Long-Base Laser Strainmeters: A Review
This document reviews the history, design, and use of the long-base laser strainmeters developed at IGPP/SIO since 1970: the longest-running and most stable examples of such instruments. We describe the principles of operation, limitations on dynamic range the frequency response, and major sources of error, along with the design and construction methods used. We also provide results on reliability of operation and the methods used to produce final high-quality data from what is recorded in the field: both based on close to 100 instrument-years of experience. We describe the setting of the two long-running instruments in California, at Pin~on Flat Observatory and at Durmid Hill; for both locations we summarize results on long-term strain, tidal strains, and various strain ``events'': the last almost all related to earthquakes, since the results of these high-quality measurements have shown few departures from steady strain accumulation with time.