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Moment Tensor and Rupture Model for the 1949 Olympia, Washington, Earthquake and Scaling Relations for Cascadia and Global Intraslab Earthquakes

¹ URS Group, Inc.
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We reanalyzed the 13 April 1949 Olympia, Washington, earthquake by using digitized records and first-motion polarities from long-period seismograms. The moment-tensor mechanism is normal faulting with a down-dip-trending T axis similar in style to other Cascadia intraslab earthquakes. The total seismic moment is 1.3 x 10²⁶ dyne cm (M_w 6.7) and the hypocenter depth is 60 km. Additional inverse modeling for the kinematic rupture process assuming the steeply east-dipping fault plane from the moment tensor resulted in a slightly higher total moment of 1.9 x 10²⁶ dyne cm (M_w 6.8). The earthquake ruptured to the south with at least two subevents.

The combined area of asperities and seismic moment for the 1949 earthquake was compiled with those from the 1965 Seattle-Tacoma and the 2001 Nisqually earthquakes and with those from Japan and Mexico to develop a source-scaling relation separate from shallow global strike-slip earthquakes. We infer that deeper intraslab earthquakes have a significantly smaller combined area of asperities than those compiled for shallower strike-slip earthquakes with the same seismic moment. This difference in rupture area leads to a 3- to 5-fold increase in stress drop for earthquakes with seismic moments between 10²⁴ and 10²⁸ dyne cm.

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