Strands of the east-striking Devils Mountain Fault cross the northern tip of Whidbey Island at Dugualla Bay and north side of Ault Field ( Whidbey Island Naval Air Station ) . Just four miles ( 6 km ) south the city of Oak Harbor straddles several stands of the Utsalady Point Fault ( UPF ) as they head roughly east-southeast towards Utsalady Point at the north end of Camano Island . And in between these two the Strawberry Point Fault ( SPF ) skirts the south side of Ault Field , splits into various strands that bracket Strawberry Point , and then disappear ( possibly ending ) under the delta of the Skagit River . Both the SPF and UPF are said to be oblique-slip transpressional ; that is , the faults show both horizontal and vertical slip as the crustal blocks are pressed together . These faults also form the north and south boundaries of uplifted pre-Tertiary rock , suggesting that the faults come together at a lower level , much like one model of the Seattle and Tacoma faults , but at a smaller scale . Marine seismic reflection surveys on either side of Whidbey Island extend the known length of these faults to at least 26 and 28 km ( about 15 miles ) . The true length of the UPF is likely twice as long , as it forms the southern margin of an aeromagnetic high that extends another 25 km to the southeast . Trenching on the UPF ( at a scarp identified by LIDAR ) shows at least one and probably two Holocene earthquakes of magnitude 6.7 or more , the most recent one between AD 1550 to 1850 , and possibly triggered by the 1700 Cascadia earthquake . These earthquakes probably caused tsunamis , and several nearby locations have evidence of tsunamis not correlated with other known quakes
1700 Cascadia earthquake
1700 Cascadia earthquake
Puget Sound faults