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