ANALYSIS OF RESIDENTIAL DAMAGE CAUSED BY THE 23 AUGUST 2011 LOUISA COUNTY, VIRGINIA, EARTHQUAKE

The magnitude 5.8 earthquake that struck central Virginia on 23 August 2011 caused significant residential damage to parts of Louisa County, in the form of damaged brickwork, sheetrock, and foundations. In the days following the earthquake, Louisa County officials compiled a Microsoft Excel™ spreadsheet of residential damage (i.e., name, address, phone, assessed value of damage, and property owner damage description), in order to qualify for federal disaster relief. The county was approved for federal disaster assistance last November.

A copy of the county spreadsheet was obtained, imported into a Microsoft Access™ database, and subsequently modified to include additional parameters (e.g., property type, geographic coordinates, pre-earthquake assessed property value, square footage, etc.) derived from several sources, including the Louisa County Real Estate Assessment Database and Google Earth™. The database contains accurate and complete data on 858 dwellings (5.3% of the housing units in Louisa County).

Residential damage was analyzed using both Microsoft Access and ArcView™ GIS. Within Access, data was queried for possible damage correlations (e.g., factors such as dwelling age and condition may have affected the amount of damage dwellings sustained in the earthquake) and displayed as tables, PivotCharts, or PivotTables. ArcView was used to produce contoured map images of the data to look for possible damage correlations and develop a better understanding of the data distribution.

The damage assessment total for the included residential dwellings came to $13.1 million dollars. The percent of damage (i.e., assessed damage amount ($) / pre-earthquake improved property value ($) X 100) each dwelling sustained was used to analyze the distribution of damage in the county. Areas to the SW and NE of the epicenter sustained the greatest percent of damage. If the data is limited to dwellings constructed after 1973 (when building codes became more uniform), half of the dwellings that sustained more than the countywide average of 12.9% damage are located on the Chopawamsic Formation, with the worst of the damage occurring in a zone just east of Quail that trends ~N7^oW, is 5 km long, and has a maximum width of 1.6 km. Additional factors that may have affected the amount of dwelling damage will be explored.