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Agana Fault Adelup Fault FINAL TECHNICAL REPORT LIDAR AND FIELD MAPPING OF QUATERNARY FAULTS IN THE U.S. TERRITORY OF GUAM Prepared for U.S. Geological Survey National Earthquake Hazards Reduction Program Award Number G11AP20058 July 2015 URS Corporation Seismic Hazards Group 1333 Broadway, Suite 800 Oakland, California 94612 FINAL TECHNICAL REPORT LIDAR AND FIELD MAPPING OF QUATERNARY FAULTS IN THE U.S. TERRITORY OF GUAM Principal Investigators Judith Zachariasen Susan Olig URS Corporation (Award G09AP00048) 1333 Broadway, Suite 800, Oakland, CA 94612 (510) 874-1749 (phone); (510) 874-3268 (fax); [email protected] Program Elements I and III Northern California Region Keywords: Quaternary fault behavior, neotectonics, paleoseismology U. S. Geological Survey National Earthquake Hazards Reduction Program Award Number G11AP20058 July 2015 This research was supported by the U.S. Geological Survey (USGS), Department of the Interior, under USGS Awards G11AP20058. The views and conclusions contained in this document are those of the Principal Investigators only and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government. i ABSTRACT The U.S. territory of Guam is the largest, most-populated, and southernmost of the Mariana Islands, a chain of volcanic islands and seamounts that constitute the Mariana Arc, and it lies in the midst of a complex plate boundary zone in the western Pacific Ocean. Guam is located on the Mariana plate, a microplate of the Philippine Sea plate, and is flanked on the east and southeast by the Mariana subduction zone, on the west by an active spreading center, and on the south by a series of right- lateral transform faults. Several large historic earthquakes, including an M 7.7 event in the Mariana subduction zone in 1993, have affected Guam. Furthermore, there are numerous surface faults on the island that appear to displace Quaternary materials. The tectonic setting and the presence of Quaternary faults suggest that Guam could face a significant seismic hazard from the subduction zone and/or crustal faults. Little work to date, however, has been done in identifying and assessing the seismic potential of the crustal faults. This proposal aims to address the lack of data about onshore Quaternary faults in Guam with the goal of providing more accurate information about the sources that contribute to Guam’s seismic hazard. The reports presents a new map of the Quaternary faults of Guam. Based on analysis of island-wide Lidar, bathymetric data, aerial photographs, and field mapping we have compiled, at a scale of 1:50,000, a map of lineaments that do or may represent Quaternary faults. We also include a compilation of fault data where obtainable, including geometry, sense of slip, and level of activity. This map of Guam’s active faults may be used as a first step in characterizing the onshore sources of seismic hazard. ii TABLE OF CONTENTS 1.0 Introduction........................................................................................................................................... 1 2.0 Tectonic Setting .................................................................................................................................... 2 2.1 Regional Seismicity .......................................................................................................................... 2 2.2 Geology of Guam .............................................................................................................................. 3 2.3 Crustal Faults .................................................................................................................................... 3 2.4 Quaternary Geology and Tectonics ................................................................................................... 4 2.5 Fault Characterization ....................................................................................................................... 5 3.0 Results .................................................................................................................................................. 7 3.1 Methods ............................................................................................................................................ 7 3.2 Fault Map .......................................................................................................................................... 8 3.3 Fault Characteristics .......................................................................................................................... 9 3.3.1 Ritidian Fault Zone .................................................................................................................... 9 3.3.2 Anderson Air Force Base Fault ................................................................................................ 11 3.3.3 Machanao fault ......................................................................................................................... 11 3.3.4 Finegayan Fault ........................................................................................................................ 12 3.3.5 Tamuning-Yigo Fault ............................................................................................................... 12 3.3.6 Agana Fault .............................................................................................................................. 14 3.3.7 Adelup fault ............................................................................................................................. 15 4.0 Discussion ........................................................................................................................................... 16 5.0 Conclusions......................................................................................................................................... 16 6.0 References .......................................................................................................................................... 17 iii LIST OF TABLES AND FIGURES Table 1. Radiocarbon Data from Crystal Springs South Trenches 1 and 2, August 2010 Table 2. Source characteristics of Quaternary faults in Guam Plate 1. Map of Quaternary faults on Guam Figure 1. Location map and tectonic setting of Guam. Figure 2. Physiographic provinces, structural blocks, and faults. Figure 3. Quaternary fault map (this study) over geologic map (Siegrist et al., 2007) Figure 4. Map of Ritidian and AAFB faults, northern Guam. Figure 5. Topographic profiles across the Ritidian fault, northern Guam. Figure 6. Photos of Ritidian fault. Figure 7. Marine terraces displaced by Ritidian fault at Ritidian Point. Figure 8. Topographic profiles of marine terraces at Ritidian Point. Figure 9. Topographic profiles across the AAFB fault, northern Guam. Figure 10. Marine terraces displaced by AAFB fault at Ritidian Point Figure 11. Topographic profiles of marine terraces across AAFB fault, northern Guam Figure 12. Map of Machanao and Finegayan faults, northern Guam. Figure 13. Topographic profiles across Machanao and Finegayan faults Figure 14. Marine terraces displaced by Machanao fault, northeast Guam Figure 15. Photographs along Machanao fault Figure 16. Topographic profiles of marine terraces and across Machanao fault Figure 17. Map of Tamuning-Yigo fault, northern Guam Figure 18. Topographic profiles across Tamuning-Yigo fault Figure 19. Topographic map and profiles of marine terraces and Tamuning-Yigo fault, Agana Bay Figure 20. Photographs along the Tamuning-Yigo fault, central Guam Figure 21. Map of Agana and Adelup faults Figure 22. Topographic Profiles across the Adelup fault iv Figure 23. Photographs along Adelup fault, Pago River, central Guam. Figure 24. Photographs along Adelup fault, Pago Bay, central Guam. Figure 25. Photographs along Adelup fault: Offset sea-level notches at Pago Point, central Guam v 1.0 INTRODUCTION Guam, a U.S. territory, is the largest and southernmost of the Mariana Islands, a chain of volcanic islands and seamounts that constitute the Mariana Arc (Figure 1). It is located on the Mariana plate, a microplate of the Philippine Sea plate, between an active subduction zone and a spreading center. As such, it is in a tectonically active region that is the locus of seismic activity. Several large historic earthquakes have affected Guam, and there are numerous surface faults on the island that appear to displace Quaternary materials. The tectonic setting and the presence of Quaternary faults suggest that Guam could face a significant seismic hazard from the subduction zone and/or the onshore faults. Any thorough seismic hazard assessment should include characterizations of both regional and local sources. To date, no modern field-based map of the active faults of Guam exists, and there is very little information about the age of fault activity. In this project, we have begun to address the paucity of data about onshore crustal fault characteristics by undertaking a detailed Lidar and field mapping investigation of the Quaternary faults on Guam to identify them, accurately determine their location, characterize their sense of slip, and obtain preliminary information regarding their level of activity. We have developed map of Guam’s active faults at a scale of 1:50,000 as a first step in better characterizing the crustal sources of seismic hazard. We have also developed preliminary characterizations of some of Guam's fault for use in seismic hazard assessment.
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