WEBVTT Kind: captions Language: en-US 00:00:00.320 --> 00:00:05.280 Hello. Thank you for inviting me. I’m Lori Dengler, emeritus professor of 00:00:05.280 --> 00:00:10.240 geology at Humboldt State University. And I’ve been working on earthquake 00:00:10.240 --> 00:00:16.827 and tsunami hazards on California’s North Coast for over four decades. 00:00:18.880 --> 00:00:23.120 Today I want to talk about the 1906 earthquake in Humboldt County. 00:00:23.120 --> 00:00:28.880 Now, I know that many of you might think that 1906 has been kind of hacked 00:00:28.880 --> 00:00:33.656 to death and there really isn’t much more to learn from that earthquake. 00:00:33.680 --> 00:00:38.000 I would like to say no. I really think it’s an extraordinary 00:00:38.000 --> 00:00:41.736 event, and it’s always worth going back to. 00:00:41.760 --> 00:00:46.616 Humboldt County, if you look at our location over here on the right, 00:00:46.640 --> 00:00:53.576 is at the northern end of both the rupture zone and the damage. 00:00:53.600 --> 00:01:00.536 In fact, it sort of straddles the area of significant damage versus felt. 00:01:00.560 --> 00:01:04.240 And we’re fortunate to actually have quite a bit of information 00:01:04.240 --> 00:01:09.250 about what happened in Humboldt County in 1906. 00:01:11.440 --> 00:01:16.080 This is one of many models of slip during the 1906 earthquake. 00:01:16.080 --> 00:01:20.296 I believe this is after Thatcher 1997. 00:01:20.320 --> 00:01:26.560 The heights of the bars are the relative slip, and the numbers are in feet. 00:01:26.560 --> 00:01:31.016 This was a illustration I used at the Humboldt County Fair 00:01:31.040 --> 00:01:36.080 a number of years ago. And you can see that the height 00:01:36.080 --> 00:01:41.040 of the bars and the number there in the Cape Mendocino area in the vicinity 00:01:41.040 --> 00:01:46.216 of the triple junction are among the highest anywhere in the region. 00:01:46.240 --> 00:01:52.400 So I’m going to show you a little evidence for why I believe that is real – 00:01:52.400 --> 00:01:56.296 that we really did have significant slip on the North Coast, 00:01:56.320 --> 00:02:01.395 and why this northern termination is really so interesting. 00:02:04.320 --> 00:02:08.640 We’re fortunate to have a rather extraordinary photographic record 00:02:08.640 --> 00:02:12.720 of what happened in Ferndale, the largest community in 00:02:12.720 --> 00:02:17.840 Eel River Valley at that time, because of a woman – Edna Garrett. 00:02:17.840 --> 00:02:22.800 Edna and her husband Ed were photographers – one of three 00:02:22.800 --> 00:02:29.200 photographic studios in Ferndale at the time of the 1906 earthquake. 00:02:29.200 --> 00:02:34.720 But only Edna, after being rudely awakened early that morning, 00:02:34.720 --> 00:02:39.840 pulled out her equipment and proceeded to document what happened 00:02:39.840 --> 00:02:44.136 in the Eel River Valley on that extraordinary day. 00:02:44.160 --> 00:02:48.936 These are some of her photographs of structural damage. 00:02:48.960 --> 00:02:55.840 On the upper level, on the upper left and the upper right, you see the two 00:02:55.840 --> 00:03:01.016 brick buildings in Ferndale at the time, both significantly damaged. 00:03:01.040 --> 00:03:05.120 The building on the left was subsequently destroyed. 00:03:05.120 --> 00:03:09.360 The one on the right continued until the 1990s. 00:03:09.360 --> 00:03:12.776 And I will get back to that building later. 00:03:12.800 --> 00:03:18.456 You can certainly see significant damage to both the unreinforced 00:03:18.480 --> 00:03:21.384 and to the wood-frame structures. 00:03:24.640 --> 00:03:30.216 Edna also documented liquefaction in the area. 00:03:30.240 --> 00:03:35.040 And on the left is my most favorite liquefaction photograph – 00:03:35.040 --> 00:03:39.736 I like the field clothes – in the Eel River Valley. 00:03:39.760 --> 00:03:45.840 And another photograph on the right, also on a tributary of the Eel. 00:03:45.840 --> 00:03:52.880 Spectacular liquefaction, April 1906. Ground is very saturated. 00:03:52.880 --> 00:03:55.077 Not surprising. 00:03:58.160 --> 00:04:02.320 She also went out to near the coast, near Centerville Beach, and took this 00:04:02.320 --> 00:04:06.720 photograph of a very large landslide. Landslides are not at all surprising 00:04:06.720 --> 00:04:11.360 in Humboldt County. They occur even in the absence of earthquakes. 00:04:11.360 --> 00:04:16.880 We saw very similar failures to this after the 1992 earthquake 00:04:16.880 --> 00:04:20.815 in the same relative area. 00:04:24.160 --> 00:04:32.216 This is the surface fault rupture – the map by Mathes in the 1908 00:04:32.240 --> 00:04:38.240 Lawson report. The northernmost documented surface rupture. 00:04:38.240 --> 00:04:42.480 Carol Prentice certainly knows much more about this than I do. 00:04:42.480 --> 00:04:47.816 She provided Mathes’ notes and interpretations for me. 00:04:47.840 --> 00:04:53.520 No question that there was surface rupture in 1906. 00:04:53.520 --> 00:04:57.416 You can still see this today. 00:04:57.440 --> 00:05:05.496 And whether this is truly the main strand of the San Andreas or some 00:05:05.520 --> 00:05:12.456 auxiliary strand, I’ll let you geologists continue to argue it out. 00:05:12.480 --> 00:05:18.799 But we did have significant offset in 1906. 00:05:21.440 --> 00:05:24.640 One of the extraordinary things about Humboldt County, 00:05:24.640 --> 00:05:30.776 and particularly Ferndale – we call Ferndale the Victorian village, 00:05:30.800 --> 00:05:37.280 and 70% of the buildings that existed in 1906 are still there today. 00:05:37.280 --> 00:05:42.560 The photograph on the top is one of Edna Garrett’s views. 00:05:42.560 --> 00:05:50.320 And we were able to take exactly the same spot in 2006, and you certainly 00:05:50.320 --> 00:05:54.800 get a feel that – except for the absence of the telephone poles, 00:05:54.800 --> 00:06:00.880 which were removed during a filming – a movie in the 1970s – wires were 00:06:00.880 --> 00:06:06.376 put underground at that point, it looks essentially the same. 00:06:06.400 --> 00:06:16.640 But you can actually visit Ellin Beltz’s terrific tour of Main Street in Ferndale. 00:06:16.640 --> 00:06:20.240 And she goes through exactly what happened to pretty much 00:06:20.240 --> 00:06:23.979 every one of these buildings in 1906. 00:06:25.440 --> 00:06:28.616 I’ve taken just a few examples here. 00:06:28.640 --> 00:06:34.080 Here we have what was the general mercantile in 1906. 00:06:34.080 --> 00:06:38.000 And, in 1902, it was the Valley Grocery. 00:06:38.000 --> 00:06:42.136 In 1906, it’s a very new building. 00:06:42.160 --> 00:06:44.856 Just a little over a year old. 00:06:44.880 --> 00:06:51.176 And you can see the parapet failure and the many windows that broke. 00:06:51.200 --> 00:06:58.696 In 1992, very similar damage. Not as much of the parapet wall failed. 00:06:58.720 --> 00:07:03.040 Not as many of the glass windows, but probably the glass windows were 00:07:03.040 --> 00:07:08.696 tempered in 1992. So that’s not a very good indication of relative strength. 00:07:08.720 --> 00:07:12.640 But I would argue that the greater parapet wall failure, 00:07:12.640 --> 00:07:17.360 especially with young mortar, suggests that, at least here, 00:07:17.360 --> 00:07:23.896 the ground shaking was stronger in 1906. 00:07:23.920 --> 00:07:30.800 Here we have 393 Main Street in 1906 on the left. 00:07:30.800 --> 00:07:34.616 The exact same building in 1992. 00:07:34.640 --> 00:07:43.049 Significantly more damage to wood- frame structures in 1906 than in 1992. 00:07:44.720 --> 00:07:52.160 This is the only interior photo that Edna Garrett took back in 1906 on the left. 00:07:52.160 --> 00:07:57.840 Fortuitously, we have the interior of the exact same building in 1992, 00:07:57.840 --> 00:08:04.160 illustrating that items off shelves are going to look pretty similar 00:08:04.160 --> 00:08:09.040 once you get over an intensity of 7, and you certainly couldn’t differentiate 00:08:09.040 --> 00:08:14.227 which of these two experienced the stronger shaking. 00:08:16.640 --> 00:08:20.616 I love to compare the liquefaction scenes. 00:08:20.640 --> 00:08:24.720 We did observe several places of liquefaction in ’92. 00:08:24.720 --> 00:08:27.920 Here you can see my daughter, who is now 31, 00:08:27.920 --> 00:08:33.496 exploring some of the failures back in ’92. 00:08:33.520 --> 00:08:39.840 Clearly, a whole different order of magnitude in 1906. 00:08:41.760 --> 00:08:50.136 One of the things I did around the centennial was to look very closely at 00:08:50.160 --> 00:08:55.440 the historic records of ground shaking in 1906 in Humboldt County. 00:08:55.440 --> 00:09:01.416 I compiled this in the 2008 BSSA publication. 00:09:01.440 --> 00:09:08.800 And one of the lines that I think is pretty easy to determine is between 00:09:08.800 --> 00:09:16.080 intensity 7 and 6, just by looking at the mentions of chimneys down. 00:09:16.080 --> 00:09:21.440 So, if you look at the yellow-and-higher zone in these two, there’s no question 00:09:21.440 --> 00:09:28.936 that, in Humboldt County, 1906 spread a much wider swath of destruction. 00:09:28.960 --> 00:09:36.720 And I think we can credibly argue for intensity 9 over a fairly large area 00:09:36.720 --> 00:09:42.080 of the Eel River Valley. I argued that there’s a little area 00:09:42.080 --> 00:09:51.736 of 9 in ’92 in the Petrolia area. No question 1906 was the strongest 00:09:51.760 --> 00:09:58.144 historic earthquake that we have information about on the North Coast. 00:10:02.400 --> 00:10:08.800 One of the aspects of 1906 that’s of most interest to me is the sort of 00:10:08.800 --> 00:10:13.280 connection between the world of the San Andreas and the world of 00:10:13.280 --> 00:10:21.360 the Cascadia subduction zone. In the six weeks following 1906, 00:10:21.360 --> 00:10:26.776 there were at least 22 earthquakes reported felt in Humboldt County. 00:10:26.800 --> 00:10:33.256 I’d argue that many of these are probably located within the Gorda Plate 00:10:33.280 --> 00:10:38.856 or on the Mendocino Fault rather than on the San Andreas. 00:10:38.880 --> 00:10:46.880 The two largest aftershocks recorded – or, the largest aftershock recorded 00:10:46.880 --> 00:10:56.296 in 1906 on April 22nd was probably in the Gorda Plate – a 6.7. 00:10:56.320 --> 00:11:00.800 Now, we have lots of earthquakes in Humboldt County, but 22 in a six-week 00:11:00.800 --> 00:11:05.176 period – well, that only happens after a really strong earthquake. 00:11:05.200 --> 00:11:12.560 In fact, in the whole year prior to 1906, we have no record of an earthquake 00:11:12.560 --> 00:11:19.440 being felt in Humboldt County. So we’re looking at a significant 00:11:19.440 --> 00:11:26.856 excitation in the – at the northern end of the rupture zone. 00:11:26.880 --> 00:11:32.560 Quite likely that this period of elevated activity continued for years 00:11:32.560 --> 00:11:40.525 afterwards – a 6.3 and a 6.7 in 1909 may well be part of that too. 00:11:42.480 --> 00:11:50.320 So I want to leave you with thinking again about this connection between 00:11:50.320 --> 00:11:56.000 the world of the San Andreas and the world of the Cascadia subduction zone. 00:11:56.000 --> 00:12:04.080 And get more of you to perhaps re-look at data from 1906, come up with new 00:12:04.080 --> 00:12:13.656 models. We know that, in 1906, we did not get a Cascadia earthquake. 00:12:13.680 --> 00:12:20.160 In 1700, the Cascadia earthquake probably did not trigger 00:12:20.160 --> 00:12:23.440 a San Andreas earthquake. But there’s certainly evidence – 00:12:23.440 --> 00:12:29.680 both Dorothy Merritts looking at terraces in the King Range thought there 00:12:29.680 --> 00:12:35.600 might be some linkage, and more recent work by Chris Goldfinger suggests that 00:12:35.600 --> 00:12:41.096 sometimes a big event on one triggers an event on the other. 00:12:41.120 --> 00:12:45.816 So plenty of food for thought. Lots more to do. 00:12:45.840 --> 00:12:53.736 And I want to end with thanking the many historical societies and historians, 00:12:53.760 --> 00:12:58.880 newspaper archives, and so forth that have preserved information 00:12:58.880 --> 00:13:03.976 on what happened in 1906. Big thanks to Carol Prentice 00:13:04.000 --> 00:13:08.536 for her information on what happened at Shelter Cove. 00:13:08.560 --> 00:13:15.416 Pete Palmquist – late archivist and photographer at Humboldt State 00:13:15.440 --> 00:13:18.480 for preserving the photographs of Edna Garrett. 00:13:18.480 --> 00:13:23.736 And, of course, Kathy Moley for helping me in 1992. 00:13:23.760 --> 00:13:26.960 Please get back to me with questions or comments, 00:13:26.960 --> 00:13:29.492 and thank you for inviting me.