Slip distribution of the February 6, 2023 Mw 7.8 and Mw 7.6, Kahramanmaraş, Turkey earthquake sequence in the East Anatolian Fault Zone

At four seventeen in the morning on February sixth, twenty twenty-three, the ground beneath Kahramanmaraş began to move. By the time it stopped, more than fifty thousand people were dead and a rupture had torn across southern Turkey for three hundred ten kilometers. The question that followed — not why it happened, but exactly where the fault moved, by how much, and what that leaves behind — is what Barbot and colleagues set out to answer. Their tools were satellites orbiting hundreds of kilometers overhead, and their answers carry a warning that goes beyond twenty twenty-three. To understand what broke, you first need to know what was there to break. The East Anatolian Fault, or EAF, is a left-lateral strike-slip fault — meaning that if you stood on one side of it and watched the other side move, it would slide to the left. It runs roughly three hundred kilometers through southern Turkey, branching diffusely toward the Dead Sea Fault and the Cyprus Arc. Compare it to its more famous cousin, the North Anatolian Fault, which is right-lateral and stretches about one thousand two hundred kilometers, and you start to see the difference: the EAF is shorter, slower slipping, and geometrically messier. Barbot and colleagues describe it as a relatively immature fault system — not a casual label, but a specific set of properties. Segmentation, a low long-term slip rate, and diffuse branching mean the system accumulates stress over long intervals and can produce surprisingly large ruptures when those segments link up. The historical record makes this concrete. Individual EAF segments have hosted large earthquakes before — the Amanos segment in five hundred twenty-one, the Pazarcık segment in fifteen thirteen, the Erkenek segment in eighteen ninety-three — but parts of the fault had remained locked for more than a century. The twenty twenty Mw six point eight Elazığ earthquake was a recent signal of the fault's restlessness, but as Barbot and colleagues note, the twenty twenty-three rupture propagated close to but did not fully overlap the twenty twenty rupture. That gap between them is something we'll return to. The point for now is that the EAF's long silence was not safety. It was stored energy. To map where that energy was released, the team combined three independent observation streams. The first was Synthetic Aperture Radar, or SAR, from the Sentinel One and ALOS Two satellites. SAR interferometry, or InSAR, measures how the radar phase of the ground changes between two satellite passes, which is exquisitely sensitive to surface motion. But right at the fault trace, where deformation is largest, the phase fringes become too dense to interpret. So the team switched to pixel-offset tracking: cross-correlating the amplitude images from before and after the earthquake to measure how far the ground actually shifted. The second stream was optical pixel tracking using Sentinel Two imagery at ten meter resolution, processed with the MicMac library to estimate horizontal surface displacements from before-and-after scenes. The third was high-rate Global Navigation Satellite System data — fifty-one CORS-TR stations recording at one hertz, plus seventeen additional continuous stations — processed with GAMIT/GLOBK software. Station EKZ1, just two kilometers from the Çardak fault, recorded up to four point four meters of westward displacement. These three streams are complementary by design: InSAR excels across broad areas but can fail in the immediate rupture zone; pixel tracking is robust at large near-fault offsets; and GNSS anchors everything in absolute meters and real time. Now for what those instruments revealed. The Mw seven point eight mainshock produced a rupture three hundred ten kilometers long — roughly the distance from Boston to Philadelphia — torn open in minutes. Coseismic slip extended from the surface down to about fifteen kilometers depth, concentrated between three and seven kilometers. The geodetic seismic moment was five point four times ten to the twentieth Newton-meters, corresponding to Mw seven point eight. Maximum slip reached eight meters at depth, concentrated on the Pazarcık segment. But here is where the physics gets interesting: surface slip peaked at only about six meters — roughly twenty-five percent less than the slip at depth. This shallow slip deficit is not a measurement artifact. It reflects the genuine mechanical behavior of the fault's uppermost layers, which deform partly through distributed cracking and aseismic creep rather than clean rupture. For the people living above that zone, distributed shallow deformation can mean more widespread ground damage than a clean surface break would produce. Along strike, the slip was highly segmented. Large slip asperities sat on the South Amanos, North Amanos, Pazarcık, and Erkenek segments, separated by releasing step-overs — places where the fault steps sideways, creating a gap or basin between two fault strands. These step-overs are geometrical discontinuities, and they controlled where the rupture sped up and where it slowed down, producing a waxing-and-waning pattern that Barbot and colleagues interpret as a start-stop signature of fault bends and morphological gradients. The GNSS displacements for the mainshock were explained with a variance reduction — essentially the fraction of observed signal captured by the model — of eighty-five percent. Different satellite geometries agreed well; the ALOS Two interferograms showed variance reductions of seventy-three and eighty percent. Nine hours after the mainshock, the Mw seven point six aftershock struck. And this is where the story gets more complex. This was not a simple continuation of the Mw seven point eight rupture along the same fault. It was a one hundred fifty kilometer rupture of nearby, geometrically distinct faults — the Çardak and Savrun faults, plus a previously unidentified, immature fault crossing the Nurhak complexity. The two events share the same sense of motion, left-lateral strike-slip, but their fault surfaces are disconnected and differently oriented. The geodetic moment for the Mw seven point six event was three point three times ten to the twentieth Newton-meters, and maximum coseismic slip reached eleven to twelve meters on the Çardak fault — higher than the mainshock's peak, concentrated even closer to the surface. The GNSS variance reduction for this event was ninety-seven percent, one of the best fits in the study. Then came February twentieth: the Mw six point four Antakya aftershock. Smaller, more confined. The InSAR inversion yielded a twenty-five kilometer along-strike rupture, with maximum slip of zero point ninety-three meters at eight point three kilometers depth, on a fault striking southwest — consistent with the mapped Antakya Fault. This serves as a reminder that the fault system south of the mainshock was still releasing stress weeks later. Across all three events, Barbot and colleagues found a consistent spatial pattern in the aftershocks: they cluster at fault bends and segment boundaries, and around the periphery of high-slip zones — not inside them. The high-slip patches had already released most of their accumulated stress. The surrounding areas had not. That is where aftershocks live. It is a physically clean picture: stress concentrates at the edges of rupture, not in its wake. Which brings us to the seismic gap. The Mw seven point eight mainshock propagated northward and tapered off into the Pütürge segment, stopping roughly forty kilometers south of where the twenty twenty Mw six point eight Elazığ earthquake ruptured. Aftershocks link the two events at depths of ten to twenty kilometers, but the shallow fault — the zone between the surface and about ten kilometers depth — remains unbroken across those forty kilometers. The twenty twenty Elazığ earthquake itself showed a shallow slip deficit of sixty percent, even larger than the twenty twenty-three mainshock's twenty-five percent. Together these observations point to a stretch of fault that has not slipped recently and may be accumulating stress. Barbot and colleagues explicitly raise the possibility of another Mw approximately six point eight event on the Pütürge segment and call for dedicated instrumentation of the area to assess its seismic potential. That is the practical conclusion of this work. Space-based geodesy — SAR pixel tracking, InSAR, optical cross-correlation, and high-rate GNSS — can map a complex, multi-segment rupture sequence within days of the event, producing a detailed slip picture precisely when it matters most for emergency response and hazard assessment. The three hundred ten kilometer tear of February sixth is now mapped in detail: where slip peaked, where it was shallow, and where it was arrested by geometry. The edges of that map — the Pütürge gap to the north, and the diffuse southern termination where the East Anatolian Fault branches toward the Antakya Fault and the Dead Sea Fault system — are the places that demand attention now. The twenty twenty-three earthquake was catastrophic. The fault system it belongs to has more to say. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.