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Earthquake Types & Analysis 1 Earthquakes are events of sudden ground-shaking, which in many cases can be very devastating. 2 What type of boundaries does California have? Northern California is along a subduction zone due to converging plates. Which plates are converging? Southern California is along a transform fault boundary. What direction are the 2 plates moving? 3 Background • Most earthquakes are caused by sudden movements on faults or by volcanic eruptions. Of course plate boundaries are characterized by faults of different types. • These sudden movements are described by the elastic rebound theory. 4 Elastic Rebound Theory • Energy builds - The crust will stretch and deform. • Release - Build up is released at rupture point • Slippage - The breaking crust is felt as vibrations • Back to normal Energy is released and the rock returns to its normal shape 5 3 Types of Faults: • Reverse faults occur at convergent plate boundaries. • Normal faults occur at divergent plate boundaries. • Strike-slip faults occur at transform plate boundaries. 6 Reverse Faults 7 Normal Fault Dixie Valley-Fairview Peaks, Nevada earthquake December 16, 1954 8 Strike Slip Faults 1906 San Francisco Earthquake Before earthquake manure pile was under window where farmer shoveled it out from inside Fault runs right under corner of barn After earthquake manure pile moved over about 10 feet 9 SEISMIC WAVES • Most earthquakes are caused by sudden movements on faults or by volcanic eruptions. • The place where the disturbance starts below the surface is called the FOCUS. • Most foci are restricted to depths less than about 100 km, except along subduction zones where foci can be as deep as 700 km. • The kinetic energy (energy of motion) of the disturbance radiates in all directions away from the focus in the form of seismic waves. • The wavefront first reaches the surface at the epicenter, which is directly above the focus, or very nearly so. Epicenter Focus 10 How Earthquake vibrations are measured: The Seismograph • Seismogram: the recorded image from a seismograph 11 WAVE TYPES: Body Waves • BODY WAVES: These can pass into the deeper layers of the earth, beneath the lithosphere. There are two types of body waves, P-waves and Swaves. Body waves are NOT responsible for the ground-shaking during an earthquake. P-waves: "P" stands for primary. These waves are the fastest of the wave types. After passing through the deep interior of the earth and reemerging at the surface, P-waves are the first to arrive. P-waves are longitudinal waves. 12 WAVE TYPES: Body Waves S-waves: "S" stand for secondary. These are the second fastest kind of seismic wave. S-waves are transverse waves. This kind of motion can only be support by materials that are rigid. For this reason, S-waves cannot travel through liquids. After passing through the deep interior of the earth, Swaves re-emerge at the surface only along paths that do not intersect the core. S-waves that intercept the core are stopped, the energy is absorbed completely, and none of the energy reaches the opposite side of the earth. Thus, a "shadow zone" is formed at the surface of the earth, defined by the area where Swaves are not detected. These waves helped scientists determine the structure of the Earth’s core. 13 14 WAVE TYPES: Surface Waves SURFACE WAVES: These are restricted to the lithosphere, or surface of the Earth, and are responsible for most of the actual ground-shaking. Most of the kinetic energy of an earthquake is released in the form of surface waves. In fact practically all of the damage done by an earthquake is caused by surface waves. There are 2 types of surface waves. Two types: L-Waves and R-waves 15 WAVE TYPES: Surface Waves L-waves: "L" comes form the name of the mathematician, Love, who discovered these types of waves. However, it's helpful to think of "L" as standing for "lateral," which relates to the nature of the resulting groundshaking. The ground moves laterally, or shimmies side-to-side with no vertical component. These are the most damaging type of waves. They can literally move the ground beneath a building faster than the building itself can respond, effectively shearing the base off of the rest of the building. 16 WAVE TYPES: Surface Waves R-waves: "R" stands for Rayleigh, the English physicist who first predicted this kind of seismic movement. Here, it's helpful to think of "R" as standing for "rolling," which relates to the nature of the ground-motion, like the motion of an anchored boat as it bobs up and down on sea waves. R-waves behave like sea-waves. As an R-wave passes beneath a building, for instance, the building first moves up and is then set back down. 17 18 • 19 VELOCITY The velocity (speed and direction) of the p-wave and s-wave are 2 key pieces of information when determining the event of an earthquake. These 2 important pieces of information used to determine the place, magnitude, and time of an earthquake. The velocity of the P-wave is nearly twice the velocity of the S-wave. At any place on the surface where both the P-wave and the S-wave can be detected, there is a delay in time between the arrival of the P-wave and the arrival of the S-wave. From the perspective of an observer watching a seismometer, the first indication that an earthquake has happened somewhere is the arrival of the Pwave. A short period of time passes before the arrival of the S-wave. 20 Richter Scale The Richter scale is a scale that expresses the magnitude of an earthquake. Each step on the scale represents a 30 fold increase in the amount of energy released. An earthquake with a magnitude of 6.0 would be 306 times more powerful than an earthquake with a magnitude of 1.0. Using the distance and the amplitude of an earthquake from 3 earthquake stations, we can calculate the magnitude of an earthquake on the Richter scale. 21 #36-39?? • When looking at a seismogram, we first see the arrival of the p-wave. The next wave to arrive is the s-wave. The time in between the arrival of the 2 waves is known as the s-p interval. This s-p interval is used to calculate the distance to the epicenter of the earthquake. After the p & s waves arrive, then the surface waves arrive. These are the 22 waves that make the large squiggles on the right. AMPLITUDE • Amplitude is the maximum amount of vertical distance of the seismogram, and reflects the amount of groundshaking. Notice that most of the "ground-shaking" is associated with the surface waves. 23 Determining the Distance to the Epicenter The red line shows how the S-P interval relates to the distance to the epicenter. For each seismic station determine the distance to the epicenter using the graph. For each seismic station, find the S-P interval time on the vertical scale, move across horizontally to the red line, then vertically 24 down to the distance scale. Record your distance for each station in your data table. The exercise refers to earthquakes in and around the Caribbean basin, an area that experiences many earthquakes every year. The map shows the locations of earthquakes (epicenters) in the Caribbean region. Note how many noteworthy earthquakes (Magnitude > 3.5) occurred in just the twenty-two years from 1967 to 1999. Note also the relationship between the epicenters and plate boundaries. 25 26 BACKGROUND All three types of plate boundaries can be found around the edges of the Caribbean plate, although the divergent type of boundary is restricted to a very small region west of Jamaica, just south of the Cayman Islands. Near the eastern margin of the Caribbean plate there are recently active volcanoes in the lesser Antilles. These are associated with a convergent plate boundary and caused by subduction of the North American plate beneath the eastern edge of the Caribbean plate. 27 PRACTICE Here are 2 seismographs. Find the arrival time, s-p interval, and amplitude for each. Record these in the data table and answer the questions below: 1. Which seismometer station was closer to the epicenter, A or B? Explain briefly. 2. Which seismogram records the most ground-shaking at the seismic station? 28 3. Give two reasons why these seismograms cannot be records of the same earthquake. Determining the Distance to the Epicenter 29 Location of an Earthquake Once you have calculated the distance which the epicenter occurred from the location of the seismograph, then we can locate the actual epicenter. We must have 3 distances to find the actual location. This process of using 3 locations is called triangulation. 30 Richter Scale 31