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Acɵve Faulɵng Associated with the Southern Cascadia Subducɵon Zone Kelsey Et Al

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Acɵve Faulɵng Associated with the Southern Cascadia Subducɵon Zone Kelsey Et Al GEOL 308L Natural Hazards Field Trip 1: Faulting in the Humboldt Bay Region AcƟve FaulƟng Associated with the Southern Cascadia SubducƟon Zone Kelsey et al. (2001) Based on earthquake fault slip-rates and marine terrace upliŌ-rates, crustal faults in the North America plate may account for between 20% and 30% of the plate convergence in the Humboldt Bay region. Table of Contents The earthquake and tsunami hazard in Humboldt and Del Norte Counties, California Introduction up to about 1.4 mm/yr. during the last several hundred thousand years. Introduction ................................................................................................................................................. 3 The Cascadia subduction zone is the convergent plate boundary between the Juan de Fuca - Gorda oceanic No terraces are present along the coast at the north Historic Seismicity ........................................................................................................................................ 3 plates and the North America plate. It extends from and south ends of Humboldt Bay or in the Eel River Figure 1: Trip Map: ................................................................................................................................... 4 the northern end of Vancouver Island in Canada to Valley. These are areas of subsidence located in the Figure 2: Cascadia subduction zone Map: ................................................................................................5 near Punta Gorda in northern California. Along most of axes of large synclines. These parts of the coast exhibit its length the plate boundary intersects the sea floor drowned topography and thick sequences of late Figure 3: Historic Seismicity Map: ............................................................................................................ 5 far offshore. A fold and thrust belt associated with Quaternary sediments. The late Holocene sediments Figure 4: Cascadia subduction zone Shakemap Scenario map: ................................................................6 the subduction zone is present in the western edge of in these areas of subsidence include salt marsh peats Figure 5: CDMG Damage Assessment Map: .............................................................................................7 the North America plate. This system of large youthful and fossil forests buried beneath bay muds, deposits structures is located offshore along much of the Pacific indicative of coseismic subsidence. Prehistoric Seismicity ................................................................................................................................... 8 Northwest, but at its southern end the subduction Figure 6: Cascadia subduction zone Recurrence Intervals: .......................................................................8 Historic Seismicity: zone converges with the coast, and the fold and thrust Figure 7: Mad River Fault Zone Map: ....................................................................................................... 9 belt intersects the shoreline and extends inland. These During historic times, the coastal and offshore areas Figure 8: Cross Section Illustration: .......................................................................................................... 9 folds and faults are exposed on land along the Hum- of Humboldt and Del Norte Counties, California have boldt County coast between Cape Mendocino and Big been the single most seismically active region of the Tsunami Hazard .......................................................................................................................................... 10 Lagoon. The map on the cover provides names and lo- coterminous United States (Dengler and others, 1992). Stop 1: School Road ................................................................................................................................... 11 cations of the principal fold and thrust belt structures On average, an earthquake with strength enough to Figure 9: Fault Map: McKinleyville ......................................................................................................... 12 on land in the vicinity of Humboldt Bay. topple items off shelves has occurred every 2 - 3 years. During the past century, 24 earthquakes have caused Figure 10: Alquist-Priolo Zone Map: ....................................................................................................... 13 Evidence of recent crustal deformation at the south- damage to structures, an average recurrence of about ern end of the Cascadia subduction zone is provided Figure 11: School Road Fault Trench Map: .............................................................................................13 5 years. Most of these earthquakes have been cen- by uplifted and deformed marine terraces. Terraces Figure 12: School Road Fault Trench Log: ...............................................................................................13 tered offshore or near Cape Mendocino, sparing the like these are found from Big Lagoon south to Cape more populated Humboldt Bay region the strongest Figure 13: McKinleyville Marine Terrace Map: .......................................................................................14 Mendocino. In this zone, large landward-dipping thrust shaking levels. The 1932 and 1954 earthquakes, both .................................................................................................................................. 14 faults and folds of the subduction zone fold and thrust Stop 2: Airport Road in the magnitude 6 - 6.5 range and located less than 20 belt intersect the coast. Growth of the fold and thrust Stop 3: Wildberries Market ....................................................................................................................... 14 miles from the bay, demonstrated the vulnerability of belt structures is expressed by uplift of terraces on the area. These moderate earthquakes caused the only Stop 4: Fort Humboldt ............................................................................................................................... 15 fault-bounded blocks, tilting of terraces in the limbs of known deaths from local earthquakes and produced Figure 14: Table Bluff and South Eureka Marine Terrace e Map: ...........................................................16 folds, uplift of terraces at the crests of anticlines, and significant damage in the Arcata and Eureka areas. The down-dropping of surfaces in the troughs of synclines. Figure 15. Fault Map: South Bay ............................................................................................................. 17 same fault system could produce earthquakes as large Stop 5: Table Bluff Overlook ...................................................................................................................... 18 Age estimates and correlation of terrace segments as 7 with much greater damage. In 1992 there was an across faults and folds are based on the degree of soil earthquake in the Petrolia area that had a magnitude Glossary ..................................................................................................................................................... 18 development, the matching of terrace elevations with of M 7.1 that had several large magnitude aftershocks. eustatic sea level curves, several thermoluminescence This earthquake produced significant damage in the re- age measurements, and a volcanic ash in one of the gion, including structural damage and extensive fires. A older terraces at Table Bluff. The deformation and el- recent large earthquake occurred on June 14, 2005 and evation of the terraces indicates that most of the coast had a magnitude of 7.2 but fortunately was far enough in the Humboldt Bay region has been rising at rates of offshore to cause no damage. It did trigger a tsunami 2 3 Chaytor et al. (2004) Cascadia subducƟon zone Nelson et al. (2004) 120 o 130 o 125 o Queen CharloƩe fault BRITISH COLUMBIA Vancouver o o 50 50 S t r a it Explorer of G e Vancouver CANADA plate Island o Con rg i U.S.A. C a o Ɵnental shelf n t in S e e n a t w a l a r s d lo p SeaƩle e Puget e d Sound g WASHINGTON e Grays Harbor o Willapa f Bay C a s River c Nehalem mbia a Colu Pacic Ocean d River l i a e Portland n Netarts Bay o n 45 Juan a o s Salmon River estuary h 45 u deFuca C b d a u i Yaquina Bay c North plate d t a i Alsea Bay o c n America s a plate C z o n Pacic e Coos Bay OREGON plate Coquille River Sixes River Thrust fault at plate boundary Figure 3. Historic Seismicity Map (Rollins and Stein, Other faults Spreading ridge Crescent City 200-m isobath Gorda Lagoon Creek 2010). Deep-sea channel plate Volcano Orick Earthquake evidence Humboldt Bay Tsunami deposit Eel River Eel River basin (Loleta and lower parts of Ferndale, Cape Mendocino 40 o 40 o Mendocino fault CALIFORNIA portions of Fortuna built on fill), the Samoa Peninsula, 0 100 200 300 San Andreas km fault 120 o o Arcata Bottom, Elk River Valley, Freshwater Valley, 130 125 o Plate conguraƟon for the Cascadia subducƟon zone (CSZ). The Jacoby Creek Valley, the areas of Fields Landing imme- Juan de Fuca and Gorda plates are subducƟng northeastwardly oblique beneath the North America plate at ~36 mm/yr in the diately adjacent to the bay. Humboldt Bay region. Paleoseismic core sites (marine and terrestrial) are ploƩed as circles. Liquefaction is caused by shaking-induced compaction Figure
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