Configuration of the Seattle Urban Seismic Array for the February 28, 2001, M6.8, Nisqually earthquake and its aftershocks

Abstract

This is a description of the configuration of the Seattle Urban Seismic Array (click on thumbnail map to see full size map) at the time of the Nisqually earthquake and its aftershocks (from April 28 through March 3, 2001). The array is operated by the Geologic Hazards Team of the U. S Geological Survey. It has been recording small local earthquakes since its initial installation immediately after the Duvall earthquake in May 1996. Since then, we have been able to convert the array to all Kinemetrics K2s after retiring the aging DR-200s in June of 1998. The array stations have been moved around to suit our research interests. We had 26 instruments in urban Seattle at the time of the Nisqually earthquake. The primary research goal of the array is to record earthquake ground motion at sites scattered around the city that represent the major types of surficial geologic materials found in the city. With this information we plan to draw seismic hazard maps that will show the expected variability of ground shaking throughout the city. Our strategy to achieve our goal includes: 1) sampling ground motion at sites that represent the major geologic materials including man-made fill and alluvial surficial materials, 2) recording high-resolution ground motions from a tripartite array (SDN, SDS, and SDW) with about 200 meter spacing located on man-made fill in the SoDo (South of Downtown) industrial district, 3) recording ground motion in areas with man-made fill and alluvial surficial materials which have a high potential for liquifaction, 4) sampling ground motion at two sites on Capitol Hill (THO, and SEU) that are located near drill holes that have been logged for seismic velocity, 5) Providing long-term continuity at a few sites for comparison and reference, especially SEW and ALK, which are located on Tertiary bedrock. 6) recording ground motion at sites with similar surficial geology both inside and outside the Seattle Basin The configuration of the array is summarized in the accompanying table which is further described below.