2016年新疆呼图壁6.2级地震前b值异常特征研究

利用2009年以来中国地震台网中心地震编目资料,以2016年新疆呼图壁6.2级地震作为研究对象,采用最大似然法进行b值空间扫描计算,获取震前震中及其邻区地震b值的空间图像。研究结果显示,2016年呼图壁6.2级地震发生在震前显著低b值区域,该异常特征可能反映了地震孕育的应力积累过程,印证了b值的物理意义。     

Joint analysis of b-value and apparent stress before the 2011 MW9.0 Tohoku-Oki,Japan earthquake

Detecting tempo-spatial changes of crust stress associated with major earthquakes has implications for understanding earthquake seismogenic processes. We conducted a joint analysis of b-value and apparent stress in the source region before the March 11, 2011 M_W9.0 Tohoku-Oki, Japan earthquake. Earthquakes that occurred between January 1, 2000 and March 8, 2011 were used to estimate b-values, while source parameters of events with magnitudes of Ms5.0–6.9 between January 1, 1997 and March 8, 2011 were used to calculate the apparent stresses. Our results show that the average b-value decreased steadily from 1.26 in 2003 to 0.99 before the Tohoku-Oki mainshock. This b-value decrease coincided with an increase in the apparent stress from 0.65 MPa to 1.64 MPa. Our results reveal a clear negative correlation between the decrease in b-value and increase in apparent stress, which lasted for approximately eight years prior to the 2011 mainshock. Additionally, spatial pattern results of the relative change in b-value show that the area associated with drastic b-value decreases (25% or greater) was concentrated near the 2011 mainshock epicenter. The joint analysis of b-value and apparent stress provides a promising method for detecting anomalies that could serve as potential indicators of large earthquakes.     

Depth and region dependence of b-value for micro-aftershocks of the May 12th, 2008 Wenchuan earthquake and its tectonic implications

Micro-aftershocks with magnitude range of 1.5?4 around the Wenchuan earthquake epicenter, the southern part of the Longmenshan fault zone, exhibit good frequency-magnitude linear relationships, thus enabling b-value analysis. The average b-value for micro-aftershocks of M1.5?4 from July to December of 2008 in our local study region is about 0.88, similar to the b-value for all aftershocks of M3.0?5.5 from May, 2008 to May, 2009 along the whole Longmenshan fault zone. The similarity between the local and regional b-values possibly indicates that the southern part of the Longmenshan fault zone has similar seismogenic environment to the whole Longmenshan fault zone. Alternatively, it may also imply that b-values derived from all events without consideration of structural variation can not discriminate local-scale tectonic information. The present study shows that the b-value for the Wenchuan earthquake micro-aftershocks varies with different regions. The b-value in southwest of the Yingxiu town is higher than that in the northeast of the Yingxiu town. The high b-value in the southwest part where the Wenchuan earthquake main shock hypocenter located indicates that the current stress around the hypocenter region is much lower than its surrounding area. The b-values are also dependent on depth. At shallow depths of < 5 km, the b-values are very small (~0.4), possibly being related to strong wave attenuation or strong heterogeneity in shallow layers with high content of porosity and fractures. At depths of ~5?11 km, where most aftershocks concentrated, the b-values become as high as ~0.9?1.0. At the depth below ~11 km, the b-values decrease with the depth increasing, being consistent with increasing tectonic homogeneity and increasing stress with depth.

     

河北平原地震带b值时空变化特征

利用华北地区近44年地震资料,在区域地震序列完整性分析的基础上,用最小二乘法进行b值时间扫描计算,用最大似然法进行b值空间扫描计算。时间扫描中的b值为研究区内每个扫描窗口的平均b值,因此其变化幅度不大,基本保持在0.62~1.05之间。研究区b值空间分布范围基本维系在0.5~1.4,低b值区域为昌平—宝坻断裂段和唐山—迁安断裂段,变化范围为0.5~0.7,表明该区域地壳介质正处于相对高应力或闭锁状态,存在未来可能发生中强以上地震的潜在危险。     

The depth variations in the b-value of frequency-magnitude distribution of the earthquakes in the Garm region of Tajikistan

The depth changes in the b-value and density of the number of earthquakes in different magnitude bins (M ≥ 1.8, M ≥ 3.0, M ≥ 3.5) are analyzed using highly accurate seismological observations carried out in 1955–1991 at the Garm prognostic area in Tadjikistan. It is found that the observed b-values are controlled by the variations in the proportion between weak and strong earthquakes. Two horizons with different patterns of the b-value are identified in the Earth’s crust above and below a depth of 15–16 km. The b-value in the upper and lower horizons is close to 0.8 and 1.2, respectively. The lower horizon is marked by almost complete absence of relatively strong earthquakes with M ≥ 3.0. The observed changes in the b-value with increasing depth could probably be due to the increase in the strength of crustal material caused by the growth in temperature and confining pressure in the depth interval from 0 to 15 km. The transitional interval between the upper and lower crustal horizons (～13–18 km), which is characterized by a sharp drop in seismic activity, can probably be associated with the zone of the phase transition of crustal material from an elastic brittle state to a plastic state, as suggested by some authors. Typically, the top of this zone hosts the hypocenters of the strongest earthquakes in a given territory. The correlation is established between the crustal areas with low b-values and the locations of the strongest earthquakes in the region. It is suggested that the three-dimensional mapping of the b-value can be helpful for estimating the location, depth, and maximal magnitude of the probable strong earthquakes in seismically active regions and can be used to assess seismic risks.     

An estimation of the maximum b-value in the Gutenberg-Richter relation

An estimation of the maximum b-value in the Gutenberg-Richter relation is achieved from empirical laws of earthquake occurrence and from dimensional considerations of power law distributions of earthquake size. The deduced maximum b-value, 1.5 (or with error limits 1.30⩽b_max⩽1.64) agrees very well with maximum b-values computed from earthquake catalogues or published elsewhere in the literature. © 1999 Elsevier Science Ltd. All rights reserved.     

Continuous-cyclic variations in the b-value of the earthquake frequency-magnitude distribution

Seismicity of the Earth (M ≥ 4.5) was compiled from NEIC, IRIS and ISC catalogues and used to compute b-value based on various time windows. It is found that continuous cyclic b-variations occur on both long and short time scales, the latter being of much higher value and sometimes in excess of 0.7 of the absolute b-value. These variations occur not only yearly or monthly, but also daily. Before the occurrence of large earthquakes, b-values start increasing with variable gradients that are affected by foreshocks. In some cases, the gradient is reduced to zero or to a negative value a few days before the earthquake occurrence. In general, calculated b-values attain maxima 1 day before large earthquakes and minima soon after their occurrence. Both linear regression and maximum likelihood methods give correlatable, but variable results. It is found that an expanding time window technique from a fixed starting point is more effective in the study of b-variations. The calculated b-variations for the whole Earth, its hemispheres, quadrants and the epicentral regions of some large earthquakes are of both local and regional character, which may indicate that in such cases, the geodynamic processes acting within a certain region have a much regional effect within the Earth. The b-variations have long been known to vary with a number of local and regional factors including tectonic stresses. The results reported here indicate that geotectonic stress remains the most significant factor that controls b-variations. It is found that for earthquakes with M _w ≥ 7, an increase of about 0.20 in the b-value implies a stress increase that will result in an earthquake with a magnitude one unit higher.     

The b-value and fractal dimension of local seismicity around Koyna Dam (India)

Earthquakes began to occur in Koyna region (India) soon after the filling of Koyna Dam in 1962. In the present study, three datasets 1964–1993, 1993–1995, and 1996–1997 are analyzed to study the b-value and fractal dimension. The b-value is calculated using the Gutenberg–Richter relationship and fractal dimension D _corr. using correlation integral method. The estimated b-value and D _corr. of this region before 1993 are found to be in good agreement with previously reported studies. In the subsequent years after 1995, the b-value shows an increase. The estimated b-values of this region are found within the limits of global average. Also, the pattern of spatial clustering of earthquakes show increase in clustering and migration along the three zones called North-East Zone, South-East Zone (SEZ), and Warna Seismic Zone. The earthquake events having depth ≤5 km are largely confined to SEZ. After 1993, the D _corr. shows decrease, implying that earthquake activity gets clustered. This seismic clustering could be helpful for earthquake forecasting.     

川东南长宁地区地震活动及b值演化特征

利用全国统一目录和流动台站目录,研究了四川盆地东南部长宁地区的地震活动特征和b值的空间分布特征.研究结果显示,长宁地区的地震活动在时间上呈现明显的分段特征,地震活动在2015年后明显增强;在空间上,长宁地区的地震活动主要集中在以28.3°N为界限的南、北2个地区,对于这2个区域的b值演化,计算结果显示出不同的分段特征....     

Structural heterogeneity of the media and macroseismic field formation: Racha,April 29, 1991 earthquake

The points with normal, anomalously low, and anomalously high shaking intensities are recognized in the spatial distribution of macroseismic effects from the 1991 Racha earthquake, Greater Caucasus. Distribution of these points in the epicentral area is not random. Comparison between this distribution and the results of local tomography reveals that seismic wave velocities do not increase in the upper layers (from 0 to 3 km) beneath the points with anomalously high intensity, while a sharp increase in velocity is observed in the depth interval from 6 to 9 km. An original method of b-value mapping is suggested. Application of the method demonstrates that anomalously low intensities correlate to high b-values. This likely reflects higher intensity attenuation associated with higher b-value.     

张家口-渤海断裂带西段及中西段b值时空扫描

张家口-渤海断裂带作为华北平原地区重要的活动断裂带,地震活动频繁,是我国地震监测预测重点区域之一。本文选取该断裂带西段及中西段1970-2016年的地震目录,采用最大似然法进行时间扫描,分析显示研究区b值为0.28-1.52,其随时间变化的特点是在大地震发生前降至最低,震后逐渐恢复;研究区空间扫描结果显示,该区b值的平均值为0.93,其中怀安-万全盆地北缘断裂和蓟运河断裂平均b值较低,反映该区域应力水平较高。综合以上结果,本次研究揭示出研究区地震危险性的时间和空间差异,为对研究区地震危险性评价提供基础数据。     

b-value and fractal dimension variations in Iran

Iran sits on a region with a high intrinsic level of seismic activity due to its tectonic setting. Through statistical examination of the earthquakes listed in the catalogue from International Institute of Earthquake Engineering and Seismology (IIEES), this research attempted to calculate some seismicity factors and find correlation between them. A preliminary analysis indicated changes in the b-value of the Gutenberg-Richter relationship over the study region. Thus, the study area was divided into five zones (Alborz, Zagros, Azerbaijan, Central and East) and b-value was computed for each zone. Considering faulting mechanism styles and the b-values in the region, it was found that the lowest b-values belong to the thrust events and strike-slip faulting earthquakes have intermediate values. These findings support previous studies. Furthermore, results of b-value calculation were used for the estimation of accumulated differential stresses (σ₁–σ₂) over each zone. Overall, the b-value for Iran is averagely low which signifies the high stress tectonic regime in this region. Also, by having calculated fractal dimension (D) in each zone, a correlation obtained showing that in Iran region, the b-value correlates to fractal dimension by D = 4.2b–2 relation which does not support Aki's (1981) speculation of D = 3b/c.     

Analysis of the 2012–2013 Torreperogil-Sabiote seismic swarm

This study analyses the temporal clustering, spatial clustering, and statistics of the 2012–2013 Torreperogil-Sabiote (southern Spain) seismic swarm. During the swarm, more than 2200 events were located, mostly at depths of 2–5 km, with magnitude event up to m_bLg 3.9 (M_w 3.7). On the basis of daily activity rate, three main temporal phases are identified and analysed. The analysis combines different seismological relationships to improve our understanding of the physical processes related to the swarm's occurrence. Each temporal phase is characterized by its cumulative seismic moment. Using several different approaches, we estimate a catalog completeness magnitude of m_c≅ 1.5. The maximum likelihood b-value estimates for each swarm phase are 1.11 ± 0.09, 1.04 ± 0.04, and 0.90 ± 0.04, respectively. To test the hypothesis that a b-value decrease is a precursor to a large event, we study temporal variations in b-value using overlapping moving windows. A relationship can be inferred between change in b-value and the regime style of the rupture. b-values are indicators of the stress regime, and influence the size of ruptures. The fractal dimension D₂ is used to perform spatial analysis. Cumulative gamma and beta functions are used to analyse the behaviour of inter-event distances during the earthquake sequence.     

Anomalously high b-values in the South Flank of Kilauea volcano, Hawaii: evidence for the distribution of magma below Kilauea's East rift zone

The pattern of b-value of the frequency–magnitude relation, or mean magnitude, varies little in the Kaoiki-Hilea area of Hawaii, and the b-values are normal, with b=0.8 in the top 10 km and somewhat lower values below that depth. We interpret the Kaoiki-Hilea area as relatively stable, normal Hawaiian crust. In contrast, the b-values beneath Kilauea's South Flank are anomalously high (b=1.3–1.7) at depths between 4 and 8 km, with the highest values near the East Rift zone, but extending 5–8 km away from the rift. Also, the anomalously high b-values vary along strike, parallel to the rift zone. The highest b-values are observed near Hiiaka and Pauahi craters at the bend in the rift, the next highest are near Makaopuhi and also near Puu Kaliu. The mildest anomalies occur adjacent to the central section of the rift. The locations of the three major and two minor b-value anomalies correspond to places where shallow magma reservoirs have been proposed based on analyses of seismicity, geodetic data and differentiated lava chemistry. The existence of the magma reservoirs is also supported by magnetic anomalies, which may be areas of dike concentration, and self-potential anomalies, which are areas of thermal upwelling above a hot source. The simplest explanation of these anomalously high b-values is that they are due to the presence of active magma bodies beneath the East Rift zone at depths down to 8 km. In other volcanoes, anomalously high b-values correlate with volumes adjacent to active magma chambers. This supports a model of a magma body beneath the East Rift zone, which may widen and thin along strike, and which may reach 8 km depth and extend from Kilauea's summit to a distance of at least 40 km down rift. The anomalously high b-values at the center of the South Flank, several kilometers away from the rift, may be explained by unusually high pore pressure throughout the South Flank, or by anomalously strong heterogeneity due to extensive cracking, or by both phenomena. The major b-value anomalies are located SSE of their parent reservoirs, in the direction of motion of the flank, suggesting that magma reservoirs leave an imprint in the mobile flank. We hypothesize that the extensive cracking may have been acquired when the anomalous parts of the South Flank, now several kilometers distant from the rift zone, were generated at the rift zone near persistent reservoirs. Since their generation, these volumes may have moved seaward, away from the rift, but earthquakes occurring in them still use the preexisting complex crack distribution. Along the decollement plane at 10 km depth, the b-values are exceptionally low (b=0.5), suggesting faulting in a more homogeneous medium.     

Gorkha earthquake (MW7.8) and aftershock sequence: A fractal approach

On April 25, 2015, Nepal was struck by the M_W7.8 Gorkha earthquake followed by an intense aftershock sequence. It was one of the most destructive earthquakes in the Himalayan arc, causing more than 8900 fatalities. In this study, we analyzed the dataset (429 events, magnitude of completeness (M_c) ≥ 4.2 local magnitude) of the first 45 days after the Gorkha earthquake to estimate the seismicity parameters b-value, D-value, and p-value. We used the maximum likelihood method to estimate the b-value and Omori-Utsu parameters, whereas the correlation integral method was applied to estimate the fractal dimension (D-value). The analysis was carried out using running and sliding window techniques. The lowest b-value (0.57 ± 0.04) and the highest D-value (1.65 ± 0.02) were computed at the time of the Gorkha earthquake, after which the b-value significantly increased to a maximum of 1.57. It again dropped to 0.93 at the time of the major aftershock on May 12, 2015. The D-value showed an initial quick drop and then decreased in a wavy pattern until the end of the study period, indicating the clustering and scattering of earthquakes in a fault region. The b-value contour map identified the eastern part of the study area as a high stress region (b = ～0.8), implying that the stress shifted to that region. The D-value contour map reveals that the seismogenic structure shifted from linear to planar in the region. The rate of aftershock decay (p = 0.86 ± 0.04) for a short period reflects that the level of stress decreased rapidly. This study helps to understand the level of stress and seismicity pattern of a region, which could be useful for aftershock studies.     

Inferring seismic hazard in Sichuan-Yunnan region of China based on the modern earthquake catalogue (1980-2019)*

Based on the modern earthquake catalogue, the incomplete centroidal voronoi tessellation (ICVT) method was used in this study to estimate the seismic hazard in Sichuan-Yunnan region of China. We calculated spatial distributions of the total seismic hazard and background seismic hazard in this area. The Bayesian delaunay tessellation smoothing method put forward by Ogata was used to calculate the spatial distributions of b-value. The results show that seismic hazards in Sichuan-Yunnan region are high, and areas with relatively high hazard values are distributed along the main faults, while seismic hazards in Sichuan basin are relatively low.     

Study of theQ-value in and around Minle Basin

The digital seimograph network set up by China and France in Zhangye of China had been operated in 1988. The Zhangye network is situated in the middle segment of Hexi corridor and Qilina mountain, which was regarded as a monitoring earthquake area. Using the records of Zhangye digital network theQ-values in and around Minle basin have been measured. The results of this study showed that theQp-values range from 500 to 780, andQs-values range from 230 to 460. TheQ-values of inside of Minle basin are higher than that around the basin. The greater parts of moderate and strong earthquakes occurred along tectonic belts around the Minle basin. Moreover, TheQ-values increased with the depth of penctration of wave ray. The attenuation of S wave is stronger than P wave in shallow layer of crust. Some problems ofQ-value change versus time before and after Sunan eathquake (M _s=5.7) have been also disscussed. These results can be applied to study and to monitor seismic danger of the Minle monitoring area.     

Seismicity and strain accumulation around Karliova Triple Junction (Turkey)

GPS studies in Turkey date back to the early 1990s, but were mostly focused on the seismically active North Anatolian Fault System (NAFS), or on the more populated Western Anatolia. Relatively few studies were made of the seismically less-active East Anatolian Fault System (EAFS), although it has the potential to produce large earthquakes. In this study, we present the results of a combination of geodetic and seismological data around the Karliova Triple Junction (KTJ), which lies at the intersection of the North- and East Anatolian Fault Systems. In particular, the geodetic slip rates obtained through block modeling of GPS velocities were compared with b-values to assess seismicity in the region. Yedisu segment, one of the best-known seismic gaps in Turkey, was specifically analyzed. The relatively low b-values across Yedisu segment verify the accumulation of seismic energy in this segment, and the GPS-derived geodetic slip rates suggest that it has the potential to produce an earthquake of Mw 7.5 across an 80-km rupture zone.Additionally, analysis of earthquake data reveals that the study area has a ductile or rigid–ductile behavior with respect to its surroundings, characterized by varying b-values. Although, seismic events of moderate- to high magnitudes are confined along the major fault zones, there are also low-seismicity zones along the eastern part of the Bitlis Suture Zone and around Yedisu. Since the high seismicity areas within the region may not accumulate sufficient stress for a large earthquake to occur, it is considered that the deformation in such areas occurs in a ductile manner. On the other hand, the areas characterized by low b-values may have the capacity of stress accumulation, which could lead to brittle deformation.     

Tidal Stress/Strain and the b-values of Acoustic Emissions at the Underground Research Laboratory, Canada

The correlation between the b-values of acoustic emissions (AEs) and the phase of the moon was investigated at the Underground Research Laboratory (URL) in Canada. The same data as those used in Iwata (2002) were examined, which showed that the occurrence of AEs is correlated with the phase of the moon. It was expected, therefore, that the b-value of the AEs would also be sensitive to tidal stress/strain fluctuations. We investigated the variation of the b-values as a function of the phase of the moon. Results show that b-values immediately following the times of full/new moon are higher than those at other times. Using AIC (Akaike Information Criterion) and random (Monte Carlo) simulations, it was confirmed that this feature is statistically significant. We also investigated whether or not there was a change in the b-values immediately before the times of full/new moon, but no statistically significant change was observed. The results suggest that the effect of stress/strain fluctuations on AE occurrences at the URL is asymmetric to the times of full/new moon.All authors are members of the Academic Robotics Group. In listing The Academic Robotics Group, the authors are endeavoring to place each of the participant institutions on an equal footing in terms of effort and authorship. M. A. Talamini is serving as presenter.     

Gutenberg-Richter <Emphasis Type="Italic">b</Emphasis>-value maximum likelihood estimation and sample size

The Aki-Utsu maximum likelihood method is widely used for estimation of the Gutenberg-Richter b-value, but not all authors are conscious of the method’s limitations and implicit requirements. The Aki/Utsu method requires a representative estimate of the population mean magnitude; a requirement seldom satisfied in b-value studies, particularly in those that use data from small geographic and/or time windows, such as b-mapping and b-vs-time studies. Monte Carlo simulation methods are used to determine how large a sample is necessary to achieve representativity, particularly for rounded magnitudes. The size of a representative sample weakly depends on the actual b-value. It is shown that, for commonly used precisions, small samples give meaningless estimations of b. Our results give estimates on the probabilities of getting correct estimates of b for a given desired precision for samples of different sizes. We submit that all published studies reporting b-value estimations should include information about the size of the samples used.