Possibility of Temporal Variations in Earth Tidal Strain Amplitudes Associated with Major Earthquakes

Abstract

Earth tidal strains observed concurrently at two stations in the western Chubu region have been analyzed to see if there are any temporal variations of their amplitude in relation to major earthquakes in the region.The tidal strain records obtained at Kamitakara during 7 years and at Inuyama during 2 years are bandpass-filtered, and correlated with the corresponding theoretical tidal function predicted for a solid earth, using least squares mainly in the time domain, to estimate the tidal admittance over the periods. The admittances determined for the two-component Kamitakara strains appear to show a consistent and gradual increase during 10-11 months prior to the central Gifu earthquake of 1969 (M=6.6), with a maximum variation reaching about 15%, and after the earthquake they seem to decrease and recover their normal value a half year later. No such significant variations as in this period can be found over the entire period. The resolved amplitudes of the semidiurnal M2 and diurnal O1 waves show similar trends. The results from the Inuyama observations involve somewhat larger fluctuations, but appear to reach a peak 3-4 months before the earthquake and then decrease to the time of its occurrence.Secular strains and tilts observed at the above two stations also indicate appreciable changes during the corresponding periods. If these temporal variations of the tidal strain amplitudes are not spurious but real phenomena, they might be interpreted as a manifestation of some precursory changes in crustal rigidity around the seismic source region.S 123

Anomalous strain response to rainfall in relation to earthquake occurrence in the Tokai area, Japan.

“…Some earthquakes are believed to be preceded by changes in the physical properties of the crust. In this context, MIKUMO et al (1977) reported an amplitude change of tidal strains at two observatories at Kamitakara and Inuyama prior to the 1969 central Gifu earthquake (M=6.6). They inferred that the change in crustal properties would have taken place in the source region.…”

Determination of instability of the fault zone model

“…The Earth is subjected to the mechanical e ects of earth tides with their highest values in the semidiurnal, diurnal, fortnightly and monthly periods. Although numerous attempts were made to use earth tides for evaluation of seismic hazards Mikumo et al, 1978Nikolaev and Nikolaev, 1993Nishimura, 1950Xiang-chu Yin et al, 1994] the problem is still well beyond solution.…”