IV. The seismic cycle

• Taiwan Lab

Introduction: geological context and issues addressed in this thesis

Introduction: geological context…

• 6.5 Ma old collision [e.g. Lin et al, 2003].

• high rates of deformation and erosion

• southward propagation of the collisionproposed to be of 55 to 90 mm/yr [Suppe 1981, 1984; Byrne & Liu, 2002].

Taiwan is an ideal place to investigate thetransition from subduction to mature collision, and for testing models ofmountain-building processes

Malaveille

Malavielle et al, 2000

C

A

B

(Shyu et al, 2005)

(Malavielle et al, 2000)

Tectonic Setting

(from Jacques Angelier)

Malaveille(Malavielle et al, 2000)

Ground displacements as measured by GPS – Interseismic

displacements before the Chichi EQ (black arrows)

– Coseismic displacements due to Chichi EQ (red arrows)

Testing the seismic cycle model from the 1999 Chi-Chi

09/20/1999, Mw=7.6 earthquake

Comparison of interseismic GPS velocities (black arrows) and postseismic (blue arrows) GPS displacements. Modeled interseismic velocities and postseismic displacements are shown in gray and light-blue vectors, respectively. Distribution of coseismic slip in color

Convergence rate 30mm/yr

LFZ

Creeping Zone

Active faults of the foothills of West Central Taiwan…

Interseismic GPS data from Yu et al (1997)Elastic dislocation modeling from Dominguez et al (2003) (fit shown on top panel for 40 mm/yr creep), Hsu et al (2002) and Loevenbruck et al (2001).

Cross-section from Yu et al (2005).

Shortening absorbed by each one of these faults?

1. Shortening absorbed by the major faults of the Western Foothills: a morphotectonic approach.

Pakuashan anticline: blind Changhua fault

Dungpuna debris-flow: Chelungpu thrust sheet

Morphotectonic investigations to assess slip

rates on major faults

Principle of our approach

Cumulated shortening

age

pretectonic stratatotal shortening

growth strata

different incrementalshortenings

Age of deformation inception

shortening

rate

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5

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1. Shortening absorbed by the major faults of the Western Foothills: a morphotectonic approach.

Finite shortening recorded

by pre-growth strata vs. incremental

shorteningrecorded by growth

strata or geomorphic markers.

1. Shortening absorbed by the major faults of the Western Foothills: a morphotectonic approach.

The finite deformation allows for calibrating the parameters in the analytical expressions

Example of analysis from northern Pakuashan: retrieving model parameters

• Depth of the decollement• Overall geometry of the fold• Hinges delimiting domains of homogeneous dips• Finite shortening from excess-area method

Methodology:

1. Shortening absorbed by the major faults of the Western Foothills: a morphotectonic approach.

Example of analysis from northern Pakuashan: testing the calibrated modelModeling incremental growth of the fault-tip fold

Model describes quite well the finite structure of the Pakuashan anticline!

We can use these calibrated formulations to translate dips measured in the field into cumulated shortening !

1. Shortening absorbed by the major faults of the Western Foothills: a morphotectonic approach.

Results for northern and southern Pakuashan, along the direction of transport

Consistent kinematics from north to south

Age of deformation initiation: 63,7 +/- 9.8 ka

Shortening rate across the Pakuashan anticline: 15.9 +/- 1.4 mm/yr

1. Shortening absorbed by the major faults of the Western Foothills: a morphotectonic approach.

Field survey of the Dungpuna debris-flow.

• Survey of the strath terrace / collecting samples to date it.

• Structural measurements

1. Shortening absorbed by the major faults of the Western Foothills: a morphotectonic approach.

Data available for analysis

• Most reliable geomorphic marker: strath terrace (not top of the deposits).

• structural model: Tiechenshan and Chelungpu faults are evolved structures splaying from a common thrust at depth.

A model of fault-bend folding may apply here

1. Shortening absorbed by the major faults of the Western Foothills: a morphotectonic approach.

Model of deformation

Application to vertical throw of the strath across surface fault traces

Results …

1. Shortening absorbed by the major faults of the Western Foothills: a morphotectonic approach.

Final results from the morphotectonic investigations in West Central Taiwan

??

Long term shortening across other more internal structures ?

Long term shortening across the range ?How does it compare to interseismic deformation?

~ 16 mm/yr

~ 15 mm/yr

16mm/yr

15mm/yr

from foreland sedimentation:

40-45mm/yr

?

Interseismic velocities

(Simoes et al, 2007)

Convergence rate 30mm/yr

LFZ

Creeping Zone

Amplitude of Horizontal Co-seismic Displacements measured from SPOT images and GPS

(Hsu et al, 2009)

Co-seismic Slip Model of Chichi Earthquake

Co-seismic deformation

(Hsu et al, 2009)

Interseismic+Co-seismic=Long Term implies a return period of Chichi EQ of about 250yr

(Dominguez et al, 2003)

2-D Analysis

Backslip model of interseismic strain (scale in m/yr)(Only Chelungpu fault is taken into account)

3-D Analysis

Early Postseismic deformation is dominated by afterslipLater on Viscous relaxation should be dominant

Time evolution of afterslip obeys frictional sliding (Perfettini and Avouac, JGR,2004)

Displacement at station I007

InterseismicCoseismic

Postseismic

Chichi, Earthquake Mw 7.6, 1999

A simple Fault Model

Locked Fault Zone

Brittle Creeping Fault Zone:

Ductile Shear Zone

Frictional sliding Ductile Shear

F: Driving Force (assumed constant)Ffr : Frictional resistanceF: Viscous resistance to

A simplified springs and sliders model

F

F = Ffr + F

F < Ffr No slip

F > Ffr Stick-slip

a-b<0

(Perfettini and Avouac, 2004b)

a-b>0

Stress transfer during the seismic cycle

F : Driving Force (assumed constant)Ffr : Co-seismic drop of frictional resistanceF: Viscous resistance

F >> Ffr

F Ffr

(Perfettini and Avouac, 2004b)

Two characteristic times are governing the temporal evolution of deformation

- Brittle creep relaxation time tr- Maxell time TM

ReferencesSimoes, M., J. P. Avouac, and Y. G. Chen (2007), Slip rates on the Chelungpu and Chushiang thrust faults

inferred from a deformed strath terrace along the Dungpuna river, west central Taiwan, Journal of Geophysical Research-Solid Earth, 112(B3).

Simoes, M., J. P. Avouac, O. Beyssac, B. Goffe, K. A. Farley, and Y. G. Chen (2007), Mountain building in Taiwan: A thermokinematic model, Journal of Geophysical Research-Solid Earth, 112(B11).

Simoes, M., J. P. Avouac, Y. G. Chen, A. K. Singhvi, C. Y. Wang, M. Jaiswal, Y. C. Chan, and S. Bernard (2007), Kinematic analysis of the Pakuashan fault tip fold, west central Taiwan: Shortening rate and age of folding inception, Journal of Geophysical Research-Solid Earth, 112(B3).

Dominguez, S., J. P. Avouac, and R. Michel (2003), Horizontal coseismic deformation of the 1999 Chi-Chi earthquake measured from SPOT satellite images: Implications for the seismic cycle along the western foothills of central Taiwan, Journal of Geophysical Research-Solid Earth, 108(B2), art. no.-2083.

Hsu, Y.-J., J. P. Avouac, S. B. Yu, Y. M. Wu, C. H. Chang, and J. Woessner (2009), Spatio-temporal slip, and stress level on the faults within the western foothills of Taiwan: implications for fault frictional properties, Pageoph.

Perfettini, H., and J. P. Avouac (2004), Stress transfer and strain rate variations during the seismic cycle, Journal of Geophysical Research-Solid Earth, 109(B6).