Double-difference adjoint tomography of the Cascadia subduction zone

Abstract:

Along the strike of the Cascadia subduction zone, the seismicity occurrence rate significantly varies, with peaks at the northern and southern ends of the Cascadia, for both the regular earthquakes located within the slab and the episodic tremor and slip (ETS) near the subduction interface. It is not clear how the wave speed variations of the slab, the overriding plate, and the mantle wedge contribute to this distinct along-strike distribution of intraslab earthquakes and ETS, partly due to the discrepancies amongst different tomographic models. This study aims at improving the image resolution and accuracy of the Cascadia subduction zone by the double-difference travel time (DDTT) adjoint tomography. The DDTT adjoint tomography minimizes the differential travel time misfit between the observed and synthetic waveforms of a station pair. With a limited number of earthquakes available in our study region but dense station coverage, we still can obtain a large number of DDTT measurements, comparable to the number of absolute travel time measurements made from hundreds of earthquakes. Our synthetic tests show that with eight local earthquakes recorded by 800 stations, more than one million measurements are obtained, and the synthetic model can be well recovered. Moreover, different from the ambient noise tomography, this study uses earthquake data that have higher signal-to-noise ratios, a broader frequency band, and both body waves and surface waves. Besides, a new inversion strategy for incorporating surface waves is proposed by starting from shorter periods (e.g., 12-25 s) to first constrain the shallow structure and then adding in longer periods (e.g., 25 – 50 s, 50-100 s, and 75-150s) to recover deeper structure. Our new model, based on DDTT adjoint tomography of earthquake data and an initial model from the latest ambient noise full-wave inversion, will reveal more detailed along-strike structural variations of the Cascadia subduction zone and provide insight into the along-strike distribution of intraslab earthquakes and ETS.