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.

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.

Of the many devastating pictures to come out of central Italy after last week’s deadly earthquake, the clock tower of Amatrice standing defiantly amid the rubble of the town has become an iconic image.

The clock tower was reportedly built in the 13th century and its solid stance defies us to understand how this remarkable structure has evaded destruction at least twice in the past 800 years.

But perhaps surprisingly, it’s not unusual for tall, ancient structures to survive earthquakes.

Unlikely survivors

Similar towers are relatively commonplace in Italy and part of the country’s charm. The town of San Gimignano, about 200km from the centre of the Amatrice earthquake, has 14 towers that date as far back as the 12th century – and have consequently survived many earthquakes big and small. Other towers can be seen in Alba in northern Italy.

Further afield, a memorable image of the Izmit earthquake in Turkey in 1999 was of the tower of the Golcuk Mosque standing forlornly among the ruins.

Photos from the 1906 San Francisco earthquake show a slender tower and an array of chimneys standing in the rubble of the city.

In many instances, however, the towers fall, as happened to the Dharahara tower during the magnitude-7.8 Nepal earthquake in April 2015.

Why do some of these slender icons survive repeated earthquakes and others fall? An article in The Economist suggested that the clock tower was better constructed than the surrounding buildings, pointing out that it even survived better than a modern school and hospital. The L'Aquila experience suggests that this is probably one part of the story.

However, the reality is more complex. Other factors can and do contribute to the resilience of buildings.

On shaky ground

It is very likely that the clock tower’s survival was influenced by the relationship between the frequency of the earthquake waves and the natural resonance of the building. To understand why, we have to consider how earthquakes interact with buildings.

Earthquakes generate seismic waves that pass through the ground. Like ocean waves, they have peaks and troughs. The frequency of the wave is related to its “period” – the time taken for one complete waveform (including a peak and a trough) to pass.

A building has a natural period that causes it to vibrate back and forth. Think of a child on a swing – a swing with short ropes will complete a full cycle much more quickly than a long swing.

The same is true of buildings with different heights. A building is effectively an upside-down pendulum and taller buildings have longer natural periods of oscillation (swinging back and forth).

The ground also has a preferred period at which it oscillates. Soft sediment in a river valley will oscillate over longer periods, and hard bedrock over shorter ones.

High-frequency (short period) earthquake waves are therefore amplified in bedrock, such as the site of Amatrice, and are the dominant frequency radiated by small to moderate and shallow earthquakes such as last week’s.

Low-frequency (long period) earthquake waves are amplified in sediment and form a greater part of the seismic energy radiated by larger earthquakes, such as the Tohuku earthquake in Japan and the Nepal quake that felled the Dharahara tower.

When the resonant frequency of the ground coincides with the resonant frequency of the building, the structure will undergo its largest possible oscillations and suffer the greatest damage. The rigidity and distribution of mass along the height of a building also have a big effect on the likely damage sustained in a given earthquake, as this governs the way the induced forces are distributed.

You can try this for yourself by experimenting with a broom handle and a 30cm ruler. Held vertically, the top of the broom handle will do little if you vigorously shake its base with small movements, whereas the ruler will oscillate under the same shaking.

Slow the shaking down and the handle will begin to whip back and forth while the ruler settles down. Place a large mass on the end of either the ruler or the broom handle and the characteristics will change.

The concept is beautifully demonstrated in a video by Robert Butler of the University of Oregon.

A resonant problem

Of course, real structures and real earthquakes are far more complex. Real structures have many natural frequencies, and earthquakes vibrate across a spread (or spectrum) of frequencies.

Destruction occurs when any of a buildings’s natural frequencies coincide with any of the dominant frequencies of the earthquake. In some situations, there may be just a few structures that avoid this dangerous combination, such as the clock tower at Amatrice, or the chimneys of San Francisco.

The characteristics of shaking at Amatrice have not yet been published, but it is highly likely that the tower is standing not only because it was built well in the first instance, but also because it is just the right size and shape to survive the frequency of shaking that occurs during Italy’s moderate-magnitude earthquakes.

This process is equally important in other regions. The magnitude-6.8 Myanmar earthquake on August 24 damaged many historic temples in the Irrawaddy Valley, but none appears to have collapsed. These high-but-squat structures are susceptible to high-frequency shaking, whereas the passage of earthquake waves through alluvium is likely to have amplified mainly low-frequency earthquake waves.

Notably, much of the damage to the temples seems to have occurred as a result of the collapse of recent cheap “restorations”.

Building practices are extremely important in mitigating the effect of shaking on buildings. Modern buildings are commonly fitted with devices to reduce the effects of resonance. Engineered solutions are available to retrospectively enhance the performance of unreinforced masonry buildings, with little impact on their aesthetics.

In Italy, this retrofitting needs to be done as quickly as possible before the next earthquake. This will be a costly exercise. Even apparently resilient medieval towers may require retrofits, because they have commonly accumulated a degree of damage.

However, Italy is a globally important cultural and tourism hub, and her earthquake-prone buildings, like those in Myanmar, are part of our collective heritage. Italy should not be left to struggle alone with the management of earthquake-prone building hazards.

Brendan Duffy, Lecturer in Applied Geoscience, University of Melbourne; Colin Caprani, Lecturer, Structural Engineering, Monash University, and Mark Quigley, Associate professor, University of Melbourne

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

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 100. Buildings have collapsed in nearby Amatrice and residents are reportedly trapped in rubble.

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.

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.