the volcano.
"Volcanic eruptions that are related to stress changes following earthquakes, or due to triggering by the seismic waves, do seem to occur," Chris Goldfinger, a marine geologist at Oregon State University, said by email. "But documentation of them is spotty at best."
Known examples, he said, include changes in geothermal activity in Yellowstone National Park in 2002 following a magnitude 7.9 earthquake in Alaska, and the eruption of an Andean volcano in 1960 after Chile's magnitude 9.5 megaquake.
Experts caution, though, that the nearly simultaneous events might simply be coincidence in the world's most seismically active country. Indonesia sits along the Pacific Ring of Fire, and its 17,500 islands are littered with tectonic threats. (Find out about life in the shadow of Indonesia's volcanoes.)
For instance, on Merapi's home island of Java alone, more than 30 volcanoes loom over 120 million people.
Tsunami Earthquake a Rare Slow Temblor?
Meanwhile, experts note that this week's tsunami was unexpectedly large for the size of the associated earthquake.
The magnitude 7.7 earthquake that triggered the tsunami occurred along the same fault zone as the magnitude 9.0 temblor that set off the devastating 2004 Indian Ocean tsunami.
Despite the less intense quake, Monday's tsunami still sent ten-foot (three-meter waves) crashing onto nearby islands, with some reports of waves as high as 19 feet (6 meters). (See pictures of the 2004 tsunami's aftermath.)
That could be because the event was a so-called tsunami earthquake, said Costas Synolakis, director of the Tsunami Research Center at the University of Southern California, who cited calculations by geophysicist Emile Okal of Northwestern University. (Okal was out of the country and unavailable for comment.)
Although many types of quakes can trigger tsunamis, a tsunami earthquake is a unique case. In such quakes, slippage of the tectonic plates is unusually slow, occurring over a far longer period than would be expected based on the intensity of the seismic shift, Synolakis said by email.
"The reason for the slowness of tsunami earthquakes is unclear," Synolakis said. "The hypothesis is that it is due to fractured or jagged rocks that slow the rupture. We just don't know, but we do know that they generate larger tsunamis, hence the common name. ... "
Monday's event was the first tsunami from a slow earthquake to be recorded by a tsunamograph, Synolakis added, which could provide valuable data in unraveling the mysteries of tsunami earthquakes.