Vagabonding on Dangerous Ground: A Geologist Walks Onto a Bar in Cascadia...

A very strange-looking sandbar at Big Lagoon in Humboldt Lagoons State Park on California's north coast.

A geologist walks INTO a bar. She may get hammered, vulcanized, laminated, stoned, cemented, bombed, or petrified. And all her drinks will be on the rocks...

But when that geologist walks ONTO a bar, she just gets sand on her feet.

Yeah, yeah, I know, shut up and stick with the science...

We're traveling north on a journey through the Cascadia Subduction Zone, exploring this unique region with an eye to the geology, and the beauty, of the region. We've explored the Redwood forests of the Eel River, and the Lost Coast where the Cascadia zone begins. Today we are looking at a unique state park along the coast north of Eureka, California, and south of Crescent City. It's called the Humboldt Lagoons State Park, and the three lagoons found there are bounded by stunning examples of baymouth bars.

Big Lagoon at Humboldt Lagoons State Park (source: Google Earth)

One of the reasons that the already infamous Cascadia Subduction Zone earthquake might not be as bad as it could be is that so few people actually live on the coast of Washington, Oregon, or Northern California. Why is this? If one looks at a map, one notes the paucity of flat lands along the coast. There are sometimes some coastal terraces and a number of small natural harbors, but for most of the distance between Vancouver Island and Cape Mendocino, the mountains rise from the sea. There's no place to build. That's not to minimize the tragedy. There will be horrible results, but the largest cities like Seattle and Portland are inland, behind the coastal ranges, and they will be spared the worst of the tsunami damage, if not the shaking. There are certainly a number of towns along the coast, but there are also long stretches with few people. One of the lightly populated stretches of coast is between Eureka and Crescent City.

Big Lagoon at Humboldt Lagoons State Park

Along Cascadia's mountainous coast, wave action is violent and constant, and cliffs are quickly worn back, forming a relatively straight coastline. But there are also a number of drowned river valleys and coves, many caused by the rise of sea level after the last ice age. There are four small examples of these valleys at Humboldt Lagoons (one of which was filled in to allow some farming). But the coastline is very straight. Why?

Intense rainfall sends sediments down the many rivers and streams, and vast amounts are added to the coastal waters. When waves encounter the coast at an angle, the swash and backwash of the water causes sediment to be transported along the coast, a process called longshore drift. There is a tremendous amount of sediment in this coastal system.

Humboldt Lagoon also include a county park at the south end of Big Lagoon. It's a nice place to see coastal erosion!

When waves hit rocky coasts, they expend their energy wearing away at the rocks. When those same waves reach a cove of open water, the wave energy is dissipated as the waves spread out. When sand is being transported around the headland and into a cove, the declining energy causes the sand to settle into a curving sandbar, called a hooked spit. The spit may grow large enough to close off the mouth of the bay, becoming a baymouth bar.

In most bays along the Cascadia coast, rivers are large enough to keep the bays open, but at Humboldt Lagoons only small streams are present. Water can simply seep through the sandbar rather than flowing out. The bays are breached only during the wettest, most intense storms. The baymouth bars are miles long, and incredibly straight (see the Google Earth image above). They don't look natural, and yet they are.

Smaller Stone Lagoon is just north of Big Lagoon.

The lagoons are a fascinating spot along the north coast, and they get a bit less attention than Redwoods National Park and the state parks. They are an important link in the ecosystem of the region, and are marvelous spots for birdwatching and looking for other animals. If you ever travel that way, the state park (and associated county park) is well worth a visit.

The next stop on our vagabonding tour of the Cascadia Subduction Zone is Crescent City at the far north end of California. We'll have some things to learn about tsunamis in California.

Magnitude 5.6 Earthquake in Northern California

There was a magnitude 5.6 earthquake in Northern California at 1:07 PM, at a depth of 20 miles. It took place in a lightly populated area, but I assume it was felt in Eureka and other north coast towns. It occurred on land between Eureka and Crescent City, roughly between Redwood Creek and the Klamath River, very close to the Hoopa Reservation.

As can be seen in the map above, earthquakes are not unusual in this area. It lies north of the triple junction where three tectonic plates come together (the Gorda, Pacific and North American). A subduction zone is forcing the Gorda Plate beneath the North American continent, and earthquakes are to be expected. There have been a series of magnitude 7 quakes in the region over the years, including the Petrolia earthquake of 1992. Ultimately the descending plate starts to melt, producing the magmas responsible for California volcanoes like Lassen Peak and Mt. Shasta.

As can be seen from the seismicity map of California above, there have been plenty of earthquakes within the last week, but there is nothing happening that is out of the ordinary. California averages several hundred earthquakes a week, although most are not felt.

Get the official details at this site: http://earthquake.usgs.gov/earthquakes/recenteqsus/Quakes/nc71734741.php#details
Keep track of all the quakes in California here:
http://earthquake.usgs.gov/earthquakes/recenteqscanv/

Magnitude 6.5 Quake Offshore of Northern California

A magnitude 6.5 quake has struck offshore of northern California, about 25 miles offshore of Ferndale and Eureka, at 4:27 PST. It was apparently felt widely across the northern part of the state. Seven aftershocks in the range of 30-4.5 magnitude have followed the main shock so far. This area is quite close to the Triple Junction where three plates, the Gorda, Pacific and North American come together. The fault motions were primarily horizontal so a tsunami is not considered likely (and it would have hit onshore by now anyway). Andrew Alden at About.com:Geology provides a nice perspective of the quake, the biggest in the lower 48 states in a couple of years.

Update, 1/10: 23 aftershocks thus far, including three that exceeded magnitude 4. Some minor damage reports, in the form of fallen chimneys and broken windows. A nice history of seismicity in the region by the Berkeley Seismological Laboratory can be seen here.