Numerical simulation Numerical simulation of strong motions for of strong motions for 1997 Colfiorito 1997 Colfiorito Mw 6.0 earthquake: Mw 6.0 earthquake: method method Ji Ji ří ří Zahradník Zahradník Charles University, Charles University, Prague Prague
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Numerical simulation of strong motions for 1997 Colfiorito Mw 6.0 earthquake: method
Numerical simulation of strong motions for 1997 Colfiorito Mw 6.0 earthquake: method. Ji ří Zahradník Charles University, Prague. Colfiorito earthquake (Umbria-Marche, Central Italy). mainshock 26 September 1997 at 09:40 GMT Mw = 6.0 strike 152 o , dip 38 o , rake -118 o - PowerPoint PPT Presentation
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Numerical simulation of strong Numerical simulation of strong motions for 1997 Colfiorito motions for 1997 Colfiorito Mw 6.0 earthquake: Mw 6.0 earthquake: methodmethod
JiJiříří Zahradník Zahradník
Charles University, PragueCharles University, Prague
Colfiorito earthquake (Umbria-Marche, Central Italy)
• mainshock 26 September 1997 at 09:40 GMT
• Mw = 6.0
• strike 152o, dip 38
o, rake -118
o
• fault size 12 x 7.5 km, bottom depth 8 km
• slip average 0.37 m (a heterogeneous model)
Capuano et al., J. of Seismology (2000)
Importance of an asperityfor the Colfiorito earthquake
entire fault (incl. geodetic data):
12 x 9 km, = 3 MPa
asperity (strong motion data): 6 x 6 km, 13 MPa
modelled with subevents of 20 MPa Castro et al., BSSA (2001)
previous stress drop estimates from strong-motion accelerograms (Rovelli et al., 1988): 20 MPa
Fault and asperities in generalFault and asperities in general
Somerville et al. (1999):
a self-similar empirical scaling, relating Mw-L
and
asperity slip / average slip = 2 (slip contrast)
asperity area / entire fault area = 0.25
Mw=6:
fault area = 104 km2 asperity area = 23 km2
Asperity modelAsperity modelEntire fault:
Average slip: D
Moment: Mo= D L2
Stress drop: MoL
Spectr. acc: A L
Asperity:
Slip 2D
Moment: Mo/2
Stress drop: 4Spectr. acc: 2A
Asperity size for ColfioritoAsperity size for Colfiorito
Capuano et al. (2000): fault area = 90 km2
Hunstad et al. (1999)
and Salvi et al. (2000): fault area = 108 km2
my asperity model = 1/4 fault: a square 5 x 5 km with 1/2 moment
rupture outside asperity is neglected
Moment (Nm) 0.5 e18Length x Width (km) 5 x 5Static Stress Drop (MPa) 10Average Slip (m) 0.8Rupture Veloc. (km/s) 2.6
Asperity model
the asperity slip 0.8 m is equivalent to the all-faultaverage slip of 0.4 m (cf. 0.37 m of Capuano et al.)
mechanism, position of asperity. A free parameter is mechanism, position of asperity. A free parameter is maximum slip velocity. maximum slip velocity.
• Variations of the HF spectral level due to source Variations of the HF spectral level due to source complexities do not require repeated source complexities do not require repeated source calculation. Instead, we use a (randomized) calculation. Instead, we use a (randomized) extrapolation of the LF acceleration spectrum. extrapolation of the LF acceleration spectrum.
and finally ... and finally ...
• Since the HF directivity of true ground motions is Since the HF directivity of true ground motions is questionable we propose composite modeling with questionable we propose composite modeling with variable extrapolation limit, hence with a variable extrapolation limit, hence with a high/intermediate/low HF directivity. high/intermediate/low HF directivity.
• The pronounced LF directivity remains unchanged The pronounced LF directivity remains unchanged unless we want to account for uncertain slip unless we want to account for uncertain slip distribution. distribution.
• As the extrapolated composite method is very fast it As the extrapolated composite method is very fast it allows easy construction of the PGA and PGV allows easy construction of the PGA and PGV simulation maps.simulation maps.
ENDEND
Summary in detail
• We investigated synthetic composite models of a finite-extent source.
• Input data: stations, 1D crustal structure, Mw, focal mechanism, position of the main asperity with respect to hypocentre (or the latter parameter is varied). The basic free parameter is maximum slip velocity.
• Deterministic composite modeling yields a clear LF directivity. It is caused by station-dependent spectral level and duration. The HF directivity is weaker since the HF spectral level (given by the subevent size) does not vary with station position, but the duration does.
• Variation of rupture time and rise time on the fault (and/or variation of the crustal model) yield variation of the HF spectral level. Effects like that do not require repeated source calculation. Instead, we use a (randomized) extrapolation of the acceleration spectrum.
• If the extrapolation starts at (or above) the corner frequency of the subevent, the HF directivity is as small as in the deterministic composite model.
• If, however, we decrease the extrapolation limit, the HF directivity increases. The radiation becomes similar to kinematic source models.
• Since the HF directivity of true ground motions is questionable we propose composite modeling with variable extrapolation limit, hence with a high/intermediate/low HF directivity.
• It is easy to do that, since the extrapolated composite method is very fast.
• The LF perturbation can be also included to account for the uncertainty of the slip distribution.
• As the extrapolated composite method is very fast it allows easy construction of the PGA and PGV maps.
How the summation works ?
• a formal exercise (parametric study)a formal exercise (parametric study)
• summation of N2 wavelets of equal duration summation of N2 wavelets of equal duration in a given time windowin a given time window