Calibration of the Global Seismographic Network Using Tides

Abstract

The Incorporated Research Institutions for Seismology/United States Geological Survey (IRIS/USGS) Global Seismographic Network (GSN) is now a fully operational scientific facility routinely used to investigate earth structure and seismicity throughout the world (Butler et al. 2004). Data recorded at GSN stations are rapidly transmitted to international agencies charged with hazard assessment and response as well as to individual scientists eager to study in detail the properties of the most recent earthquake. All who use these recordings depend upon the accuracy of the GSN's metadata provided to them through the IRIS Data Management System (http://www.iris.edu). The IRIS DMS in turn distributes response information supplied by the GSN network operators, the University of California San Diego's Project IDA (International Deployment of Accelerometers) and the USGS's Albuquerque Seismological Laboratory (ASL). To maximize the scientific return from these data and to ensure that accurate measurements may be made at the various monitoring centers that use GSN data in their analyses, both network operators subject the equipment at each GSN station to a series of stringent tests before field deployment. These tests include careful calibration of the data acquisition system. A goal of the GSN is to publish instrument responses accurate to 1%. In principle, the Earth's' tides provide an independent check of the laboratory-determined instrument response. The bulk properties of the Earth are well-enough known to compute the solid earth's response to tides to a high degree of accuracy. However, until the advent of improved ocean tidal models based upon analysis of TOPEX/Poseidon orbit cycles, the effects of ocean loading were too poorly known to be used for this purpose. Using the current generation of tidal models that effectively incorporate ocean loading as well as the solid earth tide, one may now compute a tidal series accurate to 1% in many regions. In this …