Applicability of Pseudoprobabilistic Method of Liquefaction Hazard Assessment for Nuclear Power Plants at Diffuse Seismicity Sites

For the new nuclear power plants, the hazard of liquefaction due to earthquakes should be excluded by appropriate site selection or eliminated by engineering measures. An important question is how to define a quantitative criterion for negligibility of the liquefaction hazard. In the case of operating plants, liquefaction can be revealed as a beyond-design-basis event. It is important to learn whether the liquefaction hazard has a safety relevance and whether there is a sufficient margin to the onset of liquefaction. The use of pseudoprobabilistic method would be practicable for the definition of probability of liquefaction, but it could result in overconservative results. In this paper, the applicability of the pseudoprobabilistic procedure is demonstrated for the sites in diffuse seismicity environment and for low hazard levels that are typical for nuclear safety considerations. Use of the procedure is demonstrated in a case study with realistic site-plant parameters.

Modeling Diffuse Seismicity in Probabilistic Seismic Hazard Analysis: Treatment of Virtual Faults

There is no scientific consensus on how finite virtual faults should be modeled in the probabilistic seismic hazard analysis of diffuse seismicity. Often pragmatic implementation decisions are made as a matter of convenience or computational efficiency. To better understand how differences in modeling virtual faults impact seismic hazard, we evaluated several methods of modeling such faults. For our preferred model, we test the sensitivity of the hazard to differences in fault discretization density, number of random azimuths, and cutoff distance. We propose a new computationally efficient method of modeling the random orientation of virtual faults by holding the fault plane fixed in space and rotating a virtual site around the source grid point. We show that there can be relatively large differences in seismic hazard, depending on how virtual faults are modeled, and provide guidance on how virtual-fault parameters should be selected in order to achieve a specified degree of accuracy.

CONSIDERATIONS ON THE DIFFUSE SEISMICITY ASSUMPTION

Second-order effects were historically included by the effective length method (K concept). All the studies about that methodology have been developed in frame plane, with regular rectangular frames. The new way to include those effects is the use of second-order analysis, direct analysis method or alternative simplified options. This methodology was included in ANSI AISC360 in the 2005 version and in the 2010 version. As before, the studies already developed for DAM analysis are in plane. In this paper, the K concept is revisited by numerical analysis, and extended to the 3D space. Using models of symmetric and non-symmetric industrial steel structures in plane, 3D stability analyses were developed, and the results were compared with plane behavior. Several conclusions and recommendations were exposed, resulting from the analyzed models. Keywords: Second-order analyses, steel structures, irregular 3D frames.

The tectonic source of the 1755 Lisbon earthquake and tsunami

The SW continental margin of Iberia is affected by several tectonic structures of Cenozoic to Recent age, gen-erated by the dynamics of the Iberia-Africa plate margin. This activity is testified by diffuse seismicity along the eastern portion of the Azores-Gibraltar line. The most important active structure, detected during a reflection seismic survey in 1992, is a thrust-fault, some 50 km long and with dip-slip throw of more than 1 km, located offshore Cabo de S. Vincente. A relocation of historical earthquakes in the area shows that this structure lies very close to the epicentre of the catastrophic 1755 Lisbon earthquake and that it should be the generator of the event. This submarine structure can now be studied for modelization of tsunamis and consequent risk mitigation.