RE(CALIBRATION) OF MAGNITUDE SCALES FOR THE CROATIAN SEISMOGRAPHIC NETWORK

M. Herak, S. Markušić, D. HerakDepartment of Geophysics, Faculty of ScienceUniversity of Zagreb, Zagreb

History and motivation...The first magnitude calibration in Croatia was done by D. Skoko in his Ph. D. dissertation in 1969. Two calibrating functions for recordings on 1000 kg Wiechert horizontal seismograph were derived – for distances of 2.2˚–20˚, and 20˚–170˚.

In the ‘Balkan project’ (1974), a new formula was derived. It was retained even after introduction of electromagnetic and digital seismographs:

After a significant enlargement of the network in the few first years of the 21st century, Futač (2007) checked its appropriatness, and concluded that data do not indicate the need for change.

As digital high-quality data accumulated, it became clear that the formula does not work well for short distances, significantly underestimating the magnitude.

This prompted recalibration of the ML-scale. The need to use moment magnitudes in hazard analyses, motivated efforts to routinely observe also Mw thus initiating this work.

log( / ) 2.094log 2.19LM A T

Goals and earthquakesGoals:• Calibration of ML and MWA using recent digital data from the Croatian

Seismographic Network, and • Introduction of an automatic routine

procedure for determination of MW

using spectral analyses of local and regional earthquakes recorded by the Croatian Seismographic Network

Selection of earthquakes:

Events from the Croatian Earthquake Catalogue satisfying any of the following:

Year > 2002M > 2.5 & D < 200 km (for any station)M > 3.0 & D < 300 km (for any station)M > 3.5 & D < 400 km (for any station)M > 4.0 & D < 500 km (for any station)M > 5.0 & D < 700 km (for any station)

11900 3-component BB seimograms found!

Goals and earthquakes1. Determination of seismic moment (M0) and moment magnitude (Mw)

02

32

0

( )1 4

MS f

fv

f

The source displacement spectrum looks like:

v – velocity of seismic wavesf – frequencyf0 – corner frequency – density

frequency

ampl

itude

cornerfrequency, f0

low frequencyspectral level

M0 and Mw

1. Determination of seismic moment (M0) and moment magnitude (Mw)

Displacement spectrum at receiver at the epicentral distance :

0 ( )2

32

0

( , ) ( , )1 4

ftp S f Q fM R F

D ft G h e ef

vf

If we know (or invert for!) attenuation (, Q), and assume reasonable values representing the effects caused by the radiation pattern, free surface amplification and geometrical spreading, we can fit the formula to the observed spectrum and get the scalar seismic moment (M0), and then compute the moment magnitude: Mw = 2/3 log(M0) – 6.07

– near surface attenuationQ – quality factor (for P or S-waves)RP – average radiation pattern coefficient ( 0.6)FS – free surface amplification ( 2 for SH)G = 1/gd – geometrical spreading function,

gd – geodistance

M0 and Mw

1. Determination of seismic moment (M0) and moment magnitude (Mw)

1. Determine theoretical onset times of Pg, Pn, Sg, Sn.2. LP-filter, read max. S-wave amplitudes.3. Wood-Anderson filter, read max. S-wave amplitudes4. Determine the windows for:

noise, P-waves, S-waves.

5. Compute spectra, smooth, correct for the instrument and noise.

6. Fitting theoretical curve to spectra for P- and S-waves yields estimates for: Q(f), , f0, M0, both for P- and S-waves

7. Compute MW as average of MW(P) and MW(S)

8. Save everything, next earthquake

S-waves

P-waves

M0 and Mw; ML and MWA 2. Calibration of ML and the Wood-Anderson magnitude MWA

1. Compute ML and MWA for every recording (all stations) using the best calibrating function so far, in the form:

2. Compute representative earthquake magnitude for all events for which at least three magnitude estimates exist as the median of the station magnitudes.

3. Check if there is any distance dependence – if yes adjust coefficients observing all possible constraints (e.g. anchoring of MWA at 100 km).

4. Compute station corrections (SC) and start again...

The best estimates are:

Amax is the trace amplitude in nm on the simulated Wood-Anderson seismograph (magnification 2080). This is the same MWA formula as obtained for Central Europe by Stange (2006).

max

max

log( ) log( )log( ) log( )

L L L L

WA WA km WA km WA WA

M V A B SCM A A D B SC

max

max

log( ) 1.65 log( ) 2.30log( ) 1.11 log( ) 0.001 2.00

L L

WA km km WA

M V SCM A SC

M = (Station magnitude) – (Earthquake magnitude) vs. time

MW ML MWA MW ML MWA

MW ML MWA MW ML MWA

M = (Station magnitude) – (Earthquake mgnitude) vs. time

M0 and Mw – results

MWS vs MWP

Indi

vidu

al (s

tatio

n) m

agni

tude

sEa

rthq

uake

mag

nitu

des

Corner frequency vs MW

S-waves P-waves

M0 and Mw; ML and MWA

(Event magnitude) – (Station magnitude) (with added station corrections)

Relationships...In

divi

dual

(sta

tion)

mag

nitu

des

Eart

hqua

ke m

agni

tude

s

MW vs. MWA MW vs. ML ML vs. MWA

Station correctionsSta ML MWA MwS MwP

---------------------------- KIJV 0.02 0.06 -0.03 -0.03CACV 0.04 0.06 -0.12 -0.08PTJ 0.09 0.06 -0.05 -0.00DBR 0.06 0.15 0.01 0.06STON 0.05 0.08 0.01 0.06STA 0.10 0.14 -0.01 -0.04NVLJ 0.05 0.05 -0.05 -0.08RIY -0.06 -0.12 -0.13 0.02ZAG -0.27 -0.32 -0.21 -0.16SLUN 0.06 0.07 -0.05 -0.04UDBI -0.06 -0.11 -0.12 -0.05BRJN 0.04 0.04 -0.03 -0.10SISC -0.40 -0.41 -0.20 -0.24HVAR 0.04 0.00 -0.02 0.12MORI -0.04 -0.01 -0.07 -0.08ZIRJ -0.03 -0.01 -0.03 -0.04OZLJ 0.15 0.16 -0.13 -0.01KALN 0.07 0.01 -0.05 -0.03RIC -0.04 -0.11 -0.18 -0.01KSY -0.10 -0.11 -0.17 -0.10SLNJ 0.14 0.04 -0.01 0.14----------------------------

The largest (negative) station corrections are found on thick alluvium, in river valleys close to the Pannonian basin.

Corrections for the moment magnitudes are smaller than for local magnitudes (!)

ML station corrections

Conclusions

1. Regression of Mw vs. ML using both magnitudes determined independently on the same set of stations and earthquakes is far better than regressions of locally determined ML vs. Mw determined (mostly for large and distant earthquakes!) by other agencies.

2. Proposed inversion of observed spectral shapes of local and regional earthquakes results in consistent Mw estimation using data collected by the Croatian Seismographic Network.

3. New calibrating functions for local magnitudes (ML and MWA) yield distance-independent estimates.

4. The correspondence between the three magnitudes is close to 1:1 relationship. This is encouraging, promising an easy conversion from ML to MW for older events.

5. Results obtained in studies done so far using ML as proxy for MW will most probably not significantly change after proper conversion.