2019年5月18日松原M5.1地震构造机制分析

基于区域地震台网的数字化波形资料,使用ISOLA方法对2019年5月18日吉林松原M5.1地震进行矩张量反演,研究地震的震源机制,并且收集了地震序列中M_L2.5以上地震的震源机制解,采用FMSI（focal mechanism stress inversion）方法反演震中区构造应力场。结果显示：松原M5.1地震的矩震级为4.9,矩心深度为6 km,双力偶分量为91.5%,主压应力P轴方位角、倾角分别为76°和3°,主张应力T轴方位角、倾角分别为166°和16°,震源机制解显示典型的构造地震特征;震中区构造应力场理论应力轴σ₁方位角、倾伏角分别为88.0°和0.9°,σ₂方位角、倾伏角分别为178.2°和9.6°,σ₃方位角、倾伏角分别为352.5°和80.4°,这一结果与区域构造应力场一致。推断认为区域构造应力场触发了2019年松原M5.1地震活动,地震震源机制解的北西向节面与震中区附近的第二松花江断裂现今活动性质完全一致,认为第二松花断裂可能是松原M5.1地震的发震断层。     

Magnitude-frequency distribution in the European-Mediterranean earthquake regions

The European-Mediterranean earthquake catalogue from 1901 to 1985, which comprises uniformly determined magnitudes M_S and m_B(h ≥ 60 km) of 13300 events, was used in the study of cumulative magnitude-frequency relationships N_c(M) compiled for 75 earthquake regions and 25 larger provinces. In the whole magnitude range observed, the Gutenberg-Richter formula log N_c(M) = a − bM very rarely fits the cumulative (log N_c, M) distributions. The b-values of log-linear segments of N_c(M) vary regionally from b = 0.7 to b = 1.3; averaging of all values leads to (shallow events, M_S and ).

Most distributions pertain to the Mediterranean area (b = 0.86 from the graph for shallow events) and many of them indicate the existence of characteristic earthquakes in accordance with the theoretical single-fault model. Other observed shapes of N_c(M) can be explained by the superposition of populations of different M_max values or by the presence of swarm-type activity. The observed N_c(M) distributions depend very much on the delineation of earthquake regions i.e. on the number and dimension of seismoactive faults in the investigated region.

A premonitory enhancement of medium earthquake activity (M = 4.5–5.5) can be observed only very rarely.     

Groundwater radon anomalies associated with earthquakes

Earthquake-related changes in groundwater radon have been detected at a sensitive observation site located right on a major active fault in Northeast Japan. A time-series analysis based on Bayesian statistics was successfully applied to remove background variations from the observed radon data, enabling us to examine the earthquake-related changes in detail.

We set a simple criterion of amplitude and duration for an anomaly observed in our radon data; we define an anomaly as a radon change that kept its level beyond 2σ (a standard deviation over the whole observation period) during a period longer than one day. We have observed 20 radon anomalies that satisfied this criterion from January 1984 to December 1988. Most of these anomalies have turned out to be related to large earthquakes that occurred in East Japan and its surrounding area; we have identified 12 post-seismic and 2-pre-seismic radon anomalies out of a total of 30 earthquakes with magnitude M 6.0 and hypocentral distance D 1000 km.

The typical pattern of the post-seismic anomalies is a radon decrease which started just after an earthquake, lasting for periods ranging from a few days to more than one week. The amplitude of the post-seismic anomalies depends on both magnitude and hypocentral distance, and can, in general, be expressed by a simple magnitude-distance relationships.

A possible pre-seismic anomaly was observed about one week before the largest earthquake that occurred in this region during the observation period (March 6, 1984; M = 7.9, D = 1000 km). Another possible pre-seismic anomaly was observed about three days before two nearby large earthquakes that occurred at almost the same place in a time interval of 53 min (February 6, 1987; M = 6.4 and M = 6.7, D = 130 km).     

Regional time-predictable modeling in the Hindukush–Pamir–Himalayas region

The seismic characteristic of Hindukush–Pamir–Himalaya (HPH) and its vicinity is very peculiar and has experienced many widely distributed large earthquakes. Recent work on the time-dependent seismicity in the Hindukush–Pamir–Himalayas is mainly based on the so-called “regional time-predictable model”, which is expressed by the relation log T=cM_p+a, where T is the inter-event time between two successive main shocks of a region and M_p is the magnitude of the preceded main shock. Parameter a is a function of the magnitude of the minimum earthquake considered and of the tectonic loading and c is positive (0.3) constant. In 90% of the cases with sufficient data, parameter c was found to be positive, which strongly supports the validity of the model. In the present study, a different approach, which assumes no prior regionalization of the area, is attempted to check the validity of the model. Nine seismic sources were defined within the considered region and the inter-event time of strong shallow main shock were determined and used for each source in an attempt at long-term prediction, which show the clustering and occurrence of at least three earthquakes of magnitude 5.5≤M_s≤7.5 giving two repeat times, satisfying the necessary and sufficient conditions of time-predictable model (TP model). Further, using the global applicability of the regional time- and magnitude-predictable model, the following relations have been obtained: log T_t=0.19 M_min+0.52M_p+0.29 log m₀−10.63 and M_f=1.31M_min−0.60M_p−0.72 log m₀+21.01, where T_t is the inter-event time, measured in years; M_min the surface wave magnitude of the smallest main shock considered; M_p the magnitude of preceding main shock; M_f the magnitude of the following main shock; and m₀ the moment rate in each source per year.

These relations may be used for seismic hazard assessment in the region. Based on these relations and taking into account the time of occurrence and the magnitude of the last main shock in each seismogenic source, time-dependent conditional probabilities for the occurrence of the next large (M_s≥5.5) shallow main shocks during the next 20 years as well as the magnitudes of the expected main shocks are determined.     

Source parameters of earthquakes in the reservoir-triggered seismic (RTS) zone of Koyna–Warna, Western India

New empirical relations are derived for source parameters of the Koyna–Warna reservoir-triggered seismic zone in Western India using spectral analysis of 38 local earthquakes in the magnitude range M _L 3.5–5.2. The data come from a seismic network operated by the CSIR-National Geophysical Research Institute, India, during March 2005 to April 2012 in this region. The source parameters viz. seismic moment, source radius, corner frequency and stress drop for the various events lie in the range of 10¹³–10¹⁶ Nm, 0.1–0.4 km, 2.9–9.4 Hz and 3–26 MPa, respectively. Linear relationships are obtained among the seismic moment (M ₀), local magnitude (M _L), moment magnitude (M _w), corner frequency (fc) and stress drop (?σ). The stress drops in the Koyna–Warna region are found to increase with magnitude as well as focal depths of earthquakes. Interestingly, accurate depths derived from moment tensor inversion of earthquake waveforms show a strong correlation with the stress drops, seemingly characteristic of the Koyna–Warna region.     

Rupture lengths of intraplate earthquakes in Brazil determined by relative location of aftershocks: Evidence for depth dependence of stress drops

Whether intraplate earthquakes have different average source properties, compared to interplate events, has been long debated. It has been proposed that intraplate events tend to rupture smaller areas with higher stress drops, compared to the average interplate earthquake. Here we estimate the rupture lengths of several Brazilian earthquakes by accurately locating their immediate aftershocks. The sparsity of stations in low-seismicity regions, such as Brazil, hinders accurate epicentral determination. We use cross-correlation of P, S and Lg waves to accurately locate the aftershocks relative to a reference event. In several cases, it was possible to infer the rupture length by the distribution of the early aftershocks; with the later aftershocks tending to span a larger area. We studied six different aftershock sequences using regional stations up to several hundred km distance. The mainshock occurs close to the foreshocks, which act as triggers to the main rupture. The immediate aftershocks tend to occur in a circle around a central (presumably stress-free) zone, which we interpret as the rupture of the mainshock. Published data from other events, based mainly on local networks, were added to provide an empirical relationship between rupture length and magnitude. These data suggest that stress-drops in Brazil vary mostly between 0.1 and 10 MPa, a similar range to many other studies worldwide. However, the mean stress drop (about 1 MPa) is smaller than the mean values of both interplate and intraplate events globally (mostly between 2 and 10 MPa). A possible dependence of stress drops with hypocentral depth may explain this difference: Brazilian intraplate earthquakes tend to be shallower than most other mid plate regions giving rise to smaller stress drops, on average. This result has important implications for seismic hazard estimation when GMPE equations from other intraplate regions are used in Brazil.     

Source parameters of some recent earthquakes in the Gulf of Aqaba, Egypt

Gulf of Aqaba is recognized as an active seismic zone where many destructive earthquakes have occurred. The estimation of source parameters and coda Q attenuation are the main target of this work. Fifty digital seismic events in eight short-period seismic stations with magnitude 2.5–5.2 are used. Most of these events occurred at hypocentral depths in the range of 7–20 km, indicating that the activity was restricted in the upper crust. Seismic moment, M _o, source radius, r, and stress drop, Δσ, are estimated from P- and S-wave spectra using the Brune’s seismic source model. The average seismic moment generated by the whole sequence of events was estimated to be 4.6E?+?22 dyne/cm. The earthquakes with higher stress drop occur at 10-km depth. The scaling relation between the seismic moment and the stress drop indicates a tendency of increasing seismic moment with stress drop. The seismic moment increases with increasing the source radius. Coda waves are sensitive to changes in the subsurface due to the wide scattering effects generating these waves. Single scattering model of local earthquakes is used to the coda Q calculation. The coda with lapse times 10, 20, and 30 s at six central frequencies 1.5, 3, 6, 12, 18, 24 Hz are calculated. The Q _c values are frequency dependent in the range 1–25 Hz, and are approximated by a least squares fit to the power law [ $ {Q_c}(f) = {Q_o}{(f/{f_o})^\eta } $ ]. The average of Q _c values increases from 53?±?10 at 1.5 Hz to 700?±?120 at 24 Hz. The average of Q _o values ranges from 13?±?1 at 1.5 Hz to 39?±?4 at 24 Hz. The frequency exponent parameter η ranges between 1.3?±?0.008 and 0.9?±?0.001.     

福建仙游地区高精度地震震源定位及深部断裂特征

福建仙游位于福建省东南沿海中部,其周边地区历史地震活动较平静,属于弱震区。但自从该地区的金钟水库于2010年5月下闸蓄水后,库区附近的地震活动性随之增强。为深入了解该地区的地震活动性、地震分布特征以及寻找隐伏断层,利用中国地震局提供的地震初至震相数据,使用双差定位方法对仙游地区近10年发生的地震进行重定位,获得了更为精确的震源位置,并根据重定位结果模拟深部断裂,寻找隐伏断层。结果显示：(1)重定位后的震源位置更加集中,按照发震时间可分为4个活动区,主要沿沙县—南日岛的次级断裂石苍断裂两侧北西向线性分布。(2)重定位后仙游震群的震源深度主要为8～11 km。石苍断裂左侧地震条带震源深度为6～12 km;右侧地震条带呈现明显的分层现象,上层西北侧地震较为分散,东南侧地震分布较紧凑,震源深度同左侧一样为6～12 km,而下层地震较少,震源深度为14～23 km。(3)根据重定位后的震源位置,利用奇异值分解法拟合得到三个深部断层面,其倾向均为南西向,走向为北西向,与石苍断裂和潼关断裂的倾向和走向一致。结合前人研究成果和本研究结果,推测石苍断裂并不是主发震断层,而是其两侧存在的深部断裂(高倾角隐...     

辽宁地区主要断裂活动性和地震危险性评估

基于辽宁地区主要活动断裂的几何特征和空间展布，对1980年以来辽宁地区M_L≥2.0地震的累计频次和1900年以来M_s≥5.0地震的年发生率的空间分布及其与活动断裂构造背景关系进行研究，获得了基于地震学的辽宁省内主要断裂和构造区（带）的活动性与地震危险性的初步评估结果。辽宁地区主要断裂活动性较高的有海城河断裂、金州断裂九寨—盖州北段、朝阳—北票断裂等；辽宁地区未来3年发生M_s≥5.0地震危险性较高的断裂依次有海城河断裂、金州断裂、熊岳—庄河断裂、鸭绿江断裂及赤峰—开原断裂与柳河断裂交汇处等。在判定区域地震危险性和城市地震风险时，除了依据前兆异常的空间分布，还应充分考虑区内主要构造（断裂）的活动性与地震危险性。     

The Belchatow, Poland, earthquakes of 1979 and 1980 induced by surface mining

Five seismic events occurred between August 1979 and May 1980 in the Belchatow trench area in central Poland, where large brown-coal deposits have been surface-mined since 1976. The three largest shocks had a local magnitudeM_L = 3.5 , seismic momentM_o = 1.5 · 10¹⁴ N-m and source radiusr = 350m , approximately. The tremors had maximum intensity between 5 and 6, and from the magnitude—intensity—depth relation, the focal depth was estimated to be between 1 and 5 km.

The Belchatow tremors are the manifestation of an unusual type of induced seismicity, i.e., seismicity connected with surface mining. The removal of overburden and extensive ground-water withdrawal by the mining operations seem to be direct factors responsible for stress concentration, although preexisting tectonic stress might be the most important indirect factor responsible for the origin of these events.     

岩浆泡破裂引发地震的模式——以吉林松原2013年地震群为例

2013年10月31日,吉林省松原前郭尔罗斯蒙古族自治县（44.60°N,124.18°E）发生震级为5.5级地震,此后的40 d内发生了700多次地震,其中5级以上地震5次。松原地区近年来地震活动频繁,2014年1月以来又发生4级以上地震9次、5级以上地震1次,震中处于松辽盆地油气田开采区,地震活动序列十分特殊。为了揭示松原地震的发震机制与发震模式,研究深部地质过程与地震的关系,根据此次实测的通过震中25 km长的大地电磁测深剖面,结合地热梯度、He同位素比值（³He/⁴He）、CO₂碳同位素、地震序列等资料的综合分析,发现震中地区存在两个位于不同深度的低阻体,地震发生与地幔深部岩浆活动有关;据此提出了一种新的地震发生模式——岩浆泡破裂发震模式,描述了来自地幔的基性岩浆通过向上侵入、在脆-韧性转换带附近聚集形成岩浆泡、岩浆泡破裂及岩浆泡上覆岩层中聚集能量引发岩层破裂产生地震的过程,并使来自地幔的无机成因的CO₂气在储层中形成CO₂气藏。该模式可以解释许多发生在大陆内部地震和深源地震的发生机制。     

Seismicity and source parameters of moderate earthquakes in Sikkim Himalaya

In this study, we accurately relocate 360 earthquakes in the Sikkim Himalaya through the application of the double-difference algorithm to 4?years of data accrued from a eleven-station broadband seismic network. The analysis brings out two major clusters of seismicity??one located in between the main central thrust (MCT) and the main boundary thrust (MBT) and the other in the northwest region of Sikkim that is site to the devastating Mw6.9 earthquake of September 18, 2011. Keeping in view the limitations imposed by the Nyquist frequency of our data (10?Hz), we select 9 moderate size earthquakes (5.3????Ml????4) for the estimation of source parameters. Analysis of shear wave spectra of these earthquakes yields seismic moments in the range of 7.95?×?10²¹ dyne-cm to 6.31?×?10²³ dyne-cm and corner frequencies in the range of 1.8?C6.25?Hz. Smaller seismic moments obtained in Sikkim when compared with the rest of the Himalaya vindicates the lower seismicity levels in the region. Interestingly, it is observed that most of the events having larger seismic moment occur between MBT and MCT lending credence to our observation that this is the most active portion of Sikkim Himalaya. The estimates of stress drop and source radius range from 48 to 389?bar and 0.225 to 0.781?km, respectively. Stress drops do not seem to correlate with the scalar seismic moments affirming the view that stress drop is independent over a wide moment range. While the continental collision scenario can be invoked as a reason to explain a predominance of low stress drops in the Himalayan region, those with relatively higher stress drops in Sikkim Himalaya could be attributed to their affinity with strike-slip source mechanisms. Least square regression of the scalar seismic moment (M ₀) and local magnitude (Ml) results in a relation LogM ₀?=?(1.56?±?0.05)Ml?+?(8.55?±?0.12) while that between moment magnitude (M _w ) and local magnitude as M _w ?=?(0.92?±?0.04)Ml?+?(0.14?±?0.06). These relations could serve as useful inputs for the assessment of earthquake hazard in this seismically active region of Himalaya.     

Space-time distribution patterns of destructive earthquakes in the inner belt of central Japan: Activity intervals and locations of earthquakes

Based on a block structure model of the inner belt of central Japan, an examination was conducted of the space-time distribution patterns of destructiv magnitudes M 6.4 or greater (M =Japan Meteorological Agency Scale). The distribution patterns revealed a periodicity in earthquake activit seismic gaps. Major NW—SE trending left-lateral active faults divide the inner belt of central Japan into four blocks, 20–80 km wide. The occurrenc A.D. with M ≥ 6.4, which have caused significant damage, were documented in the inner belt of central Japan. The epicenters of these earthquakes close to the block boundaries.

Using the relationship between the magnitude of earthquakes which occurred in the Japanese Islands and the active length of faults that generated them, movement is calculated for each historical earthquake. Space—time distributions of earthquakes were obtained from the calculated lengths, the latitud of generation. When an active period begins, a portion or segment of the block boundary creates an earthquake, which in turn appears on the ground surf active period ends when the block boundary generates earthquakes over the entire length of the block boundary without overlapping.

Five seismic gaps with fault lengths of 20 km or longer can be found in the inner belt of central Japan. It is predicted that the gaps will generate ea magnitudes of 7.0. These data are of significance for estimating a regional earthquake risk over central Japan in the design of large earthquake resist

The time sequences of earthquakes on the block boundaries reveal a similar tendency, with alternating active periods with seismic activity and quiet pe activity. The inner belt of central Japan is now in the last stage of an active period. The next active period is predicted to occur around 2500 A.D.     

Structure,tectonics and thermal state of the Lithosphere beneath intraplate seismic region of Latur,central India: An appraisal

Recent surge in intraplate seismicity has led to detailed geological and geophysical investigations, covering different continental segments of India including seismogenic region of Latur. A synthesis of such data sets to understand the prevailing tectonic and thermal state of the Lithosphere beneath Latur region, that witnessed a large scale human loss due to 1993 seismic activity, has revealed shallow surfacing of denser deeper crustal segments which may have resulted due to ongoing active subsurface tectonic activity like uplift and erosion since geological past. Below this region, Moho temperature exceeds 500°C, heat flow input from the mantle is quite high (29–35 mW/m²) and the asthenosphere is shallow (∼100±10 km). It is suggested that stress generated by ongoing upliftment and related subcrustal thermal anomaly is concentrating in this denser and stronger mafic crust within which earthquakes tend to nucleate. In all likelihood, the seismic activity witnessed in the region may stem from the deep crustal/lithospheric dynamics rather than the role of fluids at the hypocentral depth.     

Source parameters of the 2001 Mw 7.7 Bhuj earthquake,Gujarat, India,aftershock sequence

We present the estimated source parameters from SH-wave spectral modeling of selected 463 aftershocks (2002–06) of the 26 January 2001 Bhuj earthquake, the well-recorded largest continental intraplate earthquake. The estimated seismic moment (M_o), corner frequency (f_c), source radius (r) and stress drop (Δσ) for aftershocks of moment magnitude 1.7 to 5.6 range from 3.55×10¹¹ to 2.84×10¹⁷ N-m, 1.3 to 11.83 Hz, 107 to 1515 m and 0.13 to 26.7 MPa, respectively, while the errors in f_c and Δσ are found to be 1.1 Hz and 1.1 MPa, respectively. We also notice that the near surface attenuation factor (k) values vary from 0.02 to 0.03. Our estimates reveal that the stress drop values show more scatter (Mo^{0.5 to 1} is proportional to Δσ) toward the larger M_o values (≥10^14.5 N-m), while they show a more systematic nature (Mo³ is proportional to Δσ) for smaller M_o values (<10^14.5 N-m), which can be explained as a consequence of a nearly constant rupture radius for smaller aftershocks in the region. The large stress drops (= 10 MPa) associated with events on the north Wagad fault (at 15–30 km depth) and Gedi fault (at 3–15 km depth) can be attributed to the large stress developed at hypocentral depths as a result of high fluid pressure and the presence of mafic intrusive bodies beneath these two fault zones.     

Improved earthquake locations in the Koyna-Warna seismic zone

In estimating the likelihood of an earthquake hazard for a seismically active region, information on the geometry of the potential source is important in quantifying the seismic hazard. The damage from an earthquake varies spatially and is governed by the fault geometry and lithology. As earthquake damage is amplified by guided seismic waves along fault zones, it is important to delineate the disposition of the fault zones by precisely determined hypocentral parameters. We used the double difference (DD) algorithm to relocate earthquakes in the Koyna-Warna seismic zone (KWSZ) region, with the P- and S-wave catalog data from relative arrival time pairs constituting the input. A significant improvement in the hypocentral estimates was achieved, with the epicentral errors <30 m and focal depth errors <75 m i.e. errors have been significantly reduced by an order of magnitude from the parameters determined by HYPO71. The earthquake activity defines three different fault segments. The seismogenic volume is shallower in the south by 3 km, with seismicity in the north extending to a depth of 11 km while in the south the deepest seismicity observed is at a depth of 8 km. By resolving the structure of seismicity in greater detail, we address the salient issues related to the seismotectonics of this region.     

Source parameters of selected earthquakes on the central and western margin of Afar

Spectral analysis of 196 short-period and long-period vertical- and horizontal-component seismograms from ten earthquakes on the central and western margin of Afar is performed to determine source parameters and discuss their tectonic implications. For the earthquakes in the regions under study, the stress drop varies from 2 to 31 bar while the seismic moment varies from 2 × 10²⁴ to 154 × 10²⁴ dyn cm. In general, low stress-drop values are obtained indicating the presence of softer material (especially for central Afar) at a shallower depth. It is observed that there is an increase in stress drop with the increase in moment-magnitude which in turn is obtained from the calculated average seismic moment. Energy estimates show that the mode of energy release is different in the two regions indicating that different tectonic processes are involved in the two regions. The slip rate obtained for the Serdo area is of the order of 1.6 cm/yr and is in close agreement with the spreading rate already obtained for central Afar. Spreading rates obtained earlier and that of the present study show a low spreading rate for Afar and neighbouring regions as compared to those of the other regions of the world.     

A seismic source zone model for the seismic hazard assessment of the Italian territory

We designed a new seismic source model for Italy to be used as an input for country-wide probabilistic seismic hazard assessment (PSHA) in the frame of the compilation of a new national reference map.

We started off by reviewing existing models available for Italy and for other European countries, then discussed the main open issues in the current practice of seismogenic zoning.

The new model, termed ZS9, is largely based on data collected in the past 10 years, including historical earthquakes and instrumental seismicity, active faults and their seismogenic potential, and seismotectonic evidence from recent earthquakes. This information allowed us to propose new interpretations for poorly understood areas where the new data are in conflict with assumptions made in designing the previous and widely used model ZS4.

ZS9 is made out of 36 zones where earthquakes with M_w > = 5 are expected. It also assumes that earthquakes with M_w up to 5 may occur anywhere outside the seismogenic zones, although the associated probability is rather low. Special care was taken to ensure that each zone sampled a large enough number of earthquakes so that we could compute reliable earthquake production rates.

Although it was drawn following criteria that are standard practice in PSHA, ZS9 is also innovative in that every zone is characterised also by its mean seismogenic depth (the depth of the crustal volume that will presumably release future earthquakes) and predominant focal mechanism (their most likely rupture mechanism). These properties were determined using instrumental data, and only in a limited number of cases we resorted to geologic constraints and expert judgment to cope with lack of data or conflicting indications. These attributes allow ZS9 to be used with more accurate regionalized depth-dependent attenuation relations, and are ultimately expected to increase significantly the reliability of seismic hazard estimates.     

South Australian earthquakes, 1980–92

The seismicity of South Australia over the period 1980–92 is presented as a follow‐up to earlier studies. The South Australian seismic network has undergone a significant expansion in the last decade, and with it an increase in the number and precision of located earthquakes. The distribution of recent seismic activity is similar to the historical pattern of earthquakes and the previous instrumental seismicity maps, all of which show the three main areas as being the Flinders‐Mt Lofty Ranges, Eyre Peninsula, and the southeast. The one notable exception in the recent study is the presence of earthquake activity in the Musgrave Block, a previously aseismic region. Intensity characteristics are reported for earthquakes that were sufficiently widely felt. Fault plane solutions for three Flinders Ranges earthquakes (previously unpublished) are also presented; the focal mechanisms are consistent with predominant northeast‐southwest compression. The seismic moment method was used to estimate the seismic risk for the major population centres in terms of probability of exceedance of seismic intensity within a given period. These estimates are based on the recurrence parameters and intensity attenuation function for the region. The results place Adelaide close to the AS2121 ‐ 1979 Earthquake Code Zone I/Zone 2 boundary.