Why should Vanuatu brace for more aftershocks after this week's deadly earthquakes?

The death toll from the Vanuatu earthquake disaster continues to rise, as international search and rescue operations continue to help people affected by the initial quake and its aftershocks.

On Tuesday afternoon, a strong earthquake struck an area 30 kilometers off the west coast of Efate, Vanuatu's main island. The 7.3-magnitude earthquake, with a depth of only 50 kilometers, damaged many buildings and injured dozens in Port Vila, the capital of the Pacific country.

Since then, multiple aftershocks have occurred at roughly the same location, including a 5.5 magnitude one on Wednesday morning, adding to the destruction and casualties.

Based on past experiences, here's what we might see next.

How long can aftershocks last?

Yesterday's earthquake, with a magnitude of 7.3, is considered a major earthquake.

Multiple aftershocks of magnitude 5 in the same area are typical of an aftershock sequence — the series of smaller earthquakes that tend to follow a larger tremor, known as a mainshock.

In fact, for this particular earthquake with a magnitude greater than 7, we would expect at least some aftershocks in the 6 magnitude range as well.

In a typical aftershock sequence, there will be 10 times as many aftershocks in the 5 magnitude range, 10 times as many aftershocks in the 4 magnitude range, and so on.

This pattern can last for weeks to several months, or even longer, with the magnitude and frequency of earthquakes decreasing over time.

Is it possible for a larger earthquake to occur?

There is a very small possibility of an earthquake larger than yesterday's 7.2 magnitude earthquake. In this case, yesterday's quake was a “foreshock” — a smaller earthquake preceding a larger one.

However, this is not very common. In only about 5 percent of cases, an earthquake this large is a harbinger of a larger one.

Does the Ring of Fire have anything to do with this?

Vanuatu lies within the Ring of Fire, a belt of tectonic activity surrounding the Pacific Ocean, thanks to a series of plate boundaries around the Pacific Plate.

Its name comes from volcanism associated with subduction along the margins. Subduction occurs when one tectonic plate is pulled beneath another.

The Pacific Ring of Fire is a defining plate boundary and volcano, and plate boundaries around the Pacific Rim dip under each other, forming volcanoes and causing earthquakes. Vanuatu is located at the southern end of the Ring of Fire. (conversation)

The Pacific plate and the Australian plate meet to the immediate west of Vanuatu. The Australian plate is subducted, it is sinking beneath the Pacific plate at a rate of 80 to 90 mm per year, making it a fast-moving plate boundary.

The Solomon Islands, Fiji, and Tonga also border this plate, as does New Zealand to the south.

In areas prone to earthquakes due to subduction, there can be earthquakes of magnitude greater than 8. However, there are no earthquakes higher than this magnitude in the historical records of the Vanuatu region.

While yesterday's earthquake will continue to produce aftershocks, they will occur close to the main shock.

Of course, it is always possible that earthquakes will occur elsewhere on the plate boundary, but these distant events will not be the result of yesterday's quake.

People inspect a collapsed building following the earthquake in Port Vila. (Source: Michael Thompson) How often do earthquakes hit Vanuatu?

Earthquakes are common in Vanuatu, due to the aforementioned Ring of Fire. Since 1990, 11 earthquakes with a magnitude greater than 7 have occurred within 200 km of yesterday's event.

In 2010, a 7.5-magnitude earthquake struck an area just 40 kilometers from Port Vila and at a depth of 35 kilometers, triggering a small tsunami. However, no deaths or injuries were reported.

The last time a series of earthquakes caused a significant death toll in Vanuatu was in November 1999, when a shallow 7.5-magnitude earthquake struck off the coast of the volcanic island of Ambrym, triggering a tsunami.

At these plate boundaries, earthquakes can be 300 kilometers or so deep. The deeper the earthquake, the less damage it will cause because it is farther from the surface and therefore causes weaker shaking.

This is another reason why yesterday's earthquake near Port Vila was so devastating – it was reasonably shallow for the plate boundary zone and generated very strong ground shaking.

De Ninis works at the Center for Earthquake Research, is Vice-President of the Earthquake Engineering Society of Australia, and a committee member of the Geological Society of Australia – Victoria Section. This article originally appeared on The Conversation.

Published 19 hours ago 19 hours ago Wednesday, December 18, 2024 at 4:00 AM, Updated 19 hours ago 19 hours ago Wednesday, December 18, 2024 at 4:28 AM

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