云南省及其邻区部分中强地震应力降特征

本文用波谱的方法估算了云南省及其邻区部分中强地震的应力降,初步讨论了应力降的分布特征。并指出,不同地区不同类型的地震,其应力降也有所不同。震群地区的地震一般应力降较低,主震型的大地震往往发生在应力降高的地区,单个地震发生以后,其应力降的高低与后续地震的大小有关,这对估计地震发生的趋势有一定的的意义。     

不同地震区应力降分布与分形特征

利用 P 波初动半周期的方法测定了5个地震区的小震震源参数.所研究的5个地区和时间段,以及发生的主要地震是:鹤庆,1984-1986年,1986年3月发生5.6级地震:乌什,1985年1月-1987年9月,1987年1月发生6.7级地震;松潘,1975-1976年,1976年8月发生2次7.2级地震:唐山-卢龙,1980年1月-1984年6月,1982年10月发生6.2级地震;邢台,1976-1985年,1981年11月发生5.8级地震.结果表明:(1)5个地区的应力降数值分布和集中范围各不相同,说明积累应力的水平因地区而异,其中唐山-卢龙地区的应力水平最高.其次是邢台和鹤庆,乌什和松潘的应力水平低;(2)5个地区的应力降数值都具有分形的特征,但分维数 D 则因地区而异,其中乌什和松潘的 D 值高,约分别为1.42和1.24,其次是唐山-卢龙地区,D 值为1.06.邢台和鹤庆的 D 值低,约分别为0.73和0.84综上所述,乌什和松潘这两个7级左右大地震发生区的特点是低应力水平和高 D 值;邢台、鹤庆和唐山-卢龙这三个6级左右地震发生区的特点是,高应力水平和低 D 值.      

浙江地区水库地震与构造地震应力参数特征研究

利用浙江区域数字地震台网记录到的地震波数据,采用布龙模式下近震源记录计算震源谱参数的方法,系统测定了发生在温州珊溪水库、宁波皎口水库库区以及浙江地区构造地震共326次地震的应力降、视应力等参数。给出了两个水库区地震与构造地震的应力降与视应力参数特征,并进行了对比分析。初步研究发现,皎口地震的应力降和视应力值均低于浙江地区构造地震;珊溪水库地震与构造地震在应力降和视应力值上具有震级分段的特征:震级ML<4.0级的地震,同震级的应力降和视应力值两者相差不大,而震级ML>4.0级的地震,构造地震的应力降和视应力值均高于珊溪水库地震。此外,测定的应力降、视应力的对数与震级ML之间存在较好的线性关系。     

2008年5月12日四川汶川8．0级地震前后震源区应力水平估计

根据地震力学和数字地震学理论,利用视应力和应力降,估算了2008年5月12日四川汶川MS8.0级地震前后震源区的应力水平,结果表明,震前震源区应力值约为1.5~2.0MPa,地震破裂过程中,由于断层发生错动过头,使地震发生后震源区应力低于动摩擦力,降至-1.2~-0.1MPa。     

延怀盆地中小地震应力降时空特征初步分析

正应力降表明地震发生时错动面上构造应力的瞬间释放水平,它不但反映了地震区域震源尺度的破裂程度,而且还代表了该区域应力场的强弱。延怀盆地地处北京西北,地质构造较为复杂,一直是首都圈重要研究和监测区域。利用北京及邻区密集数字地震监测台网记录并积累的大量新的数字地震观测资料研究延怀盆地中小地震应力降情况,可以了解该地区近年来应力降背景值,为发现具有前兆意义的中小地震提供基础。本文研究在利用Atkinson和Moya方法分别已取得北京及邻区非弹性衰减系数Q值和场地     

武定6.5级地震序列的地震应力降研究

以Brune提出的中小地震圆盘形破裂位错模式为理论基础，研究了地面运动峰值参数（r.d)和（r.v)对地震应力降△σ的依赖性，并考虑地球介质Q值对峰值参数的影响，利用武定M6．5级地震序列的近场数字记录地震资料，由此导出了用峰值位移d和峰值程度v求地震应力降△σ的关系式：lg(g.d/e^-qr)=-10.56 2/3lgM0 1/3lg△σ；lg（r，v/e^-qr)=-2.566 1/3lgM0 2/3lg△σ.用上述公式测定了武定地震序列的地震应力降△σ，结果给出地震序列的应力降△σ构大多数在5．0－50MPa的范围内，是典型的高应力降序列。通过分析应力降的时空分布特征，发现应力参数与余震活动的频度及强度相关，也与地震构造背景相关。研究结果还表明用地面运动峰值参数估算地震应力降是一种新的实用性较强的方法。     

下关地区水压致裂应力测量及构造应力状态的研究(英文)

本文报导了下关附近500米深钻孔中的水压致裂应力测量结果。通过原地应力测量发现应力值随深度增加而增大;最大水平主应力方向随深度变化不大,平均方向为北20°东,与红河断裂带呈45°交角。最大主应力及最大剪应力随深度变化的梯度比华北和美国圣安德列斯断层附近的测值大得多。从测得的最大主应力值和最大剪应力值来看,下关地区处于高应力状态,发生地震的危险性是值得引起注意的。     

小江断裂带及邻区应力降特征分析

在对S波观测记录位移谱进行传播路径、场地响应和仪器响应等校正的基础上,根据Brune圆盘震源模型,利用遗传算法估算了小江断裂带及邻区335次ML2.0～5.3地震的应力降,分析了应力降的时空特征.结果表明:△σ≥10.0Mpa的高应力降地震主要分布在小江断裂带的东川、巧家及其以北地区和元谋、武定一带.小江断裂带北段、中段、南段△σ≥10.0Mpa的地震所占比例分别为10.2％、10.9％和1.6％,据此结合地震活动性分析了该断裂带不同段的强震危险性.结合震源位置和速度结构联合反演方法得到的震源位置精确定位结果,分析了应力降和震源深度变化情况及其所反映的断层深部延伸和分布情况.     

丹江口水库及邻区地震震源参数研究

在讨论震源参数和Ｍ震级关系以及论述利用地方震尾波估算地震矩方法的基础上，尝试性地系统估算了丹江口水库及邻区内４３次ＭＬ≥２．５级地震的震源参数，并讨论了丹江口水库地震的特征。丹江口水库及邻区地震震源参数的初步分析表明：丹江口及邻区属低应力降地区。水库蓄水的附加引张应力效应可能使得丹江口水库中局部以引张应变为主的地区应变迅速增加，地震应力降高于该区一般地震原有水平、从而诱发地震。此外，丹江口水库地震的破裂长度可能和震源机制的破裂类型有关。     

1998年张北地震及其较大余震的应力降

采用地震波初动半周期估算应力降的方法,计算了张北震区25次较大地震的应力降并对余震的应力降进行了分析,2月23日震区内发生的ML3.5级地震,其应力降高达26.2×105Pa,为3月11日5.6级地震的震前异常.     

用云南下关测点的水压致裂结果分析地形起伏对应力测值的影响

本文以云南下关黄草坝的水压致裂应力测量结果为例,讨论了地形起伏对云南区应力测值的影响。结果表明,地形起伏对应力测值的绝对影响随深度的增加衰减缓慢,相对影响急剧减小。100米以内深度的应力测量,地形影响可能给测值带来10%以上的误差。云南区的水压致裂应力测量深达500米,是可以排除掉地形起伏的影响的。     

Stress accumulation and release on the San Andreas fault

Summary The San Andreas fault can be divided into locked and free sections. On the locked sections accumulated slip is released in great earthquakes. On the free sections slip is occurring continuously either aseismically or during smaller earthquakes. Stress drops during earthquakes can be estimated from the ratio of short to long period amplitudes and from surface strain. Surface heat flow may provide an upper bound on the absolute stress. The failure or yield stress must reach a maximum at some depth on the fault. This maximum may occur in the near-surface brittle zone or deeper in the plastic zone of the fault. The historic distribution of seismic activity provides information on the stress level. The accumulation of strain and stress on the fault can be predicted using elastic theory. It is necessary, however, to include the viscous coupling of the lithosphere to the asthenosphere in order to fully model the problem.     

对中国大陆地壳上部应力状态的讨论

本文从走滑型断层活动所应具备的应力条件出发,讨论了走滑型断层活动区的水平主应力的上限和下限。经与现有的绝对应力测值对比,发现3000米以上深度,仅有那些有明显垂直运动的地区的应力值可能超出这个界限。3000米以下深度,实测应力值向所给出的界限内集中。本文所给出的水平应力界限可以做为对深部水平应力值的一种估计。     

Source Scaling of Inland Crustal Earthquake Sequences in Japan Using the S-Wave Coda Spectral Ratio Method

We estimate corner frequencies and stress drops for 298 events ranging from M _w 3.2–7.0 in 17 inland crustal earthquake sequences in Japan to investigate the source scaling and variation in stress drops. We obtain the source spectral ratio from observed records by the S-wave coda spectral ratio method. The advantage of using the S-wave coda is in obtaining much more stable source spectral ratios than using direct S-waves. We carefully examine the common shape of the decay of coda envelopes between event pair records. The corner frequency and stress drop are estimated by modeling the observed source spectral ratio with the omega-square source spectral model. We investigate the dependences of stress drops on some tectonic effects such as regionality, focal mechanism, and source depth. The principal findings are as follows: (1) a break in self-similar source scaling is found in our dataset. Events larger than M _w 4.5 show larger stress drops than those of smaller events. (2) Stress drops of aftershocks are mostly smaller than those of mainshocks in each sequence. (3) There are no systematic differences between stress drops of events occurring inside and outside the Niigata-Kobe Tectonic Zone in Japan. (4) Clear dependence of the faulting type on stress drops cannot be seen. (5) Stress drops of aftershocks depend on their source depth. (6) The crack size obtained from the corner frequency corresponds to the total rupture area of heterogeneous slip models for large events.     

2000年姚安地震的震源参数

根据近场小孔径观测台网记录的余震序列资料，研究了2000年1月15日云南姚安MS6.4地震序列的地震物理过程. 用地震标定律关系估算主震的地震矩M0=1.58×1018N·m，矩震级MW=6.0，平均位错=0.63m，断层长度L=16.6km，断层宽度W=5.6km. 余震序列的高精度定位结果和能量分布走向，很好地证实了主震的断层破裂走向为N50°W，震区马尾菁断裂为主震发震构造，断层错动性质以右旋走滑为主. 用横波记录资料及波谱分析方法估算出余震的震源参数: 地震矩范围为1010～1016N·m，震源破裂半径a为80～500m，地震应力降范围为0.01～9.5MPa. 较大应力降(Δσ>1.0MPa)沿主断层线性排列，大应力降(Δσ>2.0MPa)与ML≥3.0级地震相关. 余震能量释放和高应力降的地震多发生在6.0～11km的深度范围，说明在这一深度范围内最大程度地集中了地壳中的应力.     

Stress drop assessment of the August 8, 2017, Jiuzhaigou earthquake sequence and its tectonic implications*

By using a broadband Lg attenuation model developed for the Tibetan Plateau, we isolate source terms by removing attenuation and site effects from the observed Lg-wave displacement spectra of the M7.0 earthquake that occurred on August 8, 2017, in Jiuzhaigou, China, and its aftershock sequence. Thus, the source parameters, including the scalar seismic moment, corner frequency and stress drop, of these events can be further estimated. The estimated stress drops vary from 47.1 kPa to 7149.6 kPa, with a median value of 59.4 kPa and most values falling between 50 kPa and 75 kPa. The estimated stress drops show significant spatial variations. Lower stress drops were mainly found close to the mainshock and on the seismogenic fault plane with large coseismic slip. In contrast, the highest stress drop was 7.1 MPa for the mainshock, and relatively large stress drops were also found for aftershocks away from the major seismogenic fault and at depths deeper than the zone with large coseismic slip. By using a statistical method, we found self-similarity among some of the aftershocks with a nearly constant stress drop. In contrast, the stress drop increased with the seismic moment for other aftershocks. The amount of stress released during earthquakes is a fundamental characteristic of the earthquake rupture process. As such, the stress drop represents a key parameter for improving our understanding of earthquake source physics.     

Characterization of Stress Drops on Asperities Estimated from the Heterogeneous Kinematic Slip Model for Strong Motion Prediction for Inland Crustal Earthquakes in Japan

Dense strong motion observation networks provided us with valuable data for studying strong motion generation from large earthquakes. From kinematic waveform inversion of seismic data, the slip distribution on the fault surface of large earthquakes is known to be spatially heterogeneous. Because heterogeneities in the slip and stress drop distributions control the generation of near-source ground motion, it is important to characterize these heterogeneities for past earthquakes in constructing a source model for reliable prediction of strong ground motion. The stress changes during large earthquakes on the faults recently occurring in Japan are estimated from the detailed slip models obtained by the kinematic waveform inversion. The stress drops on and off asperities are summarized on the basis of the stress change distributions obtained here. In this paper, we define the asperity to be a rectangular area whose slip is 1.5 or more times larger than the average slip over the fault according to the previous study for inland crustal earthquakes. The average static stress drops on the asperities of the earthquakes studied here are in the range 6?C23?MPa, whereas those off the asperities are below 3?MPa. We compiled the stress drop on the asperities together with a data set from previous studies of other inland earthquakes in Japan and elsewhere. The static stress drop on the asperity depends on its depth, and we obtained an empirical relationship between the static stress drop and the asperity??s depth. Moreover, surface-breaking asperities seemed to have smaller stress drops than buried asperities. Simple ground motion simulations using the characterized asperity source models reveal that deep asperities generate larger ground motion than shallow asperities, because of the different stress drops of the asperities. These characteristics can be used for advanced source modeling in strong ground motion prediction for inland crustal earthquakes.     

Variation of seismic source parameters and stress drops within a descending slab and its implications in plate mechanics

A least-squares searching technique has been developed to estimate the source dimensions of intermediate and deep focus earthquakes using azimuthal variations of body wave pulse-widths. With this method and also amplitude data, modes of rupture propagation, seismic moments, and stress drops of 17 intermediate and deep focus earthquakes in the Tonga-Kermadec region have been determined in order to investigate variations in source properties and the state of stress within the descending slab there. Three different modes of rupture; unilateral, bilateral, and circular faults, are compared and tested against observations. Results indicate that the unilateral fault is the best model for most of the earthquakes studied. Stress drops of the 17 events vary within a very large range, from 20 bar to about 4.6 kbar, and change significantly with depth. The magnitude of stress drops for depths between 220 and 430 km is very much higher than at shallower depths. This change in stress drop magnitude at about 220 km-depth seems to reflect a change in material properties both in the mantle and within the slab. Two regions of high stress drop are observed at depths of about 360 and 640 km. A relative minimum of stress drop is found at about 450–560 km where the earthquake frequency is particularly high. Earthquakes at the northern end to the Tonga arc, where the Benioff zone is laterally bent, show systematically higher stress drops than other events at comparable depths, but away from the bend. Also, events in regions of low seismicity appear to have higher stress drops than those in regions of high seismicity. The upper bound of seismic efficiency is found to decrease with depth, implying an increase of frictional force with depth at the earthquake source.     

Estimation of Source Parameters for the Aftershocks of the 2001 Mw 7.7 Bhuj Earthquake, India

The source parameters for 213 Bhuj aftershocks of moment magnitude varying from 2.16 to 5.74 have been estimated using the spectral analysis of the SH- waveform on the transverse component of the three-componnet digital seismograms as well as accelerograms. The estimated stress drop values for Bhuj aftershocks show more scatter (Mo^{0.5 to 1} ∞ Δσ) toward the larger seismic moment values (log Mo ≥ 10^14.5 N-m, larger aftershocks), whereas, they show a more systematic nature (Mo³ ∞ Δσ) for smaller seismic moment (log Mo < 10^14.5 N-m, smaller aftershocks) values. This size dependency of stress drop has also been seen from the relation between our estimated seismic moment and source radius, however, this size-dependent stress drop is not observed for the source parameter estimates for the other stable continental region earthquakes in India and around the world. The estimated seismic moment (Mo), source radius (r) and stress drop (Δσ) for aftershocks of moment magnitude 2.16 to 5.74 range from 1.95 × 10¹² to 4.5 × 10¹⁷ N-m, 239 to 2835 m and 0.63 to 20.7 MPa, respectively. The near-surface attenuation factor (k) is found to be large of the order of 0.03 for the Kachchh region, suggesting thick low velocity sediments beneath the region. The estimated stress drop values show an increasing trend with the depth indicating the base of seismogenic layer (as characterized by larger stress drop values (>15 MPa)) lying in 22–26km depth range beneath the region. We suggest that the concentration of large stress drop values at 10–36km depth may be related to the large stress/strain associvated with a brittle, competent intrusive body of mafic nature.     

地震应力降与岩石破裂应力降

从应力降的静态理论,讨论了地震应力降的精度,并利用野外观测的最大位移和由地震波得到的地震矩,分别计算了十个研究较充分的地震应力降.对不同作者的结果作了比较,可以看出,应力降有因子为2的变化范围.但从统计观点看,地震应力降是分布在几巴到几百巴的数量级,大多数位于20-60巴;总结了地壳中代表性岩石在地壳温度、压力条件下,粘滑及脆性剪切破裂时的应力降,它们与温度、压力、岩性、含水矿物的存在有关.取值范围是从零到几千巴数量级;讨论了地震断层的物理状态,指出地震应力降是一种平均结果,并由于静态理论忽略了断层面破裂所消耗的能量,所以地震应力降比实际的应力降要低.