What happened in New Zealand’s magnitude 7.5 earthquake


Brendan Duffy, University of Melbourne and Mark Quigley, University of Melbourne

At least two people have died in the magnitide 7.5 earthquake that struck New Zealand’s South Island early on Monday, local time.

Preliminary modelling suggests that the earthquake was caused by a rupture of a northeast-striking fault that projects to the surface offshore.

But this may be a complex event, involving several faults on the South Island.

The northern part of the South Island straddles the boundary between the Pacific and Australian tectonic plates.

The jostling between these plates pushes up rocks that create mountains including the Southern Alps and the beautiful Seaward Kaikoura Range, one of New Zealand’s most rapidly uplifting mountain ranges.

The plate motion forces the oceanic crust of the Pacific plate beneath the Australian plate on thrust faults, and also causes the plates to slide laterally with respect to one another on strike-slip faults.

The region affected by the recent earthquake has been one of the most seismically active in New Zealand over the past few years, including earthquakes that occurred as part of the Cook Strait earthquake sequence in 2013. It is likely that these sequences are related given their close spatial and temporal association.

What slipped during the earthquake?

The preliminary analysis strongly suggests that most of the energy release during this earthquake was sourced from the rupture of a roughly 200km-long fault system. This fault system is aligned northeast and dips to the northwest, beneath the northern part of the South Island. It coincides roughly with the subduction thrust in this area.

The potential for large earthquakes on the subduction fault in the lower North Island and upper South Island of New Zealand was recently highlighted by GNS Science, New Zealand’s geological survey. It published evidence for two similar events in the Blenheim area roughly 520-470 years ago, and 880-800 years ago.

Given its setting, this latest earthquake may be structurally complex, involving a mixture of plate boundary thrusting, lateral slip on strike-slip faults, and thrusting within the Pacific plate close to the epicentre, some 15km northeast of Culverden.

The largest aftershocks suggest a mixture of thrusting and strike-slip movements.

The damage caused by the earthquake

Because the fault system was large, and the earthquake apparently started at the southwest end of the fault and propagated to the northeast, the seismic energy was released over a period of up to two minutes.

Large earthquakes produce more long period wave energy than smaller events. The 2011 Christchurch earthquake contained a lot of high-frequency energy and very strong ground accelerations, exposing more than 300,000 people to very strong to intense ground shaking.

In contrast, this recent earthquake was manifested in Christchurch as lower-frequency rolling, and due to the sparse population density in the earthquake region, roughly 3,000 people in the upper South Island experienced strong ground shaking equivalent to the Christchurch earthquake.

Reports are emerging of at least one major fracture in the ground surface that could be related to strike-slip faulting in the Clarence region.

More traces may yet be found given the complexity of the earthquake. Tide gauge analysis will help to understand if a similar trace offshore caused the tsunami.

The earthquake has also triggered liquefaction in coastal areas and in susceptible sediments, and landsliding of up to a million cubic metres along steep susceptible cliffs in the northern South Island.

There are reports of extensive road damage including in the area between Hanmer Springs and Culverden, much of State Highway 1 and even Wellington, on the North Island.

Most of this damage is probably caused by strong ground shaking, which causes weak ground to move en masse and has resulted in numerous slips and road closures in the central and northern South Island.

Earthquakes, aftershocks and the pull of the moon

Given the earthquake happened on the eve of a supermoon full moon, and the closest the Earth and moon will be since 1948, it wasn’t long before some tried to make a connection.

But the tidal triggering of earthquakes has been investigated since the 19th century and remains a challenging and controversial field.

Small amplitude and large wavelength tidal deformations of the Earth due to motions of the sun and moon influence stresses in Earth’s lithosphere.

It is possible that, for active faults that are imminently close to brittle failure, small tidal force perturbations could be enough to advance rupture relative to the earthquake cycle, or to allow a propagating rupture to travel further than it might otherwise have done.

But the specific time, magnitude and location of this or any other large earthquake has not been successfully predicted in the short-term using tidal stresses or any other possible precursory phenomenon.

Deliberately vague predictions that provide no specific information about the precise location and magnitude of a future earthquake are not predictions at all. Rather, these are hedged bets that get media air time due to the romantic misinterpretation that they were valid predictions.

Most earthquake scientists, including those that research tidal triggering of earthquakes, highlight the importance of preparedness over attempts at prediction when it comes to public safety.

To this end, GNS Science uses a system of operational earthquake forecasts to communicate earthquake risk to concerned New Zealand residents during an aftershock sequence such as we are now entering.

These forecasts are based on earthquake physics and statistical seismology. The current operational forecast indicates an 80% probability of:

A normal aftershock sequence that is spread over the next few months. Felt aftershocks (e.g. M>5) would occur from the M7.5 epicentre near Culverden, right up along the Kaikoura coastline to Cape Campbell over the next few weeks and months.

This aftershock sequence will probably (98%) include several large aftershocks (some greater than magnitude 6 have already occurred), and for each magnitude 6 aftershock we expect 10 more magnitude 5 aftershocks over the coming days and weeks.

The Conversation

Brendan Duffy, Lecturer in Applied Geoscience, University of Melbourne and Mark Quigley, Associate professor, University of Melbourne

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

Italy’s deadly earthquake is the latest in a history of destruction


Brendan Duffy, University of Melbourne; Mark Quigley, University of Melbourne, and Mike Sandiford, University of Melbourne

The Appenines region of central Italy has been struck by a deadly earthquake, with a magnitude of 6.2. The quake, which had an epicentre roughly 10km southeast of Norcia, Italy, occurred just over seven years after the 2009 L’Aquila earthquake that killed more than 300 people only 90km away.

The latest earthquake occurred at 3:36 am local time. The number of fatalities is unknown at time of writing but already exceeds 30. Buildings have collapsed in nearby Amatrice and residents are reportedly trapped in rubble.

General tectonic setting of Italy, showing seismicity over the past 10 years from the USGS catalogue.
USGS

Fracture zone

This earthquake is no surprise. Italy is prone to earthquakes; it sits above the boundary of the African and European plates. The oceanic crust of the African plate is subducting (sinking) under Italy, creating iconic natural features such as the volcano at Mount Vesuvius. These plates are converging at a rate of around 5mm each year.

Both the L’Aquila and Norcia earthquakes were located below the central Appenines, which form the mountainous spine of Italy.

The Earth’s crust under the Appenines of central and western Italy is extending; eastern central Italy is moving to the north east relative to Rome. As a result, this region experiences normal faulting: where one part of the earth subsides relative to another as the crust is stretched.

The fault systems in the central Appenines are short and structurally complex, so the earthquakes are not large by global standards, the largest almost invariably hover around magnitude 6.8 to 7.0. But because the quakes are shallow and structurally complex, and because many of the local towns and cities contain vulnerable buildings, strong shaking from these earthquakes has the potential to inflict major damage and loss of life in urban areas.

This region also seems to be particularly prone to earthquake clustering, whereby periods of relative quiet are interrupted by several strong earthquakes over weeks to decades.

A history of quakes

Both Norcia and L’Aquila feature prominently at either end of a zone of large Appenine earthquakes. This zone has produced many strong earthquakes. The latest Norcia earthquake occurred only around 90km northwest of the L’Aquila earthquake and very close to the epicentre of the 1979 Norcia earthquake, which had a magnitude of 5.9.

But the area’s earthquake history can be traced back over seven centuries. During this period, this region has been hit by at least six earthquakes that have caused very strong to severe shaking. Amatrice, so badly damaged in the most recent quake, was severely damaged in 1639. A few decades later, in 1703, roughly 10,000 people were killed in Norcia, Montereale, L’Aquila and the surrounding Appenine region in three magnitude 6.2-6.7 earthquakes.

Parts of Norcia were subsequently built upon the surface rupture created in the 1703 earthquake. Another earthquake in 1997 killed 11 people.

In this most recent event, an estimated 13,000 people would have experienced severe ground shaking, probably lasting 10-20 seconds.

The estimated damage of this latest earthquake will almost inevitably exceed US$100 million, and may top US$1 billion. Amatrice appears to be among the populated areas that were most severely affected.

What lies ahead?

The region now faces a prolonged and energetic aftershock sequence; over the first 2.5 hours following the mainshock, at least four earthquakes of around magnitude 4.5 were recorded in the region by the US Geological Survey. More than 10,000 aftershocks were recorded following the L’Aquila earthquake in 2009.

We note that within the region, there is excellent and continuously improving scientific information about the hazard. But the knowledge of the hazard has not always translated well into measures that directly reduce economic loss and fatalities in earthquakes.

Following the L’Aquila earthquake, six scientists were convicted of manslaughter for failing to inform the public adequately of the earthquake risk. Although the charges were subsequently dropped, this marked a major development in the way blame is apportioned after large natural events, particularly with regard to effective hazard communication.

Numerous vulnerable buildings remain, and the recovery process is commonly plagued by long disruptions and inadequate government funding to recover rapidly. Both the 2009 L’Aquila earthquake and this most recent quake highlight just how important it is to translate hazard assessments into improving the resilience of infrastructure to strong shaking. The focus should remain on linking science, engineering and policy, this is often the biggest challenge globally.

The Conversation

Brendan Duffy, Lecturer in Applied Geoscience, University of Melbourne; Mark Quigley, Associate professor, University of Melbourne, and Mike Sandiford, Chair of Geology & Redmond Barry Distinguished Professor, University of Melbourne

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