Fracture Energy of a Large Earthquake from the Taiwan ChelungpuFault

Earthquake associated with the seismic faulting is the most dangerous and unforeseen natural disaster. The 1999magnitude 7.7 earthquake in Chi-Chi Taiwan, induced by Chlungpu faulting, produced very large surfacedisplacement and caused huge casualty. Recovering deep cores which penetrate the fault zone is essential forstudying the physical fault structure. Cores from the region of large slip on Chelungpu fault provides us a uniqueopportunity of sampling the fault in a recent earthquake. The energy budget of the earthquake has attracted theattention of the geoscience's disciplines for understanding the mechanism of earthquake nucleation and transition.We measured the particle size distribution for the particle diameter range from several nm to 150μm of the majorslip zone by using transmission electron microscope (TEM), transmission X-ray microscope (TXM), scanning electronmicroscope (SEM), and optical microscope (OM). We identify the power law of the distribution and ascertain thesurface fracture energy. Comparing the fracture energy with the breakdown work, we obtain the fraction of theprocess of grain formation to the value of the earthquake breakdown work. The results may ascertain the implicationof the nucleation, growth and transition structure of the fault zones.

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Earthquake attracts much attentions because it is the mostdisastrous and unpredictable hazards in the world. The large earth-quake events occurred in history, such as Tokyo, San Francisco, Tan-shan and Kobe earthquakes, caused severe casualty and destructionto society. Taiwan is located at the complexity with the oblique colli-sion zone of the Eurasian continental plate and the Philippine Seaplate which is moving WNW at about 70 mm per year. The mountain-building process is still in progress and a dominant collision zone,which induces folding and fault thrusting, exists in west-centralTaiwan. In the last centuries, rapid crustal movement and widely dis-tributed active structures induced tens of large earthquakes withmagnitudes of over 7.0. The 1999 magnification 7.7 Chi-Chi earth-quake struck near the town of Chi-Chi in Nantou County in west-central Taiwan, with its epicenter about 15 km east of the surfacetrace of the thrust fault and a hypocenter depth of about 10-12 km. Itproduced very large surface rupture of 80-90 km along the Chelung-pu fault. In the south part of the fault, the vertical offsets was 2 m andincreased to 8-10 m in the north, which is the largest offset evermeasured for modern earthquake. Figure 1 shows the 8 m vertical

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01B X-ray Microscopy

Authors

Y. F. Song and Y. M. C henNational Synchrotron Radiation Research Center,Hsinchu, Taiwan

S. R. S ongNational Taiwan University, Taipei, Taiwan

H. Tanaka University of Tokyo, Tokyo, Japan

K. F. MaNational Central University, Taoyuan, Taiwan

offset of the fault created a new waterfall where the faultcrossed the Tachia River. The slip displacement reached12 m at the depth of 1 km. This earthquake is one of thelargest inland events in the past century, which destroyedmore than 100,000 buildings and results grave injury. Theexpectation of this work is to understand the physics,mechanism and controlling factors of the large earth-quake. The final goal is to alleviate the potential disasterof seismic events in the future.

Grain rupturing and gouge formation are found toexist in brittle faults at all scales. This fine-grain gouge isbelieved to govern earthquake instability. Thus investiga-ting the gouge texture property is very important to anunderstanding of the earthquake process. The actualthickness of the zone that slips during the rupture of alarge earthquake is a key seismological parameter inunderstanding energy dissipation, rupture processes andseismic efficiency. The energy budget of the earthquake isanother critical parameter which attracts the attention ofthe geoscience's disciplines for investigating the mecha-nism of earthquake. Thecharacterization of particlesize distribution, porosityand 3D structure of the faultrocks in transition from thefault core to damage zoneare related to the comminut-ing, f luid behaviour andenergy in the earthquakefaulting. The results willreveal the insight of themechanism of faulting, thenucleation, growth, transi-tion structure and perme-ability of the fault zones.

Recovering deep cores which penetrate the faultzone is essential for studying the physical fault structure.Fault zone rocks are extremely fragile and easily alteredby weathering; therefore, the surface outcrops can pro-vide only limited information. The Taiwan Chelungpu-fault Drilling Project (TCDP) has drilled three deep holesinto the slip and fault zones of the Chelungpu fault. Coresfrom the region of large slip on Chelungpu fault providesus precious materials to study the characteristics, mecha-nism and energy budget in a recent thrust fault. TheChelungpu fault is major 90 km structure that dips sha-llowly the east (30˚) and slips within bedding of thePliocene Chinshui Shale. The TCDP drilled two verticalholes 40 m apart (hole A and hole B) and a side-track fromhole B (hole C) near the town of Dakeng (Fig. 2). Thedrilling carried out continuous coring for depths of 500-2000 m for hole A, 950-1300 m for hole B and 950-1200 mfor hole C, respectively. Geophysical well logs were carriedout from hole A to collect seismic velocities, densities anddigital images.

Chelungpu fault zone is within the Chinsui shale as adamaged zone at depth of 1105 to 1115 m observed fromhole A core. The degree of fracturing increased from thetop to the bottom of the damaged zone. Near the bottomof the deformation zone, we identify a 12-cm-thick prima-ry slip zone (PSZ) based on the presence of ultra-fine-grained fault gouge at depths of 1111.23 to 1111.35 m.The geophysical logging measurements of low seismicvelocities and low electrical resistivity around the depthof 1111 m also indicate that this is the primary fault zone.The PSZ consists of several layers of slip zone associatedwith several repeating earthquakes. Each zone has a ~2 cmthickness, indicating the number of earthquake events is6. Among the slip zones, the least deformed region, whichhas the fewest cross-cutting cracks, is the 2-cm zone at

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Fig. 1: 8m vertical offset on the Chelungpu fault created thisnew waterfall where the fault crossed the Tachia River.

Fig. 2: Taiwan Chelungpu-fault Drilling Project drilled two vertical holes 40 m apart (hole A andhole B) and a side-track from hole B (hole C) near the town of Dakeng.

the bottom of the PSZ. This narrow band might be themajor slip zone (MSZ) which corresponds to the Chi-Chiearthquake.

Several models were proposed to describe the energybudget for physical process of earthquake slip. In thecommon usage slip-weakening model, the elastic strainenergy which released during an earthquake is distrib-uted between the frictional heat, EH, the product of faultarea and seismic fracture energy, EG, and the energy radi-ated as seismic waves, ER. The frictional heat is a largecomponent of the total energy budget, but it does notdirectly affect earthquake rupture dynamics. Whereas theradiation efficiency, ηR, which is the ratio of the radiationenergy to the energy available for mechanical processes,defined asηR = ER/(ER + EG), plays a significant role. For themature earthquake, the energy spent for gouge forma-tion is small, ηR is nearly equal to 1. The surface fractureenergy associated with gouge formation is a form oflatent heat required to produce an earthquake rupturesurface and related to parameters controlling rupturepropagation and processes of slip weakening. It describesthe flow of energy at rupture tip which is required tocreate a rupture surface and produce a breakdown instrength. Formation of the slip zone is associated with theseismic fracture energy, which is consumed as the earth-quake rupture propagates. The surface fracture energy forthe creation of small grains is a contribution, but may notbe the total equivalent to the seismic fracture energy. Thebreakdown work is the excess work over some minimumlevel achieved during slip and is the energy spent to allowthe rupture to advance. The breakdown work is com-posed of the surface fracture energy for gouge formationand other dissipative losses during faulting. It can be con-sidered as an equivalent to the seismic fracture energyper unit area. Comparing the surface fracture energy withthe breakdown work, we will know the relation of theenergy for processing the grain formation to the value ofthe earthquake breakdown work. The results would pro-vide the information to confirm the surface fracture ener-gy and energy budget in the 1999 Chi-Chi earthquake.

The total fracture surface area, ST, which includes thesurface area of cataclastic particles in the ultracataclasite,SUC, surface area of gouge particles in subsidiary faults ofthe damage zone, SSF, and surface area of microfracturedamage area, SMF. We analysis the particle size distribu-tion and the structure of the fault core gouge by transmis-sion electron microscope (TEM), scanning electron micro-scope (SEM), nano-transmission X-ray microscope (TXM),and optical microscope (OM) for the particle diameter

range from several nm to 150μm to determine the fac-ture surface area in the ultracataclasite associated withthe gouge formation (SUC) of MSZ. The Chelungpu fault isa 10 m thick zone of fractured rock containing 1 m thickcore of sheared cataclasite and ultracataclasite. The pri-mary slip zone is 12 cm thick. The ultracataclasite layer ofMSZ is 2 cm thickness. The components of the drillinggouge include smectite, illite, chlorite and kaolinite.Figure 3 shows the 2D phase contrast images of the TXMof hole A fault core gouge of MSZ. The particle size distri-bution of the ultracataclasite from all images is shown inFigure 4, which is consistent with the power law N(D) =aD-b, where D is particle diameter, N(D) is the number ofparticles for each size diameter per unit area (mm2), andconstants a = 0.005, b = 2.3. The grain size for calculatingthe surface area of ultracataclasite is from 50 nm to 150μmfor MSZ. The upper cut-off grain size of 150μm is obtainedby adopting optical microscope. From the particle sizeobservation using transmission electron microscope, weconsider the lower cut-off of the particle size is 50 nm.The images with grain sizes of less than 50 nm showsrounded shapes, suggesting that those small grains mightbe the results of precipitation rather than fracturing. The

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Fig. 3: TXM 2D phase contrast image of the hole A gougewithin major slip zone.

Fig. 4: Particle size distribution measured by using TEM, TXM,SEM and OM from the gouge of hole A within major slip zone.

total fracture surface area for 2 cm MSZ from hole A coreis 5.5 x 105 m2/m2 supposing the spherical grains. Assum-ing free-surface energy is 1 J/m2, and the geometricsurface should be increased by about a factor of 6.6 toaccount for non-spherical grains shapes, we obtained avalue of 4.0 MJ/m2 for surface fracture energy in MSZ. Theestimated number of events is 6. The contribution ofgouge surface energy on one earthquake event in MSZ toearthquake breakdown work (11.6 M J/m2) is quantifiedto be 6 %. This value shows that the process of grain for-mation represents about 6 % of the earthquake break-down work. We consider this estimate to be the maxi-mum assuming that there is no fracture energy occurredduring sliding. The residual part of the breakdown workprobably is heat associated with other dynamics process-es, such as fault thermo-pressurization or fault lubrication.

The surface fracture energy of gouge zone of theChelungpu fault for the Chi-Chi earthquake is identifiedabout 6% of the breakdown work. The calculated value ofradiation efficiencyηR is 0.88, which is intermediatebetween that of well-developed Punchbouwl fault inCaliforniea (ηR is nearly 1) and that of the new fracturedearthquakes in the South African mine (ηR is 0.16). Thephysical differences in the fault zones depend on thematurity and style of faulting and result the differences inthe mechanical energy absorbed during large events.When large earthquakes occur on mature fault, there isless fracturing, so the proportional amount of dissipativeenergy is smaller, compared to the more fracturingbehaviour of young faults.

The investigation of energetic budget of the faultzone gauge is pivotal for understanding the earthquakeprocess. The results of the particle size distribution mea-surement and analyzing would reveal the insight of thesurface fracture energy and the relation with the energybudget in the 1999 Chi-Chi earthquake, then apply toanalyze other faulting events and to ascertain the fracturemechanics of thrust fault system. Furthermore, it mayunderstand the relation of the nucleation, growth andtransition structure of the fault zones and then deducethe mechanism of faulting, and the nucleation of theearthquake.

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Experimental Station

Nano-transmission X-ray microscopy

Publication

K. F. Ma, H. T anaka, S. R. S ong, C. Y . Wang, J. H. H ung, Y. B. Tsai, J. M ori, Y. F. Song, E. C. Y eh, W. Soh, H. S one, L. W. Kuo and H. Y. Wu, Nature, 444, 473 (2006).

References

．B. Wilson, T. Dewerw, Z. Reches and J. Brune, Nature,434, 749 (2005). ．2 J. S. Chester, F. M. Chester and A. K. Kronenberg,

Nature 437, 133 (2005).

Contact E-mail

song@nsrrc.org.tw