M 9.0+ possible for subduction zones along ‘Ring of Fire,’ suggests new study

The magnitude of the 2011 Tohoku quake (M 9.0) caught many seismologists by surprise, prompting some to revisit the question of calculating the maximum magnitude earthquake possible for a particular fault. New research offers an alternate view that uses the concept of probable maximum magnitude events over a given period, providing the magnitude and the recurrence rate of extreme events in subduction zones for that period. Most circum Pacific subduction zones can produce earthquakes of magnitude greater than 9.0, suggests the study.

The idea of identifying the maximum magnitude for a fault isn’t new, and its definition varies based on context. This study, published online by the Bulletin of the Seismological Society of America (BSSA), calculates the “probable maximum earthquake magnitude within a time period of interest,” estimating the probable magnitude of subduction zone earthquakes for various time periods, including 250, 500 and 10,000 years.

“Various professionals use the same terminology – maximum magnitude – to mean different things. The most interesting question for us was what was going to be the biggest magnitude earthquake over a given period of time?” said co-author Yufang Rong, a seismologist at the Center for Property Risk Solutions of FM Global, a commercial and industrial property insurer. “Can we know the exact, absolute maximum magnitude? The answer is no, however, we developed a simple methodology to estimate the probable largest magnitude within a specific time frame.”

The study’s results indicated most of the subduction zones can generate M 8.5 or greater over a 250-return period; M 8.8 or greater over 500 years; and M 9.0 or greater over 10,000 years.

“Just because a subduction zone hasn’t produced a magnitude 8.8 in 499 years, that doesn’t mean one will happen next year,” said Rong. “We are talking about probabilities.”

The instrumental and historical earthquake record is brief, complicating any attempt to confirm recurrence rates and estimate with confidence the maximum magnitude of an earthquake in a given period. The authors validated their methodology by comparing their findings to the seismic history of the Cascadia subduction zone, revealed through deposits of marine sediment along the Pacific Northwest coast. While some subduction zones have experienced large events during recent history, the Cascadia subduction zone has remained quiet. Turbidite and onshore paleoseismic studies have documented a rich seismic history, identifying 40 large events over the past 10,000 years.

“Magnitude limits of subduction zone earthquakes” is co-authored by Rong, David Jackson of UCLA, Harold Magistrale of FM Global, and Chris Goldfinger of Oregon State University. The paper will be published online Sept. 16 by BSSA as well as in its October print edition.

M 9.0+ possible for subduction zones along ‘Ring of Fire,’ suggests new study

The magnitude of the 2011 Tohoku quake (M 9.0) caught many seismologists by surprise, prompting some to revisit the question of calculating the maximum magnitude earthquake possible for a particular fault. New research offers an alternate view that uses the concept of probable maximum magnitude events over a given period, providing the magnitude and the recurrence rate of extreme events in subduction zones for that period. Most circum Pacific subduction zones can produce earthquakes of magnitude greater than 9.0, suggests the study.

The idea of identifying the maximum magnitude for a fault isn’t new, and its definition varies based on context. This study, published online by the Bulletin of the Seismological Society of America (BSSA), calculates the “probable maximum earthquake magnitude within a time period of interest,” estimating the probable magnitude of subduction zone earthquakes for various time periods, including 250, 500 and 10,000 years.

“Various professionals use the same terminology – maximum magnitude – to mean different things. The most interesting question for us was what was going to be the biggest magnitude earthquake over a given period of time?” said co-author Yufang Rong, a seismologist at the Center for Property Risk Solutions of FM Global, a commercial and industrial property insurer. “Can we know the exact, absolute maximum magnitude? The answer is no, however, we developed a simple methodology to estimate the probable largest magnitude within a specific time frame.”

The study’s results indicated most of the subduction zones can generate M 8.5 or greater over a 250-return period; M 8.8 or greater over 500 years; and M 9.0 or greater over 10,000 years.

“Just because a subduction zone hasn’t produced a magnitude 8.8 in 499 years, that doesn’t mean one will happen next year,” said Rong. “We are talking about probabilities.”

The instrumental and historical earthquake record is brief, complicating any attempt to confirm recurrence rates and estimate with confidence the maximum magnitude of an earthquake in a given period. The authors validated their methodology by comparing their findings to the seismic history of the Cascadia subduction zone, revealed through deposits of marine sediment along the Pacific Northwest coast. While some subduction zones have experienced large events during recent history, the Cascadia subduction zone has remained quiet. Turbidite and onshore paleoseismic studies have documented a rich seismic history, identifying 40 large events over the past 10,000 years.

“Magnitude limits of subduction zone earthquakes” is co-authored by Rong, David Jackson of UCLA, Harold Magistrale of FM Global, and Chris Goldfinger of Oregon State University. The paper will be published online Sept. 16 by BSSA as well as in its October print edition.

Scientists anticipated size and location of 2012 Costa Rica earthquake

Andrew Newman, an associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, performs a GPS survey in Costa Rica's Nicoya Peninsula in 2010. -  Lujia Feng
Andrew Newman, an associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, performs a GPS survey in Costa Rica’s Nicoya Peninsula in 2010. – Lujia Feng

Scientists using GPS to study changes in the Earth’s shape accurately forecasted the size and location of the magnitude 7.6 Nicoya earthquake that occurred in 2012 in Costa Rica.

The Nicoya Peninsula in Costa Rica is one of the few places where land sits atop the portion of a subduction zone where the Earth’s greatest earthquakes take place. Costa Rica’s location therefore makes it the perfect spot for learning how large earthquakes rupture. Because earthquakes greater than about magnitude 7.5 have occurred in this region roughly every 50 years, with the previous event striking in 1950, scientists have been preparing for this earthquake through a number of geophysical studies. The most recent study used GPS to map out the area along the fault storing energy for release in a large earthquake.

“This is the first place where we’ve been able to map out the likely extent of an earthquake rupture along the subduction megathrust beforehand,” said Andrew Newman, an associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology.

The study was published online Dec. 22, 2013, in the journal Nature Geoscience. The research was supported by the National Science Foundation and was a collaboration of researchers from Georgia Tech, the Costa Rica Volcanological and Seismological Observatory (OVSICORI) at Universidad Nacional, University California, Santa Cruz, and the University of South Florida.

Subduction zones are locations where one tectonic plate is forced under another one. The collision of tectonic plates during this process can unleash devastating earthquakes, and sometimes devastating tsunamis. The magnitude 9.0 earthquake off the coast of Japan in 2011 was due to just such a subduction zone eaerthquake. The Cascadia subduction zone in the Pacific Northwest is capable of unleashing a similarly sized quake. Damage from the Nicoya earthquake was not as bad as might be expected from a magnitude 7.6 quake.

“Fortunately there was very little damage considering the earthquake’s size,” said Marino Protti of OVSICORI and the study’s lead author. “The historical pattern of earthquakes not only allowed us to get our instruments ready, it also allowed Costa Ricans to upgrade their buildings to be earthquake safe.”

Plate tectonics are the driving force for subduction zones. As tectonic plates converge, strain temporarily accumulates across the plate boundary when portions of the interface between these tectonic plates, called a megathrust, become locked together. The strain can accumulate to dangerous levels before eventually being released as a massive earthquake.

“The Nicoya Peninsula is an ideal natural lab for studying these events, because the coastline geometry uniquely allows us to get our equipment close to the zone of active strain accumulation,” said Susan Schwartz, professor of earth sciences at the University of California, Santa Cruz, and a co-author of the study.

Through a series of studies starting in the early 1990s using land-based tools, the researchers mapped regions where tectonic plates were completely locked along the subduction interface. Detailed geophysical observations of the region allowed the researchers to create an image of where the faults had locked.

The researchers published a study a few months before the earthquake, describing the particular locked patch with the clearest potential for the next large earthquake in the region. The team projected the total amount of energy that could have developed across that region and forecasted that if the locking remained similar since the last major earthquake in 1950, then there is presently enough energy for an earthquake on the order of magnitude 7.8 there.

Because of limits in technology and scientific understanding about processes controlling fault locking and release, scientists cannot say much about precisely where or when earthquakes will occur. However, earthquakes in Nicoya have occurred about every 50 years, so seismologists had been anticipating another one around 2000, give or take 20 years, Newman said. The earthquake occurred in September of 2012 as a magnitude 7.6 quake.

“It occurred right in the area we determined to be locked and it had almost the size we expected,” Newman said.

The researchers hope to apply what they’ve learned in Costa Rica to other environments. Virtually every damaging subduction zone earthquake occurs far offshore.

“Nicoya is the only place on Earth where we’ve actually been able to get a very accurate image of the locked patch because it occurs directly under land,” Newman said. “If we really want to understand the seismic potential for most of the world, we have to go offshore.”

Scientists have been able to reasonably map portions of these locked areas offshore using data on land, but the resolution is poor, particularly in the regions that are most responsible for generating tsunamis, Newman said. He hopes that his group’s work in Nicoya will be a driver for geodetic studies on the seafloor to observe such Earth deformation. These seafloor geodetic studies are rare and expensive today.

“If we want to understand the potential for large earthquakes, then we really need to start doing more seafloor observations,” Newman said. “It’s a growing push in our community and this study highlights the type of results that one might be able to obtain for most other dangerous environments, including offshore the Pacific Northwest.”

Scientists anticipated size and location of 2012 Costa Rica earthquake

Andrew Newman, an associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, performs a GPS survey in Costa Rica's Nicoya Peninsula in 2010. -  Lujia Feng
Andrew Newman, an associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, performs a GPS survey in Costa Rica’s Nicoya Peninsula in 2010. – Lujia Feng

Scientists using GPS to study changes in the Earth’s shape accurately forecasted the size and location of the magnitude 7.6 Nicoya earthquake that occurred in 2012 in Costa Rica.

The Nicoya Peninsula in Costa Rica is one of the few places where land sits atop the portion of a subduction zone where the Earth’s greatest earthquakes take place. Costa Rica’s location therefore makes it the perfect spot for learning how large earthquakes rupture. Because earthquakes greater than about magnitude 7.5 have occurred in this region roughly every 50 years, with the previous event striking in 1950, scientists have been preparing for this earthquake through a number of geophysical studies. The most recent study used GPS to map out the area along the fault storing energy for release in a large earthquake.

“This is the first place where we’ve been able to map out the likely extent of an earthquake rupture along the subduction megathrust beforehand,” said Andrew Newman, an associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology.

The study was published online Dec. 22, 2013, in the journal Nature Geoscience. The research was supported by the National Science Foundation and was a collaboration of researchers from Georgia Tech, the Costa Rica Volcanological and Seismological Observatory (OVSICORI) at Universidad Nacional, University California, Santa Cruz, and the University of South Florida.

Subduction zones are locations where one tectonic plate is forced under another one. The collision of tectonic plates during this process can unleash devastating earthquakes, and sometimes devastating tsunamis. The magnitude 9.0 earthquake off the coast of Japan in 2011 was due to just such a subduction zone eaerthquake. The Cascadia subduction zone in the Pacific Northwest is capable of unleashing a similarly sized quake. Damage from the Nicoya earthquake was not as bad as might be expected from a magnitude 7.6 quake.

“Fortunately there was very little damage considering the earthquake’s size,” said Marino Protti of OVSICORI and the study’s lead author. “The historical pattern of earthquakes not only allowed us to get our instruments ready, it also allowed Costa Ricans to upgrade their buildings to be earthquake safe.”

Plate tectonics are the driving force for subduction zones. As tectonic plates converge, strain temporarily accumulates across the plate boundary when portions of the interface between these tectonic plates, called a megathrust, become locked together. The strain can accumulate to dangerous levels before eventually being released as a massive earthquake.

“The Nicoya Peninsula is an ideal natural lab for studying these events, because the coastline geometry uniquely allows us to get our equipment close to the zone of active strain accumulation,” said Susan Schwartz, professor of earth sciences at the University of California, Santa Cruz, and a co-author of the study.

Through a series of studies starting in the early 1990s using land-based tools, the researchers mapped regions where tectonic plates were completely locked along the subduction interface. Detailed geophysical observations of the region allowed the researchers to create an image of where the faults had locked.

The researchers published a study a few months before the earthquake, describing the particular locked patch with the clearest potential for the next large earthquake in the region. The team projected the total amount of energy that could have developed across that region and forecasted that if the locking remained similar since the last major earthquake in 1950, then there is presently enough energy for an earthquake on the order of magnitude 7.8 there.

Because of limits in technology and scientific understanding about processes controlling fault locking and release, scientists cannot say much about precisely where or when earthquakes will occur. However, earthquakes in Nicoya have occurred about every 50 years, so seismologists had been anticipating another one around 2000, give or take 20 years, Newman said. The earthquake occurred in September of 2012 as a magnitude 7.6 quake.

“It occurred right in the area we determined to be locked and it had almost the size we expected,” Newman said.

The researchers hope to apply what they’ve learned in Costa Rica to other environments. Virtually every damaging subduction zone earthquake occurs far offshore.

“Nicoya is the only place on Earth where we’ve actually been able to get a very accurate image of the locked patch because it occurs directly under land,” Newman said. “If we really want to understand the seismic potential for most of the world, we have to go offshore.”

Scientists have been able to reasonably map portions of these locked areas offshore using data on land, but the resolution is poor, particularly in the regions that are most responsible for generating tsunamis, Newman said. He hopes that his group’s work in Nicoya will be a driver for geodetic studies on the seafloor to observe such Earth deformation. These seafloor geodetic studies are rare and expensive today.

“If we want to understand the potential for large earthquakes, then we really need to start doing more seafloor observations,” Newman said. “It’s a growing push in our community and this study highlights the type of results that one might be able to obtain for most other dangerous environments, including offshore the Pacific Northwest.”

Quake-triggered landslides pose significant hazard for Seattle, new study details potential damage

Locations of each zoom-in are shown on the map of Seattle at right. A) Coastal bluffs in the northern part of Seattle are most affected when soils are saturated. B) There are several areas along the I-5 corridor that are highly susceptible to landsliding for all soil saturation levels, such as the area shown here near the access point to the West Seattle bridge. C) The hillsides in West Seattle along the Duwamish valley are at risk of seismically induced landsliding, such as the area shown here. There are industrial as well as 59 residential buildings that could be affected by runout from landsliding in these areas. D) The coastal bluffs along Puget Sound in West Seattle on the hanging wall of the fault, such as the area shown here, are the most highly susceptible areas to landsliding in the city; numerous residential structures are at risk from both potential landslide source areas and runout. -  Allstadt/BSSA
Locations of each zoom-in are shown on the map of Seattle at right. A) Coastal bluffs in the northern part of Seattle are most affected when soils are saturated. B) There are several areas along the I-5 corridor that are highly susceptible to landsliding for all soil saturation levels, such as the area shown here near the access point to the West Seattle bridge. C) The hillsides in West Seattle along the Duwamish valley are at risk of seismically induced landsliding, such as the area shown here. There are industrial as well as 59 residential buildings that could be affected by runout from landsliding in these areas. D) The coastal bluffs along Puget Sound in West Seattle on the hanging wall of the fault, such as the area shown here, are the most highly susceptible areas to landsliding in the city; numerous residential structures are at risk from both potential landslide source areas and runout. – Allstadt/BSSA

A new study suggests the next big quake on the Seattle fault may cause devastating damage from landslides, greater than previously thought and beyond the areas currently defined as prone to landslides. Published online Oct. 22 by the Bulletin of the Seismological Society of America (BSSA), the research offers a framework for simulating hundreds of earthquake scenarios for the Seattle area.

“A major quake along the Seattle fault is among the worst case scenarios for the area since the fault runs just south of downtown. Our study shows the need for dedicated studies on seismically induced landsliding” said co-author Kate Allstadt, doctoral student at University of Washington.

Seattle is prone to strong shaking as it sits atop the Seattle Basin – a deep sedimentary basin that amplifies ground motion and generates strong seismic waves that tend to increase the duration of the shaking. The broader region is vulnerable to earthquakes from multiple sources, including deep earthquakes within the subducted Juan de Fuca plate, offshore megathrust earthquakes on Cascadia subduction zone and the shallow crustal earthquakes within the North American Plate.

For Seattle, a shallow crustal earthquake close to the city would be most damaging. The last major quake along the Seattle fault was in 900 AD, long before the city was established, though native people lived in the area. The earthquake triggered giant landslides along Lake Washington, causing entire blocks of forest to slide into the lake.

“There’s a kind of haunting precedence that tells us that we should pay attention to a large earthquake on this fault because it happened in the past,” said Allstadt, who also serves as duty seismologist for the Pacific Northwest Seismic Network. John Vidale of University of Washington and Art Frankel of the U.S. Geological Survey (USGS) are co-authors of the study, which was funded by the USGS.

While landslides triggered by earthquakes have caused damage and casualties worldwide, they have not often been the subject of extensive quantitative study or fully incorporated into seismic hazard assessments, say authors of this study that looks at just one scenario among potentially hundreds for a major earthquake in the Seattle area.

Dividing the area into a grid of 210-meter cells, they simulated ground motion for a magnitude 7 Seattle fault earthquake and then further subdivided into 5-meter cells, applying anticipated amplification of shaking due to the shallow soil layers. This refined framework yielded some surprises.

“One-third of the landslides triggered by our simulation were outside of the areas designated by the city as prone to landsliding,” said Allstadt. “A lot of people assume that all landslides occur in the same areas, but those triggered by rainfall or human behavior have a different triggering mechanism than landslides caused by earthquakes so we need dedicated studies.”

While soil saturation — whether the soil is dry or saturated with water – is the most important factor when analyzing the potential impact of landslides, the details of ground motion rank second. The amplification of ground shaking, directivity of seismic energy and geological features that may affect ground motion are very important to the outcome of ground failure, say authors.

The authors stress that this is just one randomized scenario study of many potential earthquake scenarios that could strike the city. While the results do not delineate the exact areas that will be affected in a future earthquake, they do illustrate the extent of landsliding to expect for a similar event.

The study suggests the southern half of the city and the coastal bluffs, many of which are developed, would be hardest hit. Depending upon the water saturation level of the soil at the time of the earthquake, several hundred to thousands of buildings could be affected citywide. For dry soil conditions, there are more than 1000 buildings that are within all hazard zones, 400 of those in the two highest hazard designation zones. The analysis suggests landslides could also affect some inland slopes, threatening key transit routes and impeding recovery efforts. For saturated soil conditions, it is an order of magnitude worse, with 8000 buildings within all hazard zones, 5000 of those within the two highest hazard zones. These numbers only reflect the number of buildings in high-risk areas, not the number of buildings that would necessarily suffer damage.

“The extra time we took to include the refined ground motion detail was worth it. It made a significant difference to our understanding of the potential damage to Seattle from seismically triggered landslides,” said Allstadt, who would like to use the new framework to run many more scenarios to prepare for future earthquakes in Seattle.

Quake-triggered landslides pose significant hazard for Seattle, new study details potential damage

Locations of each zoom-in are shown on the map of Seattle at right. A) Coastal bluffs in the northern part of Seattle are most affected when soils are saturated. B) There are several areas along the I-5 corridor that are highly susceptible to landsliding for all soil saturation levels, such as the area shown here near the access point to the West Seattle bridge. C) The hillsides in West Seattle along the Duwamish valley are at risk of seismically induced landsliding, such as the area shown here. There are industrial as well as 59 residential buildings that could be affected by runout from landsliding in these areas. D) The coastal bluffs along Puget Sound in West Seattle on the hanging wall of the fault, such as the area shown here, are the most highly susceptible areas to landsliding in the city; numerous residential structures are at risk from both potential landslide source areas and runout. -  Allstadt/BSSA
Locations of each zoom-in are shown on the map of Seattle at right. A) Coastal bluffs in the northern part of Seattle are most affected when soils are saturated. B) There are several areas along the I-5 corridor that are highly susceptible to landsliding for all soil saturation levels, such as the area shown here near the access point to the West Seattle bridge. C) The hillsides in West Seattle along the Duwamish valley are at risk of seismically induced landsliding, such as the area shown here. There are industrial as well as 59 residential buildings that could be affected by runout from landsliding in these areas. D) The coastal bluffs along Puget Sound in West Seattle on the hanging wall of the fault, such as the area shown here, are the most highly susceptible areas to landsliding in the city; numerous residential structures are at risk from both potential landslide source areas and runout. – Allstadt/BSSA

A new study suggests the next big quake on the Seattle fault may cause devastating damage from landslides, greater than previously thought and beyond the areas currently defined as prone to landslides. Published online Oct. 22 by the Bulletin of the Seismological Society of America (BSSA), the research offers a framework for simulating hundreds of earthquake scenarios for the Seattle area.

“A major quake along the Seattle fault is among the worst case scenarios for the area since the fault runs just south of downtown. Our study shows the need for dedicated studies on seismically induced landsliding” said co-author Kate Allstadt, doctoral student at University of Washington.

Seattle is prone to strong shaking as it sits atop the Seattle Basin – a deep sedimentary basin that amplifies ground motion and generates strong seismic waves that tend to increase the duration of the shaking. The broader region is vulnerable to earthquakes from multiple sources, including deep earthquakes within the subducted Juan de Fuca plate, offshore megathrust earthquakes on Cascadia subduction zone and the shallow crustal earthquakes within the North American Plate.

For Seattle, a shallow crustal earthquake close to the city would be most damaging. The last major quake along the Seattle fault was in 900 AD, long before the city was established, though native people lived in the area. The earthquake triggered giant landslides along Lake Washington, causing entire blocks of forest to slide into the lake.

“There’s a kind of haunting precedence that tells us that we should pay attention to a large earthquake on this fault because it happened in the past,” said Allstadt, who also serves as duty seismologist for the Pacific Northwest Seismic Network. John Vidale of University of Washington and Art Frankel of the U.S. Geological Survey (USGS) are co-authors of the study, which was funded by the USGS.

While landslides triggered by earthquakes have caused damage and casualties worldwide, they have not often been the subject of extensive quantitative study or fully incorporated into seismic hazard assessments, say authors of this study that looks at just one scenario among potentially hundreds for a major earthquake in the Seattle area.

Dividing the area into a grid of 210-meter cells, they simulated ground motion for a magnitude 7 Seattle fault earthquake and then further subdivided into 5-meter cells, applying anticipated amplification of shaking due to the shallow soil layers. This refined framework yielded some surprises.

“One-third of the landslides triggered by our simulation were outside of the areas designated by the city as prone to landsliding,” said Allstadt. “A lot of people assume that all landslides occur in the same areas, but those triggered by rainfall or human behavior have a different triggering mechanism than landslides caused by earthquakes so we need dedicated studies.”

While soil saturation — whether the soil is dry or saturated with water – is the most important factor when analyzing the potential impact of landslides, the details of ground motion rank second. The amplification of ground shaking, directivity of seismic energy and geological features that may affect ground motion are very important to the outcome of ground failure, say authors.

The authors stress that this is just one randomized scenario study of many potential earthquake scenarios that could strike the city. While the results do not delineate the exact areas that will be affected in a future earthquake, they do illustrate the extent of landsliding to expect for a similar event.

The study suggests the southern half of the city and the coastal bluffs, many of which are developed, would be hardest hit. Depending upon the water saturation level of the soil at the time of the earthquake, several hundred to thousands of buildings could be affected citywide. For dry soil conditions, there are more than 1000 buildings that are within all hazard zones, 400 of those in the two highest hazard designation zones. The analysis suggests landslides could also affect some inland slopes, threatening key transit routes and impeding recovery efforts. For saturated soil conditions, it is an order of magnitude worse, with 8000 buildings within all hazard zones, 5000 of those within the two highest hazard zones. These numbers only reflect the number of buildings in high-risk areas, not the number of buildings that would necessarily suffer damage.

“The extra time we took to include the refined ground motion detail was worth it. It made a significant difference to our understanding of the potential damage to Seattle from seismically triggered landslides,” said Allstadt, who would like to use the new framework to run many more scenarios to prepare for future earthquakes in Seattle.

Preparing for the next megathrust

Understanding the size and frequency of large earthquakes along the Pacific coast of North America is of great importance, not just to scientists, but also to government planners and the general public. The only way to predict the frequency and intensity of the ground motion expected from large and giant “megathrust ” earthquakes along Canada’s west coast is to analyze the geologic record. A new study published today in the Canadian Journal of Earth Sciences presents an exceptionally well-dated first record of earthquake history along the south coast of BC. Using a new high-resolution age model, a team of scientists meticulously identified and dated the disturbed sedimentary layers in a 40-metre marine sediment core raised from Effingham Inlet. The disturbances appear to have been caused by large and megathrust earthquakes that have occurred over the past 11,000 years.

One of the co-authors of the study, Dr. Audrey Dallimore, Associate Professor at Royal Roads University explains: “Some BC coastal fjords preserve annually layered organic sediments going back all the way to deglacial times. In Effingham Inlet, on the west coast of Vancouver Island, these sediments reveal disturbances we interpret were caused by earthquakes. With our very detailed age model that includes 68 radiocarbon dates and the Mazama Ash deposit (a volcanic eruption that took place 6800 yrs ago); we have identified 22 earthquake shaking events over the last 11,000 years, giving an estimate of a recurrence interval for large and megathrust earthquakes of about 500 years. However, it appears that the time between major shaking events can stretch up to about a 1,000 years.

“The last megathrust earthquake originating from the Cascadia subduction zone occurred in 1700 AD. Therefore, we are now in the risk zone of another earthquake. Even though it could be tomorrow or perhaps even centuries before it occurs, paleoseismic studies such as this one can help us understand the nature and frequency of rupture along the Cascadia Subduction Zone, and help Canadian coastal communities to improve their hazard assessments and emergency preparedness plans.”

“This exceptionally well-dated paleoseismic study by Enkin et al., involved a multi-disciplinary team of Canadian university and federal government scientists, and a core from the 2002 international drill program Marges Ouest Nord Américaines (MONA) campaign,” says Dr. Olav Lian, an associate editor of the Canadian Journal of Earth Sciences, professor at the University of the Fraser Valley and Director of the university’s Luminescence Dating Laboratory. “It gives us our first glimpse back in geologic time, of the recurrence interval of large and megathrust earthquakes impacting the vulnerable BC outer coastline. It also supports paleoseismic data found in offshore marine sediment cores along the US portion of the Cascadia Subduction Zone, recently released in an important United States Geological Survey (USGS) paleoseismic study by a team of researchers led by Dr. Chris Goldfinger of Oregon State University. In addition to analyzing the Effingham Inlet record for earthquake events, this study site has also revealed much information about climate and ocean changes throughout the Holocene to the present. These findings also clearly illustrate the importance of analyzing the geologic record to help today’s planners and policy makers, and ultimately to increase the resiliency of Canadian communities. “

Preparing for the next megathrust

Understanding the size and frequency of large earthquakes along the Pacific coast of North America is of great importance, not just to scientists, but also to government planners and the general public. The only way to predict the frequency and intensity of the ground motion expected from large and giant “megathrust ” earthquakes along Canada’s west coast is to analyze the geologic record. A new study published today in the Canadian Journal of Earth Sciences presents an exceptionally well-dated first record of earthquake history along the south coast of BC. Using a new high-resolution age model, a team of scientists meticulously identified and dated the disturbed sedimentary layers in a 40-metre marine sediment core raised from Effingham Inlet. The disturbances appear to have been caused by large and megathrust earthquakes that have occurred over the past 11,000 years.

One of the co-authors of the study, Dr. Audrey Dallimore, Associate Professor at Royal Roads University explains: “Some BC coastal fjords preserve annually layered organic sediments going back all the way to deglacial times. In Effingham Inlet, on the west coast of Vancouver Island, these sediments reveal disturbances we interpret were caused by earthquakes. With our very detailed age model that includes 68 radiocarbon dates and the Mazama Ash deposit (a volcanic eruption that took place 6800 yrs ago); we have identified 22 earthquake shaking events over the last 11,000 years, giving an estimate of a recurrence interval for large and megathrust earthquakes of about 500 years. However, it appears that the time between major shaking events can stretch up to about a 1,000 years.

“The last megathrust earthquake originating from the Cascadia subduction zone occurred in 1700 AD. Therefore, we are now in the risk zone of another earthquake. Even though it could be tomorrow or perhaps even centuries before it occurs, paleoseismic studies such as this one can help us understand the nature and frequency of rupture along the Cascadia Subduction Zone, and help Canadian coastal communities to improve their hazard assessments and emergency preparedness plans.”

“This exceptionally well-dated paleoseismic study by Enkin et al., involved a multi-disciplinary team of Canadian university and federal government scientists, and a core from the 2002 international drill program Marges Ouest Nord Américaines (MONA) campaign,” says Dr. Olav Lian, an associate editor of the Canadian Journal of Earth Sciences, professor at the University of the Fraser Valley and Director of the university’s Luminescence Dating Laboratory. “It gives us our first glimpse back in geologic time, of the recurrence interval of large and megathrust earthquakes impacting the vulnerable BC outer coastline. It also supports paleoseismic data found in offshore marine sediment cores along the US portion of the Cascadia Subduction Zone, recently released in an important United States Geological Survey (USGS) paleoseismic study by a team of researchers led by Dr. Chris Goldfinger of Oregon State University. In addition to analyzing the Effingham Inlet record for earthquake events, this study site has also revealed much information about climate and ocean changes throughout the Holocene to the present. These findings also clearly illustrate the importance of analyzing the geologic record to help today’s planners and policy makers, and ultimately to increase the resiliency of Canadian communities. “

13-year Cascadia study complete – and Northwest earthquake risk looms large

A comprehensive analysis of the Cascadia Subduction Zone off the Pacific Northwest coast confirms that the region has had numerous earthquakes over the past 10,000 years, and suggests that the southern Oregon coast may be most vulnerable based on recurrence frequency.

Written by researchers at Oregon State University, and published online by the U.S. Geological Survey, the study concludes that there is a 40 percent chance of a major earthquake in the Coos Bay, Ore., region during the next 50 years. And that earthquake could approach the intensity of the Tohoku quake that devastated Japan in March of 2011.

“The southern margin of Cascadia has a much higher recurrence level for major earthquakes than the northern end and, frankly, it is overdue for a rupture,” said Chris Goldfinger, a professor in OSU’s College of Earth, Ocean, and Atmospheric Sciences and lead author of the study. “That doesn’t mean that an earthquake couldn’t strike first along the northern half, from Newport, Ore., to Vancouver Island.

“But major earthquakes tend to strike more frequently along the southern end – every 240 years or so – and it has been longer than that since it last happened,” Goldfinger added. “The probability for an earthquake on the southern part of the fault is more than double that of the northern end.”

The publication of the peer-reviewed analysis may do more than raise awareness of earthquake hazards and risks, experts say. The actuarial table and history of earthquake strength and frequency may eventually lead to an update in the state’s building codes.

“We are considering the work of Goldfinger, et al, in the update of the National Seismic Hazard Maps, which are the basis for seismic design provisions in building codes and other earthquake risk-mitigation measures,” said Art Frankel, who has dual appointments with the U.S. Geological Survey and the University of Washington.

The Goldfinger-led study took four years to complete and is based on 13 years of research. At 184 pages, it is the most comprehensive overview ever written of the Cascadia Subduction Zone, a region off the Northwest coast where the Juan de Fuca tectonic plate is being subducted beneath the continent. Once thought to be a continuous fault line, Cascadia is now known to be at least partially segmented.

This segmentation is reflected in the region’s earthquake history, Goldfinger noted.

“Over the past 10,000 years, there have been 19 earthquakes that extended along most of the margin, stretching from southern Vancouver Island to the Oregon-California border,” Goldfinger noted. “These would typically be of a magnitude from about 8.7 to 9.2 – really huge earthquakes.

“We’ve also determined that there have been 22 additional earthquakes that involved just the southern end of the fault,” he added. “We are assuming that these are slightly smaller – more like 8.0 – but not necessarily. They were still very large earthquakes that if they happened today could have a devastating impact.”

The clock is ticking on when a major earthquake will next strike, said Jay Patton, an OSU doctoral student who is a co-author on the study.

“By the year 2060, if we have not had an earthquake, we will have exceeded 85 percent of all the known intervals of earthquake recurrence in 10,000 years,” Patton said. “The interval between earthquakes ranges from a few decades to thousands of years. But we already have exceeded about three-fourths of them.”

The last mega-earthquake to strike the Pacific Northwest occurred on Jan. 26, 1700. Researchers know this, Goldfinger said, because written records in Japan document how an ensuing tsunami destroyed that year’s rice crop stored in warehouses.

How scientists document the earthquake history of the Cascadia Subduction Zone is fascinating. When a major offshore earthquake occurs, Goldfinger says, the disturbance causes mud and sand to begin streaming down the continental margins and into the undersea canyons. Coarse sediments called turbidites run out onto the abyssal plain; these sediments stand out distinctly from the fine particulate matter that accumulates on a regular basis between major tectonic events.

By dating the fine particles through carbon-14 analysis and other methods, Goldfinger and colleagues can estimate with a great deal of accuracy when major earthquakes have occurred over the past 10,000 years.
Going back further than 10,000 years has been difficult because the sea level used to be lower and West Coast rivers emptied directly into offshore canyons. Because of that, it is difficult to distinguish between storm debris and earthquake turbidites.

“The turbidite data matches up almost perfectly with the tsunami record that goes back about 3,500 years,” Goldfinger said. “Tsunamis don’t always leave a signature, but those that do through coastal subsidence or marsh deposits coincide quite well with the earthquake history.”

With the likelihood of a major earthquake and possible tsunami looming, coastal leaders and residents face the unenviable task of how to prepare for such events. Patrick Corcoran, a hazards outreach specialist with OSU’s Sea Grant Extension program, says West Coast residents need to align their behavior with this kind of research.

“Now that we understand our vulnerability to mega-quakes and tsunamis, we need to develop a culture that is prepared at a level commensurate with the risk,” Corcoran said. “Unlike Japan, which has frequent earthquakes and thus is more culturally prepared for them, we in the Pacific Northwest have not had a mega-quake since European settlement. And since we have no culture of earthquakes, we have no culture of preparedness.

“The research, though, is compelling,” he added. “It clearly shows that our region has a long history of these events, and the single most important thing we can do is begin ‘expecting’ a mega-quake, then we can’t help but start preparing for it.”

13-year Cascadia study complete – and Northwest earthquake risk looms large

A comprehensive analysis of the Cascadia Subduction Zone off the Pacific Northwest coast confirms that the region has had numerous earthquakes over the past 10,000 years, and suggests that the southern Oregon coast may be most vulnerable based on recurrence frequency.

Written by researchers at Oregon State University, and published online by the U.S. Geological Survey, the study concludes that there is a 40 percent chance of a major earthquake in the Coos Bay, Ore., region during the next 50 years. And that earthquake could approach the intensity of the Tohoku quake that devastated Japan in March of 2011.

“The southern margin of Cascadia has a much higher recurrence level for major earthquakes than the northern end and, frankly, it is overdue for a rupture,” said Chris Goldfinger, a professor in OSU’s College of Earth, Ocean, and Atmospheric Sciences and lead author of the study. “That doesn’t mean that an earthquake couldn’t strike first along the northern half, from Newport, Ore., to Vancouver Island.

“But major earthquakes tend to strike more frequently along the southern end – every 240 years or so – and it has been longer than that since it last happened,” Goldfinger added. “The probability for an earthquake on the southern part of the fault is more than double that of the northern end.”

The publication of the peer-reviewed analysis may do more than raise awareness of earthquake hazards and risks, experts say. The actuarial table and history of earthquake strength and frequency may eventually lead to an update in the state’s building codes.

“We are considering the work of Goldfinger, et al, in the update of the National Seismic Hazard Maps, which are the basis for seismic design provisions in building codes and other earthquake risk-mitigation measures,” said Art Frankel, who has dual appointments with the U.S. Geological Survey and the University of Washington.

The Goldfinger-led study took four years to complete and is based on 13 years of research. At 184 pages, it is the most comprehensive overview ever written of the Cascadia Subduction Zone, a region off the Northwest coast where the Juan de Fuca tectonic plate is being subducted beneath the continent. Once thought to be a continuous fault line, Cascadia is now known to be at least partially segmented.

This segmentation is reflected in the region’s earthquake history, Goldfinger noted.

“Over the past 10,000 years, there have been 19 earthquakes that extended along most of the margin, stretching from southern Vancouver Island to the Oregon-California border,” Goldfinger noted. “These would typically be of a magnitude from about 8.7 to 9.2 – really huge earthquakes.

“We’ve also determined that there have been 22 additional earthquakes that involved just the southern end of the fault,” he added. “We are assuming that these are slightly smaller – more like 8.0 – but not necessarily. They were still very large earthquakes that if they happened today could have a devastating impact.”

The clock is ticking on when a major earthquake will next strike, said Jay Patton, an OSU doctoral student who is a co-author on the study.

“By the year 2060, if we have not had an earthquake, we will have exceeded 85 percent of all the known intervals of earthquake recurrence in 10,000 years,” Patton said. “The interval between earthquakes ranges from a few decades to thousands of years. But we already have exceeded about three-fourths of them.”

The last mega-earthquake to strike the Pacific Northwest occurred on Jan. 26, 1700. Researchers know this, Goldfinger said, because written records in Japan document how an ensuing tsunami destroyed that year’s rice crop stored in warehouses.

How scientists document the earthquake history of the Cascadia Subduction Zone is fascinating. When a major offshore earthquake occurs, Goldfinger says, the disturbance causes mud and sand to begin streaming down the continental margins and into the undersea canyons. Coarse sediments called turbidites run out onto the abyssal plain; these sediments stand out distinctly from the fine particulate matter that accumulates on a regular basis between major tectonic events.

By dating the fine particles through carbon-14 analysis and other methods, Goldfinger and colleagues can estimate with a great deal of accuracy when major earthquakes have occurred over the past 10,000 years.
Going back further than 10,000 years has been difficult because the sea level used to be lower and West Coast rivers emptied directly into offshore canyons. Because of that, it is difficult to distinguish between storm debris and earthquake turbidites.

“The turbidite data matches up almost perfectly with the tsunami record that goes back about 3,500 years,” Goldfinger said. “Tsunamis don’t always leave a signature, but those that do through coastal subsidence or marsh deposits coincide quite well with the earthquake history.”

With the likelihood of a major earthquake and possible tsunami looming, coastal leaders and residents face the unenviable task of how to prepare for such events. Patrick Corcoran, a hazards outreach specialist with OSU’s Sea Grant Extension program, says West Coast residents need to align their behavior with this kind of research.

“Now that we understand our vulnerability to mega-quakes and tsunamis, we need to develop a culture that is prepared at a level commensurate with the risk,” Corcoran said. “Unlike Japan, which has frequent earthquakes and thus is more culturally prepared for them, we in the Pacific Northwest have not had a mega-quake since European settlement. And since we have no culture of earthquakes, we have no culture of preparedness.

“The research, though, is compelling,” he added. “It clearly shows that our region has a long history of these events, and the single most important thing we can do is begin ‘expecting’ a mega-quake, then we can’t help but start preparing for it.”