近场全波形反演：芦山7.0级地震及余震矩张量解

2013年4月20日四川芦山发生了7.0级地震.该地震是自2008年汶川8.0级地震以来发生在龙门山断裂带上的最强地震,截至5月5日,四川省数字遥测台网记录到了52个（M_L≥4.0）余震.我们搜集了信噪比相对较高的宽频带数据,根据已有的三个分别适合于四川盆地和龙门山断裂东南地区、西南山区、西北山区速度模型,利用TDMT_INVC方法计算矩张量,获得了芦山主震及其16个余震的高质量矩张量解;根据P轴、B轴和T轴的倾角对主震及余震的断层类型进行了分类,结果表明：芦山地震的震级为M_w6.46（矩震级）,地震矩为0.61×10¹⁹Nm,震源深度为13 km,震源机制类型为逆断层.芦山余震类型简单,主要为逆冲型;芦山地震序列震中分布于龙门山断裂带西南端的彭县—灌县断裂上,本研究获得的主震和余震的震源机制主要为逆冲型,这表明了龙门山断裂西南端主要为逆冲特征.芦山地震震源性质与汶川地震相近,发震间隔时间短,且均发生于同一构造单元内,表明了该地震与汶川地震可能有密切关系.     

基于分频波形拟合方法的鲁甸M_S6.5地震序列震源机制解研究

2014年鲁甸M_S6.5地震造成了极大的人员伤亡和经济损失,其孕震机理和动力学成因为国内外学者广泛关注.由于目前的反演主要采用单一的反演方法进行处理,导致结果的不确定性较大,使得该地震的发震构造及孕震机理等问题仍存在较大争议.针对目前研究存在的不足,本文基于近震波形求解震源机制解的gCAP(general Cut And Paste)方法和全波形拟合方法,分别反演了鲁甸地震主震及其9次余震的震源机制解,以此评价近震数据反演震源机制解结果的可靠性与稳定性.结果表明数据方位角覆盖对gCAP解的稳定性有较大影响.针对全波形拟合方法反演过程中,低频信号稳定、高频信号解析度高的特点,采用0.01～0.05Hz和0.01～0.2Hz分频段波形拟合思路:在低频段剔除拟合差的数据,进一步在高频段进行高解析度波形拟合,从而获得主震可靠稳定的震源机制解.研究结果表明鲁甸地震主震倾角为76°～83°、滑动角为-157°～-164°,为一次高倾角走滑型地震.两种方法获得的余震震源机制解比较一致,验证了结果的可靠性.研究结果显示,鲁甸地震的高倾角破裂特征,会导致其能量释放快速且完整,这可能是导致鲁甸地震地面破坏加重及缺乏较大震级余震的主要原因.     

The 20 April 2013 Lushan,Sichuan, mainshock,and its aftershock sequence: tectonic implications

Using the double-difference relocation algorithm, we relocated the 20 April 2013 Lushan, Sichuan, earthquake (M _S 7.0), and its 4,567 aftershocks recorded during the period between 20 April and May 3, 2013. Our results showed that most aftershocks are relocated between 10 and 20 km depths, but some large aftershocks were relocated around 30 km depth and small events extended upward near the surface. Vertical cross sections illustrate a shovel-shaped fault plane with a variable dip angle from the southwest to northeast along the fault. Furthermore, the dip angle of the fault plane is smaller around the mainshock than that in the surrounding areas along the fault. These results suggest that it may be easy to generate the strong earthquake in the place having a small dip angle of the fault, which is somewhat similar to the genesis of the 2008 Wenchuan earthquake. The Lushan mainshock is underlain by the seismically anomalous layers with low-V_P, low-V_S, and high-Poisson’s ratio anomalies, possibly suggesting that the fluid-filled fractured rock matrices might significantly reduce the effective normal stress on the fault plane to bring the brittle failure. The seismic gap between Lushan and Wenchuan aftershocks is suspected to be vulnerable to future seismic risks at greater depths, if any.     

2016年1月21日青海门源MS6.4余震序列重定位和主震震源机制解

2016年1月21日01时13分13.0秒（北京时间）,青海省海北州门源县发生M_S6.4地震.为了更好地认识这次地震的发震构造,本文利用青海省地震台网和甘肃省地震台网的省级固定地震台站及部分流动地震台站记录到的波形资料,通过重新拾取震相和联合HYPOINVERSE 2000与HypoDD定位方法,对2016年1月21日青海门源地震序列M_L≥1.8的189个地震事件进行了重新定位,并采用gCAP方法分别反演了主震的双力偶机制解和全矩张量解. 定位结果显示,主震位置为37.67°N、101.61°E,震源深度为11.98 km;余震序列展布方向为SE和NW两个方向、长度约16 km,震源深度优势分布为4~14 km,断层面倾向为SW方向. 利用gCAP方法得到的矩心深度在8~9 km之间. 结合野外地质调查结果,认为该次地震事件为一次逆冲型事件,其发震断层可能为北西向冷龙岭断裂与北西向民乐—大马营断裂之间的一条盲断层,推测由于印度板块与欧亚板块的碰撞挤压使得青藏高原北缘与阿拉善地块之间的东西向挤压而造成的断层应力失稳,从而形成门源地震.     

Sensitivity of the strong ground motion time histories to a finite source model: A case study for the January 12, 2010 Haiti earthquake (Mw=7.0)

We analyze the waveforms generated by the January 12, 2010 Haiti earthquake (M_w=7.0) for its source characteristics. A 60 to 25 km source model is retrieved by the Kikuchi and Kanamori finite source inversion technique that uses broadband teleseismic body wave records. The derived rupture model points out unilateral rupture propagation commenced at the eastern side of the fault plane where the major seismic moment release occurred. The rupture front propagated westward and terminated at a site where the largest aftershocks occurred. Our estimates yield a seismic moment of M_o=8.17×10¹⁹ N m released on a 60 km-long fault plane. A patch at the eastern side of the ruptured fault plane inferred as a region of maximum moment release.     

2015尼泊尔M_S8.1地震中等余震震源机制研究

本研究利用西藏台网记录的波形数据,采用gCAP方法反演了2015年4月25日尼泊尔MS8.1大震5次中等余震(5.0≤MS≤6.5)及西藏定日MS5.9地震震源机制解.结果显示,6次地震包含2个正断、2个走滑及2个逆冲型地震.其中2个正断型地震位于主震的东北方向,即发震断层的上盘,表明该区域受到主震同震位移的影响,表现出应力拉张的变化特征;2个走滑型地震在主震破裂的东南方向上,说明随着破裂往东南方向延伸,余震的走滑分量增强;另外2个逆冲型地震位于5月12日MS7.5强余震区域,与MS7.5地震的滑移状态一致,可能与主震同震位移引起该区域处于应力挤压状态密切相关.这些结果表明,尼泊尔MS8.1主震发生后,由于同震位移的影响,不同区域处于不同的应力状态,从而使中等余震表现出不同的震源类型.     

The Lemnos 8 January 2013 (M w = 5.7) earthquake: fault slip,aftershock properties and static stress transfer modeling in the north Aegean Sea

We investigate mainshock slip distribution and aftershock activity of the 8 January 2013 M _w?=?5.7 Lemnos earthquake, north Aegean Sea. We analyse the seismic waveforms to better understand the spatio-temporal characteristics of earthquake rupture within the seismogenic layer of the crust. Peak slip values range from 50 to 64 cm and mean slip values range from 10 to 12 cm. The slip patches of the event extend over an area of dimensions 16?×?16 km². We also relocate aftershock catalog locations to image seismic fault dimensions and test earthquake transfer models. The relocated events allowed us to identify the active faults in this area of the north Aegean Sea by locating two, NE–SW linear patterns of aftershocks. The aftershock distribution of the mainshock event clearly reveals a NE–SW striking fault about 40 km offshore Lemnos Island that extends from 2 km up to a depth of 14 km. After the mainshock most of the seismic activity migrated to the east and to the north of the hypocenter due to (a) rupture directivity towards the NE and (b) Coulomb stress transfer. A stress inversion analysis based on 14 focal mechanisms of aftershocks showed that the maximum horizontal stress is compressional at N84°E. The static stress transfer analysis for all post-1943 major events in the North Aegean shows no evidence for triggering of the 2013 event. We suggest that the 2013 event occurred due to tectonic loading of the North Aegean crust.     

芦山震区地壳三维P波速度精细结构及地震重定位研究

联合芦山地震序列5285个地震的50711条P波初至绝对到时数据及7294691条高质量的相对到时数据,利用双差地震层析成像方法联合反演了芦山震源区高分辨率的三维P波速度精细结构及5115个地震震源参数.反演结果表明,芦山主震震中为30.28°N,103.98°E,震源深度为16.38km,主震南西段余震扩展长度约23km,余震前缘倾角较和缓,主震北东段余震扩展长度约12km,余震前缘呈铲形,倾角较陡.芦山震源区P波三维速度结构表现出明显的横向不均匀性,近地表处的P波速度异常与地形起伏及地质构造密切相关:宝兴杂岩对应明显的高速异常,此异常由地表延伸到地下15km深度附近,而中新生代岩石表现为低速异常;大兴附近区域亦显示出小范围的大幅度高速异常,宝兴高速异常与大兴高速异常在10km深度附近相连,进而增加了芦山震源区的高低速异常对比幅度.在芦山主震的南西、北东两段速度结构存在着较大差异,芦山主震在水平向位于宝兴及大兴高速异常所包围的低速异常的前缘.主震南西段余震主要发生在倾向北西的高低速异常转换带上并靠近低速一侧,其下盘为低速异常,上盘为高速异常.而芦山主震北东段的余震主要分布在宝兴高速体与大兴高速体之间,主发震层向北西倾斜,主发震层上方的宝兴高速异常下边界出现一条南东倾向的反冲地震带,两地震带呈"y"型分布.     

Seismic Subduction of the Nazca Ridge as Shown by the 1996–97 Peru Earthquakes

—By rupturing more than half of the shallow subduction interface of the Nazca Ridge, the great November 12, 1996 Peruvian earthquake contradicts the hypothesis that oceanic ridges subduct aseismically. The mainshock’s rupture has a length of about 200 km and has an average slip of about 1.4 m. Its moment is 1.5 × 10²⁸ dyne-cm and the corresponding M _w is 8.0. The mainshock registered three major episodes of moment release as shown by a finite fault inversion of teleseismically recorded broadband body waves. About 55% of the mainshock’s total moment release occurred south of the Nazca Ridge, and the remaining moment release occurred at the southern half of the subduction interface of the Nazca Ridge. The rupture south of the Nazca Ridge was elongated parallel to the ridge axis and extended from a shallow depth to about 65 km depth. Because the axis of the Nazca Ridge is at a high angle to the plate convergence direction, the subducting Nazca Ridge has a large southwards component of motion, 5 cm/yr parallel to the coast. The 900–1200 m relief of the southwards sweeping Nazca Ridge is interpreted to act as a "rigid indenter," causing the greatest coupling south of the ridge’s leading edge and leading to the large observed slip. The mainshock and aftershock hypocenters were relocated using a new procedure that simultaneously inverts local and teleseismic data. Most aftershocks were within the outline of the Nazca Ridge. A three-month delayed aftershock cluster occurred at the northern part of the subducting Nazca Ridge. Aftershocks were notably lacking at the zone of greatest moment release, to the south of the Nazca Ridge. However, a lone foreshock at the southern end of this zone, some 140 km downstrike of the mainshock’s epicenter, implies that conditions existed for rupture into that zone. The 1996 earthquake ruptured much of the inferred source zone of the M _w 7.9–8.2 earthquake of 1942, although the latter was a slightly larger earthquake. The rupture zone of the 1996 earthquake is immediately north of the seismic gap left by the great earthquakes (M _w ～8.8–9.1) of 1868 and 1877. The M _w 8.0 Antofagasta earthquake of 1995 occurred at the southern end of this great seismic gap. The M _w 8.2 deep-focus Bolivian earthquake of 1994 occurred directly downdip of the 1868 portion of that gap. The recent occurrence of three significant earthquakes on the periphery of the great seismic gap of the 1868 and 1877 events, among other factors, may signal an increased seismic potential for that zone.     

2013年芦山地震序列震源机制与震源区构造变形特征分析

基于四川区域地震台网记录的波形资料,利用CAP波形反演方法,同时获取了2013年4月20日芦山M7.0级地震序列中88个M≥3.0级地震的震源机制解、震源矩心深度与矩震级,进而利用应变花（strain rosette）和面应变（areal strain）As值,分析了芦山地震序列震源机制和震源区构造运动与变形特征.获得的主要结果有：（1）芦山M7.0级主震破裂面参数为走向219°/倾角43°/滑动角101°,矩震级为M_W6.55,震源矩心深度15 km.芦山地震余震区沿龙门山断裂带走向长约37 km、垂直断裂带走向宽约16 km.主震两侧余震呈不对称分布,主震南西侧余震区长约27 km、北东侧长约10 km.余震分布在7~22 km深度区间,优势分布深度为9~14 km,序列平均深度约13 km,多数余震分布在主震上部.粗略估计的芦山地震震源体体积为37 km×16 km×16 km.（2）面应变As值统计显示,芦山地震序列以逆冲型地震占绝对优势,所占比例超过93%.序列主要受倾向NW、倾角约45°的近NE-SW向逆冲断层控制;部分余震发生在与上述主发震断层近乎垂直的倾向SE的反冲断层上;龙门山断裂带前山断裂可能参与了部分余震活动.P轴近水平且优势方位单一,呈NW-SE向,与龙门山断裂带南段所处区域构造应力场方向一致,反映芦山地震震源区主要受区域构造应力场控制,芦山地震是近NE-SW向断层在近水平的NW-SE向主压应力挤压作用下发生逆冲运动的结果.序列中6次非逆冲型地震均发生在主震震中附近,且主震震中附近P轴仰角变化明显,表明主震对其震中附近局部区域存在明显的应力扰动.（3）序列整体及不同震级段的应变花均呈NW向挤压白瓣形态,显示芦山地震震源区深部构造呈逆冲运动、NW向纯挤压变形.各震级段的应变花方位与形状一致,具有震级自相似性特征,揭示震源区深部构造运动和变形模式与震级无关.（4）不同深度的应变花形态以NW-NWW向挤压白瓣为优势,显示震源区构造无论是总体还是分段均以NW-NWW向挤压变形为特征.但应变花方位与形状随深度仍具有较明显的变化,可能反映了震源区构造变形在深度方向上存在分段差异.（5）芦山地震震源体尺度较小,且主震未发生在龙门山断裂带南段主干断裂上,南段长期积累的应变能未能得到充分释放,南段仍存在发生强震的危险.     

Segmented seismicity of the M w 6.2 Baladeh earthquake sequence (Alborz Mountains, Iran) revealed from regional moment tensors

The M _w 6.2 Baladeh earthquake occurred on 28 May 2004 in the Alborz Mountains, northern Iran. This earthquake was the first strong shock in this intracontinental orogen for which digital regional broadband data are available. The Baladeh event provides a rare opportunity to study fault geometry and ongoing deformation processes using modern seismological methods. A joint inversion for hypocentres and a velocity model plus a surface-wave group dispersion curve analysis were used to obtain an adapted velocity model, customised for mid- and long-period waveform modelling. Based on the new velocity model, regional waveform data of the mainshock and larger aftershocks (M _w ?≥3.3) were inverted for moment tensors. For the Baladeh mainshock, this included inversion for kinematic parameters. All analysed earthquakes show dominant thrust mechanisms at depths between 14 and 26 km, with NW–SE striking fault planes. The mainshock ruptured a 28° south-dipping area of 24 × 21 km along a north-easterly direction. The rupture plane of the mainshock does not coincide with the aftershock distribution, neither in map view nor with respect to depth. The considered aftershocks form two main clusters. The eastern cluster is associated with the mainshock. The western cluster does not appear to be connected with the rupture plane of the mainshock but, instead, indicates a second activated fault plane dipping at 85° towards the north.     

Seismic relocation,focal mechanism and crustal seismic anisotropy associated with the 2010 Yushu MS7.1 earthquake and its aftershocks

The 2010 Yushu M_S7.1 earthquake occurred in Ganzi-Yushu fault, which is the south boundary of Bayan Har block. In this study, by using double difference algorithm, the locations of mainshock (33.13°N, 96.59°E, focal depth 10.22 km) and more than 600 aftershocks were obtained. The focal mechanisms of the mainshock and some aftershocks with M_S>3.5 were estimated by jointly using broadband velocity waveforms from Global Seismic Network (GSN) and Qinghai Seismic Network as well. The focal mechanisms and relocation show that the strike of the fault plane is about 125° (WNW-ESE), and the mainshock is left-laterally strikeslip. The parameters of shear-wave splitting were obtained at seismic stations of YUS and L6304 by systematic analysis method of shear-wave splitting (SAM) method. Based on the parameters of shear-wave splitting and focal mechanism, the characteristics of stress field in seismic source zone were analyzed. The directions of polarization at stations YUS and L6304 are different. It is concluded that after the mainshock and the M_S6.3 aftershock on April 14, the stress-field was changed.     

Moment tensors,state of stress and their relation to faulting processes in Gujarat,western India

Time domain moment tensor analysis of 145 earthquakes (M_w 3.2 to 5.1), occurring during the period 2006–2014 in Gujarat region, has been performed. The events are mainly confined in the Kachchh area demarcated by the Island belt and Kachchh Mainland faults to its north and south, and two transverse faults to its east and west. Libraries of Green's functions were established using the 1D velocity model of Kachchh, Saurashtra and Mainland Gujarat. Green's functions and broadband displacement waveforms filtered at low frequency (0.5–0.8 Hz) were inverted to determine the moment tensor solutions. The estimated solutions were rigorously tested through number of iterations at different source depths for finding reliable source locations. The identified heterogeneous nature of the stress fields in the Kachchh area allowed us to divide this into four Zones 1–4. The stress inversion results indicate that the Zone 1 is dominated with radial compression, Zone 2 with strike-slip compression, and Zones 3 and 4 with strike-slip extensions. The analysis further shows that the epicentral region of 2001 M_W 7.7 Bhuj mainshock, located at the junction of Zones 2, 3 and 4, was associated with predominant compressional stress and strike-slip motion along ∼ NNE-SSW striking fault on the western margin of the Wagad uplift. Other tectonically active parts of Gujarat (e.g. Jamnagar, Talala and Mainland) show earthquake activities are dominantly associated with strike-slip extension/compression faulting. Stress inversion analysis shows that the maximum compressive stress axes (σ₁) are vertical for both the Jamnagar and Talala regions and horizontal for the Mainland Gujarat. These stress regimes are distinctly different from those of the Kachchh region.     

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.     

Constraining the focal mechanism of the Lushan earthquake with observations of the Earth’s free oscillations

The amplitudes of the Earth's free oscillations have a close relationship to earthquake focal mechanisms. Focal mechanisms of large earthquakes can be well analyzed and constrained with observations of long period free oscillations. Although the 2013 Lushan earthquake was only moderately sized, observable spherical normal modes were excited and clearly observed by a superconductive gravimeter and a broadband seismometer. We compare observed free oscillations with synthetic normal modes corresponding to four different focal mechanisms for the Lushan earthquake. The results show that source parameters can be analyzed and constrained by spherical normal modes in a 2.3–5 mHz frequency band. The scalar seismic moment M0 has a major influence on the amplitudes of free oscillations; additionally, the strike, dip, rake and depth of the hypocenter have minor influences. We found that the synthetic modes corresponding to the focal mechanism determined by the Global Centroid Moment Tensor show agreement to the observed modes, suggesting that earthquake magnitudes predicted in this way can readily reflect the total energy released by the earthquake. The scalar seismic moment obtained by far-field body wave inversion is significantly underestimated. Focal mechanism solutions can be improved by joint inversion of far- and near-field data.     

2020年新疆于田MS6.4地震断层面快速测定及发震构造研究

本文基于新疆、 西藏区域数字地震台网波形数据, 利用gCAP反演方法和空间格点搜索算法获得2020年6月26日于田M_S6.4地震矩心的空间位置为35.649°N, 82.339°E, 深度为5 km。 最佳震源机制解节面Ⅰ走向166°, 倾角59°, 滑动角-144°; 节面Ⅱ走向26°, 倾角38°, 滑动角-55°, 矩震级为M_W6.21。 根据不同震源机制解结果, 获得中心震源机制解和标准差, 表明震源机制解较为稳定和可靠。 使用H-C方法进行地震发震断层的快速判断, 显示节面Ⅱ为发震断层面。 综合震源区地质构造特征、 余震序列的空间分布和区域构造应力场特征, 最终推断此次地震断层面为节面Ⅱ, 阿尔金断裂西段是发震断层, 震源机制解显示以正滑为主, 是一次张性破裂地震事件, 属于阿尔金断裂西段强烈活动的响应。     

利用sPn震相测定芦山MS7.0级地震余震的震源深度

利用南北地震带南段密集流动地震台阵的观测数据,采用波形互相关方法拾取Pn波走时,应用滑动时窗相关法识别sPn震相,通过sPn与Pn震相之间的走时差测定了芦山地震序列中28个M_L4.0级以上余震的震源深度.结果表明,震源深度集中在10~20 km范围内,垂直余震带的北西-南东向震源深度剖面揭示,余震分布表现出西深东浅的特点,倾角大约为39°.这些余震在空间上具有较好的线性分布特征,推测可能发生在与主震有关的破裂面上或邻近位置,由此推测主震的破裂面倾角大约为39°.根据余震的空间分布特征,认为芦山地震的发震断层并非双石-大川断裂,可能是其东侧的一条隐伏断层.     

Strong ground motion simulation of the 2003 Bam, Iran, earthquake using the empirical Green’s function method

The 2003 Bam, Iran, earthquake caused catastrophic damage to the city of Bam and neighboring villages. Given its magnitude (M _w ) of 6.5, the damage was remarkably large. Large-amplitude ground motions were recorded at the Bam accelerograph station in the center of Bam city by the Building and Housing Research Center (BHRC) of Iran. We simulated the Bam earthquake acceleration records at three BHRC strong-motion stations—Bam, Abaraq, and Mohammad-Abad—by the empirical Green’s function method. Three aftershocks were used as empirical Green’s functions. The frequency range of the empirical Green’s function simulations was 0.5–10 Hz. The size of the strong motion generation area of the mainshock was estimated to be 11 km in length by 7 km in width. To estimate the parameters of the strong motion generation area, we used 1D and 2D velocity structures across the fault and a combined source model. The empirical Green’s function method using a combination of aftershocks produced a source model that reproduced ground motions with the best fit to the observed waveforms. This may be attributed to the existence of two distinct rupture mechanisms in the strong motion generation area. We found that the rupture starting point for which the simulated waveforms best fit the observed ones was near the center of the strong motion generation area, which reproduced near-source ground motions in a broadband frequency range. The estimated strong motion generation area could explain the observed damaging ground motion at the Bam station. This suggests that estimating the source characteristics of the Bam earthquake is very important in understanding the causes of the earthquake damage.     

The Beni Haoua,Algeria, Mw 4.9 earthquake: source parameters,engineering, and seismotectonic implications

A moderate Mw 4.9 earthquake struck the Beni Haoua (Algeria) coastal area on April 25, 2012. The mainshock was largely recorded by the accelerograph network of the Centre National de Recherche Appliquée en Génie Parasismique (CGS). The same day the earthquake occurred, eight mobile short period stations were deployed through the epicentral area. In this study, we use accelerogram and seismogram data recorded by these two networks. We combined the focal mechanism built from the first motion of P waves and from waveform inversion, and the distribution of aftershocks to well constrain the source parameters. The mainshock is located with a shallow focal depth, ～9 km, and the focal mechanism shows a nearly pure left lateral strike slip motion, with total seismic moment of 2.8?×?10¹⁶ N.m (M_w?=?4.9). The aftershocks mainly cluster on a narrow NS strip, starting at the coast up to 3–4 km inland. This cluster, almost vertical, is concentrated between 6 and 10 km depth. The second part of this work concerns the damage distribution and estimated intensity in the epicentral area. The damage distribution is discussed in connection with the observed maximum strong motion. The acceleration response spectrum with 5 % damping of the mainshock and aftershocks give the maximum amplitude in high frequency which directly affects the performance of the high-frequency structures. Finally, we tie this earthquake with the seismotectonic of the region, leading to conclude that it occurred on a N–S transform zone between two major compressional fault zones oriented NE–SW.     

用接收函数建立区域模型的震源机制反演及其在芦山地震序列研究中的应用

本文提出并试验了一种基于接收函数建立区域模型进行震源机制反演的方法.选取四川地震台网记录的M≥3且信噪比高的近震波形资料,反演得到了芦山地震序列中74个地震的震源机制.通过对震源深度和震源机制的综合分析,探讨了芦山地震的发震构造和区域应力场状态.采用接收函数方法反演获取了26个台站下方的S波速度结构,对不同区域的台站反演结果进行叠加平均,以此区域平均S波速度作为本文震源机制反演使用的区域模型的S波速度;区域模型的P波速度由经验公式给出.反演稳定性测试表明,使用不同模型或对原始波形记录加入随机噪声的反演结果与原始反演相比,震源深度最大误差为1km,断层面各参数误差水平也很低,且显示的发震类型是一致的,其中随机噪声带来的误差小于模型带来的误差.主震反演得到的震源机制解为:震源深度17km,矩震级6.47;节面Ⅰ走向213°,倾角51°,滑动角98°;节面Ⅱ走向20°,倾角40°,滑动角80°;显示芦山主震可视为纯逆冲型地震,发震构造可能是某个具有较大倾角的逆冲断层,而不是低缓的推覆构造的基底滑脱面.同时本文反演获取的73个M≥3余震的震源机制绝大多数也显示了类似的发震类型,逆冲型地震为67个,占92%,具有绝对优势;走滑型地震为5个,正断型地震为1个.其中5个走滑型地震中的4个均分布在震源区的东北端.整个芦山地震序列深度集中在12~20km,且沿震源区短轴的余震深度剖面有自西向东呈逐步变浅的趋势,呈现清晰的铲形断面结构,结合本地地质构造,可以推断芦山地震序列主要发生在龙门山前山断裂以东的逆冲推覆体内的一个隐伏断裂上.P轴方位角优势方位与区域应力场及汶川震源区南段的相一致,表明芦山序列地震活动主要受区域应力场控制,且汶川震后该区应该不存在应力场变化.P轴仰角随深度分布则显示了孕震层在浅部为脆性上地壳,而深部已经进入了中地壳低速层.断层面的几何形态简单,倾角均值在不同深度保持稳定在55°左右,与主震倾角接近,这与汶川震源区南段的研究结果明显不同,揭示了龙门山断裂带南段与此次芦山发震断裂在断层面几何形态上的明显差异.