GPS strain analysis examples Instructor notes Compiled by Phil Resor (Wesleyan University) This document presents several examples of GPS station triplets for different tectonic environments. These examples are provided as possible unknown triplets for students to use in solving the instantaneous strain problem. This document includes instructor s notes and student data sheets for the three examples. Instructor s notes The examples have been chosen to illustrate different tectonic settings. The specific triplets include GPS stations with good data quality (relatively clean time series) where the instantaneous strain estimates for each triplet are consistent with the regional tectonics and well outside of the estimated uncertainty. We envision these unknown examples being used in one of two ways: 1) The examples can be approached as a sort of jigsaw where students are assigned to work on one of the examples and then share their results with the class as part of their introduction to strain or 2) They can be used later in a structural geology class associated with study of specific tectonic environments (contractional, extensional, strike-slip). We recommend having the students complete Exercise: Finding location and velocity data for PBO GPS stations (which walks them through accessing Plate Boundary Observatory data prior to this longer exercise). For each of the examples we provide a velocity map and geologic context so that students can solve for the instantaneous deformation and interpret the results in a geologic context. Related files: Exercise: Finding location and velocity data for PBO GPS stations precursor exercise for learning how to download PBO data Strain calculator two versions of the calculator are available: Excel and Matlab. The Excel version is more black box whereas the Matlab version would be more appropriate if instructor would like the students more involved in the mathematics and programing end. Document: Explanation of calculator output runs through the different output parameters with explanations and images to aid in real world understanding of results. Other related files can be found on the UNAVCO module website. The following background material can be shared with the students (or not) based on the instructor s objectives. Example 1: Olympic Peninsula compressional strain This triplet (PBO stations NEAH, P401, P403) is located at the tip of the Olympic Peninsula, west of Seattle (Fig. OP.1). The stations are located near the down dip limit of the locked portion (Chapman and Melbourne, 2009) of the Cascadia subduction zone (Fig. OP.2) and experience compression as they are forced northeastward by the down-going Juan de Fuca plate. One complication that might confuse/interest students is that the time series show effects of slow slip events (also called episodic tremor and slip) (Fig. IN.1). The majority of the slow slip occurs in the transition zone immediately to the west, but these coastal stations also move in response to Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 1
GPS strain analysis examples instructor notes the nearby slip. The data still show a fairly constant long-term velocity, however, and thus work well as a compressional example for this exercise. Another interesting thing to consider is that the triangle of stations also spans the Calawah fault (Fig. OP.3), inferred to be a left-lateral strike slip fault active in the last 15,000 years (Lidke, 2004). Figure IN.1. Time series plot of east component of location for station P403. Arrows point to likely slow slip events (also called episodic tremor and slip). (PBO data) This triplet could be used as a launching point for a discussion of the Cascadia subduction zone, the Seattle Fault, and seismic hazards in the region (e.g. Johnson et al., 1994 and more recent work). Example 2: Wasatch Fault extensional strain This triplet (PBO stations P116, P088, COON) is located along the Wasatch Front in the Salt Lake City metropolitan area (Fig. WF.1). The stations form a triangle that spans the Wasatch fault zone (Black et al., 2004) one of the most active normal faults in North America as well as several other active faults. The long term horizontal velocities for the stations are quite robust but there are some non-tectonic variations with decadal and annual periods that students might wonder about (Fig. IN.2). Elósegui et al. (2003) found that the decadal variation is consistent with elastic deformation due to changes in the water level and thus the mass of the Great Salt Lake while the annual signal most likely reflects more local site-specific effects. Figure IN.2. Time series plot of location for station COON. Annual (seasonal) signal is most clearly visible in East component. Decadal signal is most clearly visible in the vertical (PBO data). Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 2
GPS strain analysis examples instructor notes This triplet could be used as a launching point for a discussion of the Wasatch Fault, intracontinental normal faulting, and seismic hazards in the region (e.g. Chang and Smith, 2002). Example 3: San Andreas Fault shear strain This triplet (PBO stations P538, P541, P539) spans the San Andreas Fault immediately southwest of Cholame Valley or ~30 km northwest of Wallace Creek. This segment of the San Andreas Fault has seen extensive paleoseismological study (Bryant et al., 2002). The Wallace Creek site, in particular, provides a striking example of tectonic geomorphology that has been used to estimate long-term slip rate and earthquake recurrence along this portion of the San Andreas (Sieh and Jahns, 1984). The time series for these stations are very clean particularly in the horizontal. Minor annual effects can be seen in the vertical but even there the overall velocities are more robust than at many PBO sites (Fig. IN.3). Figure IN.3. Time series plot of location for station P538. Horizontal velocities are extremely well constrained and even the vertical is quite good (PBO data). This triplet could be used as a launching point for discussions of the seismic cycle including geodetic and geologic estimates of slip rate. The Southern California Earthquake Center (SCEC) has created a Wallace Creek Trail Guide (http://www.scec.org/wallacecreek/) with links to many useful resources including class exercises. Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 3
GPS strain analysis examples Student exercise Example 1: Olympic Peninsula Name: Please complete the following worksheet to estimate, calculate, and interpret the strain for a triangle defined by three GPS stations at the tip of the Olympic Peninsula, just west of Seattle (Fig. OP.1). Step 1. Estimate the strain from the velocity field Figure OP.1. Location of the GPS station triplet (NEAH, P401, P403) at the tip of the Olympic Peninsula. Basemap shows GPS velocity vectors in Stable North American Reference Frame (SNARF; more on reference frames). Horizontal vector at bottom of the plot is 25 mm/yr scale. (UNAVCO Velocity Viewer) Use the map of the velocity field (Fig. OP.1) to hypothesize (infer) the instantaneous deformation for this set of stations. Approximate Magnitude (m/yr) Approximate Azimuth Translation: Rotation direction (+ = counter clockwise, - = clockwise): Strain: Sign (+ = extension, - = contraction) Max horizontal extension Min horizontal extension Approximate Azimuth Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 1
Step 2. Find location and velocity data for your stations Access the station data from the Plate Boundary Observatory website (http://pbo.unavco.org/station/overview/neah and so on) using the method outlined in Finding location and velocity data for PBO GPS stations. Geographic coordinates using WGS 1984 datum, Stable North American Reference Frame (SNARF) Site Decimal Lat Decimal Long NEAH P401 P403 GPS site velocities relative to SNARF, expressed in mm/year Site N Velocity ± Uncert E Velocity ± Uncert Height Velocity ± Uncert NEAH P401 P403 Step 3. Calculate the instantaneous deformation Use the strain calculator provided by your instructor to find the following parameters that describe the complete deformation of the area. E component ± uncert (m/yr) N component ± uncert (m/yr) Translation Vector Azimuth (degrees) Speed(m/yr) magnitude ± uncertainty (deg/yr) magnitude ± uncertainty (nano-rad/yr) direction Rotation Magnitude (e1h) (nano-strain) Azimuth of S1H (degrees) Max horizontal extension Magnitude (e2h) (nano-strain) Azimuth of S2H (degrees) Min horizontal extension Max shear strain (nano-strain) Area strain (nano-strain) Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 2
Step 4. Interpret the results Compare your results to your estimates from Step 1 as well as other indicators of crustal strain including regional tectonics (Fig. OP.2) and active faults (Fig. OP.3). You may find the document Explanation of calculator output to be helpful in your interpretation. Did your estimates in Step 1 generally agree or not with the outputs of the strain calculator? Explain. The Juan de Fuca plate is moving toward the northeast relative to North America at an azimuth of 59 degrees with a velocity of 0.033 m/yr (UNAVCO Plate Calculator using GSRM v1.2 velocities and fixed North America for a point on the Juan de Fuca plate at 47.055154 degrees latitude and -126.258545 degrees longitude). Figure OP.2. Tectonic setting for the Olympic Peninsula. The North American Juan De Fuca Plate boundary (trench of the Cascadia Subduction zone) is the thin yellow line on the left side of the figure. (This Dynamic Planet (Simkin et al., 2006)) How does the deformation you calculated from GPS velocities in compare to the kinematics (motion) of this subduction zone? Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 3
Figure OP.3. Quaternary faults and folds in the region of the Olympic Peninsula (US Geological Survey, 2006) Although most of the relative motion between the North America and Juan de Fuca plates is accommodated by the plate boundary fault, active faults in the region (Fig. 3) attest to some permanent deformation of the overriding plate. Based on your strain calculations what sort of motion do you expect on the orange faults along the northern margin of the Olympic Peninsula (reverse, normal, left-lateral, right lateral or some combination of these (i.e. oblique slip specify a combination such as normal + right lateral))? Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 4
Example 2: Wasatch Front Name: Please complete the following worksheet to estimate, calculate, and interpret the strain for a triangle defined by three GPS stations that span the Wasatch Mountain Front in the Salt Lake City metropolitan area (Fig. WF.1). Step 1. Estimate the strain from the velocity field Figure WF.1. Location of the GPS station triplet (P116, P088, COON) spanning the Wasatch Front. Basemap shows GPS velocity vectors in Stable North American Reference Frame (SNARF; more on reference frames). Horizontal vector at bottom of the plot is 0.025 m/yr scale. (UNAVCO Velocity Viewer) Use the map of the velocity field (Fig. WF.1) to hypothesize (infer) the instantaneous deformation for this set of stations. Approximate Magnitude (m/yr) Approximate Azimuth Translation: Rotation direction (+ = counter clockwise, - = clockwise): Strain: Sign (+ = extension, - = contraction) Max horizontal extension Min horizontal extension Approximate Azimuth Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 5
Step 2. Find location and velocity data for your stations Access the station data from the Plate Boundary Observatory website (http://pbo.unavco.org/station/overview/p116 and so on) using the method outlined in Finding location and velocity data for PBO GPS stations. Geographic coordinates using WGS 1984 datum, Stable North American Reference Frame (SNARF) Site Decimal Lat Decimal Long P116 P088 COON GPS site velocities relative to SNARF, expressed in mm/year Site N Velocity ± Uncert E Velocity ± Uncert Height Velocity ± Uncert P116 P088 COON Step 3. Calculate the instantaneous deformation Use the strain calculator provided by your instructor to find the following parameters that describe the complete deformation of the area. E component ± uncert (m/yr) N component ± uncert (m/yr) Translation Vector Azimuth (degrees) Speed(m/yr) magnitude ± uncertainty (deg/yr) magnitude ± uncertainty (nano-rad/yr) direction Rotation Magnitude (e1h) (nano-strain) Azimuth of S1H (degrees) Max horizontal extension Magnitude (e2h) (nano-strain) Azimuth of S2H (degrees) Min horizontal extension Max shear strain (nano-strain) Area strain (nano-strain) Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 6
Step 4. Interpret the results Compare your results to your estimates from Step 1 as well as other indicators of crustal strain including regional tectonics (Fig. WF.2) and active faults (Fig. WF.3). You may find the document Explanation of calculator output to be helpful in your interpretation. Did your estimates in Step 1 generally agree or not with the outputs of the strain calculator? Explain. The Pacific plate is moving toward the northwest relative to North America at an azimuth of 323 degrees with a velocity of 0.049 m/yr (UNAVCO Plate Calculator using GSRM v1.2 velocities and fixed North America for a point on the Pacific plate located at 37.696318 degrees latitude and -123.009224 degrees longitude). Most of this motion is accommodated by slip along the San Andreas Fault, but deformation in the Basin and Range province contributes a significant component of westward motion. Figure WF.2. Tectonic setting for the Wasatch Front. The mountain front marks the eastern edge of the northern Basin and Range tectonic province that extends westward to the Sierra Nevada block, northward to the Snake River Plain and southward to Las Vegas. (This Dynamic Planet (Simkin et al., 2006)) How does the deformation you calculated from GPS velocities compare to the kinematics (motion) of the Basin and Range? Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 7
Figure WF.3. Quaternary faults and folds in the vicinity of the Wasatch Front near Salt Lake City (US Geological Survey, 2006) The Basin and Range province is named after the numerous mountain ranges separated by intervening basins. In the Salt Lake City area the ranges are bounded on their western sides by active faults (Fig. WF.3). Based on your strain calculations what sort of motion do you expect on these orange rangebounding faults (reverse, normal, left-lateral, right lateral or some combination of these (i.e. oblique slip specify a combination such as normal + right lateral))? Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 8
Example 3: San Andreas Fault Name: Please complete the following worksheet to estimate, calculate, and interpret the strain for a triangle defined by three GPS stations that span the central San Andreas Fault southwest of the town of Parkfield (Fig. SA.1). Step 1. Estimate the strain from the velocity field Figure SA.1. Location of the GPS station triplet (P538, P541, P539) spanning the San Andreas Fault. Basemap shows GPS velocity vectors in Stable North American Reference Frame (SNARF; more on reference frames). Horizontal vector at bottom of the plot is 0.025 m/yr scale. (UNAVCO Velocity Viewer) Use the map of the velocity field (Fig. SA.1) to hypothesize (infer) the instantaneous deformation for this set of stations. Approximate Magnitude (m/yr) Approximate Azimuth Translation: Rotation direction (+ = counter clockwise, - = clockwise): Strain: Sign (+ = extension, - = contraction) Max horizontal extension Min horizontal extension Approximate Azimuth Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 9
Step 2. Find location and velocity data for your stations Access the station data from the Plate Boundary Observatory website (http://pbo.unavco.org/station/overview/p538 and so on) using the method outlined in Finding location and velocity data for PBO GPS stations. Geographic coordinates using WGS 1984 datum, Stable North American Reference Frame (SNARF) Site Decimal Lat Decimal Long P538 P541 P539 GPS site velocities relative to SNARF, expressed in mm/year Site N Velocity ± Uncert E Velocity ± Uncert Height Velocity ± Uncert P538 P541 P539 Step 3. Calculate the instantaneous deformation Use the strain calculator to find the following parameters that describe the complete deformation of the area. E component ± uncert (m/yr) N component ± uncert (m/yr) Translation Vector Azimuth (degrees) Speed(m/yr) magnitude ± uncertainty (deg/yr) magnitude ± uncertainty (nano-rad/yr) direction Rotation Magnitude (e1h) (nano-strain) Azimuth of S1H (degrees) Max horizontal extension Magnitude (e2h) (nano-strain) Azimuth of S2H (degrees) Min horizontal extension Max shear strain (nano-strain) Area strain (nano-strain) Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 10
Step 4. Interpret the results Compare your results to your estimates from Step 1 as well as other indicators of crustal strain including regional tectonics (Fig. SA.2) and active faults (Fig. SA.3). You may find the document Explanation of calculator output to be helpful in your interpretation. Did your estimates in Step 1 generally agree or not with the outputs of the strain calculator? Explain. The Pacific plate is moving toward the northwest relative to North America at an azimuth of 323 degrees with a velocity of 0.49 m/yr (UNAVCO Plate Calculator using GSRM v1.2 velocities and fixed North America for a point on the Pacific plate located at 37.696318 degrees latitude and -123.009224 degrees longitude). Most of this motion is accommodated by slip along the San Andreas Fault. Figure SA.2. Tectonic setting for the San Andreas Fault. The San Andreas Fault (thin black line) forms the western boundary of the North American plate from the Gulf of California to the Mendocino triple junction (This Dynamic Planet (Simkin et al., 2006)). How does the deformation you calculated from GPS velocities compare to the kinematics (motion) of the plate boundary? Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 11
Figure SA.3. Quaternary faults and folds in the vicinity of the central San Andreas. The active trace of the San Andreas Fault is shown in red (US Geological Survey, 2006). The San Andreas Fault in this area slipped 3-9 meters during the 1857 Ft. Tejon earthquake. Based on your strain calculations what sort of slip do you expect occurred along this segment of the San Andreas Fault (reverse, normal, left-lateral, right lateral or some combination of these (i.e. oblique slip specify a combination such as normal + right lateral)) during the earthquake? Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 12
Selected References General Simkin, T., Tilling, R.I., Vogt, P.R., Kirby, S.H., Kimberly, P., and Stewart, D.B., 2006, This dynamic planet: World map of volcanoes, earthquakes, impact craters, and plate tectonics: U.S. Geological Survey Geologic Investigations Series Map I-2800, 1 two-sided sheet, scale 1:30,000,000, accessed 11/21/2012, from Smithsonian web site: URL: http://mineralsciences.si.edu/tdpmap/. U.S. Geological Survey, 2006, Quaternary fault and fold database for the United States, accessed 11/21/2012, from USGS web site: http//earthquakes.usgs.gov/regional/qfaults/ Example 1 Chapman, James S. and Melbourne, T., 2009, Future Cascadia megathrust rupture delineated by episodic tremor and slip: Geophysical Research Letters, v. 36, L22301. Johnson, Samuel Y., Potter, Christopher J., and Armentrout, John M., 1994, Origin and evolution of the Seattle fault and Seattle basin, Washington: Geology, v. 22, p. 71-74. Lidke, D.J., compiler, 2004, Fault number 550, Calawah fault, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, http://earthquakes.usgs.gov/regional/qfaults, accessed 11/21/2012. Example 2 Black, B.D., DuRoss, C.B., Hylland, M.D., McDonald, G.N., and Hecker, S., compilers, 2004, Fault number 2351f, Wasatch fault zone, Salt Lake City section, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, http://earthquakes.usgs.gov/hazards/qfaults, accessed 01/02/2013 09:10 AM. Chang, W.L. and Smith, R.B., 2002, Integrated seismic-hazard analysis of the Wasatch Front, Utah: Bulletin of the Seismological Society of America, v. 92, p. 1904-1922. Elósegui, P., Davis, J.L., Mitrovica, J. X., Bennett, R. A., and Wernicke, B. P., 2003, Crustal loading near Great Salt Lake, Utah: Geophysical Research Letters, v. 30, 1111 doi:10.1029/2002gl016579. Hammond, W.C. and Thatcher, W., 2004, Contemporary tectonic deformation of the Basin and Range province, western United States: 10 years of observation with the global positioning system: Journal of Geophysical Research, v. 109, B08403, doi:10.1029/2003jb002746. Parsons, T. and Thatcher, W., 2011, Diffuse Pacific-North American plate boundary: 1000 km of dextral shear inferred from modeling geodetic data: Geology, V. 39, p. 943-946. Example 3 Bryant, W.A., and Lundberg, M.Matthew, compilers, 2002, Fault number 1g, San Andreas fault zone, Cholame-Carrizo section, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, http://earthquakes.usgs.gov/hazards/qfaults, accessed 01/02/2013 10:52 AM. Sieh, K.E. and Jahns, R.H., 1984, Holocene activity of the San Andreas Fault at Wallace Creek, California: Geological Society of America Bulletin, V. 95, p. 883-896. Questions or comments please contact presor AT wesleyan.edu or ece AT unavco.org Page 13