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Fast determination of earthquake source parameters from strong motion records:
Mw, focal mechanism, slip distribution
B. Delouis, J. Charlety, and M. Vallée
Géosciences Azur CNRS/IRD Université de Nice – Sophia Antipolis
EGU General Assembly 13-18 April 2008 Vienna Austria
Fast determination of earthquake source parameters from strong motion records:
Mw, focal mechanism, slip distribution
B. Delouis, J. Charlety, and M. Vallée
Géosciences Azur CNRS/IRD Université de Nice – Sophia Antipolis
Overview and objectives:
Step 1
Step 2
Step 3
All this usingnear sourcerecords
Where are we in the time scale of Seismic Warning ?
TIME
Earthquake origin time
Earthquake detection and localisation
"Early P-wave magnitude"A few seconds
Early warning
Global CMT solutions: > 1 hour
Moment magnitude Mw2 minutes
Rapid sourceparameters
Step 1
Focal mechanism15 minutes
Step 2
40 minutesSlip distributionStep 3
Step 1: Fast Moment Magnitude Mw
0 = spectral level at low frequency
Classical way to determine Mw from the far-field body waves
Rp cor_free_surfM0 = ———————
4 3 r0
Far field (distance >> rupture dimension)
Valid for separated far-field waves (P and S) only.
This simple relation cannot be used when P, S, and near-field waves are intermixed as it is the case near the source for large earthquakes.
(Kanamori 1977)
A new approach:
Using synthetic displacement spectra generated for extended sources in the near-field domain
virtual stations
Validation with 22 earthquakes worldwide with Mw ranging from 3.9 to 7.7Using a time window of 80 sec after origin time
Step 2: Fast Focal Mechanismusing a line source model
5.5 < M < 6
6 < M < 7
7 < M < 7.5
M > 7.5
If magnitude < 5.5 point source
If magnitude > 5.5 finite line source Parameters to be inverted for:Fault strike, dip, rakeand …
N
Example 1: 1999 Chi-Chi earthquake Mw 7.6
100 km
Example 2: 1999 Izmit Mw 7.6
100 km
N
Example 3: 2000 Tottori earthquake Mw 6.7
The method has been validated for 19 earthquakes worldwide of moment magnitude 3.6 to 8.0
50 km
Step 3: Fast Slip Inversion
(under development and validation)
: hypocenter
East-dipping rupture plane
SSW NNE
SSW NNE
Test of standardized slip inversion1999 Chi-Chi earthquake Mw 7.6
: hypocenter
Conclusions
It is possible to determine a moment magnitude Mw from near source stations at distance < 100 km with an acceptable degree of accuracy (+/- 0.2), over a very wide range of magnitude (4 to 8). Expected time scale: < 2 min after origin time
The focal mechanism can be retrieved in the same conditions using a simple line source model. Expected time scale: < 15 min after origin time
A fast and automated slip inversion is feasible (presently under development and validation). Expected time scale: < 45 min after origin time