The 2016 January 25 earthquake (M w 6.3) follows in sequence from the1994 May 26 earthquake (M w 6.0) and the 2004 February 24 earthquake (M w 6.4) in the Rif Mountains and Alboran Sea. The earlier two seismic events which were destructive took place on inland conjugate faults, and the third event occurred on an offshore fault. These earthquake sequences occurred within a period of 22 yr at ∼25 km distance and 11-16-km depth. The three events have similar strike-slip focal mechanism solutions with NNE-SSW trending left-lateral faulting for the 1994 and 2016 events and NW-SE trending right-lateral faulting for the 2004 event. This shallow seismic sequence offers the possibility (i) to model the change in Coulomb Failure Function (CFF with low μ including the pore pressure change) and understand fault-rupture interaction, and (ii) to analyse the effect of pore fluid on the rupture mechanism, and infer the clock-time advance. The variation of static stress change has a direct impact on the main shock, aftershocks and related positive lobes of the 2004 earthquake rupture with a stress change increase of 0.7-1.1 bar. Similarly, the 2004 main shock and aftershocks indicate loading zones with a stress change (>0.25 bar) that includes the 2016 earthquake rupture. The tectonic loading of 19-24 nanostrain yr −1 obtained from the seismicity catalogue of Morocco is comparable to the 5.0 × 10 17 N•m yr −1 seismic strain release in the Rif Mountains. The seismic sequence is apparently controlled by the poroelastic properties of the seismogenic layer that depend on the undrained and drained fluid conditions. The short interseismic period between main shocks and higher rate of aftershocks with relatively large magnitudes (4 < M w < 5.5) implies the pore-fluid physical effect in undrained and drained conditions. The stress-rate ranges between 461 and 582 Pa yr −1 with a CFF of 0.2-1.1 bar. The computed clock-time advance reaches 239 ± 22 yr in agreement with the ∼10 yr delay between main shocks. The calculated static stress change of 0.9-1.3 bar, under pore-fluid stimulus added with well-constrained geodetic and seismic strain rates are critical for any seismic hazard assessment.

On 15 May 2020 at 11:03 (UTC) a shallow earthquake with a Magnitude Mw 6.5 struck the Central Walker Lane in the Mina Deflection region. This shallow event offers the possibility to model the change in Coulomb failure function (ΔCFF) including the historical events and to analyze the effect of fluid redistribution on fault ruptures. The static stress change modeling shows that the 1932 Cedar Mountain (Mw 7.1) earthquake increased ΔCFF on the 2020 Monte Cristo left‐lateral rupture by an average value of ∼2 bar suggesting a fault interaction in the context of conjugate strike‐slip faults. Also, the ΔCFF caused by the Monte Cristo earthquake shows a 0.1–0.5 bar increase on nearby right‐lateral fault planes at 8 km depth. In contrast to the 2019 Ridgecrest sequence (Mw 6.4 and Mw 7.1), our poroelastic stress change modeling of the 2020 Monte Cristo earthquake shows no apparent correlation between the positive stress change values and fluid redistribution along the Monte Cristo conjugated ruptures. Nevertheless, the Coulomb modeling using adapted poroelastic solutions shows that the 2020 Monte Cristo mainshock increased stresses with a value of 0.2–0.9 bar south of the Mina Deflection along the White Mountains and Fish Lake Valley fault zones. The stress values combined with a strain rate of 40–50 nanostrain/yr represent a shift of 8%–15% of the recurrence time interval for a large earthquake along the White Mountains and Fish Lake Valley faults suggesting an elevated pore‐fluid effect for faults reactivation in both undrained and drained fluid conditions.

No abstract

No abstract

No abstract

This paper focuses on the study of the temporal evolution of seismicity and the role of fluids during major earthquake sequences that occurred in the central Apennines and Southern Walker Lane belt-Eastern California Shear Zone over the last two decades: The 1997 Colfiorito sequence, the 2009 L’Aquila sequence, the 2016 Amatrice-Norcia sequence, and the 2019 Ridgecrest sequence. The availability of different high-quality seismic catalogs offers the opportunity to evaluate in detail the temporal evolution of the earthquake's size distribution (or b-value) and propose a physical explanation based on the effect of the fluid flow process in triggering seismicity. For all seismic sequences, the b-value time series show a gradual decrease from a few months to one year before mainshocks. The gradual decrease in the b-value is interpreted in terms of coupled fluid-stress intensity as a gradual increase in earthquake activity due essentially to the short-term to intermediate-term pore-fluid fluctuations. For the 2016 Amatrice-Norcia sequence and the 2019 Ridgecrest sequence, the temporal variation of the b-value during the foreshock sequence is characterized by a double b-value minimum separated by a short-lived b-value increase as observed in laboratory experiments on water-saturated rocks. Based on laboratory experiment results, the observed short–term fluctuation of the b-value is presented here as an accelerating crack growth due essentially to the fluid flow instability. Despite that the occurrence of seismic precursors could have been predictable in areas with high dense seismic networks, the different b-value time series show difficulty to establish a correspondence between the duration of the foreshock activity and the magnitude of the next largest expected earthquake. This may suggest that the spatial and temporal evolution of fluid migration controls the size of the ruptures.

No abstract

No abstract