Subtle shifts in the Earth could forecast earthquakes, tsunamis

University of South Florida graduate student Jacob Richardson stands beside a completed installation.  The large white disc is the dual frequency antenna.  A portable solar panel that powers the system is visible in the foreground. -  Photo by Denis Voytenko
University of South Florida graduate student Jacob Richardson stands beside a completed installation. The large white disc is the dual frequency antenna. A portable solar panel that powers the system is visible in the foreground. – Photo by Denis Voytenko

Earthquakes and tsunamis can be giant disasters no one sees coming, but now an international team of scientists led by a University of South Florida professor have found that subtle shifts in the earth’s offshore plates can be a harbinger of the size of the disaster.

In a new paper published today in the Proceedings of the National Academies of Sciences, USF geologist Tim Dixon and the team report that a geological phenomenon called “slow slip events” identified just 15 years ago is a useful tool in identifying the precursors to major earthquakes and the resulting tsunamis. The scientists used high precision GPS to measure the slight shifts on a fault line in Costa Rica, and say better monitoring of these small events can lead to better understanding of maximum earthquake size and tsunami risk.

“Giant earthquakes and tsunamis in the last decade – Sumatra in 2004 and Japan in 2011 – are a reminder that our ability to forecast these destructive events is painfully weak,” Dixon said.

Dixon was involved in the development of high precision GPS for geophysical applications, and has been making GPS measurements in Costa Rica since 1988, in collaboration with scientists at Observatorio Vulcanológico y Sismológico de Costa Rica, the University of California-Santa Cruz, and Georgia Tech. The project is funded by the National Science Foundation.

Slow slip events have some similarities to earthquakes (caused by motion on faults) but release their energy slowly, over weeks or months, and cannot be felt or even recorded by conventional seismographs, Dixon said. Their discovery in 2001 by Canadian scientist Herb Dragert at the Pacific Geoscience Center had to await the development of high precision GPS, which is capable of measuring subtle movements of the Earth.

The scientists studied the Sept. 5, 2012 earthquake on the Costa Rica subduction plate boundary, as well as motions of the Earth in the previous decade. High precision GPS recorded numerous slow slip events in the decade leading up to the 2012 earthquake. The scientists made their measurements from a peninsula overlying the shallow portion of a megathrust fault in northwest Costa Rica.

The 7.6-magnitude quake was one of the strongest earthquakes ever to hit the Central American nation and unleased more than 1,600 aftershocks. Marino Protti, one of the authors of the paper and a resident of Costa Rica, has spent more than two decades warning local populations of the likelihood of a major earthquake in their area and recommending enhanced building codes.

A tsunami warning was issued after the quake, but only a small tsunami occurred. The group’s finding shed some light on why: slow slip events in the offshore region in the decade leading up to the earthquake may have released much of the stress and strain that would normally occur on the offshore fault.

While the group’s findings suggest that slow slip events have limited value in knowing exactly when an earthquake and tsunami will strike, they suggest that these events provide critical hazard assessment information by delineating rupture area and the magnitude and tsunami potential of future earthquakes.

The scientists recommend monitoring slow slip events in order to provide accurate forecasts of earthquake magnitude and tsunami potential.

###

The authors on the paper are Dixon; his former graduate student Yan Jiang, now at the Pacific Geoscience Centre in British Columba, Canada; USF Assistant Professor of Geosciences Rocco Malservisi; Robert McCaffrey of Portland State University; USF doctoral candidate Nicholas Voss; and Protti and Victor Gonzalez of the Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional.

The University of South Florida is a high-impact, global research university dedicated to student success. USF is a Top 50 research university among both public and private institutions nationwide in total research expenditures, according to the National Science Foundation. Serving nearly 48,000 students, the USF System has an annual budget of $1.5 billion and an annual economic impact of $4.4 billion. USF is a member of the American Athletic Conference.

Offshore islands amplify, rather than dissipate, a tsunami’s power

This model shows the impact of coastal islands on a tsunami's height. -  Courtesy of Jose Borrero/eCoast/USC
This model shows the impact of coastal islands on a tsunami’s height. – Courtesy of Jose Borrero/eCoast/USC

A long-held belief that offshore islands protect the mainland from tsunamis turns out to be the exact opposite of the truth, according to a new study.

Common wisdom — from Southern California to the South Pacific — for coastal residents and scientists alike has long been that offshore islands would create a buffer that blocked the power of a tsunami. In fact, computer modeling of tsunamis striking a wide variety of different offshore island geometries yielded no situation in which the mainland behind them fared better.

Instead, islands focused the energy of the tsunami, increasing flooding on the mainland by up to 70 percent.

“This is where many fishing villages are located, behind offshore islands, in the belief that they will be protected from wind waves. Even Southern California residents believe that the Channel Islands and Catalina will protect them,” said Costas Synolakis of the USC Viterbi School of Engineering, a member of the multinational team that conducted the research.

The research was inspired by a field survey of the impact of the 2010 tsunami on the Mentawai Islands off of Sumatra. The survey data showed that villages located in the shadow of small offshore islets suffered some of the strongest tsunami impacts, worse than villages located along open coasts.

Subsequent computer modeling by Jose Borrero, adjunct assistant research professor at the USC Viterbi Tsunami Research Center, showed that the offshore islands had actually contributed to — not diminished — the tsunami’s impact.

Synolakis then teamed up with researchers Emile Contal and Nicolas Vayatis of Ecoles Normales de Cachan in Paris; and Themistoklis S. Stefanakis and Frederic Dias, who both have joint appointments at Ecoles Normales de Cachan and University College Dublin to determine whether that was a one-of-a-kind situation, or the norm.

Their study, of which Dias was the corresponding author, was published in Proceedings of the Royal Society A on Nov. 5.

The team designed a computer model that took into consideration various island slopes, beach slopes, water depths, distance between the island and the beach, and wavelength of the incoming tsunami.

“Even a casual analysis of these factors would have required hundreds of thousands of computations, each of which could take up to half a day,” Synolakis said. “So instead, we used machine learning.”

Machine learning is a mathematical process that makes it easier to identify the maximum values of interdependent processes with multiple parameters by allowing the computer to “learn” from previous results.

The computer starts to understand how various tweaks to the parameters affect the overall outcome and finds the best answer quicker. As such, results that traditionally could have taken hundreds of thousands of models to uncover were found with 200 models.

“This work is applicable to some of our tsunami study sites in New Zealand,” said Borrero, who is producing tsunami hazard maps for regions of the New Zealand coast. “The northeast coast of New Zealand has many small islands offshore, similar to those in Indonesia, and our modeling suggests that this results in areas of enhanced tsunami heights.”

“Substantial public education efforts are needed to help better explain to coastal residents tsunami hazards, and whenever they need to be extra cautious and responsive with evacuations during actual emergencies,” Synolakis said.

###

The research was funded by EDSP of ENS-Cachan; the Cultural Service of the French Embassy in Dublin; the ERC; SFI; University College Dublin; and the EU FP7 program ASTARTE. The study can be found online at http://rspa.royalsocietypublishing.org/content/470/2172/20140575.

Scientists reconstruct ancient impact that dwarfs dinosaur-extinction blast

A graphical representation of the size of the asteroid thought to have killed the dinosaurs, and the crater it created, compared to an asteroid thought to have hit the Earth 3.26 billion years ago and the size of the crater it may have generated. A new study reveals the power and scale of the event some 3.26 billion years ago which scientists think created geological features found in a South African region known as the Barberton greenstone belt. -  American Geophysical Union
A graphical representation of the size of the asteroid thought to have killed the dinosaurs, and the crater it created, compared to an asteroid thought to have hit the Earth 3.26 billion years ago and the size of the crater it may have generated. A new study reveals the power and scale of the event some 3.26 billion years ago which scientists think created geological features found in a South African region known as the Barberton greenstone belt. – American Geophysical Union

Picture this: A massive asteroid almost as wide as Rhode Island and about three to five times larger than the rock thought to have wiped out the dinosaurs slams into Earth. The collision punches a crater into the planet’s crust that’s nearly 500 kilometers (about 300 miles) across: greater than the distance from Washington, D.C. to New York City, and up to two and a half times larger in diameter than the hole formed by the dinosaur-killing asteroid. Seismic waves bigger than any recorded earthquakes shake the planet for about half an hour at any one location – about six times longer than the huge earthquake that struck Japan three years ago. The impact also sets off tsunamis many times deeper than the one that followed the Japanese quake.

Although scientists had previously hypothesized enormous ancient impacts, much greater than the one that may have eliminated the dinosaurs 65 million years ago, now a new study reveals the power and scale of a cataclysmic event some 3.26 billion years ago which is thought to have created geological features found in a South African region known as the Barberton greenstone belt. The research has been accepted for publication in Geochemistry, Geophysics, Geosystems, a journal of the American Geophysical Union.

The huge impactor – between 37 and 58 kilometers (23 to 36 miles) wide – collided with the planet at 20 kilometers per second (12 miles per second). The jolt, bigger than a 10.8 magnitude earthquake, propelled seismic waves hundreds of kilometers through the Earth, breaking rocks and setting off other large earthquakes. Tsunamis thousands of meters deep – far bigger than recent tsunamis generated by earthquakes — swept across the oceans that covered most of the Earth at that time.

“We knew it was big, but we didn’t know how big,” Donald Lowe, a geologist at Stanford University and a co-author of the study, said of the asteroid.

Lowe, who discovered telltale rock formations in the Barberton greenstone a decade ago, thought their structure smacked of an asteroid impact. The new research models for the first time how big the asteroid was and the effect it had on the planet, including the possible initiation of a more modern plate tectonic system that is seen in the region, according to Lowe.

The study marks the first time scientists have mapped in this way an impact that occurred more than 3 billion years ago, Lowe added, and is likely one of the first times anyone has modeled any impact that occurred during this period of the Earth’s evolution.

The impact would have been catastrophic to the surface environment. The smaller, dino-killing asteroid crash is estimated to have released more than a billion times more energy than the bombs that destroyed Hiroshima and Nagasaki. The more ancient hit now coming to light would have released much more energy, experts said.

The sky would have become red hot, the atmosphere would have been filled with dust and the tops of oceans would have boiled, the researchers said. The impact sent vaporized rock into the atmosphere, which encircled the globe and condensed into liquid droplets before solidifying and falling to the surface, according to the researchers.

The impact may have been one of dozens of huge asteroids that scientists think hit the Earth during the tail end of the Late Heavy Bombardment period, a major period of impacts that occurred early in the Earth’s history – around 3 billion to 4 billion years ago.

Many of the sites where these asteroids landed were destroyed by erosion, movement of the Earth’s crust and other forces as the Earth evolved, but geologists have found a handful of areas in South Africa, and Western Australia that still harbor evidence of these impacts that occurred between 3.23 billion and 3.47 billion years ago. The study’s co-authors think the asteroid hit the Earth thousands of kilometers away from the Barberton Greenstone Belt, although they can’t pinpoint the exact location.

“We can’t go to the impact sites. In order to better understand how big it was and its effect we need studies like this,” said Lowe. Scientists must use the geological evidence of these impacts to piece together what happened to the Earth during this time, Lowe said.

The study’s findings have important implications for understanding the early Earth and how the planet formed. The impact may have disrupted the Earth’s crust and the tectonic regime that characterized the early planet, leading to the start of a more modern plate tectonic system, according to the paper’s co-authors.

The pummeling the planet endured was “much larger than any ordinary earthquake,” said Norman Sleep, a physicist at Stanford University and co-author of the study. He used physics, models, and knowledge about the formations in the Barberton greenstone belt, other earthquakes and other asteroid impact sites on the Earth and the moon to calculate the strength and duration of the shaking that the asteroid produced. Using this information, Sleep recreated how waves traveled from the impact site to the Barberton greenstone belt and caused the geological formations.

The geological evidence found in the Barberton that the paper investigates indicates that the asteroid was “far larger than anything in the last billion years,” said Jay Melosh, a professor at Purdue University in West Lafayette, Indiana, who was not involved in the research.

The Barberton greenstone belt is an area 100 kilometers (62 miles) long and 60 kilometers (37 miles) wide that sits east of Johannesburg near the border with Swaziland. It contains some of the oldest rocks on the planet.

The model provides evidence for the rock formations and crustal fractures that scientists have discovered in the Barberton greenstone belt, said Frank Kyte, a geologist at UCLA who was not involved in the study.

“This is providing significant support for the idea that the impact may have been responsible for this major shift in tectonics,” he said.

Reconstructing the asteroid’s impact could also help scientists better understand the conditions under which early life on the planet evolved, the paper’s authors said. Along with altering the Earth itself, the environmental changes triggered by the impact may have wiped out many microscopic organisms living on the developing planet, allowing other organisms to evolve, they said.

“We are trying to understand the forces that shaped our planet early in its evolution and the environments in which life evolved,” Lowe said.

Study uncovers new evidence for assessing tsunami risk from very large volcanic island landslides

A core is extracted from the seabed. -  Russell Wynn
A core is extracted from the seabed. – Russell Wynn

The risk posed by tsunami waves generated by Canary Island landslides may need to be re-evaluated, according to researchers at the National Oceanography Centre. Their findings suggest that these landslides result in smaller tsunami waves than previously thought by some authors, because of the processes involved.

The researchers used the geological record from deep marine sediment cores to build a history of regional landslide activity over the last 1.5 million years. They found that each large-scale landslide event released material into the ocean in stages, rather than simultaneously as previously thought.

The findings – reported recently in the scientific journal Geochemistry Geophysics Geosystems – can be used to inform risk assessment from landslide-generated tsunamis in the area, as well as mitigation strategies to defend human populations and infrastructure against these natural hazards. The study also concluded that volcanic activity could be a pre-condition to major landslide events in the region.

The main factor influencing the amplitude of a landslide-generated tsunami is the volume of material sliding into the ocean. Previous efforts, which have assessed landslide volumes, have assumed that the entire landslide volume breaks away and enters the ocean as a single block. Such studies – and subsequent media coverage – have suggested an event could generate a ‘megatsunami’ so big that it would travel across the Atlantic Ocean and devastate the east coast of the US, as well as parts of southern England.

The recent findings shed doubt on this theory. Instead of a single block failure, the landslides in the past have occurred in multiple stages, reducing the volumes entering the sea, and thereby producing smaller tsunami waves. Lead author Dr James Hunt explains: “If you drop a block of soap into a bath full of water, it makes a relatively big splash. But if you break it up into smaller pieces and drop it in bit by bit, the ripples in the bath water are smaller.”

The scientists were able to identify this mechanism from the deposits of underwater sediment flows called turbidity currents, which form as the landslide mixes with surrounding seawater. Their deposits, known as ‘turbidites’, were collected from an area of the seafloor hundreds of miles away from the islands. Turbidites provide a record of landslide history because they form from the material that slides down the island slopes into the ocean, breaks up and eventually settles on this flatter, deeper part of the seafloor.

However, the scientists could not assume that multistage failure necessarily results in less devastating tsunamis – the stages need to occur with enough time in between so that the resulting waves do not compound each other. “If you drop the smaller pieces of soap in one by one but in very quick succession, you can still generate a large wave,” says Dr Hunt.

Between the layers of sand deposited by the landslides, the team found mud, providing evidence that the stages of failure occurred some time apart. This is because mud particles are so fine that they most likely take weeks to settle out in the ocean, and even longer to form a layer that would be resistant enough to withstand a layer of sand moving over the top of it.

While the authors suggest that the tsunamis were not as big as originally thought, they state that tsunamis are a threat that the UK should be taking seriously. The Natural Environment Research Council (NERC) is investing in a major programme looking at the risk of tsunamis from Arctic landslides as part of the Arctic Research Programme, of which NOC is the lead collaborator. The EU have also just funded a £6 million FP7 project called ASTARTE, looking at tsunami risk and resilience on the European North Atlantic and Mediterranean coasts, of which NOC is a partner.

The current study was funded by NERC, through a NOC studentship.

Breaking the rules for how tsunamis work

The earthquake zones off of certain coasts-like those of Japan and Java-make them especially vulnerable to tsunamis, according to a new study. They can produce a focusing point that creates massive and devastating tsunamis that break the rules for how scientists used to think tsunamis work.

Until now, it was largely believed that the maximum tsunami height onshore could not exceed the depth of the seafloor. But new research shows that when focusing occurs, that scaling relationship breaks down and flooding can be up to 50 percent deeper with waves that do not lose height as they get closer to shore.

“It is as if one used a giant magnifying lens to focus tsunami energy,” said Utku Kanoglu, professor at the Middle East Technical University and senior author of the study. “Our results show that some shorelines with huge earthquake zones just offshore face a double whammy: not only they are exposed to the tsunamis, but under certain conditions, focusing amplifies these tsunamis far more than shoaling and produces devastating effects.”

The team observed this effect both in Northern Japan, which was struck by the Tohoku tsunami of 2011, and in Central Java, which was struck by a tsunami in 2006.

“We are still trying to understand the implications,” said Costas Synolakis, director of the Tsunami Research Center at the USC Viterbi School of Engineering and a co-author of the study. “But it is clear that our findings will make it easier to identify locales that are tsunami magnets, and thus help save lives in future events.”

During an earthquake, sections of the sea floor lift up while others sink. This creates tsunamis that propagate trough-first in one direction and crest-first in the other. The researchers discovered that on the side of the earthquake zone where the wave propagates trough-first, there is a location where focusing occurs – strengthening it before it hits the coastline with an unusual amount of energy that is not seen by the crest-first wave. Based on the shape, location, and size of the earthquake zone, that focal point can concentrate the tsunami’s power right on to the coastline.

In addition, before this analysis, it was thought that tsunamis usually decrease in height continuously as they move away from where they are created and grow close to shore, just as wind waves do. The study’s authors instead suggest that the crest of the tsunami remains fairly intact close to the source.

“While our study does not preclude that other factors may help tsunamis overgrow, we now know when to invoke exotic explanations for unusual devastation: only when the basic classic wave theory we use does not predict focusing, or if the focusing is not high enough to explain observations,” said Vasily Titov, a researcher at NOAA’s Pacific Marine Environmental Laboratory and study co-author.

Tsunami caused long-term ecosystem change in the Caribbean

A detailed analysis of sediments from the island of Bonaire in the Caribbean presents convincing evidence for an extraordinary wave impact dating back some 3,300 years, even though no historical records of tsunamis exist for this island. Of particular interest are the consequences this large wave impact had on the island’s ecosystem. The sediments studied by the scientists suggested that this tsunami entirely changed the coastal ecosystem and sedimentation patterns in the area. The work by Dr. Max Engel and colleagues, from the University of Köln in Germany, is published online in Springer’s journal, Naturwissenschaften – The Science of Nature.

The Caribbean region is highly vulnerable to coastal hazards, including tropical cyclones, earthquakes, volcanoes, and tsunamis. Even though the island of Bonaire has not experienced a tsunami during the past 500 years, which is the period of historical documentation, overwash deposits from a coastal lagoon provide evidence for at least one such event in prehistory.

Engel and colleagues investigated sediment cores from Washington-Slagbaai National Park. They looked specifically at grain size distribution, carbonate content, organic matter, magnetic susceptibility and fauna. Their analyses showed that the sediments had criteria typically linked with tsunami deposits, consistent with a tsunami with a maximum age of 3,300 years.

The authors conclude: “This single catastrophic event is of long-term ecological significance. Formation of a barrier of coral rubble was triggered by the tsunami separating a former inland bay from the open sea and turning it into a highly saline lagoon which persists until today. Further studies of the geology of tsunamis, using well-dated deposits, are required over the entire Caribbean to reconstruct reliable patterns of magnitude, frequency and spatial occurrence of tsunami events and their environmental impact.”

Unearthing clues of catastrophic earthquakes


‘An inviting tale of destruction’



The destruction and disappearance of ancient cultures mark the history of human civilization, making for fascinating stories and cautionary tales. The longevity of today’s societies may depend upon separating fact from fiction, and archeologists and seismologists are figuring out how to join forces to do just that with respect to ancient earthquakes, as detailed in new studies presented at the international conference of the Seismological Society of America.



“It’s an idea whose time has come, ” said Robert Kovach, professor of geophysics at Stanford University and a leading proponent that seismology needs to be included in any framework for understanding what happened to past civilizations. Very large earthquakes may have recurrence rates that exceed 500 years, making it very difficult to assign potential hazard estimates.



Archaeoseismology, a young scientific discipline that studies past earthquakes in the archaeological record, allows scientists to broaden the time window to detect these rare seismic catastrophic events. But archaeological evidence for past earthquakes raises a lot of reservations from seismologists, some of them strongly questioning whether man-made structures can be used as earthquake indicators at all.



Controversy stems from what is seen by some seismologists as haphazard blame placed on earthquakes by archaeologists for inexplicable phenomena on an archaeological site, adding drama to the site’s history. “We need to be wary of circular reasoning” said Tina Niemi, a geologist at the University of Missouri-Kansas City, who noted the temptation to assign evidence to match a preconceived notion that an earthquake may have caused damage.



“We are indeed at a turning point with respect to archaeoseismology — either earthquake evidence in archaeological sites remains in a world of conjecture and drama or a more objective and quantitative approach gets the upper hand,” said Manuel Sintubin, professor of geodynamics at Katholieke Universiteit Leuven in Belgium.



Earlier this month UNESCO awarded a five-year grant to Sintubin and his colleagues Niemi; Iain Stewart, geologist at University of Plymouth in the United Kingdom; and Erhan Altunel, geologist at the Eskisehir Osmangazi University in Turkey, to support archaeoseismology by broadening the field’s primary focus from the Near East to include the Far East.


“The importance of this effort is to create a long-term, worldwide platform for a broad multidisciplinary discussion on archaeoseismology. Our final objective is to assure that archaeoseismology will be considered as a legitimate and complementary source of seismic-hazard information.”



There is still much to be known about ancient earthquakes. The instrumental record for seismology is short, going back 100 years. The historical seismology record is a much longer, including written documentation such as news accounts and diaries, which vary widely by culture and region. The archeoseismic record serves as the bridge between historical accounts and the paleoseismic record of Earth’s history.



“It’s important to society to understand the risks posed by earthquakes with longer repeating cycles,” said Kovach. “Unless the world was drastically different than today, then it’s inconceivable that earthquakes did not play a role in the past to affect the cultures that occupied the land along the faults, some of which we do not even know of yet,” said Kovach.



Seismologists look for evidence that suggest an earthquake’s footprint. Sintubin and Niemi cite three distinct types of evidence: faulted and displaced archaeological relics, or “cultural piercing features”; ground-shaking induced damage to buildings and damage induced by secondary phenomena, such as tsunamis; and archaeological evidence, such as repairs to man-made structures.



Kovach looks at the issue of water, such as the damming of rivers and changing elevation of coasts. His research has focused on Banbhore, which is an inland city that was once the ancient coastal city of Debal, the gateway for Islam’s advent in the Indian subcontinent. According to Kovach, the site has witnessed at least four distinct Muslim occupations and three successive reconstructions that correlate to the written record by Arab historians. “There are numerous examples in the Indus Valley that earthquakes did affect the occupying history of these sites,” said Kovach. Today, most of Pakistan and the western states of India occupy the ancient Indus Valley, which experienced the earthquakes that, according to Kovach, altered the course of civilization there over the past millennium.



Sintubin and Stewart are proposing a standardized method to study an archaeological site with the purpose of identifying ancient earthquakes and to evaluate existing archaeoseismological data. The research is currently in process for publication by the Bulletin of the Seismological Society of America. Called the Archeological Quality Factor, or AQF, this proposed evaluative approach would document a degree of certainty of an ancient earthquake recorded at a site. According to Sintubin, the approach reveals the weaknesses in any earthquake hypothesis at a site and constitutes a significant step in the overall acknowledgement of archaeoseismology as a scientific discipline. Sintubin applied the method to research conducted at an excavation in Turkey. The resulting AQF (~5%) turns out to support with some certainty the hypothesis that the region has been struck in the 7th century AD by a previously unknown major earthquake.



While some remain cautious, others are eager to refine the role of earthquakes on past cultures. “A lot can be gleaned from going back to look at old reports,” said Kovach. “Past earthquakes have left an inviting tale of destruction.”



Archaeoseismological Methodologies: Principles and Practices, SSA Annual Convention, 1:30 – 5 PM, Wednesday, 16 April, in the Hilton Hotel, Mesa C