DEEP ROOTS AND SHALLOW DAMAGE STRUCTURE OF THE DENALI FAULT ZONE FROM TOMOGRAPHIC AND LARGE-N SMALL-APERTURE ARRAY SEISMIC IMAGING

Despite being one of the longest (2000km) and oldest (100 Ma) strike-slip fault systems in the world, and situated along a major Mesozoic-aged suture zone, the Denali Fault remains poorly understood at multiple scales. At the regional scale, there is little constraint on the fault zone structure at depth and the along-strike variation of deformation patterns. At the local scale, no seismological study has been conducted to examine the fault damage zone and principal slip surface structure. Here we present results of separate seismic imaging studies of the Denali fault: regional tomography, and local-scale observational analysis of the damage zone structure using hundreds of seismometers with ~100m spacing. We image a 600km by 500km by 60km volume around the central portion of the Denali fault zone using 715,000 P and 229,000 S wave phase arrivals including data from the EarthScope TA. The tomographic results indicate that the Moho is offset by approximately 10km along the entire resolved length of the Denali fault, with the northern side having the shallower Moho depths around 30km. A shallow reversal contrast polarity in the upper 10km leads to an unusual distribution of fault zone head waves, with near-fault stations on the northern side of the fault recording head waves from nearby event and stations on the southern side recording head waves from distant events. In addition to the tomographic imaging and head wave detection, we examine data from a ~10km aperture temporary array of 196 three-component Fairfield Nodal geophones. From the amplitude distribution of the earthquake-induced trapped waves and ambient-noise-derived Rayleigh wave phase velocities, we are able to image the damage zone structure at high resolution along a segment of the Denali fault that ruptured during the M7.9 2002 event. Our results allow for the interpretation of the suture zone structure at depth, and have implications for the earthquake dynamics and tectonic history of central Alaska.