The patterns of seismic activity before large strike slip and thrust-type earthquakes were examined for intervals as great as 45 years before the main shocks. The earthquakes chosen for this investigation occurred along the northwestern, northern, and eastern margins of the Pacific, and most of these events had rupture zones that extended hundreds of kilometers. The observed distribution of prior seismicity for these earthquakes correlates with the configurations of their rupture zones and with the epicentral locations of the main shocks within the zones. An obvious and consistent feature of these spatial distributions is the relatively aseismic character of extensive portions of the rupture zones until the times of the main shocks. These reduced levels of seismic activity extended to events at least several magnitudes smaller than the main shocks and possibly many magnitudes smaller. For most of the large earthquakes examined, rupture initiated in an area of moderate seismic activity and then propagated as much as hundreds of kilometers into adjacent quiet regions. In at least several instances, rupture also terminated in a region of some prior activity. Thus prior seismicity frequently occurred near the epicenters of the main shocks and/or near the edges of the rupture zone. There are some indications that the level of this prior activity, particularly in the vicinity of the epicenter, increased as the time of the main shocks approached. Thus one important conclusion of this study is that gaps in seismicity for great earthquakes along major plate boundaries may also be gaps for smaller-magnitude acticity. Further, such gaps may commonly remain so until the time of the principal shock. That is, if premonitory activity is associated with large earthquakes, such activity should be sought along the boundaries of a seismic gap rather than in the gap itself. Along the great transform fault system near western Canada and southeastern Alaska the nature of the seismicity varies noticeably with distance northwest of the Juan de Fuca spreading center. That is, the area near the spreading center is a region of frequent moderate-size earthquakes (mM=6); further north is a region of occasional large earthquakes (M=7); even further north is a region of infrequent great earthquakes (M=8). This changing seismic regime is similar to that of the San Andreas system proceeding north from the spreading centers in the Gulf of California. Along both transform fault systems, and possibly others, the change in characteristic seismicity may be a function of the changing lithospheric structure with distance from the spreading centers.