22 December 2016
Researchers in Maine (Marvinney and Glover 2015) have created a clever earthquake risk shake map using readily available surficial geology maps and online responses from state residents. Did you feel it? That is the name of a USGS Earthquake Hazards Program interactive website that “collects information from people who felt an earthquake and creates maps that show what people experienced and the extent of damage.”
I first became interested in the Presumpscot Formation many years ago when, nearing the end of my undergraduate geology program at Grand Valley State, I needed to choose a field camp, as my school didn’t offer one. Most of my classmates chose field camps in the western U.S.: Colorado, Wyoming, Idaho, Montana, or Alaska. One lucky guy went to the British Isles.
I chose to study the rocky shores of Maine with the University of Southern Maine field camp, led by geological “one-man-band” Dr. Stephen G. Pollock. I was drawn to the program to be close to the sea, which was no trivial matter for this midwesterner who’d spent all his life to that point in “flyover country.”
The Presumpscot Formation
The Quaternary geology portion of the field camp took place inland, away from the rocky coast, where we could consider processes and materials in the associated glacio-fluvio-marine deposits. One of the most wide-spread Quaternary deposits found in the lower elevations of eastern Maine is the Presumpscot Formation, sometimes called Maine’s “blue clay.”
The Presumpscot has an interesting genesis. Glaciation of New England depressed the crust so much that, upon glacial retreat, the rising sea invaded much of eastern Maine. In time, crustal rebound lifted the region enough to expose the marine sediments. The formation is fossiliferous and contains varve-like alternating silty-clayey layers likely caused at least in part by a combination of seasonal flow variations and a high tidal range
With its clayey texture, the Presumpscot Formation is the parent material for some of the more fertile agricultural soils in New England. It also coincides with more densely populated areas.
Shaking and Site Characteristics
During an earthquake, severity of ground shaking is not solely dependent on distance from the epicenter. Various geologic materials behave differently to shear wave propagation and shaking due to variable density, water content, and other factors. From Marvinney and Glover. 2015. (link added):
Due to its fine-grained character, shear waves from seismic events travel slowly through the Presumpscot Formation in comparison to other surficial sedimentary units. Low shear-wave velocities result in amplification of seismic waves, potentially increasing local seismic hazard. Prior study (Becker and others, 2012) demonstrated that careful assignment of National Earthquake Hazards Reduction Program (NEHRP) site classifications based on shear velocities to surficial geologic units, can greatly improve earthquake loss estimations using programs like HAZUS-MH…
The following slightly modified list is from Marvinney and Glover. 2015. “Table 1. NEHRP Site Classification Categories.”
Site Classification Category, Description, (shear wave velocity m/s)
Type A, Hard Rock, (>1500 m/s)
Type B, Firm to hard rock, (760-1500 m/s)
Type C, Dense soil, soft rock, (360-760 m/s)
Type D, Stiff soil, (180-360 m/s)
Type E, Soft clays, (<180 m/s) (Presumpscot Formation qualifies)
The October 16, 2012 Earthquake in Hollis, Maine: Did You Feel It?”
The USGS has a national database, Did you feel it?, that collects online data from respondents who experience earthquakes. It’s very easy to use as I logged in with my own observations following a May 2, 2015 earthquake in Galesburg, MI. Marvinney and Glover collected responses to the M 4.0 Hollis, ME earthquake, including those described in their study:
During the October earthquake, responders to the USGS website felt slight vibrations to rolling motions. Many respondents thought “that something had hit the house” or “that the furnace had exploded.” Many also heard a deep rumbling sound. Movement was reported to have lasted for 5-30 seconds. Reported damage was minor and included broken windows, cracked plaster, and items falling off of shelves.
Marvinney and Glover then combined normalized responses from the database with a regional surficial geology map to create a shake map in GIS. Again, from their paper:
When normalized by population density within each surficial unit, we found no statistical difference in respondents’ experiences. However, when normalized for areal extent of each unit, we found that more people responded in areas underlain by NEHRP class ‘E’ materials (including the Presumpscot Formation) than for other classes. Our results suggest potentially greater intensity of ground shaking and seismic hazard in areas underlain with sediment of the Presumpscot Formation.
I highly recommend this well-written paper which, for me at least, raises questions and concerns:
1. The authors mention how poorly earthquakes in the eastern United States correlate to mapped faults, which are essentially inactive. The risk is real and hard to predict, as evidenced by the M 5.8 Mineral, Virginia earthquake of 2011. We need more, not less research in Appalachian seismicity.
2. How good is “crowd sourced” data? For example, could urban vs. rural demographics contribute to a reporting bias? Are some folks more likely to get to the USGS website than others?
3. How objective are the reports made to Did You Feel It? Might there be a tendency to under or over-react due to individual differences?
4. How good is the mapping and characterization of the Presumpscot Formation? I assume it’s good but probably could be improved.
5. The other Type E sites reporting more shaking were beaches and fill material, which may include higher risk coastal properties and urban areas.
Years ago, during a college field trip to the Appalachians, at a campground near Hanging Dog, NC, our professor, Dr. Richard Le Febfre told us about the early days of Geology in North America. Because the western part of the continent offered spectacular geology, the most excited geologists rushed out there. The geology of the Appalachian range wasn’t overlooked, but it didn’t get the attention it deserves.
Here’s to those who brave the heat, cold, rain, snow, ticks, poison ivy, Lyme’s disease, site access problems, traffic and all the rest of it to learn more about the geology of this interesting place.
Robert G. Marvinney and Hannah Glover. 2015. The influence of the Presumpscot Formation on seismic hazard in southern coastal Maine. 2015 Symposium on the Presumpscot Formation, Portland, ME.
Thompson, W.B., and Borns, H.W., Jr. (editors), 1985, Surficial geologic map of Maine: Maine Geological Survey, 1:500,000 scale.