Scientific aspects of
South Iceland earthquakes
26 July 2013
Símon Ólafsson, EERC
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
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Icelandic Strong Motion Network
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Árborg Town Hall:
The strong-motion array
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
The Árborg Town Hall:
Earthquake 29 May 2008, UTM 15:45
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Internet Site for the European Strong
motion Database (ISESD)
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
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
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Ground motion model (Stochastic method)
A(M 0 , R , R  , c ,  ,  ) 
Cp M 0 R 
2 3
1
B(c ,  ) =
1+ ( / c ) 2
 2 B(c ,  )  G( R ,  )  E ( ,  )  S ( )
Omega squared source model
1
Geometrical spreading function
R
E ( ,  )  exp   12  
Spectral attenuation
G (R ,  ) 
S ( )
Site amplification
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Selection of S-wave window
5
x 10
-4
Longditudinal acceleration
0
ACCELERATION (g)
-5
2
0
x 10
10
20
-4
30
40
50
60
70
80
60
70
80
60
70
80
Vertical acceleration
0
-2
0
5
x 10
10
20
-4
30
40
50
Transverse acceleration
0
-5
0
10
20
30
40
TIME (s)
50
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Estimation of parameters
Ds stress drop
 = R/Q ‘spectral attenuation’
Moment magnitude Mw (Hanks-Kanamori):
M0 seismic moment
Mw  (2 / 3) log M0  10.7
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Earthquake 28 May 2008 - Parameters
Parameter
Estimate
Units
_____________________________________
Mo
3.4 ×1018
Nm
Mw
6.26
Fc
0.24
Hz
r
6.4
km

0.053
s
Ds
73.0×104
Pa
u
79.4
cm
_____________________________________
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Attenuation of PGA South Iceland June 2000
1
0.9
0.8
0.7
PGA (g)
0.6
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
40
50
60
70
DISTANCE (km)
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
80
90
100
Geometric spreading function
 D21n D n
R 
 D
D1  D  D2
D2  D  D3
D  d 2  h2
d = epicentral distance
h = depth parameter
D1, D2 and D3 are used to set the limits for the
different zones of the spreading function
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Derivation of GMPE
Parseval theorem:
a rms 
1
Td
Td

0
2
a(t) dt
a rms
1

2



PGA (amax) related to rms-acceleration (arms) with peak factor (p):
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
2
A()
d
Td
amax = parms
Theoretical GMPE
 (2 7) 2/3 CP R Ds2/3
log10 (arms )  log10 
 2 
 

 1
 1
  log10    log10 ( M o )  log10 ( R)
 Td  3
 2
1
1
  1   ci      cos     3sin       si      sin     3cos    
2
2
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Attenuation for 29 May 2008
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Duration estimation
30
25
90%
75%
70%
25
DURATION (s)
DURATION (s)
20
20
15
10
85%
80%
65%
60%
15
55%
50%
10
5
5
0
0
20
40
60
DISTANCE (km)
80
100
0
0
20
40
60
80
100
DISTANCE (km)
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%).
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Ground motion simulation
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Discrete time realizations
Time domain equations for sub-filters representing discrete model.
Sub-filter
Brune
H1
Savage
H1
H2
H3
H4
Time domain discrete representation
x1 (k )  2e x1 (k  1)  e2 x1 (k  2)  2 w(k )
x1a (k )  e x1a (k  1)  w(k )
x1 (k )  x1 (k  1)  A1 x1a (k  M 1 )  B1 x1a (k  M 2  B)
 A1 x1a (k  M1 )  B1 x1a (k  M 2  B)
x2 (k ) 
1
( x1 (k )  2 x1 (k  1)  x1 (k  2))
TS2

x3 (k ) 
2
2 N p 1

n 1
z n
 nTS 
2
   / 2
2
x2 (k  n)
x4 (k )  rx4 (k  n)  (1  r ) x3 (k  m)
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
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.
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Introduction
• Examine relationship: Modified Mercally Intensity
(IMM) vs. g
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Estimating parameters
• PGA, PGV: Use for example larger og two horizontal
component, average of peak values for horizontal
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
Intensities map
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013
5
x 10
-4
Longditudinal acceleration
0
ACCELERATION (g)
-5
2
0
x 10
10
20
-4
30
40
50
60
70
80
60
70
80
60
70
80
Vertical acceleration
0
-2
0
5
x 10
10
20
-4
30
40
50
Transverse acceleration
0
-5
0
10
20
30
40
TIME (s)
50
UPStrat MAFA: 3rd General Meeting in
Selfoss, July 23-26, 2013