STRESS ORIENTATION DETERMINED FROM FAULT SLIP DATA IN HAMPEL WASH AREA, NEVADA, AND ITS RELATION TO CONTEMPORARY REGIONAL STRESS FIELD

Fault-slip data were collected from an area of relatively young faulting in a seismically active part of the Nevada Test Site 12 km NW of Mercury, Nevada. The data come primarily from intensely faulted Miocene tuffaceous sedimentary rocks in Hampel Wash, which is bounded on the north by the Quaternary ENE trending Rock Valley fault and on the south by a parallel unnamed fault. Data from faults with known sense of displacement exhibit a bimodal distribution of slip angles (rakes). Faults exhibiting steep rakes (typically 75 ° to 90 °) cluster about a N30 °-35 °E strike; most dip 65 ° to 80 ° . Faults having shallow rakes (generally less than 20 °) exhibit a wide range of strikes (from N6 °W to N80 °E) and mostly dip between 80 ° and 90 °. The predominant N30 °-35 °E strike of the steep-rake faults and the quasi-conjugate nature of a consistent subset of the shallowrake faults suggest a maximum horizontal stress orientation of about N30 °-35 °E and a least horizontal principal stress direction of N55 °-60 °W. Analysis of the data using a least squares iterative inversion to determine a mean deviatoric principal stress tensor indicatesa normal-faulting stress regime (S1 vertical) with principal stress axes in approximately horizontal and vertical directions (S1, trend = N19 °E and plunge = 82 °N; S2, N30 °E and 8 °S; and S3, N60 °W and 2 °E). The maximum horizontal stress, S2, was found to be nearly intermediate in magnitude between S1 and S3. The N60 °W least horizontal principal stress orientation obtained from the fault-slip inversion agrees with our geometric analysis of the data and is consistent with a modem least horizontal principal stress orientation of N50 °-70 °W inferred from earthquake focal mechanisms, well bore breakouts, and hydraulic fracturing measurements in the vicinity of the Nevada Test Site. This solution fits all the data well, except for a subset of strike-slip faults that strike N30 °-45 °E, subparallel to the normal faults of the data set. Nearly pure dip-slip and pure strike-slip movement on similarly oriented faults, however, cannot be accommodated in a single stress regime. Superposed sets of striae observed on some faults suggest temporal rotations of the regional stress field or local rotations within the region of the fault zone