Improving automatic location procedure by waveform similarity analysis: An application in the South Western Alps (Italy)

The accuracy of automatic procedures for locating earthquakes is influenced by several factors such as errors in picking seismic phases, network geometry, modeling errors and velocity model uncertainties. The main purpose of this work is to improve the performances of the automatic procedure employed for the “quasi-real-time” location of seismic events in North Western Italy by developing a procedure based on a waveform similarity analysis and by using only one seismic station.To detect “earthquake families” a cross-correlation technique was applied to a data set of seismic waveforms recorded in the period 1985-2002, in a small test area (1600 km²) located in the South Western Alps (Italy). Normalized cross-correlation matrices were calculated using about 2700 seismic events, selected on the basis of the signal to noise ratio, manually picked and located by using the Hypoellipse code. The waveform similarity analysis, based on the bridging technique, allowed grouping about 65% of the selected events into 80 earthquake families (multiplets) located inside the area considered. For each earthquake family a master event is selected, manually re-picked and re-located by using Hypoellipse code. Having chosen a reference station (STV) on the basis of the completeness of the available data set, an automatic procedure has been developed with the aim of cross-correlating new seismic recordings (automatically picked) to the waveforms of the events belonging to the detected families. If the new event is proved to belong to a family (on the basis of the cross-correlation values), its hypocenter co-ordinates are defined by the location of the master event of the associated family. The performance of the proposed procedure is tested and demonstrated using a data set of 104 selected earthquakes recorded in the period January 2003-June 2004 and located in the test area. The automatic procedure is able to locate, associating events with the multiplets detected by the waveform similarity analysis, about 50% of the test events, almost independently of the accuracy of the automatic phase picker and without the biasing of the network geometry and of the velocity model uncertainties.     

甘肃塌陷地震震相识别研究

选取甘肃省测震台网测定的甘肃平凉地区塌陷地震、甘东南地区天然地震各50个,同地区的30个地震事件为待测事件.采用近年来计算机领域里较先进的图像识别方法——卷积神经网络识别两种地震事件类型,设定波形通道总数的80％为地震事件分类阈值,超过分类阈值的设定为"0"或"1",即可判定该待测事件为对应的事件类型.将地震事件作为原...     

Hypocenter location accuracy and seismicity distribution in the Central Apennines (Italy)

In this paper, we analyse the seismicity distribution in the CentralApennines (Italy) using the recordings of the Rete SismometricaMarchigiana (RSM). In particular, the selected events are relocated usinga 1-D model calculated by means of an inversion procedure. The robustnessof the 1-D model and the location accuracy are tested. The capability ofthe RSM to well constrain crustal and subcrustal events in the studied areais discussed. We find that in the inner side of the chain the seismicity liesin the upper crustal layers, following the structural trend of the Apenninicbelt. A W-deepening of the events is observed in the Apenninic foredeep,where the seismicity is mainly confined in the lower crust. This evidenceimplies the deepening of the brittle to ductile transition. Some well-locatedsubcrustal events are found. Their locations seem to confirm the W-dippingsubduction of the Adriatic lithosphere beneath the Apennines.     

基于双差定位法的鄂尔多斯块体西北缘地震精定位研究

针对鄂尔多斯块体西北缘地震活动的复杂性及目前台网定位方法存在偏差的问题,采用双差定位法对2009—2019年发生在鄂尔多斯块体西北缘的地震事件进行重新定位.经研究表明,双差定位之后地震分布更集中,鄂尔多斯块体西缘的地震沿银川吉兰泰断陷带分布,北缘的地震沿河套断陷带分布,地震定位精度明显提高,这与块体周缘复杂的地质构造背...     

内蒙古东部地震形势及平均波速比分析

内蒙古东部地区(41°—51°N,115°—125°E)近几年来地震活动性增强,中强地震比较活跃。2009年,辽蒙交界和牙克石-扎兰屯地区的地震形势更加紧迫。基于内蒙古东部的地震形势分析,根据内蒙古东部地区的台站分布、断裂构造及地震空间分布特点等,划分出4个分区;在2003年到2009年的时段内,筛选出符合计算要求的地震事件482个。根据多台和达法,分别计算各个分区的平均波速比值,通过对波速比曲线进行近均值滑动滤波,分析波速比在中强地震前的变化规律,认为可以把震前波速比异常作为中强地震发生的中短期指标。     

A 15-year-Long catalog of seismicity in the Eastern Tennessee Seismic Zone (ETSZ) using matched filter detection

We present a detailed catalog of 13 671 earthquakes in the Eastern Tennessee Seismic Zone (ETSZ) that spans January 1, 2005 to July 31, 2020. We apply a matched filter detection technique on over 15 years of continuous data, resulting in arguably the most complete catalog of seismicity in the ETSZ yet. The magnitudes of newly detected events are determined by computing the amplitude ratio between the detections and templates using a principal component fit. We also compute the b-value for the new catalog and comparatively relocate a subset of newly detected events using XCORLOC and hypoDD, which shows a more defined structure at depth. We find the greatest concentration along and to the east of the New York-Alabama Lineament, as defined by the magnetic anomaly, supporting the argument that this feature likely is related to the generation of seismicity in the ETSZ. We examine seismicity in the vicinity of the Watts Bar Reservoir, which is located about 5 ?km from the epicenter of the M_W 4.4 December 12, 2018 Decatur, Tennessee earthquake, and find possible evidence for reservoir modulated seismicity in this region. We also examine seismicity in the entire ETSZ to search for a correlation between shallow earthquakes and seasonal hydrologic changes. Our results show limited evidence for hydrologically-driven shallow seismicity due to seasonal groundwater levels in the ETSZ, which contradicts previous studies hypothesizing that most intraplate earthquakes are associated with the dynamics of hydrologic cycles.     

A low magnitude seismic sequence near Isernia (Molise,Central Italy) in January 1986

Following the increase in seismic activity which occurred near Isernia (Molise, Central Italy) in January 1986, a digital seismic network of four stations with three-component, short-period seismometers, was installed in the area by the Osservatorio Vesuviano. The temporary network had an average station spacing of about 8–10 km and, in combination with permanent local seismic stations, allowed the accurate determination of earthquake locations during an operating period of about one month. Among the 1517 detected earthquakes, 170 events were selected with standard errors on epicentre and depth not greater than respectively 0.5 and 1.5 km. The most frequent focal depths ranged between 4 and 8 km, while the epicentres distribution covered a small area NE of Isernia of about 10 km². A main rupture zone could not be clearly identified from the spatial distribution of the earthquakes, suggesting a rupture mechanism involving heterogeneous materials. The activity was characterized by low energy levels, the largest earthquake, on January 18, 1986, havingM _D =4.0. The time sequence of events and pattern of seismic energy release revealed a strong temporal clustering of events, similar to the behaviour commonly associated with seismic swarms.     

Seismic expression of active extensional faults in northern Umbria (Central Italy)

This paper deals with the geometry and kinematics of the active normal faults in northern Umbria, and their relationship with the seismicity observed in the area. In particular, we illustrate the contribution of seismic reflection data (a network of seismic profiles, NNW–SSE and WSW–ENE trending) in constraining at depth the geometry of the different active fault systems and their reciprocal spatial relationships. The main normal fault in the area is the Alto Tiberina fault, NNW trending and ENE dipping, producing a displacement of about 5 km, and generating a continental basin (Val Tiberina basin), infilled by up to 1500 m with Upper Pliocene–Quaternary deposits. The fault has a staircase trajectory, and can be traced on the seismic profiles to a depth of about 13 km. A set of WSW-dipping, antithetic faults can be recognised on the profiles, the most important of which is the Gubbio fault, bordering an extensional Quaternary basin and interpreted as an active fault based on geological, geomorphologic and seismological evidence. The epicentral distribution of the main historical earthquakes is strictly parallel to the general trend of the normal faults. The focal mechanisms of the major earthquakes show a strong similarity with the attitude of the extensional faults, mapped at the surface and recognised on the seismic profiles. These observations demonstrate the connection between seismicity in the area and the activity of the normal faults. Moreover, the distribution of the instrumental seismicity suggests the activity of the Alto Tiberina fault as the basal detachment for the extensional tectonics of the area. Finally, the action of the Alto Tiberina fault was simulated using two dimensional finite element modelling: a close correspondence between the concentration of shear stresses in the model and the distribution of the present earthquakes was obtained.     

三峡水库地震活动特征研究

在已有速度模型的基础上,选取三峡水库区529个地震重新确定了速度模型和台站校正,并利用该结果对2 138次地震进行了定位。研究表明,在不同区域震源深度存在明显的差异,仙女山断裂附近的地震平均震源深度为5.6km,而巴东地区的地震平均震源深度为2.6km。以震源深度5km为界,浅于5km的地震的b值为1.16,而深于5km地震的b值为0.90,不同深度地震的频次变化与水位关系差异明显     

2014年鲁甸MS6.5地震前后地震活动性

2014年 8月3日在云南省鲁甸县发生M_S6.5地震,造成了巨大的人员伤亡和财产损失.关于鲁甸地震的起因,特别是鲁甸地震与溪洛渡水库之间的关系存在一些争议.分析中强地震前后地震活动性的变化可以为判断主震起源提供一定的依据.然而,由于鲁甸地震震中附近固定台站数目少、台站距离震中远及主震后尾波干扰等因素的影响,会造成地震目录的缺失,从而可能影响地震活动性的可靠分析.为了获得更加完备的地震目录,本文采用模板匹配方法对鲁甸地震前后小地震进行了搜索和识别.以台网目录中的541个事件作为模板,在主震之前33天到主震后5天的连续波形中识别出991个未在目录中列出的地震,使余震的完备震级从1.8降低到1.3.根据检测目录得到主震前33天内的b值为1.04±0.18,余震的b值在震后5天内由0.5左右逐渐上升至0.9附近并保持稳定;根据2009年到主震发生前的台网目录得到该区域的背景b值为0.93±0.04.即鲁甸地震前后区域地震活动b值均与背景b值接近,这与典型水库诱发的中强震具有的b值特征不相符.同时通过进一步分析台网目录,未发现溪洛渡水库蓄水前后研究区域内的地震活动性存在明显变化,这也与典型水库诱发地震区域的地震活动性特征不相符.综合以上结果,我们认为2014年鲁甸M_S6.5地震不具有典型水库诱发地震的特征.     

Using the ETAS Model for Catalog Declustering and Seismic Background Assessment

The concept of background seismicity is strictly related to the identification of spontaneous and triggered earthquakes. The definition of foreshocks, main shocks and aftershocks is currently based on procedures depending on parameters whose values are notoriously assumed by subjective criteria. We propose a method for recognizing the background and the induced seismicity statistically. Rather than using a binary distinction of the events in these two categories, we prefer to assign to each of them a probability of being independent or triggered. This probability comes from an algorithm based on the ETAS model. A certain degree of subjectivity is still present in this procedure, but it is limited by the possibility of adjusting the free parameters of the algorithm by rigorous statistical criteria such as maximum likelihood. We applied the method to the seismicity of southern California and analyzed the sensitivity of the results to the free parameters in the algorithm. Finally, we show how our statistical declustering algorithm may be used for mapping the background seismicity, or the moment rate in a seismic area.     

铜街子电站水库诱发地震的判定及最大可能诱发地震震级的估计

对于水库围堰后猛然增加的地震活动，人们普遍关心两个问题，第一是这些地震的成因，第二是这些地震能对大坝安全性带来多大的影响。位于四川省乐山地区的铜街子电站在蓄水后地震活动骤然增高。本文通过对地震活动的分布范围、震源深度、震源机制解以及地震活动与库水水位变化之间的关系判定这些地震系铜街子电站蓄水后所诱发的地震。为了估计该水库所诱发地震的震级上限，本文通过定性预测方法和定量预测方法试图对诱震震级上限给出一个估计，但各手段所给出的结果却不完全一致，这说明目前的预测手段的不完善。在对各手段给出的结果进行综合分析之后，认为铜街子电站最大可能诱发地震震级上限在５．０级左右。当然，也可能以较小震级的震群型形式出现。     

2002年夏季长白山天池火山区的地震活动研究

2002年6月以来,长白山天池火山区的地震活动明显增加. 本文利用2002年夏季布设在长白山天池火山区15套宽频带流动地震台站的记录资料,对天池火山区的地震活动进行了研究. 地震观测结果表明,2002年夏季长白山天池火山日平均地震发生频次超过30次. 地震主要位于长白山天池西南部和东北部两个区域,震源深度较浅,离地表的深度一般小于5km. 天池西南部和东北部的地震,b值存在较大的差异. 火山区地震记录的频谱分析和时频分析结果表明,这些地震主要为火山构造型地震. HSZ和DZD等台站地震记录中丰富的低频成分,可能与台站附近的局部介质或断层带有关. 我们认为2002年夏季频繁发生的地震和小震震群活动是由火山深部活动诱发的局部断裂活动引起.     

Prediction and verification of earthquakes induced by the Xiluodu hydropower station reservoir

Research has been conducted on reservoir-induced earthquakes in China since the Xinfengjiang reservoir-induced earthquakes in the 1960s. Regulations now require the risk of reservoir-induced earthquakes to be evaluated in the pre-research stage of all hydropower projects. Although nearly 40 cases of reservoir-induced earthquakes have been reported in China, analyses comparing the changes in seismic activity following reservoir impoundment with predictions are rare. In this study, we compared seismic activities observed in the reservoir area before and after the impoundment of the Xiluodu hydropower station in terms of the spatial distribution, frequency, and focal depths of the earthquakes, and clarified the correlation between their frequency/timing and reservoir level after impoundment. We then concluded that the seismic events in the head region were karst-type earthquakes, while those in the second segment of the reservoir were tectonic earthquakes. The spatial distribution of the earthquake epicenters and the seismic intensities validated some of the results for the reservoir-induced seismic risk assessment for the Xiluodu hydropower station, indicating that the proposed earthquake triggers and predictive models are reasonable. This study can provide a valuable reference for investigating the mechanism (s) of reservoir-induced earthquakes, revising reservoir-induced earthquake hazard assessment codes, and predicting the hazard zones of reservoir-induced seismicity under similar conditions.     

以空间光滑的地震活动性模型为空间分布函数的地震危险性分析方法

根据华北地区的地震目录,建立了4个空间光滑的地震活动性模型,并以这些模型为空间分布函数,将华北地震区每个地震带的地震年发生率分配到空间格点中,计算这一地区的地震危险性．结果表明,采用仪器记录地震计算得到的地震活动性模型和地震危险性结果能够反映华北地区现今的地震活动水平和地震危险性水平,符合人们对现今华北地区地震危险性的认识;采用历史破坏性地震（Mge;4.7）计算的地震活动性模型和地震危险性结果,较好地反映了华北地区中强地震活动区的地震危险性水平;以地震应变计算地震活动率,并根据点椭圆模型和线椭圆模型计算得到的地震活动性模型,能够较好地反映大地震的活动水平和空间构造特征．将根据4个模型计算得到的50年超越概率10%峰值加速度（PGA）分布加权平均,得到综合的华北地区PGA分布,并将该PGA分布与根据《中国地震动参数区划图》中综合潜源方案计算得到的50年超越概率10%的PGA分布做了比较,发现二者无本质差别,均能反映华北地震区的地震危险性水平．当然,二者也具有一定的差异:前者计算得到的符合PGAge;100 cm/s2条件的区域面积明显要比后者的大,而符合PGAge;250 cm/s2条件的区域面积则比后者的要小. 这主要是由于潜在震源区类型和空间分布函数不同造成的．      

Universality of the Seismic Moment-frequency Relation

—We analyze the seismic moment-frequency relation in various depth ranges and for different seismic regions, using Flinn-Engdahl's regionalization of global seismicity. Three earthquake lists of centroid-moment tensor data have been used the Harvard catalog, the USGS catalog, and the Huang et al. (1997) catalog of deep earthquakes. The results confirm the universality of the β-values and the maximum moment for shallow earthquakes in continental regions, as well as at and near continental boundaries. Moreover, we show that although fluctuations in earthquake size distribution increase with depth, the β-values for earthquakes in the depth range of 0–500 km exhibit no statistically significant regional variations. The regional variations are significant only for deep events near the 660 km boundary. For declustered shallow earthquake catalogs and deeper events, we show that the worldwide β-values have the same value of 0.60 ± 0.02. This finding suggests that the β-value is a universal constant. We investigate the statistical correlations between the numbers of seismic events in different depth ranges and the correlation of the tectonic deformation rate and seismic activity (the number of earthquakes above a certain threshold level per year). The high level of these correlations suggests that seismic activity indicates tectonic deformation rate in subduction zones. Combined with the universality of the β-value, this finding implies little if any variation in maximum earthquake seismic moment among various subduction zones. If we assume that earthquakes of maximum size are similar in different depth ranges and the seismic efficiency coefficient, χ, is close to 100% for shallow seismicity, then we can estimate χ for deeper earthquakes for intermediate earthquakes χ≈ 5%, and χ≈ 1% for deep events. These results may lead to new theoretical understanding of the earthquake process and better estimates of seismic hazard.     

Study on Precise Location and Structure of Earthquakes in the Shanxi Reservoir,Zhejiang Province

The Shanxi reservoir earthquakes are significant seismic events in southern Zhejiang Province in recent years, an area with fewer and weaker earthquakes. The seismicity showed an intermittent characteristic and group distribution. The epicenters located by the seismic network did not show a predominant direction and the seismogenic structure is not clear. In the study, the nonlinear imaginary wave travel time equation was linearized and solved, and the source position, initial imaginary velocity and travel ...     

新疆于田地区地震活动与强震间的应力转移

新疆于田地区位于青藏高原与塔里木盆地相交会的地方,强震频发.2008年以来,该地区陆续发生的4次M6以上强震都位于阿尔金断裂带南分支的龙木错 郭扎错断裂带附近.这些地震的发生为研究于田地区中强震活动的空间分布及其之间的相互作用及提供了很好的实例.首先,利用小震目录对区域地震活动进行分析,给出了地震活动性参数b值的空间分...     

Characteristics of Energy Accumulation and Release of Seismogenic Tectonics and Seismic Risk Analysis in Some Areas in the Tianshan Mountains, Xinjiang1

INTRODUCTIONHowtocombinethestudyofseismogenictectonicswithearthquakepredictionisanurgentscientificdifficulty .Thereexistbiggapsbetweenstudymethodsandcurrentknowledgeonseismogenitectonics ,earthquakeprediction ,seismogenesisandthephysicsofearthquakeoccurre…     

Anomalously deep earthquakes in northwestern Italy

It is usually assumed that earthquakes in intraplate regions occur in the upper crust, and northwestern Italy is generally assigned to this kind of normal seismicity. In this work, the depth distribution of the events localized in this area by the Istituto Geofisico Geodetico (IGG) seismic network in the period 1991–1997 is analyzed in detail. In particular, the location capability of the network is discussed, adopting as reference quarry blasts (for the epicentral position) and the locations obtained from a dense temporary network (for the depth estimate). Within the so-obtained error limits, the depth distribution of events show a characteristic pattern: while for most of the area covered by the network the well-located seismicity lies within the first 20 km of depth, in a band following the inner arc of the Western Alps, numerous events have anomalously large focal depths, reaching a maximum of 114 km. These depth determinations cannot be attributed to instabilities of the location procedure: different choices of the propagation models used for the hypocentral determination led to very similar depth values, always significantly larger than the standard values for the surrounding areas. A strong correlation has been found between the 3-dimensional distribution of these foci and the P-wave propagation anomalies obtained from tomographic studies, suggesting a direct link between elastic and rheological properties of lower crust and upper mantle in this area.