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Western University
Geography 2240A/B
Philip Egberts

Chapter 6: Other Earthquakes: Transform and Intraplate Faults 1.0 Introduction In the category of “others”, the earthquakes on transform faults get all the press, but very occasionally there‟s an interesting quake within a continental plate that‟s just categorized as an „intraplate‟ quake (for example the little 5.0 on 23 June 2010 just outside). We‟ll start off this chapter with a consideration of earthquakes on transform faults, and conclude with a shorter section on intraplate quakes. 2.0 Transform Fault Earthquakes: Introduction In looking at the various types of faults in Chapter 4 ( Earthquake Basics), we saw one possibility was the transform fault, where one block of rock moved laterally past another (Fig.1). The whole concept of transform faults was developed by Tuzo Wilson, a famous Canadian scientist from the University of Toro] In Chapter 3 (Plate Tectonic) we saw that transform faults are an inevitable result of shifting lithosphere segments around on a sphere. Perhaps that‟s best shown by the myriad of transform faults that bound segments of the sphere adjacent to the spreading boundary represented by the Mid-Atlantic Ridge (Fig.2). In fact, most transform faults are found on ocean floors in exactly the role shown in the figure: offsetting spreading ridges. The earthquakes that result from motions on those breaks are small simply because the ocean lithosphere is thin and little stress can be built up. So, this is not the location of dangerous transform fault seismic activity. Also, because the motion is mostly in a horizontal sense, there‟s no significant vertical displacement of water, thus no tsunami generation. The most famous transform faults are found on land. Here, transform faults appear to connect convergent and/or divergent plate boundaries in various combinations. The San Andreas Fault of California (Fig. 3) is exactly such an example (and we’ll look at it in a case study in this chapter). Because of the great thickness of continental crust crossed by these faults, sections get „locked‟ by friction while great tension builds; eventually the strength of the rock is overcome, and the release of energy produces devastating earthquakes. The „locked‟ sections which go some period of time without any significant movement (thus no significant earthquakes) are known as „seismic gaps‟. Sections that do not lock (i.e. experience a series of small motions and thus tiny earthquakes, are said to „creep‟. The progressive „jump and stick‟ motion of the Pacific Plate holding Los Angeles and the adjacent North American Plate holding San Francisco (Fig.3), dictates that in something like 10 million years the locations of those cities will be side by side although it’s unlikely that either humans or those cities will be around to ). One „step‟ along that journey was the April 18, 1906 great San Francisco earthquake (and fire). This earthquake, however, was but one of many that have resulted from episodic displacement along the fault throughout its life of about 15-20 million years. Right at the end of Chapter 4 I showed a table of “Twenty Earthquake Events with Highest Fatalities”. Thirteen of those earthquakes arose from motions on transform fault plate boundaries. While the magnitude of transform fault earthquakes that cross land is normally a bit lower than that of megathrust earthquakes, the devastation can be widespread and terrifying simply because the regions affected may be heavily populated: think south-west California or central Turkey – all heavily populated land cursed by being cut by active transform faults. 3.0 Case Study 1: The San Andreas Fault of North America In Figure 3, you‟ll see the roughly 1300 km long San Andreas transform fault to the south of the subducting Juan de Fuca plate. In Chapter 5 I showed how subduction of the Farallon Plate beneath the North American Plate has left only a few small fragments, the most interesting to us being the Juan de Fuca Plate. The San Andreas Fault developed out of this same activity. You can see in Figure 4 (west is to the bottom of the figure, north to the left ) that, as a part of the Farallon Plate – right up to the spreading ridge between the Farallon and Pacific plates - went down the subduction zone, a transform fault was the net result of that (subduction/spreading) activity. As more and more of the Farallon went under, the transform fault lengthened to the south and to the north; that transform fault is the San Andreas. That portion of the San Andreas that‟s of most concern crosses the continental crust of California. Actually, the name „San Andreas Fault‟ is a bit misleading – you get the impression there‟s one break along which all the action occurs. In fact, the San Andreas is the "master" fault of an intricate fault network that cuts through rocks of the California coastal region. The entire San Andreas Fault System is more than 1300 km long and extends to depths of at least 16 km within Earth‟s lithosphere. Just to show how effectively the surface expression of the fault may be identified, Figure 5 is a photo taken in 2000 from the Space Shuttle; the city of Palmdale (about 60 km north of Los Angeles) is toward the right side, and the Palmdale water reservoir sits right in part of the San Andreas depression; the San Andreas runs N-S right through the center of the photo. Geologists believe that the total accumulated displacement is at least 560 km along the San Andreas fault since it came into being about 15-20 million years ago. Currently, on unlocked sections of the fault system, creep amounts to roughly 2 cm per year. Rather than be limited by one seismic event for the San Andreas, we‟ll look at two events in the vicinity of San Francisco (the big 1906 San Francisco earthquake, and the much less intense – but hugely damaging – 1989 Loma Prieta earthquake) and then switch our attention to the Los Angeles region. 3.1 San Francisco: 1906 The people living in San Francisco in 1906 were used to small earthquakes, but so little geology had been done at that time that they had no idea they were living on one of the great active faults of the world – and no one had ever heard of plate tectonics. The amount of energy released – and thus the damage done – was awesome, but this event will forever be known as the event that began the scientific study of earthquakes. At 5:12 local time on the morning of 18 April 1906, an earthquake large enough to be felt and to awaken many residents rumbled across the city. This was known as a foreshock ( A small tremor that commonly precedes a larger earthquake or main shock by seconds to weeks and that originates in or near the rupture zone of the larger earthqSome 20 to 25 seconds later, the major break occurred, with its epicenter near the city. First, we‟ll look at the (dry) scientific facts, and then get to the blood and gory details of human death and social chaos! Figure 6 shows the general Bay area; the numbers beside the trace of the San Andreas fault are amounts of slippage ( in feet, of course, although the map scale is metric…Americans, Americans!) at specific locations that occurred during the quake. The length of fault that broke apart during the earthquake measures 477 kilometers – the longest single break ever witnessed on the San Andreas [By comparison, the 1989 Loma Prieta quake ( see next secti) resulted from a break over 40 km]. The Modified Mercalli Intensity was VII to IX over the rupture length, which translates to a Richter Scale magnitude of about 7.8. Offset faded to zero at both ends of the 477 km break length, but at surface in the middle, over the focus, it was 8.5 meters; at depth (at the focus), it would have been more. Scientifically, the research done after the 1906 quake was a windfall! For example, following careful surveys that measured the exact changes in positions of things like roads and fences, before and after the event, it was realized that Earth‟s lithosphere gradually and elastically distorts with accumulating plate motion until it suddenly „snaps‟ and returns to its undistorted state. This gave rise to the „theory of elastic rebound‟ ( see Fig. 4 of Ch). The Chair of the Department of Geology, University of California (Berkeley), a guy called Lawson, undertook to get a research project going to examine if the type of material upon which buildings were constructed had anything to do with the damage they experienced. His report (commonly called The Lawson Report) formed the basis for much of what is now known of earthquake damage in California. The detailed surveys show that the damage to buildings in the earthquake was strongly related to both the design and construction of the structure and the local geology - the type of soil or rock on which it was built (Fig. 7; please note that this figure was made after the 1989 earthquake of Loma Prieta, but I couldn’t find a comparable one for 1906. Disregard the hig.). Maps included in the report of apparent shaking intensity in San Francisco clearly show that some of the strongest shaking in 1906 occurred in the soft sedimentary soils of the present Marina district -- a San Francisco neighborhoods that would, some 83 years later, again (and for the same reason) be shaken hard and damaged in the 1989 Loma Prieta earthquake ( see section belo). Sketch maps (of the time) of the shaking effects show that the soft soils bordering San Francisco Bay experienced especially strong agitation. Geological observations carried out after the earthquake by individuals who walked virtually the entire length of the earthquake rupture ( which would be rather impossible these days because so much infrastructure is built) showed where and how much the fault had slipped ( those were the numbers on Figure 6). Maps of triangulation surveys carried out during 1906-1907 also documented the lithosphere movement. This information plays a key role in our current models of how much (and when) the San Andreas is expected to slip in future earthquakes. Now, let‟s look at the human aspects. Although not from the time of the 1906 quake, Figure 7 clearly shows much of the same geological detail around the Bay area as existed then: not a whole lot of bedrock, lots of alluvium (i.e. packed sand and gravel, mostly), and recent fill (plus some mud) around the edges upon which many people had built homes and warehouses. And there, of course is the problem: the degree of shaking (or seismic acceleration) is vastly different in those three types of foundation material (Fig. 8; seismic traces in different material from the same earthqu).e Obviously, any construction on solid bedrock is most likely to survive intact, while any construction on recent fill + mud is most likely to be destroyed. The shaking, rumble of breaking infrastructure, and fires started by broken gas pipes was terrifying. Eye witnesses describe individual waves of energy rolling across the city, tossing people, cars and buildings into the air (clearly, these were the visible effects of surface waves). The damage to the San Francisco region was almost complete - certainly comparable to the worst destruction we see from earthquakes along the Pakistan- India fault system, such as the tragic event of 2005 ( which we’ll look at l). Because San Francisco was a „frontier‟, thriving city, there‟s no good record of the number of residents. Many records say that about 700 people died, but research now suggests that the real figure was between 2100 and 2800. Figures 9 and 10 pretty much show the devastation after the fire that raced uncontrolled through much of the city: all the houses made of wood are gone, all the trees are gone, most of the buildings made of stone/brick/concrete are broken rubble. Fig.10 supplemental suggests that city officials of the day were just a bit frazzled! Well, this was a real „learning experience‟ for seismologist; let‟s see how much others learned, as we look at the next big event to hit San Francisco: the Loma Prieta quake. 3.2 San Francisco: 1989 It was a great late afternoon: October 17, 1989 in Candlestick Park, and Game Three of the World Series between the San Francisco Giants and the Oakland Athletics was about to begin; the crowds were opening their beer and ordering hot dogs, the teams were warming up, and the TV announcers were telling all of us at home what an amazing game we were about to witness. Who could ask for anything better! At 5:04 p.m. the TV cameras suddenly shuddered, the crowd in the park felt a jolt, and a dull roar like thunder was heard. The „locals‟ in the crowd rose in their seats and gave a short standing ovation – based on lots of experience, they knew exactly what had just happened: the P-wave of an earthquake had just passed through the park! The rest of us, of course, thought for a moment the locals were just idiots! But then the first S-wave arrived, and everything started to sway. By that time, everyone was a bit scared. As far as I know, this is the only instance of a TV audience of tens of millions that watched a major earthquake happen in real time. In fact, the timing of this event was fortunate: it seemed that those people who were not at the game were at home sprawled in front of their TVs. Traffic on the freeways – which normally would have been bumper-to-bumper at that time of day – was light. In all, 62 people died, 3757 were injured, and damage exceeded $6 billion. Clearly, this was not the dreaded “Big One” that inhabitants of the Bay Area fear – but it was bad enough! The epicenter was not under the ball park or even in the Bay Area but 80 km south, near a place called Loma Prieta in the Santa Cruz Mountains (Fig.11); a section of a fault associated with the San Andreas released, generating an earthquake of magnitude 6.9. Note: this was NOT on the San Andreas itself, but its release actually put the San Andreas under even more stress. According to scientists at the time, there had been absolutely zero foreshocks (i.e. no precursor events) indicating that a break was imminent, but post-event studies have shown otherwise. A magnetometer ( measuring variations in magnetic fiel) had been set up in the area, in conjunction with a system to communicate with submarines. Two weeks prior to the quake, background magnetic readings increased to about 20 times normal, and about 3 hours before the quake to 60 times normal. After the event, they dropped back to normal; so that‟s two anomalies. So what was that all about? When we looked at precursors to major earthquakes, we never mentioned magnetic field variations. After a whole lot of head-scratching, it seems the most likely explanation is that the magnetometer was monitoring changes in the ionization character of groundwater in the region. According to experiments done (later) in labs, when groundwater flows through very tiny (essentially molecule size) fractures in rock, a very small degree of ionization of the water is produced; magnetic fields are sensitive to ionization. As more tiny fractures developed, more flow occurred, and more ionization of the water occurred. But if the rock in the region was beginning to fracture under stress, why didn‟t it show up on seismographs? After very careful examination of the charts - long after the main event – there they were: two tiny precursor earthquakes just barely above the background readings. By the way, it turns out there was a „prediction‟ too. The morning prior to the game, one journalist for the Baltimore Sun wrote a „tongue-in-cheek‟ article in which he said: "... these are two teams from California and God only knows if they'll even get all the games in. An earthquake could rip through the Bay Area before they sing the national anthem for Game 3". The rupture covered about a 40 km length of fault – but there was no surface offset! This was a really deep rupture. True, the movement at depth was great, but by the time the energy reached within 6 km of the surface, all offset had been reduced to zero. So, it was not the sharp breaks of structures that did the damage – it was the intense shaking! This is exactly what the Lawson Report of the 1906 quake had predicted would happen if construction engineers did not pay enough attention to the type of foundation they built upon. Lawson had said that the acceleration of seismic energy waves in solid rock was small, in consolidated sediments (such as compact soil, sand or gravel) was higher, and in mud was enormous. Well, take another look at Fig. 11; in the top right inset ( OK…OK: same figure I used to demonstrate the 1906 shaking!), we see the traces of the energy waves, reflecting the „shaking‟ (or acceleration) in various materials in the Bay Area. Lawson had it nailed! Did construction engineers pay attention? Let‟s see. Note the section of the 880/80 Freeway, or Nimitz Freeway (called the Cypress Structure) that crosses mud (still Fig. 11); this is the section that collapsed. In designing this section of the highway, it was realized that a single-deck design would be safest, and also that it would be better to swing the highway around the east end of the mud flats. But they didn‟t have enough money to buy the wide right-of-way or make a very wide multi-lane single decker, so it was decided to just go with a double-decker freeway straight across the mud flats. The freeway opened in 1957, and was super-busy! The engineers appreciated that building on the mud fill foundation should require a very costly type of construction, designed to move with ground motion (i.e. for the components to be less rigid) – but they didn‟t have the money for that either. Anyway, when the 1989 earthquake occurred, the structure of the double-decker highway amplified the (already) high motion in the mud flats, and the resonance among the rigid components broke 50 of the 124 support pillars, allowing the upper deck to crash onto the lower deck (Fig.12 before, and 13 after) – and that‟s where most people were killed. The replacement newly rebuilt highway swings around the mud flats and is a wide, single deck construction; cost of construction: $4,000 per inch! Of course, it was not only a section of highway that was built on fill and poorly consolidated foundation material; buildings were too (Fig.14). The damage to infrastructure was very great and very costly. Could it happen again? Well for sure there will be other earthquakes as large and possibly larger. As I mentioned above, there certainly is the strong probability that the 1989 event did not remove stress on the San Francisco area, but increased it. The US Geological Survey had already identified sections along the San Andreas within which there had been very few earthquakes over the years ( i.e. seismic gaps: sections of active’ faults with little or no historical acti), indicating that these sections were essentially locked. One section was in the Santa Cruz Mountains, includ
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