Seismicity around the Hekla and Torfajökull volcanoes, Iceland, during a volcanically quiet period, 1991–1995

The volcano Hekla in south Iceland had its latest eruption in January–March 1991. The eruption was accompanied and followed by considerable seismic activity. This study examines the seismicity in the Hekla region (63°42′–64°18′N, 18°30′–20°12′W) during a period when the high activity related to the eruption had ceased, from July 1991 to October 1995. The aim is to define the level of the normal background seismicity of the area that can be compared to the eruption-related activity. The Hekla Volcano proper was generally aseismic during the study period. The most prominent earthquake cluster is in the neighbouring Torfajökull Volcano. The epicentres are concentrated in the western part of the caldera and west of it. The hypocentres are located at all depths from the surface down to 14?km, with highest activity at 5–12?km. Inside this cluster, in the northwest part of the caldera, is a spherical volume void of earthquakes, approximately 4?km in diameter and centred at 8?km depth. This is interpreted as a cooling magma body. Small, low-frequency events of volcanic origin were occasionally recorded at Torfajökull. This activity has mainly occurred in swarms and was most abundant during the first year of the study period, presumably reflecting some kind of connection to the 1991 Hekla eruption. Our study area also includes the easternmost section of the South Iceland seismic zone, a transform zone characterized by bookshelf faulting on transverse faults. Two lineaments of epicentres were identified, roughly corresponding to mapped faults of the South Iceland seismic zone. The hypocentres are relatively deep, mainly at 6–12?km, matching the general trend of hypocentral depth increasing toward the east. The seismicity is highest in the area of the mapped faults. However, the epicentres extend beyond them and indicate greater width of the South Iceland seismic zone, or 20–30?km rather than approximately 10?km as indicated by the length of the surface faults. The seismicity in the volcanic systems of Hekla and Vatnafjöll shows some characteristics of the South Iceland seismic zone. Epicentres are concentrated into two N–S lineaments, one of which coincides with the location of the 1987 Vatnafjöll earthquake (M_w=5.9), a strike-slip event on a N- to S-trending fault. The hypocentres of the Hekla–Vatnafjöll events are mainly at 8–13?km depth, which indicates a continuation of the depth trend of the earthquakes of the South Iceland seismic zone. The events located at Hekla proper and immediately north of it are all of low-frequency character, which can be held as an indication of volcanic origin. On the other hand, they show clear S arrivals at observing stations like normal high-frequency tectonic earthquakes.     

A FINITE ELEMENT MODEL FOR SEISMICITY INDUCED BY FAULT INTERACTION

1　IntroductionInsouthwestregionofChina ,strongearth quakesarefrequentandinteractional.This paperpersentsafiniteelementmodelforstrongearth quakesinthisregion ,whichmayprovideatheoreti calbasisforearthquakesprediction .Amongstudiesonsimplifiedmodelthatsimu latedseismicactivities,thestudiesonspringslipmassandautomaticmachinemodelarethemostrep resentative .Althoughthiskindofcalculationissim pleandhasconsideredthecrucialfactors,thusithelpsunderstandcomplexphysicalprocessofseismicactivities,itisov…     

An earth systems approach to understanding the tectonic and cultural landscapes of linked marine embayments: Avon‐Heathcote Estuary (Ihutai) and Lake Ellesmere (Waihora), New Zealand

An earth systems study of the Avon‐Heathcote Estuary (Ihutai), New Zealand, shows that tectonic activity has a marked direct and indirect control upon its geomorphology and human settlement in the area. We discuss the Late Holocene history of the embayment in relation to large earthquakes and their after‐effects. Of particular note is the rapid fluvial transfer of sand to the coast causing dune formation and a more delayed pulse of coarser sediment causing channel avulsion of the Waimakariri River. While dune system development seems to occur soon after tectonic activity, river channel avulsion, spit/barrier formation and ongoing geomorphological changes may well relate to periods of tectonic activity that occurred 100–200 years previously. The interaction between these two sediment delivery systems causes significant, and often rapid, changes to coastal geomorphology and ecosystems that have serious implications for human populations living at or near the coast. We show a more region‐wide picture of the direct and indirect effects of tectonic activity, by comparing two embayments that represent coastal points of entry at opposite ends of the Waimakariri River floodplain: the Avon‐Heathcote Estuary (Ihutai) and Lake Ellesmere (Waihora). Copyright © 2005 John Wiley & Sons, Ltd.     

顾及先验信息的模型参数反演——澜沧-耿马地震位错模型参数的求定

采用澜沧－耿马地震前后两期大地测量资料，结合发震区的地质资料和震后地表形变破坏带的调查资料，运用顾及参数先验信息的贝叶斯反演方法，联合求定了澜沧－耿马地震的两个发震断层的位错模型参数。由反演后的位借模型推算的地震应力降同用地震波资料估算的应力降属同一量级     

Significant duration predictive models developed from strong-motion data of thrust-faulting earthquakes recorded in Mexico City

Predictive models for estimating strong-motion duration in sites characterized by soft-soil profiles are presented in this paper. The models were developed using a strong-motion database that includes observations from subduction interface earthquakes that occurred from 1989 to 2020 and recorded in Mexico City, which is located at source-to-site distances up to 600 km. A linear mixed-effects regression model, which is a statistical fitting procedure that allows to consider the correlation structure of grouped data, was used to develop the predictive models. Relative significant duration was selected to measure strong-motion duration. This measure can be directly associated with the accumulation of energy of the ground movement. The proposed predictive models relate relative significant duration with moment magnitude, either hypocentral distance or closest distance to the rupture plane, and dominant period of the soil. Regression analyses were performed grouping the ground-motion data by both seismic event and site class. Model assumptions, such as homoscedasticity, normality, and linearity of effects, were verified from residual analyses. From the results, the expected value of the natural logarithm of relative significant duration was found to be ～1.2 times greater for an earthquake with a moment magnitude equal to 8.0 than for one of 6.0. An insightful discussion about the sources and character of the uncertainties detected in the proposed predictive models is also presented in this study. The predictive models proposed in this paper are of valuable application in seismic and structural engineering because they allow to circumscribe properly the dimension and randomness of strong-motion duration.     

Seismic performance of bridges with novel SMA cable-restrained high damping rubber bearings against near-fault ground motions

This study presents a novel type of shape memory alloy (SMA) cable-restrained high damping rubber (SMA-HDR) bearing, which is particularly suited to near-fault (NF) regions where the pulsing effect potentially exists in the ground motions. The working mechanism of the bearing is first described, followed by an experimental investigation on a full-scale SMA-HDR bearing specimen. The test results confirm the efficient restraining effect offered by the SMA cables, which contribute to 65% and 24.4% of the lateral load resistance and total energy dissipation, respectively, prior to the initial fracture of the SMA cables. The failure of the cables is initiated near the end grip where moderate stress concentration exists at this region. Following the experimental study, the numerical modeling strategy for the bearing is discussed, and a case study is then presented, demonstrating the application of the SMA-HDR bearings in the Datianba #2 highway bridge, a real project that first adopts the proposed bearings in the world. A simplified design process is introduced for the bridge with novel SMA-HDR bearings to mitigate the potential damage during strong earthquakes especially the NF ones. The system-level analysis on the prototype bridge shows that the novel SMA-HDR bearings equipped with ten 7×7×1.2 SMA cables in each bearing could reduce the average maximum bearing displacement (MBD) by nearly 30% compared with the conventional bridge with HDR bearings. The application of the novel SMA-HDR bearing can significantly alleviate the pounding effect, especially under the NF earthquakes. The presence of the SMA cables tends to increase the maximum force response of the piers, but this effect is minor and under control.     

北京地区小震精定位结果初步探讨

利用双差重定位方法对北京地区(39.2°N～41.2°N,115.2°E～117.6°E)2008年10月至2020年3月期间共6463个地震事件进行重新定位,得到4776个高质量地震事件的空间位置分布。定位结果均方根走时残差显著降低,震源间的相对位置精度明显提高,地震活动的成丛性与活动断层之间的关系更加密切。根据重定位结果探讨了小震活动与活动断裂及深浅构造的关系,得到以下结论:夏垫断裂小震活动强烈,重定位后震中分布明显向断裂靠拢,呈现沿断裂走向延伸的NE向线状分布。结合人工剖面探测结果推测,该区域深度剖面上小震集中带10km深度附近较宽的丛集区主要是浅部铲形断裂活动所致,14～22km深度附近相对较窄近直立的小震丛集为深部断裂活动产生。怀—涿次级盆地北缘断裂现今小震活动活跃,该区域小震深度剖面揭示该断裂为倾向南东、上陡下缓的铲形断层。结合人工剖面探测结果,分析认为该区域深浅断裂共存和非均匀的地壳结构为中强地震发生的深部构造背景。平原区内部小震分布与第四纪断陷盆地和地层厚度分布存在较强的相关性,研究区域内部昌平—海淀小震集中区、顺义小震集中区、夏垫小震丛集区和廊坊小震丛集区分别位于马池口—沙河断陷盆地、顺义断陷盆地、大厂断陷盆地和固安—廊坊—永清断陷盆地。根据小震空间密集成带分布特征来推测未知断裂,推测怀柔南部山区小震集中区下方存在隐伏断层,宝坻至夏垫断裂段之间NW向的小震集中分布区下方存在一条走向NW或者NWW的隐伏断层。     

山东乳山震群重复地震P波频散衰减特征分析

利用波形互相关方法识别乳山震群中的重复地震,挑选一组在时间上跨越2015年5月22日M_L 5.0地震的重复地震序列（repeating earthquake sequences,RES）,利用测算单台直达P波第一个周期信号频散衰减特性的方法,估算乳山震群震源区介质品质因子,结果显示,不同路径下Q_m值随时间的变化形态基本一致,即此次M_L 5.0地震前Q_m值升高,临震及震后Q_m值下降并恢复至震前水平。同时,通过对震源区近场及远场应力状态的相关性分析,认为此次M_L 5.0地震发生在远场作用下区域构造应力状态调整过程中,与区域构造的介质状态、断层分布、速度结构密切相关。     

Emergence of non-extensive seismic magnitude-frequency distribution from a Bayesian framework

Non-extensive statistical mechanics has been used in recent years as a framework in order to build some seismic frequency-magnitude models. Following a Bayesian procedure through a process of marginalization, it is shown that some of these models can arise from the result shown here, which reinforces the relevance of the non-extensive distributions to explain the data (earthquake’s magnitude) observed during the seismic manifestation. In addition, it makes possible to extend the non-extensive family of distributions, which could explain cases that, eventually, could not be covered by the currently known distributions within this framework. The model obtained was applied to six data samples, corresponding to the frequency-magnitude distributions observed before and after the three strongest earthquakes registered in Chile during the late millennium. In all cases, fit parameters show a strong trend to a particular non-extensive model widely known in literature.     

Hazard analysis of seismic submarine slope instability

To assess the risk associated with a submarine landslide, one must estimate the probability of slope failure and its consequences. This paper proposes a procedure to estimate the probability of earthquake-induced submarine slope failure (hazard) based on probabilistic seismic hazard analyses, ground response analyses and advanced laboratory tests. The outcomes from these analyses are treated in a probabilistic framework, with analytical simulations using mathematical techniques such as the first-order reliability method, Monte Carlo simulation and Bayesian updating. Fragility curves of slope failure during the earthquake (co-seismic) and after the earthquake (post-seismic) were developed in this study, and were shown to provide a clear and well-organized procedure to estimate the annual failure probability of a submarine slope under earthquake loading.