Seismicity of Garfagnana-Lunigiana (Tuscany, Italy) as recorded by a network of semi-broad-band instruments

Eight seismic stations equipped with5-second sensors have recently beeninstalled in the Lunigiana-Garfagnana areato monitor activity in this sector of theNorthern Apennines shaken by the 5.0 Mdearthquake of October 1995. The stations(RSLG network) represent an eastern branchof the RSNI (Regional Seismic network ofNorthwestern Italy) managed by Dipteris,Universita' di Genova.The installation of this dense networkallowed the operators to improve themagnitude detection level, providinginformation about the seismicity of thearea down to magnitude 1.0. Severalanalyses have been carried out to evaluatethe performance of the network and thereliability of the computed locations. Itseems that the epicentral location forevents occurring inside the network isalready constrained using 8–10 P+S phases,but usage of more readings ensuresreliability of depth and the reduction oflocation errors.Though not conclusive, as the network hasonly been fully operating for two years, apreliminary study carried out on a selecteddataset of high-quality locations confirmed that the seismicity of theGarfagnana-Lunigiana is characterised by alow-to-medium magnitude level and it issubdivided into two bands, a superficialone about 30–35 km thick and the secondbelow 50 km. This distribution, confirmedby other studies carried out in the past,reflects the complex structure of this areawhere two tectonic plates (European andAdriatic) meet.     

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.     

利用波形互相关方法识别分析灌县—安县断裂重复地震

以中国地震台网中心地震目录中的事件为模板地震,通过滑动窗口的波形互相关方法对布设在灌县—安县断裂周边17个流动地震台的连续地震记录进行处理,识别M_L0.0以上的重复地震. 然后使用结合波形互相关技术的双差算法对这些地震进行重定位,获得了243次地震的重定位结果. 结果表明: 在研究时段内,灌县—安县断裂的地震活动性呈减弱趋势; 地震震源的优势分布深度为5—15 km,震源深度剖面显示地震呈高角度向西倾斜分布; 地震震中沿NE向分布,与龙门山前山断裂的走向基本一致; 研究区内南、 北两段的地震活动性及b值存在差异,这可能与龙门山断裂带中段区域应力方向由南到北发生的WNW向到ENE向转换的构造作用密切相关.      

广东地震台网近震定位精度的经验性GT准则

利用广东地震台网的固定台和流动台记录的2次人工爆破事件,通过随机模拟台网几何形态,获得了该台网近震定位精度的经验性GT（Ground Truth）准则。结果表明：地震定位精度与台网几何形态密切相关;当台网满足以下条件时近震震中定位精度优于2km,震源深度精度优于3km,具有95%的置信度：（1）震中距小于150km的台站数≥7;（2）一级方位角空区<180°;（3）震中距小于10km的台站数≥1。在评估震源深度精度时,发现对于壳内地震,可靠的近台数据能够提高震源深度的精度,而定位时震中距较大的台站数量的增加对震源深度的精度影响很小。最后,应用该GT准则对广东地震台网产出的地震观测报告进行了评估,获得了1160个GT2参考事件。     

Earthquake activity during the 1983 Etna eruption

Seismic stations, with automatic P-picking and satellite retransmission were set up on Mount Etna following the eruption started on March 1983. Positions of the stations were chosen in order to complement the permanent telemetered network of Catania University.Comparison between locations obtained by both networks were made for earthquakes recorded by at least 5 ARGOS DCP (Data Collection Platform) stations. We observed a satisfactory agreement for events inside both networks.By merging data of both networks, it has been possible to locate more than 50 earthquakes for which separate computation was not possible due to the low number of arrivals.On 3rd-4th June a swarm of deep seismic events was observed. Hypocenters of these earthquakes are clearly located in a NNW-SSE-trending vertical zone of 5 km width at a depth of 7–36 km.Changes in the distribution of shallow seismic activity, before and after this swarm, have been observed.     

华北地区中小地震重新定位和地震活动特征研究

选取了华北地区1978—2011年289个台站记录到的24753个地震事件, 使用双差定位方法对这些地震进行了重定位, 获得了20512个地震的重定位结果, 平均定位误差在EW、 NS、 UD方向分别为0.54 km, 0.57 km和0.57 km。 相比于初始结果和绝对定位结果, 双差定位结果在平面上分布更集中, 沿断裂分布特征更加明显, 震源深度也更加合理。 通过剖面分析, 张北地震带、 唐山地震带、 邢台地震带均为高倾角断裂, 南部剖面6震源深度分布有随时间变浅的趋势。 通过与前人的研究结果进行对比, 本次研究使用的时间较长, 地震数量较大, 得到了更加可信的结果。     

Real-time envelope cross-correlation detector: application to induced seismicity in the Insheim and Landau deep geothermal reservoirs

We develop and test a real-time envelope cross-correlation detector for use in seismic response plans to mitigate hazard of induced seismicity. The incoming seismological data are cross-correlated in real-time with a set of previously recorded master events. For robustness against small changes in the earthquake source locations or in the focal mechanisms we cross-correlate the envelopes of the seismograms rather than the seismograms themselves. Two sequenced detection conditions are implemented: After passing a single trace cross-correlation condition, a network cross-correlation is calculated taking amplitude ratios between stations into account. Besides detecting the earthquake and assigning it to the respective reservoir, real-time magnitudes are important for seismic response plans. We estimate the magnitudes of induced microseismicity using the relative amplitudes between master event and detected event. The real-time detector is implemented as a SeisComP3 module. We carry out offline and online performance tests using seismic monitoring data of the Insheim and Landau geothermal power plants (Upper Rhine Graben, Germany), also including blasts from a nearby quarry. The comparison of the automatic real-time catalogue with a manually processed catalogue shows, that with the implemented parameters events are always correctly assigned to the respective reservoir (4 km distance between reservoirs) or the quarry (8 km and 10 km distance, respectively, from the reservoirs). The real-time catalogue achieves a magnitude of completeness around 0.0. Four per cent of the events assigned to the Insheim reservoir and zero per cent of the Landau events are misdetections. All wrong detections are local tectonic events, whereas none are caused by seismic noise.     

基于谱元法数值模拟研究地震判定准则

本文以宁夏区域地震台网为例,分析了波形互相关法在判定重复地震中可能存在的问题并讨论了相应的处理方法,通过构建三维非均匀体模型并利用谱元法数值模拟地震波的传播,统计了不同台站观测到的地震对波形互相关系数的分布,进一步研究了互相关系数与非均匀体性质及震源机制解之间的关系。结果表明:针对宁夏区域地震台网,利用波形互相关法判定重复地震比地震定位方法更有效;互相关系数在不同台站的取值与震源附近三维非均匀体强度和直达波与尾波的振幅比有关,对于相同的震源间距,较弱震源、较弱非均匀体或者较强振幅的直达波均会导致波形互相关系数变高,因此应选取更高的互相关系数阀值来判定重复地震。宁夏区域地震台网平均台间距为30—50 km,通过选取直达波较弱的台站或只截取尾波窗口计算互相关系数并设定较高的阀值,利用波形互相关法可有效地判定M_L1.0—3.0重复地震,进而为重复地震的监测与研究提供依据。      

龙门山断裂带周缘重复地震识别及其在台网定位评价中的应用

利用国家测震台网数据备份中心时间跨度超过7年的连续波形资料, 设定至少三个台站记录的垂直分量波形互相关系数大于0.8的地震对为一组重复地震, 通过波形互相关分析, 识别出龙门山断裂带周缘波形相关意义上的“重复地震”2790次, 构成2907组重复地震。 沿用Schaff和Richards的研究结果, 假定“重复地震”间距很小(小于1 km), 地震目录记录的重复地震对位置之差主要为地震台网的定位误差所致, 基于此误差给出了龙门山断裂带周缘地震台网的定位精度估计: 台网的水平定位精度较高, 水平定位误差约为2.8 km(2倍标准差), 且西南段台网的水平定位精度优于中北段; 垂直定位精度较差, 垂直定位误差约为10 km(2倍标准差), 现有地震定位方法对震源深度的测定有待改善。     

基于模板匹配定位法的江苏盐城附近微震检测和构造分析

利用基于GPU加速的匹配定位法和双差定位法,对江苏盐城及邻区18个台站记录的2009~2018年共10年的连续地震资料进行分析。首先从台网目录中挑选211个地震事件作为模板事件,使用匹配定位技术对江苏盐城附近连续10年的地震进行检测和识别,共识别出1349个地震事件,约为台网目录地震事件的3倍,最小完备震级由台网目录的M_L1.9降为M_L1.2。然后利用双差定位法对检测到的地震事件进行精定位,精定位的结果揭示：建湖地区的地震密集带与洪泽-沟墩断裂有关,震源深度优势分布为5~20km,断裂两侧震源深度有显著差异,断裂带倾向NW;射阳震群震源深度比建湖震群有所加深,优势分布为10~25km,震源深度由南东向西北逐渐变浅;宝应地区地震丛集分布;东台地区由于模板事件相对较少,扫描定位后,地震事件在陈家堡-小海断裂带附近零星分布。研究结果为研究盐城地区的地震活动性、发震断层的深部构造提供了基础数据支撑。     

2013年4月20日四川芦山M7.0级地震与余震精确定位及发震构造初探

使用汇集在四川台网中心的固定台站、震后架设的流动台站、周边水库台站等震中距150 km以内的震相数据,选用分层速度模型,对芦山7.0级地震及震后9天内的余震利用双差定位法进行了重新定位.给出了芦山7.0级地震的发震时刻为2013-04-20 08:02:46.8,震中位置30.278°N,102.989°E,震源深度16.67 km,给出了3324次余震的双差定位结果,并对发震构造进行了分析.结果表明:芦山地震主破裂长度约40 km,下倾宽度约20 km,破裂视面积约800 km²,主破裂沿南西走向,倾角约40°.余震震源优势深度为10~22 km.余震沿南西走向,主要集中于大邑-名山断裂上盘.     

Distribution Characteristics of the Seismicity of Zipingpu Reservoir Region after the Wenchuan Earthquake

815 earthquakes recorded by 12 seismic stations of the Zipingpu reservoir seismic network in 2009 were relocated using the double difference algorithm to analyze the seismic activity of the Zipingpu reservoir.Relocation results show that the earthquakes are concentrated relatively in three zones.The distribution characteristics of focal depth are obviously different among different concentration zones.This means earthquakes in different concentration zones may have different causes.Compared to relocation of...     

Earthquake Monitoring and Study in the Jingpohu Volcano Cluster Area

Seismicity in the Jingpohu volcanic area was investigated based on the seismic data recorded by the mobile seismic network consisting of 14 stations equipped with 24-bit broad-band 3- component seismographs around Crater Forest, Results show that there appears certain seismicity in Jingpohu and its adjacent areas with a low activity level and most of the recorded earthquakes are the volcanic-tectonic ones, The results of location indicate a dominant focal depth of 10km - 30kin, most of the earthquakes are smaller than ML2,0, and are concentrated in the area of ＂ Crater Forest＂ and on the Dunhua-Mishan fault which runs through the volcanic area. At station No. 2, which has better observation conditions, two types of events, likely associated to volcanism, were recorded; their waveform characteristics are somewhat similar to that of the long-period volcanic event and the volcanic tremor, but with different feature of frequencies.     

2003年新疆巴楚-伽师地震序列的双差法重新定位研究

将近台记录和区域台网数据联合用于双差地震定位算法,对2003年新疆巴楚-伽师MS68强震后404个ML≥35余震序列进行双差法重新精确定位,并对其进行了理论上的可行性分析. 精定位后定位结果与传统定位方法的原始定位结果进行比较发现：（1）精定位后震中分布图像更加集中,与当地的烈度考察和震源机制解结果更加协调;（2）震源深度优势分布集中在15～25km以内,与当地存在深度为10km以上低速沉积层的地质构造情况相一致;（3）精定位后定位残差由原来的394s降为024s,水平向估算误差平均可控制在11km以内,垂直向估算误差平均可控制在24km.     

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.     

2010年以来四川地区中强地震震源机制反演及深度确定

2008年5月12日四川龙门山断裂带发生了汶川8.0级地震,之后四川境内发生了两次7.0级地震(其中一个是芦山地震),为了研究汶川地震之后龙门山断裂带及周边区域的地震活动性,本研究收集了国家地震台网和四川区域地震台网2010年1月1日—2017年12月31日四川地区发生的17次M≥5.0地震以及120多次5.0>M≥4.0地震的波形资料,利用波形拟合法反演了震源机制解及区域应力场.反演结果显示,位于龙门山断裂带上的地震,震源机制以逆冲型为主,鲜水河断裂带地震震源机制以走滑型为主,而川滇块体西南部的理塘断裂、金沙江断裂附近,震源机制解以正断层为主.根据震源机制解反演得到的龙门山地区、鲜水河地区的主压应力场方向为WNW、近EW向.川滇块体的巴塘、理塘等地区,其主压应力轴方向为12°左右,接近SN向,且仰角接近40°左右.本研究利用面波振幅谱特征对震源深度进行了精确定位,定位结果与中国地震台网中心(CENC),美国地震调查局(USGS),国际地震中心(ISC)等机构地震目录进行了对比.结果显示,四川地区强震震源深度主要分布在20km以上的中上地壳.龙门山地区震源优势分布在10～20km,鲜水河断裂地震震源深度在10km左右,川滇块体西南部的理塘断裂,巴塘断裂,金沙江断裂等地区,震源深度一般在5～10km范围.     

Post-eruptive seismic activity of Mount Cameroon (Cameroon), West Africa: a statistical analysis

Recent seismological studies of the Cameroon Volcanic Line show that Mt. Cameroon is the most active centre, so a permanent seismic network of six seismographs was set up in its region between 1984 and 1986. The network was reinforced with temporary stations up till 1987, and the local seismicity was studied. Here we emphasise a statistical analysis of seismic events recorded by the permanent seismic stations. Four swarms lasting 9 to 14 months are identified at intervals of 2–3 years. Most earthquakes are felt (intensity and magnitude, respectively, less than VI MM and 5) during the first three swarms and a few during repose periods. The main focal regions are the northwest and southeast flanks, the Bimbia and Bioko regions in the South of the volcano. Hypocentres are distributed from the surface to 60 km depth indicating crustal and subcrustal activities. The subcrustal events are observed only in the southeast flank, they are the most regular earthquakes with a monthly frequency of 9 to 15 events. They are characteristic earthquakes with magnitude 2.8 ± 0.1. Between 1984 and 1992, their yearly mean time interval between successive events range from 50 to 86 hours. For that period their occurrence can be modelled as a stationary renewal process with a 3-day period. But the analysis of variance shows possible significant differences among yearly means. A Weibull's distribution confirms that the time intervals between successive deep events are not independent, and in 1993 a swarm of deep earthquakes is recorded, hence a non-loglinear magnitude/frequency relation. The deep seismicity is thought to be associated with a zone of weakness (perhaps a magmatic conduit) and may have some close relationship with the magmatic activity.     

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.     

2012年9月7日彝良地震及余震序列双差定位研究

本文提出了时域多通道相关检测函数并用其计算波形互相关走时差数据,采用双差定位法对2012年9月7日云南彝良地震和余震序列共944个地震进行重定位,得到652个重定位事件,并与目录数据的结果进行了对比.本文采用了多个准则对走时差数据进行筛选,确保定位结果稳定可靠.得到MS5.7主震的震中为27.516°N,103.951°E,震源深度6.9km;MS5.6主震的震中为27.543°N,104.023°E,震源深度7.27km;重定位结果显示,地震序列紧缩为条带状并沿附近断裂走向分布,深度总体分布较重定位前变浅,集中分布在5~8km,地震群出现轻微倾斜.东西向、南北向、深度和发震时刻的平均相对误差分别为55.2 m,43.0 m,186.7 m和0.01s,走时残差16ms.研究表明:互相关数据的结果要优于目录数据;震源深度与速度模型存在较大的相关性;确定彝良—会泽断裂为本次彝良地震序列的发震构造.     

2013-2014年山东乳山地震序列发震构造初探

采用结合波形互相关技术的双差定位方法,对2013—2014年山东乳山地震序列重新定位,通过CAP及P波初动方法确定乳山序列较大地震的震源机制,在此基础上初步探讨乳山地震序列发震构造.结果显示,乳山序列呈现NW向展布,地震密集分布在8km×3km范围,震源深度分布在4~10km,4~7km区间相对集中.较大地震震源机制的节面Ⅰ方向与序列地震优势分布方向基本一致.综合考虑精确定位结果及较大地震震源机制,并结合震区附近地震资料,初步推测乳山地震序列发震断层为NW方向、近直立的走滑型隐伏断裂.