Melbourne earthquake: what exactly happened, and what’s the best way to stay safe from aftershocks?

James Ross/AAP

Mark Quigley, The University of MelbourneA magnitude 5.8 earthquake has struck about 115 kilometres east of Melbourne in Victoria, causing damage to buildings and forcing residents to evacuate across the city. The quake, which started near Woods Point at a depth of 12km, was also felt in Sydney, Canberra, Adelaide and even as far as Launceston, Tasmania.

I and the co-author of this article, Dee Ninis, work as earthquake scientists at the Seismology Research Centre. Researching earthquakes is our life’s work. Here’s what you need to know to understand why today’s earthquake happened, and the geological conditions that triggered it.

Where was it exactly?

On-ground sensors distributed by the Seismology Research Centre have confirmed the earthquake was of a 5.8 magnitude, with an epicentre about 60km south-east of Mansfield in Victoria. The preliminary focal mechanism of this earthquake is strike-slip, meaning the rocks likely slid past each other laterally on what is probably an east-west oriented fault.

The earthquake was felt across the region at around 9.15am today. Geoscience Australia had received 32,409 felt reports as of when this article was published.
Screenshot/Geoscience Australia

Australia experiences fewer earthquakes than plate boundary regions, such as New Zealand. Many of Australia’s suspected neotectonic faults (faults which have hosted earthquakes in recent geological times) have not been thoroughly investigated, commonly due to lack of funding and resources for earthquake research.

However, earthquakes basically happen for the same reason in Australia as they do in New Zealand: there is a buildup of elastic strain energy in the crust, which eventually needs to be released. And most of this energy release occurs due to the rupture of weak zones in the crust, called faults.

Geoscience Australia hosts a database of what we think might be active faults across Australia, but few of these faults have been studied on the ground.

Most of the neotectonic faults near today’s earthquake were identified from remote elevation data — and this alone doesn’t reveal information such as when, how big and how often previous earthquakes on these faults occurred.

What we look for here is displacement at Earth’s surface, formed by movement during previous quakes. Such displacement is only caused by moderate to large earthquakes relatively close to the surface.

If it’s deep enough, it’s entirely possible for a quake to happen at a fault that never ruptures the surface — so we can’t see evidence for it. At a magnitude of 5.8 and a depth of 12km, we don’t expect today’s event to have an associated surface rupture, although it is remotely possible.

The Conversation’s readers sent in their accounts of the earthquake, which was felt across Melbourne’s suburbs.
The Conversation

Is this an unusual event?

While some early reports suggested today’s earthquake was the “largest on-land earthquake in Australia since 1997”, this isn’t the case. Australia has an earthquake of magnitude 6 or higher every six to ten years, on average. That’s based on an instrumental record going back about 150 years.

The 2016 Petermann Ranges earthquake in the Northern Territory was a magnitude 6.1 quake. And while Australia is not a tectonic plate boundary, it is still quite seismically active.

This morning’s earthquake was the largest onshore quake ever recorded in Victoria. Other recent earthquakes include two magnitude 5 quakes: one in 1996 near Mt Baw Baw, and one in 2012 near Moe.

But just because we haven’t seen such a high-magnitude earthquake in our time doesn’t mean they don’t happen. For instance, there is geological evidence for a possible magnitude 7 earthquake occurring sometime between 70,000 and 25,000 years ago, on the Cadell Fault near the Victorian town of Echuca.

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Earthquakes are more intense and frequent in plate boundary regions. The Pacific plate boundary, which passes directly through New Zealand’s South Island, lies to Australia’s east.

But despite this — and although the tectonic deformation rates across Australia are lower than the deformation rates at plate boundary regions — Australia has seen earthquakes in places you wouldn’t expect (unless you’re an earthquake scientist).

For instance, the Tennant Creek earthquake sequence in 1988 saw three separate shocks erupt within 12 hours, with magnitudes of 6.2, 6.3 and 6.6 (the main shock).

What about aftershocks?

Several aftershocks followed the main event this morning, some occurring within the hour. In an earthquake sequence, an “aftershock” is defined as an earthquake that’s smaller than and which follows the main shock. The strongest aftershocks come soon after the main event and slowly taper off.

We do expect the region around today’s earthquake epicentre to remain active, and we will probably have more felt events in the next few days. In fact, we would expect aftershocks to continue up to decades afterwards, although through time most of these will become too small to be felt (the Tennant Creek earthquake sequence of 1988 is still ongoing).

If, under unfortunate circumstances, we experience an even larger earthquake soon — then that will become the main event, and the quake from this morning will be designated a “foreshock”.

So we all have to stay alert. Even if the aftershocks aren’t as intense in magnitude, smaller quakes can still be incredibly damaging depending on their depth and location. In the 2011 Christchurch disaster, it was an aftershock of magnitude 6.3 which wreaked the most havoc, and led to many people’s deaths.

How to prepare?

In terms of personal safety, the best thing to do during an earthquake is drop to the ground, take cover and hold on. If you’re inside a house or other building, try to crawl under something sturdy to protect yourself, such as a solid table. This will help save you from anything that might fall.

If you experience a quake while you’re outside, make sure you’re as far away from buildings and other structures as possible, as these too can fall on you. You need to be in an open area. Victoria’s State Emergency Service has more recommendations on what to do, including:

• staying away from glass, windows, outside doors and walls and anything that could fall such as lighting fixtures
• not using a doorway unless you know it is strongly supported and is close to you
• keeping in mind the electricity may go out, and sprinkler systems or fire alarms may turn on.

Finally, if you’re considering any activities that might put you at risk, such as roofing, gutter cleaning, and other activities that involve the use of ladders, it is prudent to reconsider whether these are essential in the short term.

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Acknowledgment: this article was co-authored by Dee Ninis, who works as an earthquake geologist at ESS Earth Sciences’s Seismology Research Centre based in Richmond, Victoria.

Mark Quigley, Associate Professor of Earthquake Science, The University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Aftershocks hit Papua New Guinea as it recovers from a remote major earthquake

Sabin Zahirovic, University of Sydney; Gilles Brocard, University of Sydney; John Connell, University of Sydney, and Romain Beucher, University of Melbourne

Another powerful aftershock hit Papua New Guinea this weekend as the recovery effort continues following February’s deadly magnitude 7.5 earthquake, with many thousands of people dependent on humanitarian aid.

Aid organisations such as CARE Australia and UNICEF are still seeking donations. The Australian government has sent medical staff and other support to help.

Some have criticised the PNG government’s efforts as “too slow”.

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But the earthquake highlights the challenge for emerging economies like PNG in deploying relief efforts into remote areas to deal with natural disasters.

And the same geological features that make PNG a rich source of mineral deposits are also part of its earthquake problem.

The earthquake hits

The February earthquake struck the western Highlands provinces of the Pacific island nation, and a series of aftershocks, including several of magnitude 6 or more, continued to shake the region during the following weeks.

Although parts of PNG are particularly earthquake-prone (especially in the north and the islands, along the plate boundary), February’s earthquake was quite exceptional.

It occurred in a usually less active part of the plate boundary and was remarkably powerful when compared with the short (modern) instrumental earthquake record. The strength and frequency of the aftershocks has posed an additional threat to local populations and key economic infrastructure.

On average 10-20 major earthquakes (magnitudes 7 and greater) occur on Earth every year. Most of them occur far from densely populated regions, such that only a few draw media attention.

The mountainous regions of New Guinea, known as the fold and thrust belt, have been geologically active for millions of years. But the long recurrence interval of major earthquakes (every few centuries) combined with the short period of the instrument records (just a few decades) gives us the false impression that seismicity is uncommon in this region.

The February earthquake occurred due to the activation of a major fault system in the forested foothills, between the Papuan highlands to the north and the Fly River lowlands to the south.

Australia collides

The Papuan highlands have risen due to the collision between the Australian and Caroline/Pacific tectonic plates over the past five million years.

An animation of Australia’s tectonic journey as it broke away from Gondwana more than 100 million years ago. (Credit: Sabin Zahirovic)

Despite this collision, the Australian plate continues to move at about 7 cm a year to the northeast, in geological terms a quite remarkable speed, leading to a build-up of strain in the continental crust.

Much of this strain is released at the plate boundary along northern New Guinea, usually with more frequent but less powerful swarms of earthquakes. It is this motion, driven by the churning interior of our planet, that leads to major adjustments to the GPS datum and reference coordinates for the entire Australian continent.

But few people are aware that this very motion of the Australian continent is what causes the seismic and volcanic activity in New Guinea and parts of Southeast Asia.

As Australia moves northward, the entire New Guinea margin acts as a bulldozer, collecting Pacific islands, seamounts and other topographic features. New Guinea represents the leading edge of the advancing Australian continent, which causes continental crust to fold and crumple over a broad region.

This is a well-known process in plate tectonics, where the oceanic plates are known to behave quite rigidly, whereas the continental regions tend to deform over broader diffuse boundaries that resemble plasticine over geological timeframes.

When continents are squeezed during tectonic collisions, the crust crumples and folds over geological timescales. (Credit: Romain Beucher)

But the continental deformation process results in poorly defined (often due to the thick tropical vegetation cover) and intermittently active fault systems in the continent.

Over the duration of mountain building in the past five million years, the areas of highest deformation have shifted across the range. Today most of the deformation in PNG takes place north of the mountainous area, where it generates a lot of earthquakes.

Underground riches at risk

Some substantial crumpling of the continental crust still occurs across the southern foothills. The folding and thrusting has generated geologically young folds, within which a large part of PNG’s gas and oil wealth has accumulated.

The intense tectonic activity has also led to the enrichment of mineral resources, including mines sourcing gold, copper, silver, nickel, cobalt and a suite of other ore types.

Distribution of the aftershocks magnitude 4+ since the main quake (as of April 9, 2018). The size and colour (small to large, yellow to red) indicate aftershock magnitude and D+ the number of days after main shock. The white shaded ellipse represents the area of greatest slip during the main shock. Green diamonds represent the main gas fields.
USGS/Gilles Brocard, Author provided

It is this tectonic activity that determines the delicate interplay of economic benefits from raw materials, and the often-devastating and usually-unpredictable effects of natural disasters on society.

Although the February earthquake occurred at the very heart of one of the largest and newest gas fields in the country, the industrial installations, at the highest international standards, have not suffered major damage from the tremors.

But the ongoing disaster triggered a temporary halt in gas extraction, as the facilities require inspections and repairs. Unfortunately, and unusually, the earthquakes have struck in some of the most remote parts of the country.

Coping with disaster

Hela province is one of the poorest in PNG and its people are unprepared and ill-equipped to deal with a disaster of this scale. As many as half a million people were reported to be affected by the earthquake. At least 145 people reported killed.

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The Highlands Highway, the one real road into the region, was badly damaged and this is the major source of food and medicines. Many feeder roads have gone.

Papua New Guineans are resilient but it is likely that more external assistance will be needed to ensure that a physical disaster does not become a greater human tragedy.

Even so the full extent of the disaster has still to be revealed, while aftershocks continue to trigger secondary hazards including major landslides that have isolated a large number of communities.

Not only are local communities facing the immediate hazards of further earthquakes and landslides, they face a protracted and costly recovery ahead.

Sabin Zahirovic, Postdoctoral Research Associate, University of Sydney; Gilles Brocard, Post doctoral associate, University of Sydney; John Connell, Professor of Human Geography, University of Sydney, and Romain Beucher, Postdoctoral Research Associate in Computational Geodynamics, University of Melbourne

This article was originally published on The Conversation. Read the original article.

New Zealand: Three Major Tremors

The New Zealand city of Christchurch was struck by three major tremors yesterday and a large number of minor ones. These are all considered aftershocks of the first major earthquake that Christchurch suffered some time ago. There is now expected to be a period of increased seismic activity.

The following videos feature footage and updates concerning the latest earthquake developments in Christchurch.

ITALY: MASSIVE EARTHQUAKE WREAKS DEATH AND DESTRUCTION

A massive earthquake has struck central Italy, some 100 km to the east of Rome. The 5.8 magnitude earthquake epicentre is located about 5 km below the historical city of L’Aquila.

Early reports have 27 people confirmed dead, with many people still trapped and buried within the 10-15 000 damaged buildings in L’Aquila and surrounding areas. Hundreds have been injured and some 100 000 people have fled their homes fearing the usually expected aftershocks.