Theoretical researches on three-dimensional coda wave scattering problem

Based on the previous researches on scattering theory, a model of single and multiple scattering in a three-dimensional infinite medium with non-zero hypocentral distance is proposed in this paper. According to the assumptionof three-dimensional medium with numerous, statistically isotropic and uniformly distributed scatterers, we obtain the analytic form of power spectrum of coda waves for single scattering and the integral form of power spectrum of coda waves for multiple scattering.

Spatial variation of coda wave attenuation using aftershocks of the Al Hoceima earthquake of 24 February, 2004, Morocco

On 24th February 2004 a significant earthquake (Md = 6.4) occurred in the north of Moroccocausing great damage in the vicinity of Al Hoceima region. This area is characterized by a complex faulting system as a result of compressional tectonic forces. Three short period seismic stations are set in this area of interest and recordings from these stations were used in this study. In order to complete our knowledge of attenuation, 60 local earthquakes are recorded a few days after the great earthquake with magnitude Ml 2.6 - 5.0 to estimate seismic attenuation. For this purpose, we applied the single backscattering model of Aki & Chouet 1975 inthe frequency range for 1 to 8 Hz. The study of coda waves was limited to a relatively short lapse time (20 Seconds) in order to sample the earth’s crust only. The values of Qc estimated for all the three stations show a strong frequency dependent relationship of the form Qc = Q0fn, where Q0 is Qc at 1 Hz , and n represents the degree of frequency dependence, and reflects the level of crustal heterogeneities to varying degrees. The average frequency dependent attenuation relationship has been obtained which indicates that the attenuation is high in this region. Finally to conclude our work, the values of Q0 suggest that Al Hoceima area is highly heterogeneous and the n parameter indicates a meaning frequency dependence of Qc.

Attenuation of coda waves in the SW of High-Atlas area, Morocco

We investigate attenuation scattering and loss properties in Souss basin(SW of High-Atlas) as a transition zone between the High and Anti Atlas ranges. This district consists in a thinned crustal patch with shallow seismicity and loose sedimentary trenches that perform an important contribution to augment seismic attenuation. So far, no coda waves approach in our knowledge have been used to draw satisfying outputs about the attenuation properties in the region. Therefore, this update suggests to correlate the lateral changes of seismic attenuation to different characteristics and asperities i.e. seismic activity,crustal age and thickness, heat flow, and ground deformation rate. To do so, we analysed coda waves derived from waveform data of more than 23 local earthquakes from seven broadband seismometers recorded during 2010 e2012 period. As a starter, we utilized the backscattering model which defines theseismic attenuation as inversely proportional to quality factor by the equation A ?1=. QQcestimates c were deducted at various central frequency bands 1.5, 3.0, 6.0, 9.0, 12.0 and 18.0 Hz according to different lapses times. The estimated average frequency dependence quality factor gives relation Qc? 120 f1;01,while the average Qcvalues vary from 149 at 1.5 Hz to 1895 at 18 Hz central frequencies. We observed an intimate dependence between quality factor and frequency ranges, which reflects the complexity of geological/geophysical pattern in the Souss basin and the presence of a variety of heterogeneities within the underlying crust.

Monitoring Media Velocity Variations with Coda Wave Interferometry

Multiply scattered waves are sensitive to media changes owing to the effect of repeated sampling,superposition and amplification. Based on this characteristic,small-medium changes could be detected by using coda wave interferometry. In recent years,coda wave interferometry has been widely used in estimating velocity variation with high precision in areas such as seismology and non-destructive testing. This paper systematically presents the principle and research status of coda wave interferometry,and especial focus is placed on the research of media velocity variations by using repeating earthquakes,artificial sources,and ambient noise. Applications of coda wave interferometry can contribute to the more subtle understanding of dynamic evolution process in the medium.

Discussion on the Measurement Method for the Coda Wave Quality Factor

The Quality factor is the parameter that can be used to describe the energy attenuation on seismic wave. In theory, we can obtain the relationship between the change of the coda wave quality factor with time and the strong earthquake preparation process on the basis of the quality factor of a coda wave in a same ray path. However, in reality the coda wave quality factor measured by different seismic coda waves corresponds to different seismic wave ray paths. The change of the quality factor with time is related to non-elastic characteristics of the medium and the volume of scattering ellipsoid constrained by scattered wave phase fronts, besides the change of regional stress field. This paper discusses the relationship between quality factor, epicenter distance and different lapse time, and then discusses the relationship between quality factor and frequency. Furthermore the determination method of the coda wave quality factor is put forward. The improved determination method of the quality factor, which removes the influence of different earthquakes or propagation depth of scattered waves, may increase measurement precision, thus information pertaining to abnormal changes in quality factor and the relationship between the quality factor and earthquake preparation process can be acquired.

Precise Measurement of Subsurface Seismic Velocity Variation by Coda Wave Interferometry

A filed experiment was conducted continuously for three days,and the velocity variation was measured using coda wave interferometry.The measurement error is estimated to be around 10-4,which coincides well with the theoretical error.The velocity variation during this period is up to 10-3.The relationship between velocity variation and changes in air temperature,barometric pressure and solid earth tide was analyzed with linear least square fitting.The velocity has no dependence on air temperature,but displayed change of the order of 10-6～10-7 when the barometer or earth tide changed one Pa.

Moment magnitudes of two large Turkish earthquakes on February 6,2023 from long-period coda

Two large earthquakes(an earthquake doublet)occurred in south-central Turkey on February 6,2023,causing massive damages and casualties.The magnitudes and the relative sizes of the two mainshocks are essential information for scientific research and public awareness.There are obvious discrepancies among the results that have been reported so far,which may be revised and updated later.Here we applied a novel and reliable long-period coda moment magnitude method to the two large earthquakes.The moment magnitudes(with one standard error)are 7.95±0.013 and 7.86±0.012,respectively,which are larger than all the previous reports.The first mainshock,which matches the largest recorded earthquakes in the Turkish history,is slightly larger than the second one by 0.11±0.035 in magnitude or by 0.04 to 0.18 at 95%confidence level.

Attenuation characteristics of coda waves and estimation of Qc values in eastern China

For short-period near-earthquake records in eastern China,from the empirical attenuation formula of coda ground motion amplitude A with timeτ:lgA=G-2.235lgτ,using the single scattering theory modified with epicentral distance,we obtain the curve family of corrected coda amplitude Ac(r,t),and w/2Qc values for each time interval of coda. From this,Qc(f,h) values, which correspond to each observational average frequency and sampling depth,are calculated.The results substantially agree with those observational Qc values in Yunnan,Beijing and central Asia.

Monitoring Contact State of Laboratory Fault with Coda Transmission Waves

We monitored the amplitude changes of coda transmission waves around 500 kHz across the frictional interface of a simulated 1. 5-meter-long fault during normal stress holding test.We find that the amplitude of coda transmission waves increases with the logarithm of stationary contact time. Localized increase amounted to a level ranging from 4% to 16%along the fault is observed during the 1-hour experiment. We discuss that the frictional strength at mesoscopic scale,which is related to the amplitude of coda transmission waves,is responsible for the phenomenon. Combining the reported method with other complementary approaches will enhance the understanding of fault mechanism either at laboratory or on-site applications.

基于超声尾波法的钢筋混凝土腐蚀监测研究

针对现有超声无损检测方法在钢筋混凝土(reinforced concrete,RC)腐蚀损伤评价方面存在的传播速度偏差问题,提出一种利用尾波干涉(coda wave interference,CWI)表征RC腐蚀情况的监测方法。在混凝土超声传播多重散射基础上,建立CWI腐蚀损伤评价指标。搭建快速电化学试验监测了RC在0,1%,2%腐蚀率下的腐蚀过程,通过逐步CWI分析了相对速度变化Δv/v及去相关系数K_(d)的变化规律,探究了换能器布置方式对尾波腐蚀监测敏感性的影响。研究结果表明:Δv/v随着腐蚀时间而逐渐减小,K_(d)随着腐蚀时间而逐渐增大;CWI方法能够有效地监测RC结构腐蚀裂纹的扩展过程,并且具有较好的敏感性;换能器采用垂直、对侧和同侧三种不同布置方式均能检测出RC结构腐蚀开裂,垂直布置方式对RC腐蚀检测的敏感程度更高。

High frequency S wave envelope synthesis using a multiple non-isotropic scattering model: application to aftershocks from the 2008 Wenchuan earthquake

Based on the formulation of a multiple non-isotropic scattering process, a characteristic source time is introduced to define the initial impulse width of energy density at the source. An analytical expression of the initial intensity spectral density of a seismic wave is incorporated into the integral equation of seismic wave energy density. And, a recursive formula of Green＇s function is derived to obtain the higher order Green＇s function, which is included to describe the stronger non-isotropic scattering process. Then, the effect of the scattering pattern on the energy density envelope is investigated by the modified scattering theory. Significant differences arc found in the decay of the energy density envelopes with distances using different scattering patterns. The envelope synthesized by the forward dominated scattering pattern is larger than the results obtained by the isotropic and backward dominated scattering pattern. Different scattering patterns are also used to fit the observation data from the aftershocks of the 2008 Wenchuan earthquake. It is concluded that the envelopes synthesized by the forward scattering pattern can match the data better than the isotropic and backward dominated scattering cases, and a new interpretation of the coda wave is given. Finally, using the forward dominated scattering pattern, the envelope broadening of the observed data is reproduced.

Research on coda Q distribution in Beijing and its surrounding regions

Based on the single scattering model of seismic coda waves, we have calculated the Q-factor in Beijing and its surrounding regions by means of calculating the power density spectrum in frequency domain with a fixed time window. The digital seismic data of 69 earthquakes from Beijing Telemetered Seismographic Network are used.These earthquakes were recorded from January 1, 1989 to December 31,1990 at 20 stations. This paper shows the variations of the coda Q-factors in the studied region with different sites, frequency and lapse time, and the temporal change of the coda Q-factors in these two years. The results indicate that coda Q-factor depends strongly on the lapse time and frequency. It is assumed that when Qc=Q0f″, for the three time windows of 15 -30 s, 30-60 s and 60-90 s, the average values of Q0 are 48, 115 and 217; and the average values of ηare 0.89, 0. 91 and 0. 74, respectively.

On the generation and decay of the long-period coda energy of large earthquakes

Seismic coda wave is the tail portion of the earthquake record after main arrivals.Studies on the coda usually focus on high-frequency data within several hours after regional events and attribute them to the scattering effect of the heterogeneities inside the earth.Here,we use records of seven large earthquakes at globally distributed seismic stations to examine the decay of long-period(100 s to 300 s)coda in the time window of 10,000 s to 140,000 s after the origin time and fit it with a statistical model.The geometric spreading effect in the estimated initial energy and a locationindependent equivalent attenuation coefficient indicate that the long-period coda energy is less affected by the heterogeneity-induced scattering effect than that of shorterperiod coda.The coda energy can reach the earth’s inner core and can be explained by a 1D earth model,making it more effective for constraining the global attenuation model.It also has the potential to determine the magnitudes of large earthquakes and to explore the interior of planetary bodies.

Temporal variations of coda Q^（－1） associated with the dahaituoshan， Beijing， earthquake of 1990

emporal variation of coda Q-1 associated with the occurrence of Dahaituoshan earthquake (ML= 5. 4) and its variation with the seismicity in Beijing area were studied by using 90 local earthquakes from the China Digital Seismograph Network (CDSN). The results showed that there was a significant temporal change in coda Q-1 during the period from Jan. 1987 to Aug. 1991 and it was relative to the seismicity of this area. Coda Q-1 was determined for the lapse time window ranging from 10 to 60 s, frequency 1 to 16 Hz. By comparing the average values of Q-1 for different time periods, we found that in region near the mainshock area the coda Q-1 for earthquakes which occurred after the mainshock were higher than those earthquakes that occurred prior to the mainshock, while the coda Q-1 after the mainshock for the surrounding area were lower than before. We also found the temporal changes in η, the exponential in formula Q-1=Q0-1 (f/f0) -η, and Q0, the Q-factor at 1 Hz. Measurements of coda Q-1 from three components showed inconsistency in temporal variations for different frequencies.

重庆武隆地区地震尾波Q_(C)值特征

为了了解重庆武隆地区地下介质的物性特征,为预测强地面运动和评估地震灾害损失提供一定的参考。本文选取2012年1月至2022年12月重庆地震台网记录的武隆地区56次M_(L)≥2.0地震波形资料,采用Sato单次散射模型分别计算了5个台站记录的尾波衰减参数,拟合得到武隆地区地震尾波Q_(C)值与频率(f)的依赖关系为Q_(C)(f)=41.87±8.69f^(0.9568±0.0904),表明该地区为低Q_(0)值(f=1)和较高频率依赖性(η)的构造活跃区。本文还分析了整个地震序列的尾波Q_(C)和η值随时间的变化,发现在2017年武隆M_S5.0地震发生后,WUL和LAX台站记录的Q_(0)和η值存在短期的异常变化,Q_(0)值呈“V”形态,η值呈倒“V”形态,η与Q_(0)值呈反向变化,Q_(0)值经历下降-上升的快速调整过程,表明该区构造活动处于活跃期。

Analysis on the Regional Features of Coda Attenuation Before and After the 1992 Qina (M_S= 5.4, 5.1) Earthquakes in WYEPSA

Using the theoretical model of coda scattering from local earthquakes, we have studied the features of time-space variations of the attenuation rate β of coda amplitude and coda Qc-1 values before and after the Qina earthquakes (MS5.4 and MS5.1) in Dec. 1992 in West Yunnan Earthquake Prediction Study Area (WYEPSA). The study results show that the attenuation rate of coda amplitude has remarkable regional inhomogeneity in the near-field (△ 【50 km) around the main shock: β = 0.0076 s-1 and Qc-1 = 0.0056 during the main shock while (3 = 0.0209 s-1 and Qc-1 = 0.0153 during the aftershock. The Qc-1 value shows an approximate 3-foid variation before and after the main shock. We have also obtained the regional averages of β = 0.0235 s-1 and Qc-1 = 0.0153, which show that the coda attenuation in surrounding areas (△ 】 50 km) far away from the epicenter of the main shock did not change much before and after the main shock. The evolution of Qc-1 values of the main seismic sequence with time has passed

Frequency-dependent attenuation of P and S waves in Yunnan region

We analyzed digital seismogram data of 5668 earthquakes that occurred in Yunnan region between July of 1999 and December of 2003. Among the 22 seismic stations, six ones, namely Baoshan, Yongsheng, Lijiang, Heqing, Yimen, and Luquan, were selected and their attenuations of P and S waves were measured by using the extended coda-normalization method. The six stations were classified into three regions according to their location, that is, Baoshan area, Yongsheng-Lijiang-Heqing (YLH) area and Luquan-Yimen (LY) area. The values of QS-1 and QP-1 are expressed as QS-1=0.00867 f-0.86, QP-1=0.01155 f-0.93, QS-1=0.01824 f-0.92, QP-1=0.02288 f-0.92, and QS-1=0.01647 f-0.91, QP-1=0.02826 f-0.97 in Baoshan, YLH, LY areas respectively. The attenuation of YLH, LY are closer to each other, however, Baoshan area is apparently lower. Comparing attenuation in the three areas with other areas of the world using the same method, it is suggested that the attenuations of P and S waves in YLH and LY areas are close to Kanto of Japan, but much higher than southeast of South Korea. The QS-1 and QP-1 in Baoshan area are slightly higher than southeastern South Korea. Furthermore, the results indicate that our QS-1 in Yunnan area is close to others by analyzing the coda attenuation.

基于残差矩阵SVD的混凝土梁超声信号温湿度干扰去除算法

研究混凝土梁超声尾波信号的温湿度干扰去除算法。在环境温湿度自然变化的室内条件下,开展混凝土梁超声测试试验,采集受环境温湿度干扰的超声尾波信号,以信号间的归一化夹角作为信号的波动指标。构造测试信号与参考信号的偏差为残差矩阵,建立了一种基于残差矩阵SVD(奇异值分解)的信号环境温湿度干扰去除算法,根据试验的环境温湿度影响确定算法的参数,由此标定左奇异向量中的干扰方向向量,去除信号在干扰方向向量上的投影,从而实现信号的温湿度干扰去除。研究结果表明:针对超声尾波信号的温湿度干扰去除问题,建立的算法在混凝土工字梁上得到验证,信号波动指标从原来的0.14降低至0.05,降幅超过50%。

Measuring Crustal Lg-Waves Attenuation in the Northern Part of Africa

Lg-waves propagate over long distances in the northern part of Africa and are recorded on the seismic station located close to Abeche (Chad). We have applied the well-known coda analysis method developed by Aki and Chouet [1] to study the Lg-wave anelastic attenuation in this area. We found a frequency dependence: Q( f ) = (650±34) f (0.77±0.2). This result shows an average value of Q0 (Q0 ≈ 650) for earthquakes occurred in tectonic areas as the East African Rift system, Turkey, Greece and Red Sea. The higher value of Q0 (Q0 ≈ 800) is obtained for the event occurred in the middle of the stable plate close to the Tamanrasset (TAM) station in Algeria.

基于尾波分析的沥青混凝土老化研究

为了研究尾波分析法在沥青混凝土路面老化程度的检测应用,使用COMSOL建立了沥青混凝土随机骨料模型,模拟了超声波在不同老化程度下沥青混凝土中的传播过程;通过尾波分析建立超声波尾波波速变化率参数Δv/v,得到老化程度与尾波波速变化率之间的关系,并进行了沥青混凝土的老化试验与声波测试试验.研究结果表明:超声波尾波波速变化率与沥青混凝土老化程度之间存在着良好的正相关性,在不同老化程度下可通过尾波波速变化率来确定沥青混凝土的老化程度,为沥青路面老化检测提供了新的测量方式.