A magnitude 7.5 earthquake occurred at a shallow depth of 35 km in the south west of Porgera, in Papua New Guinea (PNG) at 17:44(UTC) on 25 February 2018. PNG is located between the rather stable continental plate of Australia and deep ocean basin of Pacific.

This earthquake has happened as a result of younger rock formations being pushed above older rocks as a result of Australian Tectonic Plate (south west) pressing against Pacific Tectonic plate (north east), creating compressive stresses. It is estimated that these tectonic plates move around 100mm per year toward each other, and the fault rupture in the order of 75 km occurred in this earthquake.

In June 1976, the Papua Earthquake with the magnitude of Mw 7.1 in the region killed more than 400 people. Let’s not forget that the 1998 Papua New Guinea earthquake with magnitude of 7.0 in north Papua killed more than 2,000 people and injured thousands more.
 
Vast areas of central Papua New Guinea particularly in Highlands Regions (for example Enga, Hela, Southern Highlands and Western Highlands) comprise of crystalline rock formations overlain by sedimentary rocks. There are several major mining sites in the region, such as Porgera Gold Mine, that need to be inspected for possible cracks and open pit instability.

In addition, the Liquid Nitrogen Gas (LNG) area near Port Moresby needs to be checked for probable ground subsidence and impacts on pipelines and LNG tanks since there is a chance of sliding and deformation of foundations after this earthquake.
 
The local roads and areas near rivers and lakes, such as Hegigio River and Lake Kutubu, need to be carefully assessed against landslides. Central PNG and parts of earthquake affected areas are hilly and in regions elevated 3km, with several open valleys and mangrove swamps.

In recent history, landslides have induced a significant amount of damage in central PNG to villages and people, particularly near the town of Mendi, close to the epicentre of this earthquake.

PNG experiences large rains generally up to March each year, with most of the rain falling in the northern and western regions. Therefore it is essential to access these regions, particularly those that experienced heavy rainfall and flood events within the last two weeks, since earthquakes can readily trigger landslides in already wetted and softened slopes.

The large magnitude 7.5 PNG earthquake at 03:44 am (Papua New Guinea Time) this morning has been followed by many aftershocks, with the USGS reporting about 10 with magnitudes exceeding 5.0 in the following six hours. This is typical of a shallow crustal earthquake.

These aftershocks are distributed along the Southern Highlands of PNG, from southwest of Mendi to within 125 km of Tabubil near the PNG border with West Irian, and will continue for some days or weeks. This covers a distance exceeding 100 km, as expected for an earthquake of this magnitude.

The main earthquake epicentre was near the centre of the fault rupture, which then fractured both to the south-east and to the north-west. It was a crustal thrust fault due to southwest to northeast compression, caused by the Australian continent moving north-north-east at about 70 mm per year.

It was not associated with deep subduction of one tectonic plate under another, but the relatively shallow depth of the crustal rupture is likely to cause severe shaking on the upthrown (north-east) side of the fault, along the Southern Highlands.
 
This region of PNG has a long history of earthquake activity, with magnitude 6.6 or larger events in 1910 (~M 6.9), and 1954 (~M 7.0) near today’s rupture. To the southeast along the fault, there have been earthquakes in 1936 (~M 6.9), 1947 and 1948 (both ~M 7.0), and in 2000 (~M 6.6). To the northwest, in 1922 there was another very large earthquake (~M 7.5) over the border in West Irian, Indonesia.

The magnitude-7.5 Papua New Guinea earthquake occurred in a mountainous region of high seismic hazard. The mountains are the result of the collision between the Australia and Pacific plates and the earthquake occurred along a thrust (compressional) fault that helps accommodate this plate motion.

In addition to damage caused by shaking, the earthquake is likely to have triggered landslides throughout the region. Ten aftershocks of at least magnitude-5 were recorded in the first few hours after the main earthquake; aftershocks will continue for at least several weeks and magnitude-6 or larger aftershocks would not be unexpected.

A large, magnitude 7.5 earthquake struck Papua New Guinea in the early hours of the morning. Papua New Guinea is one of the most tectonically complex regions on Earth and is shaken by earthquakes routinely, including big ones such as this. There are some characteristics of this particular event that make it stand out though.

The initial data released by the USGS suggest it was what scientists call a thrust earthquake, occurring at 35 km depth at the margin of the Southern Highlands. The estimate of the 35 km depth will not necessarily be very good, but we can always expect extensive damage when an earthquake greater than magnitude six occurs somewhere in the top 30 km of the crust. A magnitude 7.5 event as this was should have ruptured massive areas of the crust (at least 80 km long) and propagated all the way to the surface.

Thankfully, the event does not appear to have hit a large city or town. The biggest concern will be secondary events, such as landslides, flash floods from blocked rivers and aftershocks striking nearby towns.

As we have seen recently in New Zealand, earthquakes even in remote regions can cut off roads for months. Papua New Guinea is also heavily reliant on its natural resources and the Porgera mine amongst other operations would have certainly felt the shaking.

Aside, from the obvious human side to the story, thrust earthquakes are common in parts of the world where two plates are colliding. It is thrust earthquakes along large faults that build many of the mountain chains in the world. It may well be that this earthquake is a snapshot of the mountain building events that are forming the Southern Highlands even today.

The earthquake occurred where the Australian tectonic plate collides with the southernmost of a series of microplates that collectively comprise the PNG collision zone between the Australian and Pacific Plates. This complex collision zone represents a major boundary where the rigid Australian Plate converges with the New Guinea Highlands Block at about 6 to 13 mm per year.

Large earthquakes on reverse faults in the area occur in order to release this accumulated elastic strain and often are associated with regional uplift of the PNG mountain ranges and erosion due to landslides. The location and magnitude of this earthquake is not surprising and there have been many historical earthquakes in this area with similar faulting characteristics to this recent event. It should not surprise us if there are 10s to 100s of fatalities from this earthquake, as suggested from initial estimates (https://earthquake.usgs.gov/earthquakes/eventpage/us2000d7q6#pager).

These are likely to be caused primarily by damage and collapse of unreinforced brick masonry structures and the occurrence of landslides in populated areas such as river valleys. Strong aftershocks above magnitude 6 that cause more landsliding and infrastructure damage are highly likely for an earthquake of this magnitude and depth.