A Discussion on the measurement and interpretation of changes of strain in the Earth - Dislocation models for fault creep processes

Abstract

Tectonic processes in the lithosphere are plastic-elastic phenomena, in which the processes of plastic yield are in general inhomogeneous, and often strongly localized in faults, but then still, of course, at any one time inhomogeneous over the area of the fault. Motion on a fault may either be rare, sudden and seismic (stick-slip) or frequent and virtually aseismic (creep). Associated with these localized plastic strains, and dependent on their inhomogeneity, there are small elastic strains in the surrounding medium, in which sensitive strain gauges distributed over the surface, away from the fault, can give information about the inhomogeneities of plastic strain below the surface. Present needs are above all to have Earth-strain gauges giving significant continuous records over a period of years, cheap enough and sufficiently undemanding in site requirement to permit installation in considerable numbers for the observation of one tectonic or pre-seismic phenomenon, and to learn their habits and how to separate tectonically significant signals from various kinds of noise, e.g. of hydrologic origin. In planning such a layout, say, near to a strike-slip fault known to have creep activity, one sensibly assumes some simple model or models for the kind of motions on the fault which are to be looked for, and whose elastic consequences can be readily calculated. Single line dislocations moving on the fault provide such models, and enable one to see that features of the fault motion will be indicated by strain gauges in particular orientation. For example, if a travelling creep event is represented by vertical edge dislocation lines, strain gauges parallel to or perpendicular to the fault have their merits for resolving their distribution on the fault, but a strain gauge at 45° will see it coming from a greater distance (e.g. will see a slip-event of magnitude 1 mm when it is 10 km away, if a strain of 10-8 can be distinguished from noise), and will also respond to slip events which correspond to the rise of screw dislocations from below, which will not be noticed on strain gauges in the other two orientations. Gradually accumulating screw dislocations at depth make a reasonable tentative mode for the stress build up preceding an eathquake.