James P. McCalpin

Thermoluminescence dating of fault‐scarp‐derived colluvium: Deciphering the timing of paleoearthquakes on the Weber Segment of the Wasatch Fault Zone, north central Utah

The timing of middle to late Holocene faulting on the Weber segment of the Wasatch fault zone, Utah, is constrained by thermoluminescence (TL) and radiocarbon age estimates on fine-grained, fault-related colluvial sediments. The stratigraphy in two trenches excavated across fault scarps is characterized by a stack of three colluvial wedges, deposited in response to three separate faulting events, the oldest of which buried a soil developed on a middle Holocene debris flow. Thermoluminescence age estimates by the partial and total bleach methods and the regeneration method on fine-grained colluvium from the trenches agree within 1 sigma and are concordant with the radiocarbon chronology. A synthesis of the TL and 14C age estimates indicate that these three faulting events occurred sometime between 4500 and 3500, between 3200 and 2500, and between 1400 and 1000 years ago. Detailed investigation of a sequence of fine-grained, scarp-derived distal colluvium shows that much of the sediment was deposited during <600-year intervals immediately after faulting. The sedimentation rate of colluvium is inferred to increase shortly after faulting, and TL dating of these sediments provides additional information to constrain the timing of faulting events.

Reevaluation of Holocene faulting at the Kaysville site, Weber segment of the Wasatch fault zone, Utah

The 1978 Kaysville, Utah, trench excavated by Swan and others (1980) across a large graben of the Weber segment of the Wasatch fault zone was reexcavated in 1988 to reevaluate the timing and nature of Holocene faulting. Relogging of the trench reveals evidence for five or six faulting events younger than the Provo phase of Lake Bonneville (circa 13,000 14C years B.P.). Geometric reconstruction of net vertical offset in the last three events suggests a variation in coseismic vertical displacement at this site, ranging from a net of 1.4–3.4 m per event. The three latest faulting events occurred at shortly before 0.6–0.8 ka, 2.8 ± 0.7 ka, and circa 3.8–7.9 ka. Earlier events cannot be directly dated because older graben-fill sediments yielded thermoluminescence ages older than the time of deposition, and some scarp-derived colluvial wedges beneath the trench floor were not exposed. The two younger faulting events we recognize at Kaysville correlate reasonably well with faulting events on the same segment 25 km north near East Ogden, Utah, at circa 0.8–1.2 ka and 2.5–3.0 ka (Forman and others, 1991), whereas the earlier Kaysville event is significantly older than the earliest (3.5–4.0 ka) event dated at East Ogden. The 3.5–4.0 ka ground rupture recognized at East Ogden may have died out at a subsegment boundary between the two trench sites within the 61-km-long Weber segment.

Holocene paleoseismicity of the Tunka fault, Baikal rift, Russia

The Tunka fault is a major normal-oblique transform fault within the NNE trending Baikal rift that displays geomorphic evidence of recurrent Quaternary movement. A flight of six fanhead terraces of the Kyngarga River is displaced by several parallel faults and has scarps up to 32 m high at Arshan. The main fault zone is exposed in an abandoned gravel quarry about 1 km east of the Kyngarga River. Thirteen radiocarbon dates from the quarry, a shallow trench in a graben, and natural streamcuts constrain the timing of the latest two or three Holocene paleoearthquakes. The latest earthquake is bracketed by the 1024–1315 calendar years (cal. year) B.P. age of an unfaulted terrace and by the 1947–2179 cal. year B.P. age of a soil buried by scarp colluvium in the graben trench. The penultimate earthquake is also relatively well constrained, assuming that the displacement event slightly younger than 7091–7867 cal. year B.P. in the upper quarry exposure is the same as the fissuring event slightly older than 6733–7385 cal. year B.P. in the lower quarry exposure. Evidence for earlier event(s) between 9.2–12.7 ka depends on ambiguous stratigraphic evidence in the lower quarry exposure. On the basis of only the two well-dated earthquakes, the recurrence interval at Arshan may range from 2.9 to 6.8 ka for earthquake displacements of at least 1.3 m or a slip rate of 0.19–0.44 mm/yr. Intermediate-term (100 ka) and long-term (circa 500 ka) slip rates computed from terrace age estimates and fault scarp heights are 0.08–0.16 mm/yr and 0.07–0.11 mm/yr, respectively, rates that are considerably lower than the late Holocene rate and the approximately 0.5 mm/yr that might be inferred from the tectonic geomorphology of the Tunka Range front.

Sedimentology of Fault-Scarp-Derived Colluvium from the 1983 Borah Peak Rupture, Central Idaho

ABSTRACT Ten undisturbed samples of scarp-derived colluvium from the 1983 Borah Peak, Idaho, fault scarp were subjected to laboratory grain-size and fabric measurements to characterize typical colluvium from a Basin and Range fault scarp. Colluvial deposits consisted of poorly-sorted pebble gravels (mean diameter -0.3 to -3.75 ; 1.2 to 13.5 mm) with 25-79% matrix (< 4 mm). Stereograms of long-axis orientations of 115 clasts from each colluvial wedge yielded girdle-type distributions, with typical eigenvalue ratios of In S1/S2 = 0.05-0.50 and In S2/S3 = 0.3-1.0 These girdle distributions indicate preferred orientation in angle-of-repose planes coincident with colluvial wedge depositional planes. Short-axis stereograms display single or du l maxima with mean bearing parallel to transport direction and plunge perpendicular to the colluvial wedge surface. Clast fabric strength is not strongly correlated with scarp aspect (corr. coeff. = 0.03 to 0.68) or with the slope of the faulted geomorphic surface (corr. coeff. = -0.16 to +0.57), but it improves with increasing matrix content (corr. coeff. = -0.04 to -0.42). Fabrics in measured wedges are weaker than those reported for other colluviums and talus and resemble fabrics in debris flows. However, the stronger girdle tendencies and distinct orientation subgroups in scarp-derived colluvium should distinguish it from suspected debris flow deposits in fault-zone exposures.

Chapter 2A Field Techniques in Paleoseismology—Terrestrial Environments

Publisher Summary This chapter describes the common techniques used on-land to collect field data in most paleoseismic investigations, regardless of the tectonic setting. The techniques are classified into two broad categories, depending on whether the paleoseismic features are landforms or have stratigraphic expression. Basic geomorphic techniques involve locating paleoseismic features with remotely sensed imagery and making detailed topographic maps of paleoseismic landforms. Stratigraphic techniques emphasize the finding of buried faults with geophysical methods and mapping of paleoseismic deformation in subsurface exposures. The techniques described in the chapter are mostly basic methods of geologic investigation as typically applied to unconsolidated sediments by quaternary geologists. Many of the generic techniques—such as fault scarp profiling and trench logging—are developed specifically for paleoseismic studies. Other techniques—such as geomorphic mapping, shallow coring, and shallow geophysics—are developed for other types of investigations and have been adapted for use in paleoseismology.

Chapter 1 Introduction to Paleoseismology

Publisher Summary Paleoseismology is the study of prehistoric earthquakes, especially their location, timing, and size. Paleoseismology differs from more general geologic studies of slow to rapid crustal movements during the late Cenozoic (for example, neotectonics) in its focus on the almost instantaneous deformation of landforms and sediments during earthquakes. This focus permits the study of the distribution of individual paleoearthquakes in space and over time periods of thousands or tens of thousands of years. Such long paleoseismic histories help understand many aspects of neotectonics, such as regional patterns of seismicity and tectonic deformation and the seismogenic behavior of specific faults. Paleoseismology is also part of the broader field of earthquake geology, which comprises aspects of modern instrumental studies of earthquakes (seismology), tectonics and structural geology, historical surface deformation (geodesy), and the geomorphology of tectonic landscapes (tectonic geomorphology). Paleoseismologists can only study earthquakes producing recognizable deformation in the form of deformed stratigraphic units, displaced landforms, or earthquake-induced sedimentation.

Chapter 2 Field techniques in paleoseismology

Publisher Summary This chapter describes techniques used to collect field data in most paleoseismic investigations, regardless of the tectonic setting. The methods fall into two broad categories, depending on whether paleoseismic features are landforms or have stratigraphic expression. Basic geomorphic techniques include locating paleoseismic features with remotely sensed imagery and making the detailed topographic maps of paleoseismic landforms. Stratigraphic techniques emphasize the mapping of paleoseismic deformation in subsurface exposures and the finding of buried faults with geophysical methods. The chapter also discusses the importance of trench studies in identifying and dating paleoearthquakes. An ideal sequence of paleoseismic investigations is the progress from the regional scale (thousands of square kilometers), to the local scale (a few square kilometers), and then to the site scale (1 hectare to a few square meters). The approach to study paleoseismic reconstruction involves the mapping of quaternary landforms and deposits in the zone of deformation. Surficial geologic mapping helps identify deformed geomorphic surfaces and reveals trends in deformation styles and rates across landforms of different ages. Based on the estimated ages of faulted geomorphic surfaces and/or deposits, and their measured displacements, the following variables can be estimated without recourse to stratigraphic investigation: (1) fault slip or surface deformation rates, (2) displacements or tilting per event, and (3) bracketing ages for deformation events.

Earthquake Magnitude Scales

Several magnitude scales are widely used and each is based on measuring of a specific type of seismic wave, in a specified frequency range, with a certain instrument. The scales commonly used in western countries, in chronological order of development, are local (or Richter) magnitude (ML), surface-wave magnitude (Ms), body-wave magnitude (mb for short period, mB for long period), and moment magnitude (Mw or M). Reviews of these magnitude scales are given by Bath (1981), Kanamori (1983), and dePolo and Slemmons (1990); their interrelations are shown in Figure A.1.1. Throughout this book we prefer to cite magnitudes as moment magnitudes (Mw). If the type magnitude is unknown or unspecified, it it cited as Mw.

Radiocarbon Sampling Techniques

Pretreatment procedures strongly influence the radiocarbon ages of samples that contain multiple organic compounds, such as soils. Styles of pretreatment seem to vary from region to region and between laboratories, which makes comparison of radiocarbon ages between distant regions often difficult. In the western USA the fine (<125 mm) organic fraction is thought to be least susceptible to contamination by younger carbon, so it is physically concentrated from coarse-grained A horizons (Kihl, 1975). Rinses with HCl and NaOH then follow.