Natural Hazards

HAZUS Program in Vermont - Multihazard Risk Assessment

The Vermont Geological Survey(VGS), in cooperation with Vermont Emergency Management runs the HAZUS program. Since 1997, the VGS has run HAZUS-MH a software program developed by the Federal Emergency Management Agency (FEMA) that estimates potential damage from postulated earthquakes, floods, and hurricanes. HAZUS calculates damage based on default data from the census that is embedded in the program; all calculations are aggregated by census tract. In the early years, HAZUS was exclusively an earthquake module and VGS used it to run numerous scenarios throughout Vermont for specific 500 year earthquake events that were proposed by Ebel et al. (1995) of the Weston Geophysical Observatory. VGS provided HAZUS output to predict potential losses in Chittenden, Lamoille, Franklin, Caledonia, and Orange Counties. As part of Project Impact, VGS worked with the Lamoille County, Two Rivers-Ottauquechee, and the Franklin-Grand Isle Regional Planning Commissions to provide HAZUS data.

Figure 1. Click on image for a larger pdf.	Figure 2. Click on image for a larger pdf.
The two figures above are maps of scenarios used to predict earthquake damage in Addison County. The figures show the Peak Ground Acceleration (PGA) expressed as a fraction of the acceleration of gravity (Fig. 1) and the predicted economic loss for buildings in Addison County from a once-in-500 year 5.7 magnitude event (Fig. 2). The Addison County Hazard Mitigation Plan can be adopted by towns as preparation for seismic events. Lifelines and building stock can be upgraded to handle the potential ground accelerations. Towns may then receive disaster funding if an earthquake causes damage, but hopefully dollar losses will be minimized due to mitigation planning.

The default census data within the HAZUS program can be modified with site specific building information to make the damage assessments more accurate. In addition, site specific geologic information can be added to HAZUS. On the default setting, HAZUS assumes that all soils in an area are Class D “stiff” soils that do not amplify earthquake shaking. Because recent surficial geologic mapping in the Burlington Quadrangle by Wright et al. (2009) identified varved clays that may be considered Class E “soft” soils, we added this data to HAZUS. We ran HAZUS to determine the effect that a 500 year, magnitude 6.8 earthquake with an epicenter in Montreal, Quebec would have on Chittenden County with and without Class E soils. The following differences were observed:

A comparison of results using default data, site specific data and data from new surficial geologic maps. Click on images below for a larger pdf.
	The default HAZUS run assumed that Chittenden County was only underlain by Class D "stiff" soils whereas as the second run added the isolated Class E "soft" soil polygons (Figure 1). Red polygons are varved clays from surficial mapping by Wright et. al., 2009; green is the default soils data.
	When all of Chittenden County is assumed to be the default Class D soils, the HAZUS run shows the highest peak ground accelerations (pga) are found in census tracts in the northwestern part of the county which is closest to Montreal (Figure 2). PGA values that exceed 10% of gravitational acceleration (0.10 g) could cause damage in older unreinforced buildings.
	The substitution of Class E soil polygons into the HAZUS run causes the highest pga to be located in only a single census tract in the City of Burlington where ground shaking was amplified (Figure 3). This is because most of this census tract was underlain by Class E soils (varved clay from Wright et. al., 2009). The next highest pga is in the northwestern part of the county similar to the default scenario. It is apparent that other “soft” soil polygons located in larger census tracts (Figure 1) did not amplify ground shaking. Since HAZUS aggregates all pga data by census tract, the “soft” soil polygons were probably not large enough to influence pga values in the larger census tracts.

VGS also uses HAZUS as a educational tool and to support engineering studies. Two examples are cited below.

On the morning of April 20, 2002 a 5.1 magnitude earthquake occurred south of Plattsburgh, New York. Emergency Management contacted the State Geologist within 10 minutes of the event. The State Geologist then contacted Weston Observatory at Boston College to get the size and location of the event. Weston Observatory, in cooperation with VGS, manages a SDAS (Seismic Data Acquisition Station) in Waterbury. The earthquake size and location information was relayed back to VGS at 8:45 am. The VGS worked quickly to get HAZUS information into an Emergency Management press release. As a result of the press release, both WCAX Channel 3 and WVNY, Channel 22 visited VGS for interviews on the science of seismic events and the need to prepare for future events that could cause damage in Vermont.

HAZUS modeling of the 5.1 Richter magnitude earthquake predicted ground shaking slightly below the damage threshold, consistent with early reports from the actual event. A simulation of a 5.5 magnitude event at the same location predicted potentially damaging ground shaking in northwestern Vermont. In general, northwestern Vermont has a 10% chance of experiencing damaging ground shaking in a 50 year period. Click here for "A Report on the Seismic Vulnerability of the State of Vermont" by Ebel, Bedell, and Urzua.

HAZUS was used to investigate the relationship between the April 20, 2002 earthquake and slope stability. A consulting firm performing a slope stability analysis for a site in Colchester requested technical data on the level of shaking that occurred at the site during the earthquake. HAZUS was used to predict the horizontal shaking as a percent of the acceleration of gravity and to calculate vertical shaking. This information was then used in the slope stability analysis.