Date post: | 19-Jan-2016 |
Category: |
Documents |
Upload: | steven-sherman |
View: | 223 times |
Download: | 0 times |
05/12/1005/08/08 12.540 Lec 2312.540 Lec 23 1
12.540 Principles of the Global Positioning System
Lecture 23
Prof. Thomas Herring
Room 54-611; 253-5941
http://geoweb.mit.edu/~tah/12.540
05/12/10 12.540 Lec 23 2
OVERVIEW
• Final lectures: Application areas and GPS results– Analysis of contemporary motions in the Tien Shan
region of central Asia.– Start to look at the Earthscope program:
NSF/USGS/NASA program to study the dynamics of the Western United States and Alaska
05/12/10 12.540 Lec 23 3
Central Asia Analysis• Network
– Development of network– Transition from “pins” with tripods to IVTRAN fixed-heights
monuments– Continuous stations
• Analysis methods– Pre-continuous station analysis– Merger with global GPS analyses
• Motion relative to Eurasia– Methods of used to realize a Eurasia fixed frame
• Vertical motions– Vertical motions as a function of existing topographic heights
05/12/10 12.540 Lec 23 4
Network
• Development of Network– First measurements made in July 1992; just after start of
International GPS Service (IGS). Since then measurements each year and often multiple campaigns per year.
– The current network has over 1000 sites, many of them being recovery marks for the main stations (typically 3 per location)
– Network extends from Kyrgyzstan to the Kazakh Platform– In 1995, the first continuously operating stations installed.
Currently, 9 continuous sites with 2 IGS sites (POL2, SELE)– Results available on web at:
http://www-gpsg.mit.edu/~tah/cont98g/cont98.html(Pages include time-series, phase residuals, atmospheric delay estimates)
05/12/10 12.540 Lec 23 5
Central Asia Study Area
KyrgyzstanKazakhstan
05/12/10 12.540 Lec 23 6
Regional view of network
Zoom
Eurasia Fixed Frame realized using ITRF2000
50% confidence ellipses shown
05/12/10 12.540 Lec 23 7
Monument evolution
• Original monuments were mainly steel pins in bedrock which required tripod setups
• Starting in 1995: IVTRAN designed fixed-heights were installed. Simplified setup. Three recovery marks installed for each original monument.
• After 1995, measurements made on the recovery marks rather than original marks.
• With the original marks and fixed height marks, for many stations two independent velocity estimates (results shown later)
05/12/10 12.540 Lec 23 8
Analysis methods
• Prior to 1995; selected IGS from Europe, Asia and Australia included in the analysis of regional data
• After 1995 when POL2 became an IGS site and was routinely included in the IGS analysis; only local sites included in the MIT analysis
• Using GAMIT/GLOBK; regional analyses are included in the Scripps Institution of Oceanography IGS (SOPAC) analysis including orbit improvement (more important in early data). ftp://garner.ucsd.edu/pub/hfiles
• Campaigns are combined with SOPAC h-files for velocity field and repeatability analyses.
05/12/10 12.540 Lec 23 9
Analysis Method
• Types of analyses performed with combined files:– Velocity field analysis. Multiple types
• Global: Global selection of sites along with regional sites. Eurasian frame then defined using the ITRF2000 Eurasian rotation pole.
• Eurasian: Only sites from Eurasia included. Eurasian frame defined to minimize velocities of stable Eurasian sites
• In both analyses: Option to force velocities at nearby sites (<0.5 km separation) to be the same (some exceptions)
– Repeatability analysis:• Each campaign is rotated/translated to best align as a function of
time with positions and velocities from one of the velocity analyses.
05/12/10 12.540 Lec 23 10
IVTRAN Monuments
Monument components
Installed monument (SST antennas used in Central Asia
Rod length 355 mm
05/12/10 12.540 Lec 23 11
Statistical analysis• Since the network contains both continuous sites and
campaign sites, we include statistical process noise in the velocity analyses to account for temporal correlations in the time series.
• Each site has random walk process noise (2 mm)2/yr• Without process noise, velocities of continuous sites
would have sigmas of < 0.1 mm/yr; with process noise sigmas are between 0.6 and 1.0 mm/yr e.g.
Site Random Walk (mm/yr) White noise (mm/yr)
E N U E N U
POL2 0.5±0.6 3.0±0.6 -0.5±1.1 0.2±0.1 3.3±0.1 -1.1±0.2
SELE 0.9±0.8 3.7±0.8 -2.5±1.2 1.7±0.1 3.8±.0.1 -2.1±0.2
05/12/10 12.540 Lec 23 12
Frame Realization
• Realization of Eurasian frame; Two methods used– ITRF2000: Used global distribution of sites(52) on many
plates to rotate/translate frame onto ITRF2000. ITRF2000 Eurasia pole used to rotate to Eurasia fixed frame
• Fit to ITRF2000 (52-sites):• Horizontal RMS 0.8 mm/yr; √2/f 1.2• Vertical RMS 1.8 mm/yr; √2/f 2.4
– Eurasia only: Used 14 sites on stable Eurasia to define frame• Fit to Eurasian sites only(14-sites)• Horizontal RMS 0.5 mm/yr; √2/f 0.8• Vertical RMS 1.8 mm/yr; √2/f 2.3
• Notice 2 is <1 for Eurasia but >1 for global; sigmas depend on size of region considered.
05/12/10 12.540 Lec 23 13
ITRF2000 Residuals
Horizontal RMS 0.8 mm/yr
52-sites
05/12/10 12.540 Lec 23 14
Eurasia only residuals
Horizontal RMS0.5 mm/yr
14 sites
Postglacial rebound model at NYAL, ONSA
05/12/10 12.540 Lec 23 15
Differences between co-located sites• Of the approximately 400 sites with high-
quality velocity estimates, 104 sites are collated between old and new monument styles.
• Comparison:Analysis Horizontal RMS 3D-RMS
Global 1.9 mm/yr (√2/f 1.0)
2.5 mm/yr (√2/f 1.1)
Regional 1.9 mm/yr (√2/f 0.9)
2.4 mm/yr
(√2/f1.1)
05/12/10 12.540 Lec 23 16
Collocated velocity estimates
Zoom
50% confidence intervals
Differences are consistent with sigmas
05/12/10 12.540 Lec 23 17
Motion in Eurasia Frame
Motion from difference analyses
Best Estimates:Site East North mm/yr mm/yrPOL2 0.5±0.6 3.0±0.6SELE 0.9±0.8 3.7±0.8
AZOK 1.2±0.9 1.1±0.9CHUM -0.3±1.0 0.4±1.0
05/12/10 12.540 Lec 23 18
Complete Velocity Field
Field dominated by North-South compression
Also East-West extension to East
Zoom
05/12/10 12.540 Lec 23 19
Conclusions
• North of the Tienshan the motion of sites relative to Eurasian appears to be less than 1 mm/yr
• Within the Kyrgz Tienshan ~13 mm/yr North converge• East west extension across the range front of
~3 mm/yr• Height rate range of ~4 mm/yr• Median RMS scatter of position estimates 1.4-1.8 mm
horizontal and 4.6 mm height.• Consistent statistics are obtained with random walk
process noise of (2 mm)2/yr.
05/12/10 12.540 Lec 23 20
What is Earthscope
• Earthscope is Major Research Equipment (MRE) project in the National Science Foundation (NSF) with contributions from NASA and other government agencies
• A major component of this project is the Plate Boundary Observatory (PBO) which uses the Global Positioning System (GPS) to study in detail deformation of the Earth on time scales from seconds to decades.
• Plans for interferometric synthetic aperture radar (InSAR) measurements (NASA)
• Project is a major application of space based systems to study the Earth (www.earthscope.org).
05/12/10 12.540 Lec 23 21
Components of Earthscope
• The MRE portion of Earthscope is $240M over the next 5-years (started late 2003).
• There are three major components:– USArray: Moving array of 400 seismic stations that work their way
across the US; 120 new permanent stations– SAFOD: San Andreas Fault Observatory at Depth. Project to drill into
the San Andreas fault near Parkfield California.– PBO: Plate boundary Observatory; 875 continuously operating GPS
receivers and 175 bole-hole strainmeters over the Western United states
05/12/10 12.540 Lec 23 22
USArrayEach black dot represents a seismic station deployment over the next five years.
05/12/10 12.540 Lec 23 23
SAFOD
Idea is to drill into the San Andrea fault in an area with repeating earthquakes and observe the processes that occur during, before and after earthquakes
05/12/10 12.540 Lec 23 24
PBO
05/12/10 12.540 Lec 23 25
Strainmeter deployment
Instruments measure local horizontal strain on three gauges (dl/l). Measurements are converted to east and north strain components
Elaborate installation process. PBO is installing a large number of these instruments
05/12/10 12.540 Lec 23 26
Specific results from Earthscope
• Transient behavior in Earth Deformations (this and next lecture)– Postseismic: Deformation after recent California
earthquakes and relating post-seismic rates to pre-seismic values
– Effects of water
• Concerns with Earthscope
05/12/10 12.540 Lec 23 27
Examples of Geophysical Signals
05/12/10 12.540 Lec 23 28From Hetland and Hager, Relationship of geodetic velocities to velocities in the mantle,GLR, 31, L17604, doi:10.1029/2004GL020691, 2004
Velocity evolution through the seismic cycle
Weak Lower Crust
Weak Mantle
Strong lower crust
Modeled changes in velocities through the earthquake cycle.
Times are shown for early, middle and late in cycle
With strong lower crust less variation during cycle.
05/12/10 12.540 Lec 23 29
Vectors show coseismic offsets and postseismic displacements after 7-years
Postseimic estimated with two different methods.
Hector Mine Co- and Post-seismic
displacements
05/12/10 12.540 Lec 23 30
178 mm
57 mm
Time series for site LDES
Coseismic offsets
05/12/10 12.540 Lec 23 31
Time series with coseismic offset removed
28 mm
13 mm
Accumulated postseismic
05/12/10 12.540 Lec 23 32
Time series 60-day averaging
31 mm postseismic
05/12/10 12.540 Lec 23 33
Analysis of postseismic
signals
One method of analysis for signals is principle component analysis
Number of components related to number of processes acting during postseismic period
05/12/10 12.540 Lec 23 34
Large scale postseismic deformation
Figure from Andy Freed and Roland Burgmann
Examine North component of motion (averaged 60-days)
05/12/10 12.540 Lec 23 35
San Simeon Earthquake 12/22/2003
Magnitude 6.5 earthquake
Co-seismic displacement shown
05/12/10 12.540 Lec 23 36
Motion after earthquake: PBO stations installed to measure postseismic motion
05/12/10 12.540 Lec 23 37
Time series of North motion of sites near San Simeon
Grey lines show motion expected for this part of California
05/12/10 12.540 Lec 23 38
Parkfield earthquake (9/28/2004) Magnitude 6 (latitude)
9 months after San Simeon, the Parkfield Earthquake
Again showing long term transient after event
05/12/10 12.540 Lec 23 39
Longitude Motion after Parkfield
05/12/10 12.540 Lec 23 40
Full North/East Time series for Parkfield site
Pre-EarthquakeNorth 19.1±0.1 mm/yrEast -15.2±0.3 mm/yr
2006-2007 VelocitiesNorth 10.5±0.3 mm/yrEast -10.0±0.5 mm/yr
05/12/10 12.540 Lec 23 41
Summary
• Results from different regions of the world (Tien Shan and Earthscope).
• Steady state deformation showing strain accumulation
• Non-steady motions. So far postseismic deformation. Next class look at other types of non-steady motion.