We studied the M6.1, 17 May, 1993, Eureka Valley earthquake on the border between California and Nevada. This earthquake occurred at a depth of 13 km along the west side of the Eureka Valley (Fig. 1), one of the westernmost valleys in the wide zone of extension distributed across the Great Basin. The focal mechanism of the main shock indicates that the earthquake ruptured a north-northeast-striking fault, steeply dipping to the west. The aftershocks define a north-northwest trend, and include two shocks of M~5 and several of M>4. Small surface ruptures formed in the central part of the Eureka valley (arrow A1 in Fig. 1). ERS-1, 3-pass interferograms show that the 17 May, 1993, M=6.1, Eureka Valley earthquake produced an elongated subsidence basin oriented north-northwest, parallel to the trend defined by the aftershock distribution, whereas the source mechanism of the earthquake implies a north-northeast striking normal fault. The ±3 mm accuracy of the radar observed displacement map over short spatial scales, allowed identification of the main surface rupture associated with the event. These observations suggest that the rupture initiated at depth and propagated diagonally upward and southward on a west dipping, north-northeast fault plane, reactivating the largest escarpment in the Saline Range (Peltzer and Rosen, 1995).
Fig. 1: Active fault map of the Eureka Valley region over a shaded USGS topographic map. Faults are from interpretation of Landsat TM image. White contour depicts projected fault plane as modeled in this study. Light shaded area in fault plane is zone of non-zero slip. Arrow A1 shows location of surface breaks recognized in the field after the earthquake (Suzanne Hecker, unpublished data). Arrow A2 points to fault segment where seismic rupture reached the surface, as inferred from the radar data. Dashed line delineates area shown in Fig. 2. Large star indicates location of main shock, small stars, locations of aftershocks of magnitude greater than 4.5, and circles smaller aftershocks (J. Louie, personal communication). Focal mechanism of the main shock is depicted (H. K. Thio, personal communication). (After Peltzer and Rosen, 1995).
Click on figure to display entire map.
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Fig. 2: SAR interferograms formed from combination of the 14 Sep. 1992 - 23 Nov. 1992 (A), and the 23 Nov. 1992 - 8 Nov. 1993 (B) SAR images, and by double difference (C) of interferograms in (A) and (B). Black areas in (B) and (C) are zones of low coherence that have been masked before unwrapping the phase (12). They correspond to zones of major surface changes such as over sand dunes or cultivated fields, and to zones of phase ambiguities produced by overlays on steep slopes facing toward the satellite. Black masks are absent in interferogram (A) because it has not been unwrapped. The phase value is color coded and laid over the radar intensity image for reference. The interferometric baseline of the second image pair (B) being smaller than that of the first pair (A), the fringe spacing is larger in the map of Fig. 2B than it is in the map of Fig. 2A. In (A), a full color cycle corresponds to an elevation difference of 50 m. In (B), a full color cycle can be due to either an elevation difference of 78 m, 28 mm of line of sight surface displacement (half the radar wavelength), or a combination of the two. In the 3-pass interferogram (C), where the topography has been removed, a full color cycle corresponds to a displacement of the ground of 28 mm in the direction of the satellite. White lines in (C) indicate location of profiles shown in Fig. 3. (After Peltzer and Rosen, 1995).
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| Fig. 3: Profiles of surface displacement observed with the radar (solid lines) and predicted with an elastic dislocation model (dashed lines) assuming a 15 km-long, 16 km-wide fault plane striking N7 deg. E, dipping 50 deg. to the west, and a north-plunging distribution of slip on the fault plane (Fig. 4). Profiles are corrected for the geometric distortion induced by topography in radar imagery. Arrow A1 in profile AA' indicates location where surface cracks were observed in the field. Arrow A2 in profile CC' points to the place where the main rupture reached the surface. (After Peltzer and Rosen, 1995). | Fig. 4: Sketch showing a plunging slip distribution on a fault plane with dip angle a. (After Peltzer and Rosen, 1995). |
