Abstract:

We live in a time of unprecedented growth in the volume and quality of seismic data gathered world-wide. Today's global networks collectively comprise over two hundred continuously recording stations and are complemented by numerous regional networks, arrays, and temporary deployments. These instruments record ground motions as small as a few nanometers at periods ranging from hundredths to hundreds of seconds. Many of these seismograms can be obtained and analyzed within hours of an earthquake by any individual equipped with a computer and an internet connection. These modern datasets have been used to determine, with unprecedented clarity, the Earthcs inner structure. But not all of the latest advances in global seismology have come from newly acquired, and easily accessed, data.

For example, several recent results including evidence for partial melt at the base of the mantle [Vidale and Hedlin, 1998], support for fine-scale inner-core structure [Vidale and Earle, 2000], and constraints on the relative rotation rates of the inner-core and mantle [Vidale et al., 2000] rely on data collected by two venerable seismic arrays; the Large Aperture Seismic Array (LASA) operated in Montana and the NORwegian Seismic ARray (NORSAR) located near Oslo. Here we discuss the merits of these data, review these new findings, and report on an effort to rescue LASA data from aging 9-track tapes.

The ability of modern broadband seismometers to accurately record ground motion greatly surpasses the capabilities of older band-limited low-dynamic-range instruments. However, using archival data has several advantages. These databases often span long time periods; for example, NORSAR has recorded earthquakes over the last thirty years. This long recording interval increases the number of cataloged high-quality events such as deep-focus earthquakes that are known for their impulsive sources and compact codas. A long recording history captures a more complete record of seismicity, consequently increasing global coverage and recording closely located events with long temporal separation allowing time-varying phenomena to be studied.