Sub- and super-shear ruptures during the 2023 Mw 7.8 and Mw 7.6 earthquake doublet in SE Türkiye

The rupture tearing through southeastern Türkiye on February 6th, 2023, was moving faster than the shockwaves it was generating — it was outrunning its own sound. Not by a little, but by enough that the physics community has a name for it: super-shear. It's a crack in the Earth's crust breaking the seismic sound barrier. Melgar and colleagues traced exactly how it happened, kilometer by kilometer, fault segment by fault segment. The story begins with the East Anatolian Fault Zone, a roughly 580 kilometer long boundary where the Arabian and Anatolian microplates grind past each other at about 10 to 15 millimeters per year. It is a left-lateral shear zone, meaning the two sides slide horizontally past each other, and it produces frequent shallow earthquakes in the upper 20 to 25 kilometers of crust. However, it had not produced, in the instrumental record, anything like what happened that February morning. At 1:17 in the morning local time, the moment magnitude seven point eight Nurdağı-Pazarcık earthquake struck. Nine hours and seven minutes later, the moment magnitude seven point six Ekinözü earthquake followed on a separate fault structure. Melgar and colleagues treat the pair as a doublet rather than a mainshock-aftershock sequence because the second event was large, distinct, and on its own fault. Together, they produced ground accelerations exceeding one point five times the acceleration due to gravity and shaking intensities as high as Modified Mercalli Intensity eight to nine across southeastern Türkiye and northwestern Syria. The scale of destruction was staggering. Two massive earthquakes, two different faults, nine hours apart. The question was: what actually happened beneath the surface? To find out, Melgar and colleagues combined two complementary instruments. High-rate Global Navigation Satellite System receivers — essentially GPS stations sampling once per second, fast enough to track the ground moving in real time — recorded displacement directly and cleanly at long periods. Strong-motion seismometers captured higher-frequency shaking and peak accelerations. Put simply, Global Navigation Satellite System tells you how far the ground moved; seismometers tell you how violently it arrived. Together, they constrain not just the shaking but the slip, the timing, and the rupture speed. Equally important was the geometry. The team did not assume a single flat fault plane. They built a multi-fault, three-dimensional surface from aftershock relocations, mapped fault traces, and remote sensing data, meshing it into subfault elements with an average spacing of about five kilometers — four hundred eighty-two elements for the moment magnitude seven point eight model, two hundred fifty-six for the moment magnitude seven point six. Then they applied kinematic inversion: they worked backward from the recorded ground motion to reconstruct how slip evolved on the fault over time. The technique uses mathematical descriptions of how waves travel through the crust to link each patch of the fault to what the instruments recorded. Now, let's turn to the moment magnitude seven point eight rupture. The first and most striking finding is where it started. The relocated hypocenter sits about 15 kilometers south of the mapped East Anatolian Fault trace, at a depth near 12 kilometers. Melgar and colleagues infer that the rupture nucleated on a previously unmapped structure they label the Nurdağı-Pazarcık Fault, and only after that initial rupture propagated did slip transfer to the East Anatolian Fault itself, roughly ten seconds after the origin time. That matters beyond academic interest. If a moment magnitude seven point eight earthquake can begin on a fault that wasn't on the map, the maps are incomplete in ways that directly affect hazard assessment. The finding reveals source complexity that existing fault inventories simply didn't capture. Once the rupture jumped to the East Anatolian Fault, it ran. The preferred model shows roughly 350 kilometers of bilateral slip, with peak displacements reaching about 9 meters in places and slip rates as high as 1.5 meters per second. The seismic moment, which is a measure of the total mechanical energy released, comes out at six point five one times ten to the twentieth newton meters. The maximum rupture speed was 3.2 kilometers per second. The shear-wave speed at the depths where most slip occurred is roughly 3.5 kilometers per second. So at 3.2, the moment magnitude seven point eight was running at about 90 percent of that limit. Fast, but still sub-shear — still behind its own sound waves. That distinction becomes the setup for what the second earthquake did. The moment magnitude seven point six Ekinözü earthquake broke the Sürgü fault, a distinct structure to the northeast. The rupture was bilateral, propagating both east and west from the hypocenter over roughly 160 kilometers of fault. Peak slip reached approximately 6 to 7 meters, and the seismic moment was three point six four times ten to the twentieth newton meters. But the rupture speed is where this earthquake becomes scientifically remarkable. Melgar and colleagues found that westward, the Sürgü fault broke at approximately 4.8 kilometers per second. The shear-wave speed at those depths is roughly 3.7 kilometers per second. So the westward rupture was moving faster than shear waves can travel through the crust — it was super-shear. Eastward, the same fault broke at 2.8 kilometers per second, comfortably sub-shear. Think of it this way. In normal, sub-shear rupture, the fault crack moves more slowly than the stress waves it generates, so those waves can run ahead and prepare the fault ahead of the crack. Super-shear is the geological equivalent of breaking the sound barrier: the crack outruns its own waves, generating a Mach cone of seismic energy — concentrated, directional, and damaging in ways that standard ground-motion models may not fully account for. The asymmetry was not a modeling artifact. A single rupture speed could not fit the data from stations on both sides of the fault. When Melgar and colleagues enforced a uniform speed, the waveform fits degraded noticeably. The western stations demanded super-shear, while the eastern stations demanded sub-shear. Their preferred model sits on a broad low-misfit plateau between about 4.6 and 5.4 kilometers per second for the western propagation — that's a robust result, not a knife-edge. The same fault, the same earthquake, running at two fundamentally different regimes in opposite directions. What drives that directional asymmetry? Melgar and colleagues raise the question without closing it. Pre-existing fault structure, variations in stress along the fault, and the geometry of the Sürgü fault's curvature are all plausible contributors, but the data constrain the kinematics, not the mechanism. That distinction — measuring what happened precisely, without fully explaining why — is where this paper is honest about its limits. Several other open questions come into focus in the discussion. Three significant aftershocks, ranging from moment magnitude 5.6 to 6.6, struck in the first 18 minutes after the moment magnitude seven point eight. Then, nine hours later, the moment magnitude seven point six. How did the first rupture alter the stress field in ways that loaded the Sürgü fault toward failure? The aftershock relocations show a cloud extending from about 3 kilometers depth down to 25 kilometers, consistent with slip on multiple interacting structures — but the precise stress-transfer pathway between the two mainshocks remains an open problem. There is also the question of the unmapped fault. The Nurdağı-Pazarcık Fault that nucleated the moment magnitude seven point eight was not on the map before this earthquake. The inversion used long-period waveforms, so it is possible that brief, early super-shear processes on that structure exist but are muted in these records. The multi-segment, branching geometry of the East Anatolian Fault Zone raises difficult questions about how to separate signals from individual structures in remote-sensing data, and what future rupture scenarios on this system might look like. What Melgar and colleagues have produced, then, is a tightly constrained empirical record of two of the most complex large-earthquake ruptures in recent memory. The moment magnitude seven point eight started on a fault nobody had mapped, then tore 350 kilometers along the East Anatolian Fault at speeds approaching the sub-shear limit. The moment magnitude seven point six broke the seismic sound barrier heading west at 4.8 kilometers per second while staying well below it heading east. Peak slips of 8 to 9 meters and 6 to 7 meters, respectively. Ground motions exceeding one point five times the acceleration due to gravity. This doublet is now a reference event — an empirically mapped case of unmapped nucleation, directional super-shear, and multi-fault cascading on one of the world's most seismically active plate boundaries. The models Melgar and colleagues produced sharpen the basis for hazard assessment in the region. They remind us that the faults we haven't mapped may be exactly the ones we need to worry about most. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. 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