IV. The seismic cycle Taiwan Lab

Introduction: geological context… 6.5 Ma old collision [e.g. Lin et al, 2003]. high rates of deformation and erosion southward propagation of the collision proposed to be of 55 to 90 mm/yr [Suppe 1981, 1984; Byrne & Liu, 2002]. Taiwan is an ideal place to investigate the transition from subduction to mature collision, and for testing models of mountain-building processes Introduction: geological context and issues addressed in this thesis

Malaveille A B C (Shyu et al, 2005) Malavielle et al, 2000

(Malavielle et al, 2000)

Tectonic Setting (from Jacques Angelier)

Malaveille (Malavielle et al, 2000)

Testing the seismic cycle model from the 1999 Chi-Chi 09/20/1999, Mw=7 Testing the seismic cycle model from the 1999 Chi-Chi 09/20/1999, Mw=7.6 earthquake Ground displacements as measured by GPS Interseismic displacements before the Chichi EQ (black arrows) Coseismic displacements due to Chichi EQ (red arrows) Our Taiwanese colleagues are running a network of permanent stations. Making it . Prelimary studies made from the seed money show that deformation before the Chichi earthquake was indicating that the fault that ruptured was perviously locked to a depth of the order of about 15km, and was slipping aseismically farther downdip. This model is staisfying but it does not explain how the mountain was formed. To adress this question wee need to setimet long term geological defotmation

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

LFZ Creeping Zone Convergence rate 30mm/yr

Active faults of the foothills of West Central Taiwan… Shortening absorbed by each one of these faults? 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).

Morphotectonic investigations to assess slip rates on major faults Pakuashan anticline: blind Changhua fault Dungpuna debris-flow: Chelungpu thrust sheet 1. Shortening absorbed by the major faults of the Western Foothills: a morphotectonic approach.

Principle of our approach Finite shortening recorded by pre-growth strata vs. incremental shortening recorded by growth strata or geomorphic markers. Cumulated shortening pretectonic strata total shortening 1 2 3 4 5 6 7 shortening rate different incremental shortenings growth strata age Age of deformation inception 1. Shortening absorbed by the major faults of the Western Foothills: a morphotectonic approach.

Example of analysis from northern Pakuashan: retrieving model parameters The finite deformation allows for calibrating the parameters in the analytical expressions 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 model Modeling 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.

Long term shortening across other more internal structures ? Final results from the morphotectonic investigations in West Central Taiwan ?? ~ 15 mm/yr ~ 16 mm/yr Long term shortening across other more internal structures ? Long term shortening across the range ? How does it compare to interseismic deformation? 1. Shortening absorbed by the major faults of the Western Foothills: a morphotectonic approach.

Interseismic velocities  16mm/yr from foreland sedimentation: 40-45mm/yr ? 15mm/yr Interseismic velocities  16mm/yr (Simoes et al, 2007)

LFZ Creeping Zone Convergence rate 30mm/yr

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

Co-seismic Slip Model of Chichi Earthquake (Hsu et al, 2009) Co-seismic Slip Model of Chichi Earthquake

Co-seismic deformation (Hsu et al, 2009)

2-D Analysis Interseismic+Co-seismic=Long Term implies a return period of Chichi EQ of about 250yr (Dominguez et al, 2003)

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

Time evolution of afterslip obeys frictional sliding (Perfettini and Avouac, JGR,2004) Displacement at station I007 Early Postseismic deformation is dominated by afterslip Later on Viscous relaxation should be dominant

Coseismic Interseismic Postseismic Chichi, Earthquake Mw 7.6, 1999

Brittle Creeping Fault Zone: A simple Fault Model Locked Fault Zone Brittle Creeping Fault Zone: Ductile Shear Zone

A simplified springs and sliders model a-b<0 a-b>0 Frictional sliding Ductile Shear F: Driving Force (assumed constant) Ffr : Frictional resistance Fh: Viscous resistance to F = Ffr + Fh F < Ffr No slip F > Ffr Stick-slip (Perfettini and Avouac, 2004b)

Stress transfer during the seismic cycle Fh >> DFfr F : Driving Force (assumed constant) DFfr : Co-seismic drop of frictional resistance Fh: Viscous resistance Two characteristic times are governing the temporal evolution of deformation - Brittle creep relaxation time tr - Maxell time TM Fh  DFfr (Perfettini and Avouac, 2004b)

References Simoes, 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).