Long-Base Laser Strainmeters: Four Decades of Results

Over the past forty years, we have operated 11 long-base laser strainmeters (LSM's) in California and Nevada, 9 of which continue to operate. Ten of these are on the ground surface, and have lengths of 380 to 730 m; one, with length 405 m, operated in a tunnel. The first lesson learned was the importance of precisely tying the end points to the ground, which is done with an additional "optical anchor" system. Given this, all the instruments show long-term strain rates consistent with those determined from continuous GPS networks: close to major faults this rate is 0.5 microstrain/yr, and as small as 1/20 of that further away. This ability to measure secular strain changes has been obtained in geological settings ranging from weathered granite to Holocene clays. At periods less than a year, the noise levels are much less than those obtained from GPS, about 10,000 times less at periods of a day. Borehole strainmeters and LSM's have similar performance for periods from seconds to weeks; at longer periods the LSM's show stability not usually found in borehole measurements. The surface LSM's are also free of the cavity effects that make it difficult to compare borehole (and tunnel) measurements of tides with theory; nevertheless there are still significant differences, particularly in the East-West extensional strain. The response to air pressure depends on site geology. Loading by nearby seiching and anthropogenic sources is observed on some instruments, though the results are not consistent with half-space computations.  The instruments located near major faults have observed a number of episodes of rapid strain change over periods from minutes to weeks; in many cases these appear to reflect shallow creep on the fault nearby, but there are also slow-slip episodes on the San Jacinto fault at seismogenic depths: these appear to be triggered by moderate local earthquakes and larger regional ones. In almost 200 instrument-years of data no anomalous strains have been seen before earthquakes.

The supplement shows the NS LSM near Cholame, California (439 m long). The San Andreas fault is 2 km away, adjacent to the hills on the horizon.