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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 April 2008 Vienna Austria

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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

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Overview and objectives: Step 1 Step 2 Step 3 All this using near source records

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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 Mw 2 minutes Rapid source parameters Step 1 Focal mechanism 15 minutes Step 2 40 minutes Slip distribution Step 3

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Step 1: Fast Moment Magnitude Mw

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0 = spectral level at low frequency Classical way to determine Mw from the far-field body waves R p cor_free_surf M 0 = 4 3 r 0 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)

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A new approach: Using synthetic displacement spectra generated for extended sources in the near-field domain

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virtual stations

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Validation with 22 earthquakes worldwide with Mw ranging from 3.9 to 7.7 Using a time window of 80 sec after origin time

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Step 2: Fast Focal Mechanism using a line source model

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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, rake and …

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N Example 1: 1999 Chi-Chi earthquake Mw km

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Example 2: 1999 Izmit Mw km

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N Example 3: 2000 Tottori earthquake Mw 6.7 The method has been validated for 19 earthquakes worldwide of moment magnitude 3.6 to km

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Step 3: Fast Slip Inversion (under development and validation)

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: hypocenter

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East-dipping rupture plane SSW NNE SSW NNE Test of standardized slip inversion 1999 Chi-Chi earthquake Mw 7.6 : hypocenter

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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

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