Evaluation of Seismicity and Earthquake Shaking at Offshore Sites
Authors: Page Robert A. 
Published: October 2008

Introduction

In an effort to meet national energy needs through the 1980's many new continental shelf areas bordering the United States are being considered for oil and gas lease sales. Among the areas under consideration are several along the Pacific Coast that are characterized by high levels of earthquake activity. Successful development of petroleum and other mineral resources in such areas requires that earthquake hazards in the offshore environment be carefully assessed and that fixed offshore structures be designed to resist earthquakes and related geologic and hydrologic effects.

In designing a structure to resist earthquakes, the engineer relies upon earth scientists to provide information concerning 1) the probable location and magnitude of future earthquakes, 2) the probability of surface faulting, 3) the expected nature of ground shaking, 4) the likelihood of ground failure, such as liquefaction and sliding, and 5) the probability of a tsunami. This paper discusses various methods and techniques used by the seismologist to evaluate seismicity and ground shaking at offshore sites. The discussion is illustrated with examples from the continental shelf area in the Gulf of Alaska a region under consideration for petroleum lease sales, and a region of extreme earthquake hazard.

The Offshore Environment

The problem and difficulties encountered in evaluating offshore seismicity and ground motion differ from those normally encountered on land.

The seismic history for offshore areas is generally less complete than for neighboring onshore regions. Prior to the installation of regional seismograph networks (in the 1930's in parts of California and in the 1960's in southern Alaska), offshore shocks too small or distant to be felt by people on land or too small to be recorded by sensitive seismographs at great distances passed unnoticed. Although perhaps of little engineering significance, such shocks are valuable for delineating active faults, if they can be reliably located.

The accuracy of location for offshore earthquakes is generally less than for onshore events, because the azimuthal distribution of recording stations is less uniform. For an offshore shock, stations typically lie to one side of the earthquake, and any error in the assumed velocity structure for the earth biases the calculated epicenter. (The epicenter is the project

Marine seismic profiling techniques provide a relatively rapid and inexpensive means to identify subsurface faults compared to onshore geophysical techniques; however, recognized submarine faults are inaccessible to the detailed geological study that might be undertaken to decipher the deformational history of an onshore fault.

A frequent characteristic of offshore sites that is not usually present onshore is a surface layer of low-rigidity, water saturated sediments overlying more competent geologic formations. No records of strong ground motion have been obtained on the ocean bottom, and very few earthquakes have been recorded on land at a site underlain by similar low-rigidity, water-saturated sediments. As will be illsutrated, surficial sedimentary layers can drastically alter the nature of ground shaking.