The Long-Term Measurement of Strong-Motion Earthquakes Offshore Southern California
Authors: Sleefe G.E. 
Published: October 2008

This paper describes a new long-life Seafloor Earthquake Measurement System (SEMS) and presents earthquake data obtained from the seafloor near an offshore oil production field. The SEMS is a battery powered (8 year life) digital data acquisition system which telemeters data via a sonar link. Seafloor earthquake data from SEMS indicate that the seafloor vertical acceleration is nearly an order of magnitude weaker than the corresponding on-shore vertical motions. The importance of the SEMS measurements in designing earthquake resistant offshore structures is also described.

INTRODUCTION

A significant consideration in the design of offshore structures is their response to environmental stimuli. Information regarding climatic and oceanographic environmental stimuli for most offshore areas is extensive. As such, a solid data base is available for designing offshore structures that can withstand the threats of storm winds, waves, and ice floes. In regions such as offshore southern California and offshore Alaska, an equally important environmental stimulus is the seismic vibrations induced by local earthquakes. Data on the response of seafloor sediments to earthquake-induced seism city has been scarce, thereby introducing significant uncertainty into the seismic-hazards aspect of offshore structural design. To reduce this uncertainty, a program was undertaken to develop and implement instrumentation to measure seafloor seismic motions. The result of this This work performed at Sandia National Laboratories supported by the U.S. Department of Energy under contract number DE-AC04-76DP00789. program has been two-fold: the deployment of a long term, strong-motion digital seismograph offshore southern California; and the offshore recording of several significant earthquakes. Two of the earthquakes recorded by a seafloor instrument were simultaneously recorded by on-shore instruments, and by instruments located on a nearby offshore oil platform. Thus, some of the problems associated with the design of offshore earthquake resistant structures can now be addressed with supporting data.

In this paper, the most recent Seafloor Earthquake Measurement System (SEMS) will be described. This system is currently operating in 210 ft of water, 10 miles offshore Long Beach, CA, in the Beta Field. Data obtained from this unit and previous prototype SEMS will be presented. The implications of this data on the design of offshore earthquake-resistant structures will also be described.

INSTRUMENTATION DESCRIPTION

General System Overview

It has long been recognized [1] that offshore strong motion seismographs are needed to complement the onshore strong motion networks. The main difficulties associated With implementing these offshore seismographs are the remoteness of the site and the severity of the site environment. As a result, previous attempts at sea-bottom seismographs tended to be experimental in nature. Previous offshore strong-motion seismographs can be grouped into two categories: those that are tethered by a fond power and signal cable (e.g. f2]), and those that are electrically autonomous (e.g. L3). Unfortunately, the tethered-type are inappropriate for remote sites and are relatively costly to deploy and maintain. Autonomous types, while suitable for remote locations, have the added complexity of an internal power source and a data telemetry system. The use of a power source and telemetry.