Nathan A. Toké

Surface Fault Slip Associated with the 2004 Parkfield, California, Earthquake

Surface fracturing occurred along the San Andreas fault, the subparallel Southwest Fracture Zone, and six secondary faults in association with the 28 September 2004 ( M 6.0) Parkfield earthquake. Fractures formed discontinuous breaks along a 32-km-long stretch of the San Andreas fault. Sense of slip was right lateral; only locally was there a minor (1–11 mm) vertical component of slip. Right-lateral slip in the first few weeks after the event, early in its afterslip period, ranged from 1 to 44 mm. Our observations in the weeks following the earthquake indicated that the highest slip values are in the Middle Mountain area, northwest of the mainshock epicenter (creepmeter measurements indicate a similar distribution of slip). Surface slip along the San Andreas fault developed soon after the mainshock; field checks in the area near Parkfield and about 5 km to the southeast indicated that surface slip developed more than 1 hr but generally less than 1 day after the event. Slip along the Southwest Fracture Zone developed coseismically and extended about 8 km. Sense of slip was right lateral; locally there was a minor to moderate (1–29 mm) vertical component of slip. Right-lateral slip ranged from 1 to 41 mm. Surface slip along secondary faults was right lateral; the right-lateral component of slip ranged from 3 to 5 mm. Surface slip in the 1966 and 2004 events occurred along both the San Andreas fault and the Southwest Fracture Zone. In 1966 the length of ground breakage along the San Andreas fault extended 5 km longer than that mapped in 2004. In contrast, the length of ground breakage along the Southwest Fracture Zone was the same in both events, yet the surface fractures were more continuous in 2004. Surface slip on secondary faults in 2004 indicated previously unmapped structural connections between the San Andreas fault and the Southwest Fracture Zone, further revealing aspects of the structural setting and fault interactions in the Parkfield area.

Applications of airborne and terrestrial laser scanning to paleoseismology

Paleoseismic investigations aim to document past earthquake characteristics such as rupture location, frequency, distribution of slip, and ground shaking intensity—critical parameters for improved understanding of earthquake processes and refined earthquake forecasts. These investigations increasingly rely on high-resolution ( 2 /m. This situation refines interpretations of PBR exhumation rates and thus their effectiveness as paleoseismometers. Given that earthquakes disrupt Earth9s surface at centimeter to meter scales and that depositional and erosional responses typically operate on similar scales, ALS and TLS provide the absolute measurement capability sufficient to characterize these changes in challenging geometric arrangements, and thus demonstrate their value as effective analytical tools in paleoseismology.

Late Holocene slip rate of the San Andreas fault and its accommodation by creep and moderate-magnitude earthquakes at Parkfield, California

Investigation of a right-laterally offset channel at the Miller9s Field paleoseismic site yields a late Holocene slip rate of 26.2 +6.4/−4.3 mm/yr (1σ) for the main trace of the San Andreas fault at Parkfield, California. This is the first well-documented geologic slip rate between the Carrizo and creeping sections of the San Andreas fault. This rate is lower than Holocene measurements along the Carrizo Plain and rates implied by far-field geodetic measurements (∼35 mm/yr). However, the rate is consistent with historical slip rates, measured to the northwest, along the creeping section of the San Andreas fault (

Reassessment of a Slip Budget along the Parkfield Segment of the San Andreas Fault

Historically, the Parkfield segment has represented a transition in fault behavior along the San Andreas fault (saf). Despite a ∼33 mm/yr long-term slip rate along the saf, slip has not been observed on the Cholame segment since the great 1857 Fort Tejon earthquake. During that time, the Parkfield segment has experienced slip from at least six ∼ M 6.0 earthquakes, aseismic fault creep, and from minor earthquakes. Data from aseismic slip-rate studies and historical earthquake studies allow us to estimate the total slip released along this portion of the saf since 1857. Assuming the saf should slip at the long-term slip rate, a slip deficit of ∼5 m exists along the Cholame segment since 1857. The slip deficit is approximately equivalent to 1857 offsets measured on the Cholame and the southeast portion of the Parkfield segment. Thus, the slip deficit in southeast Parkfield and Cholame may be as great as the slip accommodated along these segments in 1857. The slip deficit abruptly decreases to the northwest across the Parkfield segment. It is < 2 m near the town of Parkfield and ∼0 m northwest of Middle Mountain. An ∼ M 7 event rupturing all or part of the Cholame segment and the southeastern Parkfield segment (slip decreasing to the northwest) would release the accumulated slip and is plausible. Importantly, this study also shows that the change in the pattern of strain release occurs in the middle of the Parkfield segment, rather than at its ends. Online material : Tables showing aseismic and coseismic slip estimations and calculations.

Paleoseismic and Postseismic Observations of Surface Slip along the Parkfield Segment of the San Andreas Fault

Parkfield is considered a transitional segment along the San Andreas fault (saf) between continuous fault creep to the northwest and segments to the southeast that last slipped in the great 1857 Fort Tejon earthquake. Historically, creep and recurring M 6.0 earthquakes have been observed at Parkfield, California, but the segment’s relevance in great saf earthquakes has remained uncertain. This paleoseismic study of the central Parkfield segment provides a >2000-year record of tectonically deformed fluvial and sag stratigraphy. Two fault-perpendicular excavations across a pressure ridge and a sag pond ∼200 m north of Carr Hill exposed five primary fault zones displaying apparent vertical offsets, upward splaying clay shear bands, and warped stratigraphy. Four of five fault zones extended into the uppermost stratigraphy, suggesting recent surface offset and fault creep. Several antithetic fault splays and one primary fault zone displayed upward terminations, but strong indicators of large-magnitude earthquakes with meter-scale surface offset and rupture such as filled fissures and colluvial scarp deposits were not observed. The absence of evidence for large-magnitude earthquakes does not preclude the possibility of 1857-style earthquakes extending into the Parkfield segment. However, all deformation exposed within these trenches is consistent with repeated small ground rupture and fault creep. The 2004 M 6.0 Parkfield earthquake ruptured through the site and activated at least three of the five fault zones exposed in our excavations. Comparison between 2004 vertical offset and vertical offsets within the exposed stratigraphy suggests a recurrence interval between 8 and 188 years for M 6.0 earthquakes at Parkfield. Online material : Supplemental unit descriptions, unanalyzed radiocarbon samples, and trench logs and photos.

Fault zone regulation, seismic hazard, and social vulnerability in Los Angeles, California: Hazard or urban amenity?

Public perception and regulation of environmental hazards are important factors in the development and configuration of cities. Throughout California, probabilistic seismic hazard mapping and geologic investigations of active faults have spatially quantified earthquake hazard. In Los Angeles, these analyses have informed earthquake engineering, public awareness, the insurance industry, and the government regulation of developments near faults. Understanding the impact of natural hazards regulation on the social and built geography of cities is vital for informing future science and policy directions. We constructed a relative social vulnerability index classification for Los Angeles to examine the social condition within regions of significant seismic hazard, including areas regulated as Alquist-Priolo (AP) Act earthquake fault zones. Despite hazard disclosures, social vulnerability is lowest within AP regulatory zones and vulnerability increases with distance from them. Because the AP Act requires building setbacks from active faults, newer developments in these zones are bisected by parks. Parcel-level analysis demonstrates that homes adjacent to these fault zone parks are the most valuable in their neighborhoods. At a broad scale, a Landsat-based normalized difference vegetation index shows that greenness near AP zones is greater than the rest of the metropolitan area. In the parks-poor city of Los Angeles, fault zone regulation has contributed to the construction of park space within areas of earthquake hazard, thus transforming zones of natural hazard into amenities, attracting populations of relatively high social status, and demonstrating that the distribution of social vulnerability is sometimes more strongly tied to amenities than hazards.