Geological and geomorphologic asymmetry across the rupture zones of the 1943 and 1944 earthquakes on the North Anatolian Fault: possible signals for preferred earthquake propagation direction

Abstract

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.

Evidence for a bimaterial interface along the Mudurnu segment of the North Anatolian Fault Zone from polarization analysis of P waves

“…This was based on the assumption that the damage asymmetry was produced by statisticallypreferred propagation direction of large earthquakes on the fault (Ben-Zion and Shi, 2005). The imaging results of Ozakin et al (2012) are consistent with the inference of Dor et al (2008). These set of studies indicate that large earthquakes on that section of the NAFZ tend to propagate, and hence produce larger shaking hazard, to the west.…”

“…The above studies and the results of the present work, associated with completely independent data and techniques, consistently indicate that the Mudurnu segment of the NAFZ represents a boundary between significantly different crustal units. Dor et al (2008) inferred from observed asymmetry of rock damage across the rupture zone of the 1944 earthquake on the NAFZ that the north side of the fault has faster seismic velocity at seismogenic depth. This was based on the assumption that the damage asymmetry was produced by statisticallypreferred propagation direction of large earthquakes on the fault (Ben-Zion and Shi, 2005).…”

“…Moreover, large fault zones tend to nucleate along pre-existing sutures between different geological units (e.g., Le . Field studies show that the active slip zone in large fault structures is often localized along the overall lithology contrast and/or the boundary of a damaged fault zone layer (e.g., Sibson, 2003;Sengor et al, 2005;Dor et al, 2006Dor et al, , 2008Mitchell et al, 2011).…”

Internal structure of the San Jacinto fault zone at Blackburn Saddle from seismic data of a linear array

“…We focus on these structural components because they contain information on likely properties of past and future earthquake ruptures and associated ground motion (e.g. Andrews & Ben-Zion 1997;Dor et al 2006Dor et al , 2008Brietzke et al 2009;Shlomai & Fineberg 2016).…”

Structural Properties and Deformation Patterns of Evolving Strike-slip Faults: Numerical Simulations Incorporating Damage Rheology

“…This fault core is typically parallel to the macroscopic slip vector and is surrounded by a cataclasite layer which is a few meters thick (e.g., CHESTER and CHESTER, 1998;SCHULZ and EVANS, 2000). The damage zone (DZ) around the fault core typically consists of a zone of intense damage, and possibly pulverized rocks, with a width of a few hundred meters (DOR et al, 2006(DOR et al, , 2008, which is surrounded by a broader, several kilometers wide zone, of distributed damage. The latter is probably a relic structure of the progressive coalescence and localization of the active fault zone over time (AMBRASEYS, 1970;KIM et al, 2004;SIBSON, 2003).…”