The magnitude 5.0 Robinson Point earthquake of 29 January 1995 at 03:11:22 PM UTC (28 January 1995 at 07:11:22 PM PST) occurred at a depth of 20 km roughly midway between the cities of Seattle and Tacoma; at a map location nearly coincident with the much deeper 60 km and much larger (mb = 6.5) 1965 Seattle earthquake. The Robinson Point sequence includes one foreshock (Mc = 1.8 on Jan. 28 at 14:18 UTC), the main shock, and 25 aftershocks recorded through 14 April, 1995. Aftershock epicenters cluster about the mainshock; the hypocenter distribution shows a steeply dipping distribution with the mainshock at the bottom, near 20 km (after relocation with modified station corrections). P-wave first motions from the mainshock indicate reverse faulting with east-west trending nodal planes. Figure 1 shows a map and cross section views of the sequence and the P-wave polarity focal mechanism.
Minor damage was reported in Auburn, Tacoma, and Puyallup and shaking was felt throughout western Washington to as far away as Salem, Oregon and Vancouver, British Columbia. The Point Robinson mainshock was the largest earthquake to occur in the Puget-Willamette lowland region of the Pacific Northwest since the ML = 5.6 Scotts Mills earthquake of 25 March, 1993, and was the first moderate-sized Puget Sound earthquake to be recorded by the three-component, wide dynamic range, broadband instruments added recently to the PNSN. This broadband data provides the only unclipped local records of the mainshock.
The Robinson Point sequence included relatively few aftershocks, none larger than magnitude 2.2. A comparison of the Robinson Point aftershock sequence to aftershock sequences of several other moderate-sized, well-recorded crustal earthquakes (the 1981 Elk Lake (Grant et al., 1984), 1981 Goat Rocks (Zollweg and Crosson, 1981), 1990 Deming (Qamar and Zollweg, 1990), 1993 Scotts Mills (Thomas et al., 1995), and 1989 Storm King Mtn.) provides some insight into the nature of deep crustal sources in western Washington.
Aftershock sequences associated with the four moderate-sized crustal mainshocks at depths of 15 km or less (1981 Elk Lake, 7 km; 1981 Goat Rocks, 3 km; 1990 Deming, < 5 km; and 1993 Scotts Mills 15 km) differ systematically from sequences following two moderate-sized deeper-crustal mainshocks (1995 Robinson Point, 20 km; and 1989 Storm King Mountain 18 km.) The four shallower mainshocks are followed by two general aftershock behaviors: a significant number of aftershocks (70-600 events with Mc > 0.0) and an exponential decay in event frequency with time. In contrast, the deeper mainshocks show relatively few aftershocks (< 30 events) and no significant decay in occurrence with time immediately following the mainshock. In addition, both of the deeper-crustal-mainshock sequences lack aftershocks larger than magnitude 2.4.
The close spatial relation of the Robinson Point sequence to the Seattle Fault is also of great interest. Gravity data, geologic evidence, and seismic reflection data indicate that the Seattle Fault is an east-west trending blind thrust fault dipping to the south. The surface manifestation of this fault is approximately 30 km to the north-northwest of the mainshock epicenter (see figure). Geologic observations suggest that a reverse slip earthquake with 7 m of sudden uplift of a marine terrace at Restoration Point occurred on the Seattle fault about 1100 yr ago. The cross-section shows the projected position of the Seattle Fault at depth according to a preprint of "The Puget Lowland Thrust Sheet", by T.L. Pratt et. al. Pratt's hypothesis is that the Puget Sound region lies on a north-directed thrust sheet, shown in Figure 1 at about 17 km depth. The Seattle Fault, near-vertical at the surface, becomes south-dipping at depth and joins the thrust decollement. Although the cross section shows the sequence occurring beneath the decollement and not on the Seattle fault, there are significant uncertainties about the depth of the decollement surface and the dip and position of the Seattle Fault at depth. In general, the suite of deeper crustal earthquakes in the Puget Sound lies beneath the decollement surface shown by Pratt et. al. Whether the Robinson Point earthquake sequence occurred on the Seattle Fault remains unresolved at present. The sequence may have occurred on the Seattle Fault, be associated with a conjugate fault or a splay of the Seattle Fault, or may have occurred on a previously unknown blind thrust fault.
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UW Dept. of Earth and Space Sciences ..... PNW EARTHQUAKES