26 July 2013 Símon Ólafsson, EERC Scientific aspects of South Iceland earthquakes.

26 July 2013Símon Ólafsson, EERC

Scientific aspects of

South Iceland earthquakes

Outline

• Introduction• Icelandic Strong Motion Network• Estimation of source parameters• Ground motion prediction equation (GMPE)• Discrete time realization• Conclusions

UPStrat MAFA: 3rd General Meeting in Selfoss, July 23-26, 2013

Historical earthquakes in the SISZ


Icelandic Strong Motion Network


Árborg Town Hall: The strong-motion array


The Árborg Town Hall: Earthquake 29 May 2008, UTM 15:45


Internet Site for the European Strong motion Database (ISESD)


Recent earthquakes in South Iceland


• Mw 6.0, 25 May 1987

• Mw 6.5, 17 June 2000

• Mw 6.5, 21 June 2000

• Mw 6.3, 28th May 2008

Ground motion model (Stochastic method)


p 0 20 c c3

c 2c

12

C M R(M , , R , , , ) ( , ) ( , ) ( , ) ( )

2

1( , ) =

1+ ( / )

1( , )

( , ) exp

( )

A B G E S

B Omega squared source model

G Geometrical spreading function

E Spectral attenuation

S Site amplification

R R

RR

Selection of S-wave window


0 10 20 30 40 50 60 70 80-5

0

5x 10

-4 Longditudinal acceleration

0 10 20 30 40 50 60 70 80-2

0

2x 10

-4 Vertical acceleration

AC

CE

LE

RA

TIO

N (

g)

0 10 20 30 40 50 60 70 80-5

0

5x 10

-4 Transverse acceleration

TIME (s)

Estimation of parameters


M0 seismic momentDs stress drop k = R/Q b ‘spectral attenuation’

Moment magnitude Mw (Hanks-Kanamori): M Mw ( / ) log .2 3 10 70

Earthquake 28 May 2008 - Parameters


Parameter Estimate Units _____________________________________Mo 3.4 ×1018 N mMw 6.26 Fc 0.24 Hzr 6.4 kmk 0.053 s

Ds 73.0×104 Pau 79.4 cm_____________________________________

Attenuation of PGA South Iceland June 2000


0 10 20 30 40 50 60 70 80 90 1000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

DISTANCE (km)

PG

A (

g)

Geometric spreading function


12

n nD D

D

R 1 2

2 3

D D D

D D D

2 2 D d h

d = epicentral distanceh = depth parameter

D1, D2 and D3 are used to set the limits for the

different zones of the spreading function

Derivation of GMPE


dT2

rmsd 0

1a a(t) dt

T

2

rmsd

A( )1a d

2 T

Parseval theorem:

PGA (amax) related to rms-acceleration (arms) with peak factor (p): amax = parms

Theoretical GMPE


2/32/3

10 10 10 10 10

(2 7) 1 1log ( ) log log log ( ) log ( )

2 32

Prms o

d

C Ra M R

T

1 11 ci cos 3sin si sin 3cos

2 2

Attenuation for 29 May 2008


Duration estimation


0 20 40 60 80 1000

5

10

15

20

25

30

DISTANCE (km)

DU

RA

TIO

N (

s)

0 20 40 60 80 1000

5

10

15

20

25

DISTANCE (km)D

UR

AT

ION

(s)

50%55%

60%65%

70%75%

80%85%90%

a) Duration based on 90% cumulative energy. The dotted red curve represent mean value +/- one standard deviation (+/-1s). b) The curves represent the duration function of Eqn. 3.10 for different fractions of total cumulative energy (50%, 55%, 60%, 65%, 70%, 75%, 80%, 85% and 90%).

Ground motion simulation


Discrete time realizations

Sub-filter

Time domain discrete representation

Brune H1

Savage H1

H2

H3

H4


2 21 1 1( ) 2e ( 1) e ( 2) ( )x k x k x k w k

1 1( ) e ( 1) ( )a ax k x k w k

1 1 1 1 1 1 1 2

1 1 1 1 1 2

( ) ( 1) ( ) ( )

( ) ( )a a

a a

x k x k A x k M B x k M B

A x k M B x k M B

2 1 1 12S

1( ) ( ( ) 2 ( 1) ( 2))x k x k x k x k

T

2 1

3 22 21 S

( ) ( )2 / 2

pN n

n

zx k x k n

nT

4 4 3( ) ( ) (1 ) ( )x k rx k n r x k m

Time domain equations for sub-filters representing discrete model.

Conclusions

• For the South Iceland earthquakes the near-field attenuation has a high rate of attenuation (R-2)

• Ground motion model (GMPE) has to include a geometrical attenuation term to accound for the fast rate of attenuation in the near-field

• GMPE based on a physical model can be applied to obtain ground motion parameters and for time series simulation

• For realistic simulation of ground motion time sequences a realistic estimate of duration is important.


Introduction

• Examine relationship: Modified Mercally Intensity (IMM) vs. g


Estimating parameters

• PGA, PGV: Use for example larger og two horizontal component, average of peak values for horizontal


Intensities map



0 10 20 30 40 50 60 70 80-5

0

5x 10

-4 Longditudinal acceleration

0 10 20 30 40 50 60 70 80-2

0

2x 10

-4 Vertical acceleration

AC

CE

LE

RA

TIO

N (

g)

0 10 20 30 40 50 60 70 80-5

0

5x 10

-4 Transverse acceleration

TIME (s)

26 July 2013 Símon Ólafsson, EERC Scientific aspects of South Iceland earthquakes.

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