MODERN EARTHQUAKES AND THE NORUMBEGA FAULT: A TANTALIZING ENIGMA

It is often argued that the earthquake activity of recent decades is due to reactivation of pre-existing zones of weakness in the modern tectonic stress field. One such pre-existing zone of weakness is the Norumbega Fault, which has been a favorite research target of Allan Ludman. The question that is explored in this presentation is whether the Norumbega Fault might be seismically active today and whether it could host a large earthquake in the future. There have been many epicenters of modern small earthquakes that have been mapped on or very near one of the traces of the Norumbega fault system, but there also have been many off-fault epicenters within 20-50 km of the fault. The seismicity does not neatly align along the Norumbega Fault on a regional scale, even though there are some notable local associations of seismicity with the fault. The largest earthquakes in Maine since the 1970s have been within 100 km of the Norumbega Fault, but none has been directly on the fault itself. Where focal mechanisms of earthquakes near the Norumbega Fault have been computed, they generally indicate thrust faulting with a variety of fault strikes. This is consistent with the focal mechanisms of earthquakes throughout northeastern North America, and it suggests that the Norumbega Fault is not being reactivated as a strike-slip fault today. Even so, the Norumbega Fault system may be a zone of crustal weakness where modern earthquakes can initiate, but due to the orientation of the current stress field the ruptures of these modern earthquake propagate at an angle to the Norumbega Fault. The potential for the Norumbega Fault to host a large magnitude earthquake can only be clarified in the future as more earthquake data are accumulated.