SUMMARY The east and west rupture directions of the 1943 and 1944 earthquakes on the North Anatolian Fault (NAF) are hypothesized to represent, respectively, long term preferred propagation directions on the corresponding sections of the NAF. Fault sections with preferred rupture direction are expected to have an asymmetric damage structure with respect to the slipping zone. To test the above hypothesis, we study geological and geomorphologic manifestations of structural asymmetry with respect to the active trace of the NAF along the 1943 and 1944 sections. The following fault zone elements are mapped: gouge fabric in the cm scale, fault core structure in a metre scale, and secondary faults and fault rocks in tens of metres scale. Mapping results at three sites on the 1943 rupture and one site on the 1944 rupture are consistent with accumulation of more rock damage on the south side of the 1943 section and on the north side of the 1944 section. Erosion patterns adjacent to the fault that are not correlated with the distribution of intrinsic and extrinsic erosion‐controlling variables (e.g. rock type) are interpreted as morphologic responses to the damage content of rocks and its impact on rock erodibility. The valleys of 11 rivers are parallel to the studied fault sections. About 75 per cent of the total river valleys length along the 1943 rupture is on the south side of the fault, and about 89 per cent of the total river valleys length along the 1944 rupture is on the north side of the fault. Morphometric analysis of watersheds in two correlative terrains displaced along the 1944 rupture section shows that stream erosion is considerably more intense in the terrain north of the fault, with drainage density values almost double in the north compare to the south. Badland topography at two sites along the 1943 rupture section is substantially more developed at the ∼100 m scale on the south side of fault. Our observations along the 1943–1944 rupture sections, including various types of signals that span a large range of scales, are systematically compatible with an opposite sense of damage asymmetry between the two fault sections. These observations are consistent with opposite preferred direction of ruptures for the two sections, similar to the propagation directions of the two recent earthquakes. If those rupture directions are dictated by the velocity structure at depth, we infer that the south side of the 1943 rupture has faster seismic velocity at seismogenic depth than the north side, and that the sense of velocity contrast is reversed along the 1944 rupture zone.

We present new detailed analyses of samples of pulverized Tejon Lookout granite collected from sections adjacent to the San Andreas and Garlock faults in southern California. The Tejon Lookout granite is pulverized in all exposures within about 100 m from both faults. Chemical analyses indicate no or little weathering in the collected samples, although XRD analysis shows the presence of smectite, illite, and minor kaolinite in the clay-size fraction. Weathering products may dominate in the less than 1 micron fraction. The average grain size in all samples of pulverized Tejon Lookout granite ranges between 26 and 208 microns (silt to fine sand), with the particle size distribution in part a function of proximity to the primary slip zone. The San Andreas fault samples that we studied are generally finer grained than those collected from adjacent to the Garlock fault. The particle size distribution for each studied sample from both faults follows a pseudo-power law with a continuously changing exponent, which suggests that pulverization is not simply a consequence of direct shear. The average particle size that we determined for our samples is considerably coarser than reported in previous investigations, which we attribute to possible measurement errors in the prior work. Our data and observations suggest that dynamic fracturing in the wall rock of the San Andreas and Garlock faults only accounts for about 1% or less of the earthquake energy budget.

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