|Abstract or Summary
- The state of the knowledge for fault behavior in the northwest Himalaya and California varies dramatically. In the Pakistan and Kashmir Himalaya, few data constrain the role that individual active faults play in accommodating Indo-Eurasian convergence and the relative earthquake hazard across the region. By contrast, the San Andreas fault in California is one of the best-studied fault systems in the world, although seismic hazard models have yet to incorporate certain available geologic data, such as measurements of slip-in-the-last-event. This dissertation addresses the sparsity of earthquake hazard data in the northwest Himalaya, and the problem of how best to utilize available data in hazard models for California by (1) Providing the first quantitative constraints on the latest Pleistocene slip rate and earthquake potential for the thrust front Pakistan; (2) Characterizing the rate and style of upper plate faulting in Kashmir over geomorphic (10⁴ year) time scales; and (3) Creating a standardized database of fault offsets to help test time-dependent and time-independent seismic hazard models for the Uniform California Rupture Forecast.
The Himalayan thrust front in Pakistan is defined by the Salt Range thrust (SRT), the up-dip extension of the plate boundary décollement, the Main Himalayan thrust (MHT). We constrain the convergence rate across the SRT by determining the slip rate for the Kalabagh fault (KF), a tear fault that is linked with the SRT at depth. Based on the age and offset of two alluvial fan apexes from their source canyons, we estimate a slip rate of between 9 and 27 mm/yr (~12-17 mm/yr best estimate) for the KF-SRT fault system. This rate matches well with the geodetically-constrained creep rate for the MHT at depth, suggesting the entire slip budget for the Pakistan Himalaya is accommodated at the thrust front. Because the SRT is cored by salt, the earthquake potential for the fault is inferred to be low, although evidence for seismogenic Holocene rupture on the Kalabagh fault, which is also locally lined with salt, suggests that the frontal fault ruptures in plate boundary earthquakes on the MHT. The primary implications of these findings are that convergence in the Pakistan Himalaya is focused at the thrust front rather than distributed between different faults across the plate boundary.
In the Kashmir Himalaya, multiple active faults along the plate boundary suggest that Indo-Eurasian convergence is partitioned between the thrust front and faults to the north. To test how much deformation occurs within the overriding plate, we characterized deformation for the Balapora fault, a high-angle reverse fault on the southwest side of the Kashmir Valley. Based on dated offset stream terraces and alluvial fans, the slip rate for the Balapora fault is consistently between 0.3 and 0.5 mm/yr over time scales varying by an order magnitude between about 40 ka and 400 ka. These slip rates translate to shortening rates of 0.1 mm/yr, or less than 1% of the convergence rate across the Kashmir Himalaya. Earthquake recurrence for the Balapora fault is several thousand years, which is consistent with the low slip rate for the fault. The inference is thus that, the majority of convergence in the Kashmir Himalaya is accommodated near the thrust front, as in the Pakistan Himalaya.
For California, a new database was created from thousands of measurements of slip resulting from one or more historical to prehistoric earthquakes for use in seismic hazard models. A new rating scheme characterizes the quality of the offsets. Multiple methods to estimate slip during the last event, average slip and slip-per-event are used to analyze the data. These data provide a first order check for models of earthquake behavior. With the advent of high resolution topographic datasets such as LiDAR, the new methodology serves as a template for inclusion of rapidly-accumulating topographic and paleoseismic data in California as well as to regions such as the Himalayan front, as those types of data are adopted.