University of Otago chair of Earthquake Science and geology researcher Prof Mark Stirling said he had been studying 13 ‘‘ancient, fragile landform features’’ in Massachusetts, New Hampshire, and southwestern Maine to estimate how susceptible they were to collapse or breakage during large earthquakes; and develop a map of maximum postglacial earthquake magnitudes in the regions.
It aimed to provide probabilistic seismic hazard ratings for the United States Geological Survey’s National Seismic Hazard Model.
‘‘They’re estimates of future ground shaking and their associated probability.’’
Prof Stirling said the rocks were originally suspended in the Laurentide Ice Sheet during the last ice age.
When the ice melted about 13,000-20,000 years ago, many of the rocks ended up balancing in precarious positions as the ice melted around them.
By determining the age of the formation, then using simple engineering relationships that are often used on buildings and structures, he could work out what earthquake strength it would take to shake them down.
‘‘Finding that they have been sitting fragile like that for the length of time that they have, it means there hasn’t been strong earthquake shaking in their vicinity or they would be shaken down.
‘‘In most cases, where you apply these features, it can tell you that the hazard is less than what the hazard models suggest.’’
New Zealand had similar landform features, which he had also studied.
Some were studied as part of a seismic hazard assessment for the Clyde Dam.
‘‘In our studies that we’ve done in New Zealand [and California], the rocks tell a different story to what the hazard models say.
‘‘They imply that the shaking has been less in the time period that these features have been sitting.
‘‘So when you use a hazard model to forecast what’s going to happen in the future, by incorporating this rock information, you will forecast lower levels of hazard.
‘‘It’s allowing us to refine our hazard estimates, and in some cases, reduce them.’’
