Lab 9: Satellite Geodesy (35 points) Here you will work with GPS Time Series data to explore plate motion and deformation in California. This lab modifies an exercise found here: http://www.unavco.org:8080/cws/pbonucleus/draftresources/sanandreas/ Group Exercises 1. Introduction (4 pts) Glance over the time series included later in this lab, and consider what you have learned in class, to answer these questions. a. What units of measurement are used for the X and Y axes of a GPS time series? b. What do red data points represent? What causes gaps in time-series data? c. On a N-S time series, what direction does a positive slope indicate? On an E-W time series, what direction does a positive slope indicate? 2. Analyzing time-series data to determine plate tectonic motion (15) Here you will use GPS data from two geodetic reference stations to characterize observed tectonic motion. a. Examine the station information and time series plots for BEMT and SBCC on the following pages to fill in the chart below. You can check recent updates on these stations here: http://pboweb.unavco.org/stations/?checkkey=bemt http://pboweb.unavco.org/stations/?checkkey=sbcc What is the latitude and longitude? BEMT SBCC What is the elevation? Plot each station as Δ on the map of Southern California When is the first month and year of data shown on the time-series? Are data still being collected? If not, when did data stop being collected? 1
Study the North time-series. What is the range of values on the y axis for the N plot? What direction is the station moving? Study the East time-series. What is the range of values on the y axis for the E plot? What direction is the station moving? Study the Vertical time-series. When was the station at its highest elevation? How much has the station s vertical position changed since 2004? Calculate the GPS station Velocities (slope). North-south average (mm/year) East-west average (mm/year) Vertical average (mm/year) b. With this information extracted from the time series plots, you can visualize plate movement in Southern California. Plot the north-south and east-west velocity vectors of these two stations on graph paper and determine the magnitude and direction of the resulting velocity vector. Then plot and label these vectors on the same map where you plotted the stations. c. Compare and contrast the observed movement of stations BEMT and SBCC? Which station moves faster? What factors cause the differences between the two sites? Do you notice any changes in each time series? 2
Map of Southern California 3
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Individual Exercises 3. Investigate deformation at two GPS stations (10) In this exercise you will interpret two GPS time series recorded near the San Andreas Fault. Because the majority of motion recorded by the instruments is horizontal to the Earth s surface, concentrate on analyzing the North and East components when answering the questions. a. According to the GPS time series at CARH and CAND, when did the earthquake occur? Use the conversion chart to provide the month and year. b. How much slip occurred on the fault at station CAND during the earthquake? Measure the change in position over 0.1 year when you make this estimate. How does this compare to its normal motion? c. Calculate and compare the velocities of CARH and CAND after the earthquake. Hint: Use an appropriate span of data after the start of 2006 to derive the slope. d. Why are the datasets at CAND and CARH different despite both recording the earthquake? e. Using the following equation for M, the moment magnitude, what is the magnitude of the 2004 Parkfield earthquake based on the slip that you calculated? How well does this match the measured magnitude of the earthquake (Mw 6.0)? Is it a perfect match? Why not? Use this simplified formula: where M = magnitude D = average slip in meters [and 1000 mm = 1 meter] 9
4. Seismic Hazard in Central California (6) The Parkfield section of the San Andreas Fault has not experienced a magnitude 6.0 (or above) earthquake since the 2004 event, but the North American and Pacific plates continue to grind past one another. a. Based on the data about the total slip due to the Parkfield earthquake at CAND and CARH, and the fact that the plate moves ~22mm/yr at Parkfield, how long should it take to build enough strain energy to generate an earthquake with a similar magnitude? b. Look at the diagram from the USGS illustrating when earthquakes with magnitudes similar to the 2004 events have occurred along the Parkfield section of the San Andreas Fault. How often did these earthquakes occur during the 20th century? c. Does your calculation from #1 agree with the observed value from #2? If not, why? 10