Hello. Thank you for inviting me. I’m Lori Dengler, emeritus professor of
geology at Humboldt State University. And I’ve been working on earthquake
and tsunami hazards on California’s North Coast for over four decades.
Today I want to talk about the 1906 earthquake in Humboldt County.
Now, I know that many of you might think that 1906 has been kind of hacked
to death and there really isn’t much more to learn from that earthquake.
I would like to say no. I really think it’s an extraordinary
event, and it’s always worth going back to.
Humboldt County, if you look at our location over here on the right,
is at the northern end of both the rupture zone and the damage.
In fact, it sort of straddles the area of significant damage versus felt.
And we’re fortunate to actually have quite a bit of information
about what happened in Humboldt County in 1906.
This is one of many models of slip during the 1906 earthquake.
I believe this is after Thatcher 1997.
The heights of the bars are the relative slip, and the numbers are in feet.
This was a illustration I used at the Humboldt County Fair
a number of years ago. And you can see that the height
of the bars and the number there in the Cape Mendocino area in the vicinity
of the triple junction are among the highest anywhere in the region.
So I’m going to show you a little evidence for why I believe that is real –
that we really did have significant slip on the North Coast,
and why this northern termination is really so interesting.
We’re fortunate to have a rather extraordinary photographic record
of what happened in Ferndale, the largest community in
Eel River Valley at that time, because of a woman – Edna Garrett.
Edna and her husband Ed were photographers – one of three
photographic studios in Ferndale at the time of the 1906 earthquake.
But only Edna, after being rudely awakened early that morning,
pulled out her equipment and proceeded to document what happened
in the Eel River Valley on that extraordinary day.
These are some of her photographs of structural damage.
On the upper level, on the upper left and the upper right, you see the two
brick buildings in Ferndale at the time, both significantly damaged.
The building on the left was subsequently destroyed.
The one on the right continued until the 1990s.
And I will get back to that building later.
You can certainly see significant damage to both the unreinforced
and to the wood-frame structures.
Edna also documented liquefaction in the area.
And on the left is my most favorite liquefaction photograph –
I like the field clothes – in the Eel River Valley.
And another photograph on the right, also on a tributary of the Eel.
Spectacular liquefaction, April 1906. Ground is very saturated.
Not surprising.
She also went out to near the coast, near Centerville Beach, and took this
photograph of a very large landslide. Landslides are not at all surprising
in Humboldt County. They occur even in the absence of earthquakes.
We saw very similar failures to this after the 1992 earthquake
in the same relative area.
This is the surface fault rupture – the map by Mathes in the 1908
Lawson report. The northernmost documented surface rupture.
Carol Prentice certainly knows much more about this than I do.
She provided Mathes’ notes and interpretations for me.
No question that there was surface rupture in 1906.
You can still see this today.
And whether this is truly the main strand of the San Andreas or some
auxiliary strand, I’ll let you geologists continue to argue it out.
But we did have significant offset in 1906.
One of the extraordinary things about Humboldt County,
and particularly Ferndale – we call Ferndale the Victorian village,
and 70% of the buildings that existed in 1906 are still there today.
The photograph on the top is one of Edna Garrett’s views.
And we were able to take exactly the same spot in 2006, and you certainly
get a feel that – except for the absence of the telephone poles,
which were removed during a filming – a movie in the 1970s – wires were
put underground at that point, it looks essentially the same.
But you can actually visit Ellin Beltz’s terrific tour of Main Street in Ferndale.
And she goes through exactly what happened to pretty much
every one of these buildings in 1906.
I’ve taken just a few examples here.
Here we have what was the general mercantile in 1906.
And, in 1902, it was the Valley Grocery.
In 1906, it’s a very new building.
Just a little over a year old.
And you can see the parapet failure and the many windows that broke.
In 1992, very similar damage. Not as much of the parapet wall failed.
Not as many of the glass windows, but probably the glass windows were
tempered in 1992. So that’s not a very good indication of relative strength.
But I would argue that the greater parapet wall failure,
especially with young mortar, suggests that, at least here,
the ground shaking was stronger in 1906.
Here we have 393 Main Street in 1906 on the left.
The exact same building in 1992.
Significantly more damage to wood- frame structures in 1906 than in 1992.
This is the only interior photo that Edna Garrett took back in 1906 on the left.
Fortuitously, we have the interior of the exact same building in 1992,
illustrating that items off shelves are going to look pretty similar
once you get over an intensity of 7, and you certainly couldn’t differentiate
which of these two experienced the stronger shaking.
I love to compare the liquefaction scenes.
We did observe several places of liquefaction in ’92.
Here you can see my daughter, who is now 31,
exploring some of the failures back in ’92.
Clearly, a whole different order of magnitude in 1906.
One of the things I did around the centennial was to look very closely at
the historic records of ground shaking in 1906 in Humboldt County.
I compiled this in the 2008 BSSA publication.
And one of the lines that I think is pretty easy to determine is between
intensity 7 and 6, just by looking at the mentions of chimneys down.
So, if you look at the yellow-and-higher zone in these two, there’s no question
that, in Humboldt County, 1906 spread a much wider swath of destruction.
And I think we can credibly argue for intensity 9 over a fairly large area
of the Eel River Valley. I argued that there’s a little area
of 9 in ’92 in the Petrolia area. No question 1906 was the strongest
historic earthquake that we have information about on the North Coast.
One of the aspects of 1906 that’s of most interest to me is the sort of
connection between the world of the San Andreas and the world of
the Cascadia subduction zone. In the six weeks following 1906,
there were at least 22 earthquakes reported felt in Humboldt County.
I’d argue that many of these are probably located within the Gorda Plate
or on the Mendocino Fault rather than on the San Andreas.
The two largest aftershocks recorded – or, the largest aftershock recorded
in 1906 on April 22nd was probably in the Gorda Plate – a 6.7.
Now, we have lots of earthquakes in Humboldt County, but 22 in a six-week
period – well, that only happens after a really strong earthquake.
In fact, in the whole year prior to 1906, we have no record of an earthquake
being felt in Humboldt County. So we’re looking at a significant
excitation in the – at the northern end of the rupture zone.
Quite likely that this period of elevated activity continued for years
afterwards – a 6.3 and a 6.7 in 1909 may well be part of that too.
So I want to leave you with thinking again about this connection between
the world of the San Andreas and the world of the Cascadia subduction zone.
And get more of you to perhaps re-look at data from 1906, come up with new
models. We know that, in 1906, we did not get a Cascadia earthquake.
In 1700, the Cascadia earthquake probably did not trigger
a San Andreas earthquake. But there’s certainly evidence –
both Dorothy Merritts looking at terraces in the King Range thought there
might be some linkage, and more recent work by Chris Goldfinger suggests that
sometimes a big event on one triggers an event on the other.
So plenty of food for thought. Lots more to do.
And I want to end with thanking the many historical societies and historians,
newspaper archives, and so forth that have preserved information
on what happened in 1906. Big thanks to Carol Prentice
for her information on what happened at Shelter Cove.
Pete Palmquist – late archivist and photographer at Humboldt State
for preserving the photographs of Edna Garrett.
And, of course, Kathy Moley for helping me in 1992.
Please get back to me with questions or comments,
and thank you for inviting me.