SCEC Award Number 13062 View PDF
Proposal Category Travel Only Proposal (SCEC Annual Meeting)
Proposal Title Fragile Geologic Features in Southern California: Statistical Remnants and Representative Fragility
Name Organization
Mark Stirling GNS Science (New Zealand) Dylan Rood University of Glasgow (United Kingdom)
Other Participants
SCEC Priorities 6e, 6e, 6e SCEC Groups Geology, GMP, EFP
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
The objective of our 2013 work has been to address the hypothesis that the few rare fragile geological features (FGFs) observed at Lovejoy Buttes site in the Mojave Desert of Southern California may not provide realistic constraints on past ground motions because they are rarities (statistical remnants).The Lovejoy Buttes site is recognised as one of the most significant FGF sites in southern California due to its location close to the San Andreas Fault (about 20km). The few FGFs observed are estimated to be the order of 10,000 years in age from cosmogenic dating techniques. In our brief September 2013 field survey we observed semi FGFs (i.e. less fragile) to be more abundant than FGFs at Lovejoy Buttes, suggesting they may provide more realistic constraints on San Andreas motions than the FGFs. Semi FGFs, like FGFs, were identified on the basis of being disconnected from the surrounding country rock and showing obvious signs of fragility. Field-based estimates of quasi static toppling accelerations for the semi FGFs are in the range of 0.35-0.6g, as opposed to the few FGFs which have estimates of less than 0.3g. Semi FGFs therefore show lesser discrepancies to ground motion estimates from the US national seismic hazard model for return periods comparable to the assumed age of the features (1000-10,000 years). Our field survey will be completed in 2014 if SCEC support is forthcoming, and we will then be able to reassess the upper limits of ground motion at Lovejoy Buttes for return periods of 1000-10,000 years.
Intellectual Merit We consider our project to be both important and timely, given that SCEC is now at a time when the most fragile of FGFs in Southern California are being used to constrain estimates of seismic hazard in the Cybershake project, and specific rupture segmentation scenarios are being defined to reconcile the presence of FGFs near major active faults. In short, we consider interpretations limited to the most fragile FGFs may underestimate the hazard for SCEC, and that our study will provide a safer approach to verification of seismic hazard models. Our SCEC work is also complimented by studies of FGFs and semi FGFs at critical facilities in New Zealand and California.
Broader Impacts The educational value of our SCEC-funded work is that is a good example of integrative, multidisciplinary approaches to solving a significant scientific problem: how to achieve seismic hazard model verification. The melding of geology, geomorphology, engineering principles, seismology, and nuclear physics (cosmogenic dating) to develop criteria for constraining seismic hazard models requires multidisciplinary skillsets and integrative abilities. SCEC Interns will need to “think beyond the square” in order to move forward and get ahead in this competitive science world, and our study would be a good case history for them to be aware of.
Exemplary Figure Figure 2. Graph of the number of semi FGFs plotted as a function of the quasi-static estimate of PGA required to topple the FGF (fragility) at Lovejoy Buttes. The reddish curve labeled “Number” shows the number of FGFs of a given fragility, whereas the blue curve labeled “Cumulative Number” shows the number of FGFs with a given fragility greater than or equal to a given value. Note that the 0.3g estimates are based on the two known FGFs at the site, and therefore define the lower limit of fragility. The upper limit of fragility is based on features that still give the overall appearance of fragility, rather than being (e.g.) resistant outcrops which would provide meaninglessly high fragility estimates.