A new ground motion prediction
equation (attenuation relation) for Japan
based on the 2011 Tohoku-oki
earthquake records
Nobuyuki Morikawa and Hiroyuki Fujiwara
National Research Institute for Earth Scienceand Disaster Prevention (NIED), Japan

Background We have constructed a database of strong-motion records andhave obtained a new attenuation relation. (Kanno et al., 2006;BSSA)
 The

Tohoku-oki mega-earthquake (Mw=9.0) on March 11,
2011, is the largest event which many strong-motion records
were obtained. (over 2,000 records in Japan)

 We must consider Mw9-class mega-earthquakes (e.g. Nankai

trough earthquake) in our seismic hazard assessment.

A new attenuation relation directly applicable up to Mw=9earthquakes is required for the “Next Generation National
Seismic Hazard Maps for Japan”.

Strong-Motion Data during the M9 Earthquake
vs Existing Japanese Attenuation Relations○Assumed source fault
based on HERP (2011)

Observed (on the ground)
Si & Midorikawa (1999)：fop inter-plate earthquakes
Kataoka et al. (2006)：for subduction-zone earthquakes
Kanno et al. (2006)：for shallow (<30km) earthquakes
Satoh (2010)：for inter-plate earthquakes (Pacific plate)Strong-Motion Data
 Update the strong-motion database of Kanno et al. (2006) byadding recent (after the 2003 Tokachi-oki EQ) records.
Up to end of 2009 and the 2011 Tohoku-oki main shock

NIED (K-NET and KiK-net), JMA, PARI

 Target strong-motion parameters:
JMA seismic intensity (I)Peak ground acceleration (PGA)Peak ground velocity (PGV)
5% damped acceleration response spectra (SA; 0.05-10s)
 Data used in the regression analysis
Earthquake: Mw>=5.5 & number of records>=5

Station: X<=200 km & installed on the ground surface
X: closest distance to the source fault

Modeling The main part follows to that in Kanno et al. (2006)
 We use only two parameters, moment magnitude (Mw) andclosest distance to source fault (X) in “base model”.
 We apply “additional correction terms” in order to express otherphenomena such site amplification and so on.
 We obtain regression coefficients for individual earthquake

category (subduction-zone earthquakes and shallow crustalearthquakes).
 We exclude the focal-depth dependency from the “base model”.
 We think that the difference of attenuation term can be

expressed by using the earthquake category instead of the focal
depth.

Modeling of Magnitude Termlog A  a i  b  X  log( X )iai can be determined for each earthquake“ai” saturates at Mw=8 or moreModeling of Magnitude Term
Model-1
Amplitude saturation is approximated by using Mw2 term(based on Fukushima, 1996)
Model-2
Amplitudes are completely saturated at Mw >= Mw0
Model-1※ This figure is based on Fukushima (1996)

Model-2

Base Model
Model-1
log A = a1k（Mw－Mw01 )2+ b1kX – log (X+d110e1・Mw) + c1k

Model-2
log A = a2kMw02 + b2kX – log (X+d2・10e2Mw02) + c2k
Mw02 = min (Mw, Mw0)

”k” is for subduction-zone or shallow crustal earthquakes


Mw0 & e are assumed to be independent of the period andearthquake category
By trial and error approach, the following parameters fixed as
Mw01 = 16.0, e1 = 0.3,
Mw02 = 8.3, e2 = 0.5

Model-1

Obtained Base ModelModel-2

Mw=9.0, Mw=8.0, Mw=7.0 , solid: subduction, dashed: crustal, thin: Kanno et al. (2006)

Comparison of Base Model with
Model-1
Observed Data

Model-2

Additional Correction Term-1-Amplification by deep sedimentary layersIn order to model amplification characteristics by deep
sedimentary layers, we examine the relation between residual(=log [obs/pre]) and top depth of the layer with Vs=l at
observation site (Dl) obtained from the “underground structure
model of deep sedimentary layers for whole Japan”.

• “pre” is calculated from “base model”.
• The structure model consists of 6 major layers on the seismic
bedrock whose Vs = 3100 m/s or more.
1st layer: Vs = 600 m/s (engineering bedrock)
2nd layer: Vs = 1100 m/s
3rd layer: Vs = 1400 m/s
4th layer: Vs = 1700 m/s
5th layer: Vs = 2100 m/s
6th layer: Vs = 2700 m/s

Additional Correction Term-1-Amplification by deep sedimentary layers-

We modeled as;

qdGdeep (  residual )   pd  log( Dl / Dl 0 )  qd

Dl  Dl 0
Dl  Dl 0

Dl : Top depth of the layer with Vs=l (m/s) at the site [in m]

Additional Correction Term-1-Amplification by deep sedimentary layersI

ＰＰＧＧ
ＡＶSA (0.05～10s）

s: Standard deviation

Additional Correction Term-2-Amplification by shallow soft soilsRelation between residual (=log [obs/pre]) and average S-wave
velocity up to 30m depth at observation site (Vs30 in m/s)•“pre” is calculated from “base model + Gdeep”

We modeled as;

 ps  log Vs30  qs Vs30  Vs300
Gshallow (  residual )   ps  log Vs300  qs Vs30  Vs300

Additional Correction Term-2-Amplification by shallow soft soils-IＰＰＧＧＡＶSA (0.05～10s）

s: Standard deviation

Conclusions
 We suggest a new attenuation relation (groundmotion prediction equation) for Japan directly
applicable up to Mw=9 by using the strong-motion
records of the 2011 Tohoku-oki mega-earthquake.
 We examine two different base models to express
amplitude saturation at large magnitude, but wecannot decide which model is better yet.
 We also suggest additional terms corresponding tosite amplification that can be correct not only shallow
soft soils but also deep sedimentary layers.

Further Problems
 Our new model cannot be constrained at near source
region for large earthquakes (X<40km & Mw>7)
because there is no strong-motion records in thisregression analysis.
PGA Subduction-zone EQs

PGA Shallow Crustal EQs

 We should add many strong-motion records of aftershocks of the Tohoku-oki earthquakes.

Thank you for your attention !