STATIC STRESS TRANSFER FROM HISTORICAL AND HYPOTHETICAL EARTHQUAKES IN NORTHERN ROCKIES TO THE YELLOWSTONE VOLCANIC SYSTEM

Static stress transfer from an earthquake to a magma reservoir could promote volcanic eruption. Here we examine effects of four large historical earthquakes in Northern Rockies, two mainshocks (Mb 6.3 and Mb 7.0) and the largest aftershock (Mb 6.5) of the 1959 Hebgen Lake sequence, and the 1983 Borah Peak (M_W 6.9), on the Yellowstone volcanic system. The two mainshocks of the 1959 Hebgen Lake sequence, five seconds apart from each other, occurred 50 km northwest of the center of the youngest caldera of Yellowstone at 10-15 km depths and possibly reactivated one or more E-W striking Laramide thrust faults in a dextral normal sense. The largest aftershock of the sequence is a normal faulting event that occurred 30 km north of the center of the caldera at 10 km depth. The 1983 Borah Peak earthquake ruptured a SE striking fault plane in a normal sense at 13.7 km depth 280 km east of the Yellowstone caldera. We model normal stress change imparted on the Yellowstone magma reservoir due to each earthquake to determine whether these mechanisms enhanced or inhibited the potential for eruption. An elastic dislocation model is used for calculation. In our model, the magma reservoir is approximated as a 60 km long, NE-SW striking vertical plane extending from 4 km to 14 km depths, based on a recent tomographic model. Our results show that the first mainshock of the Hebgen sequence had virtually no effect on the magma reservoir, and the second mainshock decreased the normal stress on the center of the model reservoir by ~0.20 bar (0.02 MPa). The aftershock, on the other hand, increased the normal stress on the center of the magma reservoir by ~0.19 bar, which could clamp the reservoir. The effect of the 1983 Borah Peak event did not reach the Yellowstone area. We also simulate a hypothetical M_W 7.6 event along each major fault in Northern Rockies with mechanisms typical to each location, determined by the fault attitude and past earthquakes.