Keith I. Kelson

Representative Styles of Deformation along the Chelungpu Fault from the 1999 Chi-Chi (Taiwan) Earthquake: Geomorphic Characteristics and Responses of Man-Made Structures

The Chi-Chi earthquake provides dramatic evidence of the damaging effects of surface ground deformation to buildings, lifelines, and other facilities. Much of the building damage is associated with surface faulting and folding along the Chelungpu thrust fault. Our detailed surveying at representative sites along the fault shows that the rupture commonly is a relatively simple 1- to 4-m-high scarp with minor hanging-wall deformation and localized (but severe) uplift, folding, and graben formation along the scarp crest. For individual scarps, the width of deformation is about 10 to 20 times the net vertical displacement. Distributed secondary faulting and folding on the hanging wall occurred as much as 350 m from the primary fault. Near the northern end of the rupture, growth of a pre-existing 1-km-wide late Quaternary anticline produced severe ground rupture along multiple thrusts and backthrusts but only minor tilting between fault strands. The pattern of building damage coincides with the pattern of geologic deformation, with severe damage along large fault scarps and lesser but still significant damage attributable to distributed secondary surface deformation on the hanging wall. Rupture-related building damage on the footwall occurred next to the prerupture fault trace, where the hanging wall bulldozed onto the footwall. The width of this damage zone is related to the local horizontal shortening along the fault and generally is less than 10 m. Building zonation along reverse faults should account for this pattern of surface deformation. In addition, buildings with massive foundations locally influenced the style and location of near-surface deformation, producing variations in fault strike or accentuated secondary deformation on the hanging wall. Manuscript received 15 January 2001.

Timing of Late Holocene Paleoearthquakes on the Northern San Andreas Fault at the Fort Ross Orchard Site, Sonoma County, California

Paleoseismic trenching within Fort Ross State Historic Park provides data on the late Holocene rupture history of the North Coast segment of the northern San Andreas fault. The 1906 earthquake ruptured through the Fort Ross Orchard site, which is characterized by a narrow shutter ridge and associated linear trough containing latest Holocene sediments. Trenches across the northeast-facing fault scarp exposed sediments interpreted as scarp-derived colluvium and possible fissure-fill deposits, and tentative upward fault truncations that provide evidence of three possible surface ruptures prior to 1906. Coarse-grained scarp-derived colluvial sediments were deposited after individual surface-rupturing earthquakes that predate the 1906 rupture. Radiocarbon analyses of 31 detrital radiocarbon samples collected from the colluvial deposits constrain the timing of earthquakes over the past approximately 1000 years. Based on stratigraphic ordering and a statistical comparison of radiocarbon dates using the OxCal program, we estimate (at a 95% confidence level) that three pre-1906 surface ruptures at the Orchard site occurred at a.d. 1660–1812, a.d. 1220–1380, and a.d. 1040–1190. Previous trenches at the nearby Fort Ross Archae Camp site are consistent with these dates and further suggest the occurrence of an earlier event between a.d. 555 and 950. Collectively, the Fort Ross Orchard and Archae Camp sites suggest pre-1906 ruptures at a.d. 1660–1812, a.d. 1220–1380, a.d. 1040–1190, and a.d. 555–950. The time windows for these ruptures are consistent with results from other sites on the North Coast segment of the San Andreas fault. However, additional information on the late Holocene history of rupture events on adjacent fault segments is needed to evaluate whether the long-term behavior of the San Andreas fault involves a mix of large, 1906-type ruptures and shorter, segment-specific ruptures.

Late Quaternary Fold Deformation along the Northridge Hills Fault, Northridge, California: Deformation Coincident with Past Northridge Blind-Thrust Earthquakes and Other Nearby Structures?

Paleoseismic investigation of the Northridge Hills fault in the northern San Fernando Valley, California, helps assess the timing and style of near-surface late Quaternary deformation in the epicentral area of the 1994 Northridge earthquake. The Northridge Hills fault, a 15-km-long, north-dipping reverse fault, exhibits geomorphic evidence of late Quaternary surface deformation, including topographic scarps across late Quaternary fluvial terraces and aligned alluvial-fan apices on the footwall block. We excavated one 40-m-long trench and six test pits, and drilled nine boreholes across a 2-m-high scarp developed on a probable Holocene fluvial terrace adjacent to Aliso Canyon Wash. A continuous clayey gravel identified in the trench, test pits, and boreholes defines a south-facing monocline with 6 ± 1 m of vertical separation across the Northridge Hills fault. Based on pedochronology, the clayey gravel ranges in age from 6 to 30 ka. The borehole data also suggest that an unconformity developed on the Plio-Pleistocene Saugus Formation is warped into a monocline that has 13 ± 2 m of vertical separation across the fault. These preliminary data yield a dip-slip rate of 1.0 ± 0.7 mm/yr for the Northridge Hills fault. The absence of distinct scarp-derived colluvium in trench exposures at the base of the scarp and secondary brittle fracturing or faulting suggests that the monocline is related to folding during small, incremental uplifts rather than large uplifts that generate distinct scarp relief. We postulate that such uplift could be produced via moderate-magnitude earthquakes ( M W 6¼) on the Northridge Hills fault, or secondary deformation induced by earthquakes on other faults. For instance, evidence of surface uplift near the trench site during or following the 1994 earthquake suggests that all or part of the observed deformation is a result of secondary slip on the Northridge Hills fault produced by movement on the underlying Northridge blind reverse fault or other nearby large structures. Based on our geologic investigations, the distribution of aftershocks following the 1994 earthquake, and pre- and post-1994 leveling and geodetic surveys, we interpret that the Northridge Hills fault underwent triggered slip during the 1994 earthquake.

Fault Rupture Assessments for High-Pressure Pipelines in the Southern San Francisco Bay Area, California

The San Andreas, Hayward, and Calaveras faults are major active faults that traverse the San Francisco Bay area in northern California, and may produce surface rupture during large earthquakes. We assessed the entire Pacific Gas & Electric Company natural gas transmission system in northern California, and identified several locations where primary pipelines cross these faults. The goal of this effort was to develop reasonable measures for mitigating fault-rupture hazards during the occurrence of various earthquake scenarios. Because fault creep (e.g., slow, progressive movement in the absence of large earthquakes) occurs at the pipeline fault crossings, we developed an innovative approach that accounts for the reduction in expected surface displacement, as a result of fault creep, during a large earthquake. In addition, we used recently developed data on the distribution of displacement across fault zones to provide likely scenarios of the seismic demand on each pipeline. Our overall approach involves (1) identifying primary, high-hazard fault crossings throughout the pipeline system, (2) delineating the location, width, and orientation of the active fault zone at specific fault-crossing sites, (3) characterizing the likely amount, direction, and distribution of expected surface fault displacement at these sites, (4) evaluating geotechnical soil conditions at the fault crossings, (5) modeling pipeline response, and (6) developing mitigation measures. At specific fault crossings, we documented fault locations, widths, and orientations on the basis of detailed field mapping and exploratory trenching. We estimated fault displacements based on expected earthquake magnitude, and then adjusted these values to account for the effects of fault creep at the ground surface. Fault creep decreases the amount of expected surface fault rupture, such that sites having high creep rates are expected to experience proportionally less surface displacement during a large earthquake. Lastly, we modeled the expected amount of surface offset to reflect the distribution of offset across the fault zone, based on data from historical surface ruptures throughout the world. Where specific fault crossings contain a single primary fault strand, we estimated that 85% of the total surface offset occurs on the main fault and the remainder occurs as secondary deformation. At sites where the pipeline crosses multiple active fault strands in a broad zone, we consider complex rupture distributions. Using this approach yields realistic, appropriately conservative estimates of surface displacement for assessing seismic demands on the pipelines.Copyright

A Unique Pipeline Fault Crossing Design for a Highly Focused Fault

This paper describes the development of a unique pipeline fault crossing design upgrade for a 22-inch (559 mm) diameter Pacific Gas & Electric Company (PG&E) gas transmission line where it crosses the Calaveras fault near Sunol, California. The new design is capable of withstanding significant levels of horizontal fault offset while minimizing the deformation demands experienced by the pipeline. This unique design concept is applicable to fault crossings with well defined fault locations and highly localized fault offset profiles (e.g., for this fault, 85% of the offset is expected to occur within ±5 feet (±1.5 m) from the center of the fault trace, which was precisely located by field trenching studies). Relative to the original fault crossing design, the new design provides a more favorable “local” fault crossing angle “β” (β = 73° for the original design vs. β = 95° for the new design). The angle change is accomplished by installing an offset section of the pipeline adjacent to the fault such that the fault crosses the pipeline in the middle of a tangent section in the nearest offsetting leg. The four bends used to fabricate the offset section are cold bends with an average radius of 76.4 feet (23.3 m). The entire mitigated section of the pipeline is buried in a select sand trench. For this design configuration, right lateral fault motion results in (a) a “closing” action within the two adjacent cold bends located on either side of the fault and (b) a net tension force in the pipe (due to the obtuse β value) centered on the tangent section of the offsetting leg containing the fault crossing. The net tension force in the offsetting leg results in an “opening” action within the two adjacent cold bends on either side of the fault. By adjusting the local fault crossing angle β, the “bend opening” action that results from pipe extension across the fault can be made to nearly offset the “bend closing” action induced by the transverse component of the fault offset. The use of a select sand backfill in the retrofit section allows the bends to engage the soil with relatively low transverse and longitudinal resistance thereby enhancing the overall flexibility/compliance of the fault crossing design. Implementation of this unique design concept at the Calaveras fault crossing increased the amount of fault offset required to damage the pipeline from about 7 inches (18 cm) for the “as-built” design to well over 90 inches (2.3 m) for the retrofit.Copyright