gps shield - isnet

1GPS ShieldISA / ISNET Workshop Teheran12.9.2013 Andreas Hoechner

GPS Shield


Historical earthquakes


Early Warning Challenge Consider two scenarios for Padang, Sumatra:

Scenario 'West'

Scenario 'East'

Same epicenter and magnitude!


0 200 400

Distance from trench (km)

-200

0

200

400

600

Upl

ift (

cm) tren

ch

isla

nds

land

Scenario B

Scenario A

Profile sketch


How can we discriminate between the two scenarios? 'GPS-Shield'→

● Earthquake deforms earth's surface and seafloor

● GPS measures deformation on islands and at the coast

● Clever placing of stations allows reconstruction of the tsunami source


1. Produce synthetic earthquake signal. Calculate displacements at virtual GPS stations.

2. Apply random errors to displacements. This simulates finite accuracy of GPS real-time measurements.

3. Invert disturbed displacements at virtual stations for rupture parameters.

4. Using inverted rupture parameters, calculate Uz and compare with Uz from direct signal.

Can we determine earthquake parameters (i.e., slip distribution) using real-time GPS observations ?

Resolution analysis test procedure 1d


Absolute displacements:vertical: ±5 cmhorizontal: ±2.5 cm

Relative displacements:vertical: ±1.5 cmhorizontal: ±0.5 cm

Conservative resolution: ×2

Pessimistic resolution: ×4

Assumed GPS resolutions


One station at Padang


Two stations at Siberut


Stations at Siberut and Padang


Realistic resolution

Conservative

Pessimistic

Resolving power with distance from trench

Sobolev et al., EOS, 2006JGR, 2007


GPS-shield

GPS-gradientmeter

Controlstation

GPS buoys are used where there are no islands


GPS-shield applied to W/E scenarios

Forward Inversion

Forward Inversion

'West'

'East'


GPS-shield applied to W/E scenarios

'West' 'East'


How much time?

GPS signals almost approach static valuesAfter 3 min:

Static values fully established. Rupture parameters resolved.After 6 min:

Uz

Synthetic displacement seismograms


Inversion every 30 s


Inversion every 30 s

Sobolev et al. 2007 JGR


Resolving power in space: Checkerboard test


Sensitivity analysis in Magnitude

● GPS-inversion works well for large earthquakes

● Small earthquakes generate much smaller displacements (logarithm of moment!), which could drop below noise level of GPS

● Static displacement decays much faster with distance than seismic waves

● Test: Construct large number of scenarios, add different noise levels to synthetic data, perform inversion (databank matching) for different constellations of GPS-stations

● Quality criterium: Magnitude recoverd ± 0.2 units, epicenter mismatch < 50 km


Resolvable magnitude for 'ideal' GPS-shield

Diff. GPS noise level:“realistic”: horizontal +- 0.5 cm vertical +- 1.5 cm


Resolving power for Geophon-positions



Resolving power of local Padang array



Global applicability


• Reliable prediction of tsunami wave heights for Padang can not be provided using traditional, earthquake-magnitude-based methods.

• For Sumatra, GPS-arrays placed at the Mentawai islands, will provide excellent resolution for the position and the height of the initial wave, and, thus, will be able to discriminate between dangerous- and less-dangerous scenarios for Padang.

• In the case of Java, GPS arrays deployed at the southern coast (~ 250 km from trench), will still provide good estimation of the maximum uplift and seismic moment (<10%).

• Qualified warning can be issued within 5 minutes after the shock, leaving another 15-20 min before tsunami hits the coast.

Conclusions

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