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Published November 28, 2017 | public
Journal Article Open

Ten kilometer vertical Moho offset and shallow velocity contrast along the Denali fault zone from double-difference tomography, receiver functions, and fault zone head waves


We examine the structure of the Denali fault system in the crust and upper mantle using double-difference tomography, P-wave receiver functions, and analysis (spatial distribution and moveout) of fault zone head waves. The three methods have complementary sensitivity; tomography is sensitive to 3D seismic velocity structure but smooths sharp boundaries, receiver functions are sensitive to (quasi) horizontal interfaces, and fault zone head waves are sensitive to (quasi) vertical interfaces. The results indicate that the Mohorovičić discontinuity is vertically offset by 10 to 15 km along the central 600 km of the Denali fault in the imaged region, with the northern side having shallower Moho depths around 30 km. An automated phase picker algorithm is used to identify ~ 1400 events that generate fault zone head waves only at near-fault stations. At shorter hypocentral distances head waves are observed at stations on the northern side of the fault, while longer propagation distances and deeper events produce head waves on the southern side. These results suggest a reversal of the velocity contrast polarity with depth, which we confirm by computing average 1D velocity models separately north and south of the fault. Using teleseismic events with M ≥ 5.1, we obtain 31,400 P receiver functions and apply common-conversion-point stacking. The results are migrated to depth using the derived 3D tomography model. The imaged interfaces agree with the tomography model, showing a Moho offset along the central Denali fault and also the sub-parallel Hines Creek fault, a suture zone boundary 30 km to the north. To the east, this offset follows the Totschunda fault, which ruptured during the M7.9 2002 earthquake, rather than the Denali fault itself. The combined results suggest that the Denali fault zone separates two distinct crustal blocks, and that the Totschunda and Hines Creeks segments are important components of the fault and Cretaceous-aged suture zone structure.

Additional Information

© 2017 Elsevier B.V. Received 5 March 2017, Revised 30 August 2017, Accepted 2 September 2017, Available online 9 September 2017. The study was supported by the National Science Foundation (grants EAR-1315340, EAR-1314875, and EAR-1550328). We thank Cliff Thurber, Roland Bürgmann, Fan-Chi Lin, and Keith Koper for feedback on various aspects of the work, and Pieter-Ewald Share for useful suggestions for particle motion visualization. The arrival time and waveform data were acquired from the Alaska Earthquake Center. Figures were made using GMT (Wessel and Smith, 1998), Matlab, and Python, including ObsPy (Beyreuther et al., 2010). We thank and support the Open Access and Open Source initiatives. The facilities of IRIS Data Services, and specifically the IRIS Data Management Center, were used for access to waveforms, related metadata, and/or derived products used in this study. IRIS Data Services are funded through the Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation under Cooperative Agreement EAR-1261681. Data from the TA network were made freely available as part of the EarthScope USArray facility, operated by Incorporated Research Institutions for Seismology (IRIS) and supported by the National Science Foundation, under Cooperative Agreements EAR-1261681 and EAR-1251193. We thank reviewers Patrick Brennan and Ken Ridgway for thorough constructive reviews, and editor Kelin Wang for efficient handling of the manuscript.

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August 21, 2023
August 21, 2023