{"id":3988,"date":"2010-08-08T07:30:47","date_gmt":"2010-08-08T12:30:47","guid":{"rendered":"http:\/\/all-geo.org\/highlyallochthonous\/?p=3988"},"modified":"2010-08-08T08:00:59","modified_gmt":"2010-08-08T13:00:59","slug":"friday-ish-focal-mechanisms","status":"publish","type":"post","link":"https:\/\/all-geo.org\/highlyallochthonous\/2010\/08\/friday-ish-focal-mechanisms\/","title":{"rendered":"Friday-ish focal mechanisms"},"content":{"rendered":"

\"A<\/span>Obviously, my holiday has slightly disrupted my weekly seismic updates, but the magnitude 4.8 earthquake that shook Wyoming on Thursday<\/a> did catch my eye, mainly because I was right in the area only six weeks ago<\/a>. <\/p>\n

\"\"

Location of Magnitude 4.8 earthquake in Wyoming, Aug 5.<\/p><\/div>\n

The earthquake was located just to east of Grand Teton National Park<\/a>, just south of Yellowstone National Park and if, anything, even more beautiful. They host as perfect a range of mountains as you’re ever likely to see; this vista is possibly only matched by a western view of the Southern Alps of New Zealand.<\/p>\n

\"\"

Part of the Teton Range, from the shore of Jackson Lake. Photo: Chris Rowan, 2010<\/p><\/div>\n

The Teton Range is on the eastern edge of the Basin and Range<\/a>, a region of extension that has created a series of fault-bounded mountain belts across a large region of the western US, and the topography here is also created by extensional faulting. With that in mind, I looked up the focal mechanism<\/a> fully expecting to see an extensional fault plane solution. Surprisingly, however, this is not what I found; there is a small component of extension, but the focal mechanism is largely strike-slip<\/a>, indicating two bits of crust sliding past each other, rather than being pulled apart.<\/p>\n

\"\"<\/p>\n

The rupture was at a shallow depth (5 km) and not actually located in the Teton Range, but further east in the Gros Ventre range, a region with several northwest-southeast trending escarpments, suggesting this earthquake was due to right lateral shear on a NW-SE fault. The USGS Quaternary fault map for Wyoming<\/a> indicates a few faults with similar trends, and that might have recently-produced fault scarps, further to the east (such as the South Granite Mountains Fault System<\/a>). But the sense of movement on these faults is reported to be extensional, meaning that this earthquake seems inconsistent with even fairly local tectonics.<\/p>\n

So, it’s a bit of a mystery; but remember that a magnitude 4.8 has very little regional tectonic significance (it’s a relatively low energy event), so it could be just relieving a bit of local shear strain built up by deformation around it. Of course, it is always possible that some strike-slip displacement did occur across the faults to the east as well, but has gone unrecognised. <\/p>\n

I’m planning to post bit more about the larger scale tectonics in the Grand Teton region next week, as I return to blogging about my Yellowstone trip. Meanwhile, just to add to the topicality, Julia is reporting<\/a> that, in the latest act in a long-running financial dispute with Washington, the Wyoming state governor is is threatening to sell some state-earned parts of the National Park. All I can say is: don’t do it, Wyoming! <\/p>\n

And next week, I will actually post a Friday Focal Mechanisms on the Friday. Maybe.<\/p>\n","protected":false},"excerpt":{"rendered":"

A brief note on Thursday’s earthquake in Wyoming Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[13,231,5],"tags":[785,782,258,257],"class_list":["post-3988","post","type-post","status-publish","format-standard","hentry","category-earthquakes","category-focal-mechanisms","category-geohazards","tag-earthquakes","tag-geology","tag-grand-teton","tag-wyoming"],"_links":{"self":[{"href":"https:\/\/all-geo.org\/highlyallochthonous\/wp-json\/wp\/v2\/posts\/3988","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/all-geo.org\/highlyallochthonous\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/all-geo.org\/highlyallochthonous\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/all-geo.org\/highlyallochthonous\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/all-geo.org\/highlyallochthonous\/wp-json\/wp\/v2\/comments?post=3988"}],"version-history":[{"count":8,"href":"https:\/\/all-geo.org\/highlyallochthonous\/wp-json\/wp\/v2\/posts\/3988\/revisions"}],"predecessor-version":[{"id":4016,"href":"https:\/\/all-geo.org\/highlyallochthonous\/wp-json\/wp\/v2\/posts\/3988\/revisions\/4016"}],"wp:attachment":[{"href":"https:\/\/all-geo.org\/highlyallochthonous\/wp-json\/wp\/v2\/media?parent=3988"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/all-geo.org\/highlyallochthonous\/wp-json\/wp\/v2\/categories?post=3988"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/all-geo.org\/highlyallochthonous\/wp-json\/wp\/v2\/tags?post=3988"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}