Creep on the Sargent Fault over the Past 50 Yr from Alignment Arrays with Implications for Slip Transfer between the Calaveras and San Andreas Faults, California

ABSTRACT The 55-km-long Sargent fault connects the creeping Calaveras fault with the locked San Andreas fault through the Santa Cruz Mountains west of Gilroy, California. The position of the Sargent fault between these two faults may have implications for slip transfer and strain accumulation between a creeping and locked fault. The detection and measurement of creep on the Sargent fault would indicate where interseismic strain is accumulating adjacent to these neighboring faults. In 1969, two alignment arrays separated by 3.7 km were installed across the central section of the Sargent fault to investigate potential creep. These arrays were measured in 1970 and 1975, and comparison of these measurements yielded a creep rate of 3.4 ± 0.6 mm/yr across two fault strands in the northern array; results from the southern array were never published. In 2019 and 2020, we resurveyed both arrays using a total station and analyzed the results to determine accumulated fault creep. Our results show that between 1970 and 2020, a period of 49.3 yr, the northern array was dextrally offset 164 ± 25 mm across the same two fault strands that were active in the 1970s, yielding an average creep rate of 3.3 ± 1.3 mm/yr. Thus, it appears that the 5 and 50 yr creep rates at this site are similar. The southern array, which may not span the entire fault zone, was dextrally offset 84 ± 13 mm across two fault strands between 1970 and 2019, yielding an average creep rate of 1.7 ± 0.8 mm/yr over 48.9 yr. These recent surveys document continued creep on the Sargent fault, which may reduce seismic strain accumulation and therefore seismic hazard. However, continued aseismic slip on this fault may result in the redistribution of stress and strain to adjacent faults and should be an area of continued study.

Tidal Calibration of Multicomponent Borehole Strainmeters and Validation of the Method Using Surface Waves

We conducted in-situ calibration of fifteen multicomponent borehole strainmeters deployed in and around the expected focal zones of the Nankai megathrust earthquake. The in-situ calibration method compares tidal strain observed by the borehole strainmeters with predicted tidal strains from the solid Earth’s tide and oceanic tidal loading. Then we obtained a calibration matrix to transfer observed strain data to the regional strain field. We estimated the oceanic tidal loading accurately using a Green’s function, which takes the depth of deployment into consideration. We calculated four sets of calibration matrices using combinations of any three of a group of four gauges as well as a calibration matrix using all four gauges. The estimated calibration matrix was validated by comparing observed seismic strain waves after applying the calibration matrix with theoretical seismic strain waves excited by the 2010 Chile earthquake (Mw 8.8). The in-situ calibration was found to be appropriate for all eleven Ishii-type borehole strainmeters and for one of the four Gladwin Tensor Strainmeters (GTSMs). It was also effective with respect to two shear strains for two of the other three GTSMs.

Study on Inelastic Strain-Based Seismic Fragility Analysis for Nuclear Metal Components

The main purpose of this study is to investigate the feasibility of the seismic fragility analysis (FA) with the strain-based failure modes for the nuclear metal components retaining pressure boundary. Through this study, it is expected that we can find analytical ways to enhance the high confidence of low probability of failure (HCLPF) capacity potentially contained in the conservative seismic design criteria required for the nuclear metal components. Another goal is to investigate the feasibility of the seismic FA to be used as an alternative seismic design rule for beyond-design-basis earthquakes. To do this, the general procedures of the seismic FA using the inelastic seismic analysis for the nuclear metal components are investigated. Their procedures are described in detail by the exampled calculations for the surge line nozzles connecting hot leg piping and the pressurizer, known as one of the seismic fragile components in NSSS (Nuclear Steam Supply System). To define the seismic failure modes for the seismic FA, the seismic strain-based design criteria, with two seismic acceptance criteria against the ductile fracture failure mode and fatigue-induced failure mode, are used in order to reduce the conservatism contained in the conventional stress-based seismic design criteria. In the exampled calculation of the inelastic seismic strain response beyond an elastic regime, precise inelastic seismic analyses with Chaboche’s kinematic and Voce isotropic hardening material models are used. From the results of the seismic FA by the probabilistic approach for the exampled target component, it is confirmed that the approach of the strain-based seismic FA can extract the maximum seismic capacity of the nuclear metal components with more accurate inelastic seismic analysis minimizing the number of variables for the components.

Seismotectonic Analysis of the 2017 Moiyabana Earthquake (MW 6.5; Botswana), Insights from field investigations, aftershock and InSAR studies

<p>The 3 April 2017 M<sub>W</sub> 6.5, Moiyabana (Botswana) earthquake occurred in the continental interior of the African plate and in a seismogenic region previously considered as stable. We analyse the mainshock and aftershock sequence based on a local seismic network and local seismotectonic characteristics. The earthquake rupture geometry is constrained with more than 1,000 aftershocks recorded over a period of three months and from the InSAR analysis of Sentinel-1 images (ascending orbit). The mainshock (25.134 E, 22.565 S; depth 22 ± 3 km) was followed by more than 500 events of magnitude M ≥ 0.8 recorded in April 2017 including the largest aftershock (M<sub>W</sub> 4.6 on the 5 April 2017). Focal mechanism solutions of the mainshock and aftershocks display predominance of NW-SE trending and NE dipping normal faulting. Stress inversion of focal mechanisms obtained from the mainshock and aftershock database are compatible with a NE-SW extension under normal faulting regime. The InSAR study shows fringes with two lobes with 4 to 6 cm coseismic slip on a NW-SE elongated and 30-km-long surface deformation consistent with the mainshock location and normal faulting mechanism. The modelling of surface deformation provides the earthquake rupture dimension at depth with ~ 1 m maximum slip on a fault plane striking 315°, dipping 45°, -80° rake and with M<sub>o</sub> 7.12 10<sup>18</sup> Nm Although the seismic strain rate is of low level, the occurrence of the 2017 Moiyabana earthquake, followed by an aftershock sequence in the central Limpopo belt classifies the intraplate region as an active plate interior. </p>

Dynamic parallel-to-grain compressive properties of three softwoods under seismic strain rates: tests and constitutive modeling

Softwood is commonly used in timber structures, whose dynamic compressive properties along the grain direction under seismic rate actions are of major concern. In the present study, 108 samples were examined from three species of softwoods (spruce, Dahurian larch and pine) and subjected to four levels of strain rates (10−4 s−1, 10−3 s−1, 10−2 s−1 and 0.1 s−1) under parallel-to-grain monotonic and repeated compression. The seismic strain rate effects on softwood were investigated by observing the failure mechanisms, engineering stress-strain curves, initial crushing strength, elastic modulus and peak strain. Results demonstrated the high sensitivity of softwood to seismic strain rates. Finally, a rate-dependent constitutive model was developed to predict the stress-strain relationship (both monotonic and repeated loading cases) of softwood under different seismic strain rates.

The source scaling of swarm-genic slow slip events

<p>Slow slip events (SSEs) are slow fault ruptures that do not excite detectable seismic waves although they are often accompanied by some forms of seismic strain release, e.g., clusters of low- and very-low frequency earthquakes, and/or episodic or continuous non-volcanic tremor (i.e. tremor-genic SSEs) and earthquake swarms (swarm-genic SSEs). At subduction zones, increasing evidence indicates that aseismic slip and seismic strain release in the form of non-volcanic tremor represent the evolution of slow fracturing. In addition, aseismic slip rate modulates the release of seismic slip during tremor-genic SSEs. No general agreement has been reached, however, on whether source duration-moment scaling of SSEs is linear or follows that of ordinary earthquakes (cubic). To date, investigations on the source scaling has been based on global compilations of tremor-genic SSEs while no studies have looked into the source scaling of swarm-genic SSEs.</p><p>We present the first compilation of source parameters of swarm-genic slow slip events occurring in subduction zones as well as in extensional, transform and volcanic environments. We find for swarm-genic SSEs a power-law scaling of aseismic to seismic moment release during episodes of slow slip that is independent of the tectonic setting. The earthquake productivity, i.e., the ratio of seismic to aseismic moment released, of shallow SSEs is on average higher than that of deeper ones and scales inversely with rupture velocity. The inferred source scaling indicates a strong interplay between the evolution of aseismic slip and the associated seismic response of the host medium and that swarm-genic SSEs and tremor-genic SSEs arise from similar fracturing mechanisms. Depth dependent rheological conditions modulated by fluid pore pressure, temperature and density of asperities appear to be the main controls on the scaling. Large SSEs have systematically high earthquake productivity suggesting static stress transfer as an additional factor in triggering swarms of ordinary earthquakes. Our data suggest that during the slow slip evolution the proportion of seismic strain release is always smaller than the aseismic part although transient changes in stress and fault rheology imparted by swarm-genic SSEs can lead to delayed triggering of major and devastating earthquakes like in the Tohoku, Iquique and L’Aquila cases. The evidence of source scaling reported here will help constraining theoretical models of SSEs rupture propagation and seismic hazard assessments that should take into account the new scaling between aseismic and seismic moment release. </p>

Experience in synchronous observation of seismic-strain oscillations of the Earth by the spaced laser interferometers

The first results of unique experiments on the synchronous registration of seismic-strain oscillations of the Earth’s surface by three laser interferometers-deformographs (strainmeters) spaced 6740 km apart are presented. Two 100-meter laser interferometers at the Fryazino site (Moscow Region) and the 18-meter laser strainmeter at the observation point of Karymshina (Kamchatka Peninsula) were applied. The frequency-stabilized and thermally controlled lasers and the interferogram registration systems of compensation and modulation types providing an absolute instrumental resolution of 0.1-0.01 nm were used. The results of data analysis in sessions of synchronous operation of these instruments during 2016-2020 were obtained and discussed.