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1. Variations of Earthquake Properties Before, During, and After the 2019 M7.1 Ridgecrest, CA, Earthquake

   https://doi.org/10.1029/2020GL089650  

   Cheng, Yifang; Ben‐Zion, Yehuda (September 2020, Geophysical Research Letters)

   Abstract We attempt to clarify processes associated with the 2019 Ridgecrest earthquake sequence by analyzing space‐time variations of seismicity, potency values, and focal mechanisms of earthquakes leading to and during the sequence. Over the 20 years before theM_w7.1 mainshock, the percentage of normal faulting events decreased gradually from 25% to below 10%, indicating a long‐term increase of shear stress. TheM_w6.4 andM_w7.1 ruptures terminated at areas with strong changes of seismic velocity or intersections with other faults producing arresting barriers. The aftershocks are characterized by highly diverse focal mechanisms and produced volumetric brittle deformation concentrated in a 5–10 km wide zone around the main ruptures. Early aftershocks of theM_w7.1 event extended over a wide area below typical seismogenic depth, consistent with a transient deepening of the brittle‐ductile transition. The Ridgecrest earthquake sequence produced considerable rock damage in the surrounding crust including below the nominal seismogenic zone. 

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2. Long- and Short-Term Stress Interaction of the 2019 Ridgecrest Sequence and Coulomb-Based Earthquake Forecasts

   https://doi.org/10.1785/0120200169  

   Toda, Shinji; Stein, Ross S. (July 2020, Bulletin of the Seismological Society of America)

   Abstract We first explore a series of retrospective earthquake interactions in southern California. We find that the four Mw≥7 shocks in the past 150 yr brought the Ridgecrest fault ∼1  bar closer to failure. Examining the 34 hr time span between the Mw 6.4 and Mw 7.1 events, we calculate that the Mw 6.4 event brought the hypocentral region of the Mw 7.1 earthquake 0.7 bars closer to failure, with the Mw 7.1 event relieving most of the surrounding stress that was imparted by the first. We also find that the Mw 6.4 cross-fault aftershocks shut down when they fell under the stress shadow of the Mw 7.1. Together, the Ridgecrest mainshocks brought a 120 km long portion of the Garlock fault from 0.2 to 10 bars closer to failure. These results motivate our introduction of forecasts of future seismicity. Most attempts to forecast aftershocks use statistical decay models or Coulomb stress transfer. Statistical approaches require simplifying assumptions about the spatial distribution of aftershocks and their decay; Coulomb models make simplifying assumptions about the geometry of the surrounding faults, which we seek here to remove. We perform a rate–state implementation of the Coulomb stress change on focal mechanisms to capture fault complexity. After tuning the model through a learning period to improve its forecast ability, we make retrospective forecasts to assess model’s predictive ability. Our forecast for the next 12 months yields a 2.3% chance of an Mw≥7.5 Garlock fault rupture. If such a rupture occurred and reached within 45 km of the San Andreas, we calculate it would raise the probability of a San Andreas rupture on the Mojave section by a factor of 150. We therefore estimate the net chance of large San Andreas earthquake in the next 12 months to be 1.15%, or about three to five times its background probability. 

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3. The Role of Stress Transfer in Rupture Nucleation and Inhibition in the 2023 Kahramanmaraş, Türkiye, Sequence, and a One-Year Earthquake Forecast

   https://doi.org/10.1785/0220230252  

   Toda, Shinji; Stein, Ross S (January 2024, Seismological Research Letters)

   Abstract We probe the interaction of large earthquakes on the East Anatolian fault zone, site of four Mw ≥ 6.8 events since 2020. We find that the 2023 Mw 7.8 Pazarcık shock promoted the Mw 7.7 Elbistan earthquake 9 hr later, largely through unclamping of the epicentral patch of the future rupture. Epicentral unclamping is also documented in the 1987 Superstition Hills, 1997 Kagoshima, and 2019 Ridgecrest sequences, so this may be common. The Mw 7.7 Elbistan earthquake, in turn, is calculated to have reduced the shear stress on the central Pazarcık rupture, producing a decrease in the aftershock rate along that section of the rupture. Nevertheless, the Mw 7.7 event ruptured through a Çardak fault section on which the shear stress was decreased by the Mw 7.8 rupture, and so rupture propagation was not halted by the static stress decrease. The 2020 Mw 6.8 Doğanyol–Sivrice earthquake, located beyond the northeast tip of the Mw 7.8 Pazarcık rupture, locally dropped the stress by ∼10 bars. The 2023 Mw 7.8 earthquake then increased the stress there by 1–2 bar, leaving a net stress drop, resulting in a hole in the 2023 Pazarcık aftershocks. We find that many lobes of calculated stress increase caused by the 2020–2023 Mw 6.8–7.8 earthquakes are sites of aftershocks, and we calculate 5–10 faults in several locations off the ruptures brought closer to failure. The earthquakes also cast broad stress shadows in which most faults were brought farther from failure, and we observe the beginnings of seismicity rate decreases in some of the deepest stress shadows. Some 41 Mw ≥ 5 aftershocks have struck since the Mw 7.8 mainshock. But based on these Coulomb interactions and on the rapid Kahramanmaraş aftershock decay, we forecast only about 1–3 Mw ≥ 5 earthquakes during the 12–month period beginning 1 December 2023, which is fortunately quite low. 

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4. Complex multi-fault rupture and triggering during the 2023 earthquake doublet in southeastern Türkiye

   https://doi.org/10.1038/s41467-023-41404-5  

   Liu, Chengli; Lay, Thorne; Wang, Rongjiang; Taymaz, Tuncay; Xie, Zujun; Xiong, Xiong; Irmak, Tahir Serkan; Kahraman, Metin; Erman, Ceyhun (December 2023, Nature Communications)

   Abstract Two major earthquakes (M_W7.8 and M_W7.7) ruptured left-lateral strike-slip faults of the East Anatolian Fault Zone (EAFZ) on February 6, 2023, causing >59,000 fatalities and ~$119B in damage in southeastern Türkiye and northwestern Syria. Here we derived kinematic rupture models for the two events by inverting extensive seismic and geodetic observations using complex 5-6 segment fault models constrained by satellite observations and relocated aftershocks. The larger event nucleated on a splay fault, and then propagated bilaterally ~350 km along the main EAFZ strand. The rupture speed varied from 2.5-4.5 km/s, and peak slip was ~8.1 m. 9-h later, the second event ruptured ~160 km along the curved northern EAFZ strand, with early bilateral supershear rupture velocity (>4 km/s) followed by a slower rupture speed (~3 km/s). Coulomb Failure stress increase imparted by the first event indicates plausible triggering of the doublet aftershock, along with loading of neighboring faults. 

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5. Why Do Great Continental Transform Earthquakes Nucleate on Branch Faults?

   https://doi.org/10.1785/0220240175  

   Stein, Ross S; Bird, Peter (September 2024, Seismological Research Letters)

   Abstract The five Mw≥7.8 continental transform earthquakes since 2000 all nucleated on branch faults. This includes the 2001 Mw 7.8 Kokoxili, 2002 Mw 7.9 Denali, 2008 Mw 7.9 Wenchuan, 2016 Mw 7.8 Kaikōura, and 2023 Mw 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 Mw∼7.8 and 1857 Mw∼7.9 San Andreas earthquakes, but not for the 1990 Mw 7.7 Luzon, 2013 Mw 7.7 Balochistan, and 2023 Mw 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 Mw 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. 

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