Spatial patterns of seismicity before some mainshocks appear skewed to one side or toward one end of the pending surface of rupture. Several examples of thrust-type earthquakes, for example, indicate stronger patterns of prior seismicity within the overthrusting block. Similarly some aftershock patterns appear skewed to one side of the rupture surface or in a direction away from the location of prior seismicity. For some earthquakes, therefore, the zones of most significant activity before and after the mainshock do not coincide and, thus, are difficult to explain in terms of loading processes or physical properties that are completely symmetric about the plane of rupture. Furthermore, there are some indications that these asymmetric phenomena may repeat in the same or in nearby locations. These observations appear to be explainable in terms of either: (1) in asymmetric distribution of rock properties about the fault zone with consequent asymmetric seismic response to stress accumulation, or (2) a fundamental directionality to the process of tectonic loading resulting possibly from earlier crustal or plate movement in a nearby region. These two explanations are not incompatible and may in fact be related.