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Geography 2240A/B
Philip Egberts

Chapter 5 Megathrust Earthquakes and Tsunami 1.0 Introduction 1.1 Megathrust Earthquakes In Chapter 4 we looked at the geometry of various fault types, including a ‘thrust fault’ (Fig.1), where one rock block is clearly thrust up and over the other by the compressive stress on the system. On a much larger scale is a ‘megathrust fault’; here, we are dealing with the same relative action (one block moving up slope relative to the other), but we are considering the whole boundary zone between a subducting and an overriding plate rather than a specific, easily defined plane. In a simple schematic, Figure 2 shows an example of megathrust geometry; if you need a ‘real’ scenario, assume that the right-hand plate is the North American Plate and the left-hand one is the Juan de Fuca Plate. This would be the current situation on the west coast of British Columbia, where the uplift portion represents Vancouver Island. During the whole life time of a subduction zone (many tens of millions of years), the compressive activity between the plates never stops. However, the two plates typically do not slip continually and gently past each other as though they were well-lubricated. Typically, the high degree of friction between two massive plates means that they get stuck (or ‘locked’) at various contact points for some period of time, then suddenly release, and jump ahead. Figure 2 shows one such ‘jump’ as the friction energy that was locking them was overcome by the stress energy in the plates ( think about the section you read in the previous chapter about ‘elastic rebound’ for a demonstration of the release of stored ).ress energy So, during the time the plates are locked, huge stress is accumulating because – as we noted above - the plates themselves don’t stop their overall compression. That means that the rocks in the vicinity of the thrust zone must be greatly deformed. In Figure 2 we see that deformation expressed as progressive uplift and bending of the front edge (locked edge) of the right-hand plate. When the point is reached that the stress accumulated in the front end of the deformed plate is sufficient to cause the ‘lock’ to break, that sudden release of energy produces a megathrust earthquake. The world’s largest recorded earthquakes have all been megathrust earthquakes; these are the events responsible for release of something in the order of 90% of Earth’s seismic energy. I can’t find a really ‘tight’ definition of a megathrust earthquake, so let’s accept this:  occurs at an interplate zone where one plate subducts beneath another  occurs upon the sudden release of a previously locked section  has a magnitude greater than 7.0 and commonly in the range of 9.0. Having mentioned magnitude, we need to remember that for magnitudes up to about 7 the Richter Scale is just fine; for magnitudes higher, we start with the Richter Scale to get some idea of magnitude, but we get a final number by using the Moment Magnitude Scale (reviewed in Chapter 3). The big deal with high number magnitudes is that the great energy release produces shaking that lasts much longer than that of low magnitudes. Well, what about energy release: would a whole bunch of magnitude 2 earthquakes release the same energy as one magnitude 9 event? I guess we need to put some limits on our hypothesis here: let’s say we’re considering a subduction zone where there is a history of one magnitude 9 megathrust earthquake every 500 years. How many magnitude 2 earthquakes would it take over 500 years to release the same energy as one magnitude 9? The amount of energy released increases about 40 times with every unit increase on the magnitude scale. So, there would have to be 40x40x40x40x40x40x40 of these magnitude 2 quakes to release the same energy as one megathrust earthquake of magnitude 9. That equates to about one million magnitude 2 earthquakes every day for 500 years. Doesn’t happen! How often do megathrust earthquakes occur? The 500- year limit we used in the previous example is actually not a ridiculous number. In fact, they seem to happen anytime between 200 and 800 years for any given segment of a subduction fault zone that’s prone to periodic locking. As much as anything, I suppose that’s indicating the amount of stress energy a rock can store before it ruptures. There are many factors involved; as we run through various case studies, we’ll watch for any frequency evidence. What might the evidence be of past megathrust earthquakes? There are three main points of evidence:  Tsunami evidence. A tsunami is a series of large waves that travel outward from the point of a sudden, large, vertical displacement of water. The waves can have enormous dimensions and travel through complete ocean basins readily (We will look at tsunami in great detail later in the ch. The type of chaotic deposit made by a tsunami is now well known and recognizable in the geologic record. The sudden upward motion of a plate edge by a megathrust action, with its large water volume displacement, is the ideal way to generate a massive tsunami.  Submerged coastlines. We can recognize rapidly submerged coastlines because of the drowned trees and other vegetation within the sedimentary sequence. We can date the organic matter, thus get a fairly decent idea of the time of the tsunami and thus the earthquake. It is, of course the rapid release of stress (see Fig. 2 again) that suddenly pops the coastline under water.  Large underwater landslides. Particularly on the edges of continental shelves great deposits of rather loosely compacted sediments accumulate, the sediments originating from the discharge of large rivers. Any large shaking 1 event – particularly one that lasts as long as a magnitude 9 earthquake could generate – will bring down those sediments in enormous landslides. Of course, the fact there’s been a bunch of megathrust earthquakes in the distant past doesn’t mean there will ever be another in the exact same location. But we can tell easily if that’s the case. For example, we know for sure there will be a megathrust earthquake off the west side of Vancouver Island; the plate subduction there is still very active, and we can see ample evidence of the stress accumulating in the rocks by the deformation and uplift going on. So, it’s a matter of knowing what the plate tectonic model is, and recognizing the symptoms of stress accumulation. 1.2 T sunami 1.2.1 Basics 3 Before we can productively consider case studies of megathrust earthquakes, we need to consider tsumai, since the most destructive component of these very dynamic events originate from dame and casualties produced by the tsunami alone. This is a large sub-section and it’s well-worth spending some good learning time Tsunami is the name given to a set of ocean waves caused by any large, abrupt displacement of water. The word tsunami is a Japanese word, represented by two characters: tsu, meaning, "harbor", and nami meaning, "wave"; both singular and plural uses of the word have the same spelling. In the past, tsunami were sometimes referred to as "tidal waves" by the general public and as “seismic sea waves” by the scientific community. The term "tidal wave" is a misnomer; tsunami are unrelated to the tides. The term "seismic sea wave" is also misleading. "Seismic" implies an earthquake-related generation mechanism; while the vast majority of tsunami are indeed related to earthquakes, they may also be caused by a non-seismic event, such as a landslide, a violent volcanic island eruption or meteoroid/asteroid impact. 2 Figure 4 illustrates the terminology used to describe tsunami. These terms are obviously the same as used to 4 describe any other type of wave, including light waves and seismic waves. However, tsunami are unlike wind- generated waves, which many of you may have observed on a local lake or at a coastal beach. The wind-generated swell one sees at a beach, for example, spawned by a storm and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 200 km and period on the order of one hour. In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr. Tsunami not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses. 1.2.2 Measurement of Tsunami Magnitude Various scales are used for measurement of tsunami magnitude. These scales define the magnitude of a tsunami in terms of the logarithm of the maximum wave amplitude observed locally. One scale is called the Imamura- Iida scale, in which tsunami magnitude is calculated as a function of the maximum wave height measured in the open ocean (Table 1). This may be difficult to apply strictly simply because of the topography of the shore line hit by the wave influences greatly the attenuation of the wave height as it moves in. 1.2.3 How do Earthquakes Generate Tsunami? Tsunami can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are 3 associated with Earth's crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created. Large vertical movements of Earth's crust can occur at plate boundaries. Plates interact along these boundaries called faults. Around the margins of the Pacific Ocean, for example, denser oceanic plates slip under continental plates in a process known as subduction. Subduction earthquakes are particularly effective in generating tsunami. 1.2.4 How do Landslides, Volcanic Eruptions, and Asteroids Generate Tsunami? A tsunami can be generated by any disturbance that displaces a large water mass from its equilibrium position. In the case of earthquake-generated tsunami, the water column is disturbed by the uplift or subsidence of the sea floor. Submarine landslides, which often accompany large earthquakes, as well as collapses of volcanic edifices, can also disturb the overlying water column as sediment and rock slump down-slope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can create an impulsive force that uplifts the water column and generates a tsunami. Conversely, super- marine landslides and cosmic-body impacts disturb the water from above, as momentum from falling debris is transferred to the water into which the debris falls. Generally speaking, tsunamis generated from these mechanisms, unlike the Pacific-wide tsunami caused by some earthquakes, dissipate quickly and rarely affect coastlines distant from the source area. 1.2.5 What Happens to a Tsunami as it Approaches Land? As a tsunami leaves the deep water of the open ocean and travels into the shallower water near the coast, it transforms. Scientists have discovered that a tsunami 5 travels at a speed that is related to the water depth - hence, as the water depth decreases, the tsunami slows (Fig. 5). The tsunami's energy flux, which is dependent on both its wave speed and wave height, remains nearly constant. Consequently, as the tsunami's speed diminishes as it travels into shallower water, its height grows. Because of this ‘shoaling effect’, a tsunami, imperceptible at sea, may grow to be several meters or even tens of meters in height near the coast. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, or a series of long-period breaking waves. 4 1.2.6 What Happens When a Tsunami Encounters Land? The behavior of a tsunami after it reaches shore is also different from that of a normal wave. Sometimes the level of water at the coast recedes noticeably just before a tsunami strikes. This phenomenon is called draw-down, and it is deadly for anyone who walks out to 6 see why the water dropped (Figure 6 the maximum 'draw down' in this plot occurs just after the first tsunami wave). But, like most other things in nature, at other times the first sign of a tsunami is a rise in water level. Because of its long wavelength, it can take a long time for a tsunami to crest and then recede. The water level may rise, reach a peak, and stay at the peak for several minutes. It can also take as much as an hour for the successive crests in a series of tsunami to reach shore. Note (in Figure 6) that just prior to arrival there is a general and slow rise in sea level (like the tide coming in). Then following the first wave (in this case, the first wave was very great, but it is not always so; commonly, the second or third wave will be greatest), there is a 'hissing sound' -always described by survivors at the shore - as the water is drawn out of a harbor or bay, to be followed by a virtual wall of water as the second wave approaches. In this case, which was a monitoring of the tsunami generated by the Krakatau volcanic eruption and measured in Jakarta, the time between first and second waves was 2 hours. OK, so as a tsunami approaches shore it begins to slow and grow in height. Just like other water waves, tsunami begin to lose energy as they rush onshore - part of the wave energy is reflected offshore, while the shoreward- propagating wave energy is dissipated through bottom friction 7 and turbulence. Despite these losses, tsunami still reach the coast with tremendous amounts of energy. Tsunami have great erosional potential, stripping beaches of sand that may have taken years to accumulate and undermining trees and other coastal vegetation. Capable of inundating, or flooding, hundreds of meters inland past the typical high-water level, the fast-moving water associated with the inundating tsunami can crush homes and other coastal structures. Tsunami may reach a maximum vertical height onshore above sea level, often called a run-up height, of 10, 20, and even 30 meters. The highest tsunami 5 wave ever experienced occurred in Lituya Bay, Alaska, following an immense rockfall into the head of the bay. The wave swept through the bay with a height of 150 meters and a speed between 150 and 210 km/hour. It surged to an incredible height of 524 meters above sea level along the head of the bay (Fig.7). Amazingly, once people knew what evidence to look for, they found clear evidence of two much, much higher wave surges marking events in 1936 and 1874. In the 1958 event, a couple of tourists, anchored for the night at one side of the bay, awoke as the tsunami pulled their boat (snapping the steel anchor chain) into the air, well over the tops of the trees, over the bars at the mouth of the bay, and dumped it (and them) into the Pacific Ocean; they survived! 1.2.7 Pacific Tsunami Warning Center (PTWC) The next event I’d like to mention is the one which prompted the USA in particular – but, really, all the Pacific Rim countries – that a warning system must be designed and activated by everyone. But I’ll tell you about the event after I tell you about the warning system – so you’ll see what a difference it could have made. The first effort was called the “Seismic Sea 8 Wave Warning System” but by 1949 it had morphed into the PTWC. The headquarters is at Ewa Beach, Hawaii, and the center is administered by the USA’s National Oceanic and Atmospheric Administration (NOAA). The objective of PTWC is to detect and locate major earthquakes in the Pacific region, to determine whether they have generated tsunami, and to provide timely and effective information and warnings to the population of the Pacific to minimize the hazards of tsunami. 9 Functioning obviously begins with detection, and the early part of that is completely automatic. Any seismograph in any one of the 26 participating countries that detects an earthquake of magnitude 6.5 or higher automatically sends a signal to PTWC headquarters (Fig.8). As data arrive, the location is determined; if the location 6 is in a spot where generation of a tsunami is a possibility, an automatic tsunami watch is issued to all Civil Defense agencies of participating countries. Then they wait for data from tidal gauges (Fig.9) in the region of the earthquake to see if a tsunami has actually developed. If the stations report that there is no observed tsunami activity, the tsunami watch is canceled. If these stations report that a tsunami has been generated, a tsunami warning is issued for areas that may be impacted in the next hour. At this time the public is informed of the ensuing danger by the emergency broadcast system. Evacuation procedures are implemented, and sea-going vessels are advised to head out to sea, where in deep waters they will not be affected by the tsunami. Now the event! As 1 April 1946 began in the Aleutian Islands, two large subduction ruptures occurred in the Aleutian Trench (Fig. 10), one right after the other, and shook the region severely. The 10 largest of the resulting earthquakes was estimated at the time to be magnitude 7.4 but has since (after paleoseismic research) been set at magnitude 8.1. During the quake, a large section of seafloor was uplifted rapidly along the fault where the quake occurred, producing a large, Pacific-wide tectonic tsunami. The most detailed and well documented accounts of the 1946 Aleutian tsunami come from Scotch Cap, located on Unimak Island, and the Hawaiian Islands. Despite its enormous size at Scotch Cap, the 1946 tsunami had little effect on the Alaskan mainland, due to the presence of the Aleutian Islands, which absorbed the brunt of the tsunami's power, shielding the mainland. Approximately 48 minutes after the earthquake, a 30.5 m tsunami struck the area of Scotch Cap. The tsunami completely destroyed the newly built (concrete and reinforced steel) US Coast Guard lighthouse, surging over the costal cliff to a height of 42 m above sea level. All five members of the lighthouse crew were killed. Racing across the entire Pacific Ocean at 780 km/h, at 11 approximately 7 7 a.m., less than five hours after the earthquake in Alaska, the first of several tsunami waves reached the Hawaiian Islands. The tsunami caught Hawaii completely unaware (Fig.11: not a very good picture, but I’d say the photographer was in a bi), as therry destruction at Scotch Cap prevented the transmission of any warning message until it was too late. The tsunami waves produced extensive destruction along the shorelines of the Hawaiian Islands, especially at Hilo, on the big island of Hawaii, where the city's entire waterfront was destroyed. Wave heights across the Islands reached an estimated maximum of 17 m, 11 m and 10 m on Hawaii, Oahu, and Maui, respectively. The tsunami inundated areas up to 0.8 km inland in some locations. A total of 159 people were killed as a result of the tsunami in Hawaii. 2.0 Case Study 1: The Cascadia Subduction Megathrusts On January 26 1700 a megathrust earthquake of magnitude approximately 9.0 ripped apart the locked section of the Cascadia Subduction Zone just to the west of Vancouver Island. The resulting tsunami became a thing of North American Indian legend, and a thing of death and destruction as far away as Japan and China. This is a great place to begin actual case studies because this was certainly not the first event of its kind in that region, and we are more than a little anxiously awaiting the next. 2.1 Cascadia Plate Tectonics The west coast of North America has every type of plate tectonics you’d ever want to see, thus every category of earthquake is there somewhere! In order to ‘set the scene’ for our megathrust earthquake discussion, we need to quickly review regional plate tectonics over the past few million years. Follow along on Figure 12. About 80 mya (million years ago), spreading activity dominated the Pacific 12a Ocean; the Pacific plate existed to the west, the Kula to the north and the 12b Farallon plate to the east (Fig.12a). Both the Kula and the Farallon were subducting under the North American Plate (convergent boundar; the boundaries between the Pacific plate and the Kula and between the Pacific plate and the Farallon 12c 12d were spreading centers (divergent bounda. The 8 action was pretty intense, and by 40 mya both the Kula and Farallon plates were fragmenting and disappearing down their respective subduction zones (Fig.12b). At 20 mya, the Kula was gone and only two remnants of the Farallon were left (Fig.12c). So, beginning about 40 million years ago, the spreading ridge between the Pacific plate and the Farallon plate touched the North American plate. Fragmentation of the Farallon plate - by an increasing number of transform faults perpendicular to the spreading ridge – became more obvious. Today, only a few fragments of the original Farallon plate remain unsubducted, themselves given plate names (Fig.13): Explorer ( far northern fragment, commonly grouped with the Juan de Fuica), Juan de Fuca ( the large), Gorda ( on the southern tip, also commonly grouped with the Juan de Fuc), and Cocos ( a fairly large fragment to the west of Mexico and Central America; we’ll not mention it fur). As each plate fragment of the original Farallon was subducted ( i.e. the ones already gone by to), the net effect of the various tectonic movements was to produce a strong transform motion along the edge of the North American plate; the major transform boundary to the north is the Queen Charlotte Fault and to the south, the San Andreas Fault – but we’ll look at transform faults and their earthquakes later. In the past few million years, as the Juan de Fuca was subducted more and more, the spreading center between the Pacific Plate and the Juan de Fuca moved closer to the subduction trench, with the conseq
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