Chapter 5 Megathrust Earthquakes and Tsunami PDF

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Chapter 5 Megathrust Earthquakes and Tsunami 1.0 Introduction In Chapter 4 we looked at 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 there 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 Fig 1. Thrust fault. Modified from Dawes and Dawe (2013), Basics – Geologic structures. Retrieved from http://commons. wvc.edu/rdawes/G101OCL/Basics/structures. html.

‘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 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 stress 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. Fig 2. Development of a megathrust earthquake. Modified from USGS (2005), Surviving a Tsunami—Lessons from Chile, Hawaii, and Japan. Retrieved from http://pubs.usgs.gov/circ/c1187/c1187.pdf.

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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 30 times with every unit increase on the magnitude scale. So, there would have to be 30x30x30x30x30x30x30 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 chapter). The type of chaotic deposit made by a tsunami is

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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

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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 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 that 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. 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 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.

Fig 3. Spreading history of the northeastern portion of the Pacific Ocean. Modified from Dutch (2001), The North Pacific and the West coast of North America. Retrieved from https://www.uwgb.edu/dutchs/plate tec/kula.htm.

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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! You can see that many plates have been subducted under the North American plate over the last 80 million years in figures 3 and 4 (You do not need to memorize the names and movements of all these plates). I will simplify by saying that an older plate called the Farallon has been subducted under the North American plate. The net effect of this 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.

Fig 4. Tectonic plates off of the west coast of North America. By B.C. Institute of Technology and Department of Civil Engineering (n.d.),

Part of the old Farallon plate is now called the Juan Earthquakes. Retrieved from http://commons.bcit.ca/ civil/students/earthquakes/unit1_02.htm. de Fuca plate. 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 (Fig. 4). Because the Juan de Fuca plate is fairly young, it is still warm and buoyant. This means that the subducting slab doesn’t readily sink and just skims the bottom of the continental lithosphere of the North American Plate. It tends to get stuck temporarily, with the result that stress builds at the sticking points. When release occurs, the resultant earthquakes are very large. These are the megathrust earthquakes. Figure 5 shows the Juan de Fuca Plate descending (subducting) beneath the edge of the North American Plate, together with the locations of a few generalized earthquakes, their magnitudes and frequency of recurrence. Note the location of the 1700 earthquake. Fig 5. Cascadia subduction zone. Modified from USGS (n.d.), Cascadia earthquake sources. Retrieved from http://geomaps.wr.usgs.gov/pacnw/pacnweq/pdf/subd_eqpg.pdf.

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2.2 The 1700 Megathrust Event 2.2.1 The Evidence In 1805 Lewis and Clark – the legendary explorers of western North America – wrote about the many strange stumps of large trees sticking up out of the salt-rich mud of the Columbia River flats near the Pacific Ocean. They also noted the abundance of chaotically strewn boulders and rocks that had no apparent local origin. Today, we recognize these features, respectively, as a forest of redwood trees drowned by a sudden land subsidence, and the disorganized deposit of a tsunami. Starting about 1865, an attempt was made to record stories from North American natives; because they had no written language, it was felt that most of their history was bound to be lost. A consistent thread running through the stories from Pacific coast Indians was reference to a time of great shaking and flooding. After father/grandfather/greatgrandfather history assemblages, it was decided that the stories recounted a massive earthquake/tsunami event somewhere in the range of 1700. In 1987 geologists reported evidence of dramatic land subsidence in the past (i.e. the same drowned trees covered by salt-water mud noted by Lewis and Clark) that could only be produced by a large earthquake and a subsequent tsunami. A few years later, scientists carbon agedating tree rings found that the drowned trees had been alive and well throughout 1699, but were dead by a few months later. They estimated the earthquake took place in January 1700. Japan has a documented history of tsunami dating back to the 1500s. Japanese geologists started quietly searching through those records and found one ‘orphaned’ tsunami (i.e. no documented associated earthquake); when they modeled the time of tsunami arrival against travel time through the Pacific Ocean, they determined that the earthquake which generated the tsunami would have taken place in the evening of Tuesday 26 January 1700. Amazing detective work! In the end, all data consistently pointed to that time, with a location in the relatively shallow part of the subduction trench not far west of present-day Seattle. Fig 6. Modelled rupture area for the 1700 Megathrust event. Modified from Atwater et al. (2005), The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America. Retrieved from http://pubs.usgs.gov/pp/pp 1707/chapters/10_Cascadia_92-105.pdf.

2.2.2 The Event The earthquake was a single event; the rupture broke the whole length of the Cascadia subduction zone (1100 km) to a width between 50-150 km, and an offset averaging 20 meters. Working through the Moment Magnitude Scale, those numbers equate to a magnitude of 9.0 or a bit higher. The region of the break is shown in Figure 6 (refer back to Fig. 2 to recall how megathrust earthquakes form). 5

Figure 2 suggests great regions of subsidence along the coastal lands. When coastal lands sink below sea-level, plants and trees are killed because they cannot live in salt water (Fig. 7). Eventually, the area fills in with salty mud and becomes a marsh; it is the trees for these regions that were carbon age-dated (above). Smaller plants were also buried during the subsidence (and the ensuing tsunami); these have been uncovered by trenching (Fig. 8) and also carbon age-dated. Fig 7. Formation of a ghost forest. By Polet (2005), Cascadia earthquake of 1700. Retrieved from http://gsc350.wikispaces.com/Cascadia earthquake1700.

Obviously, this megathrust earthquake – like any other – caused upheaval of a very large block of ocean floor crust. The motion drove the overlying water column, causing the tsunami. Within minutes, the tsunami waves inundated the nearby coasts, but they also traveled across the Pacific Ocean. Working from mapped deposits, the wave heights on the coast of Washington State are estimated to have been 10 meters and 5 metres on the coast of Japan. A group of scientists have tried to define the frequency of seismic activity on the Cascadia Subduction Zone over the past 9800 years. There have been 18 megathrust earthquakes in this time period. The quiet periods between earthquakes lasted 200 to 1000 years. The last two occurred in ~1500 AD and 1700 AD. They found no pattern in the frequency of the Fig 8. Peat layer from 1700 megathrust earthquake. By earthquakes. It goes when it goes! Leonard et al. (2004), Coseismic subsidence in the 1700 great Cascadia earthquake: Coastal estimates versus elastic dislocation models. Retrieved from http://gsabulletin.gsapubs.org/content/116/5-6/655.figures-only.

2.4 The Cascadia Subduction Zone Today and Tomorrow The overall current subduction rate (horizontal component) is about 40 mm/y, but that doesn’t produce any movement on the locked portion, which extends to about 60 km downslope below the ‘lip’ of the slab. Below the locked zone (i.e. toward the continent) is the ‘transition’ zone (Figure 9), where some motion occurs, and below that there is complete response to the compression stress, more or less continuously, without significant earthquakes.

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During August 1999 the equivalent of a magnitude 6.7 earthquake occurred deep beneath Vancouver Island and northwest Washington State; the movement took place over about 1 month and in many small ‘silent’ steps. All of that motion took place in the transition zone – nothing moved in the locked portion. Since 1999 very sensitive seismographs

Fig 9. Transitional and locked seismic zones of the Cascadia Subduction Zone. Modified from Thorpe (2009), Earthquakes: signs point to greater Peninsula effect from the 'big one'. Retrieved from http://www.peninsuladailynews.com /article/20090823/news/308239992.

have been put in place, and it is obvious that the period of ‘silent’ creep takes place roughly once every year (Fig. 10; the vertical lines indicate the displacement steps). That sounds great – the fault is creeping along instead of taking one big release jump. But that’s not at all true! In fact, since all the motion is taking place below the locked zone, more and more stress is being placed on the locked zone. There is a general feeling that one of these years, during a period of ‘silent’ creep, the ‘lock’ on the upper zone will be released and the anticipated megathrust earthquake will occur. When will that occur? Flip a coin! I have to admit to a high level of ‘awareness’/anxiety each time I visit Vancouver Island.

Fig 10. Silent creep recorded by the movement of the ALBH GPS site, located in the Cascadia subduction zone. Note the tremors associated with the creep. By GPS Reflections Research Group (n.d.), Station ALBH. Retrieved from http://xenon.colorado.edu/spotlight/index.php?product=spotlight&station=albh.

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3.0 Case Study 2: The Sunda Megathrust Fault Zone In this case study we will look at a more recent megathrust event – that of December 26, 2004. This was the second-largest seismic event in 40+ years (Chile, 1960, was a bit greater). A megathrust earthquake of moment magnitude 9.2 ripped open a section of the Sunda Subduction Fault Zone, setting in motion the most devastating tsunami in recorded history. It roared around the Indian Ocean leaving a death toll of 186,983 and 42,883 missing and presumed dead (for a total of 229,866). In many respects, this is a view of what a megathrust event in the Cascadia zone could look like today. 3.1 Plate Tectonic Setting The Indo-Australian plate (subdivided into the Burma and Sunda subplates) moves northward at 40-50 mm/y into the southeastern portion of the Eurasian plate. In 2004 an earthquake ruptured at the boundary (Fig. 11). The oblique motion between the IndoAustralian plate and the Burma and Sunda subplates caused a plate sliver (called the Andaman microplate) to shear off parallel to the subduction zone. So, with all these plate boundaries being active, this is one very confusing tectonic mess! Anyway, some great earthquakes have occurred here in historic time (don’t memorize): 1797: moment magnitude 8.4; 1844: magnitude 9; 1861: magnitude 8.5; then a little 7.8 in 1907. All of these were in the southwestern part of the subduction zone. In March 2005, following the 2004 event (which we’ll get to in a bit), was a magnitude 8.6 in the same region as the 1861 and 1907 quakes. Throughout all of these energy releases, there has been no record of a tsunami anything close to that of 2004 – which is unusual in itself.

Fig 11. Movement of the India and Australia plate and locations of earthquakes from Dec. 2004 through to Feb. 2005. Modified from USGS (2005), Sumatra-Andaman Islands Earthquake of 26 December 2004 Magnitude 9.1. Retrieved from http://earthquake.usgs.gov/earthquakes/eqarchives/poster/2004/20041226.pdf.

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3.2 The 2004 Main Event The rupture began at a very precise point (yellow star in Fig. 11) at 59 minutes after midnight GMT (Greenwich Mean Time), or 7:59 a.m. local time, at a depth of about 30 km on a fault plane gently dipping at about 8° eastward. This was north of the sections broken in the previously mentioned events. Aftershocks – which continued for many weeks and amounted to the greatest ‘swarm’ of earthquakes ever recorded (Fig. 12) – clearly showed the rupture had slipped an average of 5 m over an awesome length of 1300 km (the longest known rupture) and a maximum 240 km of fault width. When the moment magnitude calculations were done, it was discovered that the total energy released by the main event was equal to the cumulative total of all global earthquakes for the previous ten years! It seems incredible, but the release of energy (and the shift of lithosphere mass that went with it) actually altered Earth’s rotation – not by much, of course. Computer models say that Earth’s day instantly shortened by 2.68 microseconds, and that Earth’s oblate shape decreased just a bit. The effect is short-lived, however. The natural ‘wobble’ of Earth on its axis plus the fact that Earth’s day increases just a tiny amount every year (as the planet’s rotation slows) together will eventually offset any effects of the earthquake. Fig 12. Earthquake swarm after the 2004 megathrust earthquake. From Lay et al. (2005), The Great Sumatra-Andaman Earthquake of 26 December 2004. Science 308: 1127-1133. Retrieved from https://www.eeducation.psu.edu/earth520/content/l7_p5.html.

3.3 The Tsunami Event [Be aware that a main section about tsunami follows in section 4.0 of this chapter] A section of ocean floor from 600 to 800 km north of the quake epicenter uplif...