̽��ֱ�� of Cambridge - tsunami

Opinion: ̽��ֱ��rapidly populating coastal region from the Gulf to Pakistan faces a huge tsunami risk

cjb250 — Fri, 07 Apr 2017 07:27:43 +0000

That tsunamis can cause death and devastation has become painfully clear over the past two decades. On Boxing Day, 2004, a magnitude 9 earthquake off the coast of Sumatra caused waves several metres high to devastate the Indian Ocean – killing more than 230,000 people in 14 countries. In 2011, another magnitude 9 earthquake, this time off Japan, produced waves up to 20 metres in height, flooding the Fukushima nuclear reactor. It killed more than 15,000 people.

A new study, published in Geophysical Journal International, by my colleagues and me suggests that a 1,000km long fault at the northern end of the Arabian Sea may pose a similar threat.

̽��ֱ��Makran, as the southern coastal region of Iran and Pakistan is known, is a subduction zone. In such regions, one of the Earth’s tectonic plates is dragged beneath another, forming a giant fault known as a “megathrust”. As the plates move past each other, they can get stuck, causing stress to build up. At some point the stress becomes high enough that the megathrust breaks in an earthquake.

This was exactly what caused the Sumatra 2004 and Tohoku 2011 earthquakes. When a megathrust moves suddenly, the whole seafloor is offset and the water has to move out of the way over a huge area. This sets off waves with particular characteristics that can cross entire oceans: tsunamis. ̽��ֱ��phenomenon, along with their potentially large size, makes subduction zone earthquakes particularly dangerous.

̽��ֱ��Makran region. Adapted from NASA photo.

But just because a part of a subduction zone produces earthquakes doesn’t mean that the whole megathrust can move in one go. We often see that stress builds up at different rates on different parts of the fault, with some parts sliding smoothly past each other. How much of a megathrust can move in one go is important because it determines the size of the resulting earthquake. ̽��ֱ��amount that the Makran megathrust can move in earthquakes has been a longstanding question, but the hostile climate and challenging politics of the region have made research there difficult.

We know that the eastern part of the Makran megathrust (in Pakistan) can produce large earthquakes. A magnitude 8.1 quake off the coast of western Pakistan in 1945 caused a tsunami which killed about 300 people along the coasts of Pakistan and Oman. There have been several smaller earthquakes on the megathrust since, including a magnitude 6 in February this year.

If the western part of the Makran (in Iran) also produces earthquakes – and the whole Makran megathrust were to move in one go – it could produce a magnitude 9 earthquake, similar to those in Sumatra and Tohoku.

However, we have never actually recorded a subduction earthquake in this part of Makran. In fact, there are only records of one candidate quake from 1483 – and the actual location of this is disputed. But it’s important to keep in mind that just because we haven’t seen an earthquake doesn’t mean that there couldn’t be one – particularly since the intervals between earthquakes are often hundreds or thousands of years. Historically, not many people have lived in the remote Iranian Makran, a desert which killed Alexander the Great’s army. So earthquakes might simply not have been documented.

GPS data

We used new data to look for tell-tale signs of a possible earthquake. Imagine a piece of paper on a table. If you hold one end and push the other end towards it, the paper crumples up and the distance between the two ends gets shorter. If you let go, the paper flattens out. ̽��ֱ��fixed end is like a megathrust which is stuck. Indeed, if the Arabian plate is stuck, and stress is building up, southern Iran will be squeezed and shortened. We can look for evidence of this shortening by using a more accurate version of the GPS systems found in smartphones. My coauthors from the National Cartographic Centre in Iran have set up a network of GPS stations to measure how fast different parts of Iran are moving relative to Arabia.

We found that the velocities fit with Iran being shortened near the coast, suggesting that stress is indeed building up – and meaning there could be a large subduction earthquake in the future. This fits with recent work looking at large boulders along the coast of Oman, thought to have been deposited by tsunamis. ̽��ֱ��locations of these boulders suggest that the tsunami which brought them there would need to have come from a subduction earthquake, either in western Makran or along the entire subduction zone – including Pakistan. These boulders were probably deposited in the last 5,000 years, but we can’t know for sure.

̽��ֱ��2004 Sumatra tsunami strikes Ao Nang, Thailand. David Rydevik/wikipedia, CC BY-SA

This is a hazard that people need to be aware of, particularly those living in coastal regions around the Arabian Sea. Rapid urbanisation along the Omani and Pakistani coasts in recent years has increased the population exposed to earthquakes and tsunamis in the Makran. Karachi, at the eastern end of the subduction zone, is now a megacity and home to around 25m people. Much of Muscat, the Omani capital, is less than 10 metres above sea level, making it vulnerable to tsunamis. ̽��ֱ��port of Gwadar in Pakistan, which was badly damaged in a 1945 earthquake, is also undergoing massive development.

To help protect these people, and make sure that they are properly prepared, we need to understand this hazard better. Education and early warning are both key – exercises testing the Indian Ocean Tsunami Warning System are a step in the right direction, especially if they engage the public.

At the moment, we can only say that a large earthquake in the Makran is consistent with the limited data which we have available. By continuing to work with scientists in Iran and Pakistan to make more measurements I hope that in the future we will have a much better idea of what to expect from this subduction zone.

Camilla Penney, PhD Candidate in Geophysics, ̽��ֱ�� of Cambridge

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

In recent years, tsunamis have devastated coastal regions. Writing in ̽��ֱ��Conversation, Camilla Penney, PhD Candidate in Geophysics at ̽��ֱ�� of Cambridge, looks at the risks faced by Gulf states and what can be done to mitigate them.

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Scientists explain scale of Japanese tsunami

amb206 — Fri, 24 Aug 2012 09:35:54 +0000

Scientists at Cambridge ̽��ֱ�� have developed a model that may show why some tsunamis – including the one that devastated Japan in March 2011 – are so much larger than expected. ̽��ֱ��Japanese tsunami baffled the world’s experts as it was far bigger than might have been anticipated from what is known about the deep sea earthquakes that create long waves out in the ocean.

In a paper published today (24 August 2012) in the journal Earth and Planetary Science Letters, Professors Dan McKenzie and James Jackson of Cambridge’s Department of Earth Sciences describe for the first time the added factor that may have made this tsunami so severe: a huge collapse of soft material on the sea bed resulted in a far greater movement of water than would have been caused by the earthquake alone.

Tsunamis occur when an earthquake rapidly changes the shape of the sea floor, displacing the water above it. ̽��ֱ��earthquake itself is the abrupt rupture of a fault surface separating rocks that have steadily been bending like a loaded spring, before suddenly overcoming friction and slipping, releasing the elastic energy. In the case of the Japan earthquake, the fault is the plate boundary, allowing the Pacific sea floor to slide beneath Japan. ̽��ֱ��wave formed at the sea surface as the sea floor moves can cause untold damage when it hits shore.

“As the plates move against each other, the rocks on their boundaries slowly bend under the pressure, until they eventually crack and slide on faults. When they do, there is an upwards and outwards movement that takes just a few seconds: a movement of 10 metres is a large earthquake and out at sea this causes a tsunami,” said Professor Jackson.

“But data from the Japanese earthquake show a movement of more than 60 metres. Rocks can bend - but they cannot bend to that extent and, anyway, the rocks that moved were sloppy sediments with little strength. This suggests that something else was taking place to increase the movement several fold. It was this massive movement that caused the tsunami that swamped the coast of Japan and beyond with such terrible consequences.”

Important advances in technologies for monitoring movements on the sea bed, plus a huge investment by the Japanese government, mean that the world’s scientific community has access to an unprecedented level of data about what happened in March 2011 some seven km under the sea and around 70 km off the coast of Japan.

By interpreting data gathered in the lead-up to and aftermath of the Japanese tsunami, as well as during the event itself, the Cambridge scientists have shown that the squeezing together of two plates in the earth’s crust not only resulted in a fracture but also caused a massive collapse of the debris that had built up on the sea bed as tectonic movements scraped loose sediment into an unstable wedge.

“When the wedge of material collapsed, the leading edge split off and shot forward a bit like a pip shooting out of a giant pair of tongs. In essence, what happened was a release of both the elastic energy stored in the rocks and the gravitational energy contained in the wedge-shaped build-up of debris,” said Professor Jackson.

̽��ֱ��extra movement of the sea bed at the toe of the wedge enhanced the shape of the huge wave created at the surface of the sea, which travelled towards Japan.

̽��ֱ��research throws a light on other unusually large tsunamis that have long puzzled scientists, including those that struck Nicaragua in 1992, Sumatra in 2004, and Java in 2006. A comparison of data from these events with that from the recent Japanese tsunami reveals that they have much in common, strongly suggesting that these disasters too occurred as a result of the release of gravitational as well as elastic energy.

“These events share a number of unusual features, including large displacements, suggesting that they resulted partly from the collapse of debris. We hope that our research represents a step forward in understanding how large tsunamis occur and in what circumstances they are likely to happen,” said Professor Jackson.

Tsunamis are caused by earthquakes under the seabed. Some tsunamis – including the disaster that hit Japan last year – are unexpectedly large. Cambridge scientists suggest that their severity is caused by a release of gravitational energy as well as elastic energy.

We hope that our research represents a step forward in understanding how large tsunamis occur.

Professor James Jackson

REUTERS/Mainichi Shimbun

A wave approaches Miyako City from the Heigawa estuary in Iwate Prefecture

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