{"615366":{"#nid":"615366","#data":{"type":"event","title":"Granular Physics of Rock Friction at Low Slip Rates","body":[{"value":"\u003Ch4\u003E\u003Cstrong\u003EThe School of Earth and Atmospheric Sciences Presents Dr. Behrooz Ferdowsi, Princeton University\u003C\/strong\u003E\u003C\/h4\u003E\r\n\r\n\u003Cp\u003EGranular Physics of Rock Friction at Low Slip Rates\u003C\/p\u003E\r\n\r\n\u003Cdiv\u003E\r\n\u003Cp\u003EModeling earthquake fault slip requires reliable constitutive relations\u0026nbsp;describing friction. A commonly accepted empirical framework, known as\u0026nbsp;\u0026ldquo;Rate- and State-dependent Friction\u0026rdquo; (RSF), suggests that frictional\u0026nbsp;strength depends on the fault slip rate and (history dependent) \u0026lsquo;state\u0026rsquo;\u0026nbsp;variable.\u0026nbsp;\u003C\/p\u003E\r\n\r\n\u003Cp\u003EAlthough none of the empirical RSF laws proposed thus far,\u0026nbsp;including RSF \u0026ldquo;Aging\u0026rdquo; and \u0026ldquo;Slip\u0026rdquo; versions, adequately describes the full\u0026nbsp;range of laboratory friction data, the Slip law is clearly superior, and does an excellent job of modeling both velocity steps and slide-holds\u0026nbsp;rock laboratory loading protocols.\u0026nbsp;\u0026nbsp;Despite this, and unlike the Aging\u0026nbsp;law, there is no clearly-established physical basis for the Slip law.\u003C\/p\u003E\r\n\r\n\u003Cp\u003EIn this seminar, I will first provide an overview of the RSF laboratory\u0026nbsp;observations and the empirical (standard) RSF modeling framework. I will\u0026nbsp;discuss the shortcomings of the standard model and that at the moment,\u0026nbsp;unfortunately, no physics-based constitutive relation exists for rock\u0026nbsp;friction.\u0026nbsp;\u003C\/p\u003E\r\n\r\n\u003Cp\u003EIt is noteworthy that natural fault zones typically contain a\u0026nbsp;localized shear zone, also known as the granular gouge layer, and that\u0026nbsp;laboratory experiments on even initially bare rock surfaces develop a\u0026nbsp;gouge layer through mechanical wear.\u0026nbsp;\u003C\/p\u003E\r\n\r\n\u003Cp\u003EBased on this observation, I have\u0026nbsp;developed a granular physics-based simulation to investigate the origins\u0026nbsp;of RSF in rocks. In my model, I have intentionally left out\u0026nbsp;time-dependent plasticity at the grain contact-scale, that is\u0026nbsp;traditionally thought to be the primary origin of RSF in the standard model.\u0026nbsp;\u003C\/p\u003E\r\n\r\n\u003Cp\u003EI will show results from the granular simulations that reproduce\u0026nbsp;and explain robust features of real rock and gouge friction data.\u0026nbsp;Namely, the granular model captures: (i) the functional form of the\u0026nbsp;transition to new values of the dynamic friction following a change in\u0026nbsp;shearing velocity; (ii) logarithmic-with-time healing of the frictional\u0026nbsp;interface and its dependence on prior shear rate, during the load-point\u0026nbsp;hold.\u0026nbsp;\u003C\/p\u003E\r\n\r\n\u003Cp\u003EThese laboratory observations currently have no other\u0026nbsp;first-principles or physics-based explanations. The success of the\u0026nbsp;granular model seems to be arising from the logarithmic-with-time (slow)\u0026nbsp;compaction and slow relaxation dynamics in the model that is a hallmark\u0026nbsp;of granular materials and disordered solids.\u003C\/p\u003E\r\n\r\n\u003Cp\u003EI will next discuss how I am working to further unify the search for\u0026nbsp;constitutive relations for friction and deformation of rocks with the\u0026nbsp;recent exciting developments within the broader community of soft condensed matter physicists for a state variable and an Equation of\u0026nbsp;State for granular systems. Such a state variable and equation of state\u0026nbsp;are essential for confidently applying laboratory-derived friction laws\u0026nbsp;to fault slip in the Earth.\u0026nbsp;\u0026nbsp;\u003C\/p\u003E\r\n\r\n\u003Cp\u003EIn addition to earthquake fault zones, the\u0026nbsp;RSF behavior is ubiquitously observed in friction of disordered Earth\u0026nbsp;materials and interfaces, including ice-on-rock, sediments on Earth\u0026rsquo;s\u0026nbsp;surface and damaged rocks in the shallow crust.\u0026nbsp;\u003C\/p\u003E\r\n\r\n\u003Cp\u003ETherefore, the\u0026nbsp;implications and applications of my work are broad. I will discuss some\u0026nbsp;of my near future research plans related to Earth\u0026rsquo;s near-surface\u0026nbsp;processes, in addition to earthquake source physics.\u003C\/p\u003E\r\n\u003C\/div\u003E\r\n","summary":null,"format":"limited_html"}],"field_subtitle":"","field_summary":"","field_summary_sentence":[{"value":"A seminar by Dr. Behrooz Ferdowsi, School of Earth and Atmospheric Science"}],"uid":"34458","created_gmt":"2018-12-13 19:23:19","changed_gmt":"2019-02-14 15:46:01","author":"nlawson3","boilerplate_text":"","field_publication":"","field_article_url":"","field_event_time":{"event_time_start":"2019-02-19T11:00:00-05:00","event_time_end":"2019-02-19T11:50:00-05:00","event_time_end_last":"2019-02-19T11:50:00-05:00","gmt_time_start":"2019-02-19 16:00:00","gmt_time_end":"2019-02-19 16:50:00","gmt_time_end_last":"2019-02-19 16:50:00","rrule":null,"timezone":"America\/New_York"},"extras":[],"hg_media":{"617834":{"id":"617834","type":"image","title":"B. 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Ferdowsi.png","image_path":"\/sites\/default\/files\/images\/B.%20Ferdowsi.png","image_full_path":"http:\/\/hg.gatech.edu\/\/sites\/default\/files\/images\/B.%20Ferdowsi.png","mime":"image\/png","size":45148,"path_740":"http:\/\/hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/images\/B.%20Ferdowsi.png?itok=2cKfpY4d"}}},"media_ids":["617834"],"groups":[{"id":"364801","name":"School of Earth and Atmospheric Sciences (EAS)"}],"categories":[],"keywords":[{"id":"175623","name":"EAS Seminar"}],"core_research_areas":[],"news_room_topics":[],"event_categories":[{"id":"1795","name":"Seminar\/Lecture\/Colloquium"}],"invited_audience":[{"id":"78761","name":"Faculty\/Staff"},{"id":"177814","name":"Postdoc"},{"id":"174045","name":"Graduate students"},{"id":"78751","name":"Undergraduate students"}],"affiliations":[],"classification":[],"areas_of_expertise":[],"news_and_recent_appearances":[],"phone":[],"contact":[{"value":"\u003Cp\u003EAndrew Newman\u003C\/p\u003E\r\n","format":"limited_html"}],"email":[],"slides":[],"orientation":[],"userdata":""}}}