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1. Two Foreshock Sequences Post Gulia and Wiemer (2019)

   https://doi.org/10.1785/0220200082  

   Dascher-Cousineau, Kelian; Lay, Thorne; Brodsky, Emily E. (July 2020, Seismological Research Letters)

   Abstract Recognizing earthquakes as foreshocks in real time would provide a valuable forecasting capability. In a recent study, Gulia and Wiemer (2019) proposed a traffic-light system that relies on abrupt changes in b-values relative to background values. The approach utilizes high-resolution earthquake catalogs to monitor localized regions around the largest events and distinguish foreshock sequences (reduced b-values) from aftershock sequences (increased b-values). The recent well-recorded earthquake foreshock sequences in Ridgecrest, California, and Maria Antonia, Puerto Rico, provide an opportunity to test the procedure. For Ridgecrest, our b-value time series indicates an elevated risk of a larger impending earthquake during the Mw 6.4 foreshock sequence and provides an ambiguous identification of the onset of the Mw 7.1 aftershock sequence. However, the exact result depends strongly on expert judgment. Monte Carlo sampling across a range of reasonable decisions most often results in ambiguous warning levels. In the case of the Puerto Rico sequence, we record significant drops in b-value prior to and following the largest event (Mw 6.4) in the sequence. The b-value has still not returned to background levels (12 February 2020). The Ridgecrest sequence roughly conforms to expectations; the Puerto Rico sequence will only do so if a larger event occurs in the future with an ensuing b-value increase. Any real-time implementation of this approach will require dense instrumentation, consistent (versioned) low completeness catalogs, well-calibrated maps of regionalized background b-values, systematic real-time catalog production, and robust decision making about the event source volumes to analyze. 

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2. Rupture Process of the 2019 Ridgecrest, California Mw 6.4 Foreshock and Mw 7.1 Earthquake Constrained by Seismic and Geodetic Data

   https://doi.org/10.1785/0120200108  

   Wang, Kang; Dreger, Douglas S.; Tinti, Elisa; Bürgmann, Roland; Taira, Taka’aki (July 2020, Bulletin of the Seismological Society of America)

   null (Ed.)

   ABSTRACT The 2019 Ridgecrest earthquake sequence culminated in the largest seismic event in California since the 1999 Mw 7.1 Hector Mine earthquake. Here, we combine geodetic and seismic data to study the rupture process of both the 4 July Mw 6.4 foreshock and the 6 July Mw 7.1 mainshock. The results show that the Mw 6.4 foreshock rupture started on a northwest-striking right-lateral fault, and then continued on a southwest-striking fault with mainly left-lateral slip. Although most moment release during the Mw 6.4 foreshock was along the southwest-striking fault, slip on the northwest-striking fault seems to have played a more important role in triggering the Mw 7.1 mainshock that happened ∼34  hr later. Rupture of the Mw 7.1 mainshock was characterized by dominantly right-lateral slip on a series of overall northwest-striking fault strands, including the one that had already been activated during the nucleation of the Mw 6.4 foreshock. The maximum slip of the 2019 Ridgecrest earthquake was ∼5  m, located at a depth range of 3–8 km near the Mw 7.1 epicenter, corresponding to a shallow slip deficit of ∼20%–30%. Both the foreshock and mainshock had a relatively low-rupture velocity of ∼2  km/s, which is possibly related to the geometric complexity and immaturity of the eastern California shear zone faults. The 2019 Ridgecrest earthquake produced significant stress perturbations on nearby fault networks, especially along the Garlock fault segment immediately southwest of the 2019 Ridgecrest rupture, in which the coulomb stress increase was up to ∼0.5  MPa. Despite the good coverage of both geodetic and seismic observations, published coseismic slip models of the 2019 Ridgecrest earthquake sequence show large variations, which highlight the uncertainty of routinely performed earthquake rupture inversions and their interpretation for underlying rupture processes. 

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3. The July 2019 Ridgecrest, California, Earthquake Sequence Recorded by Creepmeters: Negligible Epicentral Afterslip and Prolonged Triggered Slip at Teleseismic Distances

   https://doi.org/10.1785/0220190293  

   Bilham, Roger; Castillo, Bryan (January 2020, Seismological Research Letters)

   Abstract We report sequential triggered slip at 271–384 km distances on the San Andreas, Superstition Hills, and Imperial faults with an apparent travel-time speed of 2.2 ± 0.1  km/s, following the passage of surface waves from the 4 July 2019 (17:33:49 UTC) Mw 6.4 and 6 July 2019 (03:19:53 UTC) Mw 7.1 Ridgecrest earthquakes. Slip on remote faults was not triggered instantaneously but developed over several minutes, increasing in duration with distance. Maximum slip amplitudes varied from 10  μm to 5 mm within minutes of slip nucleation, but on the southernmost San Andreas fault slip continued for two months and was followed on 16 September 2019 by a swarm of microearthquakes (Mw≤3.8) near Bombay Beach. These observations add to a growing body of evidence that fault creep may result in delayed triggered seismicity. Displacements across surface faults in the southern epicentral region and on the Garlock fault in the months following the Ridgecrest earthquakes were negligible (<1.1  mm), and they are interpreted to characterize surface strain adjustments in the epicentral region, rather than to result from discrete slip on surface faults. 

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4. 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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5. Slip Complementarity and Triggering between the Foreshock, Mainshock, and Afterslip of the 2019 Ridgecrest Rupture Sequence

   https://doi.org/10.1785/0120200037  

   Qiu, Qiang; Barbot, Sylvain; Wang, Teng; Wei, Shengji (June 2020, Bulletin of the Seismological Society of America)

   null (Ed.)

   ABSTRACT We investigate the deformation processes during the 2019 Ridgecrest earthquake sequence by combining Global Navigation Satellite Systems, strong-motion, and Interferometric Synthetic Aperture Radar datasets in a joint inversion. The spatial complementarity of slip between the Mw 6.4 foreshock, Mw 7.1 mainshock, and afterslip suggests the importance of static stress transfer as a triggering mechanism during the rupture sequence. The coseismic slip of the foreshock concentrates mainly on the east-northeast–west-southwest fault above the hypocenter at depths of 2–8 km. The slip distribution of the mainshock straddles the region above the hypocenter with two isolated patches located to the north-northwest and south-southeast, respectively. The geodetically determined moment magnitudes of the foreshock and mainshock are equivalent to moment magnitudes Mw 6.4 and 7.0, assuming a rigidity of 30 GPa. We find a significant shallow slip deficit (>60%) in the Ridgecrest ruptures, likely resulting from the immature fault system in which the sequence occurred. Rapid afterslip concentrates at depths of 2–6 km, surrounding the rupture areas of the foreshock and mainshock. The ruptures also accelerated viscoelastic flow at lower-crustal depths. The Garlock fault was loaded at several locations, begging the question of possible delayed triggering. 

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