ISSO

ISSO is an International Organization, on a voluntary basis, devoted to seismic hazard assessment, seismic risk mitigation, earthquake forecast/prediction

POSITION STATEMENT ON EARTHQUAKE HAZARD ASSESSMENT AND DESIGN LOAD FOR PUBLIC SAFETY

Date of Issue: August 6, 2012

Summary

In view of the devastation produced by large earthquakes and associated phenomena exemplified by the 2004 Sumatra earthquake and tsunamis, the 2008 Wenchuan earthquake in China, the 2010 Haiti earthquake, and the 2011 Tohoku earthquake and tsunamis in Japan, it is imperative that structures should be designed and constructed to withstand the largest or Maximum Credible Earthquake (MCE) events that include or exceed such historic events; and the public should be advised to be prepared and ready for such possible events beforehand. These are the most dangerous and destructive events that can happen at any time regardless of their low frequencies or long recurrence intervals. Therefore, earthquake hazard assessment to determine seismic design loads should consider the MCE events. Emergency management policy should consider scenarios for possible MCE events.

The traditional Deterministic Seismic Hazard Analysis (DSHA) using MCE has been successfully used for determining seismic design loads in California with confidence since the early 1970’s to the present time and its enhanced variation, neo-DSHA (NDSHA), published in 2001 for Italy, has proved reliable when its estimates were compared with actual data for the most recent Northern Italy May 2012 earthquakes. Therefore, DSHA or NDSHA should be used for public safety policy and determining design loads. The current Probabilistic Seismic Hazard Analysis (PSHA) approach is unacceptable for public safety policy and determining design loads for the following reasons:

1. Many recent destructive earthquakes have exceeded the levels of ground motion estimates based on PSHA and shown on the current global seismic hazard map. Seismic hazards have been underestimated here.
2. In contrast, ground motion estimates based on the highest level of PSHA application for nuclear facilities (e.g., the Yucca Mountain site in USA and sites in Europe for the PEGASOS project) are unrealistically high as is well known. Seismic hazards have been overestimated here.
3. Several recent publications have identified the fundamental flaws (i.e., incorrect mathematics and invalid assumptions) in PSHA, and have shown that the result is just a numerical creation with no physical reality. That is, seismic hazards have been incorrectly estimated.

The above points are inherent problems with PSHA indicating that the result is not reliable, not consistent, and not meaningful physically. The DSHA produces realistic, consistent and meaningful results established by its long practice and therefore, it is essential that DSHA and its enhanced NDSHA should be adopted for public safety policy and for determining design loads. Seismic sources that most impact the site should be used for designing strategic and public buildings as well as critical structures. Seismic sources that most impact a region should be used for emergency management for that region. Such considerations would reduce the risk of large devastation and loss of human life in future earthquakes, and should be used for all critical cases to secure public safety. Critical cases would include situations where the consequences of failure (i.e., risks) are too costly and intolerable as illustrated below by some recent examples.

POSITION STATEMENT