Six magnitude 6.0 or larger earthquakes struck the Markham Valley region of Madang and Morobe provinces in Papua New Guinea of October 1993. The first and largest of the sequence measured Mw 6.9 and struck on 13 October; a Mw 6.7 shock occurred 12 days later. These earthquakes were centered beneath the Finisterre Range, north of the valley. They were caused by thrust faulting on the Ramu–Markham Fault, a major plate boundary between the Finisterre Terrane and Australian plate. At least 60 people were killed; many because of landslides and mudflows that destroyed villages downhill of the range.
Papua New Guinea lies along a seismically active and fast-moving convergent plate boundary between the Australian and Pacific plates where the convergent rate is 10–11 cm (3.9–4.3 in) per year. The boundary is rather complex due to interactions with the Bismarck and Solomon Sea plates sandwiched between. The Solomon Sea plate subducts in particular, may be exhibiting double subduction along; northward along the Finisterre Terrane and New Britain Trench, and southward north of the Papuan Peninsula.
The Ramu–Markham Fault is a major thrust fault representing a plate boundary and suture zone between the Finisterre Terrane and Australian plate. This fault has an average dip of 30–40° to the north, and extends to a depth of 90 km (56 mi) beneath the Finisterre Range. The terrane comprise rocks of the Paleogene to early Neogene volcanic arc, and is overlain by Miocene to Plio-Pleistocene limestones. The Finisterre Terrane accreted onto mainland Papua New Guinea about 3 to 3.7 million years ago, thrusting the block to form the Finisterre Range. The range has a maximum elevation of 4,000 m (13,000 ft). Bedrock landsliding is frequent within the range, caused by rainfall and earthquakes on thrust faults. Historical earthquakes near the mountains date back to 1876; locals have described strong shaking in the region.
The earthquake sequence comprised six events of moment magnitude 6.0 or larger with epicenters in the Finisterre Range. The first and largest occurred at 12:06 PGT on 13 October, measuring Mw 6.9. Its epicenter was 47 km (29 mi) north–northeast of Kainantu, and occurred at a depth of 21.4 km (13.3 mi). The second largest event occurred southwest of the first, on 25 October at 20:27 PGT. It measured Mw 6.7 and occurred at a depth of 21.0 km (13.0 mi). These were the largest ever recorded around the Ramu Markham Fault zone; due to their close proximity, are considered a doublet earthquake. They were caused by thrust faulting on a north-dipping fault with the Finisterre Range representing the up-thrusted hanging wall.
Research into the source of these earthquakes are contradictory. Geoffrey A. Abers and Robert McCaffrey inferred that these earthquakes occurred at depths greater than can the down-dip extension of the Ramu–Markham Fault. They most likely occurred on a deeper fault that reaches the surface about 100 km (62 mi) south despite their epicenters 10–15 km (6.2–9.3 mi) northeast of the Ramu Markham Fault surface trace in the Markham Valley. G. Pegler and others contended that these earthquakes occurred on the Ramu–Markham Fault, with the fault dipping steeply for several kilometers before flattening out at a depth of 20 km (12 mi) of less.
Although the earthquakes implies rupture on a shallow-angle thrust fault at mid-crustal depths, GPS data showed horizontal surface deformation of up to 20 cm (7.9 in) across the 5 km (3.1 mi) wide valley. These data demonstrates that near the surface, the Ramu–Markham Fault has a steep ramp geometry, before flattening out to a horizontal mid-crustal detachment at depth. While most of the earthquakes in the sequence occurred on the detachment at 20–25 km (12–16 mi) depth, the GPS data suggest coseismic slip must have occurred on the ramp and extended to within several hundred meters of the ground surface. Seismic waveform modelling suggest that the two largest earthquakes likely accounted for the deformation.
About 60–65 people were killed and 200 were injured. The earthquakes triggered some 4,700 landslides with a total surface area of 55 km2 (21 sq mi); these landslides caused most of the casualties. These landslides were reported on the border of Morobe and Madang provinces; some were 2,000–2,500 ft (610–760 m) long and a half-mile wide. When they blocked rivers such as the Ume River, earthquake-dammed lakes formed.
In an initial report by the Associated Press, some 40 people were killed, and one official in Papua New Guinea said as many as 160 people were dead or missing after mudslides from the mountains reaches the villages downhill. These mudslides buried villages and their residents; more than 600 people's homes were destroyed.
A total of 1,122 homes were destroyed, 24 schools closed, and electrical services were disrupted immediately following the earthquake shaking. The United Nations Department of Humanitarian Affairs said family gardens were destroyed which threatened the local food resource and worsened an existing humanitarian crisis caused by an earlier drought. The Papua New Guinea government said in a preliminary report that 60,000 people were affected and thousands of people were displaced. Many people were relocated to shelters including about 9,600 people in the upper Markham Valley.
Because land routes were destroyed by landslides, aid was largely delivered by air transport. Seven commercial aircrafts were also involved. The government activated its National Disaster and Emergency Services for aerial and damage assessments, set aside K450,000 for emergency relief activities, and created a trust fund for donations. Thousands of people in the mountainous region were evacuated to shelters due to the threat of additional landslides and landslide lakes along blocked streams. Some relief organisations and state government supported the relief work; the Australian government contributed supplies, US$57,000 in cash, and dispatched a Lockheed C-130 Hercules aircraft. The British and Japanese governments, and Caritas Germany also provided cash and supplies.
List of earthquakes in Papua New Guinea
The International Seismological Centre has a bibliography and/or authoritative data for this event.