The Deformation Front of the Seattle Fault Near Downtown Seattle: Constraints from New Active Source Seismic Data

Session: Advances in Upper Crustal Geophysical Characterization [Poster]
Type: Poster
Date: 4/29/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Seismic reflection data, acquired in July 2019 between Puget Sound and Lake Washington, provide constraints on the northern limits for the Seattle fault zone. Pinpointing the fault’s location is critical for hazard assessments for the downtown Seattle region, as uplift from a possible M7 earthquake is expected to extend north to the deformation front. The new seismic data were acquired with an accelerated weight drop source and recorded with both a 120-channel seismograph and three-component Nodal sensors. The cabled recording system provides reflection images to upwards of 800 m depth via geophone planting and base plates placed along city streets. Nodal data, although sparse, provide longer-offset recordings that enable reflection imaging to greater depths. The Nodal data allowed for seismic imaging beneath Interstate-90 by placing sources and receivers on both sides of the highway. This objective was critical, as aeromagnetic data place the deformation front beneath I-90. Seismic results show north-dipping Tertiary strata associated with a monocline lie mostly to the south of I-90. To the north of the interstate, we observe mostly flat-lying Tertiary and younger strata. Identifying this transition from dipping to flat-lying Tertiary strata and projecting the fault to the land surface using past estimates of the fault dip suggests the Seattle fault zone extends north to the downtown Seattle region. Unpublished magnetic data acquired on Lake Washington (R. Blakely, personal comm), borehole logs showing deformed glacial strata near downtown Seattle, and past seismic survey results (Pratt et al., 2015) support our interpretation. Integration of new seismic data with past marine, land-based reflection profiles and magnetic grid data allow for a refined fault interpretation of the Seattle fault beneath this dense urban region. Complementary shear wave velocities provide key high frequency site response estimates within the fault zone.

Presenting Author: Lee M. Liberty

Authors