We present photomosaics and logs of the walls of trenches excavated for a paleoseismic study at Thousand Palms Oasis (Fig. 1). The site
is located on the Mission Creek strand of the San Andreas fault zone, one of two major active strands of the fault in the Indio Hills along the
northeast margin of the Coachella Valley (Fig. 2). The Coachella Valley section is the most poorly understood major part of the San Andreas
fault with regard to slip rate and timing of past large-magnitude earthquakes, and therefore earthquake hazard. No large earthquakes have
occurred for more than three centuries, the longest elapsed time for any part of the southern San Andreas fault. In spite of this, the Working
Group on California Earthquake Probabilities (1995) assigned the lowest 30-year conditional probability on the southern San Andreas fault
to the Coachella Valley. Models of the behavior of this part of the fault, however, have been based on very limited geologic data.
The Thousand Palms Oasis is an attractive location for paleoseismic study primarily because of the well-bedded late Holocene
sedimentary deposits with abundant layers of organic matter for radiocarbon dating necessary to constrain the timing of large prehistoric
earthquakes. Previous attempts to develop a chronology of paleoearthquakes for the region have been hindered by the scarcity of in-situ 14C-dateable
material for age control in this desert environment. Also, the fault in the vicinity of Thousand Palms Oasis consists of a single trace
that is well expressed, both geomorphically and as a vegetation lineament (Figs. 2, 3). Results of our investigations are discussed in Fumal et
al. (2002) and indicate that four and probably five surface-rupturing earthquakes occurred along this part of the fault during the past 1200
years. The average recurrence time for these earthquakes is 215 ± 25 years, although interevent times may have been as short as a few
decades or as long as 400 years. Thus, although the elapsed time since the most recent earthquake, about 320 years, is about 50% longer than
the average recurrence time, it is not necessarily unprecedented.
