Relating Microseismicity to Fault Geometry at the Rattlesnake Ridge Landslide

The Rattlesnake Ridge rockslide, a slow-moving 3 million cubic meter translational landslide, developed beginning in October of 2017 on the Rattlesnake Hills anticline outside Union Gap, WA. The initial acceleration of the slide mass raised concern over the stability of the ridge resulting in road closures, mandatory evacuation of a nearby residential area, and placement of physical barriers to block rockfall onto Interstate 82. The slide accelerated for about six months, reaching a peak slip rate of ~74 cm/week, then gradually slowed down to half that rate by the end of 2018. We deployed 40 3C nodal seismometers around the Rattlesnake Ridge landslide body for a four-month period between March and July of 2018 to study microseismicity generated by the landslide body. The resulting dataset contains over 1 million small-magnitude repeating and non-repeating earthquakes with dominant frequencies of 30-40 Hz. We determine high resolution earthquake locations from this dataset. Over a year-and-a-half of continuous subsidence has exposed the eastern bounding fault, which appears to be the primary slip surface where these earthquakes occur. We also utilize terrestrial lidar to scan the exposed portion of the eastern slip surface and compare it to earthquake locations. The exposure of the eastern bounding fault presents a unique opportunity to study how fault geometry influences the distribution and character of microseismicity.