|Session 4.3 - Fault Rupture Hazard|
An Interim Classification of New Zealand’s Active Faults for the Mitigation of Surface Rupture Hazard
Interim Guidelines for mitigating the impacts of building on, or near, active faults have recently been promulgated in New Zealand. In the Guidelines, the defining fault-avoidance criterion for an active fault is its average recurrence interval of surface rupture. Here we present an interim classification of most of New Zealand’s on-land active faults based on the fault-avoidance recurrence interval classes defined in the Guidelines. In assigning faults to specific fault-avoidance classes, we give preference to fault-specific recurrence interval data which, in general, is most complete for the principal faults in New Zealand. For the remainder, and majority, of active faults, where recurrence interval data are generally either less constrained or non-existent, we assign fault-avoidance classes, though with less confidence, based on an iterative combination of available fault-specific data, the use of fault-scaling relationships, and comparisons with similar better studied faults. For each fault we also note the level of confidence with which the fault is assigned to a particular fault-avoidance class. This indicates the precision (or lack thereof) with which active faults can presently be classified according to the fault-avoidance criteria defined in the Guidelines. It also highlights those faults where better constrained recurrence interval data will have the most benefit towards mitigating surface rupture hazard.
Keywords: active faults, recurrence interval, surface rupture hazard
Interim Guidelines for mitigating the impacts of building on, or near, active faults have recently been promulgated in New Zealand. In the Guidelines, the surface rupture hazard of an active fault is defined by the fault’s average recurrence interval of surface rupture, and the complexity of its surface rupture trace. Examples of the latter are illustrated in this paper via a suite of photographs of historic and pre-historic surface ruptures of active faults in New Zealand. These images also highlight three important points relevant to the mitigation of surface rupture hazard: a) the location of past surface rupture along an active fault is generally a reliable guide as to the location of future surface rupture along the fault, b) surface rupture deformation is most intense at the fault, but, in places, significant "off fault" deformation can occur, and c) the extent of surface rupture deformation along an active fault can often be geographically defined to a level compatible with planning and engineering needs.
Paper 156: [Read]
Keywords: active fault, surface rupture hazard, earthquake
Earthquake fault rupture of the ground surface increases the risk of collapse for buildings which straddle that fault compared with other buildings. The lack of any clear regulatory constraints to control this additional risk was identified as a deficiency in the current building control regime in New Zealand by the Parliamentary Commissioner for the Environment during his investigation into this issue reported in 2001.
A working group was established under the auspices of the Ministry for the Environment to prepare a set of planning guidelines to address this deficiency. This paper discusses the technical considerations used to prepare a risk-based interim guideline which aims to provide planning authorities with a framework to prepare planning controls for this aspect of land development. The paper briefly outlines the seismological issues such as fault recurrence intervals and complexity, the engineering issues relating to building importance classifications and consistency of risk posed by an event, and the proposed mitigation measures, including planning controls on development and land subdivision.
The paper reflects the approach taken in developing the draft Interim Guidelines (hereafter referred to as ‘the Guidelines’). As such it portrays ‘work in progress’ with further amendment and wider consultation expected before the final guideline is available for use by planning authorities to address the issue of building over or adjacent to active faults.
Keywords: Near Faults; Risk-based; Planning Controls; Recurrence Interval Classes; Near Fault Building Importance Categories
G.J. Huftile, S.C. Lindvall, L.W. Anderson, L. Gurrola and M.A. Bell
A trench across the main strand of the Red Mountain fault, California, presents the problem in calculation of slip rates as both sides of the fault are uplifting. The Red Mountain fault is a reverse fault in the Transverse Ranges of California. It is part of a system of north-dipping reverse faults including the San Cayetano and the Santa Susana faults. The slip rates have been measured at 7.4±3.0mm/y on the San Cayetano fault, and 5.9±3.8mm/y on the Santa Susana fault. This work represents the first slip rate measurement on the Red Mountain fault. The footwall block of the Red Mountain fault has the stage 3c high stand terrace, confirmed using oxygen isotopes. This terrace has uplifted from –39m elevation at 45ka, to +168m today, an uplift rate of 4.6mm/y. The trench, geological mapping, and borehole drilling show that the hanging-wall block has uplifted 34m (0.75mm/y, 1.7mm/y slip on a 25º–dipping fault) relative to the footwall block in that time. We argue that the slip rate on the Red Mountain fault is the combination of these rates, and thus has serious implications to tectonics of the Transverse Ranges and the relative seismic hazard represented by the fault.
Keywords: paleoseismology, active faulting, reverse faulting, trenching, California, Transverse Ranges