Stress tensor inversion using detailed microearthquake information and stability constraints: Application to Olfus in southwest Iceland

Using detailed microearthquake data, we present a stress tensor inversion s cheme with new methods for selecting the fault planes and allowing for erro rs in the focal mechanisms. The nonuniqueness of earthquake focal mechanism s is accounted for in our inversion scheme through the introduction into th e inversion of a range of well-fitting focal mechanisms for each event. The range of focal mechanisms significantly improves the quality of the estima ted stress tensor. Relative localization of clusters of microearthquakes is used to obtain information about which nodal plane could be correct fault plane. The clusters frequently fall on a common fault plane, and if there a re acceptable focal mechanisms where one nodal plane has orientation simila r to the common plane, we assume this is the correct fault plane for the ev ent. If there is no predefined fault plane, we utilize a simple Mohr-Coloum b failure criterion to obtain a physical choice of fault plane between the two nodal planes in the focal mechanism. The nodal plane with highest relat ive instability is chosen as the fault plane. Differences between the insta bility and the standard slip angle criterion are investigated. The new inve rsion scheme has been applied to microearthquake data from the Olfus area i n the vicinity of the southwest Iceland triple junction. We estimate an obl ique strike-sip state of stress, maximum horizontal stress at N30 degrees E , and minimum horizontal stress at N60 degrees W, with significant normal f aulting influence. The instability fault selection criterion predicts very well the orientation of faults mapped by relative localization.