Olympia and Chehalis that may extend to the Columbia River, and there has been a suggestion that the Tacoma Fault may connect with the White River—Naches River fault on the east side of the Cascades.) The uplift and basin pattern is continued to the west and southwest by the Grays Harbor Basin, Willapa Hills Uplift, and Astoria Basin, but it is not known if these are bounded by faults in the same manner as in the Puget Sound region. Structural models Thrust sheet hypothesis It is believed that all of these faults, folds, basins, and uplifts are related. According to the preeminent model, the "Puget Lowland thrust sheet hypothesis", these faults, etc., occur within a sheet of crust about 14 to 20 km deep that has separated from and is being thrust over deeper crustal blocks. Most of this thrust sheet consists of the Crescent Formation (corresponding to the Siletz River volcanics in Oregon and Metchosin Formation on Vancouver Island), a vast outpouring of volcanic basalt from the Eocene epoch (about 50 million years ago), with an origin variously attributed to a seamount chain, or continental margin rifting (see Siletzia). This "basement" rock is covered with sedimentary deposits similar to the Chuckanut Formation, and more recent (typically Miocene) volcanic deposits. The Seattle uplift, and possibly the Black Hills uplift, consist of Crescent Formation basalt that was exposed when it was forced up a ramp of some kind. This ramp could be either in the lower crustal blocks, or where the thrust sheet has split and one part is being forced over the next. Faults and folds may develop where the thrust sheet is being bent, or where the leading edge is thrust over softer, weaker sedimentary deposits, and breaks off and slumps. If, as this model suggests, the various faults are interconnected within the thrust sheet, there is a possibility that one earthquake could trigger others. This prospect is especially intriguing as a possible explanation of a cluster of seismic events around 1100 years ago. Seismotectonic modeling In the previous study seismicity, surface geology, and geophysical data were modeled in order to examine the fault structuring of the upper crust. Another model (of , USGS Open-File Report 99–0311) – not so much in competition with the first as complementing it – used seismic and other data to create a 3-D tectonic model of the whole crust; this was then analyzed