A Study on Factors Controlling Fault Evolution and Surface Rupture Propagation Based on Geometric and Kinematic Analyses

Abstract

The ways that brittle faults grow and evolve is important in many practical fields of structural geology such as fluid flow and earthquake hazards. Fault zone evolution has been mainly studied by using results of 1) the scaling relationships between displacement and other fault elements, 2) field-based case studies on fault zones on different scales, and 3) the spatial and temporal accumulation of slip from earthquake surface ruptures. However, it is always a great challenge to model fault zone evolution based on these approaches since a variety of controlling factors can affect a fault during its evolutional history. This study focuses on how the results of each research approach can be improved with the consideration of various controlling factors. Where fault scaling relationships are concerned, mature fault zones and their complicated internal architectures may be one of the most difficult areas of statistical analysis. This means that a clearer definition and classification of fault zone architectures should be constructed based on the identical viewpoint and research method. Advanced field techniques and data accumulation methods are examined to help more accurately define fault zone architecture, in particular damage zones, using the distribution of cumulative fracture frequency. Results show that changes in slope gradient of cumulative frequency distribution can be a useful criterion in accurately defining the width of damage zones and some internal properties of fault zones. In addition, damage zones are classified into along-fault, around-tip and cross-fault damage zones based on different descriptions of an exposed fault zone as well as their three dimensional location around a segmented fault system. Next, various factors that control fault evolution are examined and discussed based on the interpretation of the evolutional history of each fault zone. These results indicate that a variety of factors, such as lithology, tectonic environment, deformation localization/distribution, and segment linkages, can affect fault growth. In particular, mature faults tend to grow by fault-parallel segment linkage and/or deformation localization, whereas fault-perpendicular growth occurs mainly by newly formed branch faults and/or deformation distribution. It is important to realize that these different factors can influence each other, and can multiply and complexly affect a fault. The effects of each factor can be various depending on the spatial and temporal evolution of a fault. Lastly, earthquake behavior along mature faults, particularly propagation and/or termination of earthquake surface ruptures at fault discontinuities, are examined focusing on the slip variation between two slip components. The results show that slip patterns change from slip deficit (decrease in both slip components), through slip compensation (increase of vertical slip with horizontal slip decrease), to slip neutral as the ratio of length to width of linkage between two segments increases. The results of this study will help us to gain a better understanding of mature fault zone properties and their relationships to fault evolution. Also, detailed analysis of slip patterns and related kinematics at damage zones can give us very useful information for coseismic rupture processes and earthquake hazard assessment at fault discontinuities.