The relationship between Wadati‐Benioff Zone geometry and P, T and B axes of intermediate and deep focus earthquakes

For 865 earthquakes with focal depths exceeding 90 km this paper presents a concise global summary of the distributions of P, T, and B axes of their focal mechanisms with respect to the downdip, along-strike, and normal axes of Wadati-Benioff zones. The focal mechanisms are best double-couple solutions for the centroid moment tensors reported by the Harvard group, with the P, T, and B axes corresponding to the principal axes of the individual moment tensors. We find, as did previous investigators, that events below 300 km have predominantly downdip P axes, with 50% of all P axes of deep focus events lying within 28° of downdip. For events between 90- and 300-km depth there are regional differences, but most regions possess events with nearly downdip P axes or events with nearly downdip T axes. In almost all regions having downdip P or T axes, Bingham statistics show that the greatest concentration of P or T axes lies somewhat below the plane of the Wadati-Benioff zone. For events of all focal depths in nearly all regions, B axes cluster along the strike direction, with 50% occurring within 38° of the strike. From these regional results we define a “typical” focal mechanism as one having the B axis within 30° of the along strike and having either the T or P axis within 30° of the downdip. Such a focal mechanism might occur in response to a stress tensor having its greatest or least principal stresses channeled approximately along the downdip direction of the subducting slab, with the intermediate principal stress lying horizontally within the slab, controlled possibly by slab bending. However, by this criterion, only 29% of the focal mechanisms for deep and intermediate events are typical. Thus the majority of the individual events cannot be explained simply as occurring in response to a downdip and slab-normal greatest and least principal stress system, with a horizontal along-strike intermediate principal stress. A few regions, such as Java and the Marianas, are clearly anomalous in that they possess significantly fewer typical focal mechanisms than do most other regions.