Downdip Extension of Large Intraslab Earthquakes and Its Engineering Implications

Date: 4/24/2019

Time: 08:30 AM

Room: Pine

Previous studies suggested a global correlation between the 600^o-700^oisothermal depth and the bottom of seismogeneic zone of the oceanic lithosphere (e.g., Craig et al., 2014). If the large intermediate depth and deep focus earthquakes occur along the existing faults before the subduction (e.g., Jiao et al., 2000), the maximum downdip extension of these events is limited. Here, we first test this hypothesis using the 2017 Mw 8.2 Mexico earthquake, the largest intraslab earthquake with close-fault modern observations. The downdip extensions inferred from published slip models varies from 30 km (Okuwaki and Yagi, 2017) to about 60 km (Melger et a., 2018). We constrain its slip history with local strong motion and high rate GPS waveforms, as well as body and surface waves in teleseismic distances. In particular, 3D Green’s functions are used to model the earth response. The uncertainty of rupture downdip extensions will be specially investigated using a subject-oriented finite fault inversion approach. We illustrate the engineering implications of this hypothesis by simulating intraslab earthquakes for the Japan Trench and for Cascadia using a 1D velocity and attenuation model representative of each structure. We test the scenarios with moment magnitude M 6.2, 6.8, 7.4 and 8.0, and use a combination of 1) the temperature related brittle deformation limitation of the oceanic plate, 2) expected dip angle of outer-rise events, and 3) the plate unbending mechanism to place the limits on the maximum width of rupture. Using the inferred maximum width, we found magnitude saturation of PGA and PGV for intraslab earthquakes in Cascadia but no evident magnitude saturation for the Japan case. We interpret this magnitude saturation as the result of their difference in the pre-assigned maximum downdip extension of the scenario slip models. Finally, using the same approach, we estimate the expected saturation magnitude for several subduction zones.

Presenting Author: Chen Ji

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