Stress Changes due to Recent Seismic Events in the Central Apennines (Italy)

The Central Apennines, Italy, are characterized by moderate seismic activity on normal faults, oriented in directions parallel to the Apenninic chain. The subject of this study is the Umbria-Marche Apennines, a segment approximately 200-km long, where three main seismic events occurred in the last three decades. The 1979 Norcia earthquake was a M_w = 5.8 event, taking place at the south end of the considered segment. The 1984 Gubbio earthquake was a M_w = 5.6 event which took place at the north end. The 1997-1998 Colfiorito sequence constituted 8 main shocks with magnitudes M_w between 5 and 6 and epicenters located between the Gubbio and the Norcia earthquake areas. A model made of an elastic half-space is considered, in which the seismic sources are represented by rectangular dislocations which have the appropriate values of source parameters, and in which the static stress field produced by each event is calculated. The analysis of the Coulomb stress change (ΔC) as a function of time shows that the coseismic stress transfer and fault interaction played an important role in the region during the past three decades: 7 earthquakes of the 9 considered took place where ΔC>0. Such an interaction has been confirmed by the analysis of the aftershocks in the Colfiorito zone post September 26, 1997: about the 61% of the aftershocks considered took place where ΔC>0. The comparison between the ΔC^s due to the coseismic stress transfer and the rate ΔĊ^t due to the tectonic stress allows us to quantify the time advance of the earthquakes. The ΔC^s pattern shows positive values in two areas that can be regarded as historical seismic gaps.     

Static stress changes and fault interaction during the 1997 Umbria-Marche earthquake sequence

We study the static stress changes caused by moderatemagnitude earthquakes that occurred in Umbria-Marcheduring a seismic sequence which started on September3, 1997, with a M_L 4.7 foreshock and consisted ofeight earthquakes whose magnitudes range between 5.0and 6.0. The earthquakes occurred on normal faultsstriking in the Apennine direction and dipping at lowangles towards the SW. The goal is to verify if stresschanges induced by each mainshock can explain theoccurrence of subsequent events. Our results show thatthe foreshock slightly increased the Coulomb stress onthe first mainshock fault plane. The distribution ofseismicity that followed the foreshock is clustered inthe area of Coulomb stress increase comprised betweenthe two faults which ruptured in opposite directionsduring the two largest shocks of September 26. Thelocations and the geometry of the three largestearthquakes agree well with the pattern of Coulombstress changes suggesting elastic interaction betweenthese faults. However, we were not able to model thewhole sequence of M_L 5.0 events in terms ofCoulomb stress changes. The difficulties are due tothe similarity of fault plane solutions for eventslocated very close to each other and in the hangingwall of the mainshock rupture planes. Our results showthat normal stress changes agree better with thespatial pattern of the whole sequence of moderatemagnitude events. If previous ruptures unclamp thefault planes of subsequent earthquakes, fluid flow canplay a dominant role in promoting earthquakes duringthe seismic sequence.     

Regional seismicity triggered by the Ms=7.8 Tangshan event on July 28, 1976 and the static stress field change

We studied the seismicity before and after the MS=7.8 Tangshan event on July 28, 1976 (39(28(N, 118(11(E) and the results show that in 3 regions outside of the source zone, seismicity rate increasing were observed, which was significant in 0.99 significance level using Z-statistic test and was proposed to be triggered by the MS=7.8 Tangshan earthquake. The magnitude of the greatest triggered event was 5.5. The epicenter distances of these earthquakes were several ten kilometers to 300 km. The static stress change (CFS of Coulomb failure was calculated using an elastic dislocation model in half space and the (CFS on the major rupture directions in these three regions were positive.     

Damage‐viscoplastic consistency model for rock fracture in heterogeneous rocks under dynamic loading

This paper presents a damage‐viscoplastic consistency model for numerical simulation of brittle fracture in heterogeneous rocks. The model is based on a combination of the recent viscoplastic consistency model by Wang and the isotropic damage concept with separate damage variables in tension and compression. This approach does not suffer from ill‐posedness, caused by strain softening, of the underlying boundary/initial value problem since viscoplasticity provides the regularization by introducing a length scale effect under dynamic loading conditions. The model uses the Mohr–Coulomb yield criterion with the Rankine criterion as a tensile cut‐off. The damage law in compression is calibrated via the degradation index concept of Fang and Harrison. Thereby, the model is able to capture the brittle‐to‐ductile transition occurring in confined compression at a certain level of confinement. The heterogeneity of rock is accounted for by the statistical approach based on the Weibull distribution. Numerical simulations of confined compression test in plane strain conditions demonstrate a good agreement with the experiments at both the material point and structural levels as the fracture modes are realistically predicted. Copyright © 2009 John Wiley & Sons, Ltd.     

从新疆地区主余震活动看地震静态应力触发模型

选取新疆地区1970年以来15次MS6.0以上地震作为源地震,MS4.0以上余震作为目标余震,对地震静态应力触发模型进行检验分析.结果表明,地震静态应力触发模型在新疆地区存在问题,80％的源地震发生后,其△CFS正区目标余震数量少于负区目标余震数量;33％的源地震发生后,其正区目标余震数量远少于负区目标余震数量(前者不...     

2010年4月14日玉树MS7.1地震对余震的触发研究

研究了2010年4月14日青海玉树7.1级地震产生的静态库仑破裂应力变化对余震及区域地震活动空间分布的影响.在考虑震源区附近区域构造应力场的基础上,由震源区构造应力和主震破裂产生的应力叠加,计算得到最容易破裂的余震断层面,进而计算玉树主震在上述余震断层面上产生的库仑破裂静态应力变化.结果表明,库仑应力变化图像与余震分布较为吻合,说明玉树主震对大部分余震有触发作用.利用相同方法计算了玉树主震对周边更大范围内地震的应力触发影响,发现大部分区域地震活动空间分布与库仑应力变化图像基本一致,中小地震大多发生在正向触发区域,但量值较小.对余震断层面与主震完全一致的情形进行了同样的计算,与前述方法计算结果的对比显示,考虑区域应力场的计算方式所得库仑破裂应力变化图像,能够更好地与余震及区域中小地震空间分布图像相吻合.     

鲜水河断裂带库仑应力演化与强震间关系

以鲜水河断裂带为研究区,首先验证了该断裂带上1893年以来M6.7以上地震的相互触发作用,然后采用更符合实际的分层黏弹介质模型研究强震震后黏滞松弛引起的库仑应力变化对后续地震的影响,并基于负位错理论计算鲜水河断裂带10个断层段的震间长期构造加载作用引起的断层上的应力积累.在此基础上,讨论同震、震后、震间效应引起的累积库仑应力变化与区域强震活动的关系,给出断层上库仑应力随时间的演化.结果表明,鲜水河断裂带上1893年以来发生的7次强震均在其前面一系列强震及构造应力加载的驱使下发生,同震、震后、震间三方面效应均引起了鲜水河断裂带不可忽略的库仑应力变化.对断层上的库仑应力状态的研究,可进一步为揭示地震的发生规律、找寻危险断层段提供线索.     

基于InSAR同震形变观测反演2010年新西兰南岛Mw7.1 Darfield地震同震破裂分布

2010年9月4日新西兰南岛Canterbury平原发生了M_w7.1地震,震源深度约为10 km.本次地震发生在一条震前不为人所知的断层上.我们利用覆盖整个震区的合成孔径雷达(SAR)观测资料,通过干涉处理分析获得雷达视线向(LOS)同震形变场;以此资料为约束反演了断层的几何参数以及同震破裂分布.结果显示,该地震造成四条相对独立断层的破裂.大部分的地震矩释放发生在Greendale断层(编号1-4),其错动以右旋走滑为主,最大破裂约为8.5 m.其它三条断层中,经过震源的逆冲断层最大破裂为5.1 m (编号6),位于Greendale断层以西的逆冲断层最大破裂为3.5 m (编号5),位于Greendale断层北面的走滑断层最大破裂为1.9 m(编号7).反演的Greendale断层地表滑动与地质调查得到的地表破裂在形态和数值上均吻合较好.本次地震释放的地震矩为5.0×10¹⁹N·m,矩震级为7.1.板块边界带形变场分析表明,Darfield地震的发生受边界带应变分配在该地区残留构造应力场控制,其复杂性体现了区域构造应力场的特点.地震对其周围地区的应力场影响较大,库仑应力增加区与余震分布有一定对应关系,并在2011年Christchurch 6.3级地震发震断层区域造成约0.1bar的库仑应力增加,对此地震有一定的触发作用.     

Influence of the 2008 Wenchuan earthquake (Mw 7.9) on the occurrence probability of future earthquakes on neighboring faults

The Longquan–Shan fault and the Huya fault are two major neighboring faults of the Longmen–Shan fault zone where the 12 May 2008 Wenchuan earthquake (M_w 7.9) occurred. To study the influence of the Wenchuan event on these two active faults, we calculate changes of Coulomb stress on the Longquan–Shan fault and the Huya fault caused by the Wenchuan mainshock. Our results indicate that the Coulomb stress in the northern section (Zone A) of the Longquan–Shan fault is increased by 0.07–0.10 bars, that in the middle section (Zone B) by 0.04–0.11 bars, and that in the southern section (Zone C) shows almost no change. For the Huya fault, the Coulomb stress is decreased by 0.01–0.03 bars in the northern section (Zone A), 0.10–0.35 bars in the middle section (Zone B), and nearly 0.5 bars in the southern section (Zone C). The epicenter distribution of small earthquakes (M_L ⩾ 1.5) on the Longquan–Shan fault and the Huya fault after the Wenchuan earthquake is consistent with the distribution of the Coulomb stress change. This implies that the Wenchuan earthquake may have triggered small events on the Longquan–Shan fault, but inhibited those on the Huya fault. We then use the rate/state friction law to calculate the occurrence probability of future earthquakes in the study region for the next decade. They include the distribution of b-values, magnitude of completeness (M_c), the background seismicity rate, a value of Aσ_n and the duration for the transient effect (t_a) in the study region. We also estimate the earthquake occurrence probabilities on the neighboring faults after the Wenchuan earthquake. Our results show that, the occurrence probability of future earthquakes in the Longquan–Shan has a slight increase, being 7% for M ⩾ 5.0 shocks during the next decade, but the earthquake probability in the Huya region is reduced obviously, being 5–20%, 7–26% and 3–9% for M ⩾ 5.0 shocks during the next decade in sections A, B and C of the Huya fault, respectively.     

Potential of future seismogenesis in Hebei Province (NE China) due to stress interactions between strong earthquakes

Northeast China, a densely populated area, is affected by intense seismic activity, which includes large events that caused extensive disaster and tremendous loss of life. For contributing to the continuous efforts for seismic hazard assessment, the earthquake potential from the active faults near the cities of Zhangjiakou and Langfang in Hebei Province is examined. We estimate the effect of the coseismic stress changes of strong (M ⩾ 5.0) earthquakes on the major regional active faults, and mapped Coulomb stress change onto these target faults. More importantly our calculations reveal that positive stress changes caused by the largest events of the 1976 Tangshan sequence make the Xiadian and part of Daxing fault, thus considered the most likely sites of the next strong earthquake in the study area. The accumulated static stress changes that reached a value of up to 0.4 bar onto these faults, were subsequently incorporated in earthquake probability estimates for the next 30 years.