Five years ago a tremendous tragedy fell upon people in countries bordering the Indian Ocean. I'm referring of course to the 26 December 2004 earthquake and tsunami that claimed over 230,000 lives. Today I will reflect upon the event and how far we've advanced since then with an emphasis on how space technologies can aid in tsunami warning. Currently, tsunami warning systems do not rely heavily upon space technologies other than for communications, but there are several promising new techniques that could dramatically improve our tsunami detection capabilities in the future.
The massive 2004 earthquake occurred at the Sunda Trench, which is a subduction zone about 300 kilometers west of the Sumatra, Indonesia where the Indo-Australian Plate slides beneath the Burma Plate in the eastern Indian Ocean. The shaking from the huge earthquake lasted nearly 10 minutes (the longest ever observed), and the fault ruptured along an area spanning almost 1600 kilometers (also the longest ever observed). As the image to the right illustrates, seismic waves were detectable crossing the planet twice over. It was either the second or third largest earthquake ever recorded, depending on how you compute its size. At magnitude 9.5, the great 1960 Chile earthquake reigns supreme, but the 2004 Sumatra-Andaman event was either slightly larger (9.3) or slightly smaller (9.1) than the runner up magnitude 9.2 1964 Alaska earthquake. Both the 1960 and 1964 earthquakes produced deadly ocean-crossing tsunamis that led to the formation of the international Pacific Tsunami Warning System, with the Pacific Tsunami Warning Center (where I work) as its main operations center.
The International Charter on Space and Major Disasters was activated three times for the Indian Ocean tsunami and earthquake disasters, and over $7 billion USD in foreign aid poured into Indian Ocean countries to help the 2 million people who were displaced or killed. It was the worst natural disaster in modern times. The loss of life due to the huge tsunami could have been avoided if there had been an Indian Ocean Tsunami Warning System in place in 2004. A tsunami warning system is a complex marriage of politics, sociology, science, and technology. It involves having the technical means to detect earthquakes and tsunamis, the political means to share data and information, and the sociological means to educate the citizenry on what to do when they are alerted to danger. Unfortunately, no such system existed in the Indian Ocean at the time of the 2004 disaster. Since 2004, the Indian Ocean, Caribbean Sea, and Mediterranean Sea/northern Atlantic have joined the Pacific Ocean by establishing their own tsunami warning systems under the UNESCO/IOC umbrella.
What does all of this have to do with space? Potentially a lot. Currently, we rely upon a sparse network of seismic stations located on land and sea level stations located mostly on shore to detect earthquakes and tsunamis. In some places, the network densities are great enough that we can quickly detect and characterize events, but in other locations the density is poor, which impedes response time. In situ measurements like these are the best way to get high quality readings of exactly what the earth and ocean are doing, but they are only point measurements of what is really a continuous wave field. Since 70% of the Earth's surface is covered by oceans (through which the tsunamis travel), we are missing most of the story. Remote sensing of the Earth from space holds promise to revolutionize our characterization of geophysical hazards since we can get a bird's eye view of what's happening without having to wait for individual land-based stations to pick up signals.
Everyone remembers Kennedy's famous 1961 speech where he urged Congress to "commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the earth." It's not as well remembered that in that same speech Kennedy also charged Congress with allocating millions of dollars to use space satellites for world wide communications and weather observation. Thankfully, Congress delivered on all three counts. It's easy to see how satellite weather and communication satellites have revolutionized modern society. The shift from relying upon discrete surface measurements of weather at sparse stations to global observation platforms in space completely changed how we monitor meteorological phenomena. The same could be true for geophysical phenomena like earthquakes and tsunamis.
The figure on the right shows what is possible with sea level altimetry. If a satellite with an altimeter that measures sea surface height is in the right place at the right time, it can see a large tsunami from space. Recent work also points to the uefulness of satellite-based radar and microwave radiometers to detect tsunami intensity and direction from surface roughness. Basically, the wave can be tracked by looking at how choppy the sea looks even if an accurate height measurement cannot be determined. In order for either of these strategies to be useful for real-time tsunami warning, we'll need a constellation of satellites with instruments capable of detecting tsunamis. The Iridium NEXT constellation promises to host an array of scientific payloads, including radio altimeters that could be useful for tsunami detection.
Ionospheric seismology is perhaps the most promising new remote sensing technique to advance our detection and characterization abilities of earthquakes and tsunamis. Seismic surface waves and tsunamis deform the Earth's surface vertically by micrometers to centimeters. That deformation dynamically couples to the atmosphere, where it induces atmospheric infrasound and gravity waves. These waves propagate all the way through the atmosphere to the ionosphere. Seismic signatures in the ionosphere were first noticed during the 1964 Alaska earthquake but weren't well studied until the 2001 Peru earthquake and tsunami. Any electromagnetic wave that passes through the ionosphere can be used to detect these types of signals. This includes satellite- and ground-based radar systems as well as the radio signals used by GPS stations. By measuring the changes in the time it takes for the GPS signals to arrive at the receiver as they travel through Earth's atmosphere, scientists can derive a surprising amount of information about the Earth's ionosphere, including any seismic perturbations that might be there.
NASA and others are also using ground-based GPS measurements to characterize the actual land deformation caused by earthquakes and tsunamis in order to better characterize the tsunami source and hence better predict its propagation. Similarly, InSAR is widely recognized as a promising remote sensing technique to image large-scale land changes from space. InSAR can image dynamic changes on the Earth's surface such as ice movements, volcanic eruptions, landslides, earthquakes, and floods. Despite many high-profile recommendations from groups such as the National Academies and NASA, the US has not joined other countries in establishing its own InSAR satellites. Luckily, the DESDynI mission is on the books for a 2012-2013 launch. What we really need is to get more operational rather than research satellites with the type of InSAR capabilities that DESDynI will have, but there seems to be a disconnect between the research (i.e., NASA) and operational (i.e., NOAA) satellite operations in the US. I've also not seen much treatment of geophysical hazards like earthquakes and tsunamis from the Global Earth Observation System of Systems (GEOSS) initiative.
The holy grail in the seismic hazard business is predicting earthquakes (and hence tsunamis) before they happen. Currently that's just not possible. However, there is hope on the horizon. Mounting evidence is starting to support the conclusion that changing rock stresses beneath the Earth's surface emit ultra low frequency (ULF) electromagnetic signals that are detectable with magnetometers either on land or in space. This could provide warning hours to weeks in advance of earthquakes. A company in California called QuakeFinder is working very hard to establish enough monitoring stations to make this possible. They even operated the QuakeSat satellite from 2003-2004 as a proof of concept for remotely detecting ULF signals from space. They are not alone in this pursuit. The French have launched the DEMETER satellite to detect electromagnetic emissions from earthquakes. It's had some success too, as it did pick up a signal 7 days prior to the 2009 Samoa earthquake. The Russians also have a long history of research into this subject and have even launched the COMPASS-2 satellite to study earthquake precursors.
Maybe someday, in addition to in situ measurements, we'll rely upon a number of ground- and space-based remote sensing monitoring methods to provide advance warnings of large earthquakes and tsunamis. I'd estimate that this reality is one to two decades away, though.
The views expressed in this post are mine alone and don't necessarily represent those of the National Oceanic and Atmospheric Administration.
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