Indonesia is one of the most seismically active places on Earth, and New Guinea sits at one of the most complicated pieces of that puzzle. Three major tectonic plates - the Pacific, the Indo-Australian, and the Southeast Asia lithospheric - meet along fault systems that crosshatch the island. Unlike the better-known Sunda Trench along the southern arc of Indonesia, where the Indo-Australian plate slides beneath the Sunda plate, the geology of New Guinea involves a messy oblique collision that has pushed the Central Range up from what was once a shallow sea. The 1981 epicenter lay in this collision zone. The focal mechanism was poorly constrained at the time but is now thought to have been a reverse fault - the kind of slip that happens when one block of crust is shoved up and over another.

A 6.7 earthquake would not, in most places, kill 300 people. What made this one catastrophic was where it happened and what came next. The highlands above the epicenter were steep, deeply weathered, and saturated by the rains that fall continuously in this country - up to 10,000 millimeters a year in places. The shaking triggered what seismologists call coseismic landslides, and in Papua's terrain those landslides were enormous. Whole hillsides detached and poured into the narrow valleys below. More than 150 homes were destroyed outright by the debris flows alone; villages where people had been sleeping simply vanished under earth and trees. Survivors woke to find their neighbors gone, their gardens stripped, their paths obliterated. The New York Times, reporting from Jakarta, called the quake "strong," though the understatement was almost comic against what had happened on the ground.

The debris from the landslides blocked roads and paths throughout the affected area. More than 2,000 people were cut off from the outside, reachable only on foot or by helicopter, in a region that even in ordinary conditions was served by dirt airstrips and missionary aviation. Relief took days to arrive. The official death toll of 305 people is almost certainly low; many of the missing were presumed buried in debris that was never excavated. Counting the dead in remote Papuan valleys was, and still is, difficult - census records were incomplete, whole hamlets might contain only a few dozen people, and the mountainside that covered them would not yield them back. The 1,000 missing number represents the limit of what anyone could confirm. What lies beneath those slopes stayed there.

Papua's seismic hazard did not begin or end in 1981. A stronger 6.9 earthquake had struck the same broad region in 1976. A 7.1 followed in 1989 near Lake Sentani. A series of devastating events has since occurred along the Papuan plate boundary. Each one has followed the same basic pattern: moderate magnitudes producing outsized damage because the terrain amplifies the hazard. The Sunda Trench far to the west captures most of the headlines - the 2004 Indian Ocean earthquake and tsunami, which killed more than 227,000 people, being the most devastating event in recent Indonesian memory - but the slow-grinding plate boundaries beneath Papua continue to generate earthquakes that kill mostly through landslides and collapsed structures in places where building codes are irrelevant and satellite communications are recent.

The 1981 earthquake left little trace in global memory. It produced no iconic photograph, no televised search-and-rescue, no international donor conference. In the year it happened, Indonesia was still administered under the New Order government of Suharto, and Irian Jaya was a restricted province; foreign journalists were largely kept out. What remains is a short entry in the USGS catalog, a sparse Wikipedia article, and local memory in the highlands where the descendants of the survivors still live. The valleys that filled that night are still prone to failure. The monsoon still saturates the slopes; the Pacific plate still grinds against the Australian. Geologically, 1981 was not an anomaly. It was an ordinary night in a country whose geography has always had the power to erase entire villages in minutes.