Geophysical investigation of the southern Puget Sound area, Washington

Abstract

Detailed surface geology and gravity maps of the Puget Sound area, combined with limited magnetic observations, earthquake epicenters, and well records, have been studied in order to determine the geologic structure of the region. The Puget Sound lowlands are a deep crustal depression separated from the Olympic Peninsula by an active fault of about 4000 m throw. The basin is divided into three main parts by an igneous horst with an older gabbroic core and younger basaltic layers. The northeastern part of the basin, centered around Seattle, has sedimentary thicknesses of about 10,000 m. The southern part, centered near Tacoma, and the western one, around Skokomish, are about 4000 m deep. Faults along the horst are active, the northern one more so. Surface geological work is hampered by a thick layer of glacial till.

Holocene fault scarps near Tacoma, Washington, USA

“…The faults, which separate areas of coastal uplift and subsidence (Fig. 2C), were first identified from geophysical lineaments (Danes et al, 1965). New geophysical data (Brocher et al, 2001) confirm that one fault zone passes beneath Seattle and another beneath Tacoma (Figs.…”

Western limits of the Seattle fault zone and its interaction with the Olympic Peninsula, Washington

“…The upper member is more magnetic than the lower member and therefore is more evident in aeromagnetic data (Blakely et al, 2009). The strong magnetic properties result in well-defi ned, linear magnetic anomalies where the Crescent Formation is folded or vertically displaced by faults, allowing potential fi eld modeling of structures in the region (e.g., Daneš et al, 1965;Gower et al, 1985;Blakely et al, 1999Blakely et al, , 2002Blakely et al, , 2009Brocher et al, 2004;Johnson et al, 2004).…”

Folklore and earthquakes: Native American oral traditions from Cascadia compared with written traditions from Japan

“…long indicated a substantial offset in basement rocks beneath Puget Sound (Danes et al 1965), no clear pattern of recent earthquake activity defining the fault has been observed. However, geological evidence of an earthquake around AD 900 (estimated magnitude 7.4) came to light in the early 1990s (Bucknam et al 1992) and the Seattle fault is now recognized as a substantial hazard to the Seattle urban area.…”