A structural model for active extension in Central Italy

This work presents a structural model for earthquake faulting in the Umbria-Marche Apennines (Central Italy). The model is derived by an integrated analysis of geological, geophysical and seismological data. At regional scale, the distribution and character of the seismicity appear to be mainly controlled by a low-angle east-dipping normal fault (Altotiberina fault, AF). The latter is the lower boundary of an active, continuously deforming hangingwall block moving toward NE. Moderate magnitude earthquakes (4 < M < 6), such as the Norcia 1979 (M = 5.9), the Gubbio 1984 (M = 5.2) and the Colfiorito 1997 (M_max = 5.9), occur within the active hangingwall block and are related to the activity of major west-dipping normal faults detaching on the AF. The geometry of the deep seismogenic structures is listric (as in the case of Colfiorito) or more complex, because of local reactivation of pre-existing low-angle thrust (e.g. Gubbio) or high-angle strike-slip faults (e.g. Norcia). For all the analysed earthquakes the rupture nucleation is located at the base of the aftershock volumes, near the line of intersection between the SW-dipping normal faults and the east-dipping AF basal detachment. The progressive increase in depth of the earthquake foci from the north–west (e.g. Gubbio, 6–7 km) to the south–east (e.g. Norcia, 11–12 km) appears to be related to the eastward deepening of the basal detachment. These seismotectonic features are relevant for determining the seismogenic potential of the Apennine active faults, which depends not only on the length of the faults, but also on the depth of the detachment zone as well.     

Estimation of seismic stress drop from the peak velocity of ground motion

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Stress Drops of the 1997–1998 Colfiorito,Central Italy Earthquakes: Hints for a Common Behaviour of Normal Faults in the Apennines

Stress drop estimates of moderate-magnitude earthquakes in the Umbria–Marche region, in the northern Apennines, exhibit a large scatter. For the two M _w 5.7 and 6.0 main shocks of 26 September 1997 near Colfiorito, several papers resulted in stress drop estimates of 20 MPa, but values as low as 2–3 MPa were proposed as well. Also for the largest aftershocks (M _w > 4), estimates spread from < 1 MPa up to values ten times larger. We have critically revisited methods and data used in the literature. We have specifically faced the trade-off between source and propagation effects, as we believe that it is responsible for a part of the large scatter. To keep this trade-off under control, we have applied a methodology that combines the best fit of both source spectra after Empirical Green’s Function (EGF) deconvolution and observed ground motion spectra, finding that the results of the two different data sets converge independently at the same solution. We have used ground motions observed in the Colfiorito basin, where an accelerograph and a co-located seismological broad-band station recorded three clusters of earthquakes in a broad magnitude interval (1.7 ≤ M _w ≤ 6.0). We have found that the mainshock–aftershock sequences result in stress drops of 2–5 MPa at M _w ≥ 5.6, with an average tendency to decrease at smaller magnitudes where stress drop variability increases. These findings confirm the source scaling recently assessed through Empirical Green’s Function deconvolution for another well-monitored seismic sequence of normal-faulting earthquakes, which struck the city of L’Aquila in the central Apennines in April 2009. The similar scaling law of the two areas suggests common mechanisms of stress release for the shallow normal faults in the Apennines. The propensity of smaller earthquakes to increase in variability, with a tendency toward smaller stress drops, may reflect an effect of fault strength heterogeneities for smaller size ruptures.     

Characteristics of ambient stress values for micro－earthquake sequences in Western Yunnan Earthquake Prediction Experimental Field

Ｃｈａｒａｃｔｅｒｉｓｔｉｃｓｏｆａｍｂｉｅｎｔｓｔｒｅｓｓｖａｌｕｅｓｆｏｒｍｉｃｒｏ－ｅａｒｔｈｑｕａｋｅｓｅｑｕｅｎｃｅｓｉｎＷｅｓｔｅｒｎＹｕｎｎａｎ　Ｅａｒｔｈｑｕａｋｅ　Ｐｒｅｄｉｃｔｉｏｎ　Ｅｘｐｅｒｉｍｅｎｔａｌ　ＦｉｅｌｄＪｉａ－Ｚ...     

A study on the focal parameters of Shidian M =5.9 earthquake sequences in 2001

On the basis of about 300 earthquake wave forms observed in the Shidian M _S=5.9 sequences on April 12, 2001 recorded in Kunming Digital Seismic Network, the spectra of shear wave have been used to estimate the focal parameters of these earthquake sequences. The results show that within the magnitude range of 1.5–5.3, the seismic moments are 10¹⁰–10¹⁶ N·m, the corner frequencies are 0.2–0.8 Hz, radii of the focal rupture are 200–2 500 m and the stress drops are 0.1×10⁵–20×10⁵Pa. Through the statistical analyses of variation of corner frequency f _c and stress drop Δσ with time, it is discovered that the average corner frequency of the foreshock sequences is obviously lower than that of the aftershock sequences. Contrarily, the average stress drops Δσ of the foreshock sequences are clearly higher than that of the aftershocks. It is considered that these variation characteristics of average corner frequency and stress drops before and after the main shock have index significance to the precursory information before a strong earthquake. The higher stress drops for the foreshock sequences show that the higher shear stresses have been stored in the area of main shock. After the main shock, most of the stresses have been released, so the aftershock sequences show a rupture process of lower stresses. Foundation item: Scientific and Technological Key Project of Yunnan Province (2001NG46)     

从库仑破裂应力和余震分布角度探讨汶川地震和芦山地震的关系

本文以龙门山及周边地区为研究对象,考虑区域地质构造差异、主要活动断裂带、地表附加重力影响,建立能反映地表起伏和岩石圈分层结构的龙门山地区三维粘弹性有限元模型。以地壳水平运动速率观测值为约束条件重建研究区现今构造背景应力场,在此基础上分别模拟了汶川地震和芦山地震的发生机理。通过分析同震库仑破裂应力变化与余震空间分布的关系,探讨了2次地震主震对余震的触发作用以及汶川地震对芦山地震的影响。研究表明,汶川地震和芦山地震的余震大部分由其主震触发,汶川地震对芦山地震的余震有约6.78%的触发作用。汶川地震的同震库仑破裂应力在芦山地震主震位置的增加值约为0.016MPa,如果龙门山断裂带南段库仑破裂应力年累积速率按照0.4×10-3-0.6×10-3MPa·a-1计算,汶川地震使芦山地震提前了约27-40年。计算还表明汶川地震和芦山地震的发生使鲜水河断裂带南段和虎牙断裂的库仑破裂应力增加,这些断裂带在未来发生地震的可能性增加。     

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.     

2015年11月23日青海祁连MS5.2地震震源机制解及发震构造初步探讨

采用CAP（Cut and Paste）方法反演了2015年11月23日青海祁连M_S5.2主震的震源机制解,其最佳双力偶解：节面Ⅰ走向109°、倾角58°、滑动角21°,节面Ⅱ走向8°、倾角72°、滑动角146°,矩震级M_W5.16,矩心震源深度约为9 km。结合震区的活动构造,判定发震断层面为节面Ⅰ,推测托勒山北缘活动断裂中段为此次地震的发震断裂。     

The moment magnitude <Emphasis Type="Italic">M</Emphasis><Subscript>w</Subscript> and the energy magnitude <Emphasis Type="Italic">M</Emphasis><Subscript>e</Subscript>: common roots and differences

Starting from the classical empirical magnitude-energy relationships, in this article, the derivation of the modern scales for moment magnitude M _w and energy magnitude M _e is outlined and critically discussed. The formulas for M _w and M _e calculation are presented in a way that reveals, besides the contributions of the physically defined measurement parameters seismic moment M ₀ and radiated seismic energy E _S, the role of the constants in the classical Gutenberg–Richter magnitude–energy relationship. Further, it is shown that M _w and M _e are linked via the parameter Θ = log(E _S/M ₀), and the formula for M _e can be written as M _e = M _w + (Θ + 4.7)/1.5. This relationship directly links M _e with M _w via their common scaling to classical magnitudes and, at the same time, highlights the reason why M _w and M _e can significantly differ. In fact, Θ is assumed to be constant when calculating M _w. However, variations over three to four orders of magnitude in stress drop Δσ (as well as related variations in rupture velocity V _R and seismic wave radiation efficiency η _R) are responsible for the large variability of actual Θ values of earthquakes. As a result, for the same earthquake, M _e may sometimes differ by more than one magnitude unit from M _w. Such a difference is highly relevant when assessing the actual damage potential associated with a given earthquake, because it expresses rather different static and dynamic source properties. While M _w is most appropriate for estimating the earthquake size (i.e., the product of rupture area times average displacement) and thus the potential tsunami hazard posed by strong and great earthquakes in marine environs, M _e is more suitable than M _w for assessing the potential hazard of damage due to strong ground shaking, i.e., the earthquake strength. Therefore, whenever possible, these two magnitudes should be both independently determined and jointly considered. Usually, only M _w is taken as a unified magnitude in many seismological applications (ShakeMap, seismic hazard studies, etc.) since procedures to calculate it are well developed and accepted to be stable with small uncertainty. For many reasons, procedures for E _S and M _e calculation are affected by a larger uncertainty and are currently not yet available for all global earthquakes. Thus, despite the physical importance of E _S in characterizing the seismic source, the use of M _e has been limited so far to the detriment of quicker and more complete rough estimates of both earthquake size and strength and their causal relationships. Further studies are needed to improve E _S estimations in order to allow M _e to be extensively used as an important complement to M _w in common seismological practice and its applications.     

新疆精河M_W6.3地震产生的静态应力变化研究

利用精河M_W6.3地震有限断层破裂模型,计算了精河地震产生的位移场、应力场、周围主要断层上的静态库仑应力变化以及主震对余震的触发作用。结果表明:(1)精河地震产生的地表隆升最大值约为6.6cm,沉降最大值约为1.8cm;水平位移方向呈现震中南北侧向震中汇聚、震中东西侧向外"流出"的特点。(2)精河地震产生的水平面应力场展布南北侧物质主要受到指向震中的拉张力作用,东西两侧物质主要受到因震中过剩物质东西向排出而导致的东西向挤压力作用。(3)震中西侧距震中约20km的库松木契克山前断裂中段和震中东北部距震中约50km的四棵树-古尔图南断裂西段的库仑应力加载均大于0.01MPa,即2处为地震危险区。(4)在震源深度为8~12km的余震事件中,约有85.5%处于库仑应力加载区,即受到主震的的触发作用;在深度为4~8km的余震事件中,约有87%受到主震的应力触发作用。     

四川长宁MS6.0地震区域构造应力场特征分析

2019年6月17日在四川宜宾市长宁县（28.34°N,104.90°E）发生M_S6.0地震,余震发育。本文利用区域测震台网的地震观测数据基于CAP方法计算了28°~29°N,104°~105°E范围内的14个M_S>3.0以上地震的震源机制解,结合全球矩心矩张量目录和部分前人研究结果中该区域的共27个震源机制解数据,应用MSATSI软件反演了研究区域的应力场。将研究区域按0.1°×0.1°划分成25个应力网格,最终得到9个网格的应力分布结果,大多数应力场方向稳定,根据主震所在应力网格点得到主震的断层类型为主逆冲型。本文研究成果为四川长宁地区的孕震机理、活动构造以及地震趋势判定提供了可靠的参考依据。     

Source parameters of the earthquakes of the Baikal rift system

The dynamic parameters of the earthquake source—the seismic moment, the moment magnitude, the source radius, the stress drop, and the amplitude of displacement—are determined by the amplitude Fourier spectra of the body shear waves (S-waves) for 62 earthquakes of the Baikal rift system with the energy class of K _P = 9.1–15.7. In the calculations I used the classical Brune model. The seismic moment of the earthquakes being investigated changes from 3.65 × 10¹¹ N m to 1.35 × 10¹⁸ N m, and the radii of earthquake sources vary from 390 m to 1.84 km. The values of the drop in stress Δσ grow with an increase in the seismic moment up to 1.7 × 10⁸ Pa. For the group of weak earthquakes (M _w = 1.7–3.3), extremely low values of the drop in stress 10³–10⁴ Pa are observed. The maximum amplitude of displacement in the source amounts to 5.95 m. The empirical equations between the seismic moment and the other dynamic parameters of the source are determined. The regional dependence of the seismic moment and energy class is obtained: log M ₀ ± 0.60 = 1.03K _P + 3.17. The character of the relationship between the seismic moment and the corner frequency indicates that the classical scaling law of the seismic spectrum for the earthquakes in question is not fulfilled. The obtained estimates of the dynamic parameters are in satisfactory agreement with the published data concerning the analogous parameters of the other rift zones, which reflects the general regular patterns of the destruction of the lithosphere and the seismicity in the extension zones of the lithosphere.     

Aftershocks study of the 2006 Silakhur earthquake (Zagros, Iran): seismological evidences for a pull-apart basin along the Main Recent Fault, Doroud segments

The Doroud segment of the Main Recent Fault (“MRF”) is studied by installing a local seismic network of 35 short-period stations for a period of 13 weeks from 21 June 2007 to 19 September 2007 in the Silakhur region of the Zagros continental collision zone, close to the epicenter of the 31 March 2006 Silakhur earthquake (M _w  ∼ 6.1). Our seismic network also covers the Qale-Hatam and Vanaei segments of the MRF and part of the Nahavand fault. We investigate the geometry and mechanism of the causative fault(s) of the 2006 Silakhur earthquake using aftershocks recorded by the dense local network. Most of the aftershocks in this region are located at a depth of 4–11 km, shallower depth than usual for other seismic zones in the Zagros. The distribution of the aftershocks along the course of the river indicates that older faults in the Silakhur region have been activated during the 2006 earthquake and only a few aftershocks have occurred on the Doroud fault. Tensional and compressional components in the northern part of the Doroud fault are interpreted as a pull-apart basin which has been activated by the right-lateral strike-slip movement of the Doroud fault.     

鲜水河断裂带断层间相互作用的触震与缓震效应

探讨了断层间相互作用产生的同震库仑应力改变及对地震的触发与延缓效应，并以鲜水河断裂带不同断裂段时间上连续发生的4次MS6.0以上地震为例，计算和分析了每次地震发生后，在周围其它断裂上产生的同震库仑应力改变及其对后续地震的触发，以及1973年炉霍MS7.6地震和1981年道孚MS6.9地震发生后，在其周围最易破坏失稳的微破裂上产生的同震库仑应力改变及对余震活动的影响．在其它条件保持不变的情况下，将这4次地震的累积触震与缓震效应加以定量考虑，对鲜水河断裂带各断裂段的地震潜势进行了重新计算，并与已有预测结果进行对比分析，检验和评估了鲜水河断裂带断层间相互作用触震与缓震效应的重要性．结果表明:鲜水河断裂带每次地震都发生于受其先前发生的地震影响而产生同震库仑应力增加的断裂段上，不同断层间相互作用的触震和缓震效应导致地震复发概率的改变可高达30.5％以上， 主震后的余震大多发生于同震库仑应力增加较高的微破裂上．      

The 1986 Dharamsala earthquake of Himachal Himalaya – estimates of source parameters,average intrinsic attenuation and site amplification functions

The 26th April 1986 Dharamsala earthquake (m_b 5.5) occurred in the Kangra region of Himachal Himalaya, which lies in the rupture zone of great Kangra earthquake of 1905. This was the first moderate sized earthquake to be recorded at a few sites of the strong ground motion array in the NW Himalaya. The accelerograms of this earthquake have been used to estimate its source parameters, site amplification functions and to estimate the effective shear wave attenuation factor Q_β in the frontal region of Himachal Himalaya. A double couple fault plane solution for the earthquake has been obtained based on the spectra of the transverse component of the accelerograms. The estimated values of the source parameters are seismic moment: 2.1×10²⁴ dyne-cm, static stress drop (Δσ): 36 bars, source radius (r): 2.8 km and moment magnitude (M_w): 5.4. The estimated average values of effective shear wave attenuation factor Q_β for various sites are in the range of 125 to 300 with an overall spatial average of 239. The influence of local site effects on the observed PGA values have been examined on the basis of site amplification functions.     

Stress changes on major faults caused by <Emphasis Type="Italic">M</Emphasis><Subscript>w</Subscript>7.9 Wenchuan earthquake,May 12, 2008

On May 12, 2008, a magnitude 7.9 earthquake ruptured the Longmenshan fault system in Sichuan Province, China, collapsing buildings and killing tens of thousands people. As predicted, aftershocks may last for at least one year, and moreover, large aftershocks are likely to occur. Therefore, it is critical to outline the areas with potential aftershocks before reconstruction and re-settling people as to avoid future disasters. It is demonstrated that the redistribution of stress induced by an earthquake should trigger successive seismic activity. Based on static stress triggering theory, we calculated the coseismic stress changes on major faults induced by the Wenchuan earthquake, with elastic dislocation theory and the multilayered crustal model. We also discuss the stress distribution and its significance for future seismic activity under the impact of the Wenchuan earthquake. It is shown that coulomb failure stress (CFS) increases obviously on the Daofu-Kangding segment of the Xianshuihe Fault, the Maqu and Nanping segment of the Eastern Kunlun Fault, the Qingchuan Fault, southern segment of the Minjiang Fault, Pengxian-Guanxian Fault, Jiangyou-Guangyuan Fault, and Jiangyou-Guanxian Fault. The increased stress raises the probability of earthquake occurrence on these faults. Since these areas are highly populated, earthquake monitoring and early disaster alarm system are needed. CFS increases with a magnitude of 0.03–0.06 MPa on the Qingchuan Fault, which is close to the northern end of the rapture of Wenchuan earthquake. The occurrence of some strong aftershocks, including three events with magnitude higher than 5.0, indicates that the seismic activities have been triggered by the main shock. Aftershocks seem to migrate northwards. Since the CFS change on the Lueyang-Mianxian Fault located on the NEE of the Qingchuan Fault is rather small (±0.01 MPa), the migration of aftershocks might be terminated in the area near Hanzhong City. The CFS change on the western Qinling Fault is around 10 Pa, and the impact of static triggering can be neglected. The increment of CFS on the Pengxian-Guanxian Fault and Beichuan-Yingxiu Fault southwest to the main rupture is 0.005–0.015 MPa, which would facilitate earthquake triggering in these areas. Very few aftershocks in these areas indicate that the accumulated stress has not been released sufficiently. High seismic risk is predicated in these areas due to co-seismic CFS loading. The Wenchuan earthquake released the accumulated CFS on the Fubianhe Fault, the Huya Fault, the Ha’nan-Qingshanwan Fault, and the Diebu-Bailongjiang Fault. The decrement of CFS changes on the Longquanshan Fault east to Chengdu City is about 0.002 MPa. The seismic activity will be depressed by decrement of CFS on these faults. Supported by Knowledge Innovation Program of Chinese Academy of Sciences (Grant No. KZCX-SW-153), National Natural Science Foundation of China (Grant Nos. 40574011 and 40474028)     

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.     

The 20th September 1999 Chi-Chi Earthquake (Taiwan): a case of study for its aftershock seismic sequence

The purpose of this work is to highlight some methodological aspects related to the observation of possible anomalies in the temporal decay of aftershocks temporal series following a mainshock with magnitude M ≥ 7.0. In this paper we present the results for the Taiwan seismic sequence started on 20 September 1999 (M = 7.7) by tuning some seismic parameters that show considerable variations during the aftershock decay process. In here we also present the results obtained using a fractal approach for the seismic sequence. Earthquakes belong to a class of phenomena known as multifractals. In general it is important to define the fractal dimension D, but sometimes is not useful if we are describing a natural phenomenon; so it is necessary to define D ₀ called box-counting dimension and D ₂ called correlation dimension, usually D ₀ ≥ D ₂. In the elaborations of the fractal dimensions, for this sequence, we have obtained values lesser than 1, with a greater tendency of aftershocks to clusterize in time before a large aftershock. This is coherent with the possible existence of seismic anomalies, that could occur before the large aftershock. We also report the results obtained by using the delta/sigma method described firstly in [Caccamo et al., 2005] and later applied to different seismic sequence. The observed temporal series of the aftershocks per day can be considered as a sum of a deterministic and a stochastic contribution. If the decay can be modeled as a non-stationary Poissonian process, the number of aftershocks in a small time interval Δt is the mean value n(t) Δt, with a standard deviation (δ = √n(t)Δt. Investigating both aftershock behavior and a wide spectrum of parameters may find the key to explain better the mechanism of seismicity as a whole.     

Statistical testing of the relation between burst of aftershocks and strong earthquakes

According to geological tectonics and seismic activites this paper devided North China (30°–45°N, 105°–130°E) into four areas. We analyzed the North China earthquake catalogue from 1970 to 1986 (from 1965 to 1986 for Huabei, the North China, plain region) and identified forty-two bursts of aftershock. Seven of them occurred in aftershock regions of strong earthquakes and seventeen of them in the seismic swarm regions. The relation between strong earthquakes with the remaining eighteen bursts of aftershocks has been studied and tested statistically in this paper. The result of statistical testing show that the random probabilityp of coincidence of bursts of aftershock with subsequent strong earthquakes is less than six percent. By Xu’sR scoring method the efficacy of predicting strong earthquake from bursts of aftershock is estimated greater than 39 percent. Following the method proposed in the paper we analyzed the earthquake catalogue of China from 1987 to June, 1988. The results show that there was only one burst of aftershock occurred on Jan. 6, 1988 withM=3.6 in Xiuyan of Northeast China. It implicates that a potential earthquake withM _S⩽5 might occur in one year afterwards in the region of Northeast China. Actually on Feb. 25, 1988 an earthquake withM _S=5.3 occurred in Zhangwu of Northeast China. Another example is Datong-Yanggao shock on October 18, 1989 which is a burst of aftershock. Three hours after an expected shock withM =6.1 took place in the same area. Two examples above have been tested in practical prediction and this shows that bursts of aftershocks are significant in predicting strong earthquakes. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 273–280, 1991. Part of earthquake catalogue is from Jinbiao Chen, Peiyan Chen and Quanlin Li.     

High frequency fall-off of source spectra using Q -free spectra estimation

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