New landslide maps reveal Seattle fault's earthquake history

Newly discovered maps of over 1,000 deep-rooted landslides in Washington State’s Puget Lowlands provide evidence of the most recent major earthquake along the Seattle Fault approximately 1,100 years ago. These maps may also contain traces of older earthquakes along the fault.

Maps Reveal Seattle Fault History

Landslide clusters can potentially record earthquakes if researchers can establish when the landslides took place. The recent study published in the Bulletin of the Seismological Society of America combines the location data of these landslides in the Puget Lowlands with new dates derived from measuring the landslides’ surface roughness.

This data amalgamation allowed Erich Herzig of the University of Washington and his team to find compelling evidence of the last known major Seattle Fault earthquake, estimated to be a magnitude 7 to 7.5 event. (A recent study suggested that there may have been a double earthquake in the region at that time.)

The researchers compared their new landslide map to ground motions generated by different Seattle Fault earthquake scenarios. They found that the scenario that best matches the landslide clusters is one that produces the strongest shaking in a west to east band from west Seattle to Mercer Island and the bluffs bordering Puget Sound.

Herzig stated that while other studies have refined our understanding of the overall strength or timing of the 1,100-year-old Seattle Fault earthquake, this is the first study that has attempted to characterize the locations of strong shaking.

The researchers also discovered other landslide clusters dating back 4,600-4,200 years, 4,000-3,800 years, 2,800 to 2,600 years, and 2,200 to 2,000 years that could indicate older Seattle Fault earthquakes.

Lidar Maps Landslides for Earthquakes

The researchers started by mapping over 1,000 deep-seated landslides across the Puget Lowlands using a technique called airborne lidar. Deep-seated landslides refer to those where the slide plane is located beneath the tree roots.

Airborne lidar involves using lasers mounted on a plane to measure the land surface’s shape in detail, even through vegetation. This technology has recently been able to produce maps at one meter resolution or better, which is crucial for measuring roughness.

The roughness of a landslide surface is a relatively new technique used to estimate a landslide’s age. The concept is that ground surfaces become rougher after the mass movement of rocks and soil, meaning landslide deposits are the roughest immediately after the landslide occurs and smooth out over time. By modeling this age-roughness relationship, and with information from landslides dated by other means such as carbon dating, researchers can estimate when a particular landslide occurred.

In the BSSA study, the researchers calculated roughness by measuring variations in the land surface within a 15-meter-wide circle. Carbon dating of wood at some landslides provided data for calibrating the landslide ages.

New Method for Earthquake History

The researchers found spatial patterns in the landslides that correlate with the ground motions predicted by models of Seattle Fault earthquakes. They also observed that the timing of landslides in the Puget Lowland aligns better with a model of a pulse of landslides at the time of the last major Seattle Fault earthquake, rather than a model of landslides happening steadily over time.

The researchers believe their method offers a powerful new way of extracting information about past earthquakes from landslide data. This method is best suited to landscapes with steep hills prone to landslides and infrequent but powerful earthquakes. In the U.S., this could include other locations in the Puget Lowlands area or places in the New Madrid seismic zone. Other locations around the world, such as the Hinagu Fault zone in Japan or the Hellenic arc in Greece, could also be interesting to study.