Earthquakes in the ocean: Towards a better understanding of their precursors

Published on 14 September in Nature Geoscience, the study conducted by researchers from several institutes, including IFREMER (French Research Institute for Exploitation of the Sea), CNRS and IFSTTAR, offers the first theoretical model that, based on fluid-related processes, explains the seismic precursors of an underwater earthquake. Using quantitative measurements, this innovative model established a link between observed precursors and the mainshock of an earthquake. The results open a promising avenue of research for guiding future investigations on detecting earthquakes before they strike.

A model specific to the submarine environment

The data used to construct the model presented in the article were collected from subsea observatories* deployed in the North-East Pacific fracture zones.

The researchers showed that the properties of the fluids that circulate in submarine fault zones change over time, during what is called the “seismic cycle”. This term describes the cycle during which strain accumulates along a fault until it exceeds the frictional forces that prevent the fault from slipping. An earthquake results at the moment of rupture, due to the sudden release of built-up strain. A new cycle begins with strain accumulating and continues until the next rupture occurs along the fault...

Due to their proximity to mid-ocean ridges, the fluids that circulate in the faults undergo tremendous pressure and extremely high temperatures. These fluids can reach the supercritical state. The physical properties of supercritical fluids (density, viscosity, diffusivity) are intermediate to those of liquids and gases.

The compressibility of supercritical fluid varies greatly with pressure, and, according to the study’s analysis, this change in compressibility may trigger an earthquake, occurring after a short period of foreshocks.

Seismic precursors
Seismic precursors are the early warning signs before an earthquake strikes. Many different types of earthquake precursors have been studied by the scientific community: ground movements, seismic signals, fluid or gas emissions, electrical signals, thermal signals, animal behaviour, etc.
For an event as large as an earthquake, which releases a considerable amount of energy, there must be a preparatory phase. This problem in predicting earthquakes does not lie in the absence of precursors (hindsight observations are numerous), but in the capacity to detect these forerunners before the mainshock.

The results of the model can help guide future research in the detection of seismic precursors with, ultimately, potential applications for earthquake prediction. Supercritical fluids require very specific conditions; they are also encountered on land in hydrothermal and volcanic areas, such as Iceland.

Details of the model
Under the effect of tectonic forces, two antagonistic effects are usually in play near transform faults. First, increasing shear stress tends to break rocks and weaken resistance in the transform fault. Second, decreasing pressure of the fluid contained in the fault results in an increase in the volume of the pore space between rock beds. This effect acts as a stabilising suction cup, counterbalancing the ‘weakening’ in the rock bed and delaying the triggering of an earthquake.

The efficiency of this counterbalancing mechanism depends on fluid compressibility. It is highest in the presence of fluids in the liquid state, whose low compressibility causes a dramatic decrease in fluid pressure in response to small increases in volume. Conversely, for gas-type fluids, which are highly compressible, the suction cup effect is nearly inexistent.

When a change in the ‘liquid-gas’ state of the fluid occurs during a fault slip, the counterbalancing mechanism fails, allowing a major shock to be triggered. This transition occurs over several days and has numerous signs, including many small foreshocks.

*Subsea observatories are comparable to a laboratory on the seafloor. Equipped with a series of instruments, they record many types of data that can be used to study the geophysical events that occur in the ocean.

Source: Institut français de recherche pour l'exploitation de la mer (Ifremer)

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A global surge of great earthquakes from 2004-2014 and implications for Cascadia

The last ten years have been a remarkable time for great earthquakes. Since December 2004 there have been no less than 18 quakes of Mw8.0 or greater -- a rate of more than twice that seen from 1900 to mid-2004. Hundreds of thousands of lives have been lost and massive damage has resulted from these great earthquakes. But as devastating as such events can be, these recent great quakes have come with a silver lining: They coincide with unprecedented advances in technological and scientific capacity for learning from them.

"We previously had very limited information about how ruptures grow into great earthquakes and interact with regions around them," said seismologist Thorne Lay of the University of California at Santa Cruz. "So we are using the recorded data for these recent events to guide our understanding of future earthquakes. We've gained a new level of appreciation for how one earthquake can influence events in other zones."

High on the list of areas ripe for a great quake is Cascadia, the Pacific Northwest, where the risk for great quakes had long been under appreciated. Evidence began surfacing about 20 years ago that there had been a great quake in the region in the year 1700. Since then the view of the great quake risk in Cascadia has shifted dramatically.

"We don't know many details about what happened in 1700," said Lay. There were no instruments back then to observe and record it. And so the best way to try and understand the danger and what could happen in Cascadia is to study the recent events elsewhere.

Over the last decade Lay and his colleagues have been able to gather fine details about these giant earthquakes using data from an expanded global networks of seismometers, GPS stations, tsunami gauges, and new satellite imaging capabilities such as GRACE, InSAR, and LandSAT interferometry. Among the broader conclusions they have come to is that great quakes are very complicated and idiosyncratic. Lay will be presenting some of those idiosyncrasies at the meeting of the Geological Society of America in Vancouver on Oct. 21.

"What we've seen is that we can have multiple faults activated," said Lay. "We've seen it off Sumatra and off Japan. Once earthquakes get going they can activate faulting in areas that were thought not physically feasible."

The great Sumatra-Andaman earthquake of Dec. 26, 2004, for instance, unzipped a 1,300 kilometer long segment of the subduction zone and unleashed one of history's most destructive, deadly tsunamis. Much of the rupture was along a region with very limited plate convergence. In Japan, the Kuril Islands, and the Solomon Islands, great mega-thrust ruptures have ruptured portions of the subduction zones that were thought too warm or weak to experience earthquakes.

"These earthquakes ruptured right through areas that had been considered to have low risk," said Lay. "We thought that would not happen. But it did, so we have to adjust our understanding."

Perhaps the best recent analogy to Cascadia is off the coast of Iquique, Chile, said Lay. There had been a great quake in 1877, and a conspicuous gap in quakes ever since. Like the 1700 Cascadia earthquake, there is little data for the 1877 event, which killed more than 2,500 people. In both subduction zones, the converging plates are thought to be accumulating strain which could be released in a very large and violent rupture. On April 1 of this year, some of that strain was released offshore of Iquique. There was a Mw8.1 rupture in the northern portion of the seismic gap. But it involved slip over less than 20 percent of the region that seismologists believe to have accumulated strain since 1877.

"We have no idea why only a portion of the 1877 zone ruptured," said Lay. "But clearly, 80 percent of that zone is still unruptured. We don't have a good basis for assessment of how the rest will fail. It's the same for Cascadia. We don't know if it always goes all at once or sometimes in sequences of smaller events, with alternating pattern. It is prudent to prepare for the worst case of failure of the entire region in a single event, but it may not happen that way every time."

What is certain is that studying these recent big earthquakes has given geophysicists the best information ever about how they work and point to new ways to begin understanding what could be in Cascadia's future.

Source: Geological Society of America

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Hippos-Sussita excavation: Silent evidence of the earthquake of 363 CE

The city of Hippos-Sussita, which was founded in the second century BCE, experienced two strong and well-documented earthquakes. The first was in the year 363 CE and it caused heavy damage. The city, did, however, recover. The great earthquake of 749 CE destroyed the city which was subsequently abandoned completely. Evidence of the extensive damage caused by the earthquake of 363 was found in earlier seasons. None, however, was as violent, thrilling and eerie as the evidence discovered this year. Credit: Image courtesy of University of Haifa

Silent evidence of a large earthquake in 363 CE -- the skeleton of a woman with a dove-shaped pendant was discovered under the tiles of a collapsed roof by archeologists from the University of Haifa during this excavation season at Hippos-Sussita. They also found a large muscular marble leg and artillery ammunition from some 2,000 years ago. "The data is finally beginning to form a clear historical-archaeological picture," said Dr. Michael Eisenberg, head of the international excavation team.

The past fifteen excavation seasons at Hippos-Sussita, run by archeologists from the Zinman Institute of Archaeology at the University of Haifa, have not stopped providing a constant flow of fascinating findings. The team digging at the city site -- situated east of the Sea of Galilee in the Sussita National Park, which is under the management of the Israel Nature and Parks Authority -- has grown over the years, with more and more teams and excavators from various countries joining them. This time, the security situation in the south of Israel "sent" them a Canadian team, led by Dr. Stephen Chambers, as reinforcement.

The city of Hippos-Sussita, which was founded in the second century BCE, experienced two strong and well-documented earthquakes. The first was in the year 363 CE and it caused heavy damage. The city, did, however, recover. The great earthquake of 749 CE destroyed the city which was subsequently abandoned completely. Evidence of the extensive damage caused by the earthquake of 363 was found in earlier seasons. None, however, was as violent, thrilling and eerie as the evidence discovered this year.

To the north of the basilica, the largest building in town that served as the commercial, economic and judicial center of the city, the dig's senior area supervisor Haim Shkolnik and his team unearthed the remains of several skeletons that had been crushed by the weight of the collapsed roof. Among the bones of one of the women lay a gold dove-shaped pendant.

This year, evidence was found for the first time that the great earthquake of 363 CE had destroyed the Roman bathhouse, which was uncovered by the team run by Arleta Kowalewska from Poland. Like the basilica, it too was not rebuilt. According to Dr. Eisenberg, the evidence found so far shows that the earthquake was so powerful it completely destroyed the city, which took some twenty years to be rebuilt. Among the wreckage from the bathhouse, an excellent Roman marble sculpture of a muscular right leg of a man leaning against a tree trunk was found. "It is too early to determine who the man depicted in the sculpture was. It could be the sculpture of a god or an athlete; it was more than two meters tall. We hope to find more parts of the sculpture in the coming seasons to shed some light on his identity," said Dr. Eisenberg.

Excavations were resumed in the bastion, the main defense post of the Roman period city built on the southern edge of the cliff, where the work focused on the fortified position of a projectile machine that propelled/launched ballista stones. The catapult was some eight meters long according to the size of the chamber. So far the archeologists have found a number of ballista balls that fit the massive catapult, as well as smaller balls that were used on smaller ballista machines. These machines were positioned above the bastion's vaults and were used to launch basalt ballista balls slightly smaller than soccer balls as far as 350 meters.

A section of the western part of the city's main colonnaded street, which traversed its entire length of 600 meters from east to west (the decumanus maximus) was excavated this year with the help of a Canadian team, after their planned dig in the south was cancelled. The archeologists uncovered another original piece of the wall that supported the street columns, confirming the theory that it had been a magnificent colonnaded street similar to those of the Roman East cities that were built at the peak of the Pax Romana -- the Roman era of peace during the first few centuries CE.

While working on the dig the team also invested a lot of work on the site's conservation. "I am extremely proud that we were able to organize a sizable conservation team this year as well, from our own internal budgets and with the help of the Western Galilee College in Acre. Twenty-two students from the college's Department of Conservation together with five experienced conservators under the direction of Julia Burdajewicz from the Academy of Fine Arts in Warsaw conducted the conservation work. This is one of the major tourist destinations in the northern part of the country, and as such I see this as a national mission, even if the budget comes primarily from our own sources, without government support," concluded Dr. Eisenberg.

Source: University of Haifa

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