Seismic Lessons from the Implosion of Reser Stadium’s Western Stands and the Dense 177-Node Corvallis Array

Abstract On the morning of 7 January 2022, the Western Stands of Reser Stadium, Corvallis, Oregon, was demolished. To capture this unique seismic event and inform local upper-crustal structure, we deployed a 177-node array for approximately three weeks with circular geometry centered on the stadium and a >5 km quasi-linear portion west of it and across the Corvallis fault (CF). We estimate an equivalent earthquake magnitude of Mw∼2.3 for the implosion. The stadium toppling toward the southwest and its shape generated two to three times higher peak ground accelerations (>0.1g <80 m away), and the most impulsive signals (>5 Hz) in that direction and longer duration elevated shaking (>60 s) to the northwest and southeast. Using implosion P first arrivals, we model shallow VP along the >5-km-long profile and invert Rayleigh- and Love-wave phase velocities derived from ambient noise to obtain VS on an overlapping ∼3.6 km section. Both models show a 300–500 m deep basin (VP<3 km/s, VS<1 km/s) spanning ∼2 km of the profile and starting near the stadium where Spencer Formation basement is locally uplifted. This transition from shallow basement rock to deeper basin coincides with a rapid reduction in prolonged high-amplitude shaking and high-frequency content from the implosion. The basin shallows around the CF before giving way to high-velocity Siletz basalts in the west. Analyses of teleseismic P waveforms pinpoint a rapid change in crustal structure >300 m east of the mapped CF, and an uncataloged shallow local event, identified with EQTransformer, has an epicenter west of it. These results highlight the need for more widespread near-field observations of seismic sources; a concerted effort to identify the most recently active locus within the CF zone; and more broadly, better understanding the heterogeneous crust of the Willamette Valley and its unique seismic response.