Preliminary Results of In-situ Stress Measurements along the Longmenshan Fault Zone after the Wenchuan Ms 8.0 Earthquake

Four months after the Wenchuan Ms 8.0 earthquake in western Sichuan (China),in situ stress measurements were carried out along the Longmengshan fault zone,with the purpose of obtaining stress parameters for earthquake hazard assessments. In situ stresses were measured in three new boreholes by using overcoring of the piezomagnetic stress gauges for shallow depth and hydraulic fracturing for deeper depth. The results show that the maximum horizontal stress at shallower depth (~20m) is about 4.3 MPa,oriented N19°E,in the epicenter area at Yingxiu Town,about 9.7 MPa,oriented N51°W,at Baoxing County in the southwestern Longmenshan range,and about 2.6MPa,oriented N39°E,near Kangding in the southernmost zone of the Longmenshan range. Hydraulic fracturing at borehole depths from 100 to 400 m shows a tendency towards increasing stress with depth. A comparison with the results measured before the Wenchuan earthquake along the Longmenshan zone and in the Tibetan Plateau demonstrates that the stress level remains relatively high in the southwestern segment of the Longmenshan,and is still moderate in the epicenter zone. These results provide a key appraisal for future assessment of earthquake hazards of the Longmenshan fault zone and the aftershock occurrences of the Wenchuan earthquake.