<HTML> <HEAD> <SCRIPT><!-- function fix(e) { var par = e.parentNode; e.id = ""; e.style.marginLeft = "0.42in"; var pos = e.innerText.indexOf("\n"); if (pos > 0) { while (pos > 0) { var t = e.childNodes(0); var n = document.createElement("PRE"); var s = t.splitText(pos); e.insertAdjacentElement("afterEnd", n); n.appendChild(s); n.style.marginLeft = "0.42in"; e = n; pos = e.innerText.indexOf("\n"); } var count = (par.children.length); for (var i = 0; i < count; i++) { e = par.children(i); if (e.tagName == "PRE") { pos = e.innerText.indexOf(">"); if (pos != 0) { n = document.createElement("DD"); e.insertAdjacentElement("afterEnd", n); n.innerText = e.innerText; e.removeNode(true); } } } if (par.children.tags("PRE").length > 0) { count = (par.children.length); for (i = 0; i < count; i++) { e = par.children(i); if (e.tagName == "PRE") { e.id = ""; if (i < (count-1)) { var e2 = par.children(i + 1); if (e2.tagName == "PRE") { e.insertAdjacentText("beforeEnd", e2.innerText+"\n"); e2.removeNode(true); count = count-1; i = i-1; } } } } } } else { n = document.createElement("DD"); par.appendChild(n); n.innerText = e.innerText; e.removeNode(true); } } --></SCRIPT> </HEAD> <BODY oncontextmenu="return true"> <p><img src="SeattleMapsPublic_files/image001.jpg" width="800" height="126"> </p> <table width="780" border="0" cellspacing="5" cellpadding="5" summary="layout for page print"> <tr> <td colspan="3"><p>The USGS has produced a new series of earthquake hazard maps for the City of Seattle. These 'urban seismic hazard' maps provide a much higher-resolution view of the potential for strong earthquake shaking than previously available. This new view is particularly important for Seattle, which sits atop a sedimentary basin that strongly affects the patterns of earthquake ground shaking and therefore, of potential damage. These new hazard maps incorporate shaking effects not captured in the National Seismic Hazard Maps, which include: </p> <ul> <li><em> The subsurface geologic structure</em> of the Seattle basin and its environs can amplify and lengthen the duration of strong shaking in some places. The seismic waves that shake the ground may be focused and diffused by the shape of and materials within subsurface geologic structures. </li> <li><em> Surficial and shallow deposits</em> of artificial fill and young alluvium (river deposits) may strongly amplify earthquake waves. </li> <li><em> The earthquake rupture process </em> can also cause higher ground shaking in certain directions from a fault. A large earthquake grows like a propagating crack, radiating seismic waves along the way. This can lead to a pile-up of wave energy in front of the fault and spread it out behind. </li> </ul> <p> The new Seattle Urban Seismic Hazard Maps include all of these effects. They are based on 540 computer simulations of earthquakes in a three-dimensional model of the Earth's crust. </p></td> </tr> <tr> <td width="419">The maps are 'probabilistic'-- that is, they portray the ground shaking with a certain probability of occurring or being exceeded. The map on the right depicts the ground shaking in Seattle with a 10% chance of being exceeded during a 50-year period (motions shown have an oscillation period of 1 second). These maps include the expected shaking from earthquakes that could impact Seattle: large earthquakes on the Seattle and other shallow faults, great earthquakes on the Cascadia subduction zone, and deep ones like the 2001 Nisqually earthquake. The geologic record tells us that these earthquakes occur repeatedly, but with differing intervals between them. The maps account for these different recurrence rates. Other maps of the suite show the shaking expected with a 5% and 2% probability of being exceeded in 50 years. </td> <td colspan="2"><img src="SeattleMapsPublic_files/image003.png" width="387" height="611"></td> </tr> <tr> <td colspan="3"><p>The highest hazard within the Seattle basin is found in areas of artificial fill and young alluvium (soils and sands), including Harbor Island, Pioneer Square, and in portions of the Interbay, Fremont and Montlake-University Village neighborhoods. Other areas above the basin on firmer soils, such as downtown Seattle, show elevated hazard compared to similar sites outside of the basin. Outside the Seattle basin very high hazard also is predicted in the alluvial Duwamish valley. </p> <p> Studies attempting to verify independently the variations in predicted shaking levels are ongoing. For example, scientists have shown that during the Nisqually earthquake focusing of damaging waves at the southern edge of the Seattle basin likely caused the enhanced damage to chimneys in West Seattle, and ground motions recorded throughout the city show the largest motions near Harbor Island and Pioneer Square. </p></td> </tr> <tr> <td colspan="2"><img src="SeattleMapsPublic_files/image005.jpg" width="420" height="545"></td> <td width="384"><p>The simulations done for the Seattle maps are based on a three-dimensional model of the crust in the region constructed from geophysical and geological data. Information on the depth of artificial fill and alluvium complied by the University of Washington's GeoMap Northwest project was critical to making the hazard maps.</p> <p>Seismometers deployed throughout Seattle by the USGS and the University of Washington provide key recordings of earthquakes that we use to verify the simulations. On the left, we show the observed amplification of seismic waves produced by the 2001 M6.8 Nisqually earthquake measured at seismic stations. The shaking was stronger at sites on artificial fill and alluvium, as indicated by the larger circles. These areas also had more building damage from the earthquake. Soil sites in the Seattle basin were also observed to have higher levels of shaking than sites with shallow bedrock south of the Seattle basin. Our simulations also predict strong shaking in these places. </p></td> </tr> <tr> <td colspan="3">The production of the Seattle urban seismic hazard maps represents a vast improvement in our understanding of earthquake hazards. Nonetheless, they are not a substitute for hazard assessments for locations where detailed soil profiles with depth have been determined. The Seattle maps may be downloaded from the website <a href="http://earthquake.usgs.gov/regional/pacnw/hazmap/seattle/index.php"> http://earthquake.usgs.gov/regional/pacnw/hazmap/seattle/index.php</a>. For more information see <a href="http://earthquake.usgs.gov/"> http://earthquake.usgs.gov</a> or contact Craig Weaver or Joan Gomberg at 206-553-0627, 206-616-5581, craig@ess.washington.edu, gomberg@usgs.gov. The Seattle maps are the work of A. Frankel, W. Stephenson, D. Carver, R. Williams, J. Odum, and S. Rhea. </td> </tr> </table> <p>&nbsp;</p> <p>&nbsp;</p> </BODY> </HTML>