{"id":2557,"date":"2011-08-25T00:10:15","date_gmt":"2011-08-25T04:10:15","guid":{"rendered":"http:\/\/billemory.com\/blog\/?p=2557"},"modified":"2011-08-25T07:17:33","modified_gmt":"2011-08-25T11:17:33","slug":"sunglasses","status":"publish","type":"post","link":"https:\/\/billemory.com\/blog\/2011\/08\/sunglasses\/","title":{"rendered":"sunglasses"},"content":{"rendered":"


\nQuestions to the USGS<\/strong>
\nI was seated outside on a concrete slab during the seismic event Tuesday. I live in Charlottesville, VA. ( 38.023873\u00b0, -78.461908\u00b0)
\nI am disappointed in my lack of ability to recount accurately three things. <\/p>\n

1- How long the event lasted
\n2- How much movement was there of the surface. If I had a “fixed point” a foot in the air above my concrete slab, did the earth move up and down two inches relative to my fixed point? Back and forth two inches? At what frequency?
\n3- The noise. How much noise did the SE make? Would it have been as noisy if I’d been out in an open field? Away from structures.<\/p>\n

Answers from the USGS<\/strong>
\n1. \u00a0Your first inquiry is about earthquake duration of the Virginia earthquake of August 23, 2011. \u00a0The short answer to your question is that the duration of the earthquake was about 2 seconds. \u00a0However, allow me to provide a little description of what (our) meaning of duration is. <\/p>\n

Duration can be technically defined as the time from the onset of an earthquake (the nucleation) to the cessation of shaking (when the moving fault finally runs out of steam). \u00a0This is called \u201csource duration\u201d and has a unique value. \u00a0However, the time over which observers close to the earthquake will feel the shaking of an earthquake can vary considerably depending on one\u2019s distance and azimuth from the epicenter. \u00a0Reasons include: \u00a0(1) directivity; (2) partition of energy (between P, S and surface waves); and (3) local geology. \u00a0Earthquake directivity, analogous to the Doppler effect in acoustics and optics, is the variation in duration due to one\u2019s position relative to the direction of rupture . \u00a0Complex rupturing, which is not uncommon for very large earthquakes, can intensify variations due to directivity. \u00a0This probably was not a strong factor for the Virginia earthquake. Also, P and S waves, as well as surface waves, travel with different velocities within the earth and carry different amounts of energy at different azimuths. \u00a0Observers may feel one or all of these waves with varying intensity and at different time intervals. \u00a0Finally, the time over which a seismic wave can be felt can be extended if the observer is situated over certain geologic structures, such as a sedimentary basement, which can trap energy and keep it reverberating for a while. \u00a0So the short answer is that you probably felt the earthquake for two long seconds. \u00a0However, depending on the geology of location and your distance from the epicenter, you might have felt shaking for many more seconds. <\/p>\n

2. \u00a0I have seen one seismogram at a station located 200 kms from the epicenter showing a peak displacement of 2500 microns. \u00a0That converts to 2.5 mm. or about 0.1 in. \u00a0If you are closer the amplitude may be larger. \u00a0Note that this displacement is not the permanent rupture at the fault; it is elastic response (shaking) of the earth. <\/p>\n

3. \u00a0Earthquake sounds have rarely been reported. \u00a0 It is feasible that an earthquake can be heard. \u00a0The range of human hearing is approximately from 20-20,000 Hz. \u00a0The peak energy (corner frequency) of earthquakes large enough to be felt ranges from roughly 0.10-10 Hz. \u00a0So there could be an overlap. \u00a0Although reports of earthquake noise are not common, we do get them. \u00a0They are usually described as a \u201clow rumbling\u201d, which is consistent with the overlap range of frequencies I just mentioned. \u00a0Higher frequency sounds you may have heard could have been indirect effects of shaking on structures or items within your house. <\/p>\n","protected":false},"excerpt":{"rendered":"

Questions to the USGS I was seated outside on a concrete slab during the seismic event Tuesday. I live in Charlottesville, VA. ( 38.023873\u00b0, -78.461908\u00b0) I am disappointed in my lack of ability to recount accurately three things. 1- How long the event lasted 2- How much movement was there of the surface. If I … Continue reading “sunglasses”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"advanced_seo_description":"","jetpack_seo_html_title":"","jetpack_seo_noindex":false,"footnotes":"","jetpack_publicize_message":"","jetpack_is_tweetstorm":false,"jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","enabled":false}}},"categories":[1],"tags":[],"class_list":["post-2557","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"jetpack_publicize_connections":[],"jetpack_sharing_enabled":true,"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/billemory.com\/blog\/wp-json\/wp\/v2\/posts\/2557"}],"collection":[{"href":"https:\/\/billemory.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/billemory.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/billemory.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/billemory.com\/blog\/wp-json\/wp\/v2\/comments?post=2557"}],"version-history":[{"count":3,"href":"https:\/\/billemory.com\/blog\/wp-json\/wp\/v2\/posts\/2557\/revisions"}],"predecessor-version":[{"id":2560,"href":"https:\/\/billemory.com\/blog\/wp-json\/wp\/v2\/posts\/2557\/revisions\/2560"}],"wp:attachment":[{"href":"https:\/\/billemory.com\/blog\/wp-json\/wp\/v2\/media?parent=2557"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/billemory.com\/blog\/wp-json\/wp\/v2\/categories?post=2557"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/billemory.com\/blog\/wp-json\/wp\/v2\/tags?post=2557"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}