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Ambient Noise Seismic Imaging of an Urban Fault: A Citizen Scientist-Hosted Investigation of the Seattle Fault Zone

Description: 

Sedimentary basins amplify and prolong strong shaking during large earthquakes. The city of Seattle sits atop the Seattle basin, and faces high seismic hazard posed by both the regional-scale Cascadia subduction zone and crustal faults. The Seattle fault zone forms the southern border of the basin, presenting an actively deforming basin edge with a complex internal geometry and velocity structure. Resolving the location and character of this active, urban fault is necessary for rigorous seismic hazard assessment in the city of Seattle. We deployed 100 nodal seismometers across the Seattle fault zone and basin edge in four North-South transects for one month in July 2019. All instruments were hosted by Seattle residents, and installation included educational involvement with residents and students.

We process and perform auto-correlation and cross-correlation analysis of the continuous ambient seismic noise data. Correlations are contaminated by anthropogenic activities, particularly highways (I-90 and I-5), which causes spurious arrivals in all correlations. To select the optimal correlations (i.e., minimal spurious arrivals) for generating the stack, we perform Gaussian mixture model clustering on the waveform shapes, using the correlation coefficient as a distance metric. We automate the clustering of correlations that we attribute to daytime and quiet times (nighttime and weekends). We further investigate if the re-constructed single-station correlations enable us to observe subsurface geological interfaces. In particular, we attempt to characterize the structure of the Seattle fault zone. Using one-bit spectrally whitened cross-correlations of the “quiet time” cluster, we produce broadband dispersion curves for surface waves in the 1-10 Hz frequency band. With DSurfTomo (Fang et al. 2015), we then directly invert these dispersion curves to produce 3D shear wave speed models.

Session: Constraining Seismic Hazard in the Cascadia Subduction Zone [Poster]

Type: Poster

Date: 4/20/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Natasha Toghramadjian

Student Presenter: Yes

Invited Presentation: 

Authors