Two-step interface and velocity inversion

This paper studies the computation method of two-step inversion of interface and velocity in a region. The 3-D interface is described by a segmented incomplete polynomial; while the reconstruction of 3-D velocity is accomplished by the principle of least squares in functional space. The computation is carried out in two steps. The first step is to inverse the shape of 3-D interface; while the second step is to do 3-D velocity inversion by distributing the remaining residual errors of travel time in accordance with their weights. The data of seismic sounding in the Tangshan-Luanxian seismic region are processed, from which the 3-D structural form in depth of the Tangshan seismic region and the 3-D velocity distribution in the crust below the Tangshan-Luanxian seismic region are obtained. The result shows that the deep 3-D structure in the Tangshan seismic region trends NE on the whole and the structure sandwiched between the NE-trending Fengtai-Yejituo fault and the NE-trending Tangshan fault is an uplifted zone of the Moho. In the 3-D velocity structure of middle-lower crust below that region, there is an obvious belt of low-velocity anomaly to exist along the NE-trending Tangshan fault, the position of which tallies with that of the Tangshan seismicity belt. The larger block of low-velocity anomaly near Shaheyi corresponds to a denser earthquake distribution. In that region, there is an NW-trending belt of high-velocity anomaly, probably a buried fault zone. The lower crust below the epicentral region of the Tangshan M _S=7.8 earthquake is a place where the NE-trending belt of low-velocity anomaly meets the NW-trending belt of high-velocity anomaly. The two sets of structures had played an important role in controlling the preparation and occurrence of the M _S=7.8 Tangshan earthquake. Contribution RCEG97006, Research Center of Exploration Geophysics, China Seismological Bureau, China. This project is supported by the Chinese Joint Seismological Science Foundation.     

The regional characteristics of seismic activity in China

ＴｈｅｒｅｇｉｏｎａｌｃｈａｒａｃｔｅｒｉｓｔｉｃｓｏｆｓｅｉｓｍｉｃａｃｔｉｖｉｔｙｉｎＣｈｉｎａＺｈｅｎ－ＬｉａｎｇＳＨＩ，ＪｉａｎＷＡＮＧａｎｄＸｉａｏ－ＤｏｎｇＺＨＡＮＧ（时振梁，王健，张晓东）（ＩｎｓｔｉｔｕｔｅｏｆＧｅｏｐｈｙｓｉｃｓ，Ｓ...     

A seismic gap on the Anninghe fault in western Sichuan,China

Through integrated analyses of time-varying patterns of regional seismicity, occurrence background of strong and large historical earthquakes along active faults, and temporal-spatial distribution of accu- rately relocated hypocenters of modern small earthquakes, this paper analyzes and discusses the im- plication of a 30-year-lasting seismic quiescence in the region along and surrounding the Anninghe and Zemuhe faults in western Sichuan, China. It suggests that the seismic quiescence for ML≥4.0 events has been lasting in the studied region since January, 1977, along with the formation and evaluation of a seismic gap of the second kind, the Anninghe seismic gap. The Anninghe seismic gap has the background of a seismic gap of the first kind along the Anninghe fault, and has resulted from evident fault-locking and strain-accumulating along the fault during the last 30 years. Now, two fault sections either without or with less small earthquakes exist along the Anninghe fault within the An- ninghe seismic gap. They indicate two linked and locked fault-sections, the northern Mianning section and the Mianning-Xichang section with lengths of 65 km and 75 km and elapsed time from the latest large earthquakes of 527 and 471 years, respectively. Along the Anninghe fault, characteristics of both the background of the first kind seismic gap and the seismicity patterns of the second seismic gap, as well as the hypocenter depth distribution of modern small earthquakes are comparable, respectively, to those appearing before the M=8.1 Hoh Xil earthquake of 2001 and to those emerging in the 20 years before the M=7.1 Loma Prieta, California, earthquake of 1989, suggesting that the Anninghe seismic gap is tending to become mature, and hence its mid- to long-term potential of large earthquakes should be noticeable. The probable maximum magnitudes of the potential earthquakes are estimated to be as large as 7.4 for both the two locked sections of the Anninghe fault.     

Comparison and analysis of the stress field in the source region of Tangshan and Lancang earthquake sequences

ＣｏｍｐａｒｉｓｏｎａｎｄａｎａｌｙｓｉｓｏｆｔｈｅｓｔｒｅｓｓｆｉｅｌｄｉｎｔｈｅｓｏｕｒｃｅｒｅｇｉｏｎｏｆＴａｎｇｓｈａｎａｎｄＬａｎｃａｎｇｅａｒｔｈｑｕａｋｅｓｅｑｕｅｎｃｅｓＧｕｉ－ＬｉｎｇＤＩＡＯ；（刁桂苓）Ｌｉ－ＭｉｎＹＵ；（于利民）...     

Interrelation among strong earthquakes and its application in seismic hazard analysis

The intterrelation among strong earthquakes and its application are emphatically studied in this paper. Taking North China seismic region as study area, we have investigated how a great earthquake influence other strong earthqukaes in neighbouring area? Does there exist earthqukae immunity phenomenon? If it exists, what distributional pattern did it has in space-time domain? The results show that occurrence of earthquakes withM⩾7 has cetain immunity phenomenon to earthquakes withM⩾6 in North China. Among others, the immunity area of earthquakes withM=8 is much larger than that ofM=7. For earthquakes withM⩾8, the immunity area to the earthquakes ofM=7 is larger than toM=6. Based on the above analysis, using some statistical methods, we gave the variational regularity of seismic immunity factor with space and time, and explored its concrete application in seismic hazard analysis. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 339–346, 1993.     

S-wave velocity structure in Tangshan earthquake region and its adjacent areas from joint inversion of receiver functions and surface wave dispersion*

Using the seismic records of 83 temporary and 17 permanent broadband seismic stations deployed in Tangshan earthquake region and its adjacent areas (39°N–41.5°N, 115.5°E–119.5°E), we conducted a nonlinear joint inversion of receiver functions and surface wave dispersion. We obtained some detailed information about the Tangshan earthquake region and its adjacent areas, including sedimentary thickness, Moho depth, and crustal and upper mantle S-wave velocity. Meanwhile, we also obtained the v_P/v_S structure along two sections across the Tangshan region. The results show that: (1) the Moho depth ranges from 30 km to 38 km, and it becomes shallower from Yanshan uplift area to North China basin; (2) the thickness of sedimentary layer ranges from 0 km to 3 km, and it thickens from Yanshan uplift region to North China basin; (3) the S-wave velocity structure shows that the velocity distribution of the upper crust has obvious correlation with the surface geological structure, while the velocity characteristics of the middle and lower crust are opposite to that of the upper crust. Compared with the upper crust, the heterogeneity of the middle and lower crust is more obvious; (4) the discontinuity of Moho on the two sides of Tangshan fault suggests that Tangshan fault cut the whole crust, and the low v_S and high v_P/v_S beneath the Tangshan earthquake region may reflect the invasion of mantle thermal material through Tangshan fault.     

The character and extent of seismic deformation in the focal zone of gazli earthquakes of 1976 and 1984,M>7.0

With objective of investigating the peculiarities of seismic process development and seismotectonic deformation character in the focal area of the Gazli earthquakes of 1976 (7.0<M<7.3) and 1984 (M=7.2), a local seismic network was installed. For the field observation period (May to June, 1991) more than 400 events with magnitudes –0.2<M<4.5 were recorded by at least 6 stations.Isometric presentation of earthquake hypocenters distribution allows us to define the depth and dipping planes orientation of seismoactive faults of the region.The focal mechanisms of 35 earthquakes for the period 1979–1988,M>2.8, connected to a gas extraction regime period, and 75 events 1<M<4.3 for the 1991 period (gas storage regime) are used to analyze the dynamics of seismotectonic deformation processes (SDP) in this region. It has been ascertained, that the earth's crust in the Gazli region is subject to complicated deformation processes, particularly below 4 km depth. The predominant kind of deformation is compression. Vertical velocities of deformation show uplift of most of the region during the period of field work. The maximum velocity of vertical deformations for the Gazli structure isV=0.41 mm/year. The comparison of the vertical velocities' displacements due to seismic flow with recent tectonic movements of the earth's crust has revealed their direct relation and high percentage of seismic flow contribution to the tectonic movement. The results obtained testify that the active seismic processes in the Gazli region are connected not as much as the residual stress release in the focal zone of the earthquakes 05. 1976 and 1984,M>7.0 but rather with the influence of the gas reservoir exploitational regime on the rocks with different rheologic properties.     

利用唐山地震台资料研究唐山断裂活动特征

本文研究了唐山地震台内的唐山断裂形变、断层活动协调比、断层土壤气CO₂的变化特征。结果显示,唐山地震台内的唐山断裂近35年来整体上在水平方向呈微弱的右旋张性活动,垂直方向呈正断活动,但不同时段的运动状态有所不同;综合分析认为,唐山地震台形变和流体前兆观测对附近及华北因区域地壳应力场调整引起的中小地震有一定的显示,说明该台具有一定的前兆异常显示能力;随着观测资料和经验的持续积累,在该区域发生更大地震前,有望捕捉到更显著的前兆异常。     

2009年7月24日西藏尼玛西南MS5.6地震研究

主要研究2009年7月24日西藏尼玛西南MS5.6地震的基本参数、地震序列特征、震源参数、发震构造等;利用震中附近600km范围内台站测定参数研究地震的震源机制解,与哈佛大学给出的震源机制解较一致,且与通过现场考察的发震断层走向具有一致性。研究认为本次地震发生在冈底斯山—拉萨块体内部,断裂为NNW向,主要受张应力作用产生左旋走滑正断层活动。此外还分析了震前地震学条带异常特征,结束表明,震前1年出现NW向条带非常显著,研究结论为该地区今后地震预测提供科学依据。     

短期线状集中分布地磁日变化感应电流异常机理初步解释——以2012年唐山4.8级地震为例

本文分析了2012年唐山4.8级地震前,震中附近出现的短期原地重现线状集中分布地磁日变化感应电流异常的时空变化特征,及其与地震、中-下地壳和上地幔高导层的关系,进一步证实了短期原地重现线状集中分布感应电流的走向与中-下地壳和上地幔高导层顶面界埋深走向一致,认为其机理可能是深部热流体的上涌导致壳幔高阻体出现带有上拱性质的拆离滑动,深部上涌的热流体和高导层内热流体侵入高导层内电阻相对较高的地区,高导层出现短时间高导电流通道,当地磁日变化感应电流扫描经过高导电流通道时,感应电流会呈线状集中分布于此,并基于趋肤效应分布于其顶面附近。由于重现异常是发生在震源下方中-下地壳和上地幔高导层的地震异常,且该异常不同于震源附近及其震源至地表的地震异常,因此对推进地震孕育与发生机理研究可能有一定作用。此外研究还发现,地震虽然主要位于重叠段的端部,但更有可能位于中-下地壳重叠段的端部,这一发现对日常震情跟踪中应用该异常确定未来地震位置有一定帮助。     

Characteristics of seismic activity before the MS8.0 Wenchuan earthquake

The characteristics of spatio-temporal seismicity evolution before the Wenchuan earthquake are studied. The results mainly involve in the trend abnormal features and its relation to the Wenchuan earthquake. The western Chinese mainland and its adjacent area has been in the seismically active period since 2001, while the seismic activity shows the obvious quiescence of M≥?7.0, M≥?6.0 and M?≥5.0 earthquakes in Chinese mainland. A quiescence area with M?≥7.0 has been formed in the middle of the North-South seismic zone since 1988, and the Wenchuan earthquake occurred just within this area. There are a background seismicity gap of M?≥5.0 earthquakes and a seismogenic gap of M_L?≥4.0 earthquakes in the area of Longmenshan fault zone and its vicinity prior to the Wenchuan earthquake. The seismic activity obviously strengthened and a doughnut-shape pattern of M?≥4.6 earthquakes is formed in the middle and southern part of the North-South seismic zone after the 2003 Dayao, Yunnan, earthquake. Sichuan and its vicinity in the middle of the doughnut-shape pattern show abnormal quiescence. At the same time, the seismicity of earthquake swarms is significant and shows heterogeneity in the temporal and spatial process. A swarm gap appears in the M4.6 seismically quiet area, and the Wenchuan earthquake occurred just on the margin of the gap. In addition, in the short term before the Wenchuan earthquake, the quiescence of earthquake with M_L≥?4.0 appears in Qinghai-Tibet block and a seismic belt of M_L?≥3.0 earthquakes, with NW striking and oblique with Longmenshan fault zone, is formed.     

唐山震区上地壳平均波速比分析

收集河北地震台网记录的唐山震区2012-2016年ML≥1.0地震,选取震源深度5-10km、波形信噪比较高的波形数据,拾取直达P波、S波震相到时,利用和达法计算平均波速比,并分析研究区平均波速比水平分布特征与时变特征,结果表明:唐山震区上地壳平均波速比约为1.71,且水平分布呈区域化特征,反映了研究区复杂的地壳结构。研究区内3次ML≥4.0地震前,波速比曲线均呈较显著的下降-低值-回升形态,表明该区平均波速比变化可能具有一定地球物理异常特征。     

Analysis on the Characteristics of the Kalpin MS5.3 Earthquake Sequence of December 1, 2013 and Anomalies before the Earthquake

This paper introduces the basic parameters, focal mechanism solutions and earthquake sequence characteristics of the Kalpin M_S5.3 earthquake sequence of December 1, 2013, and analyzed seismic activity before the earthquake, the adjacent tectonic features and the precursory anomaly at fixed points within a range of 200km. Research indicates:(1) The earthquake occurred on Kalpin fault, the source rupture type is thrust faulting with sinistral strike-slip component. (2) The earthquake sequence is mainshock-aftershock type, with the aftershock distribution attenuating quickly and trending NE. (3) Abnormal seismic activity before the earthquake was characterized by seismically nesting quiescence of M_S2.0-4.0 earthquakes, seismic quiescence of M_S4.0 earthquakes and seismic belts of M_S3.0 earthquakes in the Kalpin block, abnormal enhancement zone of moderate earthquakes on Puchang fault and seismological parameters. (4) Anomalies of precursory observation data at fixed stations are mainly characterized by mutation. Apart from the borehole tiltmeter in Halajun, the spatial distribution of other abnormal precursors showed a phenomenon of migration from the near field to far field and from the epicenter to the peripheries.     

Performance-based methodology for assessing seismic vulnerability and capacity of buildings

This paper presents a performance-based methodology for the assessment of seismic vulnerability and capacity of buildings. The vulnerability assessment methodology is based on the HAZUS methodology and the improved capacity- demand-diagram method. The spectral displacement (Sd) of performance points on a capacity curve is used to estimate the damage level of a building. The relationship between Sd and peak ground acceleration (PGA) is established, and then a new vulnerability function is expressed in terms of PGA. Furthermore, the expected value of the seismic capacity index (SCev) is provided to estimate the seismic capacity of buildings based on the probability distribution of damage levels and the corresponding seismic capacity index. The results indicate that the proposed vulnerability methodology is able to assess seismic damage of a large number of building stock directly and quickly following an earthquake. The SCev provides an effective index to measure the seismic capacity of buildings and illustrate the relationship between the seismic capacity of buildings and seismic action. The estimated result is compared with damage surveys of the cities of Dujiangyan and Jiangyou in the M8.0 Wenchuan earthquake, revealing that the methodology is acceptable for seismic risk assessment and decision making. The primary reasons for discrepancies between the estimated results and the damage surveys are discussed.     

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.     

Aftershocks of the San Marcos earthquake of April 25, 1989 (M S =6.9) and some implications for the Acapulco-San Marcos,Mexico, seismic potential

Aftershock activity following the April 25, 1989 (M _S =6.9) earthquake near San Marcos, Guerrero, Mexico, was monitored by a temporary network installed twelve hours after the mainshock and remaining in operation for one week. Of the 350 events recorded by this temporary array, 103 were selected for further analysis in order to determine spatial characteristics of the aftershock activity. An aftershock area of approximately 780 km² is delimited by the best quality locations. The area of highest aftershock density lies inside an area delimited by the aftershocks of the latest large event in the region in 1957 (M _S =7.5) and it partially overlaps the zone of maximum intensity of the earlier 1907 (M _S =7.7) shock. Aftershocks also appear to cluster close to the mainshock hypocenter. This clustering agrees with the zone of maximum slip during the mainshock, as previously determined from strong motion records. A low angle Benioff zone is defined by the aftershock hypocenters with a slight tendency for the slab to follow a subhorizontal trajectory after a 110 km distance from the trench axis, a feature which has been observed in the neighboring Guerrero Gap. A composite focal mechanism for events close to the mainshock which also coincides with the zone of largest aftershock density, indicates a thrust fault similar to the mainshock fault plane solution.The San Marcos event took place in an area which could be considered as a mature seismic gap. Due to the manner in which strain release has been observed to previously occur, the occurrence of a major event, overlapping both the neighboring Guerrero Gap and the San Marcos Gap segments of the Mexican thrust, cannot be overlooked.     

2017年8月8日九寨沟MS7.0地震前成都台地磁谐波振幅比异常分析

2017年8月8日四川省九寨沟县发生M_S7.0地震,成都地震基准台距此次地震震中约255km,震前该台地磁谐波振幅比出现趋势性异常。对成都台GM4磁通门磁力仪秒数据进行谐波振幅比计算,结果显示,谐波振幅比在九寨沟M_S7.0地震前表现出下降—转折—恢复上升的异常变化形态,地震发生在异常恢复期,同时,异常表现出由长周期向短周期迁移、SN向与EW向变化不同步的特征,芦山M_S7.0地震前成都台地磁谐波振幅比亦呈现了相似的异常变化特征。     

2017年九寨沟7.0级地震前不同级别地震空区的演化过程分析

结合传统地震学方法与数字地震学方法,回顾总结了日常分析预报中针对九寨沟7.0级地震开展的相关工作,梳理总结了地震前提出的甘东南地震异常信息,进一步讨论了九寨沟地震前中期、短期及临震异常特征。通过分析九寨沟7.0级地震之前区域范围内不同震级活动图像,发现甘青川交界地区5级空区在震前向震中一侧收缩、4级地震超长平静458天、3级空区在震前4天被打破的现象,同时发现,震中附近区域震源机制一致性较高,反映了震源区的高应力水平。     

Characteristics of coseismic water level changes at Tangshan well for the Wenchuan MS8.0 earthquake and its larger aftershocks

Coseismic water level changes which may have been induced by the Wenchuan M_S8.0 earthquake and its 15 larger aftershocks (M_S≥?5.4) have been observed at Tangshan well. We analyze the correlation between coseismic parameters (maximum amplitude, duration, coseismic step and the time when the coseismic reach its maximum amplitude) and earthquake parameters (magnitude, well-epicenter distance and depth), and then compare the time when the coseismic oscillation reaches its maximum amplitude with the seismogram from Douhe seismic station which is about 16.3 km away from Tangshan well. The analysis indicates that magnitude is the main factor influencing the induced coseismic water level changes, and that the well-epicenter distance and depth have less influence. M_S magnitude has the strongest correlation with the coseismic water level changes comparing to M_W and M_L magnitudes. There exists strong correlation between the maximum amplitude, step size and the oscillation duration. The water level oscillation and step are both caused by dynamic strain sourcing from seismic waves. Most of the times when the oscillations reach their maximum amplitudes are between S and Rayleigh waves. The coseismic water level changes are due to the co-effect of seismic waves and hydro-geological environments.     

Chi-square regression for seismic strength parameter relations, and their uncertainties, with applications to an M w based earthquake catalogue for central, northern and northwestern Europe

A generalized chi-square regression approach to establishempirical relations between different types of seismic strengthparameters with uncertainties in all input data is presented anddiscussed in comparison with standard least-squares techniques.The chi-square technique can consider errors of individual entriesbut can also be applied when errors are not exactly known and onlyweaker quantitative constraints can be made. It can preserve thesymmetry of the derived relations and is preferred for complexregression models. Results for three types of regression modelsare presented for (1) a linear relation between M _Sand m _bfor events in the North Atlantic Ocean; (2) a quadratic relationbetween M _w and M _L forevents in central Europe; (3) linearrelations between M _L and I ₀,with logarithmic dependency ofthe focal depth, for several regions in central and northernEurope.