Find faults far away
For our project, R/V Marcus G. Lances Towing a 12-kilometer hydrophone streamer Record the sound reflected from the depths of the ocean floor, Which is helping scientists imagine The structure of the Cascadia subduction zone And large thrust failure. But streamers are not the only way to record the return of these earthquakes.
The streamer records data as a function of the time it takes for sound to travel down from the ship’s source to a “reflector” (a mirror of seismic waves, a geological boundary of sound waves) below the seabed and return to the seafloor. Hydrophone streamer. As the ship travels along the survey line, the long streamer records the arrival of sound waves from different paths, which allows scientists to determine the speed of sound wave propagation (ie, seismic velocity or wave speed) and convert the propagation time into depth. When the target depth is much shorter than the streamer, the streamer data can determine the wave speed and depth very well, but accurate simulation of wave speed at deeper crust and upper mantle depths requires the recording of seismic energy up to hundreds of kilometers. These very long distances have two main benefits: they can record reflections from a wider range of angles compared to a single streamer, and they allow refraction from deep in the earth to be recorded.
Through a simple theoretical model of the Cascadia subduction zone, rays from a single sound source point (red star) to towed hydrophone streamer (yellow line), submarine seismograph (yellow triangle) and land seismograph (white circle) path. As R/V Lances Moving along its survey line, seismic waves are recorded from many different intersecting ray paths, and scientists will use these ray paths to add detail and accuracy to the wave velocity model. (Illustration: Nathan Miller/US Geological Survey)
The refracted energy propagates horizontally in the earth, and these “ray paths” will be used to simulate the wave velocity structure of the subducting Juan de Fuca plate and the overlying North American plate. Since it is impractical to tow hundreds of kilometers of streamers, we use stationary seismographs on land and under the sea to record these very long-distance earthquake arrivals.
The stationary seismograph has another important benefit: it can record transverse waves. Seismic energy travels through the earth in the form of pressure waves (sound waves or P waves) and shear waves (S waves), but S waves are particularly useful for measuring the mechanical properties of sediments and fault zones. S wave does not propagate through water, so Lances Streamers cannot record them directly, but static seismographs can.
The fixed sensor is set to record data from Lances Sound pulses along the measurement line. This geometry records the energy propagating in opposite directions and crossing paths. Using a method called seismic tomography, similar to medical CAT scan (“T” in “CAT” stands for tomography), these long-distance data will be used to reconstruct the P and S wave velocity images of deep underground casca The structure of the Dia subduction zone.
At sea, we record long offsets and S waves by placing seismometers on the seafloor.at Lances The expedition begins, our team is from the United States Submarine Seismograph Center (OBSIC) and U.S. Geological Survey (USGS) An undersea seismograph was deployed off the coast of Oregon. These instruments consist of sensors (seismometers and hydrophones), batteries and recording packages, buoyancy buoys and weights. Once the ship reaches the predetermined launch position, the instrument will be deployed out of the ship and sink to the sea floor due to the weight of its bottom.Later, after Lances Drag its sound source onto these instruments, and the second team is Right/right Oceania, A research ship operated by Oregon State University, recovered the seabed seismographs and then redeployed them along the route further north, offshore of Washington and Vancouver Island. Oceania Waiting now Lances Before restoring all seabed seismographs for the last time, complete towing along these lines.
Tim Kane (Woods Hole Institute of Oceanography), Chris Armerding (Scripps Institution of Oceanography) and Jenny McKee (US Geological Survey) prepare to deploy subsea seismographs from R/V Oceania(Photo: Nathan Miller/US Geological Survey)
In order to restore the seabed seismograph, the ship returns to the launch position and sends a code to the seabed seismograph through a sonic pulser. Once the seismograph receives the code, it releases its weight, becomes positively buoyant, and then rises to the surface. Then the team on the ship must find the instrument on the sea and bring it back to the ship safely. This task can be assisted in many ways: directional radio transmitters and visual signs (such as bright yellow/orange), flags, and helpful flashing lights at night. Nevertheless, finding small instruments in the vast ocean is an acquired skill, especially in rough seas and bad weather. OBSIC engineers, science party and crew Oceania Hundreds of such operations have been performed in the past; they are a highly skilled team!
The seabed seismograph was recovered, while the other seismographs were on deck awaiting redeployment. (Photo: Nathan Miller/US Geological Survey)
On land, another team led by scientists from Oregon State University, University of Oregon, South Dakota School of Mines and Technology, and U.S. Geological Survey Earthquake Disaster Project has been busy Deploy and restore hundreds of temporary seismographsNearly 30 volunteers, including undergraduates, graduate students, and community college students and faculty, rode 15 cars through the Oregon Coast Mountains and southwestern Washington, and installed seismographs along the six lines in the instrument grid. 3D way to image the structure of the seabed. Like seabed seismometers, these instruments record seismic energy from the seafloor. Lances The sound source, but from a path that extends eastward and deeper into the subduction zone, and then travels hundreds of kilometers back to the land surface. This data is needed to image the onshore part of the subduction zone and the subduction slab deeper than about 10 to 15 kilometers. They also recorded local and regional earthquakes and background seismic noise, which will provide input data for a wide range of imaging techniques. Importantly, these data provide key measurements on how near-surface sediments and rocks respond to strong vibrations during large thrust earthquakes.These measurements are an important part of USGS Earthquake simulation And earthquake disaster models to guide building codes and emergency response plans.
Deploy temporary seismographs on land. (Photo: Lexi Arlen)
Ground movement during major earthquakes is also a problem offshore. The rapid movement of the seabed during the earthquake replaced water, triggering a tsunami. If shallow faults connect giant thrust faults to the seafloor, seafloor displacement is more likely to occur, and the possibility of these shallow faults sliding in a giant thrust earthquake depends on their structure and mechanical properties. During 2018 and 2019, USGS scientists collected detailed seismic reflection images Along many of the same tracks Lances It is measuring now.Combine these high-resolution data with Lances New images of deeper structures from new seafloor seismograph data and measurements of mechanical properties in and around the fault zone will provide scientists with a comprehensive view of the shallow faults they need to simulate seafloor displacement during large thrust earthquakes.
Earthquake-induced vibrations can also cause slopes to collapse (such as submarine landslides), which in turn triggers fast-moving turbidity currents, which deposit unique sediments called turbidity currents when these currents are stationary in the deep ocean. If it is large enough, submarine landslides may also trigger tsunamis or damage submarine infrastructure, such as communication cables. The sedimentary core taken from the Cascadia seafloor contains many turbidites, some of which have been attributed to large thrust earthquakes in the past. Knowing how sensitive the sea floor is to strong vibrations and linking past records of slope damage to earthquakes requires measuring the mechanical properties of the sea floor. The ability of seafloor seismographs to record S waves passing through seafloor sediments is particularly useful for solving this problem.
This large, multi-component experiment needs to be Lances, OceaniaAnd the onshore team. Similarly, analyzing all different types of data and integrating the results into a coherent understanding of the geological processes and hazards in the Cascadia subduction zone will require the work of many scientists from many institutions in the coming months and years.
Nathan Miller is a research geophysicist at Woods Hole in the United States Geological Survey (USGS).
Pablo Canales is an associate scientist in geology and geophysics at the Woods Hole Oceanographic Institution.
Anne Tréhu is a professor at Oregon State University.
