Why and where great thrust earthquakes occur along island arcs

Abstract

For many subducting margins of the Pacific there appears to be a characteristic maximum for the source dimensions of large shallow earthquakes. The major conclusion of this investigation is that this characteristic maximum for a particular island arc or segment of an arc is strongly influenced by the geometry of the interface zone, particularly the width of the interface between underthrust and overthrust slabs of lithosphere. By width of interface we refer to the downdip dimension of the contact zone between the two abutting slabs of lithosphere. Maps displaying inferred variations in interface geometry along five major island arcs agree generally with the known locations and extent of rupture during large shallow earthquakes of this century. Ruptures of extraordinary length (>400 km) occur near gently dipping slabs of lithosphere that abut over a broad contact zone against the overthrust slab. Moderately large earthquakes (maximum rupture length, <150 km) occur along slabs that dip more abruptly and have a thin zone of contact with the overthrust lithosphere. If the major conclusion of this study is valid, then detailed knowledge of interface geometry may provide a technique for estimating the extent of the greatest shallow earthquakes likely to occur in an existing seismic gap. That is, a narrow width of interface apparently places severe restrictions on the maximum extent of rupture that can occur during an earthquake. The extent and the location of ruptures along large strike-slip faults may also be influenced principally by the width of interface (vertical extent of hypocenters for strike-slip faults). Along the two great transform fault systems near western North America (that starting near spreading centers in the Gulf of California and extending northward into the San Andreas system and that starting near the Juan de Fuca spreading center and extending northward into the Queen Charlotte Islands-Fairweather fault system) both the width of interface and the typical dimensions of large earthquakes tend to increase in size with distance from the spreading center.