Mark S. Zellman
Glenn D. Thackray
Richard W. Briggs
Ryan D. Gold
Shannon A. Mahan
Christopher B. DuRoss
2021
<div class="article-section-wrapper js-article-section js-content-section "><p>The 72‐km‐long Teton fault in northwestern Wyoming is an ideal candidate for reconstructing the lateral extent of surface‐rupturing earthquakes and testing models of normal‐fault segmentation. To explore the history of earthquakes on the northern Teton fault, we hand‐excavated two trenches at the Steamboat Mountain site, where the east‐dipping Teton fault has vertically displaced west‐sloping alluvial‐fan surfaces. The trenches exposed glaciofluvial, alluvial‐fan, and scarp‐derived colluvial sediments and stratigraphic and structural evidence of two surface‐rupturing earthquakes (SM1 and SM2). A Bayesian geochronologic model for the site includes three optically stimulated luminescence ages (<span class="inline-formula no-formula-id"><span class="MathJax_Preview"><span id="MJXp-Span-5" class="MJXp-math"><span id="MJXp-Span-6" class="MJXp-mo">∼</span><span id="MJXp-Span-7" class="MJXp-mn">12</span><span id="MJXp-Span-8" class="MJXp-mo">–</span><span id="MJXp-Span-9" class="MJXp-mn">17</span><span id="MJXp-Span-10" class="MJXp-mtext"> </span><span id="MJXp-Span-11" class="MJXp-mi">ka</span></span></span></span>) for the glaciofluvial units and 16 radiocarbon ages (<span class="inline-formula no-formula-id"><span class="MathJax_Preview"><span id="MJXp-Span-12" class="MJXp-math"><span id="MJXp-Span-13" class="MJXp-mo">∼</span><span id="MJXp-Span-14" class="MJXp-mn">1.2</span><span id="MJXp-Span-15" class="MJXp-mo">–</span><span id="MJXp-Span-16" class="MJXp-mn">8.6</span><span id="MJXp-Span-17" class="MJXp-mtext"> </span><span id="MJXp-Span-18" class="MJXp-mi">ka</span></span></span></span>) for the alluvial‐fan and colluvial units and constrains SM1 and SM2 to<span> </span><span class="inline-formula no-formula-id"><span class="MathJax_Preview"><span id="MJXp-Span-19" class="MJXp-math"><span id="MJXp-Span-20" class="MJXp-mn">5.5</span><span id="MJXp-Span-21" class="MJXp-mo">±</span><span id="MJXp-Span-22" class="MJXp-mn">0.2</span><span id="MJXp-Span-23" class="MJXp-mtext"> </span><span id="MJXp-Span-24" class="MJXp-mi">ka</span><span id="MJXp-Span-25" class="MJXp-mo">,</span><span id="MJXp-Span-26" class="MJXp-mtext"> </span><span id="MJXp-Span-27" class="MJXp-mn">1</span><span id="MJXp-Span-28" class="MJXp-mi MJXp-italic">σ</span></span></span></span><span> </span>(5.2–5.9 ka, 95%) and<span> </span><span class="inline-formula no-formula-id"><span class="MathJax_Preview"><span id="MJXp-Span-29" class="MJXp-math"><span id="MJXp-Span-30" class="MJXp-mn">9.7</span><span id="MJXp-Span-31" class="MJXp-mo">±</span><span id="MJXp-Span-32" class="MJXp-mn">0.9</span><span id="MJXp-Span-33" class="MJXp-mtext"> </span><span id="MJXp-Span-34" class="MJXp-mi">ka</span><span id="MJXp-Span-35" class="MJXp-mo">,</span><span id="MJXp-Span-36" class="MJXp-mtext"> </span><span id="MJXp-Span-37" class="MJXp-mn">1</span><span id="MJXp-Span-38" class="MJXp-mi MJXp-italic">σ</span></span></span></span><span> </span>(8.5–11.5 ka, 95%), respectively. Structural, stratigraphic, and geomorphic relations yield vertical displacements for SM1 (<span class="inline-formula no-formula-id"><span class="MathJax_Preview"><span id="MJXp-Span-39" class="MJXp-math"><span id="MJXp-Span-40" class="MJXp-mn">2.0</span><span id="MJXp-Span-41" class="MJXp-mo">±</span><span id="MJXp-Span-42" class="MJXp-mn">0.6</span><span id="MJXp-Span-43" class="MJXp-mtext"> </span><span id="MJXp-Span-44" class="MJXp-mi">m</span><span id="MJXp-Span-45" class="MJXp-mo">,</span><span id="MJXp-Span-46" class="MJXp-mtext"> </span><span id="MJXp-Span-47" class="MJXp-mn">1</span><span id="MJXp-Span-48" class="MJXp-mi MJXp-italic">σ</span></span></span></span>) and SM2 (<span class="inline-formula no-formula-id"><span class="MathJax_Preview"><span id="MJXp-Span-49" class="MJXp-math"><span id="MJXp-Span-50" class="MJXp-mn">2.0</span><span id="MJXp-Span-51" class="MJXp-mo">±</span><span id="MJXp-Span-52" class="MJXp-mn">1.0</span><span id="MJXp-Span-53" class="MJXp-mtext"> </span><span id="MJXp-Span-54" class="MJXp-mi">m</span><span id="MJXp-Span-55" class="MJXp-mo">,</span><span id="MJXp-Span-56" class="MJXp-mtext"> </span><span id="MJXp-Span-57" class="MJXp-mn">1</span><span id="MJXp-Span-58" class="MJXp-mi MJXp-italic">σ</span></span></span></span>). The Steamboat Mountain paleoseismic chronology overlaps temporally with earthquakes interpreted from previous terrestrial and lacustrine paleoseismic data along the fault. Integrating these data, we infer that the youngest Teton fault rupture occurred at<span> </span><span class="inline-formula no-formula-id"><span class="MathJax_Preview"><span id="MJXp-Span-59" class="MJXp-math"><span id="MJXp-Span-60" class="MJXp-mo">∼</span><span id="MJXp-Span-61" class="MJXp-mn">5.3</span><span id="MJXp-Span-62" class="MJXp-mtext"> </span><span id="MJXp-Span-63" class="MJXp-mi">ka</span></span></span></span>, generated<span> </span><span class="inline-formula no-formula-id"><span class="MathJax_Preview"><span id="MJXp-Span-64" class="MJXp-math"><span id="MJXp-Span-65" class="MJXp-mn">1.7</span><span id="MJXp-Span-66" class="MJXp-mo">±</span><span id="MJXp-Span-67" class="MJXp-mn">1.0</span><span id="MJXp-Span-68" class="MJXp-mtext"> </span><span id="MJXp-Span-69" class="MJXp-mi">m</span><span id="MJXp-Span-70" class="MJXp-mo">,</span><span id="MJXp-Span-71" class="MJXp-mtext"> </span><span id="MJXp-Span-72" class="MJXp-mn">1</span><span id="MJXp-Span-73" class="MJXp-mi MJXp-italic">σ</span></span></span></span><span> </span>of vertical displacement along 51–70 km of the fault, and had a moment magnitude (<span class="inline-formula no-formula-id"><span class="MathJax_Preview"><span id="MJXp-Span-74" class="MJXp-math"><span id="MJXp-Span-75" class="MJXp-msub"><span id="MJXp-Span-76" class="MJXp-mi MJXp-italic">M</span><span id="MJXp-Span-77" class="MJXp-mi MJXp-script">w</span></span></span></span></span>) of<span> </span><span class="inline-formula no-formula-id"><span class="MathJax_Preview"><span id="MJXp-Span-78" class="MJXp-math"><span id="MJXp-Span-79" class="MJXp-mo">∼</span><span id="MJXp-Span-80" class="MJXp-mn">7.0</span><span id="MJXp-Span-81" class="MJXp-mo">–</span><span id="MJXp-Span-82" class="MJXp-mn">7.2</span></span></span></span>. This rupture was apparently unimpeded by structural complexities along the Teton fault. The integrated chronology permits a previous full‐length rupture at<span> </span><span class="inline-formula no-formula-id"><span class="MathJax_Preview"><span id="MJXp-Span-83" class="MJXp-math"><span id="MJXp-Span-84" class="MJXp-mo">∼</span><span id="MJXp-Span-85" class="MJXp-mn">10</span><span id="MJXp-Span-86" class="MJXp-mtext"> </span><span id="MJXp-Span-87" class="MJXp-mi">ka</span></span></span></span><span> </span>and possible partial ruptures of the fault at<span> </span><span class="inline-formula no-formula-id"><span class="MathJax_Preview"><span id="MJXp-Span-88" class="MJXp-math"><span id="MJXp-Span-89" class="MJXp-mo">∼</span><span id="MJXp-Span-90" class="MJXp-mn">8</span><span id="MJXp-Span-91" class="MJXp-mo">–</span><span id="MJXp-Span-92" class="MJXp-mn">9</span><span id="MJXp-Span-93" class="MJXp-mtext"> </span><span id="MJXp-Span-94" class="MJXp-mi">ka</span></span></span></span>. To reconcile conflicting terrestrial and lacustrine paleoseismic data, we propose a hypothesis of alternating full‐ and partial‐length ruptures of the Teton fault, including<span> </span><span class="inline-formula no-formula-id"><span class="MathJax_Preview"><span id="MJXp-Span-95" class="MJXp-math"><span id="MJXp-Span-96" class="MJXp-msub"><span id="MJXp-Span-97" class="MJXp-mi MJXp-italic">M</span><span id="MJXp-Span-98" class="MJXp-mi MJXp-script">w</span></span><span id="MJXp-Span-99" class="MJXp-mo">∼</span><span id="MJXp-Span-100" class="MJXp-mn">6.5</span><span id="MJXp-Span-101" class="MJXp-mo">–</span><span id="MJXp-Span-102" class="MJXp-mn">7.2</span></span></span></span><span> </span>earthquakes every<span> </span><span class="inline-formula no-formula-id"><span class="MathJax_Preview"><span id="MJXp-Span-103" class="MJXp-math"><span id="MJXp-Span-104" class="MJXp-mo">∼</span><span id="MJXp-Span-105" class="MJXp-mn">1.2</span><span id="MJXp-Span-106" class="MJXp-mtext"> </span><span id="MJXp-Span-107" class="MJXp-mi">ky</span></span></span></span>. Additional paleoseismic data for the northern and central sections of the fault would serve to test this bimodal rupture hypothesis.</p></div>
application/pdf
10.1785/0120200212
en
Seismological Society of America
Holocene paleoseismology of the Steamboat Mountain Site: Evidence for full‐Llngth rupture of the Teton Fault, Wyoming
article