GEOL 308L Natural Hazards Field Trip 1: Faulting in the 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,

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 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 Pacic Ocean d River

l i a e Portland n Netarts Bay o n 45

Juan a o s Salmon River h 45 u

deFuca C b

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i Yaquina Bay

c North plate d

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a i Alsea Bay

o

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n Pacic 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 conguraƟon 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 2. Cascadia subduction zone (after Chaytor et of saturated sediments. As sand grains move to a more al., 2004 and Nelson et al., 2004). closely packed structure, water is forced upwards, momentarily causing a loss of rigidity. Lateral spread- warning for the entire west coast of the United States. ing and differential settling of the ground surface There was an earthquake Jan 10, 2010 with a magni- commonly result. Liquefaction in recent earthquakes tude of 6.5. Both the 2005 and 2010 earthquakes were was observed in the Big Lagoon spit area, Centerville located in crustal faults of the Gorda plate. Recently Beach, Eel River basin and the Mattole River basin. there was a widely felt earthquake along the Men- Credible historic accounts of liquefaction have been docino fault that happened on December 8, 2016 with reported for large areas of the Eel River Basin and a magnitude of M 6.5. This earthquake was also quite Humboldt Bay as far north as Fields Landing in the offshore, so did not produce much damage. 1906 earthquake, on the Samoa Peninsula (1923 earth- The pattern of ground shaking from historic earth- quake), King Salmon (1975), and Arcata Bottom near quakes has shown that regional geology affects seismic the mouth of the Mad River (1954). Liquefaction and intensity. Two affects are particularly noticeable in the soil amplification are complicated phenomena and Humboldt Bay region: soil amplification and liquefac- depend on characteristics of the earthquake source - tion. Soil amplification means the ground shaking is magnitude and frequency spectra - in addition to the stronger than what one would expect due to distance particular site conditions. Map S-1 from CDMG sce- alone from the epicenter. Areas particularly vulnerable Figure 1. Field Trip Route Map. The 5 field trip stops are designated as green rectangles on the main map. There nario (Toppozada and others, 1995) gives an overview are 4 inset maps that show the locations of stops 1, 2, 4, and 5. to soil amplification in recent earthquakes include the of regions most vulnerable to these phenomena. They

4 5 Del Norte

VIII Siskiyou

Humboldt VII

VIII Trinity

IX VII

Highway Quaternary Fault County Inundation Area (CA) MMI VIII II - III (Weak) IV (Light) Mendocino V (Moderate) VII VI (Strong) VII (Very Strong) Figure 5. Damage assessment for a magnitude 8.4 earthquake on the Cascadia subduction zone, modified from VIII (Severe) 50 25 0 50 the 1995 California Division of Mines and Geology (CDMG) Earthquake Planning Scenario. IX (Violent) ° km Figure 4. Cascadia subduction zone (CSZ) shakemap scenario map. USGS simulations for ground shaking using the Modified Mercalli Intensity Scale as modeled for a CSZ earthquake. include the Arcata Bottom, all of the Samoa Peninsula, reka west of Broadway. The industrial corridor west Freshwater Valley west of Three Corners, Elk River of Broadway, the Bayshore Mall and the industrial Valley west of Elk River, King Salmon, Fields Landing, facilities on the Samoa Peninsula are all at risk of these Eel River bottoms, and the westernmost rim of Eu- hazards.

6 7 Prehistoric Seismicity: F A 480-505 During the past three decades, the science of paleo- Barkley years seismology has developed techniques to ascertain (110-1150) the approximate magnitude and timing of prehistoric 434 JDF years seismic events. Of particular concern to residents of (120-720) B the Humboldt Bay region, are the Cascadia subduction NB zone megathrust (CSZ) and the faults on land which Astoria OB 340 have been mapped as active according to the Alquist- years Priolo Fault Zoning Act of 1973. The CSZ is a 1400-ki- HR (120-720) C lometer long system of faults and folds which extends HB 320 from Cape Mendocino to Vancouver Island, British years (40-720) Columbia. The main fault or megathrust is a gently D CB sloping plane that slants to the east beneath northern Rogue 220 California, Oregon, Washington, and southern Brit- Smith years ish Columbia. It breaks the sea floor surface about 45 Klamath (40-580) E Trinidad miles offshore of Eureka near the edge of the conti- Eel ? <200 NE nental shelf, reaches a depth of about 13 kilometers years? Segments beneath Humboldt Bay and is close to 25 kilometers deep beneath Willow Creek. Figure 6. Average Recurrence Intervals for Cascadia There is agreement among scientists and emergency subduction zone earthquakes based upon a record of managers that the CSZ has produced Great earth- paleoearthquakes from the past 10,000 years. quakes (earthquakes with M ≥ 8.0) in the past and is capable of producing earthquakes of magnitude 8 or greater (Atwater and others, 1995). Many believe long duration (a minute or more of very strong shak- that the most recent CSZ event (January 26, 1700) ing). A Great CSZ earthquake would be richer in long had a magnitude of about 9 (Satake and others, 1996; period motion than the moderate to large earthquakes Oleskevich and others, 1999), and produced a tsunami such have occurred in historic times. Large structures SW similar in size to the December 2004 Indonesia earth- such as buildings, dams and skyscrapers are particu- quake. It is much more difficult to predict recurrence larly vulnerable to long period motion. rates for CSZ events than historic earthquakes. Current Six thrust faults break the surface in the Humboldt evidence suggests that as few as 200 or as many as Bay region which have been mapped as active under 800 years may elapse between successive CSZ events. the Alquist-Priolo criteria: the Little Salmon, Fickle The average recurrence interval in the southernmost Hill, Mad River, McKinleyville, Blue Lake, and Trinidad part of the CSZ is about 220 years. The probability of faults. The Little Salmon fault south of Eureka appar- such an event occurring within the next year is consid- ently ruptures in conjunction with CSZ slip events ered low; but rises to as high as 35% within the next 50 (Clarke and Carver, 1992). The recurrence history of years for the northern California part of the CSZ. The the other faults is less well known; all have evidence main hazards associated with a CSZ rupture are tsuna- of several events in the past 10,000 years. Surface mi (discussed further below), high peak accelerations rupture associated with any of these faults is an ad- Figure 7. Geologic Map showing a portion of the Mad River fault zone (MRFZ). (A) The map showing thrust faults (liquefaction and soil amplification), and extremely ditional hazard that must be considered in the siting of in the MRFZ. (B) Cross section showing faults and folds of the MRFZ. 8 9 North SouthSouth City. A study of the Humboldt Bay spits gives evidence to tie the wood and concrete portion of the building Axis of the that the South spit has been overtopped at least twice together and a second completed in 1992 to bring the Wildberries Freshwater Sy ncline Jacoby Store House Eureka in the past 2000 years. Another graduate thesis from building up to the standards of the State Historic build- HSU’s geology department documented evidence of at ing code. We will get into our vehicles, head north on Arcata Arm Bay Axis of Anticline least 3 tsunami deposits in the past 3,500 years (Pat- HWY 101, and take the School Road exit. At the stop Humboldt Bay Fickle Hill fault (Humboldt Bay) ton, 2004). There is no evidence that the Samoa Penin- sign, turn right on School Road. Turn left at the first �i�le �almo� fault sula north of the town of Samoa has been significantly road and park in the parking lot on the left. Figure 8. Illustration showing faults and folds for the Humboldt Bay region. overtopped in the same time period, suggesting peak Stop 1. School Road exit HWY 101 northbound wave heights on the open coast in the 25 - 45 foot (8 - any structure or facility. The 1999 Chi-Chi earthquake Fourteen-knot currents were reported in the channel 15 meter) range from past Cascadia events. We will walk over and take a look at the fault scarp. in Taiwan ruptured a fault believed similar to the north opposite the Coast Guard Stations (Lander and others, Locate yourself on the map and see if you can identify There is also a rich body of oral history from both the coast thrust faults. The Chi-Chi earthquake illustrated 1993). At Trinidad, water was reported to have reached where the fault is. Tollowa and Yurok peoples that support past great that ground motions on the hanging wall of thrust 5.4 meters above Mean Lower Low Water. The 1992 tsunami events. The best-documented paleotsunami Here is an excerpt from Simpson and Roberts, 2006. faults are greatly amplified compared to the foot wall. Cape Mendocino earthquake produced a relatively was associated with the last great rupture on the CSZ. Many major or critical facilities in Humboldt County modest tsunami that was recorded at the tide gauge To most Humboldt County geologists, the School Yurok stories place the height of the waves at about 60 are located on or in close proximity to the hanging wall on the North Spit and at Crescent City. Although small, Road scarp of the Mad River fault is something of a feet near the mouth of Redwood Creek (Carver, 1998). of a thrust fault including the Eureka-Arcata airport, this tsunami launched several research efforts to bet- landmark. Perhaps it is the scarp’s appearance and Historic records from Japan give amplitudes of several Humboldt State University, and PG&E’s Humboldt Bay ter understand the tsunami hazard in our area. The Na- location; the steep grassy slope offsets planar terraces meters at five sites located over a 1000 km long reach Power Plant. tional Oceanographic and Atmospheric Administration along one of the main roads into the rapidly growing along the Honshu coastline. In order to produce a (NOAA) published a tsunami inundation model of the community of McKinleyville. There are two homes Tsunami Hazard: wave of that size in Japan, this earthquake likely had a Humboldt Bay region which mathematically computes built directly on the crest of the scarp just north of magnitude close to 9 (Stake and others, 1996). Twenty-four tsunamis have been observed or recorded the expected inundation levels caused by a magnitude School Road. Perhaps it’s the crazy trench log that on the North Coast since 1855. All but five were telet- 8:4 earthquake on the CSZ (Bernard and others, 1994). Inundation is only one of the hazards posed by tsu- came out of Carver and Burke’s initial research trench sunamis (the source was far away). Crescent City in Del In the model, the north and south spit bear the brunt nami. The extremely high currently velocity caused by back in the mid 1980’s, with its over-turned terrace Norte County has suffered more tsunami damage in of the impact. Both spits are overrun and the waves rapid changes in water elevation are capable of causing abrasion platform. Whatever the reason, development the past 150 years than any other area of the US West travel across Humboldt Bay flooding Woodley and In- significant erosion and damage to structures especially pressure on the otherwise prime building ground (flat coast outside of Alaska. Major damage occurred in the dian Islands. The shallowness of the bay dissipates the when the water is laden with debris. High velocity wa- marine terrace surface with ocean view) has resulted 1960 and 1964 teletsunamis and significant wave activ- wave energy and flooding on the east side of the bay is ter can cause damage even when the water height is in a high density of fault rupture hazard investigations ity was observed in 1946, 1952, and 1957. Fresh water expected only in the immediate waterfront area and at not significantly high. Docks, piers and structures built over the past 20 years. lagoons just south of Crescent City have been shown King Salmon, which sits directly across from the mouth directly on the waterfront are the most vulnerable. Up to four traces of the Mad River fault have been to be excellent paleotsunami traps, preserving not of the bay. Recently completed relative tsunami hazard maps of mapped in the McKinleyville area (Carver, Stephens, only 1960 and 1964 deposits but also thick prehistoric Although there is no historic evidence of major tsu- the Humboldt Bay region can be found at: and Young, 1985), although only the easternmost sands. nami damage in the Humboldt Bay region, there is two traces are included by the State within Alquist- http://www2.humboldt.edu/rctwg/ There is no known evidence of significant tsunami mounting paleoseismic evidence that the region has Priolo Earthquake Fault Zones. These faults form a damage to the Humboldt Bay region in historic times. been struck repeatedly in the past by significant tsu- Start: Founders Hall, Humboldt State University southwest-stepping fault pair. The northern trace is The Alaskan earthquake tsunami that damaged Cres- namis. The strongest prehistoric evidence comes from mapped as extending from the coastal bluff near the We start this trip from the parking lot next to Found- cent City in 1964 caused water to breach a ten-foot Del Norte County, the focus of two graduate theses western end of Murray Road, to the southeast beneath ers Hall. Founders Hall is the oldest building on the seawall at the Eureka Boat Basin and rise eight feet from HSU’s Geology Department (Abramson, 1998; Highway 101, and across School Road. The fault forms Humboldt State University Campus. It was built in 1921 into the street. The bay was filled with logs and debris Garrison-Laney, 1998) examine cores from several sites a relatively steep, prominent 7-meter-high scarp just of reinforced concrete and wood. It has undergone and nine changes in tidal height were reported over like the one shown below that suggest wave heights north of School Road (locally the “School Road scarp”), two seismic retrofits, the first in 1969 which attempted the night causing high current velocities within the bay. at least twice as high as the 1964 tsunami in Crescent which becomes increasingly subdued as it extends

10 11 Morphology and Structure of the Mad River fault at School Road, McKinleyville, California Morphology and Structure of the Mad River fault at School Road, McKinleyville, California GEOL 308L Field Trip 01 School and Airport Roads Simpson, G. D., and Roberts, M. A. Simpson, G. D., and Roberts, M. A. Quaternary Fault Hydrography Road Highway ° !. SHN, 2006 Field Trip Stop Carver and Burke

T5 T1 Pacific Ocean T2 Study Area T4

SHN, 1999

SHN, 2002 !.

0 500 1000 0 1000 2000 Morphology and Structure of the Mad River fault at School Road, McKinleyville, California Feet Feet Figure 1. Location map. Base map: Alquist-Priolo Earthquake Scale 1” = 1000’ Figure 4. Locations of previous trench studies. T1, T2, T4, and T5 Scale 1” = 2000’ Simpson, G. D., and Roberts, M. A. Fault Zone Map, Arcata North. refer to SHN trenches 1,2,4, and 5, which are discussed in the text. Consulting Engineers & Geologists, Inc. Consulting Engineers & Geologists, Inc. Figure 11. Fault Trench Map for the School Road fault Figure 10. Alquist-Priolo Fault Zone Map for the region trenches. of McKinleyville, California. The AP zone is outlined as dashed lines. ¤£101 southeastward across School Road. The scarp is not apparent within approximately 100 meters south of School Road. Trench studies have documented the ter- mination of the northern fault trace 150 to 200 meters !. School Rd south of School Road. The northern trace trends about N15W on the west side of Highway 101 and progres- sively bends to the southeast towards its terminus. A relatively sharp bend in the scarp is present just north Figure 12. Fault Trench log for the School Road fault

of School Road (south of Carver and Burke trench Figuretrench 5. Trench as log fromdocumented Carver and Burke study, fromby 1992 Carver FOP Guidebook. and Burke in 1989. location; Figure 4), from about N45W to N70W. In Colluvial Wedges are labeled 1-6 (each colluvial wedge the vicinity of School Road, the fault appears to strike is interpreted to be evidence for a single earthquake). about N70W, based on the trend of the geomorphic M 101, across an approximately 300- to 400-meter- ad Riv escarpment. In this area, scarp height and steepness er wide step-over. The southern trace extends toward decrease abruptly over a length of just a few tens of 1 0.5 0 1 the southeast across the mouth of Mill Creek, to the meters. km northern valley wall of the Mad River drainage. This The southern Mad River fault trace overlaps the north- scarp becomes very high (20+ meters) directly south of Figure 9. Fault Map for the first two stops. Faults are from the USGS Active Fault and Fold Database. The base- ern trace between about School Road and Highway School Road, although the scarp height may have been map is a slope map showing steeper slopes as darker gray. Roads are from the Humboldt County GIS database. 12 13 enhanced by past Mad River incision. The southern We will get back into our vehicles and drive north- 500 feet or more as Special Study Zones. Paleoseismic the west of Hwy. 101. One of these, the Old Town Bay trace has not to date been trenched. bound on HWY 101 until we exit to Airport Road. Turn work by HSU professors Gary Carver and Bud Burke in and Grill, suffered severe damage in the January 2010 right on Airport Road, turn right on Boeing Ave., turn the early 1980’s contributed to the designation of the earthquake and nearly collapsed. An additional prob- left on Concorde Dr., and finally turn left on Boeing Trinidad, McKinleyville, Blue Lake, Mad River, Fickle lem in this area is disruption to water supplies which Ave. Park at the end of Boeing Rd. Hill and Little Salmon faults as active. Other faults in will hamper firefighting abilities and damage to the Humboldt County with this designation are the Goose waterfront area from both tsunami inundation and Stop 2. Airport exit HWY 101 northbound Lake fault south of Carlotta and the San Andreas fault debris damaging structures. We will take a look at the fault scarp. Compare your in Shelter Cove. After we pass the Bayshore Mall on the right, turn left observations with the map. Can you identify the loca- Many faults and folds have formed in the compres- on Highland Drive, then left again at the entrance to tion of the fault(s) in this region? sive environment of the Cascadia fold and thrust belt. Fort Humboldt. We will walk to the field overlooking We will now head southbound on HWY 101 and drive Humboldt County is one of the very few places on the Bayshore Mall. back to Arcata. Take the Sunset Road exit and head earth where this unique geologic setting is exposed Stop 4: Fort Humboldt overlooking Bayshore Mall southwards on H Street. We will park in the Wildber- on land. Our coastal topography mimics the deforma- ries parking lot on the southwest side of the store. tion caused by these folds and faults: low areas such The Bayshore Mall opened in 1987. The buildings as the coastal lagoons and the Arcata and South arms were designed to withstand both strong shaking and Stop 3. Wildberries Marketplace of Humboldt Bay coincide with the axes of synclines, liquefaction effects. The parking areas, however, may Note how Wildberries Market is located on a rise over- downward folds in crustal material. Higher areas such become impassable due to liquefaction-induced settle- looking much of Arcata. We are now sitting atop the as Trinidad, Fickle Hill, Humboldt Hill and Table Bluff ment and lateral spreading. The tracks separating the Fickle Hill fault that extends to the southeast towards are situated on anticlines where the strata has bowed parking lot from the Chevron tank farm mark the limit Sunnybrae. The fault is rumored to break through to upwards. These anticlines typically sit on top of thrust of the modeled tsunami inundation zone. This is now the surface halfway down the hill slope in the vicinity faults where rocks on the northeast side of the fault the only active tank farm supplying petroleum prod- of the new apartment complex on the right side of the have been shoved up and over rocks to the southwest. ucts to the Humboldt Bay area. A Cascadia subduction street. A second strand of the fickle Hill fault causes Every time fault slip occurs, the adjacent folds grow a zone earthquake is expected to produce shaking levels a smaller step further south near the Jacoby Store little larger, building up the higher regions and causing sufficient to rupture at least some of the tanks. The House. The Fickle Hill fault is one of eight faults in bays and lagoons to deepen. water line serving the Samoa peninsula comes across Humboldt County that have been designated as Active Humboldt Bay at this point. A simplified tectonic sketch of the Humboldt Bay re- under the State’s Alquist-Priolo Special Studies Zone gion is shown below: Historic Note: Fort Humboldt Legislation. Active faults have either produced earth- quakes in historic times or show geologic evidence of Arcata: We will get back in our vehicles and drive south Fort Humboldt was established in 1853 on a bluff movement in the past 10,000 years. After surface fault on H street. When we are driving past the Arcata Plaza, overlooking Humboldt Bay at what was then Bucksport rupture caused significant damage in the 1971 San Fer- note how the road surface goes down in elevation as to provide protection for the gold-seekers and settlers nando (Sylmar) Earthquake, the Alquist-Priolo Special we pass the US Post Office (on the right) and Jacoby’s against Native Americans. The most famous resident of Study Zones Act required the State to map all known Storehouse (on the left). The is one more strand of the the Fort was the young Captain Ulysses S. Grant who active faults in California and to designate areas within Fickle Hill fault. Head south to Samoa Blvd. and turn served 5 months in 1854 as a 4th Infantry Captain and left. Get into the right lane and get back on HWY 101 reputedly spent more time at Ryan’s Tavern in Eureka Figure 13. Marine Terrace Map for the McKinleyville southbound. We will drive to and through Eureka until than at the Fort. After a dreary four months described region. (A) Marine Terrace Legend (ages are based we pass the Bayshore Mall. by Grant as the worst winter of his life, Grant offered upon a correlation between the Pleistocene sea-level his resignation from the army. Later, Fort Humboldt Eureka: As we drive through Eureka, note the con- curve and assumed uplift rates for the marine ter- would become the headquarters for the Humboldt Mil- struction styles. The majority of Eureka’s unreinforced races). (B) Map for the McKinleyville region. (C) Cross itary District, which included Forts Bragg and Wright masonry (URM) buildings are located in Old Town to section showing terrace surfaces and faults. in northern Mendocino County, extending north

14 15 GEOL 308L Field Trip 01 Ft. Humboldt, Bayshore Mall, & Table Bluff Quaternary Fault Hydrography Road Eureka Highway ° !. Field Trip Stop !.

¤£101

L i tt le S a lm o n

fa u lt Pacific Ocean

Humboldt Bay !. CR

Tab le B luf f fa ult Figure 14. Marine Terrace Map for the Table Bluff and southern Eureka region. Terrace age patterns are the same as in figure 13.

¤£101 through Humboldt County to Fort Ter-Waw in Klam- of the Fort and the Native American groups, includ- ath and Camp Lincoln near present-day Crescent City. ing the Wiyot, Hoopa, and Yurok of this region. In the Eel River 1 0.5 0 1 Fort Humboldt was formally abandoned in 1870 and 1980s the Surgeon’s Quarters were reconstructed and km rapidly fell into decay. Today, only the hospital building there are plans for its establishment as a period house remains out of the original fourteen structures. It is museum. Figure 15. Fault Map for the last two stops. Symbology is the same as for figure 9. now a historical museum dedicated to telling the story 16 17 We will now drive southbound on Hwy. 101. The Cenozoic - Geologic era extending from about 65 mil- highway is close to the modeled tsunami inunda- lion years ago to the present day. tion limit in this area. We pass by the communities of hanging wall wall colluvium - loose deposits, usually at the foot of a King Salmon and Fields Landing. King Salmon, located slope or cliff, brought there chiefly by gravity hanginghanging wallwall nearly opposite the entrance to Humboldt Bay, is the convergent plate boundary - a zone where two of the only community in this area expected to have signifi- foot wall footfoot wallwall cant flooding in a large tsunami. plates composing the earth’s outer layer move towards one another. A subduction zone is one type of conver- ReverseReverse fault fault ThrustThrust faultfault South of Fields Landing, the Little Salmon fault runs gent plate boundary. Strike-Slip fault parallel to and just east of the highway. Several strands lip fault of the fault run through the campus of the College of coseismic - time period during a large earthquake; usu- hanginghanging wallwall the Redwoods. ally on the order of ten’s of seconds to a few minutes. Normal fault fault footfoot walwall l Exit HWY 101 on the Hookton Road exit and take Hook- eustatic sea level curve - geologic chronology based on ton Road westwards. We will follow Hookton Road all the rise and fall of sea level during the ice ages. the way to the westernmost part of Table Bluff. We fault - planar break in rock along which one side has also includes rocks which have been altered by high regression - gradual contraction of a shallow sea result- will drive past the Hookton Slough Boat Ramp, where moved relative to the other pressures including greenstone, green schist and blue ing in the emergence of land as when sea level falls or we have found evidence for coseismic subsidence and schist. Most of the rocks in the Franciscan have been land rises normal fault - fault in which the hanging wall coincident tsunami deposits. intensely deformed and are on the order of 150 million has moved down relative to the footwall subduction zone - a convergent plate boundary at years old. Stop 5: Table Bluff which oceanic crust is being pushed beneath another reverse fault - fault in which the hanging wall We will park at the northern end of the parking lot. Holocene - geologic time period extending from 10,000 plate (continental or oceanic). Plate boundaries are has moved up relative to the footwall We will take a look to the north across Humboldt Bay. years ago to the present characterized by earthquakes and by volcanic activity thrust fault - reverse fault which has a low dip There is a great view of Humboldt Hill which is formed marine terrace - flat topographic surface in a coastal subsidence - sinking or downward displacement of (less than 30 °) by uplift along the Little Salmon fault system. area which was formed by wave erosion and subse- part of the earth’s crust strike slip fault - horizontal fault displacement; quently preserved by uplift relative to sea level syncline - a trough-like fold that is convex downward fault slip parallel to the direction of strike GLOSSARY OF GEOLOGIC TERMS USED IN THIS GUIDE interseismic - time period between large earthquakes; BOOK fault scarp - a cliff or scarp formed by movement on a may be on the order of tens of years to centuries. fault Alquist-Priolo special Study zone – the area immedi- marine terrace - sediments with a horizontal surface ately around a fault that has been mapped as active fluvial terrace - sediments with a horizontal surface formed by the deposition of marine sediments near by the California Geological Survey. Special regulations formed by the deposition of river sediments along the shore which are now elevated above the present sea apply to construction in these zones banks and flood plains which are now above the pres- level. transgression - gradual expansion of a shallow sea ent river channel. resulting in the progressive submergence of land as anticline - an arch-like fold that is convex upward Pleistocene - geologic time period extending from ap- when sea level rises or land level drops fold - a bend in rock layers or in any planar structure or proximately 1.6 million years ago to 10 thousand years surface. ago tsunami - water waves generated by the displacement of the sea floor most often caused by large earth- Franciscan Complex - the group of rocks which form post seismic - the time span immediately following a quakes beneath the sea floor. the bedrock of much of coastal Humboldt and Del large earthquake. Norte County. The Franciscan is a complex assemblage tsunami inundation zone - area flooded or expected to Quaternary - geologic time period extending from of different rock types including sandstone, basalt, and be flooded by a tsunami. approximately 1.6 million years ago to the present chert which were formed in a marine environment. It (includes Pleistocene and Holocene time) 18 19