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

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et a

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ccep

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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)