Abstract：There is widely distributed gravelly soil in triggering area of debris flow. From the observation，analysis and calculation，it is found that triggering of debris flow results from soil strength reduction. In fact，debris flow areas are often located in earthquake belt. The distribution relation between debris flow and earthquake makes us to suggest that earthquake lead to the strength reduction of gravelly soil and make triggering of debris flow. On the basis of the suggestion，triaxial shear equipment is used to make testing study for the gravelly soil in the triggering area of debris flow. The variables are water content，soil depth or confining pressure，the destroy criterion and earthquake intensity or vibration times. The cohesion and friction are chosen to measure the strength of soil. As a result，it is found that the cohesion increases first and then drops with the increment of water content，and the max value appears when the saturation degree gets up to 84%. With the increment of vibration times the cohesion decreases. For the sample with high water content，the cohesion can drop to zero with enough vibration times. As usual the friction angle decreases with increment of water content and can drop to zero only for the nearly saturated sample. The friction angle always decreases slightly with increment of vibration time. The friction angle changes with vibration times unstably. When the vibration times are lower than 20 and earthquake intensity is under 7.5 grade，the friction angle decreases slightly with the increment of vibration times，but when the vibration times are higher than 20，the friction angle of low saturated sample increases. For the high saturated sample when the vibration times arrive 50 times，the friction angle drops to zero. For the nearly saturated sample with saturated degree of 92%，the friction angle increases or decreases unstably，which is dependent on the other factors. In conclusion，the tests show that earthquake may reduce the soil strength and under some special cases it may leads soil to be liquefied. Therefore earthquake makes an important role in triggering of debris flow.
