Frequently Asked Questions
If we are authorized following the pending environmental compliance process, we plan to survey the western North America margin extending from 10-20 km (~6-12 miles) to 100-200 km (~62-124 miles) offshore from southern Oregon to Vancouver Island in early summer, 2020. See map...
Our goal is to characterize the properties of the Cascadia subduction zone from Southern Oregon to British Columbia, a region where “giant” earthquakes have occurred in the past and other smaller damaging earthquakes like the 2001 Nisqually earthquake occur as the oceanic Juan de Fuca plate steadily descends beneath the North America continent. The last great earthquake at Cascadia occurred on January 26, 1700 AD and is estimated to have been a magnitude 9, similar to the devastating 2011 earthquake in Japan. Studies of the older history of large earthquakes along the margin indicate they occur every 300 to 550 years at Cascadia and Scientists estimate an ~14% chance that the next great earthquake will happen in the coming 50 years. The deep fault plane where the largest earthquakes within the subduction zone occur is eerily quiet at present with little seismicity detected within the Oregon to Washington portion. Scientists believe this lack of seismicity reflects the “locked” state of the megathrust at present and that stresses are currently quietly accumulating as the JdF plate system steadily descends beneath North America. With little seismicity recorded from the fault zone, little is known of the properties of this giant-earthquake-generating fault interface (i.e., depth, roughness, thickness, and acoustic impedance, among others) and how these properties vary along and across the Oregon to British Columbia margin. The current observations allow for a wide range of possible future earthquake and tsunami scenarios. Comprehensive characterization of this zone is needed to quantify earthquake and tsunami hazards within the region.
The portion of the fault zone that generates the largest earthquakes is believed to be located almost entirely offshore, extending from close to the shoreline to ~100 km (55 naut. miles) from the coast. Thus marine surveys are needed to investigate its structure. The specific goals of our study are to use modern deep penetration multi-channel seismic (MCS) and ocean bottom seismometer data to image the internal structure and characterize the properties of this key offshore portion of the Cascadia Subduction Zone. This regional characterization would be used to determine whether there are any systematic relationships between properties of the subducting “lower” plate and the overriding North America plate, the areas are believed to have ruptured during past earthquakes (paleo-rupture segments), and variations along the margin in how “locked” the present-day megathrust fault zone is at Cascadia. The data would also be used to characterize variations along the megathrust that may be linked to transitions in fault properties, from the shallowest region near the deformation front, which is of most interest for tsunamigenesis, to closer to the shore where the downdip transition in the locked zone may reside. The data would also enable researchers to investigate the physical properties of the continental slope sediments and crust, which are critical for predicting the intensity of earthquake-triggered shaking along the Pacific Northwest and assessing tsunami/landslide hazards under hypothetical scenarios of future earthquake rupture. The proposed study would be the first such regional-scale seismic imaging investigation ever conducted spanning nearly the entire length of the Cascadia Subduction Zone and would move the Cascadia megathrust zone from arguably one of the least well characterized heavily populated megathrust regions in the world to one of the best.
Though separately managed, we plan to coordinate our effort with researchers from Oregon State University, the University of Oregon, the South Dakota School of Mines, and the U.S. Geological Survey, who have proposed to conduct an onshore experiment to model the subsurface beneath the continental margin and coastal ranges of Oregon and Washington through the deployment of seismometers during the same timeframe as the proposed marine survey. The onshore recordings of the offshore sources would help image the transition to where the plates are moving in slow jerks that occur approximately every 11-24 months in events known as Episodic Tremor and Slip (ETS).
Aside from the localized surveys conducted in 2012 by R/V Langseth using an 8-km (5 mile) streamer, no modern deep penetration multi-channel seismic (MCS) data have been acquired at the Cascadia Subduction Zone. (See #7 for more information about streamers) Data acquired prior to these surveys were collected in the ‘80s and ‘90s with much shorter streamers (2.6–4 km or 1.6-2.5 mile) and in most cases, with poorer quality sound sources, and thus provide a weak-to-no image of the deep megathrust earthquake fault zone at Cascadia. Long streamer (>8 km) MCS data represent major advances over the previous generation of MCS studies in the region for two primary reasons. First, the data acquired with long streamers enable application of advanced computational techniques providing clearer images of the Cascadia fault zone to much greater geologic depths beneath the seafloor than previously obtained. Second, long-streamer data enable us to determine the speed at which sound waves propagate through the sediments and crust at high-resolution, which both improves the imaging of structures and also provides information on physical properties of the fault zone. These properties are linked to whether and how different parts of the fault zone may slip in a large earthquake in the future. The proposed study would also provide the first homogeneous regional-scale characterization of the Cascadia Subduction Zone, enabling the first study of how properties of the megathrust fault zone vary along the margin in a systematic and self-consistent manner.
Modern long-offset marine seismic reflection imaging techniques provide the best tools available to scientists for illuminating the details of subduction zones to the depths of the earthquake source region and below. Over the past 2 decades, regional surveys using these tools have been conducted at many of the world’s subduction zones including regions near population centers of South and Central America, Japan and New Zealand but, to date, no such study has been conducted at Cascadia. The proposed study would provide similar data from the Cascadia margin as now exists for these other regions and would provide a modern basis of characterization useful for the various studies of earthquake and tsunami hazard that are underway at Cascadia by scientists, government agencies and other groups.
While modern deep penetration MCS studies are very limited at Cascadia, there have been other kinds of research seismic expeditions conducted in the recent past in the region. In 2018, the USGS began a series of high-resolution seismic studies using small energy sources designed to image the structure of the sediment section along the Cascadia margin above the megathrust fault zone. Earlier, from 2011 to 2014, a large suite of broad-band OBS’s were deployed as part of the Cascadia Initiative Amphibious Array experiment to listen for local and distant earthquakes with goals of imaging deep into the mantle beneath the Juan de Fuca plate and Cascadia subduction zone and to detect any small earthquakes occurring at or near the megathrust. Our proposed study complements these earlier studies and ongoing ones and would provide information on regional structure useful for advanced interpretation and analysis of these other data sets.
The projects would be funded by the National Science Foundation (NSF) with US Geologic Survey (USGS) support as a Cooperating Agency. The projects were reviewed through a peer-reviewed process with awards to researchers at Columbia University, Woods Hole Oceanographic Institution, and the University of Texas at Austin. If we are authorized following the environmental compliance review now in progress, we would use the R/V Langseth, the premier U.S. academic seismic research vessel, which is owned by the NSF and operated by the Lamont-Doherty Earth Observatory of Columbia University (read more here), and the R/V Oceanus, which is owned by the NSF and operated by Oregon State University (read more here).
If, following the environmental compliance process, our projects are approved, our data would be computer-processed and ready for scientific analysis roughly six months to one year after the cruise. It would then be placed in NSF-supported, public archives of the Marine Geoscience Data System - Academic Seismic Portal (read more here) and the consortium of Incorporated Research Institutions for Seismology (IRIS) (read more here).
Like a medical sonogram that uses sound to make an acoustic image of tissue beneath the skin, our proposed plan is to make acoustic images and derive estimates of the physical properties of features within sediment and crustal layers below the seafloor using an array of seismic airguns as our sound source. Towed hydrophone streamers and ocean bottom seismometers (OBSs) and nodes (OBNs) would detect and record the returning sound source signals.
An airgun is a device towed roughly 100 feet behind a ship that at regular intervals releases a bubble of compressed air below the sea surface. Like the pop of a balloon that creates a sound wave, an airgun creates a sound wave that travels down and into the seafloor. Hydrophone “streamers” (which act like a microphone) towed behind the ship listen for airgun signals, or “echoes”, to return from sediment and crustal layers below the seafloor. Shipboard computers arrange these echoes to make acoustic images of this layering.
OBS and OBN’s are instruments equipped with sensitive hydrophones and seismometers and digital recording packages that, when placed on the seafloor, detect and record the very subtle vibrations of the seafloor and sounds propagating through the ocean produced by natural sources like distant earthquakes and local microseisms, storms, or cetaceans, as well as from man-made sources like ship traffic and airguns.
Instruments referred to as OBSs are generally mid-size (~1x1x1 meter or 1x1x1 yard), have sensors capable of responding to lower frequency sound, and operate in shallow to very deep ocean depths (up to ~6,000 m). OBS are generally deployed from ships as free-fall instruments which sink due to the weight of an anchor. For recovery, an acoustic signal triggers the release of the anchor and the OBS floats back to the surface due to its buoyancy.
Instruments referred to as OBN are typically smaller in size (0.3x0.3x0.15 meters or ~1x1x1/2 feet) than OBSs, are sensitive to higher frequency sources, and operate in shallow to intermediate ocean depths (up to ~3,000 meters). Due to their reduced size, OBNs lack buoyancy and therefore have to be deployed on, and retrieved from, the seafloor using remotely operated vehicles (ROV).
We propose to use 36 airguns (and 4 spares) towed 12 meters (~13 yards) below the sea surface which together would release a total of 6600 cubic inches of compressed air. The sound wave generated by this bubble of compressed air would enable us to image to depths of 20 km (12.5 miles) or more beneath the seafloor where the Cascadia giant earthquake fault zone is located near the coast. Further offshore, this sound wave would enable us to image structures even deeper than the fault zone, thus linking deep geological structures within the Earth’s upper mantle to the properties of the earthquake fault zone. The sonogram-like images we would obtain to these great depths would provide new information pertaining to the properties and processes contributing to large subduction zone earthquakes along this margin. Smaller energy sources have recently been used for studies along the Cascadia margin to image the sediment layers above the fault zone, but the fault zone itself cannot be detected using these smaller sources. The energy source we plan to use is the same as the one we used during a similar study in 2012 in the region.
An excellent resource for more information on sound within the oceans is the “Discovery of Sound in the Sea” (DOSITS) website. This website is a resource developed by ocean acousticians to introduce students, public officials and the general public to the wide range of sound sources in the sea (animals, volcanoes, hurricanes, ships, etc.) as well as to how sound is used to study the seafloor and oceanographic processes. DOSITS content is based on peer-reviewed literature and high-quality sources of scientific data. Of particular relevance to those interested in our project are the following pages: seismic-airguns and sound-in-air-water.
No. These projects would be sponsored by the National Science Foundation in support of basic research to increase our understanding of fundamental earth processes related to earthquake and tsunami hazard along the Cascadia margin.
Although not funded through NSF, collaborators from the USGS and Canadian academic institutions (Dalhousie University and Simon Fraser University) would work with us to achieve the research goals, providing assistance, such as through logistical support and data acquisition and exchange. Canadian colleagues are also interested in obtaining an improved understanding of the Cascadia margin given the shared threats of a future megathrust earthquake all along the west coast of North America.
Our proposed study would be conducted following protection protocols determined by international and U.S. federal and state environmental compliance processes, detailed explanation of which can be found in the National Science Foundation's Draft Amended Environmental Assessment (EA) and in the applications for Incidental Harassment Authorizations (IHA) submitted to the National Marine Fisheries Service (NMFS) and U.S. Fish and Wildlife Service (USFWS). If our survey is authorized, a robust monitoring and mitigation plan would be followed, similar to that conducted during a similar study in the region carried out in 2012. The monitoring and mitigation plan would include the use of Protected Species Observers, passive acoustic monitoring, shutdowns of the airguns for marine mammals observed within specified distances from the source, and special mitigation efforts for killer whales (further details in next question).
In terms of fishing, no significant impacts on marine invertebrates, marine fish, and their fisheries, including commercial, recreational, and subsistence fisheries are anticipated. No adverse effects on Essential Fish Habitat (EFH) or Habitats of Particular Concern (HAPC) are expected given the short-term nature of the sound source study (approximately 37 days) and minimal bottom disturbance. In addition, we have initiated outreach efforts with the local commercial fishing community to minimize potential overlap between seafloor instrumentation deployments and fishing activities and help support coordination during our survey period.
The mitigation and monitoring measures that would be employed during the survey to reduce potential impacts to marine species, would include vessel speed reduction or minor course alteration, and the use of passive acoustic monitoring of marine mammals. In addition, Protected Species Observers (independently contracted and NMFS approved) would maintain visual watches for marine species around the vessel. They would have absolute authority to enforce any terms and conditions in authorizations, such as the IHA, including shutting down the acoustic source if protected marine species are observed entering a specified radius around the vessel. With the proposed mitigation and monitoring measures, impacts to marine species would, at most, be expected to be limited to short-term, localized changes in behavior and distribution close to the vessel. No significant impacts to marine species populations or critical habitat would be anticipated from the proposed activities.
The purpose of the environmental compliance process is to evaluate the potential effects that the proposed research has on the environment. These effects are determined by many science professionals with expertise in this area, supported by an extensive foundation of published literature. The environmental compliance process for these projects would include compliance with the National Environmental Policy Act (NEPA), Executive Order 12114, the Marine Mammal Protection Act (MMPA), the Endangered Species Act (ESA), Coastal Zone Management Act (CZMA), Essential Fish Habitat (EFH) per the Magnuson Stevens Act, and the Canadian Fisheries Act and Species at Risk Act. In addition, a permit from the Olympic Coast National Marine Sanctuary is being sought. At all levels, highly experienced individuals and agencies responsible for maintaining a balance between protecting the marine environment and advancing knowledge of our planet are evaluating the potential impacts of the proposed activities with the utmost care. Any required monitoring and mitigation measures identified through these regulatory processes would be adhered to during operations.
There were public comment periods associated with some of the regulatory processes noted above. For example, in compliance with the MMPA, the NMFS and USFWS announced public comment periods in the Federal Register in association with the Incidental Harassment Authorization processes. Public comment periods were also part of NSF’s compliance with NEPA and CZMA. Individuals and groups were invited to submit comments during the public comment periods, and were encouraged to include any supporting data or citations to help inform agency decisions.