A structural model for active extension in Central Italy

This work presents a structural model for earthquake faulting in the Umbria-Marche Apennines (Central Italy). The model is derived by an integrated analysis of geological, geophysical and seismological data. At regional scale, the distribution and character of the seismicity appear to be mainly controlled by a low-angle east-dipping normal fault (Altotiberina fault, AF). The latter is the lower boundary of an active, continuously deforming hangingwall block moving toward NE. Moderate magnitude earthquakes (4 < M < 6), such as the Norcia 1979 (M = 5.9), the Gubbio 1984 (M = 5.2) and the Colfiorito 1997 (M_max = 5.9), occur within the active hangingwall block and are related to the activity of major west-dipping normal faults detaching on the AF. The geometry of the deep seismogenic structures is listric (as in the case of Colfiorito) or more complex, because of local reactivation of pre-existing low-angle thrust (e.g. Gubbio) or high-angle strike-slip faults (e.g. Norcia). For all the analysed earthquakes the rupture nucleation is located at the base of the aftershock volumes, near the line of intersection between the SW-dipping normal faults and the east-dipping AF basal detachment. The progressive increase in depth of the earthquake foci from the north–west (e.g. Gubbio, 6–7 km) to the south–east (e.g. Norcia, 11–12 km) appears to be related to the eastward deepening of the basal detachment. These seismotectonic features are relevant for determining the seismogenic potential of the Apennine active faults, which depends not only on the length of the faults, but also on the depth of the detachment zone as well.     

Seismic source zoning and maximum credible earthquake prognosis of the Greater Kashmir Territory,NW Himalaya

We present the seismic source zoning of the tectonically active Greater Kashmir territory of the Northwestern Himalaya and seismicity analysis (Gutenberg-Richter parameters) and maximum credible earthquake (m _max) estimation of each zone. The earthquake catalogue used in the analysis is an extensive one compiled from various sources which spans from 1907 to 2012. Five seismogenic zones were delineated, viz. Hazara-Kashmir Syntaxis, Karakorum Seismic Zone, Kohistan Seismic Zone, Nanga Parbat Syntaxis, and SE-Kashmir Seismic Zone. Then, the seismicity analysis and maximum credible earthquake estimation were carried out for each zone. The low b value (<1.0) indicates a higher stress regime in all the zones except Nanga Parbat Syntaxis Seismic Zone and SE-Kashmir Seismic Zone. The m _max was estimated following three different methodologies, the fault parameter approach, convergence rates using geodetic measurements, and the probabilistic approach using the earthquake catalogue and is estimated to be M _w 7.7, M _w 8.5, and M _w 8.1, respectively. The maximum credible earthquake (m _max) estimated for each zone shows that Hazara Kashmir Syntaxis Seismic Zone has the highest m _max of M _w 8.1 (±0.36), which is espoused by the historical 1555 Kashmir earthquake of M _w 7.6 as well as the recent 8 October 2005 Kashmir earthquake of M _w 7.6. The variation in the estimated m _max by the above discussed methodologies is obvious, as the definition and interpretation of the m _max change with the method. Interestingly, historical archives (～900 years) do not speak of a great earthquake in this region, which is attributed to the complex and unique tectonic and geologic setup of the Kashmir Himalaya. The convergence is this part of the Himalaya is distributed not only along the main boundary faults but also along the various active out-of-sequence faults as compared to the Central Himalaya, where it is mainly adjusted along the main boundary fault.     

Interrelation between fault zone structures and earthquake processes

In order to develop capabilities for predicting earthquake processes on the basis of known fault zone structures and stress conditions, we need to find relations between seismogenic structures and processes. In the present paper we search for the scale dependence in various earthquake phenomena with the hope to find some structures in the earth that may control the earthquake processes. Among these phenomena, we shall focus on (1) geologic structures which play some role in nucleation and stopping of earthquake fault rupture, (2) depth ranges of the brittle seismogenic zone, (3) asperities and barriers distributed over a fault plane, (4) source-controlledf _max effect, (5) nonfractal behavior of creep events, and (6) temporal correlation between codaQ ^–1 and seismicity of earthquakes with magnitude characteristic to a given area. Our review of various scale-dependent phenomena leads us to propose a working hypothesis that the temporal change in codaQ ^–1 may reflect the activity of creep fractures near the brittle-ductile transition zone.     

Seismic Zonation of the Delhi Region for Bedrock Ground Motion

An endeavor is made to compute peak ground horizontal accelerations at bedrock level in the Delhi region due to the seismogenic sources present around Delhi. The entire area is divided into six seismogenic sources for which seismic hazard analysis is carried out using the complete and extreme part of the seismicity data. Maximum likelihood estimates of hazard parameters viz., seismic activity rate , b value and maximum probable earthquake M _max are made for each zone. The return periods and the probabilities of occurrence of various magnitudes for return periods of 50, 100 and 1000 years are also computed for each zone. The peak ground acceleration (PGA) values for 20% exceedance in 50 years are then computed for the Delhi region from each zone. The maximum PGA value considering all the zones is 0.34 g, which is due to the Mathura fault zone. The seismogenic zones V and VI, i.e., Mathura fault zone and the Sohna fault zone are observed to be contributing maximum PGA values in the Delhi region governing the isoacceleration contours computed for the region. The seismic zonation map for the PGA values at the bedrock level is obtained for the Delhi region. This can be used directly as input for the microzonation of ground motion at the surface by incorporating the local site conditions.     

松原市扶余北隐伏活动断裂地震潜势研究

扶余肇东断裂带是松辽盆地中一条重要的隐伏发震断裂带,2006年和2014年在此断裂带上发生了多次近6级地震,地震活动主要集中在断裂的东北段和查干花段。在2012—2014年的松原市活断层探测工作中,探明扶余肇东断裂带东北段经过松原市区,该段呈近EW向,具有明显的分段性和独立性,将其命名为扶余北断裂。通过三维物探资料和浅层人工探测,确定了扶余北断裂的空间展布和剖面特征,并利用联排钻孔探测和光释光测年技术,确定该断裂存在晚更新世以来的活动;利用石油物探资料获得的基岩面破裂规模,对扶余北断裂的地震潜势进行了估计,并进行了概率性地震危险性研究。     

Probabilistic seismic hazard assessment of Italy using kernel estimation methods

A representation of seismic hazard is proposed for Italy based on the zone-free approach developed by Woo (BSSA 86(2):353–362, 1996a), which is based on a kernel estimation method governed by concepts of fractal geometry and self-organized seismicity, not requiring the definition of seismogenic zoning. The purpose is to assess the influence of seismogenic zoning on the results obtained for the probabilistic seismic hazard analysis (PSHA) of Italy using the standard Cornell’s method. The hazard has been estimated for outcropping rock site conditions in terms of maps and uniform hazard spectra for a selected site, with 10 % probability of exceedance in 50 years. Both spectral acceleration and spectral displacement have been considered as ground motion parameters. Differences in the results of PSHA between the two methods are compared and discussed. The analysis shows that, in areas such as Italy, characterized by a reliable earthquake catalog and in which faults are generally not easily identifiable, a zone-free approach can be considered a valuable tool to address epistemic uncertainty within a logic tree framework.     

Occurrence probability of moderate to large earthquakes in Italy based on new geophysical methods

We develop new approaches to calculating 30-year probabilities for occurrence of moderate-to-large earthquakes in Italy. Geodetic techniques and finite-element modelling, aimed to reproduce a large amount of neotectonic data using thin-shell finite element, are used to separately calculate the expected seismicity rates inside seismogenic areas (polygons containing mapped faults and/or suspected or modelled faults). Thirty-year earthquake probabilities obtained from the two approaches show similarities in most of Italy: the largest probabilities are found in the southern Apennines, where they reach values between 10% and 20% for earthquakes of M _W ≥ 6.0, and lower than 10% for events with an M _W ≥ 6.5.     

Active faults and seismogenic models for the Urumqi city,Xinjiang Autonomous Region,China

We have studied the characteristics of the active faults and seismicity in the vicinity of Urumqi city, the capital of Xinjiang Autonomous Region, China, and have proposed a seismogenic model for the assessment of earthquake hazard in this area. Our work is based on an integrated analysis of data from investigations of active faults at the surface, deep seismic reflection soundings,seismic profiles from petroleum exploration, observations of temporal seismic stations, and the precise location of small earthquakes. We have made a comparative study of typical seismogenic structures in the frontal area of the North Tianshan Mountains, where Urumqi city is situated,and have revealed the primary features of the thrust-foldnappe structure there. We suggest that Urumqi city is comprised two zones of seismotectonics which are interpreted as thrust-nappe structures. The first is the thrust nappe of the North Tianshan Mountains in the west, consisting of the lower(root) thrust fault, middle detachment,and upper fold-uplift at the front. Faults active in the Pleistocene are present in the lower and upper parts of this structure, and the detachment in the middle spreads toward the north. In the future, M7 earthquakes may occur at the root thrust fault, while the seismic risk of frontal fold-uplift at the front will not exceed M6.5. The second structure is the western flank of the arc-like Bogda nappe in the east,which is also comprised a root thrust fault, middle detachment, and upper fold-uplift at the front, of which the nappe stretches toward the north; several active faults are also developed in it. The fault active in the Holocene is called the South Fukang fault. It is not in the urban area of Urumqi city. The other three faults are located in the urban area and were active in the late Pleistocene. In these cases,this section of the nappe structure near the city has an earthquake risk of M6.5–7. An earthquake M_S6.6, 60 km east to Urumqi city occurred along the structure in 1965.     

Regional Variations of the <Emphasis Type="Italic">ω</Emphasis>-upper Bound Magnitude of GIII Distribution in the Iranian Plateau

The Iranian Plateau does not appear to be a single crustal block, but an assemblage of zones comprising the Alborz—Azerbaijan, Zagros, Kopeh—Dagh, Makran, and Central and East Iran. The Gumbel’s III asymptotic distribution method (GIII) and maximum magnitude expected by Kijko—Sellevoll method is applied in order to check the potentiality of the each seismogenic zone in the Iranian Plateau for the future occurrence of maximum magnitude (M_max). For this purpose, a homogeneous and complete seismicity database of the instrumental period during 1900–2012 is used in 29 seismogenic zones of the examined region. The spatial mapping of hazard parameters (upper bound magnitude (ω), most probable earthquake magnitude in next 100 years (M₁₀₀) and maximum magnitude expected by maximum magnitude estimated by Kijko—Sellevoll method (max M_{K ? S}^max) reveals that Central and East Iran, Alborz and Azerbaijan, Kopeh—Dagh and SE Zagros are a dangerous place for the next occurrence of a large earthquake.     

2021年青海玛多MS7.4地震精定位和发震构造初探

2021年5月22日青海省果洛州玛多县发生M_S7.4地震。为探究本次地震的发震构造及余震分布特征,选取2021年5月1日—6月3日青海测震台网观测到的33°～36°N,97°～99.5°E空间范围内的地震观测报告,利用双差精定位方法进行双差精定位处理。重定位后整体残差平均减小了0.23,深度在5～25 km间随机分布。根据地震迁移方向和震区地质构造,认为本次地震的发震构造为昆仑山口—江错断裂,玛多—甘德东段受主震触发影响爆发一系列小震,两条断裂之间可能因为本次地震产生一定联系。本次地震产生新的断裂,突破了两条断裂之前的空区,连接到玛多—甘德断层,使两条断层交叉相连,形成新的断层构造。     

History of Modern Earthquake Hazard Mapping and Assessment in California Using a Deterministic or Scenario Approach

Modern earthquake ground motion hazard mapping in California began following the 1971 San Fernando earthquake in the Los Angeles metropolitan area of southern California. Earthquake hazard assessment followed a traditional approach, later called Deterministic Seismic Hazard Analysis (DSHA) in order to distinguish it from the newer Probabilistic Seismic Hazard Analysis (PSHA). In DSHA, seismic hazard in the event of the Maximum Credible Earthquake (MCE) magnitude from each of the known seismogenic faults within and near the state are assessed. The likely occurrence of the MCE has been assumed qualitatively by using late Quaternary and younger faults that are presumed to be seismogenic, but not when or within what time intervals MCE may occur. MCE is the largest or upper-bound potential earthquake in moment magnitude, and it supersedes and automatically considers all other possible earthquakes on that fault. That moment magnitude is used for estimating ground motions by applying it to empirical attenuation relationships, and for calculating ground motions as in neo-DSHA (Zuccolo et al., 2008). The first deterministic California earthquake hazard map was published in 1974 by the California Division of Mines and Geology (CDMG) which has been called the California Geological Survey (CGS) since 2002, using the best available fault information and ground motion attenuation relationships at that time. The California Department of Transportation (Caltrans) later assumed responsibility for printing the refined and updated peak acceleration contour maps which were heavily utilized by geologists, seismologists, and engineers for many years. Some engineers involved in the siting process of large important projects, for example, dams and nuclear power plants, continued to challenge the map(s). The second edition map was completed in 1985 incorporating more faults, improving MCE??s estimation method, and using new ground motion attenuation relationships from the latest published results at that time. CDMG eventually published the second edition map in 1992 following the Governor??s Board of Inquiry on the 1989 Loma Prieta earthquake and at the demand of Caltrans. The third edition map was published by Caltrans in 1996 utilizing GIS technology to manage data that includes a simplified three-dimension geometry of faults and to facilitate efficient corrections and revisions of data and the map. The spatial relationship of fault hazards with highways, bridges or any other attribute can be efficiently managed and analyzed now in GIS at Caltrans. There has been great confidence in using DSHA in bridge engineering and other applications in California, and it can be confidently applied in any other earthquake-prone region. Earthquake hazards defined by DSHA are: (1) transparent and stable with robust MCE moment magnitudes; (2) flexible in their application to design considerations; (3) can easily incorporate advances in ground motion simulations; and (4) economical. DSHA and neo-DSHA have the same approach and applicability. The accuracy of DSHA has proven to be quite reasonable for practical applications within engineering design and always done with professional judgment. In the final analysis, DSHA is a reality-check for public safety and PSHA results. Although PSHA has been acclaimed as a better approach for seismic hazard assessment, it is DSHA, not PSHA, that has actually been used in seismic hazard assessment for building and bridge engineering, particularly in California.     

喜马拉雅地区与天山地区地壳强地震的孕震因素探讨

利用哈佛大学GCMT数据中心和前人积累的历史地震资料(1962~2016年M W>4.0地震)以及Crust2.0地壳结构统计分析了喜马拉雅地区、天山地区的地壳区域构造与地震活动间的相关性。此外,利用GFZ地学研究中心提供的静态卫星重力模型GGM03S/EGM2008和地形模型Topo计算了2个地区的各类重力异常场,同时还模拟了不同地壳弹性参数下的重力异常场,结果表明喜马拉雅地区重力异常场在水平、垂直方向的梯度特征远大于天山地区的异常特征,且喜马拉雅地区的有效弹性板厚度Te(6~15km)小于天山地区的有效弹性板厚度Te(20~30km)。最后,利用喜马拉雅地区与天山地区的GPS震间三维形变场约束了断层运动模型,结果显示两者主前缘断裂的断层闭锁深度及应力积累状态存在较大的差异。因此认为,造成青藏高原及邻区的边界地壳区域地震活动性差异的动力学因素,与地壳有效弹性板厚度、孕震断层参数及区域应力积累状态等密切相关。     

The Seismotectonics of the Po Plain (Northern Italy): Tectonic Diversity in a Blind Faulting Domain

We present a systematic and updated overview of a seismotectonic model for the Po Plain (northern Italy). This flat and apparently quiet tectonic domain is, in fact, rather active as it comprises the shortened foreland and foredeep of both the Southern Alps and the Northern Apennines. Assessing its seismic hazard is crucial due to the concentration of population, industrial activities, and critical infrastructures, but it is also complicated because (a) the region is geologically very diverse, and (b) nearly all potential seismogenic faults are buried beneath a thick blanket of Pliocene–Pleistocene sediments, and thus can be investigated only indirectly. Identifying and parameterizing the potential seismogenic faults of the Po Plain requires proper consideration of their depth, geometry, kinematics, earthquake potential and location with respect to the two confronting orogens. To this end, we subdivided them into four main, homogeneous groups. Over the past 15 years we developed new strategies for coping with this diversity, resorting to different data and modeling approaches as required by each individual fault group. The most significant faults occur beneath the thrust fronts of the Ferrara-Romagna and Emilia arcs, which correspond to the most advanced and buried portions of the Northern Apennines and were the locus of the destructive May 2012 earthquake sequence. The largest known Po Plain earthquake, however, occurred on an elusive reactivated fault cutting the Alpine foreland south of Verona. Significant earthquakes are expected to be generated also by a set of transverse structures segmenting the thrust system, and by the deeper ramps of the Apennines thrusts. The new dataset is intended to be included in the next version of the Database of Individual Seismogenic Sources (DISS; http://diss.rm.ingv.it/diss/, version 3.2.0, developed and maintained by INGV) to improve completeness of potential sources for seismic hazard assessment.     

Mapping three-dimensional co-seismic surface deformations associated with the 2015 <Emphasis Type="Italic">M</Emphasis><Subscript>W</Subscript>7.2 Murghab earthquake based on InSAR and characteristics of crustal strain

Three-dimensional (3D) co-seismic surface deformations are of great importance to interpret the characteristics of coseismic deformations and to understand the geometries and dynamics of seismogenic faults. In this paper, we propose a method for mapping 3D co-seismic deformations based on InSAR observations and crustal strain characteristics. In addition, the search strategy of correlation points is optimized by adaptive correlation distance, which greatly improves the applicability of the proposed method in restoring deformations in decorrelation areas. Results of the simulation experiment reveal that the proposed method is superior to conventional methods in both the accuracy and completeness. The proposed method is then applied to map the 3D co-seismic surface deformations associated with the 2015 M_W7.2 Murghab earthquake using ascending and descending ALOS-2 PALSAR-2 images. The results show that the seismogenic fault is the Sarez-Karakul fault (SKF), which is dominated by NE-SW strike slips with an almost vertical dip angle. The north section and the south segment near the epicentre have obvious subsidence along with a southwestward motion in the northwest wall, and the southeast wall has northeast movement and surface uplift trend along the fault zone. The strain field of the earthquake is also obtained by the proposed method. It is found that the crustal block of the seismic area is obviously affected by dilatation and shear forces, which is in good agreement with the movement character of the sinistral slip.     

2022年门源MS6.9和2016年门源MS6.4地震序列比较分析

2022年1月8日青海省海北州门源县发生M_S6.9地震,震中距离2016年1月21日门源M_S6.4地震震中约33km,两次门源地震均发生在冷龙岭断裂附近,但在震源机制、主发震断层破裂过程及地震序列余震活动等方面显著不同。针对两次门源地震序列的比较分析,对研究冷龙岭断裂及其附近区域强震序列和余震衰减特征等具有重要研究意义。通过对比分析2022年门源M_S6.9地震和2016年门源M_S6.4地震余震的时空演化特征,发现二者在震源过程和断层破裂尺度上存在明显差异,前者发震断层破裂充分,震后能量释放充分,余震丰富且震级偏高;而后者发震断层未破裂至地表,余震震级水平偏低。综合分析两次门源地震序列表现出来的差异性,认为其可能与地震发震断层的破裂过程密切相关,且同时受到区域构造环境的影响。     

SEISMOTECTONICS OF THE 8 AUGUST 2017 JIUZHAIGOU EARTHQUAKE AND THE THREE-DIMENSIONAL FAULT MODELS IN THE SEISMIC REGION

On 8 August 8 2017, an M_S7.0 earthquake occurred in Jiuzhaigou County, Sichuan Province. Field geological investigations did not find any co-seismic surface rupture in the epicenter area, implying that the seismogenic structure is likely a hidden active fault. Based on the results of the relocated aftershocks, the seismogenic fault was simulated and characterized using the SKUA-GOCAD software. The three-dimensional model of the seismogenic fault was preliminarily constructed, which shows that the main shock of the Jiuzhaigou M_S7.0 earthquake occurred at the sharp bending area of the fault surface, similar to the geometry of the active fault that generated several major earthquakes in the Songpan area during 1973-1976. Our study suggests that high seismicity of this area may be closely related to the inhomogeneous geometry of the fault surface. In this work, we collected the historical earthquakes of M ≥ 6.5, and analyzed the geometric and kinematic features of the active faults in the study area. A three-dimensional fault model for the 10 main active faults was constructed, and its limitation in fault modeling was discussed. It could provide evidence for analyzing the seismotectonics of historical earthquakes, exploring the relationships between earthquakes and active faults, and predicting major earthquakes in the future.     

Research on the Seismotectonic Setting in Beibu Gulf Waters

In this paper we discuss the seismogenic structures of the two earthquakes occurring in the Beibu Gulf waters,based on the investigation of geological structure,faults,geophysical fields,seismicity and earthquake focal mechanism.The results show that the NEE trending faults along the boundary of the Wushi depression are the dominating seismogenic structures,and the NW trending faults are the key factors of stress concentration.The results also show that high seismicity has a significant relationship with the late Cenozoic deposition center in the Beibu Gulf,and their NE-NEE trending boundary faults are important to the identification of seismogenic structures.The special arcuate fault segment or the intersection point of the NW trending faults and NE-NEE trending boundary faults is a possible location of future earthquakes.     

Earthquake parameters and spatial distribution of coseismic effects in Southern Siberia and Mongolia

In this work we review earthquakes that happened in Southern Siberia and Mongolia within the coordinates of 42°–62° N and 80°–124° E and first propose relationships between earthquake parameters (a surface-wave earthquake magnitude M _s and an epicentral intensity(I ₀) based on the MSK-64 scale) and maximal distances from an earthquake epicenter (R _{e max}), hypocenter (R _{h max}), and a seismogenic fault (R _{f max}) to the localities of secondary coseismic effects. Special attention was paid to the study of these relationships for the effects of soil liquefaction. Hence, it was shown that secondary deformations from an earthquake were distributed in space away from an earthquake epicenter, than from an associating seismogenic fault. The effects of soil liquefaction are manifested by several times closer to a seismogenic fault, than all other effects, regardless of the type of tectonic movement in a seismic focus. Within the 40 km zone from an earthquake epicenter 44% of the known manifestations of liquefaction process occurred; within the 40 km zone from a seismogenic fault—90%. We propose the next relationship for effects of soil liquefaction: M _s = 0.007 × R _{e max} + 5.168 that increases the limits of the maximum epicentral distance at an earthquake magnitude of 5.2 ≤ M _s ≤ 8.1 as compared to the corresponding relationships for different regions of the world.     

2020年7月12日唐山5.1级地震分析

对2020年7月12日唐山5.1级地震的发震特点、地震的性质、发震构造以及破裂机制进行初步分析,推测唐山断裂可能为其控震断裂。地震前唐山地区和震中所处的华北构造区的地震活动性异常以缺震和显著平静为主,表明该区域地壳应力积累到了一定程度。分析认为：此次唐山5.1级地震属于1976年7月28日唐山7.8级大震震区内的地震起伏活动;此次地震的序列本身并不丰富,震区烈度偏低、有感范围大。     

Estimation of the Maximum Earthquake Magnitude, <Emphasis Type="Italic">m</Emphasis><Subscript>max</Subscript>

This paper provides a generic equation for the evaluation of the maximum earthquake magnitude m_max for a given seismogenic zone or entire region. The equation is capable of generating solutions in different forms, depending on the assumptions of the statistical distribution model and/or the available information regarding past seismicity. It includes the cases (i) when earthquake magnitudes are distributed according to the doubly-truncated Gutenberg-Richter relation, (ii) when the empirical magnitude distribution deviates moderately from the Gutenberg-Richter relation, and (iii) when no specific type of magnitude distribution is assumed. Both synthetic, Monte-Carlo simulated seismic event catalogues, and actual data from Southern California, are used to demonstrate the procedures given for the evaluation of m_max.The three estimates of m_max for Southern California, obtained by the three procedures mentioned above, are respectively: 8.32 ± 0.43, 8.31 ± 0.42 and 8.34 ± 0.45. All three estimates are nearly identical, although higher than the value 7.99 obtained by Field et al. (1999). In general, since the third procedure is non-parametric and does not require specification of the functional form of the magnitude distribution, its estimate of the maximum earthquake magnitude m_max is considered more reliable than the other two which are based on the Gutenberg-Richter relation.