China Seismic Research Articles

Earthquake Sequence Simulation and Hazard Analysis in the Tianshan Orogenic Belt and Adjacent Areas

ABSTRACT The Tianshan orogenic belt is located in central Eurasia. Its tectonic deformation and seismicity are primarily influenced by the far-field effect of the India–Eurasia collision. It is one of the regions with the most intensive seismicity in China. Understanding the mechanisms of earthquake nucleation in this region is a hot topic in the seismicity community. The numerical simulation of earthquake sequences is an efficient measure for comprehensively exploring earthquake occurrence characteristics, tectonic loading, and subsequence patterns. It is also an important method for predicting and mitigating earthquake disasters. This study utilized a finite-element model to investigate the evolution of regional historical earthquakes via the split-node method. Coulomb failure stress analyses from interactions of simulated historical earthquakes since 1900 indicate that the Taxkorgan fault may be prone to the next significant earthquake event.

Applied experimental research on in-situ online monitoring instrument for soil radon in fault zone

As an emerging method for seismic precursory signals in underground fluid, the reliability and stability of in-situ online monitoring devices for soil radon are crucial performance indicators that are directly related to the successful implementation of continuous monitoring of soil radon in seismic areas. This study conducted laboratory testing and field applications of a new in-situ online monitoring instrument for seismic soil radon, utilizing the experimental testing conditions provided by the radon monitoring instrument detection platform of the China Earthquake Administration and the natural experimental site at Xianshuihe Fault Zone in the China Seismic Experimental Site. The results indicate that laboratory measurement repeatability of the instrument is 5.53%, the relative intrinsic error of radon volume response activity is 1.53%, and the response capability of hourly sampling at high radon volume response activity (greater than 1.0 × 105 Bq/m3) lags behind the standard instrument by 2 h, essentially meeting the requirements for seismic observation. Under field conditions, the instruments exhibit good synchronization in response at different depths at the same location. However, further improvements are needed to enhance the consistency of long-term operation in the field, as well as the adaptability to outdoor self-powering, data transmission, and environmental conditions.

Background and clustering characteristics of recent seismicity in Southwestern China

SUMMARY This study analysed seismicity in southwestern China (1 January 2008 to 30 June 2021) using the earthquake catalogue compiled by the China Earthquake Network Center and four different space–time Epidemic-Type Aftershock Sequence models: the 2-D point-source (PS) model, the 2-D finite-source (FS) model, the 3-D PS model and the 3-D FS model. Our objective was to understand the features of the background seismicity and the patterns of earthquake clusters to better evaluate the regional seismic hazard. We carefully investigated the aftershock sequences that followed 7 of the 10 MS ≥ 6.0 earthquakes that have struck this region since the occurrence of the 2008 Wenchuan MS 8.0 earthquake [i.e. the Panzhihua (31 August 2008; MS 6.0), Yaoan (9 July 2009; MS 6.0), Lushan (20 April 2013; MS 7.0), Ludian (3 August 2014; MS 6.5), Jinggu (7 October 2014; MS 6.6), Kangding (11 November 2014; MS 6.3) and Yangbi (21 May 2021; MS 6.4) earthquakes]. Our results revealed the following. (1) The background seismicity level for natural earthquakes is usually stable but can experience sudden change due to major events, such as the 2014 Ludian MS 6.5, and the 2014 Jinggu MS 6.6 events. Such changes in the background rate can reach 50 per cent. (2) Reservoir-induced earthquakes substantially increase the level of regional seismicity, indicating that they cannot be ignored when analysing natural seismicity and evaluating regional earthquake hazards. (3) Events triggered directly by the main shock occur mostly in regions adjacent to areas with large coseismic slip, showing a pattern complementary to the main shock ruptures.

Three dimensional velocity model and its tectonic implications at China Seismic Experimental Site, eastern margin of the Tibetan Plateau

Three dimensional velocity model and its tectonic implications at China Seismic Experimental Site, eastern margin of the Tibetan Plateau

Multiscale structures of crust-mantle beneath the South China block and their geodynamic implication

The South China block is an ideal location in which to study the impact of paleo-Pacific plate subduction on the upper mantle and crustal structures of the overriding plate, as well as the mechanism of lithospheric thinning. By integrating data from permanent seismic stations of the China Seismic Network and a dense seismic array at Xuefeng Mountain, this study employed P- and S-wave receiver functions (RFs) to image multiscale structures of crust-mantle across the South China block, thereby providing constraints for a comprehensive understanding of its tectonic evolution. The main findings are as follows: (1) The thickness of lithosphere and crust gradually decreases from northwest (NW) to southeast (SE). (2) Beneath the Xuefeng Mountain uplift zone, the P-to-S conversion from the 660 km discontinuity arrives earlier than that predicted from a one-dimensional model, other weak discontinuities can be traced in the upper mantle, including the lithosphere-asthenosphere boundary, the crust becomes thinner, and the Conrad discontinuity becomes deeper. Referring to previous results from tomography and geological reconstructions, we deduce that lithospheric delamination might have occurred beneath the Xuefeng Mountain uplift zone, and the delaminated lithosphere penetrated the upper mantle and reached the mantle transition zone.

Heterogeneities of the lithospheric thermal structure and rheology control Cenozoic intracontinental deformation in southeast China

It still remains elusive how the rheology of the lithosphere influences intracontinental deformation. We constructed a thermo-rheological model of the lithosphere in southeast China, a typical intracontinental deformation area, to address this issue. The thermal model was derived from updated heat flow data assuming steady-state heat conduction, using xenoliths and Pn seismic velocity data as constraints. This thermal model was integrated with a compositional and structural model of the lithosphere to estimate the strength distribution. Remarkable heterogeneities in the thermo-rheological structure were obtained. Most of southeast China is characterized by a large mantle heat flow and high temperature, thin and weak lithosphere, except for the western Yangtze Craton where a cold and rigid lithosphere exists. The lithosphere thins southeastward from 200 km in the craton interior to 70 km in the northern South China Sea. Both the ‘jelly sandwich’ and ‘crème brûlée’ rheological profiles for the continental lithosphere exist in southeast China, depending on the geothermal condition. The seismicity in southeast China is restricted to areas cooler than 600 °C, which are rheologically weak and characterized by strength transition zones. The Mesozoic flat-slab subduction of the Paleo-Pacific Ocean and far-field effects of the Cenozoic Indo-Asia collision influence the thermo-rheological pattern in southeast China, and the thermo-rheological heterogeneities control the Cenozoic intracontinental deformation.

Extent of the lower crustal flow constrained by the rheological structure at the eastern boundary of the Chuan-Dian Rhombus Block

The Chuan-Dian Rhombus Block is the main channel for materials escaping southeastward from the Tibetan Plateau. Verifying the extent of the lower crustal flow and its impact on the lithospheric deformation, isostatic state, and seismicity in the Chuan-Dian Rhombus Block is of vital importance. Based on data from the China Seismic Experimental Site, the current study analyzes the rheological structure of the eastern boundary of the Chuan-Dian Rhombus Block (i.e., the Xianshuihe–Xiaojiang Fault System). The results reveal a low-viscosity zone of 1020 Pa∙s, with a thickness of approximately 20 km in the middle–lower crust beneath the Xianshuihe fault. The low-viscosity zone extends southward and gradually thins until it basically disappears at the Xiaojiang fault. Our results indicate that the extent of the lower crustal flow is restricted in the Xianshuihe fault and the northernmost part of the Anninghe–Zemuhe fault and does not extend to the Xiaojiang fault. The crust of the Xianshuihe fault is notably thickened and in an isostatic disequilibrium state. The Xiaojiang fault maintains a relatively thick seismogenic layer and isostatic equilibrium.

Support Vector Machine-Based On-Site Prediction for China Seismic Instrumental Intensity from P-Wave Features

Support Vector Machine-Based On-Site Prediction for China Seismic Instrumental Intensity from P-Wave Features

Temporal distribution model and occurrence probability of M ≥ 6.5 earthquakes in North China Seismic Zone

Temporal distribution model and occurrence probability of M ≥ 6.5 earthquakes in North China Seismic Zone

Relocation of the 8 January 2022 Menyuan (Ms 6.9), China, earthquake sequence: A conjugated type of rupturing event with a seismic gap in the Qilian-Haiyuan fault in northeast Tibet

On 8 January 2022, an earthquake with Ms 6.9 struck Menyuan County, Qinghai Province, China, which provides an opportunity to investigate the seismotectonics of the Qilian-Haiyuan fault in northeast Tibet. We collect seismic observation data from China Seismic Network Center from 8 January to 6 February 2022 around the 2022 Menyuan mainshock. The double-difference relocation algorithm is applied to relocate 904 earthquakes, and 820 high-precision hypocenter locations are obtained. The mainshock epicenter was located at 10.2 km depth in the Tuolaishan fault. The aftershock sequence extended towards nearly E-W along the Tuolaishan fault and NW-SE along the Lenglongling fault, parts of the Qilian-Haiyuan fault, but there were fewer aftershocks at the beginning of the rupture near the intersection between the Lenglongling and Tuolaishan faults, suggesting a conjugated type of rupturing with a seismic gap. Integrating with focal mechanism solutions of mainshock and one large aftershock, we suggest that the rupture is a left-lateral strike-slip event with a high dip. The stress change around the faults intersection. The 1986 Ms 6.4 earthquake may trigger the 2016 Ms 6.4 earthquake, and then the extrusion stress could transfer to around the 2022 Ms 6.9 earthquake source area due to the Indo-Eurasian collision. Previous tomographic models revealed low-velocity anomalies under the Menyuan earthquakes, reflecting that fluids could decrease friction coefficient and normal stress on the fault planes and further promote the generation of these earthquakes. These fluids could be contained in the lower crustal flow and mantle upwelling flow under northeast Tibet.

USTC-Pickers: a Unified Set of seismic phase pickers Transfer learned for China

Current popular deep learning seismic phase pickers like PhaseNet and EQTransformer suffer from performance drop in China. To mitigate this problem, we build a unified set of customized seismic phase pickers for different levels of use in China. We first train a base picker with the recently released DiTing dataset using the same U-Net architecture as PhaseNet. This base picker significantly outperforms the original PhaseNet and is generally suitable for entire China. Then, using different subsets of the DiTing data, we fine-tune the base picker to better adapt to different regions. In total, we provide 5 pickers for major tectonic blocks in China, 33 pickers for provincial-level administrative regions, and 2 special pickers for the Capital area and the China Seismic Experimental Site. These pickers show improved performance in respective regions which they are customized for. They can be either directly integrated into national or regional seismic network operation or used as base models for further refinement for specific datasets. We anticipate that this picker set will facilitate earthquake monitoring in China.

Benchmark on the accuracy and efficiency of several neural network based phase pickers using datasets from China Seismic Network

Seismic phase pickers based on deep neural networks have been extensively used recently, demonstrating their advantages on both performance and efficiency. However, these pickers are trained with and applied to different data. A comprehensive benchmark based on a single dataset is therefore lacking. Here, using the recently released DiTing dataset, we analyzed performances of seven phase pickers with different network structures, the efficiencies are also evaluated using both CPU and GPU devices. Evaluations based on F1-scores reveal that the recurrent neural network (RNN) and EQTransformer exhibit the best performance, likely owing to their large receptive fields. Similar performances are observed among PhaseNet (UNet), UNet++, and the lightweight phase picking network (LPPN). However, the LPPN models are the most efficient. The RNN and EQTransformer have similar speeds, which are slower than those of the LPPN and PhaseNet. UNet++ requires the most computational effort among the pickers. As all of the pickers perform well after being trained with a large-scale dataset, users may choose the one suitable for their applications. For beginners, we provide a tutorial on training and validating the pickers using the DiTing dataset. We also provide two sets of models trained using datasets with both 50 Hz and 100 Hz sampling rates for direct application by end-users. All of our models are open-source and publicly accessible.

An Aftershock Deletion Method Based on Fault Buffer Zone

The existence of aftershocks in an earthquake sequence can impact the analysis of the mainshock. In this study, we present a method for deleting an aftershock sequence based on the spatial relationship between earthquakes and faults. This method improves the performance of space window selection in the classical K-K method by eliminating aftershocks with an ideal fault buffer zone. The determination of fault buffer zones is based on a trial-and-error analysis of 69,714 earthquake records from the China Seismic Network Center (CENC) collected between 1980 and 2020. We selected 20 typical big earthquakes (ML7.0–8.0 or ~Ms6.6–8.0; for earthquakes above magnitude Ms7 or ML7.2, ML is approximately equal to Ms) as the mainshocks to establish the fault buffer zones. We also propose an empirical formula to determine the distance of the fault buffer zone by counting the aftershock deletion effect at different buffer distances. Compared with the classical K-K method, our method considers the correlation between the spatial distribution of aftershocks and faults, eliminates earthquake groups that are not related to the mainshock, greatly reduces the spatial range of aftershocks, improves the performance of deleting aftershocks of different magnitudes, and provides a new rule and reference for aftershock deletion.

Focal mechanism of Luding M 6.8 earthquake, September 2022 and analysis of the loading role of the tectonic stress on the seismogenic fault

To reveal the seismogenic mechanism of the Luding earthquake, we employed the 118 China Seismic Network stations to collect the P-wave polarity data from each station, which was then used in the P-wave first motion approach to calculate the focal mechanism solution of the M6.8 Luding earthquake that occurred on September 5, 2022. We have also studied the loading effect of tectonic stress on the Luding earthquake fault based on the stress field data for the research area. The results indicate that this earthquake was a strike-slip type, the nodal plane I: strike 167°, dip Angle 78°, slip Angle 2°; Nodal plane Ⅱ: strike 77°, dip Angle 88°, slip Angle 168°. The two fault planes’ instability coefficients of the Luding earthquake are examined considering the region’s background stress field’s condition. The nodal plane I in the Moho circle is discovered to practically coincide with the Coulomb failure line and the tangent point of the Moho circle, indicating that this nodal plane has a high instability coefficient compared to the nodal plane II. The conclusion is that the nodal plane I has a higher likelihood of being the seismogenic fault plane, which is congruent with the seismogenic fault plane suggested by the aftershock distribution, the earthquake radiation energy distribution of a single station, and seismic intensity distribution. The Luding earthquake’s focal mechanism is highly like the theoretical focal mechanism of the fault situated at the location where the Coulomb failure line intersects the Mohr circle, demonstrating that background stress is what caused the earthquake. The substantial fault instability and similarity between the solved and theoretical focal mechanisms make it easier to comprehend the loading effect of tectonic stress on the Luding earthquake fault.

A comparative study of seismic tomography models of Southwest China

The margin of the Tibetan Plateau of Southwest China is one of the most seismically active regions of China and is the location of the China Seismic Experimental Site (CSES). Many studies have developed seismic velocity models of Southwest China, but few have compared and evaluated these models which is important for further model improvement. Thus, we compared six published seismic shear-wave velocity models of Southwest China on absolute velocity and velocity perturbation patterns. The models are derived from different types of data (e.g., surface waves from ambient noise and earthquakes, body-wave travel times, receiver functions) and inversion methods. We interpolated the models into a uniform horizontal grid (0.5° × 0.5°) and vertically sampled them at 5, 10, 20, 30, 40, and 60 km depths. We found significant differences between the six models. Then, we selected three of them that showed greater consistency for further comparison. Our further comparisons revealed systematic biases between models in absolute velocity that may be related to different data types. The perturbation pattern of the model is especially divergent in the shallow part, but more consistent in the deep part. We conducted synthetic and inversion tests to explore possible causes and our results imply that systematic differences between the data, differences in methods, and other factors may directly affect the model. Therefore, the Southwest China velocity model still has considerable room for improvement, and the impact of inconsistency between different data types on the model needs further research. Finally, we proposed a new reference shear-wave velocity model of Southwest China (SwCM-S1.0) based on the three selected models with high consistency. We believe that this model is a better representation of more robust features of the models that are based on different data sets.

Estimation of broadband ground motion characteristics considering source parameter uncertainty and undetermined site condition in densely populated areas of Pingwu

Three destructive earthquakes occurred in Pingwu and Songpan, Sichuan Province, China, between August 16 and 23, 1976. Due to the seismic monitoring capability at that time, the ground motion characteristics of these earthquakes are very vague. Realistc and reliable strong ground motion input plays an important role in seismic building design and urban-scale earthquake damage simulation. This study reproduces the main broadband ground motion characteristics of the 1976 Ms7.2 Songpan earthquake in densely populated areas of Pingwu. The empirical Green’s function method and finite difference method are used to simulate high-frequency and low-frequency ground motion, respectively, and the broadband ground motion is obtained by superposition within the frequency range. In addition, in combination with the “Recipe” source parameter scheme, various uncertainties in the source parameters are considered, including the source mechanism, source depth, asperity parameters, etc. We obtain 36 kinds of broadband ground motion at six typical locations in the Pingwu area. Moreover, we test the rationality of the obtained broadband ground motion by ground motion prediction equations(GMPEs), and the broadband ground motions are consistent with the local ground motion characteristics. The results show broadband ground motions obtained from the scenario earthquake in this paper can meet the destructive capacity of earthquakes of this magnitude. The hybrid method can effectively compensate for the lack of long-period components of the original empirical Green function method. This research also proves that the peak ground acceleration (PGA) of ground motion is mainly contributed by high-frequency ground motion components. Long-period ground motion contributes most to the peak ground velocity (PGV), and about twice the contribution of high-frequency ground motion. Concerning the Chinese seismic intensity scale (GB/T 17742-2020) and China Seismic Ground Motion Parameter Zoning Map (GB18306-2015), the basic fortification intensity in the Pingwu area is VIII. In this paper, the seismic intensity of PWN is VI-VII, indicating that the buildings at this location are less likely to be damaged after the earthquake. The seismic intensity of other regions is VII-IX and buildings are more likely to be damaged during the earthquake at these locations. There are many mountains and valleys in the Pingwu area, and the probability of landslides, debris flows, and other disasters after an earthquake is very high, and we should give special attention to the impact of secondary disasters caused by earthquakes. It is necessary to prevent dammed lakes and other disasters caused by landslides and debris flows.

CSESnet: A deep learning P-wave detection model based on UNet++ designed for China Seismic Experimental Site

Accurate detection of P-wave arrivals has important applications in real-time seismic data processing, such as earthquake monitoring and earthquake early warning. The Sichuan and Yunnan regions, where the China Seismic Experimental Site (CSES) is located, has frequent strong earthquakes and large amount small earthquakes, resulting in serious earthquake disasters. In this paper, we modify the UNet++ network structure and use 490,000 event waveform data and 78,000 noisy data from the CSES as the data set, and analyze the effects of the training set quality, labeled data and loss function on the model performance to obtain a new P-wave detection model-CSESnet. The recall, precision and F1 score of this model are 94.6%, 85.4% and 89.7%, respectively. The tests in Beijing Capital Circle (BCC) indicates the performance of the CSESnet decrease little and has good generalization. The test in Luxian M6.0 earthquake shows that CSESnet can also predict the P-wave arrival times of large earthquakes and process strong motion data very well. CSESnet provides a new detection model to improve the earthquake detection capability in CSES.

Simulation of strong earthquake characteristics of a scenario earthquake (MS7.5) based on the enlightenment of 2022 MS6.9 earthquake in Menyuan

The Menyuan area is an important transportation hub in the Hexi Corridor. The Menyuan MS6.9 earthquake that occurred on January 8, 2022 had a major impact on the local infrastructure and transportation of this region. Due to the high possibility of similar strong earthquakes occurring in this area in the future, preliminary assessment of the seismic intensity characteristics of destructive earthquakes in this region is essential for effective disaster control. This paper uses the empirical Green′s function (EGF) method as a numerical simulation tool to predict the ground motion intensity of Datong Autonomous County under the action of the scenario earthquake (MS7.5). Seismic records of aftershocks of the 2016 Menyuan MS6.4 earthquake were used as Green’s functions for this simulation. The uncertainties associated with various source parameters were considered, and 36 possible earthquake scenarios were simulated to obtain 72 sets of horizontal ground motions in Datong County. The obtained peak ground acceleration (PGA) vs. time histories of the horizontal ground motion were screened using the attenuation relationships provided by the fifth-edition of China's Seismic Ground Motion Parameter Zoning Map and the NGA-West2 dataset. Ultimately, 32 possible acceleration-time histories were selected for further analysis. The screened PGA values ranged from 78.8 to 153 cm/s2. The uncertainty associated with the initial rupture point was found to greatly affect the results of the earthquake simulation. The average acceleration spectrum of the selected acceleration-time history exceeded the expected spectrum of a intermediate earthquake, which means that buildings in Datong County might sustain some damage should the scenario earthquake occur. This research can provide reliable ground motion input for urban earthquake damage simulation and seismic design in Datong County. Growing the dataset of small earthquakes recorded in this region will facilitate the large-scale simulation of ground motions under different earthquake scenarios.

Strong historical earthquakes and their relationships with the Tan-Lu fault system and modern seismicity in eastern China

The largest earthquake (Ms 8½) in eastern China occurred near Tancheng in 1668. Six M ≥ 6 earthquakes occurred in this region in historical time. We re-examine the primary historical archives from this region, adding independent constraints such as maps of active faults, local topography, intensity attenuation laws for China, and relevant administrative practices in archiving data, to reassess source parameters (epicenter, magnitude, or maximum intensity). We use the seismic density index—a measure of the clustering of seismicity weighted by source magnitude—to identify candidate zones of present-day clustering of seismicity than may represent a long-term memory of past historical events in an intraplate setting. We update at least one of the source parameters for four of the seven events. The main shock and two aftershocks of the 1668–1672 Ms 8½ Tancheng earthquake sequence locate along the strike of the Tan-Lu fault zone, each associated with significant and contiguous seismic density anomalies whose elliptical contours are elongated along its strike. A third major aftershock (Ms ≈ 6) in this sequence locates some 200 km away from the fault zone. Six of the seven historical events have seismic density anomalies that cannot be explained by aftershocks of modern seismicity (ML ≤ 5.3), and hence are candidate zones for long-lived aftershock sequences. In contrast, there are no significant seismic density anomalies along the section of Tan-Lu fault south of Suqian, despite palaeo-seismic evidence for a Ms 8½ earthquake around 6280 B.C., providing an upper bound to long-term memory of great earthquakes in this region.

Time-Dependent Seismic Hazard Assessment Based on the Annual Consultation: A Case from the China Seismic Experimental Site (CSES)

Time-Dependent Seismic Hazard Assessment Based on the Annual Consultation: A Case from the China Seismic Experimental Site (CSES)