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.     

Study of seismicity in the NW Himalaya and adjoining regions using IMS network

The Reviewed Event Bulletin (REB) of the International Data Center (IDC) has been used in order to investigate the seismicity of the Northwest Himalaya and its neighboring region for the time period June 1999 to March 2015 within the geographical coordinates 25–40° N latitude and 65–85° E longitude. We have used a very precisely located earthquake dataset recorded by the International Monitoring System (IMS) Network containing 7,583 events with body wave magnitudes from 2.5 to 6.3. The study area has been subdivided into six regions based on the Flinn-Engdahl (F-E) seismic and geographical regionalization scheme, which was used as the region classifications of the International Data Center catalog. The examined region includes NW India, Pakistan, Nepal, Xizang, Kashmir, and Hindukush. For each region, Magnitudes of completeness (Mc) and Gutenberg-Richter (GR) recurrence parameters (a and b values) have been estimated. The Gutenberg-Richter analysis is preceded by an overview of the seismotectonics of the study area. The obtained Mc values vary from 3.5 to 3.9. The lower value of Mc was found mainly in Xizang region whereas the higher Mc threshold is evident in Pakistan region. However, the b values vary from 1.19 to 1.48. The lowest b value is recorded in Xizang region, which is mostly related to the Main Karakoram Thrust (MKT) fault, whereas the highest b values are recorded in NW India and Kashmir regions, which are mostly related to the Main Frontal Thrust (MFT) fault. The REB for the selected period has been compared to the most renowned bulletin of global seismicity, namely that issued by the National Earthquake Information Center (NEIC) of the United States Geological Survey (USGS). A study of 4,821 events recorded by USGS in the study region indicates that about 36 % of seismic events were missed and the catalog is considered as complete for events with magnitudes ≥4.0. However, both a and b values are obviously higher than those of IMS catalog. The a and b parameters in the Gutenberg-Richter magnitude–frequency relationship have been utilized to forecast the probability of future earthquakes of different magnitudes and returned periods (recurrence intervals).     

Reliability of determining the parameters of Gutenberg-Richter distribution for weak Earthquakes in Garm, Tajikistan

The behavior of the Gutenberg-Richter distribution (the frequency-magnitude relation for the earthquakes) in response to advancing the system of seismic observations and applying various methods for estimation of the Gutenberg-Richter b value is studied. The long highly accurate time series of seismological measurements at the Garm geophysical testing site in Tajikistan is an input for the study. We analyzed the data from the detailed catalogue of earthquakes with magnitude M = 0?C5 that occurred within the test site from 1955 through 1992. It is shown that the changes in the design of measurements both as to the number of observation sites and as to the measurement capacities during the development of the seismic network in 1955?C1966 have strongly affected the content of the catalogue. Our study revealed spatiotemporal changes in the minimum magnitude of completeness, which were associated with advancing the design of observations. We have analyzed the accuracy of the calculation of the b value as a function of the reliability of determining the actual minimal magnitude of completeness (the cutoff magnitude) for the catalogue (Mc), the method of calculation of the b value, and the volume of the spatial sample of the earthquakes used. It is established that the pattern of horizontal distribution of the b value is closely correlated to the tectonic structure of the test area, while its vertical distribution distinctly reflects the crustal waveguide at a depth of 9?C17 km, which has been previously revealed approximately in this depth interval according to other seismological data. This suggests that, in the first approximation, the lateral and vertical variations in the b value observed in our study are caused by the changes in the strength properties of the crustal material, namely, by the increased plasticity of the rocks within a thick sedimentary layer and in the waveguide.     

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.     

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.     

A summary of seismic activities in and around China in 2021

In this article, we review the general characteristics of seismicity in and around China and the overall statistics of earthquake damage in 2021, focusing on several significant events and related scientific topics. Among them, the largest event is the M_S 7.4 Madoi earthquake in Qinghai Province, northwest China. The event marks another M_S ?≥ ?7 earthquake occurring near the boundary of the Bayan Har Block that has ended a remarkable quiescence of the M_S ?≥ ?7 earthquakes within the Chinese mainland. In addition, the M_S 6.4 Yangbi earthquake in Yunnan Province, southwest China draws the most attention because of its abundant foreshocks, which are well recorded by the densely distributed seismic stations in the surrounding regions. Regarding this event, we review several recent publications focusing on the Gutenberg-Richter b-value change and the physical mechanism of foreshocks associated with this sequence. The M_S 6.0 Luxian earthquake in Sichuan Province, southwest China has caused serious damage with a relatively low magnitude, partly because the focal depth of the mainshock is relatively shallow (3.5 ?km). It is another strong earthquake occurring within the southeast Sichuan basin with low historical seismicity yet has increased significantly since 2015, probably due to shale gas development and associated hydraulic fracturing.     

对汶川地震余震序列中最大余震事件的讨论

本文针对2008年5月12日四川汶川M_W7.9地震后的余震目录,采用2004年Shcherbakov和Turcotte提出的最大余震震级推断法,给出了最大余震震级的估计值。结果表明,采用现有的余震数据,在大震级区间可能存在一定数量的余震缺失,这造成了余震数据与Gutenberg-Richter定律曲线在大震级区间的差异,导致推断的最大余震震级与使用目录中的最大余震震级存在明显差异。利用震级与断层长度或地表破裂长度之间的经验关系可以看出,由灌县—江油断裂的破裂尺度得到的震级与推断最大余震震级基本一致。根据前人给出的断层摩擦失稳时间模型和Brune近断层质点运动模型,本文进一步探讨了汶川地震北川—映秀断裂对灌县—江油断裂的触发作用,认为北川—映秀断裂破裂所辐射出的S波可以在短时间内使得灌县—江油断裂发生失稳,进而产生宏观破裂并形成地震,因此由灌县—江油断裂破裂形成的地震可能为汶川地震的最大余震。      

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。     

Characteristics of seismic activity before the MS8.0 Wenchuan earthquake

The characteristics of spatio-temporal seismicity evolution before the Wenchuan earthquake are studied. The results mainly involve in the trend abnormal features and its relation to the Wenchuan earthquake. The western Chinese mainland and its adjacent area has been in the seismically active period since 2001, while the seismic activity shows the obvious quiescence of M≥?7.0, M≥?6.0 and M?≥5.0 earthquakes in Chinese mainland. A quiescence area with M?≥7.0 has been formed in the middle of the North-South seismic zone since 1988, and the Wenchuan earthquake occurred just within this area. There are a background seismicity gap of M?≥5.0 earthquakes and a seismogenic gap of M_L?≥4.0 earthquakes in the area of Longmenshan fault zone and its vicinity prior to the Wenchuan earthquake. The seismic activity obviously strengthened and a doughnut-shape pattern of M?≥4.6 earthquakes is formed in the middle and southern part of the North-South seismic zone after the 2003 Dayao, Yunnan, earthquake. Sichuan and its vicinity in the middle of the doughnut-shape pattern show abnormal quiescence. At the same time, the seismicity of earthquake swarms is significant and shows heterogeneity in the temporal and spatial process. A swarm gap appears in the M4.6 seismically quiet area, and the Wenchuan earthquake occurred just on the margin of the gap. In addition, in the short term before the Wenchuan earthquake, the quiescence of earthquake with M_L≥?4.0 appears in Qinghai-Tibet block and a seismic belt of M_L?≥3.0 earthquakes, with NW striking and oblique with Longmenshan fault zone, is formed.     

Earthquake Source Zones in Northeast India: Seismic Tomography, Fractal Dimension and b Value Mapping

We have imaged earthquake source zones beneath the northeast India region by seismic tomography, fractal dimension and b value mapping. 3D P-wave velocity (Vp) structure is imaged by the Local Earthquake Tomography (LET) method. High precision P-wave (3,494) and S-wave (3,064) travel times of 980 selected earthquakes, m _d ≥ 2.5, are used. The events were recorded by 77 temporary/permanent seismic stations in the region during 1993–1999. By the LET method simultaneous inversion is made for precise location of the events as well as for 3D seismic imaging of the velocity structure. Fractal dimension and seismic b value has been estimated using the 980 LET relocated epicenters. A prominent northwest–southeast low Vp structure is imaged between the Shillong Plateau and Mikir hills; that reflects the Kopili fault. At the fault end, a high-Vp structure is imaged at a depth of 40 km; this is inferred to be the source zone for high seismic activity along this fault. A similar high Vp seismic source zone is imaged beneath the Shillong Plateau at 30 km depth. Both of the source zones have high fractal dimension, from 1.80 to 1.90, indicating that most of the earthquake associated fractures are approaching a 2D space. The spatial fractal dimension variation map has revealed the seismogenic structures and the crustal heterogeneities in the region. The seismic b value in northeast India is found to vary from 0.6 to 1.0. Higher b value contours are obtained along the Kopili fault (~1.0), and in the Shillong Plateau (~0.9) The correlation coefficient between the fractal dimension and b value is found to be 0.79, indicating that the correlation is positive and significant. To the south of Shillong Plateau, a low Vp structure is interpreted as thick (~20 km) sediments in the Bengal basin, with almost no seismic activity in the basin.     

Seismicity and eruptive activity at Fuego Volcano, Guatemala: February 1975 –January 1977

We examine seismic and eruptive activity at Fuego Volcano (14°29′N, 90° 53′W), a 3800-m-high stratovolcano located in the active volcanic arc of Guatemala. Eruptions at Fuego are typically short-lived vulcanian eruptions producing ash falls and ash flows of high-alumina basalt. From February 1975 to December 1976, five weak ash eruptions occurred, accompanied by small earthquake swarms. Between 0 and 140 (average ≈ 10) A-type or high-frequency seismic events per day with M > 0.5 were recorded during this period. Estimated thermal energies for each eruption are greater by a factor of 10⁶ than cumulative seismic energies, a larger ratio than that reported for other volcanoes.Over 4000 A-type events were recorded January 3–7, 1977 (cumulative seismic energy ≈ 10⁹ joules), yet no eruption occurred. Five 2-hour-long pulses of intense seismicity separated by 6-hour intervals of quiescence accounted for the majority of events. Maximum likelihood estimates of b-values range from 0.7 ± 0.2 to 2.1 ± 0.4 with systematically lower values corresponding to the five intense pulses. The low values suggest higher stress conditions.During the 1977 swarm, a tiltmeter located 6 km southeast of Fuego recorded a 14 ± 3 microradian tilt event (down to SW). This value is too large to represent a simple change in the elastic strain field due to the earthquake swarm. We speculate that the earthquake swarm and tilt are indicative of subsurface magma movement.     

Is clustered seismicity an indicator of regional stress? Insights from earthquake sequences in Yongning-Luguhu faulted basin,Southwest China

Using hypocenter relocation, moment tensor inversion, stress field inversion, and fault slip tendency analysis, this study systematically investigated three M5.5–5.8 earthquake sequences that occurred after 2000 in the Yongning-Luguhu faulted basin in the middle of the Lijiang-Xiaojinhe fault zone within the Sichuan-Yunnan block, Southwest China. Our results show that since the 2008 Wenchuan Earthquake, the tectonic stress pattern in this area may have changed and that b-values estimated for the earthquake sequences show evidence of an increasing trend in stress in the study area. Seismicity in the small-scale faulted basin adjacent to the large-scale fault zone is a possible indicator of regional stress. We also note that the aftershocks of the M5.7 earthquake sequence in 2012 and the M5.5 earthquake sequence in 2022 show relatively clear fluid diffusion-triggering characteristics. Overpressure of deep fluids is still the main factor driving seismic activity in the region, and we propose that the background tectonic stresses have not yet reached critical levels.     

A sequence of seismic activity in the Kanto area precursory to the 1923 Kanto earthquake

The Kanto earthquake (M=7.9) that occurred along the Sagami Trough in the Sagami Bay on 1 September 1923 was one of the most disastrous earthquakes in Japanese history. The Kanto area includes Metropolitan Tokyo and Yokohama which are densely populated, and hence it has been a matter of great concern, from the viewpoints of earthquake prediction and disaster prevention, whether or not the 1923 Kanto earthquake was preceded by precursory seismicity. A study using the most complete lists of earthquakes catalogued recently by Utsu and the Japan Meteorological Agency reveals that seismic activity in the Kanto area was appreciably higher before and after the Kanto earthquake, and that the Kanto earthquake was preceded by a sequence of anomalous seismic activity, quiescence, and foreshocks. Such higher activity before and after the Kanto earthquake is contrasted with low seismicity during the recent 30-year period. A model is proposed to explain the precursory seismic activity, subsequent quiescence, and foreshocks for the Kanto earthquake. In the model, the transition from precursory seismic activity to quiescence is ascribed to time-dependent fracture due to stress-aided corrosion. Foreshocks are related to an acceleration of premonitory slip shortly before the mainshock slip.     

Preliminary report of the September 5, 2022 MS 6.8 Luding earthquake,Sichuan, China

The 2022 M_S 6.8 Luding earthquake is the strongest earthquake in Sichuan Province, Western China, since the 2017 M_S 7.0 Jiuzhaigou earthquake. It occurred on the Moxi fault in the southeastern segment of the Xianshuihe fault, a tectonically active and mountainous region with severe secondary earthquake disasters. To better understand the seismogenic mechanism and provide scientific support for future hazard mitigation, we summarize the preliminary results of the Luding earthquake, including seismotectonic background, seismicity and mainshock source characteristics and aftershock properties, and direct and secondary damage associated with the mainshock. The peak ground displacements in the NS and EW directions observed by the nearest GNSS station SCCM are ～35 mm and ～55 mm, respectively, resulting in the maximum coseismic dislocation of 20 mm along the NWW direction, which is consistent with the sinistral slip on the Xianshuihe fault. Back-projection of teleseismic P waves suggest that the mainshock rupture propagated toward south-southeast. The seismic intensity of the mainshock estimated from the back-projection results indicates a Mercalli scale of VIII or above near the ruptured area, consistent with the results from instrumental measurements and field surveys. Numerous aftershocks were reported, with the largest being M_S 4.5. Aftershock locations (up to September 18, 2022) exhibit 3 clusters spanning an area of 100 km long and 30 km wide. The magnitude and rate of aftershocks decreased as expected, and the depths became shallower with time. The mainshock and two aftershocks show left-lateral strike-slip focal mechanisms. For the aftershock sequence, the b-value from the Gutenberg-Richter frequency-magnitude relationship, h-value, and p-value for Omori’s law for aftershock decay are 0.81, 1.4, and 1.21, respectively, indicating that this is a typical mainshock-aftershock sequence. The low b-value implies high background stress in the hypocenter region. Analysis from remote sensing satellite images and UAV data shows that the distribution of earthquake-triggered landslides was consistent with the aftershock area. Numerous small-size landslides with limited volumes were revealed, which damaged or buried the roads and severely hindered the rescue process.     

Analysis on the Characteristics of the Kalpin MS5.3 Earthquake Sequence of December 1, 2013 and Anomalies before the Earthquake

This paper introduces the basic parameters, focal mechanism solutions and earthquake sequence characteristics of the Kalpin M_S5.3 earthquake sequence of December 1, 2013, and analyzed seismic activity before the earthquake, the adjacent tectonic features and the precursory anomaly at fixed points within a range of 200km. Research indicates:(1) The earthquake occurred on Kalpin fault, the source rupture type is thrust faulting with sinistral strike-slip component. (2) The earthquake sequence is mainshock-aftershock type, with the aftershock distribution attenuating quickly and trending NE. (3) Abnormal seismic activity before the earthquake was characterized by seismically nesting quiescence of M_S2.0-4.0 earthquakes, seismic quiescence of M_S4.0 earthquakes and seismic belts of M_S3.0 earthquakes in the Kalpin block, abnormal enhancement zone of moderate earthquakes on Puchang fault and seismological parameters. (4) Anomalies of precursory observation data at fixed stations are mainly characterized by mutation. Apart from the borehole tiltmeter in Halajun, the spatial distribution of other abnormal precursors showed a phenomenon of migration from the near field to far field and from the epicenter to the peripheries.     

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.

     

Precursory Seismic Quiescence before the 1994 Kurile Earthquake (Mw = 8.3) Revealed by Three Independent Seismic Catalogs

—We have found that the M _w = 8.3 Kurile earthquake on October 4, 1994 followed an outstanding seismic quiescence starting 5–6 years before the mainshock near the ruptured area. We have analyzed three independent seismic catalogs Institute of Seismology and Volcanology, Hokkaido University (ISV), Japan Meteorological Agency (JMA) and International Seismology Center (ISC). In spite of selecting different magnitude bands and time windows all three catalogs presented the common feature of the seismic quiescence. This fact strongly suggests that the seismic quiescence should not be a man-made change but actually occurred. Moreover we have confirmed that the seismic quiescence was the most significant and the earthquake was the largest in the past twenty-five years in this region. Therefore we confidently interpret this seismic quiescence as an indication of a preparation process for the M _w = 8.3 Kurile earthquake.     

晋冀蒙交界地区地震活动性特征分析

采用较为完整的地震资料,分析晋冀蒙交界地区地震活动性特征,结果表明,公元1929年以后,研究区的6.0级以上地震主要分布在河套地震带和张家口-渤海地震带上,在北纬40°线及附近区域形成优势分布区域,1970年以来的中等以上地震主要分布在内蒙古和林格尔-山西大同-河北张北形成的三角形区域内。研究区2005年9月以来中等地震持续1 304天的平静现象被2009年3月山西原平4.2级地震打破,岱海断陷1997年11月以来至今中等地震持续平静,统计结果显示,研究区和岱海断陷中等地震显著平静对研究区特别是河套地震带中强地震具有明显的指示意义,为显著的震兆标志,多种迹象表明,晋冀蒙交界三角形区域和呼和浩特-包头断陷为有利发震区域,发震强度或为6.0级左右。     

Seismic Inventory Simulation and Conformance Test Based on the Monte Carlo Method——A Case Study of the Fenhe-Weihe Belt

Artificial earthquake catalogue simulation is one of the ways to effectively improve the incompleteness of the existing earthquake catalogue,the scarcity of large earthquake records and the improvement of seismological research.Based on the Poisson distribution model of seismic activity and the Gutenberg-Richter magnitude-frequency relationship, the Monte Carlo method which can describe the characteristics of the stochastic nature and the physical experiment process is used.This paper simulates the future seismic catalogues of the Fenhe-Weihe seismic belt of different durations and conducts statistical tests on them. The analysis shows that the simulation catalogue meets the set seismic activity parameters and meets the Poisson distribution hypothesis,which can obtain a better simulated earthquake catalogues that meets the seismic activity characteristics.According to the simulated earthquake catalogues,future earthquake trends in this region are analyzed to provide reference for seismic hazard analysis.     

基于蒙特卡洛方法的地震目录模拟及相符性检验——以汾渭地震带为例

人工地震目录模拟是改进现有地震目录不完备性、弥补大地震记录稀缺,以及完善地震学相关研究的有效途径之一。本文基于地震活动的泊松分布模型、古登堡-里克特震级-频度关系,利用能较逼真描述具有随机性质事物特点及物理实验过程的蒙特卡洛方法,模拟汾渭地震带未来30、50、100年等不同时长的地震目录,并对其进行统计检验。分析表明,模拟地震目录符合设定的地震活动性参数和泊松分布假设特征。依据模拟地震目录,对未来该区域地震趋势进行了分析,以期为地震危险性分析提供参考。