Reports

Earthquake Hazard Maps for Maryland

1999, Reger, J.P.

Open File Report 99-03-01

Executive Summary

In 1998, the Maryland Emergency Management Agency (MEMA) initiated the Maryland Earthquake Program to determine how vulnerable the population of Maryland is to earthquakes. The first step in that Program is to identify Maryland's seismic hazard through development of "ground shaking" maps.

The maps prepared by the Maryland Geological Survey in this project were derived from the U.S. Geological Survey's National Seismic Hazard Mapping Project database of ground-shaking parameters for the United States. The national database contains 12 values for each of approximately 150,000 sites in the conterminous United States: peak ground acceleration and spectral accelerations having periods of 0.2 second (short period), 0.3 second, and 1.0 second (long period) for 10, 5, and 2 percent probabilities of exceedance (PE) in 50 years. The acceleration parameters are measured in units of percent g, the acceleration due to gravity. The 1997 National Earthquake Hazard Reduction Program (NEHRP) Recommended Provisions define the maximum considered earthquake ground motion (MCE) as an earthquake having a 2 percent PE in 50 years, which is equivalent to an annual probability of exceedance of 0.0004, or 1 in 2,500. The 1997 NEHRP Provisions utilize the short- and long-period accelerations for the MCE as the basis for determining its design recommendations.

The Maryland ground-shaking, or "seismic hazard," maps were developed from 152 data points taken from the national database. Four maps depicting probabilistic ground motions for the maximum considered earthquake were prepared by plotting and contouring the data by hand at a scale of 1:500,000 (1 inch equals approximately 8 miles). Those maps were then digitized. The 1:500,000 scale allows for much more detail than the 1:7,000,000-scale seismic hazard maps produced by the U.S. Geological Survey. The digital files for the four digitized maps and the Maryland database are available separately from the Maryland Geological Survey.

Perhaps the most significant finding about Maryland's seismic hazard is the categorization of all but the northeastern corner of the state as a "region of negligible seismicity with very low probability of collapse of the structure," which is defined by the 1997 NEHRP Provisions as "areas having MCE ground motions equal to or less than 0.25g (short period) and 0.10g (long period). For only 7 of 105 grid sites in Maryland is the short period 0.2 second spectral acceleration at 2 percent PE greater than 0.25g, with the maximum being 0.28g, and all 105 sites have a long period 1.0 second spectral acceleration less than 0.08g. The seismic hazard in these areas is generally the result of earthquakes with Mb (body wave magnitude) 5.5. According to the 1997 NEHRP Provisions, in such a region it is not considered necessary to specify seismic-resistant design on the basis of the MCE ground motion. The ground motion computed for such areas is determined more by the rarity of the event with respect to the chosen level of probability than by the level of motion that would occur if a small but close earthquake actually did occur.

Maryland's earthquake hazard, as expressed in terms of the probability of specified levels of ground shaking, is still low for the vast majority of the state, and requires no special seismic design considerations of structures. Only northeastern Maryland (mainly parts of Harford and Cecil Counties) exceeds the thresholds specified by the NEHRP Provisions by which certain critical and/or high-occupancy structures may warrant seismic design considerations in their construction. Even for those areas, site specific investigation may show that not to be necessary on a case by case basis.