We estimate the strength of fractured rocks in-situy by analysing fluid-induced microseismicity. Such an analysis is able to provide us with a completely new feature of naturally fractured rocks. The strength corresponds to the value of critical pressure in the pore space that must be exceeded in order to trigger earthquakes. We assume that during hydraulic injection experiments in boreholes, microseismicity is mainly triggered by a diffusive process of pore pressure perturbation. An analytical solution can be applied to find time-dependent pore pressure perturbations in rocks. Characteristics of the spatio-temporal evolution of microseismic clouds can be then used to estimate minimum and maximum pressures necessary to trigger earthquakes as well as the full spectrum of rock strength. We verify the approach using numerical data and apply it to real data of injection-induced microseismicity from Hot-Dry-Rock tests in crystal<br>line rocks and a hydro-fracturing experiment in sediments. We find that quite low critical pressures are characterising the strength of pre-existing cracks for all data sets analysed. The strength of the rock at sedimentary environment is found to be at least one order of magnitude smaller than for crystalline rocks.


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