Chasing Earthquakes and Tsunami in Humboldt County and the Northwest Coast

October 17, 2018 Applied Geography Geology Physical Geography Technology
Dr. Lori Dengler assembles the core sampler that will be used to bore into layers of peat and mud at Mad River Slough.

There is a region in California that experiences more frequent damaging earthquakes and tsunami than anywhere on the U.S. West Coast outside Alaska. It is a region where subduction is still active and catastrophic earthquakes over 8 magnitude are capable of producing tsunami up to 15m (45 feet) high. Entire strips of coastline and even patches of forests have been submerged under water as other landscapes have been lifted higher by these tectonic events that have left recent footprints throughout the state’s northwest coast. This is a region that more resembles the Oregon coast, a region seismologists and geologists label the Cascadian Subduction Zone.

Here, accomplished award-winning geophysicist Dr. Lori Dengler guides us through these broken landscapes on a field trip sponsored by the California Geographical Society in the spring of 2015. A series of buckles and thrusts in the crust are evident here, north of where the north-south trending San Andreas system is cut off by the east-west Mendocino Fault. This Cascadian Subduction Zone megathrust belt runs about 700 miles from Cape Mendocino to Vancouver Island. The plate boundary gently slopes from the ocean floor about 45 miles offshore to about 8 miles below Humboldt Bay, then deeper below the surface farther inland. The result is a youthful fold and thrust belt landscape that includes at least 6 active thrust faults in the Humboldt Bay region.

Each fault breaks with occasional earthquakes that have been thrusting up deformed marine terraces and downwarping the bays and marshes. The greatest events have dropped the bays and lagoons after marsh peats were established during centuries of relative stability. These lowlands are susceptible to liquefaction, soil amplification, subsidence, and tsunami inundation during these spasmodic events. Thick layers of mud and other sediment are suddenly deposited, burying the old peat and building up a new surface where another stable layer of peat can grow. A few more centuries pass until the next mega earthquake continues these cycles that have been correlated in time with similar tectonic shifts along the Oregon and Washington coasts. The last major event (January 1700) was so large, it sent a teletsunami that was still several meters high when it arrived in Japan. There are even Native American (Yurok) stories of how these megaquakes and their tsunami flooded bays in the region before recorded history.

The good news is this tectonic activity has produced diverse and scenic topographic features that give unique beauty to this quiet coast of cool mist. The bad news is there have been 24 tsunami recorded on this northwest coast since 1855, though 19 came from great distances, and Crescent City has suffered more tsunami damage than any West Coast city outside Alaska. And though smaller damaging earthquakes are likely within the next few years, seismologists estimate the probability of the big one at about 15-20% within 50 years. Join us as we explore evidence of these compressional forces and catastrophic events from the top of the thrusts and anticlines into the downwarped lagoons and bays.

Special thanks to Dr. Lori Dengler, Humboldt State University, and the California Geographical Society.

If you are looking for more, consult Chapters 2 and 3 of our book and try these sources:

Active Fault. Standing on the Fickle Hill Fault Zone where the crust is thrust upward, looking southwest across this active fault where Arcata and then Humboldt Bay are buckled downward.
Broken Arcata. Looking northeast from near Arcata Plaza toward the Fickle Hill Fault, where rocks are thrust upward to build the distant hill we were standing on in the previous photo.
Tectonic Landscapes. Looking southwest from near Arcata Plaza, the crust is buckled again along this second step of the Fickle Hill Fault, dropping farther down toward Humboldt Bay.
Halophyte History. Different halophyte species mark high marsh and low marsh surfaces that are established during centuries between major episodic subsidence and inundation events at Mad River Slough.
Assembling the Equipment. Dr. Lori Dengler assembles the core sampler that will be used to bore into layers of peat and mud at Mad River Slough.
Coring the Deposits. Reliable boots and some elbow grease are required to rotate the core sampler into the alternating layers of sediment and peat that have accumulated at Mad River Slough.
Analyzing the Sample. Dr. Dengler shows off the sharp contrasts between alternating peat layers that become established between sudden subsidence events that leave layers of mud and other sediments.
Manila Dunes. Welcome to the Manila Dunes on the Samoa Peninsula.
Sand Barriers. These Manila Dunes are on the north part of the Samoa Peninsula that blocks northern Arcata Bay from the open ocean and shields direct hits from dangerous tsunami.
Protective Dunes. The largest dunes on the Samoa Peninsula are up to 50-60 feet, capable of blocking even some of the higher tsunami, though such events might easily overtop the lower elevations south of Samoa.
Stable Dunes. Beach pine and Sitka spruce are established with a mix of other vegetation on the more stable and highest Manila Dunes on the Samoa Peninsula.
Dune Plants Hold On. Species such as beach strawberry, beach pea, and even an endangered Menzies’ wallflower compete with invasive European beach grass within the approximately 1,000 feet of foredunes between the ocean and the higher Manila Dunes.
Sand Sources. We are reminded that most of the sand on these beaches and dunes was originally weathered, eroded, and transported from the mountains to the coast along rivers and streams.
Vulnerable PG&E power facilities around King Salmon have included remnants of the state’s first nuclear power plant (1963), which was closed in 1976 and was decommissioned in 1988 after Cascadia subduction zone risks were realized.Vulnerable Landscapes.
Soon to be Overtopped? The peninsula and dunes are too thin and low here to block major tsunami events.
Potential Dangers. Tsunami entering the bay south of Samoa could slosh currents and debris that would cause major damage to port facilities in Humboldt Bay.
Tsunami Pathways. Located just on the opposite side of the entrance to Humboldt Bay, the community of King Salmon is particularly vulnerable to damaging tsunami.
Tsunami Escape Route. In 2011, King Salmon was recognized as a Tsunami-Ready Community with tsunami evacuation drills and access to Buhne Point during such emergencies.

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Rob O'Keefe

Rob O’Keefe is the creator and primary webmaster for our site. He has worn many hats in his professional career including newspaper reporter, business analyst, GIS analyst, webmaster and interpretive naturalist. He holds degrees in geography, journalism as well as certificates in GIS and computer programming. Rob has been involved with the Rediscovering The Golden State text and web site since 2005.

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