Areal strain of solid earth tides observed in Ogdensburg, New Jersey

Improvements of extensometer measurements and a method of comparing observed tidal strain with theory, almost independent of ocean loading, produce an excellent result of strain of solid earth tides at Ogdensburg, New Jersey. Earth tides are well recorded on an array of three horizontal extensometers oriented at N 29°30′E, N 48°30′E, and S 48°E, which is installed in a deep mine located at Ogdensburg, New Jersey. The extensometers are of the Benioff fused quartz type and are calibrated by a Michelson interferometer, using an Ne-He laser as a monochromatic light source. The present calibration has repeatability with a maximum error of less than 5%. A 29-day series for each extensometer of the array, from 00h 00m 00s February 16 to 23h 00m 00s GMT March 16, 1966, was analyzed. Corrections were made for the effects of instrumental drift, barometric pressure, and thermal variation on the extensometers. The latitudinal-, meridional-, and shear-strain components were resolved from the extensometer array and analyzed by the least-squares method of tidal analysis for the M2 and O1 constituents. The observed areal strain of solid earth tides at Ogdensburg, which is nearly independent of the load strain caused by the ocean tidal loading off the Atlantic coast based on the Boussinesq solution, confirms the theoretical tidal strain based on the Gutenberg-Bullen earth model with a New York-Pennsylvania crustal structure, which has the characteristic number values h = 0.621 and l = 0.084.