Mild Displacements of Boulders during the 2019 Ridgecrest Earthquakes

Session: Observations From the 2019 Ridgecrest Earthquake Sequence [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Strong seismic waves from the July 2019 Ridgecrest, California, earthquakes displaced rocks in proximity to the M7.1 mainshock fault trace at several locations. In addition, boulders were displaced at the Wagon Wheel Staging Area (WWSA), approximately 4.5 km southeast of the southern terminus of the M6.4 foreshock rupture and 9 km southwest of the nearest approach of the M7.1 mainshock surface rupture. Some boulders appear to have slid along essentially flat surfaces, which suggests that dynamic stresses overcame the coefficient of friction. Other boulders appear to have rocked within their sockets. In both cases, we estimate total peak dynamic accelerations between 0.5 and 1 g, commensurate with modified Mercalli intensity 9, using simple mechanical models. It is unclear if the strongest shaking at this location occurred during the M6.4 foreshock or the M7.1 mainshock. The inferred accelerations are higher than predicted mainshock ground motions at WWSA, although local high accelerations could have been generated by path, site or source effects. Gaps between boulders and their sockets are easily visible in the immediate aftermath of earthquakes and provide an quick indication of strong shaking. More importantly, the gaps quickly fill with surficial debris. Boulders may thus potentially be extracted to examine gap fillings associated with past earthquakes, providing a new datable paleoseismic method.

Presenting Author: Norman H. Sleep

Authors