Your Advisor Said, Get the GPS Data and Plot it. Jeff Freymueller Geophysical University of Alaska Fairbanks

Transcription

1 Your Advisor Said, Get the GPS Data and Plot it Jeff Freymueller Geophysical University of Alaska Fairbanks

2 Actual GPS Data GPS Data are phase and pseudorange data that measure distances to satellites. are es,mated from these measurements. displacements during earthquakes, etc are es,mated from the posi,ons.

3 GPS Data Products GPS Data vs. GPS Data Products Orbits and clocks What are they? Where are they? What formats/etc are they in? Where can you find for them? What do you need to know to use them?

4 Depend on Products Used The you for a GPS site for a given day depends not only on the data, but also on what products and models are used in the es@ma@on process: Satellite orbits Satellite clock biases or subnetwork/baseline defini@on Eleva@on angle cutoff of the data, plus weigh@ng A mul@tude of calibra@on models The inversion is linearized about an a priori es@mate, so your star@ng values can also maser. An example from my own solu@on system: solve 15jun25DenaliSummit -orbfiletype pos -pcmodeltype abs.atx -use_domes -use_azel -tropmap GMF -apriori_trop GPT -elevmin 10 -oceanload tpxo7only_cm -strategy LOCALTIE -subnet $SUBNETS/DenaliSummit.subnet -ambresol >& 15jun25DenaliSummit.daily

5 Some Important Processing Models Tropospheric delay Hopfield, Saastamoinen, Lanyi, Niell, GMF, VMF1 (well known) Up to ~70 cm amplitude Ocean Tidal Loading Response of solid earth to changing load of Antenna Phase Center with Phase center is the point on the antenna that we actually measure distances to It is an imaginary point in space, not a physical point

6 Models and Evolve with Time Many of the models used are adopted by community and thus standard All of the models can be improved, and there has been steady improvement over the last 30 years. Factor of ~30 improvement in GPS accuracy from 1985 to 2015! My is that a further factor of 3-10 improvement will come in the next years from improved modeling and mul@- GNSS.

7 Consequences Today s analysis of data for July 15, 2010 gives slightly different answers than we got 5 years ago for the same data. Velocity products are mostly consistent across sources, series are less so. All products are implicitly or explicitly versioned. All products will improve with the next release of a more consistent reference frame. Users need to know what went into deriving the products that they use, but most don t.

8 Reference Frame The reference frame defines the coordinate system on a moving, deforming Earth Origin and how it moves (geocenter) Coordinate axis orienta@ons Defini@on of zero velocity, how coordinate axes rotate ITRF maintains a no- net- rota@on condi@on, which is geologically arbitrary Currently: ITRF2008 and progeny Soon: ITRF2014/2015 Next ITRF will include a seasonal posi@oning model

9 Plate Boundary Processes in ITRF Subtract the of the North American plate = Veloci@es rela@ve to North America Mike Craymer, Geode/c Survey of Canada

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11 Western North America of western North America results from a combina@on of: Extension across Basin and Range Shear on San Andreas fault system Subduc@on strain in Cascadia and Alaska Distributed deforma@on in N. Canada and Alaska

12 Sources of GPS Products IGS: GNSS Service UNAVCO: Analysis for Plate Boundary Observatory and related data JPL: Time series and of global and regional SOPAC: Global and other including their submission to the IGS GPS Explorer/READI: A NASA- funded consor@um genera@ng products for the western lower 48 University of Nevada: A highly automated analysis of as much data as they can get their hands on (>13,000 sta@ons!)

13 UNAVCO (PBO) Products Products page: hsp://pbo.unavco.org/data/gps PBO Time Series Viewer Velocity series Some other things (troposphere, etc) Products are based on daily (all data for a given day). If you want kinema@c/high rate solu@ons at sub- daily intervals, you need to go elsewhere or analyze them yourself.

14 UNAVCO provides a velocity field based on the PBO data analysis Somebody has to judge whether the mo@on of a site is really linear or not Veloci@es are provided in mul@ple reference frames: NAM08: North America- fixed (model by G. BlewiS) IGS08: Updated GPS coordinate model consistent with ITRF2008 (obsolete) IGS05, SNARF

15 Go to the UNAVCO Website hsp://pbo.unavco.org/data/gps

16 Plot the PBO

17 Time Series Stored on a site by site basis Mul@ple solu@ons: NMT, CWU, PBO combined Versions with different products pbo.final_nam08: Final orbits only pbo.rapid_nam08rapid orbits only pbo.nam08: Final orbits where available, otherwise rapid Two different formats,.pos and.csv

18 Go to the UNAVCO Website pp://data- out.unavco.org/pub/products/

19 Plot a PBO Time Series

20 the GPS have simple and easy related to long- term tectonics But usually not in detail. Differences include: from locked faults Postseismic following large earthquakes Transient (slow slip events) Volcanic, GIA, etc

21 SF Bay Area

22 Block Model d Alessio et al. (2005, JGR), doi: /2004JB003496

23 Modeling We can model effect of locked faults using elas,c disloca,on theory with an half- space There are simple expressions for 2D There are standard computer codes for 3D The same codes work for earthquake (coseismic) A simple approach is to represent by a combina@on of rigid block mo@on and backslip to cancel the mo@on on the shallow fault (Savage and Burford, 1973) Backslip represents the slip deficit of the shallow fault

24 Interseismic vs. Coseismic 40 Interseismic Between earthquakes Fault-parallel velocity (mm/yr) Distance from fault (km) Coseismic During earthquake Fault-parallel displacement (m) Distance from fault (km)

25 Calculate by 0 Slip V -V Slip 0 0 Slip V Depth D D V/2 Velocity Velocity Depth Depth D + = V/2 V/2 Velocity -V/2-5D Distance 5D -5D Distance 5D -5D Distance 5D

26 2D Strike Slip Fault V(x) = S atan (x x f ) π D V(x) = fault-parallel velocity at position x Savage and Burford (1973) S = long-term slip rate D = locking depth x = perpendicular distance of site from fault x f = position of fault In this formulation, a site on the fault has zero velocity the reference frame is fault-centered For a far-field reference frame, add or subtract S/2

27 Changing the Locking Depth