Abstract
The paleoseismic record for Quaternary-active faults bordering the east side of the Seattle metro area is incomplete. A full characterization of modern moderate-magnitude seismicity may help fill the gap in our knowledge of recent activity on these faults. The Tokul Creek fault (TCF) is a northeast trending, oblique, strike-slip fault near Fall City, Washington. A series of earthquakes (M=3.8 max) likely occurred on the TCF (8/8/2023). The oblique, strike-slip Cherry Creek fault (CCF) is near and sub-parallel to the TCF. Our past studies indicate the 1996 Duvall earthquake swarm (M=5.3 max) likely occurred on the CCF. This is supported by the predominantly left-lateral strike slip focal mechanisms for these quakes and the spatial correlation of the quakes with the location of TCF and CCF. These two faults are likely conjugate to the northwest-trending, right-lateral strike-slip southern Whidbey Island fault zone (SWIF) west of the TCF and CCF lineaments. An eastside up component for the TCF and CCF system suggests these structures partly accommodate uplift of the Cascades to the east. The Monroe fault zone (MF) is an east-striking set of reverse faults mapped from the Sultan area to the west through Monroe where the MF joins the SWIF near Lords Hill. A series of (M=4.6 max) quakes occurred on 12 July 2019 near the MF and below Snohomish River Valley west of Monroe. The reverse focal mechanism for the mainshock on an east-west trending fault is consistent with the kinematics of the MF mapped at the surface to the east. Because the MF location is tenuously located in this area and the earthquakes are relatively deep, the correlation of the MF with as the 2019 earthquakes is only moderate and requires further study. The Explorer Falls basin (EFB) is a Pleistocene composite structure (8 km wide and 16 km long) in the Lake Roesiger and Granite Falls Quads. The EFB is bounded by the east-west-trending Carpenter Creek fault (CaCF) on the north and the Three Lakes Hill fault (TLF) on the south. The CaCF generally aligns with a linear band of seismicity south of the CaCF, which contains many reverse fault focal mechanisms, consistent with modern reverse movement the CaCF and TLF. Because of the low magnitude and the diffuse nature of the earthquakes, the correlation of the CaCF and TLF with these recent quakes is deemed tentative and requires further study.