Lab 6: Earthquake Focal Mechanisms (35 points) Group Exercise 1. Drawing Nodal Planes (8 pts) The outline map below labeled Figure 4.60a shows the positions of two earthquakes recorded on the Mid-Atlantic ridge system. Lower hemispheres of focal spheres of earthquakes 1 and 2 are also shown in Figures 4.60b and 4.60c. a. Draw the nodal planes on Figures 4.60b and 4.60c. Remember that the nodal planes must be perpendicular to each other. You can't do this very accurately but you can draw the nodal planes with sufficient care to indicate that the nodal planes are perpendicular to each other and do divide the focal sphere into four sectors of equal size. b. Identify the fault types that generated earthquakes 1 and 2. c. Using the Figure 4.60a, determine which nodal plane is most likely to be the fault plane in each case. Briefly explain how you came to your conclusions. d. From the tectonic setting, what are the depth ranges you would expect for earthquakes 1 and 2? 1
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Individual Activities In this section you will examine the October 1999 southern California Hector Mine earthquake. The maps and description below will give you some idea of where the earthquake took place and shows the pattern of aftershocks. The picture above shows the spatial relation of the Hector Mine earthquake to the major faults in California. It is close to the Garlock fault (labeled) and the San Andreas fault system (to the southwest). This excerpt from the Southern California Earthquake Data Center (http://www.data.scec.org/chrono_index/hectoreq.html) describes the quake: "At 2:46:44 am on the morning of Saturday, October 16, 1999, most of southern California, as well as parts of Arizona and Nevada, shook and rattled in the seismic wake of the largest earthquake to strike the area since the M 7.3 Landers earthquake of June 28, 1992. Originally measured at magnitude 7.0, this earthquake was centered in a remote part of the Mojave Desert The Hector Mine earthquake was preceded by a small cluster of foreshocks that begin about 20 hours before the onset of the mainshock. The largest of these foreshocks was a magnitude 3.8 tremor that occurred at 7:41 pm PDT on October 15. These foreshocks were in the same location as a cluster of aftershocks triggered by the 1992 Landers earthquake. When the mainshock struck, just before 2:47 am PDT, the rupture was somewhat slow in starting. But within about 10 seconds it was over, having ruptured in both directions (bilaterally) from the epicenter: north along the Lavic Lake fault for about 15 kilometers, and south along the Lavic Lake fault and the central Bullion fault for another 26 kilometers." 3
Below is a zoomed image of the fault location with topography, faults, nearby cities, aftershocks, and other significant seismicity. From http://www.data.scec.org/chrono_index/hectoreq.html: Hector quakes as yellow solid circles; Lavic Lake fault, surface rupture shown in red. 1992 Landers quakes as black circles; 1992 surface rupture shown in orange. TriNet stations as green triangles; (red epicenters 10/26/99) Egill Hauksson, Caltech. 4
1. Developing a fault plane solution data set from seismic records (8 pts) The table below lists the following quantities for a particular earthquake: (1) code of seismic station; (2) epicentral distance (Δ, in degrees of arc) of that station from the earthquake; (3) azimuth (Az) from earthquake to station; and (4) the sense of the first P- wave first motion (compression [C] or dilatation [D]) observed at that station. Follow the instructions in a-b to develop a data set from which you will plot a focal mechanism. Station Δ ( ) i ( ) Az ( ) First Motion NEW 14 358 GW01 18 65 D GW02 18 147 C GW03 18 160 D HKT 18 99 GW05 19 250 C GW04 19 336 C FFC 23 22 GW08 25 62 D GW10 25 240 D DWPF 30 92 COLA 36 338 PAYG 43 140 SJG 47 97 GW06 55 232 D GW09 55 254 C NNA 59 134 GW07 67 160 D YAK 70 333 ESK 74 33 KONO 76 25 GW11 87 224 D a. The table below lists the "take-off" angle i for various values of epicentral angle Δ. Use that table to determine i for the P-wave ray which travels from the earthquake to each of the stations in the list above and enter the values into the appropriate column. 5
Δ ( ) i ( ) 13 47 15 45 17 43 18 39 21 35 23 32 25 30 27 29 29 29 31 29 33 28 35 28 37 27 41 26 45 25 49 24 51 23 55 22 59 22 63 21 67 20 71 19 75 18 79 17 83 16 87 15 91 14 95 14 b. On an attached page, seismic recordings of this earthquake are given for several stations. Pen motions in the upward direction on the record indicate compressional motions while pen motions in the downward direction indicate dilatational motions. Examine these records and determine for each station whether the first P-wave motion is compressional or dilatational. Mark each recording with a "C" or "D" and an arrow pointing to the wave pulse you used to make the determination. Then use these observations to complete the "First Motion" column in the table above. 6
2. Plotting and interpreting the focal mechanism (19 pts) a. Use the data from the table you filled in above to plot the Cs and Ds on tracing paper placed over your stereonet. Use separate symbols for compression and dilatation and make a key for this on your plot. Be sure and trace the outer circle of the stereonet and mark N, S, E, and W along this circle. Also, remember that the take-off angle is measured FROM THE VERTICAL. That is, it is the angle from the center of the stereonet. It is NOT a dip angle. b. Fit two great circles through the distribution of first motions so that the quadrants of compressional and dilatational first P-wave motions are separated by these planes. These two planes must be perpendicular to each other. Ask for help if you're not sure how to do this. Also, all your data within each quadrant should describe the same motion, either compression or dilatation, not both. These planes represent the "nodal planes" for the earthquake radiation pattern meaning that they are planes along which there is little or no P- wave generation. c. From your "focal-plane solution" determined in a and b above, determine the type of faulting responsible for this earthquake. Was it normal, thrust, or strike-slip? d. Draw on your stereonet the sense(s) of motion which could account for this focal-plane solution. e. From the information given about the Hector Mine Earthquake, determine which of the nodal planes is the fault plane. Justify your answer. Is the slip right-lateral or left-lateral? f. Given your answer to e, make a simple interpretation of the Hector Mine fault in the greater context of California tectonics (i.e. a transform-faulted plate margin). You might want to address the following in your answer: is the Hector Mine fault a part of the San Andreas fault zone, or is it a part of a different fault system? 7
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