Geology 101 Name(s): Lab 7: Earthquakes When the stresses in a rock (which may or may not already be faulted) exceed the tensile strength of the rock, the rock ruptures at a point called the focus or hypocenter. This focus may lie on a pre-existing fault or be on a new fault entirely. This sudden release of energy is called an earthquake. What is felt during an earthquake are the vibrations of the solid earth caused by the passage of seismic waves. These type of waves are elastic because they do not cause any permanent deformation of the rocks they pass through. The energy that propels these waves is called, predictably enough, seismic energy, and this energy propogates through rock away from the focus in all directions (spherically). While the waves are in earth material, there are two types: P- (or primary) waves and S- (or secondary) waves. P-waves travel through rock by alternately compressing and dilating the rock in the direction of motion. Practically, a P-wave feels like a jolt, like a truck has hit the side of the building you’re in. S-waves travel through rock by whipsawing the rock at right angles to the direction of wave motion. An S-wave produce a rolling type of motion, similar to the rocking of a boat on the ocean. Figure 7-1. Diagram of earth movements produced by (a) P-waves and (b) S-waves. The location of the focus of an earthquake is defined by the epicenter, which is the position on the surface of the earth vertically above the focus (this is measured in latitude and longitude), and by the focal depth, the distance (in kilometers) from the epicenter to the focus. Seismographs are instruments that record vibrations in the earth as seismic waves arrive at a seismic station. Seismograms are the recorded trace of the ground motion at the station (see figure 7-2). 1. Form a line of at least 10 people. Face outwards in the same direction and stand shoulder-to-shoulder. Station someone else as a timer (get a stopwatch) and, upon a signal from the timer, the person at one end should be shoved (nottoo hard) into his or her neighbor. The push will then be transmitted down the line. When the motion reaches the opposite end of the line, the timer should record the time. This models motion associated with a P-wave. a. P-wave travel time: Keep everyone in the line, but this time, link by holding hands at arm’s length from your neighbor. The person at one end of the line should sway backwards and this motion should propagate down the line (similar to “crack the whip”). Again, time the length of motion and record it. This models motion associated with an S-wave. b. S-wave travel time: c. Which wave travels faster? 2. What would happen to the speed of the P-wave if everyone linked at the elbow, rather than stood shoulder-to-shoulder? 3. What property of the shaken rocks would question 2 model? 4. Do P-waves (or any seismic wave for that matter) travel faster through igneous rocks (density > 2.5 g/cm3) or through sedimentary rocks (density < 2.5 g/cm3). What does density have to do with speed? When an earthquake occurs, seismologists must quickly determine both the magnitude and location of the earthquake (why? For instance, to be able to predict if a tsunami will occur). Locating the magnitude 5.2 Duvall earthquake of May 2, 1996 (9:04 PDT) The location of an earthquake can be determined using triangulation. A seismograph is an instrument which records the exact time when the seismic waves of an earthquake arrive at the seismograph station (called the arrival time). If the time of the actual fault rupture which generated the earthquake (the origin time) is known, then the time the seismic waves took to travel the distance between the focus and the seismograph station (the travel time) can be calculated by simple subtraction: Travel Time = Arrival Time - Origin Time The arrival time and origin time of an earthquake are recorded not in local time, but in Greenwich Mean Time (GMT), which is broadcast via radio signal to the seismograph. Of course, if you know the scale of the seismogram, then it is easy to just read off the travel time.5. Use the three seismograms (the recorded trace of ground motion) from seismic stations JCW, GSM and GMW (see figure 7-2) to determine the travel time of the P-waves at each station (the S-waves, of course, came later). Note the units of the travel times. JCW: GSM: GMW: 6. In order to calculate a distance between the epicenter and the seismograph station from the travel time derived above, you need to know the velocity of the P-wave. What formula connects distance, velocity and time? Distance to epicenter = Now that you have the formula, and the additional information that P-waves travel, on average, at a velocity of 6.5 kilometers/second through the crust, calculate the distance (in kilometers) to each seismograph station from the epicenter. JCW: GSM: GMW: On the map provided (figure 7-3), use a compass (no, not the north/south kind) to draw a circle around each seismograph station, setting the circle size using the scale on the map and the distances calculated in question 5. In other words, since you can't tell in what direction the seismic waves came from, the circle represents every possible point the earthquake's epicenter could have been. Note that the intersection of three circles will give you a single point; that is the epicenter of the earthquake. Note why this process is called triangulation and also note that if your circles don't intersect, pick an "average" spot in the middle of where the circles come closest together.Figure 7-2 Seismograms from seismic stations JCW, GSM and GMW.Figure 7-3. Map of the Seattle area showing seismic stations JCW, GSM and GMW. 7. How far from Seattle is the epicenter (in kilometers), and in what direction? 8. For many earthquakes, the circles drawn from the seismic stations do not intersect. Give two reasons for this (hint: do earthquakes occur at the surface?).There are several different numerical magnitude systems based on a logarithmic relation between ground motion and the amount of motion recorded on the seismogram; each unit increase in magnitude represents a tenfold increase in the duration of shaking and/or the amplitude of ground motion. Note that it does not mean that every unit increase represents a tenfold increase in the energy of an earthquake. These different systems yield slightly different