Global S&T Development Trend Analysis Platform of Resources and Environment
| DOI | 10.1126/science.abg4479 |
| Illuminating tremors in the deep | |
| William Wilcock | |
| 2021-02-26 | |
| 发表期刊 | Science
 |
| 出版年 | 2021 |
| 英文摘要 | The paucity of seismic stations in the ocean limits sustained seafloor seismic and pressure observations that are needed for rapid earthquake detection, early warning of damaging ground shaking, and tsunami prediction and verification. Because establishing infrastructure in the oceans is expensive, there is a big advantage to methods that use undersea telecommunication cables. On page 931 of this issue, Zhan et al. ([ 1 ][1]) describe a new approach to monitor earthquakes, oceanic swell, and potential tsunamis. It relies on observing changes in the polarization of the light that is used to transmit data through the optical fibers in cables on the seafloor. This approach requires no new infrastructure or instrumentation, but instead relies on utilizing observations already made to extract the telecommunications data received at the end of the cable. The finding should spur implementation of an array of cable sensing technologies for geophysical monitoring.
Polarized light is used to transmit data through telecommunication cables because it doubles the capacity of each optical fiber. Imperfections make fibers birefringent—that is, light is refracted in two different directions, and polarized light travels at slightly different speeds depending on the orientation of polarization. As a result of birefringence, the orientation and degree of polarization, known as the state of polarization, change as light travels along an optical fiber. Because the birefringence of a fiber is sensitive to changes in temperature and mechanical stress from squeezing, bending, and stretching, the polarization of the light emerging from a fiber changes with time. To extract the telecommunications signal, the receiver at the end of the cable continuously monitors the state of polarization. For cables on land, changes in the polarization are dominated by noise from temperature fluctuations, air flow, and anthropogenic activity.
![Figure][2]
Seafloor cables and earthquakes
Map centered on the Pacific Ocean shows the distribution of earthquakes of magnitude ≥6 in the U.S. Geological Survey catalog from 1970 through 2021, the location of submarine cables from TeleGeography's telecom resources licensed under Creative Commons CC BY-NC-SA 3.0, and regions within 10° of stations in the Global Seismic Network (light transparent areas).
GRAPHIC: CENTER FOR ENVIRONMENTAL VISUALIZATION, UNIVERSITY OF WASHINGTON
Zhan et al. discovered that in the thermally stable setting of the seafloor, the state of polarization can be used to observe geophysical signals. Using data from a cable connecting Los Angeles, California, to Valparaiso, Chile, they detected the body waves generated by moderate- to larger-sized earthquakes with predicted maximum ground displacements as small as 0.1 mm. They also observed an unanticipated later wave that traveled either in the ocean or as an interface wave along the seafloor. Polarization is not equally sensitive to earthquakes at all seismic frequencies, and the sensitivity appeared to vary with geographic location, suggesting a dependence on ocean depth or seafloor properties. For earthquakes in the Middle America Trench, the amplitudes of signals at 0.15 to 0.35 Hz increased with the predicted amplitude of ground motions. More work is required to understand how the birefringence responds to seismic waves and how this response is reflected in the signal integrated along the cable's length. However, the findings of Zhan et al. show that polarization monitoring has the potential to characterize earthquake size.
Continuous signals were also detected at around 0.06 Hz, a frequency that correlates with the amplitude of storm-generated seismic noise at coastal stations. These signals, known as primary microseisms, are generated by the seafloor pressures from ocean swells on the shallow continental shelf near either end of the cable. The observations of Zhan et al. do not extend down to the frequencies of 0.001 Hz necessary to detect tsunamis, but given that the amplitude of seafloor pressure variations from coastal swells is similar to that of damaging tsunamis in deep water, the technique has promise for this application. Future efforts to stabilize the environment of the shore stations, characterize the land segments of cable routes, and combine polarization measurements from the many communication channels (wavelengths) used in the typical submarine cable should improve signal-to-noise ratio and expand the bandwidth of observations.
Polarization monitoring adds to an exciting array of new technologies that exploit submarine telecommunication cables for seismic and tsunami observations. Ultrastable laser interferometry senses the strains from seismic waves by measuring changes in the time it takes light to travel the full length of a fiber ([ 2 ][3]). Unlike polarization monitoring, it requires specialized equipment in the shore station, but has the advantage of localizing deformation of the cable if observations are made from both ends. Distributed acoustic sensing also measures changes in strain through the effect on backscattered light. It turns a dedicated optical fiber into a sensor somewhat akin to a line of closely spaced and very broadband horizontal seismometers that can extend up to about 100 km from the interrogating electronics ([ 3 ][4]). The scientific potential of this approach has been demonstrated in several submarine cables ([ 4 ][5]–[ 6 ][6]), and it is being adopted commercially to monitor the nearshore portions of cables to identify sections at risk of failure. Science Monitoring and Reliable Telecommunications (SMART) cable systems would incorporate a package of conventional sensors, including accelerometers, seismometers, and pressure gauges, into the optical repeaters that are spaced 50 to 100 km apart along submarine cables ([ 7 ][7]). The concept was first proposed over a decade ago ([ 8 ][8]) and is now being planned for several locations, including a cable ring linking Portugal, the Azores, and Madeira. However, the approach is disruptive to established practices in the telecommunications industry and has yet to achieve traction on major transoceanic cables.
The current distribution of seafloor cables is not uniform but is quite well configured for monitoring the seismically active subduction zones that extend around the Pacific rim and along several other coastlines (see the figure). In regions where there are no cables, particularly the southern oceans, efforts to develop long-term autonomous seafloor seismometers ([ 9 ][9]), operate floating sensors ([ 10 ][10]), and deploy strategically placed arrays of temporary seismometers ([ 11 ][11]) are critical for improving the resolution of seismic images of the Earth's structure and the completeness of global earthquake catalogs. Where cables do cross ocean basins, they could transform academic seismology by providing new observations in regions that are presently poorly observed. Subduction zones are costly to instrument offshore with dedicated warning systems ([ 12 ][12]), but many of the most hazardous coincide with regions where telecommunication cables land. Here, distributed acoustic sensing could be supported by repurposing decommissioned commercial cables and by adding spare fibers to the nearshore portions of new installations. Polarization monitoring and ultrastable laser interferometry could extend observations further offshore using cables that run both perpendicular and parallel to coastlines. Incorporating fiber-sensing technologies into effective earthquake and tsunami warning systems will require that the signals are accurately quantified through extensive co-sited observations with conventional seismic and pressure sensors. Obviously, this can be most comprehensively achieved by making the new cables “SMART” with these sensors incorporated along their path where they would also contribute to warning. Given that SMART cables will also provide observations to constrain how climate change is affecting the temperature of the deep ocean, ocean circulation, and sea-level rise ([ 7 ][7]), it is time for cable suppliers and technology companies to step forward and work with scientists and governments to make them a reality.
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| 领域 | 气候变化 ; 资源环境 |
| URL | 查看原文 |
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| 文献类型 | 期刊论文 |
| 条目标识符 | http://119.78.100.173/C666/handle/2XK7JSWQ/315923 |
| 专题 | 气候变化 资源环境科学 |
| 推荐引用方式 GB/T 7714 | William Wilcock. Illuminating tremors in the deep[J]. Science,2021. |
| APA | William Wilcock.(2021).Illuminating tremors in the deep.Science. |
| MLA | William Wilcock."Illuminating tremors in the deep".Science (2021). |
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