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We present new in situ observations of systematic asymmetry in the pattern of damage expressed by fault zone rocks along sections of the San Andreas, San Jacinto, and Punchbowl faults in southern California. The observed structural asymmetry has consistent manifestations at a fault core scale of millimeters to meters, a fault zone scale of meters to tens of meters and related geomorphologic features. The observed asymmetric signals are in agreement with other geological and geophysical observations of structural asymmetry in a damage zone scale of tens to hundreds of meters. In all of those scales, more damage is found on the side of the fault with faster seismic velocities at seismogenic depths. The observed correlation between the damage asymmetry and local seismic velocity structure is compatible with theoretical predictions associated with preferred propagation direction of earthquake ruptures along faults that separate different crustal blocks. The data are consistent with a preferred northwestward propagation direction for ruptures on all three faults. If our results are supported by additional observations, asymmetry of structural properties determined in field studies can be utilized to infer preferred propagation direction of large earthquake ruptures along a given fault section. The property of a preferred rupture direction can explain anomalous behavior of historic rupture events, and may have profound implications for many aspects of earthquake physics on large faults.

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.