Long-period ground motion

simulation of great Nankai Trough,Japan, earthquakes
Takahiro MAEDA, Nobuyuki MORIKAWA,
Shin AOI, and Hiroyuki FUJIWARA(NIED)

The Nankai trough earthquakes

series of smaller earthquakes in short time period

single larger event ruptured at once
the Nankai trough earthquakes have various
occurrence patterns

Central Disaster Prevention Council (2011)

Long period ground motion• The long-period ground motions cause damage to highrise and large-scale structures.
– 1985 Michoacan, Mexico earthquake M8.0
– 2003 Tokachi-oki, Japan earthquake (Mw8.3)– 2011 Tohoku, Japan earthquake (Mw9.0)(USGS)(Sapporo Fire Bureau)(youtube)Long-Period Ground Motion Hazard Maps

• It is difficult to determine the source model ofthe anticipated earthquake.
• For improving seismic hazard assessment to
prepare for the anticipated Nankai Troughearthquake, it is important to understand apossible range of ground motion caused byuncertainty of source model.
Earthquake Research Committee (2009)

Earthquake Research Committee (2012)

Objective• Simulate long-period ground motions for anticipated Nankai Troughearthquake (M8-9) using various source models and analyze avariability of predicted long-period ground motions.
• For selecting source models,– Assuming various source parameters: rupture area, asperity (area oflarge slip) location, and hypocenter.

– Assuming the large slip near the trough following the lesson from the2011 Tohoku earthquake.

• The long-period ground motions are simulated by the finite
difference method using characterized source model and recently
developed three-dimensional velocity structure model.

Characterized source model• The characterized source model is used in the long-period groundmotion simulation.
• Three kinds of parameters– Outer source parameters• Shape and area of the whole fault– Inner source parameters• Heterogeneity of the source fault– Other source parameters• Rupture process• These parameters are determined based on a ‘recipe’ for predicting
strong ground motion (Irikura and Miyake, 2001)

Flowchart of determination of source parameters
for subduction-zone interplate earthquakes

Earthquake Research Committee (2009); Fujiwara et al. (2009)

Characterized source model for the Nankai
trough earthquakes• Source areas ( 14 cases) Single-segment earthquake• Nankai (ANNKI: Mw 8.5), Tonankai (ATNKI: Mw 8.2), Tokai(ATOKI: Mw 8.0), Hyuga-nada (AHGND: Mw 8.3), and along
the trough (ATRGH: Mw 8.1)
• Along the trough area has 3 cases (a, b, c).

 Multi-segment earthquake (simultaneous rupture ofmulti segment)
Multi-segment earthquakes
2-segment rupture

3-segment rupture– 2-segment rupture• Nankai + Tonankai (ANNI1: Mw 8.7)
• Tonankai + Tokai (ANNI2: Mw 8.4)

– 3-segment rupture• Nankai + Tonankai + Tokai (ANNI3: Mw 8.8)
4-segment rupture– 4-segment rupture• 3-segment + along the trough (ANNI4: Mw 8.9)
• 3-segment + Hyuga-nada (ANNI5: Mw 8.9)

– Seismic moment are calculated using scaling model

Characterized source model for the Nankai
trough earthquakes• Hypocenter (3 cases)– West: boundary of Hyuda-nada and Nankai– Center: boundary of Nankai and Tonankai– East: boundary of Tonankai and Tokai• Asperity (2 cases)– Deeper and shallower cases– Total area of asperities is 20 % of source fault area
Multi-segment earthquakes
2-segment rupture

3-segment rupture(Murotani et al., 2008)– The number of asperity is one for along the trough,and three (area ratio is 2:1:1) for Nankai, Tonankai,
Tokai and Hyuga-nada.

4-segment rupture

55 scenarios

Characterized source model for the Nankai
trough earthquakes• Source time function– Kostrov-like slip velocity time function : Nakamura& Miyatake (2000)• Rupture velocity– 2.7 km/ s

Multi-segment earthquakes
2-segment rupture

4-segment rupture

3-segment ruptureVelocity structure model• Japan Integrated Velocity Structure Model

7th layer lower surface: sedimentary wedge

17th layer upper surface: Moho discontinuity

18th layer upper surface: Philippine Sea plate

21th layer upper surface: Pacific plate

Earthquake Research Committee (2012)

Finite difference method•

3-D finite difference (FD) simulation with discontinuous grids (Aoi and Fujiwara, 1999)•Grid spacing– 200m in horizontal, 100m in vertical (shallower than 8km)
– 600m in horizontal, 300m in vertical (8km – 70km)•Area of the 3-D simulation– 1100 km x 700 km x 70 km (The total number of grid points = 2 billion)•Inelastic attenuation (Graves, 1996) : reference period of 5 sec•

Valid period of the FD simulation : more than about 5 second– Since the characterized source model does not include short-wavelength heterogeneity.

•Computation time
– 36 hours/ 1 scenario•

60000 steps (120 Hz)•Itanium 1.66GHz×256Core•Memory 130 GB

Velocity waveforms @ OIT

DEEPSHALLOWDEEPSHALLOW

Smaller amplitude for west-hypocenter scenarios

SHALLOWAsperity

Hypocenter

EASTDEEPCENTERWEST

Simultaneous rupture of 3 and 4 segments(30 scenarios)

Large S waves from deeper asperities of the Nankai area
Later phases from the along trough ‘a’ area

Velocity waveforms @ OSK

DEEP
SHALLOWDEEP
SHALLOWSHALLOW

Asperity

Hypocenter

EAST

DEEP

CENTER

WEST

Simultaneous rupture of 3 and 4 segments(30 scenarios)

Larger amplitude for west-hypocenter scenarios
Later phases from the along trough ‘a’ area

Velocity waveforms @ AIC

DEEP
SHALLOWDEEP
SHALLOWSHALLOW

Asperity

Hypocenter

EAST

DEEP

CENTER

WEST

Simultaneous rupture of 3 and 4 segments(30 scenarios)

Larger amplitude for west-hypocenter scenarios
Later phases from the along trough ‘c’ area

Velocity waveforms @ TKY

DEEPSHALLOWDEEP
SHALLOWSHALLOW

Asperity

Hypocenter

EASTDEEPCENTER

WEST

Simultaneous rupture of 3 and 4 segments(30 scenarios)Long duration for west-hypocenter scenarios

Character of the waveforms

OIT• Smaller amplitude for west-hypocenter scenarios• S waves from deeper asperities of the Nankai area
• Later phases from the along trough ‘a’ area

OSK

• Larger amplitude for west-hypocenter scenarios• Later phases from the along trough ‘a’ area

AIC

• Larger amplitude for west-hypocenter scenarios• Later phases from the along trough ‘c’ area

TKY

• Long duration for west-hypocenter scenarios

The hypocenter location tends to have a greater impact onlong-period ground motions than the asperities location andthe rupture areas.

Velocity response (damping factor of 5%)

The difference of seismic moment between the 3-segment and 4segment ruptures is small by a factor of two
the large scattering indicates the hypocenter and asperity location
have greater effect on simulation results than the rupture area.

Frequency distribution of Peak ground velocity(PGV) and Velocity response (Sv)Interquartilerange (IQR)

PGVMedianMaximum

Sv(T=5s)

Sv(T=10s)
the scenarios with the along trough area
generates significantly large PGV andvelocity response at OSK and AIC

Maps of Peak ground velocity (PGV) and
Velocity response (Sv)Maximumthe sedimentary wedge greatly contributes toMedian the generation of long-period ground motions

Interquartile range (IQR)

Long-period ground motions in the Kanto basin are
relatively large for most cases.

Long-period ground motions in the Osaka and Nobibasins are greatly influenced by a few specific scenarios.

Maps of PGV and Sv(without 4-segment scenarios)
Maximum

MedianInterquartile range (IQR)

Maps of PGV and Sv(before and after applying new findings)
New findings• Hyuga-nada• Along the troughEffect of slip velocity function• Change the slip velocity function for along the trough area
– Kostrov-like slip velocity time function (Nakamura and Miyatake, 2000)– Boxcar-like slip velocity time function
PGV ratio (Boxcar / Kostrov)PGV ratio

PGV ratio

 PGV values decrease by a factor of two locally.
 Selection of appropriate source time function is important.

Summaries• We simulated long-period ground motion of the Nankai
trough earthquakes using 55 scenarios with various
possible source parameters.
• The hypocenter location (rupture start point) tends tohave a greater impact on long-period ground motions
than the asperities and the rupture areas.
• Long-period ground motions in the Osaka and Nobibasins are greatly influenced by a few specific scenarios
including the along trough area.
• Long-period ground motions in the Kanto basin are
relatively large for most cases.
• New findings will change the possible range of longperiod ground motion.

Future tasks• Selection of appropriate source time function
for the along trough area.
• Improvement of source model, velocity
structure model, and simulation method.
– Introducing some kind of heterogeneity into thecharacterized source model.
– Modeling shallow lower velocity layers.
– etc...