Critical sensitivity of load/unload response ratio and stress accumulation before large earthquakes: example of the 2008 Mw7.9 Wenchuan earthquake

The Load/Unload Response Ratio (LURR) method is often defined as the ratio between Benioff strains released during the time periods of loading and unloading, corresponding to earth tide induced Coulomb Failure Stress change on optimally oriented faults. According to the method, anomalous increase in the time series of LURR usually occurs prior to occurrence of a large earthquake. Previous studies have indicated that the stress field that existed before a large earthquake has strong influence on the evaluation of LURR. In order to augment the sensitivity of LURR in measuring the criticality of stress accumulation before an earthquake, we replace the circular region usually adopted in LURR practice with an area within which the tectonic stress change would mostly affect the Coulomb stress on a potential seismogenic fault of a future event. Coulomb stress change before the hypothetical earthquake is calculated based on a simple back-slip dislocation model of the event. Retrospective test of this new algorithm on the 2008 Mw7.9 Wenchuan earthquake shows remarkable enhancement of the LURR precursory anomaly. To illustrate the variation of LURR time series associated with our choice of identified areas with increased Coulomb stress before the earthquake, we calculate the spatial distributions of LURR within a circular region of 700 km radius centered at epicenter of the event. Comparing the spatial LURR distributions of different periods, the change of LURR within the Coulomb stress increase areas looks more prominent than the others: it remains at a low level for most of the time and markedly increases few years before the quake. This result further shows the validity of the Coulomb stress algorithm. Unlike circular regions, areas of increased Coulomb stress with anomalously increased LURR values before a large earthquake could provide a relatively more precise estimation of the criticality of the ensuing event.