Lab 4 Earthquakes doc (1)

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GEOL101 Dynamics of the Earth – Fall 2023 Name: Emily Thomson Laboratory 4: Earthquakes Section: Introduction Earthquakes generate shaking and vibrating of the land surface. Such a phenomenon commonly is produced when Earth material (rocks) ruptures during brittle failure (breaking) along an old or new fault releasing stored up energy as ground displacement seismic waves. Think back to the Plate Tectonics lab, all three of the plate boundaries are capable of producing earthquakes. The Earth’s plates are not in constant motion, instead they move in sudden bursts and each burst results in an earthquake. It is important to note that not all earthquakes are generated by movement along brittle faults. In fact, earthquakes can be generated during volcanic eruptions and nuclear explosions. Here, for the sake of simplicity, we only consider earthquakes generated during rupture along a new or old fault. Earthquakes can occur at a variety of depths in the Earth’s crust. The depth where they generate from is called the focus or hypocenter (figure 1). Located directly above the focus on the earth’s surface is the epicenter (figure 1) . When enough energy is stored along a fault to overcome the strength of the rock, it will break releasing energy as seismic waves that travel away from the focus in all directions as spheres (figure 1). A common analogy for this is dropping a pebble into water and watching the ripples (waves) travel away from where the pebble was dropped. Seismic waves are generally strongest at the focus and gradually grow weaker further away from the rupture site. Figure 1: Block diagram illustrating the locations of focus and epicenter along a fault. Seismic waves propagate away from the focus as the earthquake occurs. Question 1. What type of fault is shown in Figure 1 (strike slip, normal, or reverse)? How do you know? How does one side of the fault move relative to the other? It is a strike slip and I can tell by the direction of the arrows. One side of fault moves forwards and one backwards, in opposite directions

Seismic Waves Seismic waves are disturbances that elastically distort the material they travel 1 through. Meaning after a seismic wave has passed through a portion of the Earth it returns to its original form. Seismic waves include body and surface waves. The former type of wave emanates spherically from the focus traveling entirely within the interior of the Earth while the latter travels along the surface of the Earth. Body waves are compressional or P-waves and shear or S-waves. Surface waves are Love and Rayleigh waves. For the purposes of this lab we are going to focus on body waves. P-waves or primary waves are compressional, meaning that the p-wave energy moves outward from the focus it produces a series of contractions and expansions (Figure 2b) in the direction of wave movement. You could think about this like a slinky being pushed and pulled. Typical P-waves speeds range from 5 to 8 km/s, but they can be much lower near the ground surface. The speed at which they travel depends on part of the Earth’s interior it’s traveling through. Generally, the speed of the P-waves increases with depth. S-waves or secondary or shear waves have an up/down and/or side-to-side motion (Figure 2c) as the seismic wave energy moves outward from the focus. The motion of S-waves is similar to shaking one end of a rope. S-waves are not able to travel through liquid like P-waves. They have average speeds of around 3.5 km/s in crustal material like granite, but S-wave speed can be much lower near the ground surface. Figure 2: a. A section of undisturbed material within the Earth. b. The same section of Earth material but with a P-wave passing through. Notice that in areas of contraction the squares are smaller and in areas of expansion they are bigger as the wave moves through the material. Wave movement or propagation is from left to right. c. S-wave moving through the same section of Earth material. As the wave propagates from left to right, notice that the size of the squares does not change, rather they move up and down.

Question 2. Based on the descriptions of the motion associated with P and S waves above and in Figure 2, hypothesize which wave type causes the least intense ground shaking and therefore the least damage to buildings and why. I predicts that P waves cause the least intense ground shaking because they only travel in a push pull method, while S Waves travel in all directions therefore having a greater amplitude. Seismic Stations Seismic stations house the equipment seismologists use to record ground motion from 2 earthquakes. Globally, there are thousands of stations on land and on water recording in real time. Seismic stations consist of a seismometer (records the ground movement), a computer, communication equipment (antenna and gps), and often a solar panel to power everything (Figure 3). Figure 3: Schematic illustrating how a seismic station receives, records, and transmits data from an earthquake. usarray.org/about/how Figure 4 demonstrates how a Seismometer can record up and down motion (not used for practical purposes anymore). A weight hanging from a spring is attached to the seismometer frame and when an earthquake occurs the relative motion between the weight and the moving Earth provides a measure of the ground motion. The movement is recorded onto a seismogram with a pen attached to the weight. More modern seismometers record and store the data digitally, by inducing electrical currents from a magnet moving along with the ground. Seismograms allow us to visualize the arrival times of the different seismic waves, P and S. Figure 5 is an example of what seismograms look like. The y-axis or vertical is acceleration of the seismic waves and the x-axis is time in seconds. Because P-waves travel faster than S-waves they arrive at the station first and

are recorded first (blue arrow, Figure 5). S-waves travel a bit slower and arrive at the seismic station after P-waves, but because of the way S-waves travel (up and down/side-to-side) they produce greater accelerations (y-axis) or higher peaks on the seismogram (red arrow, Figure 5). If you have ever experienced an earthquake you have likely felt the S-wave but probably not felt the P-wave. Figure 4: A classic seismometer based on weight suspended on a spring. 3 Figure 5: Example of a seismogram with the P-wave arrival in blue and the S-wave arrival in red. P-waves travel faster and arrive at the station first, so the first peak on the seismogram is the P-wave. The second major peak is the S-wave arrival. Seismologists and scientists use the difference between the P and S-wave arrival times (called the S-P time interval ) to determine the distance the station is to the epicenter of the earthquake. The closer the station is to the epicenter the closer the P and S-wave arrivals will be and vice versa. In Figure 5 the S-P time interval is 9 seconds (34 s – 25 s = 9 seconds). We use the S-P interval to determine the distance the seismic station is away from the epicenter with the help of graph 1.

To determine the distance between a seismic station and the epicenter of an earthquake, find the S-P separation, in this example 9 sec on the y-axis and follow that time over until it intersects the S-P line on the graph. Follow this intersection down to the x-axis and read the distance, in this example the distance is ~ 90 km. Meaning the earthquake occurred 90 km away from the seismic station in any direction. Unfortunately, we do not yet know in what direction. For this we need data from at least three stations. Graph 1: Distance is on the x-axis in km and time in seconds is on the y-axis. For this lab we are only going to use the S-P line that plots distance vs. time. 4

Figure 6: Three seismograms from a 2019 earthquake, time in seconds is on the x-axis. Blue arrows indicate the P-wave arrival and red arrows the S-wave arrival. Figure 6 is showing seismograms from three seismic stations, LUG, CAA, and ALP, the x-axis is showing time in seconds. For each station the P-wave arrival is indicated with a red arrow and the S-wave arrival with a red arrow. 5 Question 3. Based on the spacing of the P and S-wave arrivals, hypothesize which station is furthest to the epicenter? Why? S wave is furthest from the epicenter because they are slower than the P waves, and the larger the arrival time difference, the further the location is

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