Shiann-Jong LeeShiann-Jong Lee11, Dimitri Komatitsch, Dimitri Komatitsch2,32,3, Yu-Chang Chan, Yu-Chang Chan11, Bor-Shouh Huang, Bor-Shouh Huang11 and Jeroen Tromp and Jeroen Tromp44

1 Institute of Earth Science, Academia Sinica, Taipei, Taiwan1 Institute of Earth Science, Academia Sinica, Taipei, Taiwan

2 CNRS and INRIA Magique-3-D, Laboratoire de Mod2 CNRS and INRIA Magique-3-D, Laboratoire de Modéélisation et d'Imagerie en Glisation et d'Imagerie en Gééosciences UMR 5212, osciences UMR 5212,

UniversitUniversitéé de Pau et des Pays de l'Adour, France de Pau et des Pays de l'Adour, France

3 Institut universitaire de France, 103 boulevard Saint-Michel, 75005 Paris, France3 Institut universitaire de France, 103 boulevard Saint-Michel, 75005 Paris, France

4 Department of Geosciences, Princeton University, Princeton, New Jersey, USA4 Department of Geosciences, Princeton University, Princeton, New Jersey, USA

Effects of realistic topography on

seismic wave propagation:

Small- and large-scale topography effects

in northern Taiwan

The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of the 10th Anniversary of the Chi-Chi Earthquake

September 21~26, 2009

OutlinesOutlines

• IntroductionIntroduction

• Small-scale topography effect topography effect

• Large-scale topography effect topography effect

• Topography effect vs. source complexityTopography effect vs. source complexity

• Discussions and SummaryDiscussions and Summary

IntroductionIntroduction• Topography influences ground motion as is observed from data recorded Topography influences ground motion as is observed from data recorded

during and after real earthquakes and from numerical simulations. However, during and after real earthquakes and from numerical simulations. However, the effects of the effects of realistic topographyrealistic topography on ground motion have not been clearly on ground motion have not been clearly characterized in numerical simulations.characterized in numerical simulations.

• To accommodate high-resolution realistic topography data from the To accommodate high-resolution realistic topography data from the LiDAR LiDAR Digital Terrain Model (DTM)Digital Terrain Model (DTM), which has a resolution close to , which has a resolution close to 2 m2 m, we use the , we use the SEM to simulate seismic wave propagation for frequencies up to 10 Hz in the SEM to simulate seismic wave propagation for frequencies up to 10 Hz in the Shamao Mountain areaShamao Mountain area..

• Furthermore, recent publications have mainly focused on implications for Furthermore, recent publications have mainly focused on implications for ground motion in the mountainous regions themselves, whereas the impact ground motion in the mountainous regions themselves, whereas the impact on on surrounding low-lying areassurrounding low-lying areas has received less attention. has received less attention.

• In order to investigate the detailed interaction between large-scale In order to investigate the detailed interaction between large-scale topography and nearby areas, we study on an example of topography and nearby areas, we study on an example of Taipei basinTaipei basin and and the the Central Mountain Range (CMR)Central Mountain Range (CMR) which are located in northern Taiwan. which are located in northern Taiwan.

Small-scale topography effectsSmall-scale topography effects

Shamao Mountain areaShamao Mountain area LiDAR DTM data (1m)LiDAR DTM data (1m)

40-m DEM

LiDAR DTM (1m)LiDAR DSM (1m)

Aerial Aerial topographic topographic mapmap

The Spectral-Element Method (SEM)The Spectral-Element Method (SEM)

• Accuracy of a

Pseudospectral Pseudospectral

MethodMethod

• Flexibility of a

Finite-element MethodFinite-element Method

• Develop more then 20 years ago in

Computational Fluid Dynamics

27 nodes element

Spectral-element meshesSpectral-element meshes

Lee et al., 2009

SnapshotsSnapshots

P wave P wave

S waveS wave

Shamao mountain

North component, frequency up to 10hz

Positive velocity

Negative velocity

Hypothetical source:

• Magnitude: ML 5.0

• Double-couple source

• Strike 40°; dip 80°; rake -90°

• Located at 4.92 km depth

Synthetic waveform comparisonSynthetic waveform comparison

Vertical component Velocity waveforms

Peak Ground AccelerationPeak Ground Acceleration

(a) LiDAR DTM (2 m)(a) LiDAR DTM (2 m) (b) 40 m DEM(b) 40 m DEM

* The PGA values are calculated from the norm of the three components of the acceleration vector.

Realistic topography effects Realistic topography effects

on ground motionon ground motion

PGA amplification factor: subtract the PGA value for the model without topography from the value for the model with topography, dividing the result by the PGA value for the model without topography, and multiplying it by 100 to obtain a percentage

Relative change in PGARelative change in PGA

Source frequency

Wavefield type

Sourcedepth

Subsurface model

Summary of small-scale topography effectsSummary of small-scale topography effects• For small-scale topography study, we combined LiDAR DTMLiDAR DTM data and an

improved spectral-element meshspectral-element mesh implementation to accommodate high-resolution topography in the Yangminshan regionYangminshan region in northern Taipei.

• The average distance between points at the top of the SEM mesh was approximately 2 m, which enabled us to calculate the response of seismic waves up to a maximum frequency of approximately 10 Hz.

• PGA increases at mountain tops and ridges, whereas valleys usually have a reduced PGA. In some locations the PGA value decreases rapidly just beneath the tops of mountain ridges. Increased PGA values are also found in parts of valleys where brooks have eroded the ground surface, resulting in steep topography.

• Topographic effects also strongly depend on the source frequency and wavefield type.

• These results demonstrate that high-resolution, realistic surface topography needs to be taken into account for seismic hazard analysis, especially in dense population mountainous areas.

Large-scale topography effectsLarge-scale topography effects

Lee et al., 2009

Taipei basin SEM mesh

Realistic topography

Snapshot and PGV distributionSnapshot and PGV distribution

3-D wave-speed model3-D wave-speed model

3-D wave-speed model3-D wave-speed model

+ topography+ topography

+ basin+ basin

(b) – (a) = Residual(b) – (a) = Residual

Sna

psh

ot (

T =

14

sec)

PG

V

PGV amplification factor: subtract the PGA value for model (b) from the value for the model (a), dividing the result by the PGA value for the model (a), and multiplying it by 100 to obtain a percentage.

Synthetic waveforms along A-A’ profileSynthetic waveforms along A-A’ profile

Topography effect vs. source depth

15 km depth 40 km depth 2 km depth

Topography effect vs. source complexityTopography effect vs. source complexity

Large subduction zone earthquake scenarios

For finite-fault rupture scenarios (b), (c) and (d):

• The fault plane is 51 x 31km, divided into 1581 subfaults (of size 1 km x 1 km)

• The slip on the fault plane is considered uniform (84 cm) with a constant rake angle of 121°.

• The rupture velocity is assumed to be constant and equal to 2.5 km/s.

• For each subfault we use a Gaussian source time function with a half-duration of 1 second.

Cen

tral m

ount

ain

rang

e (C

MR

)

SnapshotsSnapshots

Vertical component, Velocity wavefieldFrequency up to 1Hz in acceleration

Positive velocity

Negative velocity

Rupture area of finite-fault source

fictitious seismic stationlocated in CMR

Synthetic waveformsSynthetic waveforms

Vertical component of velocity waveforms

Point source

Bilateral rupture

Eastward rupture

Westward rupture

Synthetic waveforms Frequency spectra

P SV

Topography effect vs. source complexity

Point source Bilateral rupture Eastward rupture Westward rupture

SummarySummary• We investigated the effects of large-scale topography associated with the Central

Mountain Range (CMR) in northern Taiwan on strong ground motion in the Taipei basin.

• Results show that variations in source depth modulate the influence of topography on ground motion in neighboring low-lying areas. If a shallow earthquake occurs in the I-Lan region, we find that the CMR significantly scatters the surface waves and therefore reduces ground motion in the Taipei basin. However, when we move the hypocenter deeper, topography scatters the body waves, which subsequently propagate as surface waves and spread into the Taipei basin.

• We also investigated several hypothetical rupture scenarios of subduction zone earthquakes occurring off the northeast coast of Taiwan. Results shown that the effects of topography on ground motion vary depending on the source rupture process.

• Our simulations show that topography has different effects depending on the scenario: it may or may not reduce ground motion in Taipei depending on the directivity, location, and depth of the event.

• These results illustrate the fact that topography should be taken into account when assessing seismic hazard.

Thank you Thank you for your attention~for your attention~

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Cumulative kinetic energy (ECumulative kinetic energy (Ekk))

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Influence of attenuationInfluence of attenuation