Spatial and Temporal Evolution of Microseismicity of the Rattlesnake Ridge Landslide

Date: 4/26/2019

Time: 01:45 PM

Room: Elliott Bay

In October of 2017, a 3 million cubic meter translational landslide, known as the Rattlesnake Ridge rockslide, developed over the course of two months on the Rattlesnake Hills anticline outside Union Gap, WA. 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 concrete filled shipping containers to block rockfall onto Interstate 82. The slide accelerated until early April reaching a peak slip rate of 74 cm/day, approximately two orders of magnitude faster than “typical” plate boundary faults, and then has gradually slow down to half that rate by the end of 2018. To study microseismicity associated with slide motion, we deployed 40 3C Nodal seismometers around the Rattlesnake Ridge landslide body for a four-month period between March and July of 2018. Initial analysis reveals frequently occurring, very small magnitude earthquakes with dominant frequencies of 30-40 Hz. Because these events often exceed background noise levels, and have impulsive arrivals, we use STA/LTA on 118 days of seismic data to compile an initial set of earthquake detections. We then use these events as waveform templates and cross-correlate them through the remaining dataset to search for repeating earthquakes. After summing the daily cross correlations for a particular template, we consider anything with a summed network cross correlation of greater than eight times the median absolute deviation as a repeater. The resulting families of repeating events are subsequently culled such that no family shares more than 20% of detections with any other family. This process results in 30,000+ repeating earthquakes per day constituting 60+ families. Crude location estimates suggest that these events occur both along the bounding, left-lateral oblique fault on the slide margin and within the slide body itself. Here we report on the locations, occurrence patterns, and evolution of waveform character of this repeating earthquake dataset.

Presenting Author: Amanda M. Thomas

Authors