Large Historical Earthquakes Research Articles

Revisiting Slip Deficit Rates and Its Insights Into Large and Slow Earthquakes at the Nankai Subduction Zone

AbstractThe Nankai subduction zone presents significant seismic and tsunami risks, given its historical earthquakes exceeding magnitude 8 and the expectations of similar future events. Slow earthquakes, common at the shallow and deep plate interface, result from different frictional properties linked to interplate slip deficit accumulation. This study estimates slip deficit rates at the Nankai subduction zone using land and ocean‐bottom geodetic data. Previous estimates encountered limitations, often smoothing slip deficits, omitting observational error differences between ocean‐floor and land data, and relying on homogeneous structure models. To address these issues, we employ a novel trans‐dimensional reversible jump Markov Chain Monte Carlo algorithm. This approach dynamically adjusts slip parameters, accommodating data resolution and producing a flexible slip distribution without predetermined spatial constraints. Additionally, it automatically weights data for observational errors and integrates elastic Green functions from a 3D structure of the Nankai region. Our results provide a finer, heterogeneous slip distribution, improving estimates in inland regions. However, limitations remain offshore in areas with sparse data. We revised the spatial distribution of Nankai slow earthquakes and confirmed a good agreement with intermediate slip deficit rates, identifying coupled and uncoupled regions. High slip deficit rates align with rupture areas of historic large earthquakes. Slow earthquakes occur at frictionally weak plate interfaces, and shallow slow earthquakes may result from subducting relief heterogeneities with important pore fluid pressure effects. We introduce an updated distribution of slip deficit rates for the Nankai subduction zone, considering observed slip deficit rates and the fast and slow earthquake occurrence.

Quantifying Creep on the Laohushan Fault Using Dense Continuous GNSS

The interseismic behavior of faults (whether they are locked or creeping) and their quantitative kinematic constraints are critical for assessing the seismic hazards of faults and their surrounding areas. Currently, the creep of the eastern segment of the Laohushan Fault in the Haiyuan Fault Zone at the northeastern margin of the Tibetan Plateau, as revealed by InSAR observations, lacks confirmation from other observational methods, particularly high-precision GNSS studies. In this study, we utilized nearly seven years of observation data from a dense GNSS continuous monitoring profile (with a minimum station spacing of 2 km) that crosses the eastern segment of the Laohushan Fault. This dataset was integrated with GNSS data from regional continuous stations, such as those from the Crustal Movement Observation Network of China, and multiple campaign measurements to calculate GNSS baseline change time series across the Laohushan Fault and to obtain a high spatial resolution horizontal crustal velocity field for the region. A comprehensive analysis of this primary dataset indicates that the Laohushan Fault is currently experiencing left-lateral creep, characterized by a partially locked shallow segment and a deeper locked segment. The fault creep is predominantly concentrated in the shallow crustal region, within a depth range of 0–5.7 ± 3.4 km, exhibiting a creep rate of 1.5 ± 0.7 mm/yr. Conversely, at depths of 5.7 ± 3.4 km to 16.8 ± 4.2 km, the fault remains locked, with a loading rate of 3.9 ± 1.1 mm/yr. The shallow creep is primarily confined within 3 km on either side of the fault. Over the nearly seven-year observation period, the creep movement within approximately 5 km of the fault’s near field has shown no significant time-dependent variation, instead demonstrating a steady-state behavior. This steady-state creep appears unaffected by postseismic effects from historical large earthquakes in the adjacent region, although the deeper (far-field) tectonic deformation of the Laohushan Fault may have been influenced by the postseismic effects of the 1920 Haiyuan M8.5 earthquake.

Use of tsunami observations for resolving coseismic slip of recent and historic large submarine earthquakes

Tsunamis generated by seafloor displacements accompanying large submarine earthquakes provide sensitivity to absolute slip position and distribution for offshore faulting analogous to that of geodetic observations for landward faulting. Tsunami recordings at deep‐water and near‐shore ocean bottom pressure sensors and tide gauges, along with runup and inundation measurements, can now be reliably modeled using detailed bathymetric structures and robust numerical codes. As a result, tsunami observations now play an important role in quantifying coseismic slip distributions for large submarine earthquakes in subduction zones and other tectonic environments. Applications of joint modeling or inversion of seismic, geodetic and tsunami observations for recent major earthquakes are described, highlighting the specific contributions of the tsunami observations to source model resolution. Tsunami observations provide unique information on the up‐dip extent of earthquake coseismic slip on subduction zone megathrust faults and occurrence of near‐trench slip, which are usually not well constrained by seismic and land‐based geodetic signals. Tsunami signals also help to detect offshore slow slip that is not evident in seismic or land‐based geodetic data and to balance geophysical constraints on ruptures that extend from on‐shore to off‐shore. Tsunami runup measurements and stratigraphic deposits further provide unique constraints on large earthquake ruptures that occurred prior to modern geophysical instrumentation.

A 3D Seismotectonic Model and the Spatiotemporal Relationship of Two Historical Large Earthquakes in the Linfen Basin, North China

The Shanxi Graben is a transitional zone between the Ordos Block and North China Plain with complex structures and frequent earthquakes. Six earthquakes with M ≥ 7.0 have been recorded in the area, including the 1303 Hongtong M 8 and 1695 Linfen M 7.8 earthquakes in the Linfen Basin. Research on these two large earthquakes, closely related in time and space, is lacking. Our objective was to use deep seismic reflection profiles and 3D velocity structure data from previous research, along with seismological observation results, to interpret the geological structure near the source of the two earthquakes. A 3D geometric model of the seismogenic fault was constructed, and the relationships among the deep and shallow structures, deep seismogenic environment, and two large earthquakes were explored. Differences in seismogenic environment between the southern and northern Linfen Basin were identified. The distribution of small earthquakes in the southern Linfen Basin was scattered, and the overall distribution was at depths <25 km. The small earthquakes in the northern part of the basin were dense and concentrated at depths of 25–35 km. Low-velocity layers at an approximate depth of 15–20 km in the southern basin led to differences in seismogenesis between the two regions. Based on the area of the 3D geometric model of the Huoshan Fault, the maximum magnitude of an earthquake caused by fault rupture is Mw 7.7, so the magnitude of the 1303 Hongtong earthquake might be overestimated. Numerical simulation results of Coulomb stress showed that the 1303 Hongtong earthquake had a stress-loading effect on the 1695 Linfen earthquake. The change in Coulomb rupture stress was 1.008–2.543 bar, which is higher than the generally considered earthquake trigger threshold (0.1 bar). We created a new 3D source model of large earthquakes in the Linfen Basin, Shanxi Province, providing a reference and typical cases for risk assessment of large earthquakes in different regions of the Shanxi Graben.

Timing and characteristics of co-seismic surface ruptures in the Yadong rift, southern Tibet

The Yadong-Gulu rift (YGR) is the most prominent and seismically active of the seven main ∼ NS-trending rifts in southern Tibet. Although the morphology of the southern YGR clearly indicates it has witnessed large earthquakes in the past, and despite its significant late Quaternary throw rates of ∼1 mm/yr, no large historical or instrumental earthquakes have been reported, including in the southernmost Pagri half-graben, in contrast to the northern part of the rift which is highly seismically active. Here, geomorphic characteristics helped us constrain the timing of a paleoearthquake that produced surface ruptures along the Pagri half-graben, used to document its past activity and evaluate its seismic hazard. We demonstrate that the co-seismic surface ruptures extend for ∼65 km along the Yadong normal fault, with a maximum vertical displacement ranging from 2 to 4.0 ± 0.1 m. Based on empirical relationships between magnitude, surface rupture length, and fault displacement, we suggest that this event may correspond to a Mw6.9–7.2 earthquake. Combined with previous studies, our radiocarbon (14C) and Optically Stimulated Luminescence (OSL) ages from three pits within the earthquake wedge across the surface ruptures constrain the paleoearthquake timing at 3470-2056 years BP. We suggest that the southern YGR currently has a high regional seismic hazard for a Mw6.8–7.1 earthquake, considering the significant throw rates and long timespan since the last strong event. Furthermore, we suggest that such different seismic activity and throw/extension rates between the southern and northern YGR may be explained by different upper crustal rheology behavior and mid-crustal structure.

Internal deformation of the North Andean Sliver in Ecuador-southern Colombia observed by InSAR

Summary In the Northern Andes, partitioning of oblique subduction of the Nazca plate beneath the South American continent induces a northeastward motion of the North Andean Sliver. The strain resulting from this motion is absorbed by crustal faults, which have produced magnitude 7 + earthquakes historically in the Andean Cordillera of Ecuador and southern Colombia. In order to quantify the strain in that area, we derive a high-resolution surface velocity map using InSAR time-series processing. We analyzed 6 to 8 years of Sentinel-1 data and combined different satellite line-of-sight directions to produce a reliable velocity map in the East direction. We use interpolated GNSS data to express the velocity map with respect to Stable South America and remove the long-wavelength pattern due to the post-seismic deformation following the 2016 Mw 7.8 Pedernales earthquake. The InSAR velocity map finds high E-W shortening strain rates along N-S trending structures within the Western Cordillera and the Interandean valley, with little deformation taking place east of them. This result strengthens the previous proposition of a ∼350 km long Quito-Latacunga tectonic block, forming a restraining bend in the overall right-lateral strike-slip fault system accommodating the northeastward escape motion of the North Andean Sliver. However, the high spatial resolution provided by InSAR indicates that previously proposed boundaries for this block need to be revised. In particular, InSAR results highlight high strain rate (>300 nstrain/yr) along undescribed active structures, south and west of the proposed limits for the Quito-Latacunga block, respectively in Peltetec and Ibarra regions. Interestingly, the two areas with the largest strain rates spatially correlate with the proposed areas of large historical earthquakes. Modeling of the InSAR and GNSS velocities in these areas suggests shallow coupling and high slip rates on structures which, previously, were not identified as active. We also demonstrate a slow-down of the shallow aseismic slip on the Quito fault after the Pedernales earthquake, suggesting that stress changes following large megathrust events might trigger transient slip behaviors on crustal faults. The high-resolution strain map provided by this work provides a new basis for future tectonic models in the Ecuadorian and southern Colombian Andes, and will contribute to the seismic hazard assessment in this highly populated area of the Andes.

A model for the hydrothermal tremor source of the Mefite d’Ansanto (Italy) CO2 non-volcanic emissions in the intermediate frequency band (1–15 Hz)

In the Summer 2021 a seismic passive survey was carried out at Mefite d’Ansanto (Italy), well-known for the cold non-volcanic and lethal CO2 emissions. Mefite is located close to that part of Irpinia region hosting the large historical earthquakes’ faults, that generated the destructive magnitude 6.8 earthquake of 1980 and have been related to the CO2 leakage by the scientific community. The survey was conducted by installing a small short-period seismic array with the purpose of determining the wavefield features and detecting the possible signature of the regional stress variations. By analyzing the acquired data, we have determined the properties of the seismic wavefield associated with the emission vents, in the intermediate frequency band, 1–15 Hz, which was found to be composed of a stationary background component and an intermittent higher energy one. Basing on our results, considerations on the medium properties and the deep source available information, we have depicted a schematic model of the shallow seismic source: the background components are linked to the shallower activity of the hydrothermal system, e.g. the bubbling, while the intermittent ones are likely generated by the passage of the overpressured gas, trapped in the first-tens-meters’ layers, after overcoming the internal cohesion charge. The characteristics of the wavefield that we have defined, refer to a condition in which no regional earthquakes occurred and could be considered as a basis to identify possible significant variations linked to CO2 emissions and to the regional stress changes.

A future scenario earthquake and ground motion hazards for Kathmandu, Nepal

Since Nepal is an earthquake-prone country due to the collision of the Indian and Eurasian plates, it is crucial for its capital of Kathmandu to evaluate ground motion hazards from a future large earthquake. For this purpose, we constructed a realistic scenario earthquake with realistic rupture parameters in a likely location. To obtain the location, a new distribution of the rupture zones of large historical earthquakes along the Main Himalayan Thrust (MHT) was obtained by integrating the distribution from a previous study and the results of trench surveys. Global Navigation Satellite System observations indicated that the plate boundary is strongly coupled from the southern boundary of the MHT to a depth of approximately 10 km and there is almost no lateral change in the coupling. This implies that all regions along the MHT have similar rates of strain increase. Therefore, it is most probable that the rupture zone of the oldest previous event will rupture as a scenario earthquake. In the new distribution, the 1255 earthquake is the oldest. However, large earthquakes occurred in 1934 and 2015 within its rupture zone. Thus, we adopted the area obtained by removing the 1934 and 2015 rupture zones from the western part of the 1255 rupture zone. The relationship between the rupture area size and seismic moment of the 2015 earthquake lies between the scaling formulas for crustal earthquakes and plate-boundary earthquakes, but is closer to the former. Therefore, using this and the scheme for characterized source models, we determined realistic rupture parameters. We then simulated broadband ground motions in Kathmandu using these rupture parameters, our 3-D velocity structure models, and a hybrid method combining the finite-difference method and the stochastic Green’s function method. We obtained the peak ground accelerations (PGAs) of simulated ground motions, and calculated the seismic intensities in the Modified Mercalli intensity scale from the PGAs as indexes of hazards for Kathmandu. Intensities IX coincide with the center of the Kathmandu Valley, and intensities VIII and VII are found in the area surrounded by the sedimentary boundary and the southernmost part of the valley.Graphical abstract

Intrinsic and scattering attenuations of the Sichuan-Yunnan region in China from S coda waves

Seismic attenuation is a fundamental property of the Earth's media. Attenuation structure for the complicated geological structures with strong seismicity in the Sichuan-Yunnan region is poorly studied. In this study, we collected 108,399 waveforms of 11,517 local small earthquakes with magnitudes between 1.5 and 3.5 from January 2014 to September 2021 in the Sichuan-Yunnan region and its adjacent areas. We employed an envelope inversion technique for separating the intrinsic and scattering attenuations of the S coda wave, and obtained the intrinsic and scattering attenuation structures for frequencies between 0.25 and 8.00 Hz. The attenuation structures correlate well with the geological units, and some major faults mark the attenuation variations where historic large earthquakes have occurred. The regional average attenuation shows a negative frequency dependence. The average scattering attenuation has a faster descending rate than the average intrinsic attenuation, and is dominant at low frequencies, while at high frequencies the average intrinsic attenuation is stronger. The lateral variation in the intrinsic attenuation is consistent with the variation in heat flow, the scattering attenuation may be related to the scatter distribution and size. The total attenuation is consistent with the previous studies in this region, and the separate intrinsic and scattering attenuation may be useful in understanding regional tectonics and important in earthquake prevention and disaster reduction.

Active Tectonics, Quaternary Stress Regime Evolution and Seismotectonic Faults in Southern Central Hispaniola: Implications for the Quantitative Seismic Hazard Assessment

AbstractPresent‐day convergence between Caribbean and North American plates is accommodated by subduction zones, major active thrusts and strike‐slip faults, which are probably the source of the historical large earthquakes on Hispaniola. However, little is known of their geometric and kinematic characteristics, slip rates and seismic activity over time. This information is important to understand the active tectonics in Hispaniola, but it is also crucial to estimate the seismic hazard in the region. Here we show that a relatively constant NE‐directed shortening controlled the geometry and kinematics of main active faults in southern central Hispaniola, as well as the evolution of the Quaternary stress regime. This evolution included a pre‐Early Pleistocene D1 event of NE‐trending compression, which gave rise to the large‐scale fold and thrust structure in the Cordillera Central, Peralta Belt, Sierra Martín García and San Juan‐Azua basin. This was followed by a near pure strike‐slip D2 stress regime, partitioned into the N‐S to NE‐SW transverse Ocoa‐Bonao‐La Guácara and Beata Ridge fault zones, as well as subordinate structures in related sub‐parallel deformation corridors. Shift to D2 strike‐slip deformation was related to indentation of the Beata Ridge in southern Hispaniola from the Early to Middle Pleistocene and continues today. D2 was locally coeval by a more heterogeneous and geographically localized D3 extensional deformation. Defined seismotectonic fault zones divide the region into a set of simplified seismogenic zones as starting point for a seismic hazard modeling. Highest peak ground acceleration values computed in the Ocoa Bay establish a very high seismic hazard.

Local site effects and seismic microzonation around Suban Area, Curup Rejang Lebong, Bengkulu deduced by ambient noise measurements

BackgroundThe Suban area of Curup Rejang Lebong is a tourist region in Bengkulu Province, Indonesia, close to the active Ketaun and Musi faults, which are segments of the Sumatra Fault System (SFS). However, no studies have been conducted in this area to assess how geological structures affect seismic ground motions and contribute to seismic hazard and risk assessment.MethodsThe first study of seismic microzonation in the Suban area of Curup City by ambient noise measurements was conducted at 100 sites, spaced ~ 1 km apart, with 60 min of data acquisition for each site. All microseismic data were processed using the Horizontal to Vertical Spectral Ratios (HVSR) method.ResultsThe HVSR method revealed the amplification factors (A0) ranging from 1.23 to 8.26 times, corresponding to natural frequency (f0) variations between 1.24 and 9.67 Hz. About 13% and 55% of the sites show high (6 ≤ A0 ≤ 9) and medium (3 ≤ A0 ≤ 6) amplifications, respectively, predominantly in the western parts of the study area, consistent with a high seismic vulnerability index (Kg). Furthermore, we also estimated the ground shear strain (GSS) of the region using the Kanai method with two large historical earthquakes at the Ketahun segment in 1943 (Mw 7.4) and the Musi segment in 1979 (Mw 6.0). The Kg value is consistent with the GSS values and indicates areas of severe damage during the historic earthquakes.ConclusionsThus, the western parts of the Suban region are vulnerable to severe damage from an earthquake. These findings could provide valuable insights for future planning and risk management efforts aimed at minimizing the impact of earthquakes in the Suban region.

Holocene paleoearthquakes activity in the Nieru graben, southern Tibet

The Yadong-Gulu Rift (YGR) is the longest and most prominent rift in southern Tibet. While 14 large historical earthquakes have occurred on the northern-central segment of the YGR, historical or paleoearthquake records along the southern segment remain poorly understood. The spatial discrepancy of large earthquakes has significant implications for assessing seismic hazards along the entire YGR. To address this knowledge gap, this study presents the paleoseismic events for the first time in the Nieru graben located along the southern segment of the YGR. The excavation of a paleoseismic trench reveals two Holocene earthquakes with calibrated radiocarbon dates constraining the timing of the two events to be after 4084-3954 cal yr BP. The paleoearthquake had a magnitude of M 7. The study shows that the southern segment of the YGR experienced strong paleoseismic events, except for the Relong graben. Considering the high present extension rate of the southern rifts and the magnitude of the paleoseismic events along the Nieru graben, the southern segment of the YGR holds the potential for producing large earthquakes. Furthermore, the ongoing tectonic deformation in the area, coupled with stress changes from large earthquakes on Himalayan thrust faults, may increase the likelihood of future rupture along the active faults in the Nieru graben.

Broad fault zones enable deep fluid transport and limit earthquake magnitudes

Constraining the controlling factors of fault rupture is fundamentally important. Fluids influence earthquake locations and magnitudes, although the exact pathways through the lithosphere are not well-known. Ocean transform faults are ideal for studying faults and fluid pathways given their relative simplicity. We analyse seismicity recorded by the Passive Imaging of the Lithosphere-Asthenosphere Boundary (PI-LAB) experiment, centred around the Chain Fracture Zone. We find earthquakes beneath morphological transpressional features occur deeper than the brittle-ductile transition predicted by simple thermal models, but elsewhere occur shallower. These features are characterised by multiple parallel fault segments and step overs, higher proportions of smaller events, gaps in large historical earthquakes, and seismic velocity structures consistent with hydrothermal alteration. Therefore, broader fault damage zones preferentially facilitate fluid transport. This cools the mantle and reduces the potential for large earthquakes at localized barriers that divide the transform into shorter asperity regions, limiting earthquake magnitudes on the transform.

Uncertainties in Intensity-Based Earthquake Magnitude Estimates

Abstract Estimating the magnitude of historical earthquakes is crucial for assessing seismic hazard. Magnitudes of early-instrumental earthquakes can be inferred using a combination of instrumental records, field observations, and the observed distribution of shaking intensity determined from macroseismic observations. For earthquakes before 1900, shaking intensity distributions often provide the only information to constrain earthquake magnitude. Considerable effort has been made to develop methods to estimate the magnitude of moderate-to-large historical earthquakes using shaking intensities derived from macroseismic data. In this study, we consider earthquakes in California with known instrumental magnitudes to explore uncertainties in estimating the magnitude of historical earthquakes from intensity information alone. We use three California-specific intensity prediction equations (IPEs) and an IPE based on a global ground-motion model (GMM) to determine optimum intensity-based magnitudes for 33 moderate-to-large California earthquakes between 1979 and 2021. Intensity-based magnitudes are close to instrumental magnitudes on average. However, intensity-based magnitudes for individual events differ by as much as 2.2 magnitude units from instrumental magnitudes. This result reflects the weak dependence of ground motions and shaking intensities on moment magnitude and their strong dependence on stress drop. Considering the intensity distributions of the 1906 San Francisco and 1989 Loma Prieta earthquakes, we show that information that could constrain rupture length is discarded when considering only the 2D decay of intensity with distance. We also show that ground-motion intensity conversion equations used in a GMM-based approach may cause a systematic overestimation of large historical earthquake magnitudes. This study underscores both the reducible and potentially irreducible uncertainties associated with using intensity data to estimate magnitudes of historical earthquakes using IPEs and highlights the value of using additional information to constrain rupture dimensions. Using intensity observations alone, moment magnitude uncertainties are typically on the order of a full unit.

The potential for palaeoseismic and palaeoclimatic reconstructions from Lake Tennyson, North Canterbury, New Zealand

ABSTRACT Lake Tennyson’s basin and sedimentary record were assessed for multiple branches of paleoenvironmental research. A geophysical investigation of the lake depositional setting was supported by physical and chemical analysis of multiple sediment cores. The basin reaches 60 m depth and several subaerial landforms identified were likely deposited from seismic activity. Deformed sediments and turbidites in gravity cores are attributed to one or more large (pre)historic earthquakes, and episodic grain size changes in sediment cores are related to either seismic and/or climatic/meteorological processes. A hummocky slump deposit located on the western lake bed margin is linked to widespread sediment deformation that was likely caused by movement of the Awatere Fault. Radiocarbon dating constrains the most recent major deformation event between the late thirteenth century CE and mid-twentieth Century. Folded turbidites in the stratigraphy suggest multiple sedimentary deformation episodes occurred during the last millennium. Micro-XRF elemental signatures suggest proxies of past water column mixing (windiness proxy) can be reconstructed via identification of changing redox conditions. Records from Lake Tennyson show potential for several palaeoenvironmental research applications, and combining them with basin morphostratigraphy and land-based palaeoclimate and palaeoseismic evidence can enhance understanding of environmental change in the northern South Island.

How large peak ground acceleration by large earthquakes could generate turbidity currents along the slope of northern Japan Trench

Deep-sea turbidite has been used to determine the history of occurrence of large earthquakes. Surface-sediment remobilization is a mechanism of the generation of earthquake-induced turbidity currents. However, the detailed mechanism of surface-sediment remobilization caused by earthquake ground shaking is unclear. To understand how high peak ground acceleration (PGA) caused by a large earthquake can remobilize surface sediments, we determined the age of a surface-sediment core recovered from the mid-slope terrace (MST) of the inner slope of the Japan Trench in northern Sanriku to determine turbidites generated by large historical earthquakes and calculate the PGAs of these earthquakes using an empirical attenuation relation commonly used in Japan. Small offsets in radiocarbon ages and excess 210Pb activities between turbidite and hemipelagic muds suggest that the turbidites in the core resulted from surface-sediment remobilization. 137Cs and excess 210Pb chronologies indicate that the three uppermost turbidites in the core are correlated with three large historical earthquakes, namely the 1968 common era (CE) Tokachi-oki, the 1933 CE Showa–Sanriku, and the 1896 CE Meiji–Sanriku earthquakes. Calculation of PGAs for large historical earthquakes along the northern Japan Trench indicates that a PGA of > 0.6 g is necessary for turbidite deposition in the MST basin. This threshold is larger than that reported for central Sanriku and may vary spatially. Moreover, turbidites in the MST deposits are more frequent in the northern Japan Trench than in the central Japan Trench, suggesting that the occurrence of three types of large M8-class earthquakes in the northern Japan Trench might have contributed to the frequent occurrence of large PGAs.

Seismic swarms in the Pollino seismic gap: Positive fault inversion within a popup structure

Seismic swarms frequently occur along continental fault systems and their relation with large earthquakes is often contradictory. Such a case is documented in the Pollino mountain range of southern Italy, a decoupling zone where the belt-normal stretching drastically rotates accommodating the differential SE-retreat of the Ionian slab. The paucity of historical large earthquakes has led to hypothesize the presence of a seismic gap. A long-lasting seismic swarm that climaxed with a ML = 5.2 earthquake in October 2012 was therefore thought as a possible signal of an impending large earthquake filling the gap. Seismicity data collected during a 4-years long monitoring are a powerful microscope to look through the seismic swarm. In this study, we present accurate relocations for 2385 earthquakes and high-resolution Vp and Vp/Vs models of the fault system. Seismicity occurred on two separate normal faults that were formerly part of a thrusts and back-thrusts system, originally formed as a pop-up at restraining bends of the Pollino fault, a wrench fault system that inverted the original left lateral sense of slip accommodating a differential motion induced by the southward retreat of the Ionian slab.

Seismic Activities and Hazards of the Seismic Gaps in the Haiyuan Fault in Northeastern Tibet: Insights From Numerical Modeling Earthquake Interaction

AbstractThe 2022 Mw 6.6 Menyuan earthquake occurred in the Qilian‐Haiyuan fault system in northeastern Tibet. To investigate the mechanism behind the 2022 Menyuan earthquake and the seismic hazards (SHs) of the Tianzhu seismic gap, the spatiotemporal stress variations on the Qilian‐Haiyuan fault zone were assessed before and after 2022 by modeling the stress changes caused by 12 historical large earthquakes during the last century. The results showed that the 2022 Menyuan earthquake was delayed due to a stress shadow caused by the 1927 Mw 8.3 Gulang earthquake. The stress shadow also covers the middle section of the Tianzhu seismic gap and inhibits its seismic activities to some extent. By preventing the rupture of the Tianzhu seismic gap in a single event, this stress shadow may reduce the risk of a future earthquake with a magnitude of more than Mw 7.7 in the Tianzhu seismic gap (length = approximately 260 km). The western portion of the Tianzhu Seismic Gap (length = approximately 18 km) was stress‐loaded with the peak ΔCFS value of approximately 375.15 kPa due to the 2016 Mw 5.9 and 2022 Mw 6.6 Menyuan earthquakes, indicating its increased SHs. The eastern portion of the Tianzhu seismic gap (length = approximately 149 km) was stress‐loaded with the peak ΔCFS value of approximately 1,035 kPa due to the 1927 Mw 8.3 Gulang and 1920 Mw 8.5 Haiyuan earthquakes, indicating its increased SHs. Therefore, more efforts should be made to prepare for the future hazards prevention in the eastern portion of the Tianzhu seismic gap.

RESEARCH ON THE CHARACTERISTICS OF PRESENT CRUSTAL DEFORMATION IN BEIJING AREA BASED ON GNSS

Abstract. We use the GAMIT/GLOBK software to process more than 100 GNSS sites observation covering the Beijing area from 1999–2021 to obtain the three-dimensional velocity field of present-day crustal movement, and the deformation and strain characteristics of Beijing were calculated and comprehensively analyzed by combining with historical earthquakes. The results show that: 1) The present-day horizontal crustal motion of Beijing is dominated by the SEE direction with a rate of 5-10 mm/a with relative to the stable Eurasian reference frame, and the northern and southern areas show uplift as a whole with a rate of 1–3 mm/a, while the central plain area shows overall subsidence, with a maximum subsidence rate of about 130 mm/a. 2) The horizontal deformation is mainly N-E/N-W extension, accompanied by weak shear activity. 3) The N-W trending faults show sinistral strike slip movement as a whole, with about 1.1–2.0 mm/a, and N-E trending faults show dextral strike slip with a rate of less than 1 mm/a. 4) The historical large earthquakes are distributed within the positive and negative gradient zones of the first shear strain rate, transition zones of the rotation rate, and are mostly distributed at the edges of the high and low value zones of the second shear strain rate, and the edges of the high value zones of the east-west strain rate also have a background of large earthquakes.

Airborne LiDAR-Based Mapping of Surface Ruptures and Coseismic Slip of the 1955 Zheduotang Earthquake on the Xianshuihe Fault, East Tibet

ABSTRACTSurface ruptures and coseismic slip distributions of large earthquakes are the keys to understanding earthquake rupture processes, analyzing rupture history of associated faults, and assessing earthquake hazards. Detailed mapping of surface ruptures of large historical earthquakes is needed but is difficult in remote regions. The 1955 Ms 7.5 Zheduotang earthquake occurred in a prominent restraining bend of the central sinistral Xianshuihe fault and within a high-relief and densely vegetated mountain range. This study characterizes the 1955 earthquake surface rupture via 50 cm resolution airborne light detection and ranging (LiDAR) data, combined with field investigations. Our mapping of geomorphic features showed that relatively fresh mole tracks and fault scarps are still preserved beneath the dense vegetation cover. The 1955 earthquake ruptured at least 30 km in length, which consisted of four sections separated by stepovers or changes in the strike. Overall, the multistranded rupture was more complex than that of other historical earthquakes to the northwest along the Xianshuihe fault but consistent with its local structural setting. We collected 47 LiDAR-derived and 48 field-based left-lateral (with minor vertical) measurements. The clustering of the smallest offsets suggests that the average sinistral coseismic displacement of the 1955 earthquake was ∼2.1 m based on LiDAR-derived data and ∼1.5 m based on field measurements. This difference highlights the ambiguity and difficulties associated with surface rupture investigations of historical earthquakes decades after the event. The rupture length (∼30 km) and average sinistral displacement (1.5–2.1 m) suggest a moment magnitude of 6.6–7.4 based on empirical relationships of strike-slip earthquakes worldwide. The magnitude is smaller than the widely accepted Ms 7.5 in the catalog, suggesting that the previously reported magnitude was possibly overestimated. Our data have implications for the seismic hazard evaluation of the Sichuan–Tibet railway, which passes through the northern part of the ruptured fault.