Gravity anomaly and the geoid height at plate boundaries - classification of strain accumulation process and its geophysical interpretation

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1.Theoretical work.We find an intriguing reciprocity relations between the static deformation excited by a point dislocation in a SNREI earth and those generated by external forces, such as tidal force, surface loading and surface shear forces. the relations enable us to compute the coseismic potential change in a spherically symmetric earth model for the March 28, 1964 Alaska earthquake(mW=9.2). We consider two cases : (1) one is with a barrier where no slip occurs on the fault plane and (2) the other without it. Numerical results clearly indicated that the geoid swells above the subsurface fault while giving a notch over the hidden barrier. Although order of magnitude of these changes are marginal for the present satellite altimetry, there is a good chance of detecting such co-seismic geoid changes if we carefully process satellite altimeter data.2. Analysis of observational dataWe analyzed a gravity anomaly around the North-Izu Fault System (NIFS), Japan Numerical simulation using the theory of coseismic gravity change with geological fault parameters reproduced the local gravity anomaly excellently : positive anomalies over a compressed zone.while negative anomalies over the dilated areas. We conclude the resultant gravity anomaly is the product of repeated coseismic gravity change during a geological timescale because earthquakes will occur on the same fault with the same scheme. However small a coseismic gravity change may be, it is likely to pile up in a systematic way over a long time.