Geophysical Journal International

SummaryThe widespread, multi-year crustal deformation induced by megathrust earthquakes (Mw8+) is primarily controlled by the combined effects of continuous aseismic slip on the fault plane (afterslip) and viscoelastic relaxation driven by coseismic stress perturbations in the upper mantle. However, till today it remains a considerable challenge to separate these two mechanisms in geodetic observations. We derived the first 3-year GNSS observations following the 2021 Chignik Mw8.2 earthquake to investigate the mechanisms of postseismic deformation. We established a model capable of simultaneously simulating afterslip and viscoelastic relaxation, and constrained the upper mantle rheology beneath the Alaska Peninsula. The best-fit model effectively reproduces the GNSS observations and reveals a notable viscosity difference between the mantle wedge and the oceanic asthenosphere, with steady-state viscosities of $3 \times {{10}^{18}}$ Pa s and $4 \times {{10}^{19}}$ Pa s, respectively. The inferred mantle wedge viscosity beneath the Alaska Peninsula is lower than the values reported for south-central and southeastern Alaska, suggesting an eastward increase in viscosity along the subduction zone. Two main patches of afterslip are identified during the first 3 years. The patch of up-dip afterslip overlaps with the 1938 Chignik Mw8.3 earthquake rupture zone, and demonstrates a close spatial correlation with the slow slip event in 2018. The above new results enhance our insights into the spatial variability of regional rheology and slip behavior along the Alaska-Aleutian subduction zone.

SummaryNew Q (1/attenuation) models of the Central Alpine Fault provide unprecedented resolution to 20 km depth by incorporating new t* measurements from dense temporary seismograph deployments in the area. The models reveal significant heterogeneity in the crust, with the main Q features broadly similar along-strike the Alpine Fault but varying at length scales of 10-30 km. Accounting for heterogeneity is an important step towards understanding the seismic cycle of M7+ Alpine Fault earthquakes. Our models show the Alpine Fault as a southeast-dipping zone of very (<300) to moderately (600-900) low Q, contrasting sharply with high Q values (Qp>600, Qs>1000) within the Western Province bedrock and high Q values (Qp∼900, Qs∼1200) associated with uplifted Alpine schists to the east. The wealth of previous geologic and geophysical studies along this section of the Alpine Fault support a detailed interpretation of the observed Q values. We interpret the low Q values along the Alpine Fault as resulting from enhanced fracturing within the brittle crust with a proportion of these fractureslikely filled with fluids, which further enhance seismic attenuation through viscous dissipation. In the ductile crust (below ∼8 km depth), low Q values (<400) are likely predominantly caused by grain-size reduction from very high total shear strain and by small amounts of metamorphic fluids. Low Q values of 200-400 at 20-40 km depth downdip of the Alpine Fault and the generally low Q (<600) within the crustal root farther from the Alpine Fault, suggest increasing role of metamorphic fluids relative to that of grain-size reduction with depth and distance from the fault. The updated model also reveals a newly identified zone of low Q east of the Main Divide, approximately 40 km southeast of the Alpine Fault trace. This zone of low Q indicates significant strain accumulation on faults striking along the eastern flank of the Southern Alps, some of which have produced M6+ earthquakes in recent history. These faults represent a considerable seismic hazard for the South Island. The improved dataset and recent velocity models from temporary deployments also allow us to investigate the influence of the initial velocity model on the resulting t* measurements and Q models.

SummaryThe bulk component of the electrical conductivity of a porous material is related to the (connected) porosity and saturation by power-law functions defining the first and second Archie's laws. Recently, it was shown that for porous materials with fractal characteristics, the power-law exponent of Archie's law could be related to the fractal dimension of such materials. Similarly, the real and imaginary parts of the complex-valued surface conductivity are not just proportional to the specific surface area and saturation of the material but to power law functions of these properties defining two additional “interfacial” Archie's laws, which are called the third (saturated case) and fourth (unsaturated case) Archie's laws in this paper. These new laws have been poorly recognized and studied so far. A number of porous materials and especially clay-rich media are multiscale materials characterized by broad distributions of particle and pore sizes. We extend Archie's laws concept to describe the complex conductivity of such materials. We use both numerical simulations in fractal porous materials as well as published experimental datasets to propose a unified physical interpretation of the exponents entering the four Archie's type power-law relationships, which offer an updated complex conductivity model for natural porous media.

SummeryThe seismic hazard due to higher magnitude Himalayan earthquakes largely depends on the geometry of the underthrusting Indian Plate beneath the Himalayas, i.e., the Main Himalayan Thrust (MHT). For an objective assessment of seismic hazard in the central Himalayan seismic gap, we determine the geometry of the Main Himalayan Thrust (MHT) along 4 ∼SW-NE oriented arc-normal seismic profiles covering the central Himalayan seismic gap. We use teleseismic P- wave coda autocorrelation on waveforms recorded at 117 broadband seismic stations spread along these profiles, with an interstation spacing of 3-5 km. The results show that along these seismic profiles, the MHT is mostly of flat-ramp-flat geometry. However, the mid-crustal ramp of the MHT shows variations in its location, dip angle, and width. We also observe variations in the MHT near the Main Frontal Thrust (MFT) and Main Boundary Thrust (MBT). The observed variations in the MHT geometry within the central Himalayan seismic gap thus suggest the possibility of along-strike segmentation of the Himalayan arc, and different seismic hazard scenarios may be present during any possible higher magnitude earthquake in the central Himalayan seismic gap.

SummaryRayleigh wave is widely used for characterizing shallow subsurface structures. The conventional Rayleigh-wave methods rely on the manual picking of dispersion curves, and the dispersion curves of multi-component data are usually merged manually. The manual processing of multi-component Rayleigh waves reduces the efficiency of the method, especially when the data size and the number of modes are large. To overcome these limitations, we develop an energy-based clustering method, namely the Energy-Density-Based Spatial Clustering of Applications with Noise (E-DBSCAN) algorithm. The E-DBSCAN algorithm extracts energy clusters and dispersion curves from a single dispersion image. It considers the dispersion-energy values of the surface wave and is able to pick the dispersion curve more reliably compared with the conventional DBSCAN algorithm. We propose a two-step clustering approach for the automatic picking of multi-mode dispersion curves from multi-component data: we first extract the energy clusters in the dispersion spectra of horizontal- and vertical-component data using E-DBSCAN, respectively, and combine them in the frequency-velocity domain. Then we extract multi-modal dispersion curves from the combined multi-component energy clusters with E-DBSCAN or DBSCAN. Numerical results show that our proposed method has fairly high accuracy and estimates more abundant multi-modal dispersion curves than the single-component method. Two field examples, including an active-source and an ambient-noise dataset, prove the validity of our method and the outperformance of multi-component results compared with the single-component results. Our proposed method has a relatively low dependence on parameter selection and is also applicable to multi-offset data, which is valuable for picking multi-modal dispersion curves.

SummaryRecently, there has been an increasing interest in employing rotational motion measurements for seismic source inversion, structural imaging and ambient noise analysis. We derive reciprocity and representation theorems for rotational motion. The representations express the rotational motion inside an inhomogeneous anisotropic earth in terms of translational and rotational motion at the surface. The theorems contribute to the theoretical basis for rotational seismology methodology, such as determining the moment tensor of earthquake sources.