G102 - Plate Tectonics and Earthquakes (2) PDF

Why is That There? (Plate tectonics has some explaining to do) - YouTube https://www.youtube.com/watch?v=wqQUp9cU2WU Transcript: (00:04) This video examines the major features on Earth’s surface relative to the distribution of important earth processes. Any conceptual model of how Earth works, like...

Why is That There? (Plate tectonics has some explaining to do) - YouTube https://www.youtube.com/watch?v=wqQUp9cU2WU Transcript: (00:04) This video examines the major features on Earth’s surface relative to the distribution of important earth processes. Any conceptual model of how Earth works, like plate tectonics, must be able to explain these patterns. When we refer to features, we are mostly focusing on the physical characteristics of the ocean floor and mountain ranges on the continents. (00:26) Processes in this case involve earthquakes and volcanic eruptions. We'll start with this figure. Its a global relief model of Earth’s surface that illustrates the topography of the land and the bathymetry of the ocean basins and lets us see the contrasting elevations of features on the continents and on the ocean floor. (00:44) The darker brown colors indicate higher elevations on land. Darker shades of blue signify the deepest parts of the ocean basins. Lighter blue colors indicate shallow conditions. The first feature we will highlight is a relatively wide oceanic ridge system that can be traced through all the ocean basins. While higher than the surrounding ocean floor, the crest of the ridge system is still about two and a half kilometers (or about 8000 feet) below sea level. (01:15) As you learn more about plate tectonics, you will discover how such a large feature could form on the ocean floor. Elsewhere, in the interiors of the ocean basins, we can sometimes find narrow, linear chains of islands. These represent submarine volcanoes that have grown tall enough to rise above the sea surface. (01:33) The most famous of these are the Hawaiian islands. When we look more closely, we can recognize that this island chain has relatively few islands and is mainly composed of seamounts or submarine volcanoes. Here are a couple of other island chains in the Pacific Ocean. As we will discover, these island chains form very differently from the oceanic ridges. (02:02) The very deepest parts of the oceans are found in narrow trenches, typically along the margins of some ocean basins. For example, we can identify a trench along the western edge of the South American continent. Trenches are also associated with another feature we want to introduce, island arcs. These are also chains of islands but they are crescent shaped rather than a linear shape, so they form a curved line on Earth’s surface. (02:31) These are common in the western Pacific, and one of these trenches, the Mariana Trench, represents the deepest part of the world’s oceans at 11 kilometers depth (or 36,000 feet). This oblique view of the Mariana trench makes it clear that the island arc is set a couple of hundred kilometers west of the trench and is actually made up of both islands (here in green) and seamounts (in the brown colors). (02:59) Plate tectonics needs to explain this gap between the trench and the location of the arc as well as the different processes that give us both curved chains of islands and linear island chains. In contrast to the examples we just described, there are many oceanic margins that do not have trenches. Instead, these locations feature shallow continental shelves that step down into the ocean basin. (03:29) This is what that looks like along the east coast of the North American continent. The continental shelf is actually the true edge of the continent. This gives us something else to explain, why are some continents and oceans separated by trenches while others are not? Talking about the continents, let’s take a closer look at where some of the tallest mountains are located. (03:51) There are only three places on Earth where we find mountains of over 6 kilometers in height or around 20,000 feet or more. There are hundreds of them in the Himalayas, there are dozens in the Andes Mountains of South America, and there is one in North America, Denali, in Alaska What set of geological conditions created these apparently unusual settings? If we take a closer look at North America, we find that the elevations of the mountain ranges increase to the west with the ten tallest peaks on the continent, all in Alaska. (04:34) How will plate tectonics help us explain this pattern? Shifting gears from features to processes, let’s look at where we find earthquakes. This map shows the distribution of moderate to large earthquakes over a recent 30 day period. You may notice that this isn’t a random pattern and that earthquakes seem to be concentrated near some of the features we discussed earlier. (04:58) Let’s do the same thing for recent volcanic eruptions. And by recent, we are thinking about volcanoes that erupted in the last 10,000 years. Again, how do these patterns compare to those we've seen earlier? Let’s put the earthquakes and volcanoes side by side, which features listed toward the bottom of the slide best match up with the distribution of earthquakes and volcanoes? (Pause the video here to think about it if you like for a few moments) Both earthquakes and volcanoes are adjacent to trenches found near continental margins and island arcs (05:47) and they appear common in association with mountain ranges. In contrast, oceanic ridges are outlined by a modest number of earthquakes, especially in the Atlantic Ocean. But not too many volcanoes. If we look at the wider continental shelves, these locations are much quieter, hence they are often referred to as passive margins. (06:14) Geologists collectively label processes such as earthquakes, volcanic eruptions, and mountain building as examples of tectonic activity and divide the outer layers of Earth into tectonic plates based on these patterns. As you will soon learn, the relative motions of these plates cause these processes, cause the earthquakes and volcanoes, and create the features we described earlier. (06:38) The largest plates are typically named after an associated continent or ocean. This lesson sets the scene for a more in depth discussion of plate tectonics. By the time you have finished, you should be able to explain how features such as oceanic ridges and island arcs form and how the distribution of these features is likely to change in Earth’s future. (07:02) Now, how confident are you that you could successfully address the learning objective for this video? Tectonic Plates - YouTube https://www.youtube.com/watch?v=7nxITuot-ko Transcript: (00:02) We are all moving, all the time. Even as we sleep, we move; as we sit in class, we move. We are all passengers on top of a tectonic plate that is slowly traveling across Earth's surface. In this lesson, we will describe what we mean when we use the term "tectonic plate", and we will complete an exercise so you can quickly sketch your own map of the world's major plates. (00:25) Having that map in your head will make it easier to understand the concept of plate tectonics, perhaps the most important idea in all of geology. Earth can be divided into three major compositional layers. The crust, mantle and core. The crust is a thin skin, underlain by a much thicker mantle layer. (00:45) The mantle surrounds a two-part core composed of a solid metallic inner core and an outer core with a molten mix of metals. A tectonic plate is composed of both the outer layer of crust and another layer in the uppermost mantle. Together, these two parts are relatively rigid and form a mobile slab of lithosphere up to a few hundreds of kilometers thick. (01:04) These plates move over an underlying layer in the upper mantle known as the asthenosphere. This layer can deform due to the presence of a small percentage of molten minerals. This allows the rocks in the asthenosphere to flow very, very, very slowly and the rigid plates of the lithosphere ride above this layer. (01:20) Keep in mind that the mantle is 2900 kilometers thick, so we are just talking about rocks in the very uppermost part of the mantle. Most of the mantle, about 90%, is not even involved in the lithosphere and asthenosphere. So, the lithosphere makes up a shell that surrounds the Earth that is a few hundred kilometers thick. However, it's not one complete piece. (01:41) Imagine an orange where we divided up the skin into a series of separate pieces that fit together. Earth is much like that, with the pieces of orange representing different tectonic plates. These tectonic plates move around relative to each other, generating earthquakes, volcanoes, and mountain belts at their boundaries. (01:57) We can look for these zones of tectonic activity to identify the locations of plate boundaries on Earth's surface. Before we sketch the locations of the plate boundaries on our map, let's first look at the distribution of some recent moderate to large earthquakes. This map shows the location of more than 1600 earthquakes of magnitude 5 or greater that have occurred in one year. (02:16) Now, before we go any further, get yourself a blank map of the world. Like this one, or this one, or this one. If you don't have one handy, pause the lesson and go and ask the Google to find one for you. So, now that you are armed with your map, let's find some plate boundaries using these earthquake locations. We will start in the Atlantic Ocean. (02:39) We can connect the dots to identify the approximate location of the plate boundary along here. Now, you try doing something similar, starting about half-way between South Africa and Antarctica. Moving east, where is the plate boundary? You might have drawn it like this, and then wondered where to go from there. You could either continue northward, or you could follow the line of earthquakes to the southeast. (03:12) Both lines represent plate boundaries. Now we have two boundaries. One that separates Africa and South America or North America and Europe. And another that separates Africa and Antarctica or Australia and Antarctica. Now let's try something similar for the Pacific Ocean. (03:33) How many plates can we separate out using this data. We can begin here near New Zealand, and continue clockwise around the rim of the Pacific until we get to South America. Now we have some options. We could do something like this. And then we could use our inside knowledge to complete a few small plates. Such as here, and here and again, down here. (04:17) We have a few other gaps to fill where the data is more limited. Now what's left, can you identify the location of a plate in the western Pacific Ocean? We have some additional earthquakes along the northern margin of the Indian Ocean and through parts of Asia. (04:42) Can you finish off the location of those plates? Go ahead and pause the lesson and give it a try. Now let's see what you came up with. Here is a map we cooked up earlier. These are the major plate boundaries. Most plates are named for their most important geographic feature, such as a continent or ocean. We have the Pacific, North American South American African the Antarctica plate, the Eurasian plate, the Indian-Australian plate, and the Nazca plate. (05:29) These are the eight biggies. We can identify a few other smaller plates as well. Such as the Cocos, and the Caribbean the Scotia the Philippine and then the Arabian plate. That brings us to the end of this lesson. How confident are you that you could successfully complete tasks associated with the two learning objectives. (06:04) Ok, that's it for us. We're off to practice making more plate boundary maps. Convergent Plate Boundaries - YouTube https://www.youtube.com/watch?v=75di2vdSg5U Transcript: (00:00) When geology shows up in the news it is often a result of something that happened at a convergent plate boundary. For example, earthquakes and volcanic eruptions are common at these locations. Our objectives for this lesson are to make some observations of features found at convergent boundaries and then use those observations to make interpretations of the geological processes that occur at these locations. (00:21) Then we will explain why the processes we described produce similarities and differences among the three common types of convergent boundaries. We'll begin by explaining the physical features associated with a convergent boundary formed when a plate composed of oceanic lithosphere is consumed below a continental plate. (00:37) First we often have a descending plate that is pushed down into the mantle. In this example we have a plate composed of oceanic lithosphere that turns downward along the trench which represents the boundary between two plates. We also have an over-riding plate that remains at Earth's surface. In this case the over-riding plate is composed on continental lithosphere and has a line of volcanoes some distance from the trench Both plates rest on the asthenosphere in the upper most part of the mantle. (01:05) Plates move toward each other along convergent boundaries. This map illustrates divergent boundaries in red, transform boundaries in green and convergent boundaries in blue. We can find examples of where oceanic lithosphere is consumed below continental lithosphere here Where the Pacific Plate is destroyed below the North American plate in Alaska and where the Nazca plate collides with the western margin of South America And where the Indian-Australian plate is consumed below the Eurasian plate. (01:34) This map shows the locations of more than 1600 earthquakes of magnitude 5 or greater that occurred in a single year. Note that there are continuous zones of earthquake activity associated with our three examples of oceanic-continental convergent boundaries. Zooming in on two convergent boundaries on either side of the Caribbean plate, we can take a closer look at where exactly these earthquakes are occurring. (01:59) In this image we have trenches here and here, associated with convergent boundaries between the Cocos, Caribbean and North American plate. Our map suggests that there are lots of earthquakes clustering between the trench and the western margin of Central America. But if we look more carefully, what we realize is that most of these earthquakes occur at some depth. (02:18) We want you to be able to explain the processes that cause the earthquakes to occur and to account for their distribution in sloping zones reaching from the trench down to several hundred kilometers below the surface. Next we are going to zoom in to these two trenches to take a closer look. We'll start in Central America Notice the red triangles on the maps and the yellow triangles on the section. (02:40) All of these triangles indicate the presence of volcanoes located approximately 200 kilometers behind the trench. The volcanoes form a line known as a volcanic arc that trends parallel to the trench. We see a similar pattern to the west of the Puerto Rico trench but this time because the volcanoes form islands they are known as an island arc. (02:59) Our explanation of geologic processes at convergent boundaries will have to be able to explain why volcanoes are present and why the arc is consistently located several hundred kilometers from the trench. We call it subduction when one plate descends into the mantle below another plate. The feature that results is known as a subduction zone. (03:17) The oceanic lithosphere sinks below the continent as the rocks of the oceanic plate are more dense than those of the continents. Water-rich sediment rests on top of the ocean floor and the minerals of the oceanic crust often contain water in their crystal structure. Earthquakes occur as rocks in the descending plate fracture and high pressures cause minerals to undergo changes in composition and crystal structure. (03:39) As the plate descends into the mantle, it is compressed and the water is squeezed out of the sediment and minerals. At a depth of approximately 100 kilometers, the water enters the much hotter mantle rocks immediately above the subduction zone. The addition of the water causes these mantle rocks to melt. (03:54) Melting generates magma that rises to the surface to form volcanoes. Finally the force of the collision produces mountains along the leading edge of the over-riding plate. Similar processes occur where two plates of oceanic lithosphere collide. One significant difference is that we no longer have a contrast in the composition of the converging plates. (04:13) So what determines which plate undergoes subduction? While each plate is composed of similar rocks, the rocks are often of different ages. Relatively young oceanic lithosphere is warmer and less dense than older lithosphere Consequently when two oceanic plates converge the plate with older lithosphere undergoes subduction. (04:33) We see this in the western Pacific ocean where the older Pacific plate in subducting below the Philippine plate that is composed of rocks more than a 100 million years younger. Just like in the ocean/continent convergence, earthquakes occur in the descending plate and the rocks of the plate are compressed to squeeze out water which enters the much hotter mantle rocks overlying the subduction zone and causes them to melt. (04:56) The principal difference is that magma rising to the surface now forms an island arc, rather than a volcanic arc, and there are no associated mountains. Otherwise, the processes are similar. The third type of boundary is represented by continent-continent collision. Before the two continents can collide we have a typical ocean-continent boundary but eventually the ocean basin is destroyed and the two continents converge. (05:23) This is the process that occurred before the Indian subcontinent could collide with the rest of Eurasia about 10 million years ago. Continental crust is too buoyant to descend into a subduction zone so there is no source for magma generation and subsequent volcanism. Instead, the leading edge of each continent stacks up to produce unusually thick continental crust and results in higher elevations. (05:43) similar to those observed in the Himalaya mountains today. Finally we will leave you with some simple analogs for the characteristics of convergent plate boundaries. We can use some basic geo-gestures with the addition of some very geological oven mitts to model the processes of the three boundary types. (06:05) We also summarize the basic characteristics of each boundary below the figures. How well do you think you can respond to the learning objectives of the lesson? Divergent Plate Boundaries (or How do you make an ocean?) - YouTube https://www.youtube.com/watch?v=g4DdNw-Zd2Y Transcript: (00:00) Earth is over four billion years old but all of the rocks underlying the world’s ocean basins are much younger and formed less than 200 million years ago. Earth constantly recycles its ocean basins and the first part of this process is the formation of new oceanic lithosphere at divergent plate boundaries. (00:15) In this video we will discuss the geological features and processes that characterize divergent plate boundaries and talk about how ocean basins are created. We’ll start by reviewing some observations about divergent plate boundaries. Plates move away from each other along divergent boundaries as shown by the red lines on this map. (00:33) We can find examples of these boundaries in every major ocean basin. In contrast, this map shows the locations of hundreds of moderate to large earthquakes that occurred in a single year. Note that there are discontinuous lines of shallow earthquakes associated with divergent boundaries in the Pacific, Indian, and Atlantic oceans. (00:57) Geoscientists use the term – bathymetry – to refer to the depth and topography of features on the ocean floor. Divergent plate boundaries are characterized by the presence a continuous oceanic ridge that forms a submarine mountain range that can be traced through the major ocean basins. The ridge is typically more than a thousand meters higher than the surrounding sea floor. (01:16) Observations of the ridge system reveal that it is a source of volcanic activity. Finally, divergent plate boundaries are where we find the youngest oceanic lithosphere. These rocks are forming today as volcanic and plutonic igneous rocks. The age of the ocean floor gets progressively older moving away from the divergent boundary. (01:36) We can compare the bathymetry and age of the ocean floor on these spinning globes. It is relatively easy to identify the divergent boundaries toward the centers of the ocean basins using the features we have just described. These patterns are even more apparent when we superimpose both maps. So, our observations are that plates move apart at divergent plate boundaries and these boundaries are typically home to oceanic ridges, young ocean floor and earthquake activity. (02:23) Next we will use these and other observations to interpret the evolution of divergent plate boundaries and describe how oceanic basins are formed. If we are to form an ocean, we have to start without one. So we begin by examining a location where a continent is being split apart. Then we will look closely at the features of a narrow, young ocean, before turning our attention to the familiar wide ocean basins of the Atlantic and Pacific. (02:49) The Atlantic Ocean formed after the break-up of the supercontinent Pangaea. Similarly today, plate tectonic forces are acting to break apart the African continent. The first stage of this process is marked by an initial stretching and thinning of the continental crust as the mantle rises to form a regional uplift. (03:06) With time, the crust thins and breaks apart on normal faults to form a feature known as a rift valley. The rift valley may contain volcanoes or lava flows fed by magma rising from the mantle. This process is currently happening in Africa as plate tectonics is gradually slicing off a sliver of the eastern part of the continent. (03:24) Note the concentration of volcanoes marking the presence of the rift valley extending southward into the continent from the Red Sea region. Tectonic forces are pulling the continent apart. We can see landforms known as “fault scarps” that form steps in the land surface where faults cut across the landscape. (03:42) The fault scarps are aligned perpendicular to the stretching of the continental crust. With continued stretching, the elevation of the rift valley floor will drop below sea level and it will be flooded to form a narrow ocean. The rift that holds the Red Sea started to open approximately 30 million years ago as the African and Arabian plates began to separate but it didn’t begin filling with seawater until about five million years ago. (04:08) We see evidence of volcanism in the Red Sea in newly formed volcanic islands in the southern segment of the basin, just north of the east African rift. There is actual oceanic crust between the two neighboring plates so we can consider this a legitimate plate boundary. Consequently the Red Sea represents a very youthful ocean basin. (04:27) With sufficient time, the ocean basin expands in width from a few hundred kilometers across to several thousand. Just how wide the ocean becomes will depend on the rate of plate motions and the length of the process. Much of the Atlantic and Pacific oceans began opening at about the same time and contain rocks of similar ages. (04:44) However, the Pacific is much wider because the plates underlying the ocean basin have been separating faster than the North American and African plates. So, to summarize... Plates separate along rift zones associated with oceanic ridges. This process is driven by mantle convection. Faulting in the rift zone generates shallow earthquakes along the ridge. (05:04) As the plates move apart, a gap opens up that is filled by magma rising from the asthenosphere below. Magma forms new, young oceanic crust. Magma that reaches the surface forms lava with a basaltic composition or it may solidify at depth to form the plutonic igneous rock like gabbro. The high heat flow results in lower density along the ridge and produces higher elevations, explaining the presence of the ridge system. (05:28) The earliest or oldest oceanic crust forms adjacent to the continents and the latest or youngest crust is along the ridge, toward the center of the ocean basin. You can test your knowledge of the things discussed in the video by drawing a labeled sketch of the features and processes of divergent plate boundaries on a blank piece of paper. (05:47) Go on, give it a go. So, we had two learning objectives for this lesson. How well do you think you can respond to each? Transform Plate Boundaries - YouTube https://www.youtube.com/watch?v=tuKNtQ7Hupg Transcript: (00:00) Transform plate boundaries represent some of the most dangerous geological features in North America but they can be so deceptively modest in their physical appearance that we build major US cities on top of them. In this video we will introduce you to transform plate boundaries and discuss the geological features and processes that characterize them. (00:18) We’ll start by reviewing some major examples of these features. Plates slide past each other along transform boundaries, here illustrated by the green lines. While they are rarer than their divergent and convergent cousins, they can be recognized in several locations, typically forming links between longer segments of convergent or divergent boundaries. (00:37) Let’s take a look closer look at some of these examples. The most famous example of a transform plate boundary runs through California and separates the North American plate from the Pacific Plate. We will return to take a closer look at this boundary in the second half of the video. Some transform boundaries occur almost exclusively in the oceans and are largely hidden from view. (00:58) For example, the small Scotia plate is separated from the Antarctic and South American plates by transform boundaries along its north and south margins. The Caribbean plate is about the same size as the Scotia plate and its northern edge is a transform boundary separating it from the North American plate. (01:14) Movement along a fault parallel to this boundary resulted in a magnitude-7 earthquake that struck the nation of Haiti in 2010. An estimated 3 million people were affected by the earthquake which resulted in widespread destruction and many deaths in the capital of Port au Prince. Some transform boundaries are much smaller than these examples. (01:32) Examination of the ridge system in any ocean basin reveals that it is often offset by short transform boundary segments. Plates move in opposite directions along these segments, making them a source of shallow earthquakes in the ocean basins. These offsets are essential for allowing the rigid tectonic plates to move over a spherical Earth. (01:51) By way of analogy, imagine trying to wrap a ball with a rectangular sheet of paper. We have to make many small cuts in the paper to fit it to the ball’s rounded surface. Earth’s plates are rigid slabs of rock that have to make similar adjustments to slide at different rates and in different directions over the planet’s surface. (02:09) These small transform segments represent some of those adjustments. In previous videos we have described the features of convergent and divergent plate boundaries. The mountains, volcanoes, and rift valleys of these boundary types produce dramatic landforms. But what about transform plate boundaries? What features can we expect to see where these boundaries cut across a continent? Let’s take a look at the transform plate boundary as represented by the San Andreas fault system in California. (02:35) The San Andreas fault system links to oceanic ridge systems from the coastal waters of Oregon in the North to the Gulf of California in the South. The North American continent is actually on two plates. Most of it is, as expected, on the North American Plate, but there is a sliver of the continent that represents the eastern margin of the Pacific Plate. (02:55) If we look more closely we can see that two major cities lie adjacent to the transform boundary on opposing plates. Los Angeles can be found west of the boundary on the Pacific plate, while San Francisco is located just east of the fault on the North American plate. The relative plate motions will slowly move the site of Los Angeles northward along the boundary so that it will eventually be parallel with San Francisco. (03:16) Given enough time, it will continue its journey northward and may eventually collide with the convergent boundary south of Alaska. In the meantime, earthquakes of various sizes occur along the San Andreas fault system, posing a potential danger for any cities nearby. The transform boundary is so unassuming that communities are built right on top of it. (03:35) Both Los Angeles and San Francisco sprawl outward over the location of boundary. Segments of the fault have ruptured to produce three major earthquakes in the last 400 years. The most recent event occurred in 1906 and resulted in a fire that destroyed large sections of San Francisco. The city burned for three days, destroying nearly 500 city blocks, leaving 250,000 homeless and killing more than 500 people. (03:59) The same event today would be disastrous, with the potential to kill thousands of people and costing many billions of dollars in damages. Let’s take a closer look at what the boundary in the desert of southern California. The white rectangle represents the field of view for the next image. Zooming in from above on a segment of the fault in the desert we can make out a linear trace that runs northwest-southeast through the image. (04:23) Examining the feature on the ground may reveal a linear fault scarp produced by erosion along the line of the boundary or may just show a relatively nondescript surface with little to indicate that a major plate boundary is present. Unlike the depictions in some movies, movement along a transform plate boundary doesn’t generate a gap or chasm at Earth’s surface. (04:45) While earthquakes may occur along these boundaries, there are no volcanoes, rift valleys or mountain ranges characteristic of other boundary types. Ok, let’s see if you can identify the expression of the Alpine Fault which represents a transform plate boundary in New Zealand. Can you identify where the fault is located? How about now? We can see it here on the western side of the Southern Alps on the South Island of New Zealand. (05:17) We had two learning objectives for this lesson. How well do you think you can respond to the learning objectives? Earthquake Hazards I: Ground Failure - YouTube https://www.youtube.com/watch?v=h3l94aZSbGM Transcript: (00:01) A magnitude 7.8 earthquake struck San Francisco early on the morning of April 18, 1906. The event marked the first time that the consequences of a major US natural disaster were recorded on film. A subsequent report demonstrated that the earthquake occurred on the northern half of a single continuous structure that we now know as the San Andreas Fault. (00:21) The 1906 earthquake and subsequent fire killed more than 3000 San Francisco residents and left over half the city’s population homeless. Today, nearly 40 million people live in California with many of them concentrated in major cities located close to the San Andreas and other fault systems. What can we do to mitigate the potential damages and loss of life from a future dangerous earthquake? To answer that question we need to understand the nature of earthquake hazards. (00:47) When earthquakes occur the earth shakes for seconds or maybe even minutes. However much of the damage with earthquakes is not a result of the shaking itself but may be driven by other factors such as the design of buildings and the underlying geology. Our objective for this lesson is to familiarize you with the principle hazards associated with earthquake activity and the factors that can magnify their impact. (01:08) One obvious consequence of shaking associated with an earthquake is the collapse of buildings and other structures. Rapid vertical and horizontal movements can shift homes off their foundations, collapse multi-story office blocks and destroy elevated roadways. The colors on this map indicate the severity of the shaking associated with the San Francisco earthquake. (01:30) The red color indicates that shaking was most intense along the fault, and the transition from red to yellow to green indicates that shaking decreased as we get farther to the east. This graph illustrates that instruments at some stations near the source of the 1994 Northridge earthquake recorded ground shaking that approached 1g. (01:50) A vertical acceleration of 1g is sufficient to overcome gravity, throwing objects into the air. Cities in areas of seismic risk typically have strict building regulations that require that new structures withstand accelerations of 0.5g or more with little damage. Even 60 miles or 95 kilometers from the epicenter, there is sufficient shaking to cause damage to older buildings. (02:12) But shaking isn’t just about the distance from the fault. Look again at this map of the 1906 earthquake. Notice this red area here around Santa Rosa. Even though Santa Rosa was more than 30 kilometers from the fault, is suffered some of the worst damage as a result of the earthquake. The underlying geology and building design can combine to exaggerate or dampen the effects of earthquake shaking. (02:34) or example, the 49-story Transamerica Pyramid shook for more than a minute during a major earthquake but no one was seriously injured and the tower was undamaged. Buildings or structures have a natural resonance or frequency of vibration related to the motion of an earthquake. The greatest damage occurs when ground motion matches the resonance of a building. (02:58) For example, taller structures will exhibit greater oscillations back and forth with slower ground motions. In contrast, short structures, like two-story homes, will vibrate more violently with faster ground motions. The same earthquake will produce different shaking effects in different earth materials. (03:20) Bedrock typically produces more frequent, smaller vibrations – the type of shaking that is more likely to damage shorter structures such as two-story homes. Weaker materials such as muddy soils can amplify ground shaking and produce larger, low resonance vibrations that are more dangerous for multi-story structures. (03:41) Elsewhere the shaking may cause ground failure. Landslides can occur in areas with steep slopes, blocking roads and burying homes under debris. Liquefaction occurs when shaking causes the compaction of sediment increasing water pressure and causing water and water-saturated material to be ejected at the surface. (04:04) In this example, a block of granite sinks into the underlying saturated sands when the material is shaken vigorously. Local features like these sand boils or sand volcanoes are often produced. Elsewhere, liquefaction results in subsidence of the land surface and causes objects to collapse into the slurry of water and sediment. (04:26) In the most extreme events this can cause whole apartment buildings collapse as the ground beneath them gives way. Fault movement during earthquakes can result in adjacent pieces of the land surface being displaced by up to several meters. Some blocks may move up or down, while others shift from side to side. (04:45) This is one of the easier hazards to avoid, the general rule is “Don’t build anything on or near an active fault.” We see examples of the effects of ground shaking and ground failure following the magnitude 9.2 Great Alaska earthquake of 1964. This remains the largest US earthquake ever recorded. Liquefaction, subsidence, and landslides swallowed buildings and left behind a chaotic urban landscape. (05:13) The town of Valdez was moved to a more stable location following the earthquake. We was one major learning objective for this lesson. How confident are you that you could complete this task? Earthquakes https://www.youtube.com/watch?v=_yo6ZzerU2c Transcript: (00:00) [music] Earthquakes can have large and deadly impacts on communities worldwide. Many of the world’s major population centers, such as San Francisco and Tokyo, experience earthquakes on a regular basis. In this video tutorial we will discuss the locations, patterns, causes, and consequences of earthquakes worldwide. (00:27) As discussed in the Folds & Faults Video tutorial, faults are planes along which rocks under stress have broken and moved. After a fault has formed, successive application of stress can cause continued movement of the rocks along each side of the fault. If that stress is released through continual movement, we say the fault creeps. (00:51) However, when friction causes the fault to stick, stress will build, and the rocks on either side of the fault will deform elastically under that stress. When eventually the stress is high enough to overcome the friction, and the fault slips, the release of the built-up stress energy causes the Earth to shake: an earthquake. The spot along the fault where the friction is first overcome and the slip happens is called the focus of the earthquake. (01:18) The spot on Earth’s surface directly above the focus, the place that will first receive the energy waves that emanate out of the focus, is called the epicenter. The energy that is released as the fault slips and the rocks rebound back to their original shape moves in the form of waves that travel outward in all directions. We call the waves that travel through the solid rock, body waves, and these can travel all the way through the Earth to be picked up by seismic stations on the other side of the planet. (01:44) When these body waves arrive at the surface they will generate surface waves, which move outward from the epicenter in all directions along Earth’s surface (the boundary between the rock surface and the atmosphere or water). As discussed in the Plate Tectonics and Global Impacts and Folds & Faults video tutorials, plate boundaries are regions where stresses, such as compression, tension, and shear, are continually applied. (02:14) Extensive fault systems exist along all these boundaries, and earthquakes concentrate here globally. These earthquakes nicely outline the edges of plates and let us see the various shapes and sizes of plates globally. What we can also see in these images is that earthquakes at divergent plate boundaries where oceans are opening up or along transform plate boundaries where plates slide past each other are a bit different than those happening where plates converge. (02:40) Convergent plate boundaries have much wider zones of earthquake activity. And when we color code earthquakes based on depth, we can see why. Over subducting plates, we would expect shallower earthquakes near the top of the subducting plate, near the trenches, and progressively deeper and deeper focus earthquakes as the plate descends. (03:02) In fact the earthquake data confirm this, and maps like these provide an image of the location of the subducting plate and the geometry of the subduction zones. We can determine how much stress builds up and is released during an earthquake by studying the way the ground moves after the rupture. Scientists who study earthquakes are called seismologists and record the ground movement using instruments known as seismographs. (03:26) These devices can measure movement in three directions and record that motion as a two-dimensional wave form on a piece of paper called a seismogram. A simple seismograph can be described as a hollow box attached to the ground so it moves in exactly the same direction and motion as the ground. Dangling in the middle of the box from the top is a heavy weight that is attached to a thin line. (03:51) The weight is pulled towards Earth’s center by gravity and because of its mass will stay in place during an earthquake while the box moves with the Earth around it. We can then measure how much distance is created in each dimension of the box between the weight and the sides of the box as the box moves with the Earth. Thus we can record accurate motion of the Earth. Here is a sample seismogram from an earthquake. (04:13) What do we see? The first wave to appear, here, causes a different intensity ground shaking than the second wave, which appears here. After a fault ruptures at depth, there are two body waves that emanate outward. The fastest body waves are the first to arrive at the seismograph stations on the surface and are called primary waves or P waves. They are compressional waves that travel on average 7 km/s. (04:36) These energy waves are transmitted as the material through which they’re traveling alternately compresses and expands. Sound waves are also compressional waves. Because all substances (solids, liquids, and gases) are capable of compression and expansion, compressional waves can travel through all materials. We can hear sound traveling through the air, water, or solid rock. (04:59) The second type of body waves are shear waves and slower, on average 3.5 km/s. These waves arrive second at the seismograph station and are called secondary waves or S waves. As you see here with the slinky, shear waves cause the individual slinky pieces to move up and down relative to each other as the energy passes through. (05:24) Because only solid materials are capable of shear, S waves can travel only through solids. Going back to the seismogram, what we see here is the arrival of the compressional P wave, which is like a hammer punching up out of the ground. Following behind is the S wave. Both of these waves will produce surface waves that emanate outward from the epicenter. (05:46) The surface waves move along Earth’s surface and cause the Earth to move up and down and sideways in three directions. It’s the motion of the surface waves that cause the greatest amount of damage to buildings during earthquakes. In general, surface waves will dissipate the further one gets from the epicenter of an earthquake. (06:07) However, surface wave intensity is impacted by the type of material they travel through. For example, as this shake map of San Francisco shows, when each of these rock materials is subjected to earthquake waves, different amounts and types of shaking result. Solid bedrock shakes least – unconsolidated sand shakes more – and unconsolidated mud, bay fill, shakes the most (like jello). (06:34) Finding the most stable ground to build on is a key part of minimizing damage during earthquakes. It’s also important to prevent structures from being built across multiple materials that shake differently. For example, during the 1989 Loma Prieta earthquake in California, for one of the highways that fell down in the East Bay, one support was built on sand, and the other on mud. (06:56) The two supports shook with different motions during the earthquake and pulled the material supported between them apart. In addition to the ground material impacting the intensity of shaking, sometimes the geology of an area can intensify shaking through interference. Interference is the meeting of two waves arriving from different directions. (07:14) When the two waves meet, they add to each other. If they meet crest to crest, or trough to trough (in phase), they increase the overall wave height. For earthquakes that increase means more shaking. When the two waves meet crest to trough (out of phase), they decrease the wave height. That decrease means less shaking. (07:38) Interference happens during an earthquake when the seismic waves bounce off a hard object underground, like the edge of a basin, or the side of a granite intrusion. Areas between the epicenter and the reflection surface will experience two waves arriving from different directions – the original earthquake waves and the reflected waves. (07:57) The pattern that interference makes on the surface is similar to a checkerboard – in black squares waves meet out of phase and shaking is reduced; in white squares waves meet in phase and shaking is increased. In the center of black squares people may not feel any shaking at all. In the center of white squares, shaking may be so intense that structures fall. Evidence that interference has occurred can be seen when you view this checkerboard pattern of destruction in an area, like a housing development, where all structures and ground material are similar. (08:27) In addition to the local soils, rocks, and geology, the type of building material will also impact how severely a particular earthquake is experienced by the people living in an area. Urban areas prone to earthquakes will often develop strict building codes to minimize these impacts. (08:47) For example, if a building’s foundation is sunk deep and solidly into the bedrock, it will move with the ground and be less likely to break apart. If the walls of the house are bolted to the foundation, they’ll move with the foundation. If the walls are bolted to each other, it further connects everything so it all moves as one. If we build houses out of flexible material, like wood, they’re less likely to break as they bend. (09:08) If we minimize the use of rock tiles and ornamental door headers, we’re less likely to have these fall and cause damage during an earthquake. Sometimes it’s not the way a building is built, but the size of the building that impacts the amount of damage during an earthquake. Resonance is the matching of the period (or frequency) of an earthquake wave with the natural vibrational period (or frequency) of an object, like a building. (09:33) All buildings vibrate at a period (number of seconds per vibration) that depends mainly on the height of the building. If the period of an earthquake is the same as the natural period of the building structure, shaking increases more and more with each wave’s arrival, until the structure falls apart. You can see examples of resonance when you push a child on a swing (if your pushing period matches the child’s swinging period, the child will swing higher and higher) or when an opera singer matches the frequency of a sound wave to the frequency of a wine glass and makes it shatter. After an earthquake, you would (10:05) see evidence of resonance if you noticed that all buildings of a particular height experienced more damage than other buildings, shorter or taller. What are some other things that contribute to damage during an earthquake? Unconsolidated sands and muds can behave like liquids when shaking and heavy buildings on top can sink into them, a process called liquefaction. (10:41) Fires spread easily when folks are cooking over open flames and buildings are made of wood. Seiches or large standing waves can form in lakes or pools or lagoons and flood the surrounding land. Landslides can be triggered. And if the earthquake happened underwater at a fault that experienced vertical displacement of the seafloor, like at subduction zones, then it’s possible to create a tsunami or splash wave that emanates out and can travel thousands of kilometers across the ocean. Tsunami can also be created by landslides that fall into the ocean thus creating a vertical (11:20) displacement of seawater and a splash. Since the amount of damage done during an earthquake can be impacted by ground material and building design, how do we accurately measure the amount of energy released in an earthquake and assign a particular earthquake a measurement that we can compare globally with other earthquakes? The first scale devised to measure earthquake intensity was the Mercalli Intensity Scale, and it provided a scale based solely on the subjective experience of the people in the area of the earthquake. So even though the (11:50) actual energy release and size of the body waves might be the same from one location to the other, if there were no people living in one area and no buildings, it would not even appear on the scale; if ground material amplified shaking and building design was not engineered for earthquakes, then it would appear very high on the scale. (12:12) This scale is very useful for comparing damage from one part of the planet to the other, but not for comparing energy release. In 1935, Charles Richter and others at Caltech in Pasadena, California, devised a new scale, the Richter Scale, which measured the amplitude of arriving earthquake waves on a seismogram recording. Based on the local rocks of Southern California and the type of seismograph instrument used, seismologists developed a paper scale that allowed them to calculate the energy released in an earthquake based on its distance away and the amplitude on the seismogram. In the Richter Scale, for each approximately 33-fold (12:47) increase in energy, we move up 1 number in magnitude. So a magnitude 7 earthquake represents a release of approximately 33 times more energy than a magnitude 6 earthquake and 33 x 33 or approximately 1000 times more energy than a magnitude 5 earthquake. In other words, it would take 1000 magnitude 5 earthquakes to release the same amount of energy as one magnitude 7 earthquake. (13:21) *Note: the amplitude of the seismogram wave increases 10 times for each magnitude, which means of course that not all earthquakes can be recorded on a single seismograph. Some will be too big and run off the drum – others too small to see – so multiple seismograph instruments would be needed to record the various magnitude earthquakes that would exist in any one area. The benefit of the Richter scale is that earthquake magnitude can be determined quite quickly after an earthquake just by measuring the amplitude on the seismogram recording and calculating as well how far away the earthquake was. The drawback to the Richter scale is that it (13:52) was designed around a particular instrument and location and thus doesn’t translate uniformly to other earthquakes elsewhere in the world that might have different rocks and different instruments. It also loses its accuracy for high magnitude earthquakes. As a result, the Moment Magnitude Scale was developed and is the one now used by seismologists around the world. (14:13) The Moment Magnitude Scale calculates magnitude from amount of slip along the fault, depth and length of fault rupture, and strength of rocks that broke. With this new scale, a magnitude 7.1 from one location represents the same energy release as another location. (14:33) However, it does take time to gather the data necessary to calculate the moment magnitude. As a result, usually after an earthquake, the first magnitude given right away is the Richter magnitude, based on the seismogram recording, and then a few days or weeks later that number is refined to the more accurate moment magnitude. To determine the exact location of an earthquake, we look at the arrival time difference of the P and S waves. (15:01) Because we know the speed of each, we also know how far away we have to be from the epicenter of an earthquake to get a particular time difference in their arrival – the further away, the larger the arrival time difference, the closer, the smaller the arrival time difference. Once we determine the exact distance, we can draw a circle around the seismic station with a radius equal to that distance. The earthquake happened somewhere on that circle. (15:24) What else to we need to pinpoint the exact location? Two more seismic stations reporting their data, so we have three circles, which will intersect in only one location, the epicenter of the earthquake. One of the common comments I receive from students the week we study earthquakes in class is surprise at the “coincidence” that while we’re studying earthquakes, there happens to be an earthquake somewhere in the world that impacted an urban center and is written up in the news. (15:56) Is it really a coincidence? From the USGS Earthquake Hazards Program website, from 2000 to 2018, there have been on average 1-2 magnitude 8 earthquakes per year; 14 magnitude 7-7.9, approximately 140 magnitude 6-6.9; and approximately 1600 magnitude 5-5.9. You can see that the annual number seems to jump 10 times for each magnitude drop. So 4-4.9 would be about 16,000; 3-3. (16:33) 9, 160,000; and less than 3.0 is over 1.6 million. On any given week (7 days), the planet will experience thousands of earthquakes, and there’s bound to be one of those that makes the news. Pause now. [music] For more information, continue on to the next video in this series. [music] Pacific Northwest Earthquakes—3 Types (Educational) - YouTube https://www.youtube.com/watch?v=_belQwGNolY Transcript: (00:01) The Pacific Northwest is host to 3 kinds of tectonic earthquakes. Magnitude nine Cascadia megathrust earthquakes. Magnitude 6.5 to seven deep earthquakes. And shallow crustal fault earthquakes with magnitudes up to 7.5. What causes them? We will show how subduction of the Juan de Fuca plate, Basin- Range extension, invasion of crustal blocks from California, and the resistance of the Canadian crustal buttress combined to produce these earthquakes. (00:34) We begin by looking at Western North America's active plate boundaries. The San Andreas Fault is a strike-slip transform fault zone along which the Pacific plate is sliding Northwest at about 3.5 centimeters per year with respect to interior North America. The Pacific plate drags on that plate margin and segments of the boundary rupture sporadically, releasing moderate to large earthquakes. (01:00) The most famous was the 1906 magnitude 7.8 San Francisco earthquake, a 480 kilometer long rupture. North of the San Andreas Fault, the Juan de Fuca plate dives beneath, and pushes on, the margin of the North American Plate. Rates of convergence range from 3 to 5 centimeters per year. On January 26, 1700 a magnitude 9 Great earthquake ruptured the full 1,000 kilometer length of that plate boundary. (01:28) Magnitude 8 to 9 great earthquakes rupture the subduction zone on average every three to five hundred years. How do we know energy is building for a future great earthquake? The ongoing compression of the Cascadia continental margin is being measured by GPS. Data, represented by arrows shows coastal stations moving northeast faster than inland stations. (01:52) Looking at this in cross-section we use a spring to represent the elastic nature of the overriding plate locked to the diving plate. Graphs above show the motions of three GPS stations as the continental margin is compressed and elastic energy builds. When friction between the plates is overcome, the leading edge of the North American plate lurches seaward creating an earthquake and tsunami. (02:14) Next we look deeper at earthquakes within the subducting Juan de Fuca plate itself. The 1949 magnitude 6.8 Olympia earthquake caused eight deaths. Fortunately it occurred during school vacation as ten schools were severely damaged and subsequently closed. The hypocenter of the Olympia earthquake was 52 kilometers deep at least seven kilometers below the subduction zone plate boundary and thus within the subducting Juan de Fuca plate. (02:45) As the subducting plate bends beneath Puget Sound, the upper part stretches under tension and fractures in normal faults. In 1965 a magnitude 6.5 deep earthquake occurred 60 kilometers below Seattle killing seven people and causing 100 million dollars in damage. The 2001 magnitude 6. (03:10) 8 Nisqually earthquake was identical to the Olympia earthquake and may have occurred on the same normal fault. Strong ground shaking in the Puget Sound basin lasted over 30 seconds and caused one death, 400 injuries, and damaged 40 bridges and 300 thousand buildings. (03:31) Deep earthquakes occur every 20 to 30 years beneath the Puget Sound and can occur along the length of Cascadia where the subducting plate bends to dive more steeply. Aside from the forces applied by the Pacific and Juan de Fuca plates along the boundaries more is going on within the internally deforming North American plate as the subduction zone is locked and elastic energy builds. (03:54) We expect to see deformation of the continental margin as shown by these yellow arrows. However, this deformation is reversible because the edge of the North American plate jumps back to the west during each great earthquake before it locks again and begins reloading. If the load-and-release Great earthquake cycle was the only deformation, the observed GPS motions would be this simple pattern shown by the yellow arrows. (04:15) However, the measured GPS motions are more complex indicating that Cascadia deformation is a far more interesting story. Let's look at the Basin and Range. Extension stretches from Utah into southeastern Oregon, to the west the Sierra Nevada block is dragged north northwest by the Pacific plate and is pushing on the Oregon Coast Range block, that in turn pushes on the Washington crustal block. The resisting Canadian coast mountains halt the motion. (04:49) These forces rotate much of Western Oregon and Washington slowly around a pivot point in northeast Oregon. Unwinding 16 million years lets us see the permanent deformation caused by this rotation the resulting deformation has broken the crust by normal, strike-slip, and thrust faults with motion histories that are the focus of ongoing research. (05:16) An important example is the Seattle fault zone that extends across the heavily populated southern Puget Sound area. Trenching ,shown in the photo, provides direct evidence of southside-up thrust fault displacement from a major magnitude 7 or 7.5 earthquake on the Seattle fault about 1,000 years ago. (05:36) This animation depicts an exaggerated thrust faulting along the Seattle fault zone. Modeling of ground motions that would result from a magnitude 7.2 earthquake is sobering. Areas within a few miles of the Seattle fault, including much of Seattle would experience 20 seconds of severe ground shaking, an amount shown in this full-scale shake table experiment. (06:00) How does shaking from the shallow Seattle earthquake differ from a deep one like that in 2001 Nisqually Earthquake? Because the Nisqually hypocente was 52 kilometers deep, the seismic energy spread out beyond the Puget Sound rather than being concentrated near the epicenter as it would for a shallow earthquake. A magnitude 7 event has up to 20 seconds of severe groun d shaking. (06:22) A subduction-zone megathrust earthquake, like Japan experienced in 2011, will have the broadest impact. A magnitude 9 earthquake releases the equivalent energy of four magnitude 7 earthquakes every second over a rupture interval of 4 minutes. Projected ground shaking for that event will be severe at the coast but still very strong in the urban corridor where ground shaking can last for over six minutes with slow back-and-forth emotions, particularly challenging for tall and long structures. The tsunami comes ashore 15 to 20 (06:54) minutes after ground shaking stops. The Pacific Northwest is a region of high earthquake risks because of megathrust earthquakes on the plate boundary, deep earthquakes within the subducting plate, and shallow crustal earthquakes. By putting them all together scientists are able to create an earthquake-hazard map for the region. (07:18) Amplified ground motions in the sediment filled bands of the urban corridor are of concern for all earthquakes. Exterior walls of unreinforced masonry buildings constructed before modern building codes often crumble during earthquakes. Instinctually people run out of buildings during ground shaking and are often killed or injured by falling debris. The life-saving response is to drop, cover, and hold until shaking stops Earthquake and tsunami education along with construction of earthquake resilient buildings and infrastructure are vitally important in a seismically active region