Talk:1935 Helena earthquake

Date

The infobox says 19th, but the text says 18th, when it first struck. Does the infobox need changed? PumpkinSky talk 12:56, 8 July 2012 (UTC)

We tend to use UTC for dates, which in this case means 19th, but the lead shows dates both in UTC and local time - the USGS and NGDC both use UTC. Mikenorton (talk) 13:05, 8 July 2012 (UTC)

Oh, if that's the convention, that's fine. PumpkinSky talk 13:11, 8 July 2012 (UTC)

Location

Curious about "Basin and Range Province" - I know squat about earthquake physiography, but the map at that page says it's section 22 here, but Helena is more in the area numbered 19, apparently the Rocky Mountain System (? ) (Helena is roughly 46 degrees north latitude, 112 degrees west longitude). So is there an error? Montanabw^(talk) 05:28, 9 July 2012 (UTC)

The physiographic province does end in southern Idaho but the tectonic zone does not, as I understand it. I'll take another look this evening. Mikenorton (talk) 07:11, 9 July 2012 (UTC)

May need a ref here, although the ranges of central Montana east of the Rockies front do have some characteristics of Basin & Range, I don't think the terminology is used. The Lewis & Clark Fault Zone is an old (PreCambrian) shear with continued "adjustments". Vsmith (talk) 10:47, 9 July 2012 (UTC)

Definitely does not appear to be connected in any way to the Great Basin; Southern Idaho is a quite a ways off and on the other side of the Continental Divide. That's got to count for something. (Helena is at the foot of the Divide, about 20 miles east of the nearest pass) Montanabw^(talk) 20:00, 9 July 2012 (UTC)

The largest historical earthquake in the state was the 1959 Yellowstone earthquake which was caused by slip on a normal fault, as was the 1983 Borah Peak earthquake in neighbouring Idaho. In this report on the 'Seismic Hazard Susceptibility in Southwestern Montana' it states "Southwestern Montana is situated in the Montana-Idaho basin and range segment of the Basin and Range physiographic province." It further states "All earthquakes of magnitude greater than 5.5, and most identified late Quaternary faults found in Montana, exist south of the Lewis and Clark Zone". I'll add this source and reword the section some. Mikenorton (talk) 20:22, 9 July 2012 (UTC)

Still undue emphasis on the Basin and Range here. The seismic hazard paper you reference doesn't mention the Helena quake (unless I'm missing something). It discusses the 3 stress fields, Mid-continent, Yellowstone hot spot and Basin & range. The Dillon area (Borah Peak fault) is in the very SW part of the state and is more influenced by the B&R stress field. The Bozeman area, especially the Hogben Lake fault is right next to Yellowstone and the Clarkson Valley is just north of Bozeman. All are well south of Helena. The Helena quake was related to two faults on the northeast margin of the complex Lewis and Clark Fault Zone and the movement on the faults was strike-slip rather than dip-slip more typical methinks of the Basin & Range faults. The first sentence of the section over-emphasizes the B&R. If needed it should be reworded according to your ref to emphasize the three stress fields involved in the area. A reader would think that Helena (on the east side of the continental divide) is being included in the Basin & Range. Also is that "extensional tectonics" bit supported? Vsmith (talk) 01:38, 10 July 2012 (UTC)

Oops, I did miss something - the Helena quake is mentioned in that ref in the isoseismal lines map discussion - but not any detail. Vsmith (talk) 01:51, 10 July 2012 (UTC)

I understand your concerns, the earthquake was in the 'Intermountain Seismic Belt' which is not exactly Basin and Range, but it is extensional in style and closely related - I've found another three sources ([1], [2], [3]) and will rewrite using those, but not tonight. Mikenorton (talk) 22:12, 10 July 2012 (UTC)

Whatever you guyz sort out, just get it right. (smile) Montanabw^(talk) 23:01, 10 July 2012 (UTC)

More refs is good - and those pdfs have outstanding maps. I had run across the Stickney, et al paper while trying to get a handle on the Lewis and Clark Fault Zone ... seems it should have an article. The seismology folks tend to have a different perspective on the Basin & Range - Maybe use "Intermountain Seismic Belt which is related to the Basin and Range structures" or something like that. Both of those pdf maps show the importance of the Yellowstone "track" and I interpret their usage of Basin & Range (stress field) as "overprinting" on the older Rocky Mtn structures. Vsmith (talk) 01:33, 11 July 2012 (UTC)

Not knowing seismology and only a little geology, all I can add is that "intermountain seismic belt" sounds like what we'd say up here.  ;-) Montana doesn't consider itself in the Great Basin in any way, shape or form, so if we are, tectonically, Vsmith's suggested phrasing will ease our Montana souls... would the article or research sources on the 1959 earthquake by Hebgen Lake (near Yellowstone Park) be of any help? The park is almost precisely due south of Helena, so MIGHT be in the same general category (?). Not sure if this helps because I lack expertise in this area, but western Montana worries a lot about earthquakes in general (we have earthquake drills in the schools) and so some of the lingo is in the air out here, and "Basin and Range" isn't...  ;-)

OK, I had another go at it. The 1959 earthquake is very much related to Yellowstone hotspot area, rather than the ISB itself (although the hotspot lies very close to it). I'm not sure whether to mention the hotspot as it's not directly related to the earthquake. Mikenorton (talk) 21:17, 11 July 2012 (UTC)

That'll work, very good. And probably don't need to add to the complexity here (in this article) regarding the Yellowstone hotspot unless that stress field is implicated by a source. Could mention other quakes associated with the ISB if a discussion of quake history of the region to get a regional perspective perhaps. Maybe in the Modern day concerns section. Vsmith (talk) 22:08, 11 July 2012 (UTC)

The area remains seismically active; there was a noticeable jiggle in 2005, ironically, the school with the collapsed wall was named in honor of the fellow who was superintendent of schools during the 1935 quake! See here. Probably a newspaper story out there too, but I'm on the slow dialup and it won't jump out unless I go to the bloated Helenair.com newspaper web site to look. Incidentally, here is a really fun collation of various news accounts someone may want to add. Montanabw^(talk) 06:32, 13 July 2012 (UTC)

More sources

Spokane Hills, where a major north-south fault line runs at the eastern edge of the Helena Valley

The north end of the Spokane Hills (blue range farthest from camera) and Hauser Lake, location of another fault line

This may help add more geologic info to the article: http://www.co.lewis-clark.mt.us/departments/disaster-and-emergency-services/hazards/earthquakes.html At that page, you will note there is a fault map, may be handy. See http://www.co.lewis-clark.mt.us/fileadmin/user_upload/Safety/DES/susceptibility.gif and http://www.co.lewis-clark.mt.us/fileadmin/user_upload/Safety/DES/faults.jpg

This appears to be a page of technical specs as well as a huge list of links that may also help: http://www.co.lewis-clark.mt.us/departments/disaster-and-emergency-services/hazards/earthquakes.html Includes a list of photos, no idea if PD or free use, am home on the slow dialup tonight so can't do much digging. Montanabw^(talk) 05:53, 9 July 2012 (UTC)

Here is a photo of the Spokane Hills. The Lewis and Clark County page has a blurry blued-out photo with them labeled "fault bounded Spokane Hills" , and I know one of the major fault lines runs roughly along their base. See line farthest east on this map. Not sure if relevant to the 1935 quake, but if a color photo helps anything, feel free to add. Putting it here for now along with another shot that includes Hauser Lake, where there is another fault line. Montanabw^(talk) 06:29, 9 July 2012 (UTC)

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