The March 31, 2020, M 6.5 Stanley, Idaho Earthquake: Bookshelf Faulting?

On March 31, 2020, a magnitude 6.5 earthquake occurred about 19 miles northwest of Stanley, Idaho. The epicenter of this earthquake lies on a potential northern extension of the Sawtooth fault. The fault plane solution as well as the moment tensor solution suggests that it was a strike-slip faulting. The spatial distribution of the aftershocks indicates it was a sinistral slip along the N-S fault plane. The epicentral area is located in the western part of the Centennial Tectonic Belt, which is an area of southwest-northeast extension north of the Snake River Plain. This part of the Centennial Tectonic Belt is characterized by well-developed active normal faults. If the epicentral area is undergoing shear, it should be in a dextral sense because of a difference in the rate of rotation as in the case of the Centennial Shear Zone. We suggest that the sinistral slip of the Stanley event was a result of bookshelf faulting caused by the dextral shear in the epicentral area. To test this hypothesis, we employ kinematic block modeling. In these models, GPS surface velocities and earthquake slip vector data are inverted to solve for the best-fit rotational poles of modeled tectonic blocks in the study area. If a tectonic block adjacent to the hypocenter has the best-fit pole within or near the block (i.e., a small-radius rotation), it suggests that the Stanley earthquake resulted from bookshelf faulting.