Velocity contrast and 10km vertical Moho offset across the Denali fault from double-difference tomography and fault zone head wave analysis

Abstract:

We present tomographic images of lithospheric structure along the Denali fault in central Alaska based on double-difference inversions of earthquake arrival times. We discretize the region with a uniform grid spacing of 3km within a 600km by 500km by 60km volume. We invert for V_P, V_S, and hypocenter location using data from 5634 earthquakes recorded at 326 stations, incorporating 715,000 P and 229,000 S wave phase arrivals. The use of this large dataset provides resolution throughout the crust and into the upper mantle, with diminishing resolution below 50km depth as determined with checkerboard tests and calculation of the inversion derivative weight sum. 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. This indicates that the Denali fault is likely a deep lithospheric structure which penetrates into the upper mantle. The shallow crustal velocity structure of the Denali fault is more complicated with high-velocity plutonic bodies and low-velocity subsidiary fault zones, though the northern side of the fault generally has slightly lower velocities. In order to bolster the tomographic images we analyze more than 100 events recorded at 55 near-fault stations to find fault zone head waves, which offer a clear indication of a sharp across-fault velocity contrast. In addition to picking head waves manually using horizontal particle motion, we run an automated picker over the entire dataset using no assumptions about likely head wave distributions. Most of the head wave detections are located on the northern side of the fault fault near the town of Healy, though the source-receiver geometry may be suboptimal for detection in other portions of the fault zone. Taken together, the tomographic and head wave results have important implications for the shallow crust, deeper lithospheric structure, and tectonic history of the Denali fault and the Central Alaskan Interior.