多种震级及其巧妙之处

震级是对地震大小的量度。在地震活动性分析等实际的工作中,人们通常认为 “一个地震只有一个震级” 。很多人会有这样的疑惑:为何使用多种震级？震级为何不能统一？在地震监测工作中,为何震级之间不能相互转换？为何同一地震的不同震级会有差别？多种震级该如何使用？本文讲述了震级的测定原理、震级的巧妙之处,并对上述等11个问题进行讨论,以便于科研人员和管理人员在实际工作中,准确地测定震级,正确地使用震级。      

如何正确使用地震的震级

震级是表示地震自身大小的一个量,是地震的基本参数之一,正确使用震级,对于科学研究、地震预报、地震应急等工作至关重要.本文从震级的发展过程、震级的单色性、震级统一的不可行性、震级转换的危害性、震级优选的科学性等方面,论述了震级的使用方法,以便技术人员和管理人员在实际工作中能够正确地使用地震的震级.     

传统与现代震级标度

首先阐述并比较传统震级(M_L、m_b、m_B、M_S及M_JMA)所用数据、计算方法及适用范围,系统梳理两种现代震级标度M_e和M_W的理论基础及计算方法。然后根据使用数据及适用场景不同,介绍M_WW、M_WC、M_Wb、M_wp及M_dt等矩震级测定方法;同时,随着越来越丰富的GPS观测数据,基于高频GPS观测也越来越多地被用于地震震级快速测定工作。最后对这些震级测定方法的时效性进行对比分析,讨论不同震级标度的适用场景及其稳定性。     

云南地震台网震级对比研究

利用云南地震台网记录的云南及其周边地区2000～2017年的宽频带数字地震资料,按照新的震级国家标准《地震震级的规定》(GB17740-2017)的测定方法,使用同一套软件,对地方性震级M_L、面波震级M_S、宽频带面波震级M_S(BB)、短周期体波震级m_b、宽频带体波震级m_B(BB)和矩震级M_W重新进行人工测量。并分别用一般线性回归和正交回归方法,对不同震级之间的关系进行对比,给出它们之间的经验关系式。研究结果表明:(1)当M4.5时,各种震级之间相差不大,使用地方性震级M_L可以较好地表示地震的大小,也能够更加充分地反映区域特性;当4.5≤M8.0时,宽频带面波震级M_S(BB)和矩震级MW均能较好地表示地震的大小,但矩震级M_W的测定需要一定时间,因此在速报工作和大震应急中,可以使用M_S(BB)表示地震的大小; M_S(BB)的测定方法与国际接轨,消除0.2的震级偏差。(2)对于面波震级MS和宽频带面波震级M_S(BB),由于面波测量的位置、计算公式和量归函数不同,M_S比M_S(BB)系统偏高0.2左右;短周期体波震级mb较宽频带体波震级m_B(BB)整体偏小0.2左右,主要区别在于仿真模式。(3)宽频带面波震级M_S(BB)和宽频带体波震级m_B(BB)均在垂直向原始宽频带记录上直接测定,取消波形仿真环节,另外,相比测定m_B(BB)震中距要求大于5度,许多台站被限制,M_S(BB)更利于区域台网测定。(4)当3.5≤ML≤6.5时,M_L较M_W整体偏大; M_S≥3.5时,M_S也较M_W整体偏大,且均随着震级增加,偏差值呈上升趋势。(5)当M≥8.0时,面波震级出现饱和现象,使用矩震级M_W表示M≥8.0地震的大小。     

地震的震级

简要介绍了地方性震级Mi、体波震级mb、面波震级Ms和矩震级Mw的定义及其测定方法,分析了它们的优点和缺点,并对震级饱和效应及其产生的原因作了介绍和解释。文章指出,矩震级是一个表征地震绝对大小的量,它与地震震源的物理过程直接关联,不会饱和;与传统上使用的其他震级标度相比,矩震级具有明显的优点,是当今国际地震学界推荐优先使用的震级标度。     

如何正确使用地震的震级

震级是表示地震自身大小的一个量，是地震的基本参数之一，正确使用震级，对于科学研究、地震预报、地震应急等工作至关重要。本文从震级的发展过程、震级的单色性、震级统一的不可行性、震级转换的危害性、震级优选的科学性等方面，论述了震级的使用方法，以便技术人员和管理人员在实际工作中能够正确地使用地震的震级。     

不同震级标度转换关系研究概述

简述了中国学者近年来在地震震级转换关系方面的研究成果与进展,比较了中国台网测定的各种震级与国外主要地震机构测定的地震标度之间的差别与各震级间的转换关系。在前人研究的基础上,统一给出了矩震级与各种震级标度之间的对应关系：在国外震级标度中,6.5≤M≤7.6时,mB≈MW;5.2≤M≤8.0时,mS≈MW;4.2≤M≤7.0时,mL≈MW;4.0≤M≤6.0时,mb≈MW。在上述震级区间,MW与mB、mb、ML和MS的偏差≤0.2。经过推导,我国常规震级标度与矩震级的之间的对应关系为：4.5≤M≤5.8时,mb≈MW;4.5≤M≤7.5时,mB≈MW;4.5≤M≤7.0时,ML≈MW;MS≈MW;MS7=MW。在上述震级区间,MW与mB、MS的偏差≤0.2,与MS7偏差为0;与ML偏差为0.22。上述结论可为地震预测及相关领域的研究提供参考。     

从震级偏差看震级问题

震级为地震的基本参数之一,由于测定震级普遍存在偏差等问题,测定震级的稳定性问题一直令人关注。该文就震级偏差的起因进行了讨论,认为产生偏差的主要原因是震级本身问题和地球内部结构的各向异性导致地震波能量记录失真。     

中国地震台网震级的对比

采用线性回归和正交回归方法,利用中国地震台网1983-2004年的观测资料,对中国地震局地球物理研究所测定的地方性震级ML、面波震级MS与MS7、长周期体波震级mB、短周期体波震级mb进行对比,给出了它们之间的经验关系式.研究结果表明:①由于不同的震级标度反映了地震波在不同周期范围内辐射地震波能量的大小,因此对于不同大小的地震,使用不同的震级标度更能客观地描述地震的大小.当震中距小于1 000 km时,用地方性震级ML可以较好地测定近震的震级.当地震的震级M＜4.5时,各种震级标度之间相差不大.当4.5＜M＜6.0时,mB＞MS,MS标度低估了较小地震的震级,因此用mB可以较好地测定较小地震的震级.当M＞6.0时,MS＞mB＞mb,mB与mb标度均低估了较大地震的震级,用MS可以较好地测定出较大地震(6.0＜M＜8.5)的震级.当M＞8.5时,MS出现饱和现象,不能正确地反映大地震的大小;②在我国境内,当震中距＜1 000 km时,ML与区域面波震级MS基本一致,在实际应用中无需对它们进行震级的换算;③虽然MS与MS7同为面波震级,但由于所使用的仪器和计算公式不同,MS比MS7系统地偏高0.2～0.3级;④对于长周期体波震级mB和短周期体波震级mb,虽然使用的计算公式相同,但由于使用的地震波周期不同,对于mB=4.0左右的地震,mB与mb几乎相等,而对于mB≥4.5的地震,则mB＞mb.     

Low CodaQcin the Epicentral Region of the 2001 Bhuj Earthquake ofMw7.7

On 26 January, 2001 (03:46:55,UT) a devastating intraplate earthquake of M_w 7.7 occurred in a region about 5 km NW of Bhachau, Gujarat (23.42°N, 70.23°E). The epicentral distribution of aftershocks defines a marked concentration along an E-W trending and southerly dipping (45°) zone covering an area of (60 × 40) km². The presence of high seismicity including two earthquakes of magnitudes exceeding 7.7 in the 200 years is presumed to have caused a higher level of shallow crustal heterogeneity in the Kutch area; a site lying in the seismic zone V (zone of the highest seismicity for potentially M8 earthquakes) on the seismic zoning map of India. Attenuation property of the medium around the epicentral area of the Bhuj earthquake covering a circular area of 61,500 km² with a radius of 140 km is studied by estimating the coda-Q_c from 200 local earthquakes of magnitudes varying from 3.0–4.6. The estimated Q₀ values at locations in the aftershock zone (high seismicity) are found to be low in comparison to areas at a distance from it. This can be attributed to the fact that seismic waves are highly scattered for paths through the seismically active and fractured zone but they are well behaved outside the aftershock zone. Distribution of Q₀ values suggests that the local variation in Q₀ values is probably controlled by local geology. The estimated Q₀ values at different stations suggest a low value of Q=(102 ± 0.80)^*f^{(0.98 ± 0.02)} indicating an attenuative crust beneath the entire region. The frequency-dependent relation indicates a relatively low Q_c at lower frequencies (1–3 Hz) that can be attributed to the loss of energy due to scattering attenuation associated with heterogeneities and/or intrinsic attenuation due to fluid movement in the fault zone and fluid-filled cracks. The large Q_c at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneity is expected. Based on the attenuation curve estimated for Q₀=102, the ground acceleration at 240 km distance is 13% of 1 g i.e., 0.13 g agreeing well with the ground acceleration recorded by an accelerograph at Ahmedabad (0.11 g). Hence, it is inferred that the Q₀ value obtained from this study seems to be apt for prediction of ground motion for the region.     

汶川地震前后波速比变化特征的再研究

收集整理了四川省地震台网1990—2012年产出的地震直达波震相数据, 利用单台多震和达法对2008年汶川M_S8.0地震前后四川地区的波速比变化特征进行了分析研究(共筛选出13个数据量较为丰富、 连续性较好的台站). 结果显示: 其中位于龙门山断裂中北段西侧的4个台站, 震前出现长达7年左右的中长期波速比低值异常; 其它9个台站的波速比震前变化基本稳定. 震前波速比出现异常的4个台站的分布与汶川M_S8.0地震孕震区范围大体一致, 从而为研究汶川M_S8.0地震前是否存在地壳介质特性的时间变化过程, 提供了有力的判定依据.      

Vp and Vp/Vs structures in the crust and upper mantle of the Taiwan region, China

A tomographic study of the V _p and V _p/V _s structures in the crust and upper mantle beneath the Taiwan region of China is conducted by simultaneous inversion of P and S arrival times. Compared with the previous tomographic results, the spherical finite difference technique is suitable for the strong heterogeneous velocity structure, and may improve the accuracy in the travel time and three-dimensional ray tracing calculations. The V _p and V _p/V _s structures derived from joint inversion and the relocated earthquakes can provide better constraints for analyzing the lateral heterogeneity and deep tectonic characters in the crust and upper mantle. Our tomographic results reveal significant relations between the seismic wavespeed structure and the tectonic characters. In the shallow depth, sedimentary basins and orogen show distinct wavespeed anomalies, with low V _p, high V _p/V _s in basins and high V _p, low V _p/V _s in orogen. As the suture zone of Eurasian Plate and Philippine Sea Plate, Longitudinal Valley is characterized by a significant high V _p/V _s anomaly extending to the middle-lower crust and upper mantle, which reflects the impact of rock cracking, partial melting, and the presence of fluids. In the northeast Taiwan, the V _p, V _p/V _s anomalies and relocated earthquakes depict the subducting Philippine Sea Plate under the Eurasian Plate. The high V _p of oceanic plate and the low V _p, high V _p/V _s atop the subducted oceanic plate extend to 80 km depth. Along the east-west profiles, the thickness of crust reaches 60 km at the east of Central Range with eastward dipping trend, which reveals the eastward subduction of the thickened and deformed crust of the Eurasian continental plate. Supported by Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KZCX3-SW-234-2), National Basic Research Program of China (Grant No. 2007CB411701), National High Technology Research and Development Program of China (Grant No. 2006AA09A101-0201) and National Natural Science Foundation of China (Grant Nos. 40804016, 40704013)     

Vp/Vs ratios in the Yellowstone National Park region, Wyoming

In this paper we study the variation of V_p/V_s and Poisson's ratio (δ) in the Yellowstone National Park region, using earthquakes which were well recorded by a local seismic network. We find that the average V_p/V_s value within the geothermally active Yellowstone caldera is about 7% lower than in the area outside the caldera. Within the caldera itself there may be a further 2–7% reduction of V_p/V_s in the hydrothermally active Norris Geyser Basin, the Upper and Lower Geyser Basins, and the Yellowstone Lake and Mud Volcano regions. After considering various possible causes for V_p/V_s changes, such as geologic and structural differences, thermal effects, partial melting, and hydrothermal activity, we conclude that the most plausible explanation for the observed V_p/V_s reduction is the presence of hot-water at temperatures and pore-pressures near the water steam transition in the caldera geothermal reservoirs.     

Earthquake prediction on the basis of Vp/Vs variations — a case history

On November 7, 1976, an earthquake of the strike-slip fault type, and of magnitude 6.7, occurred at two mountainous localities, Yanyuan and Ninglang, in the border region of the Szechuan and Yunnan Provinces, China. One year before the earthquake, a prediction was made as to the magnitude and location of the impending earthquake by the present authors, on the basis of the results of a general survey of an area of about 20,000 km² for V_p/V_s ratio variations. The prediction of time of occurrence was made afterwards by the combined analysis of results of observations of some other precursory phenomena. The actual occurrence of the event was generally considered as being in agreement with the prediction.The present study may be taken as a new example for the detection of V_p/V_s ratio variations prior to an earthquake of the strike-slip fault type. By considering the difference between the shear rupture and stick-slip motion and the anisotropy induced by dilatancy, a preliminary discussion is made concerning the related results.     

兴蒙造山带诺敏河火山群地壳厚度与波速比研究

利用布设于兴蒙造山带诺敏河火山群地区的宽频带流动地震台站资料，基于接收函数方法，获取了该地区的地壳厚度与波速比值.研究结果显示，该地区的地壳厚度介于32~38 km，莫霍面深度在空间上分布特征与五大连池为中心的火山带分布具有较好的一致性：沿着火山带延展方向地壳较薄.该地区的波速比介于1.74~1.84，波速比在空间上与地壳厚度变化具有一致性：高波速比主要集中于靠近五大连池火山带地区，向诺敏河火山和小古里河火山延展.研究认为：诺敏河火山与五大连池火山带可能具有相同的岩浆来源，可能与富钾岩石圈地幔拆沉作用造成的地幔热物质上涌有关.研究区地壳厚度与波速比呈现负相关关系，暗示该地区可能发生过岩浆底侵作用.     

Petrogenetic grid for ultrahigh-pressure metamorphism in the model system CaO-MgO-SiO2-CO2-H2O

Abstract Petrogenetic grids for ultrahigh-pressure (UHP) metamorphism were calculated at different X_co2 conditions in the model system CaO-MgO-SiO₂-CO₂-H₂O involving coesite (Co), diopside (Di), dolomite (Do), enstatite (En), forsterite (Fo), magnesite (Ms), quartz (Qz), talc (Tc), tremolite (Tr) using a published internally consistent thermodynamic data set. Two P-T grids at X_co2= 0.01 and 0.5 are described. In the calculated P-T grid at X_co2= 0.01, four out of 10 stable invariant points, Co-En-Ms-Tc, Co-Di-En-Tc-Tr, Co-Di-Ms-Tc-Tr and Di-En-Ms-Tc-Tr lie within the stability field of coesite. If the fluid phase has X_co2= 0.5, no invariant point is stable under UHP conditions. Some magnesite-bearing assemblages are stabilized by the following three reactions: Di + Ms = Do + Fo + CO₂, Ms + Tr = Do + Fo + CO₂+ H₂O and Ms + Tc = Fo+ CO₂+ H₂O at X_co2= 0.01 and by reaction Ms + Tc = Fo + CO₂+ H₂O together with these three at X_co2= 0.5. Ten possible UHP assemblages for mafic and ultramafic compositions at very low X_co2 conditions include the following: Co-Do-Ms, Co-Di-Ms, Co-Di-Tc, Di-Ms-Tc, Di-En-Tc-, Di-En-Ms, Co-Di-En, Di-En-Fo, Di-Fo-Ms, Di-Do-Fo. Among them, talc-bearing assemblages are restricted to X_co2 < 0.02 and their high-P limit is 31.7 kb (749°C) at X_co2= 0.01. Dolomite-magnesite-silica assemblages have large P-T stability fields even if X_co2 is as low as 0.1, and could occur in cold subduction zones with very low geothermal gradients. Reported UHP coesite-dolomite assemblage is restricted only to a calc-silicate rock interlayered with marble where X_co2 is relatively higher; no such assemblage appears for mafic and ultramafic rocks with low X_co2 evidenced by the occurrence of diopside (or omphacite) at the expense of dolomite + coesite. The effect of X_co2 on the stability of coesite-dolomite-magnesite, diopside-enstatite-magnesite, diopside-talc assemblages is examined and the occurrence of coesite-dolomite, magnesite-bearing and talc-bearing assemblages in the Dabie UHP rocks are interpreted by employing the calculated P-T grids.     

Tectonothermal evolution of the Lesser Himalaya, Nepal: Constraints from 40Ar/39Ar ages from the Kathmandu Nappe

Abstract The Himalaya is a fold-and-thrust wedge formed along the northern margin of the Indian continent, and consists of three thrust-bounded lithotectonic units; the Sub-Himalaya, the Lesser Himalaya, and the Higher Himalaya with the overlying Tethys Himalaya from south to north, respectively. The orogen-scale, intracrustal thrusts which bound the above lithotectonic units are splays off an underlying subhorizontal dkcollement, and show a southward propagating piggy-back sequence with an out-of-sequence thrust. Among these thrusts, the Main Central Thrust zone (MCT zone) has played a major role in Himalayan tectonics. The MCT zone represents a shear zone which has accommodated southward thrusting of the Higher Himalayan crystalline thrust sheet over the Lesser Himalayan sequence for ～140 km. The Kathmandu Nappe in central Nepal has been transported over the Lesser Himalayan metasediments along the MCT zone, and is locally separated from the Higher Himalayan thrust sheet in the north by an out-of-sequence thrust. ⁴⁰Ar/³⁹Ar ages have been determined for one whole-rock phyllite and six muscovite concentrates from metasedimenta-ry rocks and variably deformed granites in the Kathmandu Nappe. These ages range from 44 Ma to 14 Ma, and suggest a record of both Eo-Himalayan (Eocene) and Neo-Himalayan (Miocene) tectonothermal events in the Tertiary Himalayan orogeny. The Miocene event was associated with translation along the MCT zone. No tectonothermal event of the Late Miocene to Early Pliocene ages have been reported near the MCT zone in southern Lesser Himalayan crystalline nappe or klippe, although such events have been documented within and around the MCT zone in the northern root zone of the Higher Himalaya. This suggests that out-of-sequence thrusting may have occurred between 14 Ma and 5 Ma, probably during the period 10-7.5 Ma. Since then the frontal MCT zone below the Kathmandu Nappe has been inactive, but the MCT zone in the northern root zone has remained active. The rapid increase in denudation rates of the Higher Himalaya since the Late Miocene may have been caused by ramping along the out-of-sequence thrust at depth.