Earthquake distribution is not random: very narrow deforming zones (= plate boundaries) versus large areas with no earthquakes (= rigid plate

Transcription

1 Earthquake distribution is not random: very narrow deforming zones (= plate boundaries) versus large areas with no earthquakes (= rigid plate interiors)

2 Tectonic plates and their boundaries today -- continents are embedded in the plates and move with them

3 Ocean-ocean subduction island arc Transform fault strike-slip motion Oceanic spreading center creation of new oceanic crust Ocean-continent subduction volcanism Continental rift break-up of a continent lithosphere lithosphere viscous mantle viscous mantle Lithospheric plates float on a viscous mantle. Deformation (e.g., earthquakes) occur at their boundaries: divergent (spreading centers), convergent (subductions), or strike-slip

4 The Global Positioning System Three steps: 1. Satellites broadcast a radio signal towards the Earth 2. Receivers record the signal and convert it into satellite-receiver distances 3. Post-processing consist of converting these distances into positions Precision: $100 receiver 100 m $10,000 receiver 1 mm

5 Principle of GPS positioning Satellites broadcast signals on 1.2 GHz and 1.5 GHz frequencies: Satellite 1 sends a signal at time t e1 Ground receiver receives it signal at time t r The range measurement ρ 1 to satellite 1 is: ρ 1 = (t r -t e1 ) x speed of light We are therefore located on a sphere centered on satellite 1, with radius ρ 1 3 satellites => intersection of 3 spheres Or use the mathematical model: " s r = 2 2 ( X s! X r ) + ( Ys! Yr ) + ( Z s! Zr ) 2 satellite 3 satellite 2 ρ 2 ρ 3 ρ 1 satellite 1 A! The receiver clocks are mediocre and not synchronized with the satellite clocks Time difference between the satellite clocks and the receiver clock Additional unknown => we need 4 observations = 4 satellites visible at the same time You are here x Earth

6 Principle of GPS positioning GPS data = satellite-receiver range measurements (ρ) Range can be measured in two ways: 1. Measuring the propagation time of the GPS signal: Easy, cheap Limited post-processing required As precise as the time measurements ~1-10 m 2. Counting the number of cycles of the carrier frequency More difficult Requires significant post-processing As precise as the phase detection ~1 mm t e t r Earth From codes: data = (t r -t e ) x c (unit = meters) x λ ~ 20 cm From carrier: data = λ x n (unit = cycles)

7 Principle of GPS positioning GPS phase equation (units of cycles): " k i (t) = # k i (t) $ f c + ( h k (t) % h i (t)) $ f + ion k i (t) + trop k i (t) % N k i + & Range model: " i k = (X k # X i ) 2 + (Y k #Y i ) 2 + (Z k # Z i ) 2 Φ = phase measurement = DATA ρ i k = geometric range = CONTAINS UNKNOWNS X i,y i,z i X k,y k,z k = satellite positions (GIVEN) t = time of epoch i = receiver, k = satellite f = GPS frequency, c = speed of light h k = satellite clock error, h i = receiver clock error ion i k = ionospheric delay, trop i k = tropospheric delay N i k = phase ambiguity, ε = phase noise Phase equation linearized Form a system of n_data equations for n_unknowns (positions, phase ambiguities, tropospheric parameters) Solve using weighted least squares (or other estimation techniques) End product: position estimates + associated covariance

8 Principle of GPS positioning Error source Phase measurement noise Satellite clocks errors Receiver clock errors Tropospheric refraction Ionospheric refraction Satellite orbits Geophysical models Geodetic models Antenna phase center Multipath Site setup None Get precise (2-3 cm) orbits Use correction tables Choose good sites! Choose good operators! Treatment Double difference or direct estimation Double difference or direct estimation External measurement or estimation of tropospheric parameters Dual frequency measurements Tides (polar and solid Earth), Ocean loading Precession, Nutation, UT, Polar motion Magnitude < 1 mm ~1 m meters m 1-50 m 2 cm to 100 m centimeters centimeters ~ 1 cm ~ 0.5 m??? Precise GPS positioning requires: Dual-frequency equipment Rigorous field procedures Long (several days) observation sessions Complex data post-processing

9 Campaign measurements Continuous measurements Field strategy: Network of geodetic benchmarks perfectly attached to bedrock -- Separation typically km 2 to 3 measurement sessions of 24 hours Advantages: Large number/density of sites with few receivers Relatively low cost Problems: Transient deformation Monumentation and antenna setup Typical setup: Antenna mounted permanently on a stable geodetic monument, measurements 24h/day, 365 days/year Site protected and unattended Data downloaded daily or more frequently if needed (and if possible) Advantages: Better long-term precision Better detection of transient signals Problems: Cost and number of sites Power and communication

10 Repeated GPS measurements show that the longitude of Algonquin (Canada) is changing at a rate of 1.5 cm/yr Continents show consistent pattern of displacement They move at speeds ~few cm/yr = the speed your fingernails grow

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12 Norabuena et al. (1999): deceleration back to at least 20 My, initiation of Andes growth Consequence of construction of the Andes? Increased friction and viscous drag as leading edge of Sa thickens?

13 Norabuena et al., Science, 1998

14 Jalisco earthquake, October 1995, Mw=8.0 Hutton et al., GJI, 2001

15 Episodic Tremors and Slip Dragert et al., Science Juan de Fuca subduction Typical interseismic strain accumulation Episodes of aseismic slip every 13 to 16 months Typically 10 day long, 5 mm surface displacement

16 Episodic Tremors and Slip Modeling shows: ~ 2 cm slip on the subduction Slip area ~50 km x 300 km Depth of ~25 km to 45 km Equivalent to a M6.7 earthquake (similar to 2001 Nisqually earthquake near Seattle) Cause? Consequences? (e.g. for future earthquakes)

17 Vertical motions: post-glacial rebound, up to cm/yr Sella et al., 2007

18 Time dependent vertical motions: hydrological loading Bevis et al., 2005 Comparison between GPS observations at Manaus (Amazonian basin, red dots) and the predicted flexure of an elastic plate under water loading. VanDam et al., 2001 A model of the peak-to-peak amplitude of vertical motions due to hydrological loading (=water + snow)