15:15-15:40 2003/6/17 NIED Symposium

Introduction (1)Proposal of median distances forground motion prediction equations

In comparing the existing ground motion prediction equations (GMPEs)
with the observed record of the 2011 Tohoku earthquake, it is becoming
clearlthatth t theth results
lt off theth equationsti
may vary significantly
i ifi
tl according
di totwhich definition of distance is applied.

Kazuki Koketsu,
Koketsu Hongjun Si,
Si and Hiroe Miyake
Peak Ground Accelerations

Earthquake Research Institute, University of Tokyo, Japan

Peak Ground Velocities

For example,
p the gground motions of the 2011 Mw 9.0 Tohoku earthquake
qlook only equivalent to those from the 2003 Mw 8.3 Tokachi-oki earthquake,
if we use fault distances.
1

Introduction (2)2Alternatives for `fault distance’ Equivalent Hypocentral Distance (Ohno(Ohno et al., 1993)

The problems originating from the definition of distances, such as the hanging
wall effect, are also becoming more evident. This study created and defined a
new term called “median“median distancedistance” and produced a new GMPE by using thesame set of data as Si and Midorikawa (1999). With this equation, we
conducted a preliminary study to analyze the advantages which can be derived
from this definition
definition.

for earthquakes where strong motion generation areas (SMGAs)
coincide with asperitiesp(large( g slipp areas))

Moi

2008 Iwate-Miyagi Nairiku earthquake Shortest Distance from SMGAs ((MidorikawaMidorikawa et al., 2012)

Foot-wall

f earthquakes where SMGA
forSMGAss do not coincide with asperities

Hanging-wall2011
Tohokuearthquake

Hanging wall effect (e.ge g., Abrahamson and Somerville,Somerville 1996; Abrahamson
and Silver, 2008; Campbell and Bozorgnia, 2008)3

fault distanceshortest distance from SMGAs

4

However, a distribution of slips or SMGAs is required, but
it cannot be obtained in advance to an earthquake.
DataEarthquake source faults, in many cases, are configured to placemain SMGAs at their centers.
centers. Their planes tend to lengthen in thealongalongl-striket ik direction
di ti andd SMGAs
SMGA tendt d tot diversify
diif towards
td theth samedirection.. However, when observed towards the downdirection
down-dip direction,
they can be approximated to be located near the center.
center. Therefore,
we defined the “median fault line” as the straight line extended fromthe midpoint of the fault plane in the alongalong-strike direction, anddefined “median distance” as the shortest distance from an
observation point to the “median fault line”line”..

In this study, we used the same set of data as used in Si andMidorikawa (19991999)) and Si (19991999)) to calculate peak groundaccelerations and velocities (PGAs and PGVs),
PGVs) both of which wereused for the estimation of GMPEs
GMPEs.. As for the distances, wereplaced the fault distances with the median distances, newly
calculated
l l t d fromfth fault
thef lt models
models.
d l . SinceSi
thi isthisi a preliminaryli istudy,t d
we did not use the earthquake data observed after Si andMidorikawa (19991999)) for the estimation of GMPEs
GMPEs.. However, weused observed records of the 2007 Chuetsu
Chuetsu--oki and 2011 Tohoku
earthquakes to examine the results of the GMPEs.
GMPEs.

2008 Iwate-Miyagi Nairiku earthquake (Hikima et al., 2009)65

Construction of GMPEs (first stage)

Construction of GMPEs (second stagestage-1)

 The

relations between median distances and PGAs or PGVs recorded
during the 1995 Kobe earthquake indicate no saturation at close
distances. Because this confirms the usefulness of median distance, weused the equation: log A = b - log Xmed - k Xmed (A: PGA or PGV, Xmed:
median distance) as the first-stage regression model for the GMPE using
median distances.
distances We applied this regression model to each earthquake
and calculated the value of coefficient b. In order for the ground motion
data at close distances to be reflected strongly, we analyzed by giving aweight
i ht off 8 forf distances
di tllessth 25 km,thank 4 forf 25 tot 50 km,k 2 forf 50 tot
100 km and 1 for over 100 km.

The coefficient b in the first stage regression model of the 1995 Kobe
earthquake shows a strong correlation with moment magnitude (Mw). Inaddition, the large variation of b remained even at the same Mw and it
became obvious that b is influenced by other variables than Mw.

Coe fficientb4534PGA5767289 56
Moment magnitude

P GV

7898

Construction of GMPEs (second stagestage-2)
Comparative analyses of the obtained GMPEs

Therefore, as in Si and Midorikawa (19991999)), we performed the second
stage of regression analysis by applying the model
model:: (D: focal depth, Si:earthquake type,
type e: constant term,term e: standard deviation,
deviation a, h, di:regression coefficient).
coefficient). Si is a dummy variable and is nonzero only when
it is any one of the crustal, interplate and intraplate type earthquakes
earthquakes..
Th results
Thelt off theth analysis
l i indicate,i di ti theinth same way as Si anddMidorikawa (19991999),), that PGA and PGV tend to rise in the case of
intraplate earthquakes, or when the focal depth is great.
great. The tendency ofsuch rise is stronger for PGA than PGV.
PGV.
103

 We have conducted comparative analysanalyse
es of the GMPEs derived in thisstudy with PGAs and PGVs observed in the 2004 Chuetsu, 2007 Chuetsu
Chuetsu-oki, 2008 Iwate
Iwate--Miyagi Nairiku
Nairiku,, and 2011 Tohoku earthquakes.
earthquakes.

 We

used strong motion data obtained by K-NET and KiK
KiK--net of NIED,JMA and TEPCOTEPCO.. We have applied 0.1 to 10 Hz band pass filter anddetermined the values of PGAs at the ground as in Si and Midorikawa(19991999)).

 Further,

we obtained the velocity waveforms by integrating theacceleration waveforms at stations where AVS30 is estimated, anddetermineddi d theh values
loff PGVs
PGV on theh basement,bwith
i h relations
l ioffamplification factors and AVS30
AVS30 as in Si and Midorikawa (1999)1999).

102

P.G..V. cm/s

P.G.A. cm/s2

M w = 8.0

7.0

102

M w = 8.0

101

7.0

6.0

Depth = 10 km
Crustal101

Depth = 10 km
Crustal

101

100

102

101

6.0

102
MED (km)

MED (km)

92004 Chuetsu earthquake

JHJR

Mw = 6.6
CrustalThis study+/-/  K-NET
KiK-netIntensity meter103102
Nilim
JHJR101100

Mw = 6.6
CrustalThis study+/-/  
101

102

102

101

100

103

MED ( k m )

103

103

K-NET
KiK-netIntensity meter
TEPCO

102

101

Mw = 6.6
CrustalThis study+/-
101

102
MED (km)

11P.G.A ( cm/s2 )analysis for the2007 Ch
Chuetsu
Chuetsut -okikiearthquake not only
confirms the validityof the GMPEs
derived in this study
study,,
but also that thehanging wall effectis resolved to someextent compared to
the results of theequations of Si andMidorikawa (1999)
using fault distances.

P.G.A ( cm/s2 )P..G.V ( cm/s ) The comparative

101

100

P.G.A ( cm/s2 )Panalysis for the2004 Chuetsu
earthquakeq
not onlyy
confirms the validityof the GMPEs
derived in this study
study,,
but also that thehanging wall effectis resolved to someextent compared to
the results of theequationsti
off Si anddMidorikawa (1999)
using fault distances.

2007 ChuetsuChuetsu-oki earthquake

K-NET
KiK-netIntensity meter
JH
JR102

 The comparative

10103

100K-NET
KiK-netIntensity meter
TEPCO
Mw = 6.6
Crustal
Si and Midorikawa (1999)+/-101102

103

Fault Dis tanc e ( k m )12

2008 Iwate
Iwate--Miyagi Nairiku earthquake2011 Tohoku earthquake (1) The comparative analysis

 The comparativefor the 2011 Tohoku
earthquake confirms thevalidity of the GMPEs
derived in this study.

analysis for the2008 IwateIwate
I t -Miyagi
Mii
earthquake not only
confirms the validityof the GMPEs
derived in this study
study,,
but also that thehanging wall effectis resolved to someextent compared tothe results of theequations of Si andMidorikawa (1999)
using fault distances.

 On the other hand, as is

13

2011 Tohoku earthquake (2)earthquake, most of theSMGAs do not coincide withasperities.

 As shown in the right map,the median fault line is agood approximation of theasperity distribution, but
not the SMGA distribution.

 If we use the dashed line As a result, the effect of thesource characteristics ofmegathrust earthquakes
can be larger than expected.

103

P..G.A. ( cm/s2)

P..G.A. ( cm/s2)

103

102

101

100

Mw = 9.1
Inter-plateThis study+/-/102
MED ( km )

103

101

100

P..G.A. ( cm/s2)
Mw = 9.1
Inter-plate
This study+/-
/102

101

101

102
MED ( km )

103

100

Mw = 9.1
Inter-plate
Si and Midorikawa (1999)+/-
/

101

102

103

Fault distance ( km )

14This study newly defined the term “median distance” and developed
new GMPEs by using the dataset of Si and Midorikawa (1999)1999).
By comparing these GMPEs with the most recent observation data,
we found that geometric issues such as the hanging wall effect will
be resolved to some extent
extent..
However, source characteristic issues of megathrust earthquakes
still remains.
remains.
Th equivalent
Thei l t hypocentral
ht l distancedi tandd shortesth t t distancedi tf
fromSMGAs also have
have the characteristics of ththee median distance but thecalculation is made easier with using the median distance.
distance.

102

101

101

102

Conclusions

 For the 2011 Tohoku

for measuring median
distances, the symbols
plotted in the left diagrams
will move to shorter
distances and the overestimation will becomelarger.

103

103

P..G.A. ( cm/s2)

well known case of the2011 Tohoku earthquake,the GMPEs using fault
distances areoverestimating theobserved values. This
problem of overestimation
remains within the GMPEs
using median distanceseven though it is weakened
slightly. Therefore, thisstudy suggests that theoverestimation is related to
not only geometric issues
g g wallsuch as the hanging
effect but also to thesource characteristics ofmegathrust earthquakes.

100

Mw = 9.1
Inter-plate
Si and Midorikawa (1999)+/-/101

102103

Fault distance ( km )

1516