Non-Technical Version of the Phase III Report:
Last year I received funding to finish a non-technical version of the Phase III report, which is planned to be submitted (in some form) to a popular scientific journal. Unanticipated delays in the Phase III report, as well as resolving the earthquake deficit problem (discussed next), led to a year-long postponement of this work.
A Mutually-Consistent Probabilistic-Hazard Source Model:
In the first formally-reviewed draft of the Phase III report, the earthquake deficit made apparent in the Phase II report still existed for the geologically-based seismic-source model. In trying to translate this problem into layman's terms (for the non-technical version of Phase III) I was driven to dig deeper and deeper into the exact source of this disparity. Specifically, I investigated whether a simple hazard source model could be constructed which: simultaneously satisfies the best estimate of regional moment-rate and the historically observed seismicity; incorporates geological data directly; does not invoke unprecedentedly large earthquakes or any unknown aseismic deformation; and does not reside at the margin of significance in terms of uncertainties. The answer is yes.
In a paper co-authored by David Jackson and James Dolan (copies available upon request), we outline the very simple procedure for constructing this source model. We also discuss how it differs from that in the Phase II report, and outline the factors that contributed the apparent earthquake deficit. In particular, we show that a careful adherence to the conservation of seismic-moment rate, an allowance for geologically-feasible multiple- segment ruptures, and accounting for magnitude uncertainties are among the important factors.
Study of Nonlinear Sediment Response:
It has been known since at least 1898 that sediments can amplify earthquake ground motion relative to bedrock. For the weak motion of small earthquakes, sediment amplification is well understood in terms of linear elasticity. For the damaging ground motion produced by larger earthquakes, however, there has been a long-standing debate. The view of geotechnical engineers, based largely on laboratory studies, is that Hooke's law breaks down at larger strains causing a reduced (nonlinear) amplification. Seismologists have traditionally remained skeptical, on the other hand, because the relatively few strong-motion observations were consistent with linear elasticity. Although more recent earthquake studies have demonstrated nonlinear behavior under certain conditions (mostly related to liquefaction), its significance for the type of stiff-soil sites found in the greater Los Angeles region has remained a controversial and important question.
We (Field et al., 1997; 1998) have addressed this long-standing question regarding nonlinear sediment response at stiff-soil sites in the Los Angeles region by testing whether sediment amplification was similar between the Northridge earthquake and its aftershocks. Comparing the weak- and strong-motion site response at 15 sediment sites, we have found that amplification factors were significantly less for the main shock implying systematic nonlinearity. The difference is largest between 2 and 4 Hz (a factor of two), and is significant at the 99-percent confidence level between 0.8 and 5.5 Hz. The inference of nonlinearity is robust with respect to the removal of possibly anomalous sediment sites, how the reference-site motion is defined, and with respect to corrections for finite-source effects. Nonlinearity is also suggested by the fact that the four sediment sites that contain a clear fundamental resonance for the weak-motion exhibit a conspicuous absence of the peak in the strong-motion.
Although we have taken the first step of establishing the presence of nonlinearity, it remains to define the physics of nonlinear response, and to test the methodologies presently applied routinely in engineering practice. The inference of nonlinearity implies that care must be exercised in using empirical Greens functions at sediment sites to study large earthquakes.
While this study represents significant progress, several issues remain unresolved. Because several different disciplines are currently working on various aspects of the problem, I have organized a SCEC monthly science seminar and workshop to bring these various disciplines together (Scheduled for Jan. 29-30, 1998). It is hoped that the unprecedented diversity of disciplines in attendance will establish points of agreement and disagreement, stimulate crossbreeding, and identify priorities for future research.
Publications During the Last Year:
Field, E.H., and SCEC (1998) Understanding Earthquake Ground Motion and Seismic Hazard Assessment, A Non-Technical State of the Art Review; a longer version will be a SCEC report and a shorter version will be submitted to a popular science journal.
Field, E.H., D. D Jackson, and J. F. Dolan (1998), A Mutually Consistent Seismic- Hazard Source Model for Southern California, submitted to Science.
Field, E.H., P.A. Johnson, I.A. Beresnev, and Y. Zeng (1997). Nonlinear Sediment Amplification During the 1994 Northridge Earthquake, Nature, Dec. 11, 1997.
Field, E.H., Y. Zeng, P.A. Johnson, and I.A. Beresnev (1998). Pervasive Nonlinear Sediment Response Observed During the 1994 Northridge Earthquake, Subm. to J. Geophys Res.