Characterizing Subsurface Structural Features of the Bellingham Basin by Gravity and Magnetic Modeling by Rahul Bhattacharya A thesis submitted to the Graduate Faculty of Auburn University In partial fulfillment of the requirement for the Degree of Master of Science Auburn, Alabama December 14, 2019 Copyright 2019 by Rahul Bhattacharya Approved by Lorraine W. Wolf, Chair, Professor of Geophysics Mark G. Steltenpohl, Professor of Geology Haibo Zou, Professor of Geology i Abstract This study uses gravity and magnetic data modeling to identify the geometry of the Bellingham basin, Washington, and its deformational history. The study also provides additional insight into three newly identified Holocene-active faults, Drayton Harbor, Birch Bay, and Sandy Point, that are linked with ongoing basin deformation. The models resulting from this analysis show the Bellingham basin as an arcuate- shaped, east-west-trending basin. The extent of the basin to the west is not revealed owing to a lack of offshore data. The Cascade Mountains form the basin’s eastern boundary. On the east side of the basin, the Boulder Creek fault forms the basin’s northern boundary, which may have been reactivated by ongoing crustal block rotation. Wavelength filtering of magnetic field data show distinct magnetic lineations, or “contacts,” that are assumed to result from juxtaposed rock units. Several of the contacts align with the Drayton Harbor, Birch Bay and Sandy Point faults, and suggest that the faults continue into the basin. Based on cross-sectional modeling and the filtered maps, the study postulates minimum lengths of 30, 30, and 24 km for the Drayton Harbor, Birch Bay, and Sandy Point faults, respectively. In this study, the Sandy Point and Birch Bay faults have been modeled as reverse faults with the northern side up. Drayton Harbor, although mapped as south-side up in previous work along the western shoreline, does not exhibit significant offset in its projected position onto the cross-sectional models. This result suggests that the Drayton Harbor fault does not extend far ii into the basin, or alternatively, the sense of slip along the fault is predominantly strike-slip as it continues into the basin. Epicenters of recorded earthquakes in the Bellingham basin appear closely aligned with the Birch Bay and Sandy Point faults, supporting the idea that these faults are currently active. Similarly, the moderately large (M = 5.0) 1990 Deming, WA, earthquake, which occurred ~ 7 km south of the Boulder Creek fault, along with scattered seismicity throughout the deep basin, provides evidence of ongoing deformation related to crustal block rotation and north-south directed compression. Results from this study and previous work argue that these crustal faults within the Bellingham basin should be considered in assessing seismic hazard in the Puget Sound region. iii Acknowledgments I would like to thank my advisor Dr. Lorraine Wolf for her guidance and help in this project. The challenges in this project were easy to overcome because of her constant direction and support. I would also like to thank my committee members Dr. Mark Steltenpohl and Dr. Haibo Zou for their valuable suggestions and insights in this work. I appreciate the Geosciences Advisory board for providing financial support in this project. I would also like to thank Auburn University College of Science and Mathematics (COSAM) for providing travel support to present my research results at the Symposium on the Application of Environmental and Engineering Geophysics (SAGEEP) 2019, Portland. I am thankful to Richard Blakely for providing the gravity data used in this study. I would also like to thank M. Anderson and J. Taylor for additional gravity data collection and their processing that improved the database. I appreciate Dr. Stephanie Rogers for helping me in exploring the ArcGIS database for the Pacific Northwest. Furthermore, I am grateful to USGS for supporting the gravity data collection by USGS NEHRP grant (USGS- G11AP20042) to Dr. L. Wolf. Also, this work would not be easily accomplished without the help of the Geosoft Oasis MontajTM technical support division. Finally, I would like to thank my family for their constant support and motivations. iv Table of Contents Abstract .................................................................................................................................................. ii Acknowledgments ................................................................................................................................ iv Introduction ........................................................................................................................................... 1 Geologic Setting ..................................................................................................................................... 5 Previous Studies .................................................................................................................................. 11 Methodology ........................................................................................................................................ 19 Gravity and Magnetic Data ............................................................................................................ 19 Data Processing ............................................................................................................................... 20 Wavelength-Separation Techniques .............................................................................................. 21 Data Modeling ................................................................................................................................. 22 Results .................................................................................................................................................. 23 Gravity Grid Maps ......................................................................................................................... 23 Magnetic Grid Maps ....................................................................................................................... 24 Wavelength-Filtered Maps ............................................................................................................. 29 Vertical Derivative Map/ Upward Continued Map ..................................................................... 29 Tilt Derivative Map ...................................................................................................................... 30 Cross-Sectional Profiles .................................................................................................................. 34 Cross-Sectional Profile A-Aꞌ........................................................................................................ 34 Cross-Sectional Profile B-Bꞌ........................................................................................................ 35 Discussion ............................................................................................................................................ 40 Geometry of the Bellingham Basin ................................................................................................ 40 Active Faults within the Bellingham basin ................................................................................... 41 Boulder Creek fault (BCF) .......................................................................................................... 41 Vedder Mountain fault (VM) ....................................................................................................... 42 Drayton Harbor fault (DH) ......................................................................................................... 42 Birch Bay Fault (BB) ................................................................................................................... 43 Sandy Point fault (SP) ................................................................................................................. 44 Implications for earthquake hazard .............................................................................................. 45 v Conclusion ........................................................................................................................................... 47 References ............................................................................................................................................ 49 vi List of Tables Table 1. Geologic units and tectonic units of the study area from Brown and Dragovich (2003) and Taylor (2013). ......................................................................................................................................... 8 Table 2. Modeled rock units for the cross-sectional profiles A-A' and B-B' with their lithologies, densities, and magnetic susceptibilities. ............................................................................................... 39 Table 3. Fault lengths within and outside of the Bellingham basin by Kelsey et al. (2012) compared to the results of this study. ........................................................................................................................ 45 vii List of Figures Figure 1. A: Tectonic map of the study area (red rectangular outline) showing relative motion of crustal blocks in the Cascadia forearc (Sherrod et al., 2008; Wells et al., 1998). The study area is situated between the northward-migrating Oregon coast