Map of fault scarps formed in unconsolidated sediments, Tooele 1 degree by 2 degrees quadrangle, northwestern Utah

Abstract

This report is one of a series of studies of western Utah (Anderson and Bucknam, 1979; Bucknam and Anderson, 1979b), eastern Nevada (Barnhard, 1985), and central Nevada (Wallace, 1979) that shows the distribution, relative age, and amount and extent of surface rupture on Quaternary fault scarps. Previous paleoseismicity studies in the Tooele 1°x2° quadrangle include reports by Gilbert (1928), Bucknam (1977), Anderson and Miller (1980), Bucknam and others (1980), and Everitt and Kaliser (1980). Geologic and geophysical data pertinent to paleoseismicity of the Tooele 1°x2° quadrangle can be found in reports by Mikulich and Smith (1974), Cook and others (1980), Smith and Bruhn (1984), and Arabasz and others (1987). The present study is a continuation of mapping fault scarps in 1°x2° quadrangles in the Basin and Range province; the purpose is to determine the youngest surface-faulting age and distribution of fault scarps for earthquake-hazards analysis. The accompanying map shows 10 areas containing late Quaternary fault scarps formed on unconsolidated sediments in the Tooele 1°x2° quadrangle, northwestern Utah. The faulted sediments include sandy, pebbly, to bouldery colluvium, alluvium, and lacustrine deposits. They are inferred to have been deposited in Quaternary time during the latest phase of basin-filling sedimentation. Because the map shows only scarps on unconsolidated sediments, it does not show all faults that could produce earthquakes. A comprehensive map of all Quaternary faults would also include those in bedrock and at the contact of bedrock and alluvium, as well as Quaternary faults that appear only as lineaments in surficial deposits. Initial study involved photointerpretation of high-quality 1:60,000-scale black and white aerial photographs of the map area; offsets as small as 1 m could be detected from these photos if the faults cut planar surfaces of considerable extent. A map of suspected fault scarps, identified from the photointerpretation, was compiled at a scale of 1:250,000 and served as a guide for subsequent field studies in conjunction with a similar map from Bucknam (1977). We have chosen not to connect discontinuous segments of aligned fault scarps unless there is compelling evidence of continuous surface rupture. Field studies consisted of (1) confirming scarps as fault related and deleting from the preliminary map those features that were not fault related or that had no surface offset (lineaments), (2) searching for stratigraphic indicators of amount and age of offset, and (3) measuring surface profiles according to procedures described in Bucknam and Anderson (1979a). We profiled selected fault scarps where they were sufficiently continuous and unmodified by fluvial deposition or erosion. In the early morning or late afternoon when the sun was at 1ow angles, additional field searches were made for scarps resulting from ground ruptures that had not been identified during the photo interpretation phase of the study. No additional scarps were found. Techniques that yield the relative age of fault scarps can be used as a guide to the frequency and location of relatively large earthquakes. For large regions, geologically determined rates of seismic activity in the Basin and Range province are generally in good agreement with rates determined from historic seismicity (Bucknam and Algermissen, 1984). Fault-scarp studies such as this have been used as a guide in locating boundaries of seismic source zones used in probabilistic earthquake hazard estimates of the Basin and Range province (Thenhaus and Wentworth, 1982; Bucknam and Thenhaus, 1983). We will first discuss the geologic setting of the map area and attributes of fault-scarp morphology, and then discuss the individual fault zones.