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12.540 Principles of the Global Positioning System

Lecture 23

Prof. Thomas Herring

Room 54-611; 253-5941

tah@mit.edu

http://geoweb.mit.edu/~tah/12.540

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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

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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

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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)

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Central Asia Study Area

KyrgyzstanKazakhstan

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Regional view of network

Zoom

Eurasia Fixed Frame realized using ITRF2000

50% confidence ellipses shown

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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)

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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.

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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.

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IVTRAN Monuments

Monument components

Installed monument (SST antennas used in Central Asia

Rod length 355 mm

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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

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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.

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ITRF2000 Residuals

Horizontal RMS 0.8 mm/yr

52-sites

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Eurasia only residuals

Horizontal RMS0.5 mm/yr

14 sites

Postglacial rebound model at NYAL, ONSA

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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)

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Collocated velocity estimates

Zoom

50% confidence intervals

Differences are consistent with sigmas

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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

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Complete Velocity Field

Field dominated by North-South compression

Also East-West extension to East

Zoom

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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.

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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).

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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

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USArrayEach black dot represents a seismic station deployment over the next five years.

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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

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PBO

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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

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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

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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.

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Vectors show coseismic offsets and postseismic displacements after 7-years

Postseimic estimated with two different methods.

Hector Mine Co- and Post-seismic

displacements

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178 mm

57 mm

Time series for site LDES

Coseismic offsets

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Time series with coseismic offset removed

28 mm

13 mm

Accumulated postseismic

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Time series 60-day averaging

31 mm postseismic

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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

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Large scale postseismic deformation

Figure from Andy Freed and Roland Burgmann

Examine North component of motion (averaged 60-days)

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San Simeon Earthquake 12/22/2003

Magnitude 6.5 earthquake

Co-seismic displacement shown

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Motion after earthquake: PBO stations installed to measure postseismic motion

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Time series of North motion of sites near San Simeon

Grey lines show motion expected for this part of California

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Parkfield earthquake (9/28/2004) Magnitude 6 (latitude)

9 months after San Simeon, the Parkfield Earthquake

Again showing long term transient after event

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Longitude Motion after Parkfield

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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

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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.