The City of Mercer Island and GeoMapNW recently completed geologic map of Mercer Island.

Mercer Island, an 8-km-long island, sits in the middle of Lake Washington and is accessed by I-90.  Modeling of ground shaking during a Magnitude 6.7 Seattle fault earthquake predicts that the island would be isolated for days.  Water to the City's 23,000 residents is supplied by a pipe suspended under I-90, which is susceptible to failure during such shaking.  As part of the City's mitigation efforts, the utility department needed a geologic map to begin assessing its vulnerability.

(right-click as "save as" if you have difficulty viewing online, these are very large files)

The geologic map is based on several data sources:

1. a subsurface database consisting of: 2963 Explorations:

2. new field mapping, and
3. geomorphic analysis

 The data in the subsurface database is accessible from our homepage using our "Online Geodatabase" and zooming in to Mercer Island.

2963 Explorations:

• 1277 borings

• 1362 test pits 

• 144 other 

• 180 outcrops

From: 

• 742  Geotech docs

• 91  different authors

(right-click as "save as" if you have difficulty viewing online, these are very large files)

The new geologic map of Mercer Island is substantially different than the old map (from 1962) because of new technology, abundance of data, and more time for map making than was available for the original map.  A comparison of the two maps shows the differences. Seventy-six percent of the land area was geologically reclassified.  The following are differences from the 1962 map:

• Seattle fault zone added, although no scarps were found
• 16% more areas of weak ground
• 2 times more sandy ground
• More paleolake beds identified
• Fill mapped as overprints
• Colluvium and landslide debris mapped as overprint (>20% of the map area)
• Landslide scarps added
• More geologic units mapped (31 vs. 7)
• More detailed geology (204 vs. 22 polygons)

With a digital geologic map now complete, derivative maps can be made that highlight important features such as strength of geologic materials and permeability.  In order to highlight these features, and the need for new geologic mapping, we show comparisons between the 1962 map and the new map of Mercer Island for each of the derivative maps.

(right-click as "save as" if you have difficulty viewing online, these are very large files)

The map of degree of consolidation shows where the weak vs. strong ground is located. Since this map shows the near surface geologic material, one still needs to evaluate the thickness of weak material and site-specific conditions to assess impact.  Thickness information can be derived by using the subsurface database.  Assessments of potential property impacts can be made by licensed geologists and engineers.  This type of map is used for evaluating behavior of geologic materials during earthquake shaking (this is one of the important input parameters), for planning level facility siting studies, and for evaluating foundation conditions.  For example, the City of Mercer Island could use this map with an overlay of major water lines to look for potentially vulnerable locations in the event of earthquake shaking.

(right-click as "save as" if you have difficulty viewing online, these are very large files)

(right-click as "save as" if you have difficulty viewing online, these are very large files)