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Coda attenuation analysis of Zagreb area, Croatia
Iva Dasović, Marijan Herak and Davorka HerakAndrija Mohorovičić Geophysical Institute
Department of Geophysics, Faculty of ScienceUniversity of Zagreb
Supported by
The Foundation of the Croatian Academy of Sciences and Arts
Seismicity of Croatia – Croatian Earthquake Catalogue: 373 BC - 2010
Stations and earthquakes
PTJ: 2004-2010, 70 earthquakesZAG: 2001-2010, 71 earthquakes
Δ < 120 km, ML ≥ 2.0, h < 30 km
The single backscattering model
o The coda-Q method (Aki and Chouet, 1975)
fQtfm
ctfStfA
–2
–e,
EnvelopeSource function
Propagation term, m=2 for body waves
Attenuation
tfQf
)f(Slntt,fAlnc
–
n
c ff
QfQ
00)(
Linear regression
Q0 = Qc(f = f0 = 1 Hz)
flnnQlnQln c 0 Linear regression
Procedure
tWL = time window length (30 s)
tL = lapse time → shifted
windowstS = S-wave travel time
fl (Hz) f (Hz) fu (Hz)
1 1.5 2
2 3 4
4 6 8
6 9 12
8 12 16
12 18 24
16 24 32
Band-pass filters [fl,fu] and central frequencies f
Das
ović
et a
l. (a
ccep
ted
for p
ublic
ation
in S
tud.
Geo
phys
. G
eod.
)
Results: Qc (tL, f)
Smoothed values of Qc (tL, f) in contour-graph
Q0 and n depend on tL
h = ½ vst
40 80 120 160 200 240h (km)
40 80 120 160 200 240h (km)
Q0 and n in coastal Croatia (the External Dinarides)
Dasović et al. (accepted for publication in Stud. Geophys. Geod.)
Q0 and n depend on tL – different referent frequency f0?
Dasović et al. (accepted for publication in Stud. Geophys. Geod.)
lnPGA475 vs. Q0 → 11 stations: coastal region + Zagreb area
tL (s) R (km) ρ p
40 76 -0.80 95%
50 95 -0.92 95%
60 114 -0.86 95%
70 133 -0.82 95%
80 152 -0.49 85%
ρ = correlation coefficient
p = statistically significant confidence level
PGA475 = peak ground acceleration for the return period of 475 yr at the
station site
R = radius equivalent to tL for average vs = 3.8 km/s
Conclusions
o Q0 and n indicate highly heterogeneous medium
o Coda-Q is lapse time dependent
o A marked change of Q0 and n at about 100 km depth (upper mantle)
o Different choice of f0 might “eliminate” lapse time dependence of Qc
o Very convincing correlation between Q0 and PGA475 (seismicity)