Peeking below Columbia River flood basalts with high-resolution aeromagnetic data: implications for central Washington earthquake hazards

The largest crustal earthquake in Washington's recorded history (M 6.8) occurred in 1872 in the vicinity of Lake Chelan. Numerous smaller earthquakes (>1000 earthquakes since 1971 with 1.0 ≤ MW ≤ 4.3) continue to occur 20 km south of Lake Chelan near the town of Entiat, yet little is known about active structures responsible for this ongoing deformation. A 2011 aeromagnetic survey may provide insights. The survey was flown with a fixed-wing aircraft along flight lines spaced 400 m apart and at an altitude 250 m above terrain or as low as safely possible. The survey illuminates two distinct magnetic patterns. Northwest of Entiat, broad, subdued magnetic anomalies are caused by weakly magnetic, pre-Tertiary basement rocks striking generally NW. Magnetic lineaments are associated, for example, with the NW-striking Entiat fault, the structural margin of the Chiwaukum graben, which is well represented by gravity anomalies. Southeast of Entiat, high-amplitude, short-wavelength magnetic anomalies are caused by strongly magnetic rocks of the Miocene Columbia River Basalt Group (CRBG) exposed throughout this region. Northwest-striking basement anomalies, so clear NW of Entiat, are not obvious SE of Entiat, yet there is no reason to believe basement structures do not extend beneath CRBG. We used matched filtering methods to illuminate the crustal framework of the Entiat earthquakes beneath CRBG. We selected two sub regions, one over pre-Tertiary basement NW of Entiat (sub region 1), the other over CRBG SE of Entiat (sub region 2). We modeled each sub region by fitting layer parameters to power spectra determined from magnetic anomalies (Phillips, 2007). A strongly magnetic layer was determined 470 m below the aircraft in sub region 2, which we interpret as the average top of CRBG. This interpretation is supported by the absence of a similar magnetic layer in sub region 1, where CRBG is in fact absent. Using this determination, we designed a matched filter to subdue magnetic anomalies originating from the depth of CRBG and applied it to the entire area. The filter has little effect on regions NW of Entiat, where CRBG is not present, but significantly modifies anomalies over CRBG. We interpret the filtered anomalies as being caused by pre-Tertiary basement rocks concealed beneath CRBG. We then analyzed the filtered anomalies using curvature analysis (Phillips et al., 2007) to develop a map of pre-Tertiary structures. Entiat earthquakes are occurring where NW-striking basement structures are offset by a NE-striking basement lineament. Entiat earthquakes are loosely aligned along the lineament, and most first-motion solutions are consistent with a NE-striking fault plane. The lineament is sub parallel to a NW-side-up topographic scarp 5 km SSW of Entiat observed in the field and in new LiDAR data. Although the lineament is evident in basement magnetic anomalies, it also appears in anomalies filtered to emphasize shallow sources, suggesting that the source of the lineament penetrates CRBG. These associations suggest an active NE-striking fault that deforms topography, penetrates Miocene CRBG, and roots into pre-Tertiary basement. Phillips, J.D., 2007, U.S. Geological Survey Open-File Report 2007-1355. Phillips, J.D., Hansen, R.O., and Blakely, R.J., 2007, Expl. Geophys., v. 38, p. 111-119.