{"id":1957,"date":"2026-07-23T13:09:49","date_gmt":"2026-07-23T13:09:49","guid":{"rendered":"https:\/\/bkbc.net\/index.php\/2026\/07\/23\/small-faults-add-to-seattles-quake-story\/"},"modified":"2026-07-23T13:09:49","modified_gmt":"2026-07-23T13:09:49","slug":"small-faults-add-to-seattles-quake-story","status":"publish","type":"post","link":"https:\/\/bkbc.net\/index.php\/2026\/07\/23\/small-faults-add-to-seattles-quake-story\/","title":{"rendered":"Small Faults Add to Seattle\u2019s Quake Story"},"content":{"rendered":"
\"An<\/figure>\n

The winter earthquake of 923 or early 924 CE<\/a> remains the benchmark event in the Seattle Fault Zone. It lifted shorelines around Puget Sound and gave geologists one of the most decisive records of the fault\u2019s power.<\/p>\n

But that shoreline-lifting earthquake is only part of the zone\u2019s history. A recent Geological Society of America Bulletin <\/em>study<\/a> led by Stephen J. Angster<\/a>, a geologist with the U.S. Geological Survey, looks past the most famous Seattle fault earthquake to examine evidence of earthquakes on lesser-known secondary faults.<\/p>\n

At Lytle Beach on Bainbridge Island and at Vasa Park near Bellevue, subtle landforms and trench records suggest evidence of smaller ruptures has been preserved in the landscape but not fully recognized.<\/p>\n

At Lytle Beach, the first clue was a small, raised surface. Angster described the \u201clocalized uplifted terrace\u201d as \u201cthe first feature we saw that drew our eye to that area.\u201d Similar features had already helped geologists read other secondary faults in the Seattle Fault Zone.<\/p>\n

For instance, Angster said the Toe Jam Hill fault<\/a> became one of the better-known examples when lidar helped reveal its scarp<\/a> through dense vegetation. Later work found smaller uplifted terraces near similar secondary structures, which Angster said may indicate separate earthquake events focused on smaller faults rather than the larger regional rupture.<\/p>\n

However, Lytle Beach stood out in a different way, as its fault dips south, in contrast to the zone\u2019s better-known secondary faults, which dip north.<\/p>\n

<\/a>Faults Hidden in the Fold<\/strong><\/h3>\n

Finding the localized uplifted terrace was only the beginning for Angster, who used lidar to uncover scarps and lineaments through the region\u2019s forest cover.<\/p>\n

His team also utilized ground-based magnetic transects across the Lytle Beach fault to look for changes that may reveal displacement beneath the surface. In addition, they gained a more direct view of disturbed sediments by excavating the Rose Hip trench across the newly identified Lytle Beach scarp and analyzing evidence from the earlier Vasa Park trench.<\/p>\n

In the Rose Hip trench<\/a> at Lytle Beach, Angster said the team found glacial deposits dating to roughly 15,000 years ago. Above them were lake sediments left behind as ice retreated. The trench also preserved an old layer of soil that formed after the lake dried. \u201cThat whole package was folded,\u201d he said. \u201cThe only way you could fold those is mostly by a tectonic fault.\u201d<\/p>\n

\"A
A regional map of the Seattle Fault Zone shows the Lytle Beach and Vasa Park fault scarps, along with the uplifted shore platform associated with the 923 CE earthquake. A new study used mapping, geophysics, trenching, and dating methods to investigate secondary faults within the broader fault zone. Credit: Angster et al., 2026, https:\/\/doi.org\/10.1130\/B38333.1<\/a>, CC-BY-4.0<\/a><\/figcaption><\/figure>\n

The trench record showed evidence of two surface-rupturing earthquakes on the Lytle Beach fault. The older event occurred between 11,240 and 10,430 calibrated years before present, whereas the younger event occurred after 1663 CE, likely in the early nineteenth century. (\u201cCalibrated years before present\u201d refers to<\/a> dates arrived at via radiocarbon dating, relative to the year 1950 as the \u201cpresent.\u201d)<\/p>\n

\u201cI thought the trenching on Lytle Beach was surprising, that we found two events, because it was such a relatively subtle feature that wasn\u2019t really identified before,\u201d Angster said.<\/p>\n

At Vasa Park, the team found evidence of one past earthquake that occurred sometime between 11,380 and 7,400 calibrated years before present. That range overlaps with the older Lytle Beach event and raises the possibility of a longer rupture along the Blakely Harbor fault. However, the evidence in Angster\u2019s study better supports separate ruptures on the two secondary faults.<\/p>\n

<\/a>A Longer Record of Smaller Ruptures<\/strong><\/h3>\n

Harold Tobin<\/a>, an earthquake scientist at the University of Washington who was not involved in the study, called the work \u201cexciting new research.\u201d<\/p>\n

He said the study shows there is \u201croom to accommodate smaller earthquakes\u201d that do not reshape shorelines like the 923 or 924 event but are \u201cstill big enough to be damaging earthquakes.\u201d The Angster paper, he said, examines \u201cadditional earthquakes not accounted for in the shoreline uplift record centered on the 923 or 924 event. These smaller earthquakes may have happened more recently or more often.\u201d<\/p>\n

\n
\n

\u201cSubtle features, relatively small or fault traces that we might have ignored before, can be an important part of the story.\u201d<\/p>\n<\/blockquote>\n<\/figure>\n

For Tobin, the value also extends beyond Puget Sound. The study \u201cshines a light for other people working in cities and urbanized settings,\u201d he said, because it shows that \u201csubtle features, relatively small or fault traces that we might have ignored before, can be an important part of the story.\u201d<\/p>\n

By comparing the dated events at Lytle Beach, Vasa Park, and other secondary faults, the authors estimated that these faults may have ruptured roughly every few hundred years during the late Holocene. Angster cautioned that it\u2019s not quite clockwork. \u201cThe secondary faults appear, especially within the last 2,500 years, to rupture more frequently, and that\u2019s where that 350-year interval comes from,\u201d he said. But the estimate rests on a limited record.<\/p>\n

Though the work doesn\u2019t forecast the next earthquake, it gives scientists more of the past to weigh as they assess the Puget Lowland. Tobin said the paper \u201ccertainly beg[s] more research\u201d because the Seattle Fault Zone contains many strands that still need to be studied. Earthquake hazards should remain \u201csomething real\u201d for the public and civil planners.<\/p>\n

\u201cWe don\u2019t know when they are going to come,\u201d Tobin said. \u201cObviously, we can go decades without any significant earthquakes, as we have since 2001. But when we least expect it, one will happen, and we just have to be prepared.\u201d<\/p>\n

Angster framed the findings more conservatively: \u201cThis study doesn\u2019t really change the hazard with the Seattle Fault. It just provides more insight into how it behaves.\u201d<\/p>\n

\u2014Jason Collins, Science Writer<\/p>\n

Citation:<\/strong>\u00a0Collins, J. (2026), Small faults add to Seattle\u2019s quake story,\u00a0Eos, 107, <\/em>https:\/\/doi.org\/10.1029\/2026EO260238<\/a>. Published on 23 July 2026.<\/h5>\n
Text \u00a9 2026. The authors.\u00a0CC BY-NC-ND 3.0<\/a>
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.<\/h6>\n

Source: Eos<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

The winter earthquake of 923 or early 924 CE remains the benchmark event in the Seattle Fault Zone. It lifted shorelines around Puget Sound and […]<\/p>\n","protected":false},"author":11,"featured_media":1958,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[324],"class_list":["post-1957","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-science","tag-eos"],"_links":{"self":[{"href":"https:\/\/bkbc.net\/index.php\/wp-json\/wp\/v2\/posts\/1957","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/bkbc.net\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/bkbc.net\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/bkbc.net\/index.php\/wp-json\/wp\/v2\/users\/11"}],"replies":[{"embeddable":true,"href":"https:\/\/bkbc.net\/index.php\/wp-json\/wp\/v2\/comments?post=1957"}],"version-history":[{"count":0,"href":"https:\/\/bkbc.net\/index.php\/wp-json\/wp\/v2\/posts\/1957\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/bkbc.net\/index.php\/wp-json\/wp\/v2\/media\/1958"}],"wp:attachment":[{"href":"https:\/\/bkbc.net\/index.php\/wp-json\/wp\/v2\/media?parent=1957"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/bkbc.net\/index.php\/wp-json\/wp\/v2\/categories?post=1957"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/bkbc.net\/index.php\/wp-json\/wp\/v2\/tags?post=1957"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}