Introduction to the Special Issue on the 2012 Haida Gwaii and 2013 Craig Earthquakes at the Pacific-North America Plate Boundary (British Columbia and Alaska)

Abstract

The 27 October 2012 M w 7.8 Haida Gwaii thrust earthquake and the 5 January 2013 M w 7.5 Craig strike‐slip earthquake are the focus of this special issue. They occurred along the transform boundary between the Pacific and North American plates (Fig. 1). The most identifiable feature of the plate boundary, the strike‐slip Queen Charlotte fault, might be viewed as typical of continent–ocean transform faults because it separates the continental crust of the North American plate from oceanic crust of the Pacific plate for most of its length. However, the current relative plate motion of about 5 cm/yr is highly oblique to the Queen Charlotte fault, causing a transpressive plate boundary in the region. Figure 1. Location map for the 2012 M w 7.8 Haida Gwaii and 2013 M w 7.5 Craig earthquakes, which occurred on the Pacific–North American plate boundary. Approximate rupture zones for the two events are shown in yellow, whereas other significant events on the plate boundary are depicted by their aftershock zones (red, 1949 Queen Charlotte fault; orange, 1970 Cape St. James; blue, 1972 Sitka). The epicentral positions are given by stars, and the focal mechanisms are plotted in a lower‐hemisphere projection. The black arrows indicate Pacific–North American relative motion. The Queen Charlotte terrace is located west of the Queen Charlotte fault and is interpreted to be a narrow accretionary prism. (Inset) The earthquakes occurred offshore the western margin of northern North America. Barbs indicate subduction of the Juan de Fuca (JDF) plate and smaller plates beneath North America at the Cascadia subduction zone (CSZ). Transform plate boundaries between continental and oceanic crust are often depicted as both simple and common. In reality, such continent–ocean transform boundaries are neither. For example, the San Andreas fault in California carries continental crustal fragments along with it on the seaward side …