Tsunami Scattering and Earthquake Faults in the Deep Pacific Ocean
Total Page：16
File Type：pdf, Size：1020Kb
Special Issue—Bathymetry from Space Tsunami Scattering and Earthquake Faults in the Deep Pacific Ocean Harold O. Mofjeld • This article has been published in Oceanography, Volume 1 Volume This article in Oceanography, has been published NOAA, Pacific Marine Environmental Laboratory Seattle, Washington USA Rockville, MD 20851-2326, USA. Road, 5912 LeMay The Oceanography machine, reposting, or other means without prior authorization of portion photocopy of this articleof any by Christina Massell Symons, Peter Lonsdale Scripps Institution of Oceanography • La Jolla, California USA Frank I. González, Vasily V. Titov NOAA, Pacific Marine Environmental Laboratory • Seattle, Washington USA Introduction Tectonic processes in the deep ocean occur over an Eruptions of island and submarine volcanoes can immerse range of temporal and spatial scales. The also create tsunamis and are of great interest in under- shortest in time are from seconds to minutes during standing how volcanic islands and seamounts form. 7 which earthquakes and landslides occur. The recur- Examples of recent volcanic events that have or could , Number rence interval between earthquakes, submarine land- generate tsunamis in the Caribbean Sea are the eruptions slides, and volcanic eruptions may be tens, hundreds, of the submarine Kick’em Jenny Volcano near Grenada 1 or thousands of years. At the far end of the range are and the Soufriere Volcano on Montserrat. Other exam- All rights reserved.Reproduction Society. The Oceanography 2003 by Copyright Society. The Oceanography journal, a quarterly of the tens of millions of years over which oceanic plates ples are the eruptions and landslides from Vulcano, and form at the spreading centers and drift, sometimes other islands near Italy, and the worrisome movement of thousands of kilometers with speeds of a few centime- the South Flank on the Island of Hawaii. ters per year, to the subduction zones. It is also over In this paper, we will discuss two very different these long time scales that chains of volcanic islands projects involving geological features in the Pacific and seamounts form and over even longer time scales Ocean (Figure 1). The first focuses on the effects of that major asteroids may impact the Earth. These seamounts and other wave scatterers on tsunamis processes create a host of smaller-scale geological fea- propagating across the Pacific, while the second is a tures, including rift valleys, earthquake fault scarps, study of deep-water earthquake faults near the Peru- submarine landslide deposits, and abyssal hills, which Chile and Tonga-Kermadec Trenches in the South cover vast areas of the ocean floor. Using a wide vari- Pacific. What is common about these ongoing projects ety of methods, scientists are developing a better is that they both rely on accurate, fine-detail bathyme- understanding of these geological features and the try over large areas of the deep ocean. The bathymetry prohibited. Send all correspondence or Society is strictly to: [email protected], processes that create them. is needed to establish the existence of geological fea- When a submarine earthquake occurs on the conti- tures in a region and then to quantify their spatial dis- nental shelf or slope, one consequence can be the gen- tributions, sizes, orientations, shapes, and ranges of eration of a tsunami that then propagates across the depth. Limitations in the accuracy and spatial resolu- ocean and impacts distant shorelines as well as the tion of the bathymetry also limit what can be learned nearby coast. These earthquakes can generate land- about the features and their effects of oceanographic slides that in turn increase the overall amplitude of the processes. In turn, this leads to a set of requirements for resulting tsunamis. This apparently occurred during new bathymetric data that are needed in order for the the deadly 1946 Alaska Tsunami in Hawaii, which led research to advance. to the creation of the U.S. Tsunami Warning Centers. The recent 1998 Papua New Guinea earthquake and Trans-Oceanic Tsunamis tsunami killed over 2,000 people. Ship surveys carried Tsunamis are long gravity waves that are generat- out soon after this event revealed a large, newly ed in the ocean by abrupt geophysical events such as formed landslide scarp (cliff) immediately offshore of earthquakes, landslides, or volcanic eruptions. the tsunami inundation zone. A volumetric analysis of Sometimes these processes work in conjunction the slide and tsunami modeling has shown that the to enhance the amplitudes of the tsunamis. There is landslide contributed substantially to the tsunami. also evidence that huge tsunamis have occurred in the Oceanography • Vol. 17 • No. 1/2004 38 Figure 1. The Pacific Ocean basin is rimmed by plate tecton- ic subduction zones and their associated ocean trenches and chains of volcanic arcs, popu- larly known as the “ring of fire”. The largest earthquakes in the world are generated in sub- duction zones. Submarine earthquakes, eruptions, and landslides generate tsunamis, which can bring catastrophic flooding to coastal areas in a matter of minute or hours. The bathymetric features identified here play a role in generating, scattering, or focusing these waves, as discussed in the text. abrupt bottom movements that generated them have Table 1 large horizontal scales. Smaller-amplitude source Representative tsunami phase speeds c = √gH based events tend to have smaller spatial scales, but their on long gravity waves where g is the acceleration of tsunamis are usually significant only near the source. gravity (g = 9.8 m/s2) and H is the local water However, the ground movements associated with depth. major subduction earthquakes can have finer-scale structure. Localized areas of stronger ground move- Depth Phase Speed ment, called asperities, often occur. Indeed, model sim- (m) (m/s) (km/hr) ulations of tsunamis are used to estimate the source distribution of the tsunamis by tuning the simulations 50 22 80 to tsunami observations from coastal tide gauges and offshore pressure gauges. 100 31 113 Multiple reflections and partial wave trapping, especially near the source, produce an extended wave 500 70 252 train of many waves even though the original source 1000 99 356 was a single impulse. The later waves in a tsunami form very complicated patterns in which it is difficult 2000 140 504 to determine the relationship of a later wave to the ini- tial source. Hence, only the observations of the first few 3000 171 617 waves at a site are used for comparison with model 4000 198 713 simulations and estimation of the earthquake source parameters. 5000 221 797 The phase speed at which tsunami waves move is given by the long wave formula. As shown in Table 1, 6000 242 873 these waves travel across the deep ocean at hundreds of kilometers per hour. Hence it takes a number of hours for a tsunami to traverse a major ocean, whereas the same tsunami impacts the local coast within a few geological past when asteroids struck the Earth. minutes. The period T of a tsunami (the time elapsed Tsunamis are called long waves because their wave- between the passage of two successive wave crests) is lengths (the distances between successive wave crests) determined by the wavelength and the phase speed c are much longer than the water depth. through the dispersion relation = cT. The typical peri- Tsunamis have such long wavelengths (tens to ods of trans-oceanic tsunamis range from about 2 to 90 hundreds of kilometers in the deep ocean) because the minutes. Oceanography • Vol. 17 • No. 1/2004 39 If the oceans were of constant depth, the first waves of trans-oceanic tsunamis would arrive at an impact site by a great circle route on the globe. However, broad-scale variations of the water level cause the tsunami waves to refract with a tendency for the wave crests to turn toward shallower water. Using wave ray-tracing methods and coarse bathymetric grids, travel-time maps were constructed by tsunami warning centers to predict when tsunamis would arrive at impact sites from the major source areas around the Pacific Ocean that have a history of pro- ducing dangerous tsunamis. Tsunami Wave Scattering In the late 1980s, the NOAA/Tsunami Research Program at the Pacific Marine Environmental Figure 2. Model simulation of the 1996 Andreanof Laboratory began a program to develop numerical Island tsunami showing the instantaneous sea level pat- models with very high spatial resolution. This ongoing tern 5 hours and 35 minute after the earthquake, which R&D effort is part of the National Tsunami Hazard was located at the red dot, in the Aleutian Islands of Mitigation Program and has two main goals. The first Alaska. The land areas of Western North America are in is to create accurate tsunami inundation maps for U.S. the upper right; the Hawaiian Islands in the lower left. coastal communities bordering the Pacific Ocean, to be Note that the tsunami propagates as a train of waves, not used by emergency managers in developing evacua- just a single impulse, and that this train is refracted and tion plans. scattered in several directions by ocean bottom features. The second goal is to provide the U.S. Tsunami Of particular importance to coastal California, Oregon, Warning Centers with an improved tsunami forecasting and Washington states is the scattering pattern indicat- system, based on real-time reporting bottom pressure ed by the arrow (middle of the right hand edge), where a gauges (tsunameters) deployed in the deep Pacific and bathymetric ridge known as the Mendocino Escarpment a rapid-access database of tsunami model simulations acts as a wave guide and directs energy at nearby coastal with unit-amplitude sources in the tsunami source cities. Taken with permission from Mofjeld et al. (2001). areas around the Pacific Region. Called SIFT for Short- Figure 3.