It is important to estimate both the absolute values and stress and its orientation at seismogenic depths, because it controls earthquake generation. To overcome the problem that we do no have a single method, with which we can estimate both stress values and orientation, we need to combine all the possible methods and compare the results to establish realistic and reliable methods to estimate stress at great seismogenic region. We will review the results at two site: 1) Stress measurements at the region with very shallow earthquakes and and 2) Stress measurements after the 1999 Chi-Chi earthquake (Taiwan).
The state of stress provides us insights into understanding the mechanisms of generation and propagation of large earthquakes. Several methods have been proposed to estimate stress state at depths: In-situ stress measurements, core stress measurements and stress estimates from earthquake focal mechanism analysis. It is possible to estimate stress state at seismogenic depths (deeper than 10 km) by earthquake focal mechanism analysis. However, it is still necessary to estimate absolute value of stress at great depths, because earthquake focal mechanism analysis provides us only stress ratios and orientation. In-situ measurements and core stress measurements have advantages and disadvantages. Hydraulic fracturing stress measurements will be difficult to be performed at great depths due to breakouts and has fundamental problem regarding maximum stress. Core stress measurements need to have physical basis on the assumption on the stress estimates. To overcome the problem that we do no have a single method, with which we can estimate both stress values and orientation, we need to combine all the possible methods and compare the results to establish realistic and reliable methods to estimate stress at great seismogenic region.
We will review the results at two site: 1) Stress measurements at the region with very shallow earthquakes and and 2) Stress measurements after the 1999 Chi-Chi earthquake (Taiwan).
