{"id":478,"date":"2017-11-09T16:54:20","date_gmt":"2017-11-09T21:54:20","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/12-1-what-is-an-earthquake-2\/"},"modified":"2023-07-04T13:08:30","modified_gmt":"2023-07-04T17:08:30","slug":"what-is-an-earthquake","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/what-is-an-earthquake\/","title":{"raw":"12.1 What is an Earthquake?","rendered":"12.1 What is an Earthquake?"},"content":{"raw":"

Earthquake Shaking Comes from Elastic Deformation<\/h1>\r\nEarthquakes occur when rock ruptures<\/strong> (breaks), causing rocks on one side of a fault to move relative to the rocks on the other side. Although motion along a fault is part of what happens when an earthquake occurs, rocks grinding past each other isn't what creates the shaking. In fact, it could be said that the earthquake happens after <\/em>rocks have undergone most of the displacement. Consider this: if rocks slide a few centimetres or even metres along a fault, would that motion alone<\/em> explain the incredible damage caused by some earthquakes? If you were in a car that suddenly accelerated then stopped, you'd feel a jolt. But earthquakes aren't a single jolt. Buildings can swing back and forth until they shake themselves to pieces, train tracks can buckle and twist into s-shapes, and roads can roll up and down like waves on the ocean. During an earthquake, rock is not only slipping\u2014it's also vibrating like a plucked guitar string.\r\n\r\nRocks might seem rigid, but when stress is applied, they may stretch. If there hasn't been too much stretching, a rock will snap back to its original shape once the stress is removed. Deformation that is reversible is called elastic deformation<\/strong>.\r\n\r\nRocks that are stressed beyond their ability to stretch can rupture, allowing the rest of the rock to snap back to its original shape. The snapping back\u2014called elastic rebound<\/strong>\u2014causes the rock to vibrate, and this is what causes the shaking during an earthquake.\r\n\r\nFigure 12.3 (top) shows this sequence of events. Stress is applied to a rock and deforms it. The deformed rock ruptures, forming a fault. After rupturing, the rock above and below the fault snaps back to the shape it had before deformation.\r\n\r\n[caption id=\"attachment_473\" align=\"aligncenter\" width=\"650\"]\"\" Figure 12.3<\/strong> Elastic deformation, rupture, and elastic rebound. Top: Stress applied to a rock causes it to deform by stretching. If the stress becomes too much for the rock, it ruptures, forming a fault. The rock snaps back to its original shape in a process called elastic rebound. Bottom: On an existing fault, asperities keep rocks on either side of the fault from sliding. Stress deforms the rock until the asperities break, releasing the stress, and causing the rocks to spring back to their original shape. Source: Karla Panchuk (2017), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. View original<\/a>.[\/caption]\r\n\r\nRuptures can also occur along pre-existing faults (Figure 12.3, bottom). The rocks on either side of the fault are locked together because bumps along the fault, called asperities<\/strong>, prevent the rocks from moving relative to each other. When the stress is great enough to break the asperities, the rocks on either side of the fault can slide again. While the rocks are locked together, stress can cause elastic deformation. When asperities break and release the stress, the rocks undergo elastic rebound and return to their original shape.\r\n

Rupture Surfaces Are Where the Action Happens<\/h1>\r\nImages like 12.3 are useful for illustrating elastic deformation and rupture, but they can be misleading.\u00a0 The rupture that happens doesn't necessarily break the rock through and through. Rupture and displacement only happen along a subsection of a fault, called the rupture surface<\/strong>, although the rupture surface could extend 10s to 100s of kilometres.\u00a0 In Figure 12.4, the rupture surface is the dark pink patch.\u00a0 It takes up only a part of the fault plane<\/strong> (lighter pink). The fault plane represents the surface where the fault exists, and where ruptures have happened in the past.\u00a0 Although the fault plane is drawn as being flat in Figure 12.4, faults are not actually perfectly flat.\r\n\r\nThe location on the fault plane where the rupture happens is called the hypocentre<\/strong> or focus<\/strong> of the earthquake (Figure 12.4, right). The location on Earth's surface immediately above the hypocentre is the epicentre<\/strong> of the earthquake.\r\n\r\n[caption id=\"attachment_474\" align=\"aligncenter\" width=\"1024\"]\"\" Figure 12.4<\/strong> Rupture surface (dark pink), on a fault plane (light pink). Left: In this example, the near side of the fault is moving to the left, and the lengths of the arrows within the rupture surface represent relative amounts of displacement. Coloured arrows represent propagation of failure on a rupture surface, starting at the dark blue heavy arrow and propagating left (green arrows) then right (yellow arrows). Right: An earthquake's hypocentre (or focus) is the location on the fault plane where the rupture happens. Its epicentre (red star) is the location on the surface above the hypocentre. Source: Left: Steven Earle (2015), CC BY 4.0. Image source<\/a>. Right: Karla Panchuk (2017) CC BY 4.0.
<\/em>[\/caption]\r\n\r\nWithin the rupture surface, the amount of displacement varies. In Figure 12.4, the larger arrows indicate where there has been more displacement, and the smaller arrows where there has been less.\u00a0 Beyond the edge of the rupture surface there is no displacement at all.\u00a0 Notice that this particular rupture surface doesn't extend to the land surface of the diagram.\r\n\r\nThe size of a rupture surface and the amount of displacement along it will depend on a number of factors, including the type and strength of the rock, and the degree to which the rock was stressed beforehand. The magnitude of an earthquake will depend on the size of the rupture surface and the amount of displacement.\r\n\r\nA rupture doesn\u2019t occur all at once along a rupture surface. It starts at a single point and spreads rapidly from there. Figure 12.4 illustrates a case where rupturing starts at the heavy blue arrow in the middle, then continues through the lighter blue arrows. The rupture spreads to the left side (green arrows), then the right (yellow arrows).\r\n\r\nDepending on the extent of the rupture surface, the propagation of failures<\/strong> (incremental ruptures contributing to making the final rupture surface) from the point of initiation is typically completed within seconds to several tens of seconds. The initiation point isn\u2019t necessarily in the centre of the rupture surface; it may be close to one end, near the top, or near the bottom.\r\n
\r\n\r\n<\/a>Concept Check: What Is an Earthquake?<\/strong>\r\n\r\n
\r\n\r\nWrite the words into the correct blank.<\/strong>\r\n\r\nWhen rocks are stretched, then snap back to their original shape in a process called\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>. They vibrate like a plucked guitar string, and this is what we feel as an \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>.\r\n\r\nRocks break on a \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> surface that takes up only part of the \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> plane. (Earthquakes can happen in many locations on this plane over time.)\r\n\r\nThe location where slip happens on the fault plane within the Earth is the \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>. The \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> is the location on the surface above actual slip.\r\n\r\nFill-in-the-blank options:\r\n