Liem alerted me to the local seismic network and sent the Mach 22 seismic plot from the Jim Creek Station. A seismic signal was also recorded at the Cultus Station and Stanwood Station.
Seismic network from the Pacific Northwest Seismic Network
(Stanwood seems a bit far from Stanwood proper)
All three stations record two events at approximately the same time and at the same time that the landslide event was reported to have happened. If this signal is from the slide, it indicates that two separate events took place a few minutes apart.
I checked other stations a bit further afield and found no signal, further suggesting that the slide was the source of the seismic signal.