86. Stein, R. S., and P. Bird [2024] Why do great continental transform earthquakes nucleate on branch faults?, Seismol. Res. Lett., 95, 3406–3415, doi: 10.1785/0220240175.

Abstract. The five M_w ≥ 7.8 continental transform earthquakes since 2000 all nucleated on branch faults. This includes the 2001 M_w 7.8 Kokoxili, 2002 M_w 7.9 Denali, 2008 M_w 7.9 Wenchuan, 2016 M_w 7.8 Kaikōura, and 2023 M_w 7.8 Pazarcık events. A branch or splay is typically an immature fault that connects to the transform at an oblique angle and can have a different rake and dip than the transform. The branch faults ruptured for at least 25 km before they joined the transforms, which then ruptured an additional 250–450 km, in all but one case (Pazarcık) unilaterally. Branch fault nucleation is also likely for the 1939 M 7.8 Erzincan earthquake, possible for the 1906 M_w∼7.8 and 1857 M_w∼7.9 San Andreas earthquakes, but not for the 1990 M_w 7.7 Luzon, 2013 M_w 7.7 Balochistan, and 2023 M_w 7.7 Elbistan events. Here, we argue that because fault continuity and cataclastite within the fault damage zone develop through cumulative fault slip, mature transforms are pathways for dynamic rupture. Once a rupture enters the transform from the branch fault, flash shear heating causes pore fluid pressurization and sudden weakening in the cataclastite, resulting in very low dynamic friction. But the static friction on transforms is high, and so they are usually far from failure, which could be why they tend to be aseismic between, or at least for centuries after, great events. This could explain why the largest continental transform earthquakes either begin on a branch fault or nucleate along the transform at locations where the damage zone is absent or the fault continuity is disrupted by bends or echelons, as in the 1999 M_w 7.6 İzmit earthquake. Recognition of branch fault nucleation could be used to strengthen earthquake early warning in regions such as California, New Zealand, and Türkiye with transform faults.

Figure 1.  M_w ≥ 7:8 transform ruptures since the year 2000. (a) 2023 M_w 7.8 Pazarcık (Kahramanmaraş), Türkiye, started on the Narlı oblique normal fault, rupturing onto the left-lateral East Anatolian fault (Li et al., 2018; Jia et al., 2023); (b) 2002 M_w 7.9 Denali, Alaska, which started on the Susitna Glacier thrust fault, jumping onto the right-lateral Denali fault, and eventually onto the right-lateral Totschunda fault (Haeussler et al., 2004); (c) 2001 M_w 7.8 Kokoxili, Tibet, started on the left-lateral Taiyang Lake (Heituo) fault, rupturing onto the left-lateral Kunlun and Kunlun Pass faults (Lasserre et al., 2005); (d) 2008 M_w 7.9 Wenchuan, China, started on a 30°–40° dipping thrust extension of the Beichuan and Pengguan faults, which are oblique slip with thrust and right- lateral components (Zhang et al., 2010; Fielding et al., 2013); (e) 2016 M_w 7.8 Kaikōura, New Zealand, started on the minor Humps thrust fault, rupturing onto the North Canterbury Domain and the Marlborough fault system (Litchfield et al., 2018). NEIC, National Earthquake Information Center. The color version of this figure is available only in the electronic edition.

Figure 2.  Possible branch fault nucleation in M_w ≥ 7.8 earthquakes. (a) The 1939 M_w 7.8 Erzincan, Türkiye, epicenter and its dashed blue uncertainty ellipse lie northeast of the North Anatolian fault rupture (Di Giacomo et al., 2018), near a 10° bend in the North Anatolian fault. The unnamed branch fault was mapped by Duman and Emre (2013), but whether it slipped in the earthquake is unknown (Emre et al., 2021). We have dotted the branch fault where it is unmapped but expressed in the topography. (b) The location uncertainty ellipse for the 1906 San Francisco earthquake (Lomax, 2005) includes branch faults (Jennings and Bryant, 2010) associated with the San Andreas–San Gregorio junction. (c) Foreshocks during the 9 hr before the 1857 Ft. Tejon earthquake (Sieh, 1978a) struck near its northern mapped tip (Sieh, 1978b)—a site of mapped branch faults and the Parkfield fault section. The color version of this figure is available only in the electronic edition.

Figure 3.  Potential branch fault ruptures that did not trigger a great transform earthquake. Plotted are M_w ≥ 6:0 events since 1960 from the U.S. Geological Survey (USGS) Advanced National Seismic System (ANSS) catalog that struck within 20 km of the transforms. M_w ≥ 7:8 ruptures that followed these earthquakes are red; coastlines are blue. (a) Denali fault earthquakes until 2002 (the earthquake nucleated on the north-dipping Susitna Glacier fault, not on the Denali fault). (b) East Anatolian earthquakes until 2023. (c) Conway–Hope–Needles faults until 2016. (d) San Andreas fault until the present (the 1992 M 7.3 shock is gray because it is beyond 20 km from San Andreas). No M ≥ 6.0 events occurred within 20 km of the Kunlun fault rupture before 2001 or the Beichuan–Pengguan fault rupture before 2008. The color version of this figure is available only in the electronic edition.

Figure 4.  Schematic stress histories of connected branch and transform faults; t₁ and t₂ are rupture times. The branch fault obeys Byerlee friction, so does not fail until it reaches the failure threshold. However, once the branch fault ruptures into the transform, shear heating and pore fluid pressurization of the damage zone cause a sudden drop in dynamic friction on the transform, enabling it to rupture prematurely. In this depiction, the branch fault earthquake always triggers the transform, but this need not be the case.

Figure 5.  Surface rupture in the 1999 M_w 7.6 İzmit rupture of the North Anatolian fault (Lettis et al., 2002). This highly segmented and discontinuous rupture nucleated near or within the Gölcük extensional stepover. Although the North Anatolian fault has a higher slip rate and more net slip than the East Anatolian fault, here the North Anatolian fault is discontinuous. Because the stepover is unlikely to be a long-lived feature (King and Nábělek, 1985), cataclastite might be absent there and so deviatoric stresses could be unusually high.