基于克里金方法的区域震相走时标定技术研究

在台网比较稀疏的情况下,台站走时标定是提高低震级事件定位、识别能力的重要手段.为了提高稀疏台网的定位精度,首先利用标定事件和IASPEI1991走时表以及中国大陆走时表,计算了初至P波和Lg波到AAK,MAKZ,NIL,TLG以及WMQ等5个台站的走时残差;接着,采用非平稳贝叶斯克里金方法和走时残差数据构造上述台站的走时校正曲面;最后,通过加入和不加入走时校正定位一系列位置准确的发生在新疆地区的地震事件评估克里金走时校正的有效性.结果表明,克里金走时校正能够较大地提高稀疏台网的定位精度,同时有效地缩小误差椭圆的面积.     

昆明地震台网多事件定位问题的初步研究

根据昆明电信传输台网1983-1989年记录的区域地震初至 P 波走时资料,用参数分离和多事件定位的方法得到台网内各台站的 P 波走时校正值.这组校正值在一定程度上反映了该地区地壳上部横向不均匀结构的特征.台站校正值的地理分布明显地分为3个地区:台站最密集的滇西地区,所有台站(不包括南边的云县,施甸和畹町)的台站校正值不超过0.15s;东部地区(楚雄以东)台站分布仍较密集,但间距比滇西大,其中大部分台站显示为正延迟;南部地区包括文山、思茅、普洱以及云县、施甸和畹町,它们均显示出较大的负延迟.这与云南地区的地质构造特征基本上是一致的.走时校正后重新定位的精度有较大的提高,因此,这组台站校正值可以提供地震台网的常规定位使用.      

中国地震台网初至P波区域三维走时表的建立

基于LLNL-G3Dv3全球P波三维速度模型,应用FMM软件包计算并建立了中国地震台网990个台站的初至P波区域三维走时表．该走时表覆盖了以台站为中心的水平向20°×20°、 垂直向-5.1—80 km （向下为正）的三维空间. 其水平向间隔为0.2°,垂直向间隔为5 km．这样对于任一深度小于80 km的震源,均可以应用此三维走时表计算其到周围10°范围内台站的走时．中国地震台网初至P波区域三维走时表的建立,对于改善区域初至P波走时预测,提高地震定位精度有一定现实意义．      

华东地区台站偏差的研究

用华东地区十年地震资料和迭代方法，在华东走时表的基础上，计算出百余地震台站的作为距离的函数的走时偏差值，将这些结果当做台站校正值用于地震定位，定位精度得到明显提高。     

2011年3-4月中国及邻区地震目录

本地震目录由中国地震台网中心（CENC）提供。中国地震台网中心使用全国107个测震台站的地震数据,记录中国及邻区M≥2．5地震,各地震事件的记录台站均在5个以上。地震目录走时表使用J-B表①,     

2011年9—10月中国及邻区地震目录

本地震目录由中国地震台网中心（CENC）提供。中国地震台网中心使用全国90个测震台站的地震数据,记录了中国及邻区M≥2．5的地震,各地震事件的记录台站均在5个以上。地震目录使用的走时表为J-B表①,     

2008年8月中国及邻区地震目录

本地震目录由中国地震台网中心（CENC）提供。中国地震台网中心使用全国90个测震台站的地震数据,记录了中国及邻区M≥2．5的地震,各地震事件的记录台站均在5个以上。地震目录使用的走时表为J—B表①,     

2011年11—12月中国及邻区地震目录

本地震目录由中国地震台网中心（CENC）提供。中国地震台网中心使用全国90个测震台站的地震数据,记录了中国及邻区M≥2．5的地震,各地震事件的记录台站均在5个以上。地震目录使用的走时表为J-B表①,发震时刻为国际时,     

2007年12月～2008年1月中国及邻区地震目录

本地震目录由中国地震台网中心（CENC）提供。中国地震台网中心使用全国90个测震台站的地震数据,记录了中国及邻区M≥2．5的地震,各地震事件的记录台站均在5个以上。地震目录使用的走时表为J-B表,发震时刻为国际时,     

用于地震预警的三维实时定位方法

为了尽快定位地震,在“着未着”定位算法基础上,尝试引入三维地壳模型,构建具有三维空间格点分布的走时表.根据已触发台站的到时和未触发台站的位置信息设计概率分布函数,通过八叉树搜索方法,快速给出震源在三维空间的可能位置.使用波前追踪算法,计算中国几个地区三维地壳模型的走时网格,利用中国地震台网资料,对区域内发生的地震进行定位分析.结果显示,在一定的台网密度条件下,三维实时定位方法能在震后数秒给出震源位置,可满足地震预警要求.     

Best Practice in Earthquake Location Using Broadband Three-component Seismic Waveform Data

—?We present an earthquake location algorithm, the Broadband Waveform Regional Earthquake Location Program (BW_RELP), which utilizes phase onset times and wave azimuths recorded by three-component broadband seismic stations and an adaptive migrating grid search algorithm to find the global minimum in an arbitrary normed misfit parameter. The performance of BW_RELP is demonstrated using regional (300–800?km distant) broadband recordings to locate events in the 1995 Ridgecrest, California earthquake sequence. The purpose of this study is to introduce the BW_RELP algorithm in detail and to expand on the previous paper by Deger et?al. (BSSA, 88, 1353–1362, 1998), using one Berkeley Digital Seismic Network (BDSN) station (YBH) and two USNSN stations (ELK and MNV) which span 300–800?km in distance and 55 degrees in azimuth, to further investigate the capability of a sparse broadband network of three-component stations at monitoring a region located outside of the network, as will be the case in the monitoring of the Comprehensive Test-Ban-Treaty (CTBT) for low magnitude seismic events. We assess the capability of this sparse three-station broadband network and we compare locations estimated from phase onset time and wave azimuth measurements to a ground-truth catalog of high-quality earthquake locations derived from data recorded by the Southern California Seismic Network (SCSN). The results indicate that in the regional distance range it is possible, when an appropriate calibration event is available, to obtain absolute event locations to within 18?km as is prescribed by the CTBT.     

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。     

利用和田地震台阵数据对2008年于田7.3级地震序列重新定位

利用新疆和田地震台阵数据,应用宽频F-K分析方法,结合新疆数字地震台网部分台站数据,通过震相到时、方位角和慢度的线形化反演地震定位方法对2008年3月21日新疆于田7.3级地震及2008年3月21日~2008年8月31日ML≥4.0的余震序列中119次地震进行重新定位,重新定位结果与新疆区域地震台网定位结果有明显差异。重新定位结果表明,于田7.3级地震序列呈近NS向条带分布,与野外考察发现地表破裂带基本一致。研究结果表明,利用地震台阵结合部分区域地震台网数据,可以对台网相对稀疏地区的地震进行较为精确的定位,可为基础研究和震情趋势判定提供基础资料。     

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。     

张北地震前小震活动和余震序列精确定位及主震破裂方向的讨论

1970年至1998年1月10日张北地震前,沿NWW向发生2级以上地震24次,其分布相对集中,呈条带状。对华北台网所记上述地震的到时数据全部重新查图核对,并补充了内蒙古台网5个台的数据以改善用于定位的台站分布,并采用主地震定位法重新定位,除一个地震外,其余地震更加集中,形成一小震条带。1998年2月中国地震局地球物理勘探中心在张北震区架设10台短周期数字化地震仪,记录了大量微震。作者采用P波和S波到时差重新进行了震源定位,得到184次微震的定位结果,连同1998年1～2月张北震区3级以上地震事件定位结果一起勾画出6．2级地震的震源破裂带。由张北地震震中向SEE向延伸存在由强震构成的张家口渤海地震带。根据精确定位结果,结合修订后主震的震源机制、宏观烈度分布和层析成像资料,讨论了张北地震和小尺度地震带(震源破裂带)、震前的中等尺度小震条带以及大尺度的张家口-渤海强震带的关系。     

Broadband seismic experiment at Merapi Volcano, Java, Indonesia: very-long-period pulses embedded in multiphase earthquakes

Multiphase (MP) and low frequency (LF) earthquakes with spectral peak amplitudes at 3–4 and 1 Hz, respectively, are two common types of shallow volcanic earthquakes previously recognized at Merapi Volcano. Their mechanisms are poorly understood but MPs have been temporally associated with lava dome growth. We conducted a seismic experiment in January–February 1998, using four broadband seismographs to investigate the nature of seismic activity associated with dome growth. During our experiment, Merapi experienced mild dome growth with low-level seismic activity. We compare our data to that recorded on a local short-period (SP) network, with the following preliminary results.MP and LF events as recorded and classified on the short-period network instruments were recognized on the broadband network. Frequency spectrograms revealed similar patterns in the near summit region at widely separated broadband stations. Higher frequency spectra than previously recognized were identified for both MP and LF events, and were strongly attenuated as a function of radial distance from the source. Thus the spectral characteristics of these events as recorded on far-field stations are not fully indicative of the source processes. In particular, many events classified as LF-type appear to have much high frequency energy near the source. This aspect of these so-called LF earthquakes, and their association with very-long-period (VLP) pulses, suggests that many events identified in the far-field as LF events are in actuality a variety of the MP event and involve similar source processes. Broadband records indicated that simple large-amplitude VLP pulses were embedded in MP and LF wavetrains. From event to event these pulses were similar in their waveforms and had periods of 4 s. VLP events embedded in LF and MP earthquakes were located using particle motions. The epicenters were clustered in a central region of the dome complex, and preliminary source depths were within about 100 m of the dome surface, suggesting a source region deep within the dome or the uppermost conduit. A similar source location was established by study of MP high-frequency onsets. Our broadband data suggests that we have recorded both elastic seismic waves and a simple embedded pulse that is interpreted to represent a surface tilt at the seismometer site. The inferred tilt indicates an inflation and subsequent deflation, possibly caused by a gas pressure pulse or episodic shallow magma transport with stick-slip movement of the conduit wall.     

Locating Seismic Events in the CTBT Context

—?The verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) requires the determination of accurate location of seismic events from a fixed network of seismic stations across the globe. The requirements of possible on-site inspections mean that the goal is to place the location estimate in a zone smaller than 1000 km² that includes the true location. Because a defined set of stations will be used, corrections can be refined to represent the influence of departures from the global reference model IASPEI91. The primary stations in the International Monitoring Scheme (IMS) are mostly seismic arrays and therefore the present location scheme is based on minimisation of a misfit function built from arrival time, azimuth and array slowness residuals. The effective network will change markedly with the magnitude of the event and as a result regional information has to be integrated into the location process.     

The detection and location of low magnitude earthquakes in northern Norway using multi-channel waveform correlation at regional distances

A fortuitous sequence of closely spaced earthquakes in the Rana region of northern Norway, during 2005, has provided an ideal natural laboratory for investigating event detectability using waveform correlation over networks and arrays at regional distances. A small number of events between magnitude 2.0 and 3.5 were recorded with a high SNR by the Fennoscandian IMS seismic arrays at distances over 600 km and three of these events, including the largest on 24 June, displayed remarkable waveform similarity even at relatively high frequencies. In an effort to detect occurrences of smaller earthquakes in the immediate geographical vicinity of the 24 June event, a multi-channel correlation detector for the NORSAR array was run for the whole calender year 2005 using the signal from the master event as a template. A total of 32 detections were made and all but 2 of these coincided with independent correlation detections using the other Nordic IMS array stations; very few correspond to signals detectable using traditional energy detectors. Permanent and temporary stations of the Norwegian National Seismic Network (NNSN) at far closer epicentral distances have confirmed that all but one of the correlation detections at NORSAR in fact correspond to real events. The closest stations at distances of approximately 10 km can confirm that the smallest of these events have magnitudes down to 0.5 which represents a detection threshold reduction of over 1.5 for the large-aperture NORSAR array and over 1.0 for the almost equidistant regional ARCES array. The incompleteness of the local network recordings precludes a comprehensive double-difference location for the full set of events. However, stable double-difference relative locations can be obtained for eight of the events using only the Lg phase recorded at the array stations. All events appear to be separated by less than 0.5 km. Clear peaks were observed in the NORSAR correlation coefficient traces during the coda of some of the larger events; the local stations confirm that these are in fact aftershocks exhibiting very similar waveforms to the main events. Many of the more marginal correlation detections are not made when the calculations are repeated using shorter signal segments, fewer sensors or more distant stations. We demonstrate in addition how these almost repeating seismic sources have been exploited to detect and measure timing anomalies at individual sites within the arrays and network.     

天山东北部地震的重新定位和一维地壳速度模型的改善

根据布设在乌鲁木齐市活断层探测区内的流动宽频带地震台阵,结合区域地震台网的走时数据,利用3种不同的定位方法对新疆天山东北部地区(E85°30′～ 88°30′,N43°00′～44°40′) 2004年8月至2005年8月发生的599个地震进行了重新定位.通过比较不同方法的结果合理性,确定了适合于当地震源精定位的程序,并改善了一维地壳速度模型.结果表明:联合使用流动地震台阵和区域台网的资料,显著提高了研究区的地震定位能力,精定位后震中分布图像更加集中,展示出了天山东北部地区更为明显的与活动构造相关的条带状地震活动分布图像,除了一些与已知断层相关的地震事件外,还发现一些有待证实的活断层.     

天山东北部地震的重新定位和一维地壳速度模型的改善

We apply three methods to relocate 599 earthquake events that occurred from August 2004 to August 2005 in the northeastern Tianshan Mountains area ( 85°30’ ～ 88°30’E,43°00’ ～ 44°40’ N ) by using travel times recorded by regional seismic network and 10 portable seismic stations deployed around the Urumqi city. By comparing the reliability of different results,we determined a suitable location method,and an improved 1-D crustal velocity model of the study area. The uncertainty of earthquake location is significantly reduced with combined data of seismic network and portable stations. The relocated events are clearly associated with regional tectonics of the northeastern Tianshan Mountains area, and are also in agreement with the existence of active faults imaged by deep seismic reflection profile. The relocated seismicity discovers some potential traces of buried active faults,which need to be validated further.