Earthquakes and Plate Boundaries Deborah Jordan and Samuel Spiegel Jordan, Deborah and Spiegel, Samuel: Learning Research Development Center, University of Pittsburgh. Earthquakes and Plate Boundaries. Copyright 2013 Institute for Learning. All Rights Reserved. Used Copyright laws may prohibit photocopying this document without express permission.
Earthquakes and Plate Boundaries by Deborah L. Jordan and Samuel A. Spiegel 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Earthquakes are defined as shifts in Earth s crust. A shift causes vibrations to radiate out from the shifting area. The vibrations radiate in a variety of patterns. Sometimes these vibrations are felt at the surface of Earth. Most earthquakes are quite small and are barely felt. Larger, more violent earthquakes can cause widespread changes to the surface of Earth. Earthquakes are very common; they happen every day. In fact, an earthquake will occur today somewhere on Earth. Fortunately, most are small enough that they are not noticeable at the surface, or they happen in unpopulated areas. However, no region is entirely free of earthquakes. Earth s Layers Earth is divided into four main layers: crust, mantle, outer core, and inner core. The crust and mantle play a large role in earthquakes. The surface of Earth is the very top of the crust the top of the land and oceans. The surface is the part of Earth we see every day and where we live. The crust includes the continents (land) and ocean basins. A variety of hard igneous, metamorphic, and sedimentary rocks gather together to form segments of Earth s crust. These segments form large connected layers that are called tectonic plates. There are seven large plates and several smaller ones. Just under the crust is the mantle. This is where most of the internal heat of Earth is located. The tectonic plates float above Earth s mantle. Large convective flows of melted rock and lava in the mantle circulate heat. The circulation of this heat causes the plates to move. Each plate moves several inches each year. As the different plates move toward, away from, or past each other, they collide or slide against each other, causing earthquakes. Due to this activity at the boundaries where plates meet, earthquakes often occur along the edges of the tectonic plates.
29 30 31 32 Earthquakes at Plate Boundaries Plate boundaries are the areas where two or more plates meet. These are shown as black lines in the figure below. There are three types of plate boundaries: convergent boundaries, divergent boundaries, or transform boundaries. Earth s Tectonic Plates 33 34 35 36 37 38 39 Tectonic plates moving toward and crashing into each other form a convergent boundary. When the two plates converge, one plate is pushed down, while the other is pushed up. The movement of the two plates into each other creates strong forces. Earthquakes and volcanoes are common near these boundaries. When two oceanic plates converge, one is thrust under the other, often resulting in undersea volcanoes. When an oceanic plate pushes into a continental plate, the continental plate is lifted up, and often a mountain range is created. When two continental plates converge, the crust buckles and large mountain ranges are formed.
40 41 42 43 Tectonic plates pulling apart from each other form a divergent boundary. Forces pulling the plates apart are caused by mantle convection and gravity. As plates are pulled apart, magma flows from the mantle to fill the void, creating new crust. Divergent boundaries are found in oceans and have earthquakes at shallow depths (within 30 kilometers of Earth s surface). 44 45 46 47 48 49 50 Two tectonic plates sliding next to each other form a transform (or fault) boundary. Transform boundaries are often characterized by some of the strongest earthquakes recorded throughout history. Most transform faults are found on the ocean floor. However, a few occur on land. The San Andreas Fault in California is a transform fault. This fault line runs along the length of California from the northern coast to a southern point inland and east of San Diego. Earthquakes along faults tend to occur at shallow depths and form fairly straight linear patterns. Crust is neither produced nor destroyed. 51 52 53 Earthquakes Within Plates Earthquakes can also occur within plates; however, they are much less common. Less than 10 percent of all earthquakes occur within plate interiors.
54 55 56 57 58 59 Seismic Activity The Ring of Fire is a string of volcanoes and sites of seismic activity, or earthquakes, around the edges of the Pacific Ocean. The Ring of Fire is shaped like a horseshoe. The string of 452 volcanoes stretches from the southern tip of South America, up along the coast of North America, across the Bering Strait, down through Japan, and into New Zealand. About 90 percent of the world s earthquakes occur in this area. 60 61 62 63 64 The second most seismic region is the Alpide Belt. The Alpide Belt is a mountain range that extends along southern Eurasia. It stretches from Java to Sumatra, through the Himalayas, the Mediterranean, and out into the Atlantic. It includes the Alps, the Carpathians, the mountains of Anatolia and Iran, the Hindu Kush, and the mountains of Southeast Asia. About 5 to 6 percent of the world s earthquakes occur in this area. 65 66 67 68 69 Measuring Earthquakes Earthquake strength is measured in both magnitude and intensity. Magnitude measures the relative strength of an earthquake; that is, the energy released at the source of the earthquake. Each earthquake has one magnitude. People usually cannot feel earthquakes with magnitudes of 3.0 or less. Intensity measures the severity of an earthquake in terms of its effect on humans,
70 71 72 73 74 75 76 77 78 79 80 structures, and the land surface. The intensity of a given earthquake can vary from place to place around the source of the earthquake. Why Study Earthquakes? Earth s crust consists of several tectonic plates that float on the mantle. Heat energy forms convection currents within the mantle, causing the plates to move. As plates slide into or away from each other, earthquakes occur. The convection currents and the variety of materials in the crust interact in complex patterns. It is often hard, given current technologies, to measure the currents and materials. This makes it difficult for scientists to predict when and where earthquakes may happen. However, collecting data about magnitude and intensity of earthquakes, as well as studying patterns of occurrence, helps us to better understand and predict when and where earthquakes are likely to occur.