Among the surprises is data indicating that a secondary underwater movement amplified the original tsunami to create a wave run-up more than 60 feet high along more than a one-mile section of coastline. Data also raise the possibility that some regional geophysical characteristic may be making Java more vulnerable to tsunami earthquakes.
Researchers from across the globe will present new analyses of seismic data, field survey information and modeling results of the July 17, 2006 tsunami earthquake at the American Geophysical Union’s (AGU) fall meeting in San Francisco. Andrew Newman, an assistant professor of geophysics at the Georgia Institute of Technology’s School of Earth and Atmospheric Sciences, and Susan Bilek, an assistant professor of geophysics at the New Mexico Institute of Mining and Technology are presiding over the Dec. 11 session devoted to the July disaster.
“Sharing what we’re learning will help us to better characterize tsunami earthquakes and where they occur,” Newman said. “We’ll also be able to better assess in the future when these kind of earthquakes occur whether they are likely to create tsunamis.”
Only about 0.1 percent of earthquakes of a 6.0 or larger magnitude on the Richter scale in the past 40 years have been classified as tsunami earthquakes, but their potentially catastrophic impact demands investigation into why and where they occur, Newman explained.
A tsunami earthquake is a slow-rupturing quake that occurs near the ocean floor. It uplifts a piece of the sea floor, and that deformation displaces water that propagates out to create a tsunami. Just 30 feet of slip in tectonic plates can create a 60-foot or higher wave on shore, and local wave run-up can be even higher, Newman noted.
Seismic data on the Java event revealed that the earthquake, which measured 7.7 on the Richter scale, did indeed rupture slowly compared to a typical earthquake, Newman said. It created a deadly tsunami that hit a 300-mile stretch of Java’s coastline and claimed more than 600 lives in a 125-mile-long, high-impact area.
Newman and Bilek analyzed the distribution of aftershocks that occurred in the two weeks following the July 17 event. Most of them occurred at shallow depths and close to the trench – the area where the Australian and Sunda plates collided.
“The nature of these aftershocks was a dead giveaway that this event was a classic tsunami earthquake,” Newman explained. “Only tsunami earthquakes rupture in the shallowest portion of the trench – only a few kilometers below the sea floor.”
The July 2006 tsunami earthquake was the second such event to occur on the Java trench in the past 40 years; the previous one happened in 1994. “There may be something going on in this particular trench – some regional characteristic – that is increasing the potential for tsunami earthquakes in this area,” Newman noted. His previous study of global data from 1970 to 2000 showed no indication of any seismic characteristics that made tsunami earthquakes occur more frequently in certain regions, but the Java event has renewed his interest in this possibility.
In another presentation in the same AGU meeting session, Hermann Fritz, a Georgia Tech assistant professor of civil and environmental engineering, will report the findings of the International Tsunami Survey Team (ITST) that he led into Java about two weeks after the tsunami earthquake. The team gathered perishable data on wave run-up, inundation, damage to structures, and sediment erosion and deposition along a 350-mile stretch of coastline. They recorded numerous eyewitness accounts on video to document survival strategies, as well as wave sequence and periods.
Most areas were hit by waves not exceeding 30 feet, but the team documented greater impact to a one-mile-long stretch of coastline on the island of Nusa Kambangan. It is a restricted-access area because high-security prisons are located there. In this area, the tsunami wave size exceeded 60 feet and inundated the coastline for more than half a mile inland, killing 19 inmates.
“The violent tsunami impact on Nusa Kambangan was visualized by a sharp trimline carved into the impact zone,” Fritz said. “It shredded an entire forest and caused massive beach erosion of more than a meter vertically. The violent impact extended several miles beyond the area of island coastline we visited, but the rest was not accessible. We were the only survey team that was granted access – which was in the company of federal agents -- to the off-limits island.
“This was an unusually high wave run-up for an earthquake with a magnitude of 7.7,” Fritz explained. “So we presume there was some local forcing, or underwater earth movement, such as a sub-marine landslide or slump, that may have been triggered by the earthquake and caused a larger tsunami to hit this local area.”
Most of the tsunami’s casualties – more than 400 of them – occurred in Java’s prime beach-tourist destination of Pagandaran, which is more than 20 miles west of Nusa, Fritz said.
As is common with slow-rupturing tsunami earthquakes, most people onshore in Java did not feel any shaking when the quake occurred 125 miles offshore, Fritz noted.
“Even the trained lifeguards on the beaches had a hard time recognizing the classic tsunami harbinger of initial shoreline recession,” he added. “They didn’t notice it because of the large surf waves that day.”
Though the Pacific Tsunami Warning Center issued a tsunami warning following the earthquake, it did not result in a local warning and evacuation because of communication and other logistical problems, Fritz said. But even if there had been a local warning, people would only have had 10 to 15 minutes at most to evacuate because the earthquake occurred so close to shore, he added.
Fritz and the survey team called for better education of the public to help them recognize the precursors of a tsunami. “Even if people feel weak ground-shaking on shore, they should move to higher ground,” he said. “There’s typically some degree of ocean drawback and/or an initial higher-than-normal wave, though the first wave is rarely the biggest one…. Also, when people evacuate the beach, they need to stay away from the shore for the recommended six hours.”
Researchers, including Fritz and his survey team colleagues, are working with the United Nations Educational, Scientific and Cultural Organization (UNESCO) to improve public education about tsunamis. Public awareness has improved in Indonesia, Fritz said, but needs a boost in other at-risk areas, including the Pacific Northwest coastline of the United States, Nicaragua, Costa Rica, Puerto Rico and Peru.
Jane M. Sanders | EurekAlert!
NASA sees the end of ex-Tropical Cyclone 02W
21.04.2017 | NASA/Goddard Space Flight Center
New research unlocks forests' potential in climate change mitigation
21.04.2017 | Clemson University
The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.
Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...
The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...
Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.
Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...
Two researchers at Heidelberg University have developed a model system that enables a better understanding of the processes in a quantum-physical experiment...
Glaciers might seem rather inhospitable environments. However, they are home to a diverse and vibrant microbial community. It’s becoming increasingly clear that they play a bigger role in the carbon cycle than previously thought.
A new study, now published in the journal Nature Geoscience, shows how microbial communities in melting glaciers contribute to the Earth’s carbon cycle, a...
20.04.2017 | Event News
18.04.2017 | Event News
03.04.2017 | Event News
21.04.2017 | Physics and Astronomy
21.04.2017 | Health and Medicine
21.04.2017 | Physics and Astronomy