New Guinea lies in one of the busiest tectonic intersections on Earth, where the Australian and Pacific plates grind past and into each other along a long, complicated boundary. Along its northern coast, the action is concentrated in the collision of the Huon-Finisterre island arc terrane with the Australian continental margin. The Ramu-Markham fault zone marks the southwestern edge of that collision, and the Highlands Thrust Belt lies inland. Almost all the seismicity in northern PNG is tied to the Ramu-Markham system, where the rocks above the fault have broken into blocks separated by strike-slip faults. The 2002 earthquake was a shallow reverse event on this system - the hanging wall thrust upward, the sea above it displaced, and within minutes the water found the shore.

The Sandaun earthquake did not arrive without warning, though no one reads the warnings clearly until afterward. Thirty-six foreshocks preceded the main rupture, including a magnitude 6.2 event in October 2001 and a magnitude 6.7 near Aitape in January 2002 that killed one person outright and destroyed 450 structures in a single night. To a seismologist those numbers describe a fault zone loading toward failure. To villagers living along the coast they were simply more of what this land does. Papua New Guineans do not need to be told that the ground moves. They need infrastructure that can take the movement, which is harder to come by.

The U.S. Geological Survey estimated a maximum Modified Mercalli intensity of X - extreme - near the epicentre, tapering to strong shaking of intensity VI in Wewak, Kairiru, Ambunti, Angoram, and Aitape. Even Keerom and Jayapura across the Indonesian border felt moderate tremors. A tsunami warning was issued. In East Sepik Province, a 5.01-metre wave struck the coast and destroyed several homes. Two more lives were lost to the water. In East Aitape Rural LLG, ten homes and five water tanks were carried away. Inland, in Maprik District, additional houses collapsed. At Ubidnim, the liquefaction sent water-and-sand fountains erupting from the earth. Along the Momase coast and on several offshore islands, the land itself rose by 30 to 40 centimetres. On Tarawai Island, the uplift happened twice, the second pulse an hour after the first.

The strongest aftershock, a magnitude 6.3, struck on 16 September southwest of the mainshock. Another magnitude 6.2 hit Marienberg Rural LLG in February 2003. Hundreds of homes and buildings across Sandaun and East Sepik were damaged or destroyed, mostly in Wewak. The toll could have been far worse - by Pacific standards, a shallow 7.6 with a five-metre tsunami along a populated coast is the kind of event that has killed thousands elsewhere. The 1998 tsunami at nearby Sissano Lagoon had killed over 2,000 only four years earlier; that memory may have sent some people uphill quickly when the 2002 shaking began. It is the rare good fortune in a region where good fortune is never guaranteed.

The small uplifts measured after the quake - 30 to 40 centimetres across stretches of the Momase Region coastline - are the geological signature of what happens every time a reverse fault ruptures offshore here. Over geologic time, these small jumps add up to the terraces and raised beaches that mark the whole north coast of New Guinea. The people who live here are walking on accumulated earthquakes. Each one adds a few centimetres; each one takes what it takes. The 2002 event was the biggest since 1938 but it will not be the biggest forever, and the fault zone that produced it continues to load. Wewak continues to function as a port city; the coastline continues to climb, half a metre at a time, out of the sea that keeps trying to reclaim it.