TSK 11 Göttingen 2006 Gudmundsson & Geyer
 Effects of damage-zone thick-
 ness on fault displacement
 Poster
 Agust Gudmundsson1 Adelina Geyer2
 When viewed as ideal elastic cracks,
 seismogenic faults are often modeled as
 mode II or mode III cracks in semi-
 infinite elastic bodies or half spaces.
 These models normally assume the rock
 to be homogeneous and isotropic. Such
 assumptions may be justified and neces-
 sary when using closed-form analytical
 solutions for fault displacement. They
 are not justified, however, when we at-
 tempt to understand fault-displacement
 profiles along earthquake rupture sites
 or in paleofault studies. This follows
 because crustal segments hosting faults
 are, as a rule, not homogeneous and
 isotropic, but rather heterogeneous and
 anisotropic. In particular, the fault
 rocks commonly form layers or units
 parallel with the fault plane. Also, the
 mechanical properties of the rocks next
 to the fault change as the fault develops
 (Gudmundsson 2004). During repeated
 earthquakes in a seismogenic fault zone,
 two main rock units develop around the
 fault plane. One unit is the core, located
 next to the fault plane and normally
 composed of soft (low Young’s modu-
 lus) breccia, gouge, and other cataclas-
 tic rocks. The other unit is the dam-
 age zone, containing some cataclastic
 rocks but characterized by fractures of
 various types. Field studies show that
 the fracture frequency in the damage
 zone is often quite variable, but nor-
 mally decreases with distance from the
 1 Department of Structural Geology and Geo-
 dynamics, Geoscience Center, University Göt-
 tingen, Goldschmidtstrasse 3, D-37077 Göttin-
 gen, Germany 2 Institute of Earth Science,
 Jaume Almera, Barcelona, Spain
 Figure 1: Fault zones consist of two main
 mechanical units: a comparatively thin
 core and a much thicker damage zone. The
 effective Young’s modulus (stiffness) grad-
 ually decreases from the host rock to the
 boundary between the core and the dam-
 age zone.
 core-damage zone boundary; similar re-
 sults are obtained for microfaults in lab-
 oratory experiments (Shimada 2000).
 The higher the fracture frequency, the
 lower will be the effective Young’s mod-
 ulus (stiffness) in a direction perpendic-
 ular to the main fracture trend. The
 Young’s modulus of a damage zone thus
 normally decreases on approaching the
 fault core (Fig. 1). On the basis of vari-
 ations in fracture frequency, the dam-
 age zone associated with a fault can
 commonly be divided into several sub-
 zones or units, each with a different ef-
 fective stiffness (Gudmundsson & Bren-
 ner 2003).
 Field studies also show that as the fault
 zone evolves the core and the damage
 zone both increase in thickness. A fault
 zone composed of units (core and dam-
 age zone) with stiffnesses that are differ-
 ent from those of the host rock develops
 local stresses that may be very different
 from the far-field stresses (Gudmunds-
 son & Brenner 2003).
 1
Gudmundsson & Geyer TSK 11 Göttingen 2006
 Figure 2: Boundary-element model of fault displacement when the damage-zone thickness
 is gradually increased in 10 steps, showing the maximum normalized displacement (MND)
 in the fault center in each of the steps. Here MND = 104 MDFL , where MD is the maximum
 displacement and FL the fault length, both expressed in model length units.
 Using these observations as a basis, we
 present numerical results on how the
 fault slip in a fault zone, for given fault
 geometry and loading conditions, may
 change when the thickness of its damage
 zone increases. Our results indicate that
 when the damage-zone thickness grad-
 ually increases, the maximum displace-
 ment on the fault also increases (Fig. 2).
 It follows that the fault slip generated
 during a particular earthquake, includ-
 ing the postseismic slip, may gradually
 increase with increasing damage-zone
 thickness. Thus, for an active seismo-
 genic fault of constant rupture (trace)
 length, the ratio of the maximum dis-
 placement to the rupture length should
 decrease with time. These theoretical
 results are supported by field observa-
 tions.
 Acknowledgments This work was
 supported by a grant from the Euro-
 pean Commission through the project
 Prepared (EVG1-CT-2002-00073 PRE-
 PARED).
 References
 Gudmundsson A (2004) Effects of Young’s
 modulus on fault displacement. C.R. Geo-
 science 336, 85–92
 Gudmundsson A & Brenner SL (2003) Load-
 ing of a seismic zone to failure deforms the
 nearby volcanoes: a new earthquake precur-
 sor. Terra Nova 15, 187–193
 Shimada M (2000) Mechanical Behavior of
 Rocks under High Pressure Conditions.
 Balkema, Rotterdam
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