Appraisal of Existing Seismic Hazard Estimates for the UK Continental Shelf

The first seismic hazard estimates for the UKCS were made for HSE in 1986, with later assessments being made in 1993 and 2002.Since the work on the most recent of these studies was completed, a draft ISO standard and a NORSOK standard have been published.

Also, a medium-sized earthquake has occurred in the central North Sea, in an area previously considered to have low seismic activity.

In this report, the most recent estimates (given in report OTO 2002/005) are synthesized with those from the earlier studies, using a specially-designed rating scheme, to generate hybrid contour maps of the peak ground acceleration (pga) hazard.

The reliability of these hybrid results is assessed by comparison with provisional, newly-calculated, de-minimis hazard figures based on catalogue completeness considerations. Until such time as a uniformly-derived hazard map is developed it is recommended that the 10-4 p.a.probability of exceedance pga hazard figure at any point in the offshore UKCS east of longitude 10 W should be taken to be highest of the figures given by: the new map of allowable minimal hazard levels, the new synthesized hazard map, and the estimate given by report OTO 2002/005.

As there is no current information on hazard exposure at the lower frequencies that are of concern when designing offshore installations, recommendations are made as to how such estimates could be calculated.

The characterization of ground motion is discussed, noting the significance of the differences between hard and soft sites and current thinking on vertical motion.

Attention is drawn to policy and technical issues that remain unresolved and suggestions made as to how those issues could be addressed.