FIGURE 11.10 Site map of level line across Pleito thrust fault near townsite of Grapevine, California.

1958, 1965) and by near-field leveling (Sharp and Lienkaemper, 1982; Mei, 1984; Zhu et al., 1984). Broad-scale leveling has also demonstrated recent influx of magma into the crust in the Yellowstone area (Pelton and Smith, 1982); beneath Long Valley caldera in eastern California (Savage and Clark, 1982); beneath Mount Etna (Murray and Guest, 1982); and in the rift zone of northern Iceland (Björnsson et al., 1979). Magmatic inflation of volcanoes has been geodetically monitored for many years at Kilauea and Mauna Loa Volcanoes on Hawaii (Kinoshita et al., 1974; Decker et al., 1983) and more recently at Mount St. Helens (Chadwick et al., 1983).

One of the small-scale, geodetic methods for monitoring fairly large tilt is spirit-level optical tilting (Kinoshita et al., 1974) or dry-tilt as it is popularly termed. The dry-tilt method determines tilt of a plane defined by three or more bench marks by measuring height differences among the bench marks between two separate surveys (Sylvester, 1978; Yamashita, 1981). Both in our work and in Hawaii a shaded precision level is erected at the center of an array of at least three permanent bench marks on each of which three precise Invar leveling rods are erected simultaneously (Figure 11.16). We take care to choose sites for dry tilt that are reasonably flat; that have radial symmetry; and that are not near oil and water wells, landslides, or recently imposed construction loads such as bridges, buildings, and land fills. In our experience, the noisiest data are obtained from tilt sites on ridge crests and forested areas on bedrock, whereas data showing least noise are obtained from sites on wide open flats underlain by relatively thick deposits of alluvium.

In Hawaii the dry-tilt measurements complement those of borehole tiltmeters, and short-base (3 m) and long-base (50 m) water-tube tiltmeters. With superior equipment in good adjustment, with careful attention to detail, and with rigorous systematic measurement procedure, a resolution of from 2 to 3 µrad has been achieved in Hawaii as shown in comparative tests with the 50-m water-tube tiltmeter. In general, however, resolution of tilt with three point arrays ranges from 5 to 10 µrad (Isacks et al., 1978; Björnsson et al., 1979; Savage et al., 1979b; Decker et al., 1983; Otway et al., 1984). Therefore, the dry-tilt method is best suited for monitoring large and fairly rapid tilts, from tens to hundreds of microradians per day which may be expected to accompany magmatic inflation of volcanoes (Dzurisin et al., 1982a,b; 1983; Fiske and Shepard, 1982; Chadwick et al., 1983). For more precise determination of tectonic tilt, long L- and T-shaped and closed arrays of bench marks having apertures of from 500 to 1000 m $

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