Dr. Kimberly Blisniuk has been awarded an NSF Earth Science Post Doctoral Fellowship to carry out a research and education plan at University of California at Berkeley. She will study how deformation within continental strike-slip fault systems is partitioned along and across individual fault strands and fault zones over 1,000s to 100,000s year timescales. More specifically, the project is aimed at understanding the kinematic and mechanical evolution of the San Andreas Fault System in southern California by: (1) mapping offset landforms through wavelet analysis of high-resolution topography data and geologic fieldwork, (2) dating displaced landforms to obtain precise estimates of fault slip rates at multiple time scales by applying terrestrial cosmogenic nuclide exposure age dating of surface clasts and U-series dating of pedogenic carbonate coatings on subsurface clasts, and (3) using those observations to formulate first-order distinct element models simulating the behavior and interactions of faults observed in the field.
A particularly promising aspect of this study is that cosmogenic nuclide dating and U-series dating are relatively new dating methods that can be used to reliably date late Quaternary sediments and landforms measurably offset along active faults. Prior to the development of these methods, the dating of fault motion has only been possible at the time scales of geodetic and paleoseismological studies, typically years to thousands of years, or from total displacements of geological units over hundreds of thousands to millions of years. As these new dating techniques permit the dating of fault displacements at time scales of thousands to several hundreds of thousands of years, they provide the "missing link" that existed in geochronologic studies of fault systems. Accordingly, detailed studies of fault systems during this time interval combined with mechanical models that mimic the behavior of faults observed in the field can reveal previously unrecognized aspects of fault mechanics and fault system evolution. Such information can be used to refine and improve models for earthquake recurrence used in assessing seismic hazards. This fellowship will not only develop K. Blisniuk's skills as a researcher but it will also further her skills as an educator through her co-teaching of upper division geology courses and mentoring of undergraduate and graduate students in neotectonic and geochronologic studies at the University of California, Berkeley.
The potential for a large-magnitude earthquake (Mw ≥ 6.7) in southern California is generally considered high (Working Group on California Earthquake Probabilities, 2007). Of the numerous active faults that can generate large earthquakes in southern California, the three major fault strands (Mission Creek, Banning, and Garnet Hill, from north to south) that comprise much of the accumulated strain on the main structure of the San Andreas fault zone remains poorly constrained. Each of these fault strands cut through San Gorgonio Pass west of the Los Angeles metropolitan region. To better assess the relative importance of these faults and their potential for a major earthquake, we dated offsets at multiple sites and timescales on the Mission Creek fault strand in the central Indio Hills, near Palm Springs, Ca. Although, previous work on this strand at Biskra Palms, in the southern Indio Hills, demonstrated a slip rate between 12 and 22 mm/yr, our results from dating offset landforms since ~90 ka, ~70 ka, ~25 ka and ~2.5 ka indicate that the Mission Creek fault strand in the central Indio Hills has occurred at a relatively constant and unexpectedly high rate of ~21-25 mm/yr. These higher rates of deformation on the Mission Creek fault strand suggest it is the most active fault in the region since ~90 ka. When combined with published paleoseismic studies, which show an average earthquake recurrence interval of 225 years for the past 5 events since 900 AD (Fumal et al., 2002), these data imply an average slip-per-event of ~4.5 m. Since the last earthquake to rupture this section of the Mission Creek fault strand occurred over 300 years ago (ca. 1690), this indicates that ca. 6.0 to 7.5 m of strain may have accumulated since the last surface-rupturing event. While additional work is needed to better understand how slip along the San Andreas fault zone is partitioned in the northwestern Indio Hills, the new data underscore the seismic hazard posed by the Mission Creek fault strandin this region.
