Earthquake Science最新文献

Ambient noise tomography, when applied to a dense linear seismic array, has the capability to provide detailed insights into the fine velocity structures across diverse tectonic settings. The linear station arrangement naturally generates parallel and concentrated ray paths along the array trend. This unique geometry requires specific optimization of the inversion methodology and model parameterization. The Bayesian-based transdimensional inversion method, characterized by its fully non-linear nature and high degree of freedom in parameter settings, offers a powerful tool for ambient noise inversion. To effectively adapt this method to a linear array layout, we propose a modification to the Voronoi cell tessellation built in the transdimensional method. By introducing spatial priority to the Voronoi kernels, we strategically increased the density of Voronoi cells along the direction of the array. We then applied the modified approach to a linear seismic array in the North China Craton and validated its robustness through phase velocity images and resolution tests. Our improved non-uniform sampling technique in the 2-D model space accelerates convergence while simultaneously enhancing model accuracy. Compared with the conventional damped least-squares method, the proposed algorithm revealed a shear-wave velocity map with notable low-velocity anomalies situated in the middle and lower crust beneath the borders of the Ordos block and its surrounding orogenic belt. Aligned with the crustal structures revealed by receiver function and electrical imaging, our findings indicated that the western and eastern margins of the Ordos block had experienced intensive crustal wedge deformation and re-melting, respectively.

We build a high-resolution early aftershock catalog for the 2023 SE Türkiye seismic sequence with PALM, a seamless workflow that sequentially performs phase picking, association, location, and matched filter for continuous data. The catalog contains 29,519 well-located events in the two mainshocks rupture region during 2023-02-01–2023-02-28, which significantly improves the detection completeness and relocation precision compared to the public routine catalog. Employing the new PALM catalog, we analyze the structure of the seismogenic fault system. We find that the Eastern Anatolian Fault (EAF) that generated the first M_W7.9 mainshock is overall near-vertical, whereas complexities are revealed in a small-scale, such as subparallel subfaults, unmapped branches, and stepovers. The seismicity on EAF is shallow (<15 km) and concentrated in depth distribution, indicating a clear lock-creep transition. In contrast, the Sürgü Fault (SF) that is responsible for the second M_W7.8 mainshock is shovel-shaped for the nucleation segment and has overall low dip angles (∼40°–80°). Aftershocks on the SF distribute in a broad range of depth, extending down to ∼35 km. We also analyze the temporal behavior of seismicity, discovering no immediate foreshocks within ∼5 days preceding the first mainshock, and no seismic activity on the SF before the second mainshock.

In this study, we investigate how a stress variation generated by a fault that experiences transient postseismic slip (TPS) affects the rate of aftershocks. First, we show that the postseismic slip from Rubin-Ampuero model is a TPS that can occur on the main fault with a velocity-weakening frictional motion, that the resultant slip function is similar to the generalized Jeffreys-Lomnitz creep law, and that the TPS can be explained by a continuous creep process undergoing reloading. Second, we obtain an approximate solution based on the Helmstetter-Shaw seismicity model relating the rate of aftershocks to such TPS. For the Wenchuan sequence, we perform a numerical fitting of the cumulative number of aftershocks using the Modified Omori Law (MOL), the Dieterich model, and the specific TPS model. The fitting curves indicate that the data can be better explained by the TPS model with a B/A ratio of approximately 1.12, where A and B are the parameters in the rate- and state-dependent friction law respectively. Moreover, the p and c that appear in the MOL can be interpreted by the B/A and the critical slip distance, respectively. Because the B/A ratio in the current model is always larger than 1, the model could become a possible candidate to explain aftershock rate commonly decay as a power law with a p-value larger than 1. Finally, the influence of the background seismicity rate r on parameters is studied; the results show that except for the apparent aftershock duration, other parameters are insensitive to r.

Investigating spatiotemporal changes in crustal stress associated with major earthquakes has implications for understanding seismogenic processes. However, in individual earthquake cases, the characteristics of the stress after it reaches its maximum value are rarely discussed. In this study, we use the 2021 M_S6.4 Yangbi earthquake in Yunnan, China and events of magnitudes M_L ≥ 3.0 occurred in the surrounding area in the previous 11 years to investigate the spatiotemporal evolution of apparent stress. The results indicate that apparent stress began to increase in January 2015 and reached a maximum in January 2020. Apparent stress then remained at a high level until October 2020, after which it declined considerable. We suggest that the stress was in the accumulation stage from January 2015 to January 2020, and entered the meta-instability stage after October 2020. During the meta-instability stage, the zone of decreasing stress expanded continuously and the apparent stress increased around the Yangbi earthquake source region. These features are generally consistent with the results of laboratory rock stress experiments. We propose that apparent stress can be a good indicator for determining whether the stress at a specific location has entered the meta-instability stage and may become the epicenter of an impending strong earthquake.

The Izu-Bonin subduction zone in the Northwest Pacific is an ideal location for understanding mantle dynamics such as cold lithosphere subduction. The slab produces a lateral thermal anomaly, inducing local topographic changes at the boundary of a post-spinel phase transformation, considered to be the origin of the ‘660-km discontinuity.’ In this study, the short-period (1–2 Hz) S-to-P conversion phase S660P was used to obtain the fine-scale structure of the discontinuity. More than 100 earthquakes that occurred from the 1980s to the 2020s and were recorded by high-quality seismic arrays in the United States and Europe were analyzed. A discontinuity in the ambient mantle with an average depth of ∼670 km was found beneath the 300–400-km event zone in the northern Bonin region near 33°N. Meanwhile, the ‘660-km discontinuity’ has been pushed upward, away from the slab, possibly because of a hot upwelling mantle plume. In the central part of the subduction zone, the 660-km discontinuity is depressed to an average depth of (690 ± 5) km within the slab at approximately 150 km below the coldest slab core, indicating a (300 ± 100) °C cold anomaly estimated using a post-spinel transformation Clapeyron slope of (−2.0 ± 1.0) MPa/K. In southern Bonin near 28°N, the discontinuity was found to be further depressed at an average depth of (695 ± 5) km below the deepest event and with a focal depth of ∼550 km. The discontinuity is located where the slab bends abruptly to become sub-horizontal toward the west-southwest. Near the zone of the isolated Bonin Super Deep Earthquake, which occurred at ∼680 km on May 30, 2015, the discontinuity is depressed to ∼700 km, suggesting a near-vertical penetrating slab and an S-to-P conversion in the coldest slab core, where a large low-temperature anomaly should exist.

Seismic waves generated by an earthquake can produce dynamic perturbations in the Earth’s gravity field before the direct arrival of P-waves. Observations of these so-called prompt elasto-gravity signals by ground-based gravimeters and broadband seismometers have been reported for some large events, such as the 2011 M_W9.1 Tohoku earthquake. Recent studies have introduced prompt gravity strain signals (PGSSs) as a new type of observable seismic gravity perturbation that can be used to measure the spatial gradient of the perturbed gravity field. Theoretically, these types of signals can be recorded by in-development instruments termed gravity strainmeters, although no successful detection has been reported as yet. Herein, we propose an efficient approach for PGSSs based on a multilayered spherical Earth model. We compared the simulated waveforms with analytical solutions obtained from a homogeneous half-space model, which has been used in earlier studies. This comparison indicates that the effect of the Earth’s structural stratification is significant. With the help of the new simulation approach, we also demonstrated how the PGSSs depend on the magnitude of the seismic source. We further conducted synthetic tests estimating earthquake magnitude using gravity strain signals to demonstrate the potential application of this type of signal in earthquake early warning systems. These results provide essential information for future studies on the synthesis and application of earthquake-induced gravity strain signals.

Earthquake-induced gravity variation refers to changes in the earth’s gravity field associated with seismic activities. In recent years, development in the theories has greatly promoted seismic deformation research, laying a solid theoretical foundation for the interpretation and application of seismological gravity monitoring. Traditional terrestrial gravity measurements continue to play a significant role in studies of interseismic, co-seismic, and post-seismic gravity field variations. For instance, superconducting gravimeter networks can detect co-seismic gravity change at the sub-micro Gal level. At the same time, the successful launch of satellite gravity missions (e.g., the Gravity Recovery and Climate Experiment or GRACE) has also facilitated applied studies of the gravity variation associated with large earthquakes, and several remarkable breakthroughs have been achieved. The progress in gravity observation technologies (e.g., GRACE and superconducting gravimetry) and advances in the theories have jointly promoted seismic deformation studies and raised many new research topics. For example, superconducting gravimetry has played an important role in analyses of episodic tremor, slow-slip events, and interseismic strain patterns; the monitoring of transient gravity signals and related theories have provided a new perspective on earthquake early warning systems; the mass transport detected by the GRACE satellites several months before an earthquake has brought new insights into earthquake prediction methods; the use of artificial intelligence to automatically identify tiny gravity change signals is a new approach to accurate and rapid determination of earthquake magnitude and location. Overall, many significant breakthroughs have been made in recent years, in terms of the theory, application, and observation measures. This article summarizes the progress, with the aim of providing a reference for seismologists and geodetic researchers studying the phenomenon of gravity variation, advances in related theories and applications, and future research directions in this discipline.

On September 16, 2021, a M_S6.0 earthquake struck Luxian County, one of the shale gas blocks in the Southeastern Sichuan Basin, China. To understand the seismogenic environment and its mechanism, we inverted a fine three-dimensional S-wave velocity model from ambient noise tomography using data from a newly deployed dense seismic array around the epicenter, by extracting and jointly inverting the Rayleigh phase and group velocities in the period of 1.6–7.2 s. The results showed that the velocity model varied significantly beneath different geological units. The Yujiasi syncline is characterized by low velocity at depths of ∼ 3.0–4.0 km, corresponding to the stable sedimentary layer in the Sichuan Basin. The eastern and western branches of the Huayingshan fault belt generally exhibit high velocities in the NE-SW direction, with a few local low-velocity zones. The Luxian M_S6.0 earthquake epicenter is located at the boundary between the high- and low-velocity zones, and the earthquake sequences expand eastward from the epicenter at depths of 3.0–5.0 km. Integrated with the velocity variations around the epicenter, distribution of aftershock sequences, and focal mechanism solution, it is speculated that the seismogenic mechanism of the main shock might be interpreted as the reactivation of pre-existing faults by hydraulic fracturing.

A M_S6.8 earthquake occurred on 5th September 2022 in Luding county, Sichuan, China, at 12: 52 Beijing Time (4:52 UTC). We complied a dataset of PGA, PGV, and site v_S30 of 73 accelerometers and 791 Micro-Electro-Mechanical System (MEMS) sensors within 300 km of the epicenter. The inferred v_S30 of 820 recording sites were validated. The study results show that: (1) The maximum horizontal PGA and PGV reaches 634.1 Gal and 71.1 cm/s respectively. (2) Over 80% of records are from soil sites. (3) The v_S30 proxy model of Zhou J et al. is superior than that of Wald and Allen and performs well in the study area. The dataset was compiled in a flat file that consists the information of strong-motion instruments, the strong-motion records, and the v_S30 of the recording sites. The dataset is available at https://www.seismisite.net.

The northeastern part of the Tibetan Plateau is a region where different tectonic blocks collide and intersect, and large earthquakes are frequent. Global Navigation Satellite System (GNSS) observations show that tectonic deformation in this region is strong and manifests as non-uniform deformation associated with tectonic features. S-wave splitting studies of near-field seismic data show that seismic anisotropy parameters can also reveal the upper crustal medium deformation beneath the reporting station. In this paper, we summarize the surface deformation from GNSS observations and crustal deformation from seismic anisotropy data in the northeastern Tibetan Plateau. By comparing the principal compressive strain direction with the fast S-wave polarization direction of near-field S-wave splitting, we analyzed deformation and its differences in surface and upper crustal media in the northeastern Tibetan Plateau and adjacent areas. The principal compressive strain direction derived from GNSS is generally consistent with the polarization direction of fast S-waves, but there are also local tectonic regions with large differences between them, which reflect the different deformation mechanisms of regional upper crustal media. The combination of GNSS and seismic anisotropy data can reveal the depth variation characteristics of crustal deformation and deepen understanding of three-dimensional crustal deformation and the deep dynamical mechanisms underlying it. it.