A Report on the Seismic Vulnerability of the State of Vermont by John E. Ebel, Richard Bedell* and Aifredo Urzua# Weston Observatory Department of Geology and Geophysics Boston College Weston, MA 02193 * Now at Homestake Mining Company 1375 Greg Street Suite #105 RenO, NV \89509 # Also at Prototype Engineering 57 Westland Ave Winchester, MA 01890 Submitted to the Vermont Emergency Management Agency, July, 1995. a. I Contents Executive Summary i 1. Purpose of this Report .................. ............................................................................... 1 2. The Seismic History of Vermont .......................................................................... 3 3. Maximum Historical Earthquake Effects in Vermont ................................. 1 2 4: Seismic Hazard Models for Vermont .............................................. ................... 17 4-1 Deterministic Estimates of the Seismic Hazard in Vermont............. 18 4-2 Probabilistic Estimates of the Seismic Hazard in Vermont .................. 27 .5. Soil Effects on Strong Ground Motions ................... .......................................... 38 5-1 Ground Shaking Amplification Potential in Chittenden County, Vermont............... .................................................................................................... ...... 3 9 5-2 Estimation of the Amount of Groundshaking Amplification for Typical Soils in Chittenden County, Vermont ............. ..................................... 42 6. Examples of Site Specific Seismic Hazard in Vermont: Application to the Vermont Medical Center and to the IBM sites in Burlington, Vermont ............. ............................................................................................................. 45 7. Seismic Considerations in Building Construction Practice and Building Codes in Vermont ................................. ................................................... 49 8: Public Policy Recommendations Concerning the Earthquake Hazard in Vermont.................... ..................................... .............. ........................ ............................... 5 3 9 . References ..................................................................................................................... 5 9 Appendix A. Definitions of Technical Terms.................................................................... ................. .......................................... 64 Appendix B. The Modified Mercalli Intensity Scale .......................................... 6 9 Appendix C. Earthquake Catalogs and the Seismicity Map of Vermont.. 7 0 Appendix D. Isoseismal Maps of Those Earthquakes Which Have Affected Vermont with the Strongest Ground Shaking ............................. 7 5 Appendix E. Selection of the Once-in-500-Year Earthquakes for the Deterministic Seismic Hazard Analysis ....................... ...................................... 8 5 Appendix F. The Attenuation of Modified Mercalli Intensities With Distance From an Earthquake Epicenter in New England ................. ......... 8 6 Appendix G. Inputs for the Probabilistic Seismic Hazard Analysis.......... 87 Appendix H. Method of Estimation of Possible Ground Shaking Amplification in Chittenden County, Vermont.............................................. 92 Appendix 1. Analysis of the Modification of Seismic Ground Shaking due to Surficial Soils in Vermont ........................................................................ '9 3 Appendix J. Analysis of the Soil Effects on Strong Earthquake Ground • Motions at the Vermont Medic4l Center and the IBM Site at Essex Junction........................................................................................................................... 95 ¶ Executive Summary This report presents a summary of the seismic hazard of the State of Vermont and recommendations of those actions which should be taken to mitigate the effects in Vermont of future earthquakes. This study was requested by the Vermont Emergency Management Agency (VEMA) as part of their participation in the National Earthquake Hazards Reduction Program (NEHRP), and it addresses the question Of what earthquake effects could. potentially take place within Vermont The results of this analysis serve' as the basis for earthquake emergency planning efforts within Vermont, for decisions concerning the adoption of earthquake design requirements for buildings and other structures within, the state, for education of the population about earthquake hazards and effects, and for' any future •study of possible losses due to a strong earthquake affecting VermOnt. Sixty-three known or. possible earthquakes have been centered in Vermont from the first report in 1843 through 1993. The largest of these occurred on April 10; 1962 centered at Middlebury. and on July. 6, 1943 centered at Swanton. Each earthquake had magnitude 4.1.. No damage occurred. from the 1943 earthquake, and only a" little damage was reported in the 1962 shock.' Several larger magnitude earthquakes centered outside the state boundaries have strongly shaken Vermont, most notably earthquake.s in, 1,732 centered near Montreal and 1940 centered in the Ossipee mountains 'of New Hampshire. The former event' (estimated magnitude 5.8) took place while Vermont was very sparsely settled, and no reports of, the effects of that event from within Vermont suryi''e. Two magnitude 5.5 earthquakes in 1940 did minor damage in the northeastern part of the state. A number of other earthquakes centered inside and outside 'Vermont have shaken, at least parts of the state but did not cause damage. ' Both deterministic and probabilistic seismic hazard analyses have been computed for Vermont. In the deterministic 'analysis the magnitudes of the once-in-500-year earthquakes have been found' for the' seismically active zones in southern Quebec,' the Adirondack M6untains of New York,: pentral New Hampshire, the Charlevoix 'region down the St. Lawrence River from Quebec City, and within Vermont itself.' Earthquake scenarios where the once-in-500-year earthquake is centered, at the epicenter of the largest earthquake in each of these seismic zones are presented. In each scenario the approximate expected damage area for the earthquake is delineated. All exc,ept the Charlevoix scenario s'how' areas of damaging 1, I 7 earthquake ground shaking in the parts of Vermont closest to the epicenter. What is clear from these different earthquake scenarios is that there. is a substantial seismic, hazard in Vermont from the once-in-500-year earthquake in northeastern North America.. Probable damaging earthquake scenarios come from a number of different potential earthquake sources both . inside and outside of the .state. Furthermore, Vermont's largest population centers are' sites that are likely to experience some of the greatest ground shaking in the state if the postulated earthquakes do" 'occur. : In, the probabilistic seismic hazard analysis maps of the peak ground accelerations and, 1 Hz spectral velocities for 50 years, 100 years and 250 years that have only a 10% probability of being exceeded are computed for Vermont and vicinity and are compared to the national seismic hazard maps 'which have been published by the U.S. Geological Survey. The U.S. Geological Survey maps and those from this study agree very closely in central Vermont, but this study finds slightly higher peak acceleration values in the southeastern corner of the state and significantly higher values (up to a factor of 2) in the northwestern corner of Vermont compared to the published U.S. Geological Survey values. • ' Earthquake ground motions in Vermont can . be' locally modified by soil conditions. In particular, poorly consolidated or unconsolidated soils can . significantly amplify ground shaking relative to the bedrock below the soils, up to a factor of 3 at some frequencies of ground shaking. In an analysis' of Chittenden County in Vermont,. the distributiOn of surficial soils : suggests' that a few areas in the county could experience significant amplification of earthquake ground shaking. These areas are generally in river valleys or along Lake Champlain, including some parts of the city. of Burlington. Other areas in Chittenden County could experience minor ground shaking amplification. As ,part of this study calculations were performed 'to estimate the amount of ground 'shaking amplification,: relative to that in the bedrock, which could take place in typical soils in Vermont. Soil layers ranging in thickness from '25 feet to 200 feet were analyzed. The thinnest soil layer o'niy amplified the ground shaking at very short periods (less than .1 seconds), while the thicker soils' significantly amplified the ground shaking in the period range between '0.1 seconds and 1.0 second. 'This • '. amplification could increase the damage to 'those structures. situated on soils with properties similar to those used, in this 'analysis. 11 1. The seismic hazard at the Vermont Medical Center and the IBM facility in Burlington are examined as examples of site-specific seismic hazard 'estimates. Both sites could experience substantial ground shaking in the scenario earthquakes at Montreal and in the Adirondack Mountains, and from the probabilistid seismic hazard analysis the peak horizontal ground acceleration which has a 90% chance of not being exceeded in 50: years is 16% g. If it occurs, this level of ground shaking could cause some damage at each facility. An analysis of the soil amplification