Historical seismicity, palaeoseismicity and seismic risk in Western Macedonia, Northern Greece

Western Macedonia, Northern Greece, was a seismically quiescent region for one or more centuries, and was regarded as a nearly aseismic, rigid block inside a broad zone of distributed continental deformation and faulting, and a region of minimum seismic risk. Consequently, the May 13, 1995 destructive earthquake (M = 6.6) which hit this assumed aseismic zone was a surprise for scientists, government and population.However, historical and archaeoseismic evidence, as well as coastal change data indicate that the assumed aseismic region of Western Macedonia has been affected in the last 2,000 years by at least seven, and possibly nine destructive earthquakes. One of these earthquakes occurred in circa 1700, and probably had the same epicentre with, but higher magnitude than the 1995 shock.The earthquake in circa 1700 is deduced from historical data and is modelled on the base of a swarm of church repairs which is explained as post-seismic recovery of the broader Kozani area: except for certain well known cases of towns or areas in which religious privileges were granted, large scale repairs or reconstruction of churches during the Ottoman period were possible only after Sultan's permissions, usually following earthquakes and other calamities.It can hence be concluded that some, at least, of the apparently aseismic regions inside broad zones of distributed seismicity are hit by stronger shocks, but with longer (200 years or more) recurrence intervals than their adjacent zones. Consequently, the seismic risk of the apparently aseismic regions is certainly not low, especially since relatively long periods of seismic quiescence lead to constructions vulnerable to earthquakes.     

Expulsion of a geopressured hydrothermal system associated with destructive earthquakes and buried active faults in the Shinanogawa Seismic Belt, Japan

Abstract   The Shinanogawa Seismic Belt in the Northern Fossa Magna, Honshu Island, Japan, extends along the Shinano River, bounding the Eurasian Plate and the Okhotsk Plate. The geopressured hydrothermal system occurs widely in the Northern Fossa Magna region. Many destructive earthquakes are related to the activity of this system in the Shinanogawa Seismic Belt. Expulsion of a geopressured hydrothermal system and rising from depth along an active fault triggers the occurrence of an earthquake and opens the fault as a pathway. Anomalous areas in temperature, electrical conductivity and Cl^– concentration of groundwater trend north–east in a linear distribution, and convincingly demonstrate the presence of a buried active fault at the epicentral area of the destructive earthquake in the Shinanogawa Seismic Belt. The distribution of the major axis of the anomalous area in groundwater temperature shows a strong positive relationship with earthquake magnitude, which means that the distribution of this area may indicate the scale of earthquake fault. The linearly anomalous areas in groundwater temperature, resulting from the percolation of a geopressured hydrothermal system, that have no record of previous destructive earthquake are predicted to be areas where destructive earthquakes could occur in the future. Four potential earthquake areas are proposed and discussed in this paper, based on re-examination of active faults and seismicity in the Shinanogawa Seismic Belt.     

日本信浓川地震带超压热水系的喷溢作用与地震活动

信浓川地震带位于日本大地沟北部,地壳运动十分强烈,区内地震主要沿信浓川流域发生,并密集成带,大地构造上处于日本海板块向本州板块俯冲的边界线上。该地震带大多数地震为中强震,且均为浅源地震,地震发生伴随着明显的地下水前兆异常,震中区有强烈的超压热水系的喷溢活动。震中区地下水的温度、电导率以及主要地球化学成分呈线性异常分布,并与地震强弱或地震断裂规模有关,地震断层的规模控制了超压热水系喷溢活动的强度和规模。地震发生与超压热水系喷溢活动有着密切的成生关系,超压热水系喷溢活动使断层发生活动所需应力条件降低,诱发地震发生,同时断层活动为超压热水系向上喷溢提供通道。     

Rupture model of the great AD 365 Crete earthquake in the southwestern part of the Hellenic Arc

An M8.3 earthquake struck the southwestern part of the Hellenic Arc, near the Island of Crete, in AD 365, generating a tsunami that affected almost the entire eastern Mediterranean region. Taking into account that the time history of seismicity in this region is fairly complete for such earthquakes in the historical catalog, which can be dated as back as the 5th century B.C., there is no indication that this segment of plate boundary has been fully ruptured again. The seismic hazard associated with this part of the Hellenic Arc necessitates the evaluation of the rupture characteristics of this great event. The constraint of the faulting geometry was initially achieved by using information from seismicity, and the focal mechanisms of earthquakes that occurred during the instrumental period. A rupture model for this great earthquake is constructed by assuming an elastic medium and calculating the theoretical surface displacements for various fault models that are matched with the observed surface deformation gleaned from historical reports. The resulted fault model concerns thrust faulting with a rupture length of 160 km and a seismic moment of 5.7 × 10²⁸ dyn·cm, an average slip of 8.9 m and a corresponding moment magnitude equal to 8.4, in excellent agreement with the macroseismic estimation. The absence of such events recurrence is an indication of the lack of complete seismic coupling that is common in subduction zones, which is in accordance with the back arc spreading of the Aegean microplate and with previous results showing low coupling for extensional strain of the upper plate.     

云南地区大震活动规律研究

云南地区处于印度板块与欧亚板块中国大陆碰撞带的东缘,地壳运动剧烈,活动块体特征明显,中强以上地震频发,是研究大震活动规律的理想场所。通过过去一个世纪的6.7级以上地震活动的时空分布以及地震动力分析认为,云南地区存在的4个具东西交替活动特征的地震活跃期,可能是东、西部各自地震活跃与平静过程叠加的结果,100a左右可能出现1次东、西部同时爆发大震的时段;云南地区地震活动与外围地区存在较好的呼应关系,安达曼-缅甸弧形带的巨震活动对云南地区地震活跃期的启动有一定的指示意义,而云南东部强震也与四川西部大震密切相关,四川大震活动往往滞后于云南地区;中强地震连发—平静—首发大震可能是云南以东部为活动主体的地震活跃期的启动模式。这些认识对云南地区大震预测、地震机理以及板缘动力学研究会有所帮助     

Marine seismological observations in the Central Kurile segment before catastrophic earthquakes on November, 2006 (<Emphasis Type="Italic">M</Emphasis> = 8.3) and January, 2007 (<Emphasis Type="Italic">M</Emphasis> = 8.1)

The results of detailed seismological observations with bottom seismographs in the Central Kurile segment in August-September, 2006 are discussed. The system of six bottom seismographs was placed on the island slope of the Kurile deep-sea trench southeast of Urup Island and southwest of the Bussol Strait. Over 230 earthquakes with M _LH = 0.5–5.5 were registered in the area with a radius of 150 km around the center of the observation system at depths up to 300 km during 16 days. Records of 80 earthquakes with hypocenters in the earth crust (h = 0–30 km) beneath the island slope of the Kurile deep-sea trench were first obtained by bottom seismographs. These data are inconsistent with previous concepts of aseismicity of this zone. The discovery of the unique morphological structure of the Benioff zone beneath the central Kurile Arc represents the most important result of detailed seismological observations. The zone consists of an inner seismoactive subzone, which is located beneath the island slope of the arc at depths of 15–210 km, being characterized by an angle of incline of 50° under the latter and crosses the ocean bottom approximately 80 km away from the trench axis, and outer low-activity subzone. The latter is traceable beyond the trench almost parallel to the inner zone beginning from a depth of 50 km below the sea bottom up to a depth of approximately 300 km. Due to the slightly lower incline (∼45°) of the outer subzone, both subzones gradually converge downward. The integral thickness of the Benioff zone varies from 150 km in its upper part to 125 km at depths of 210–260 km. The medium sandwiched between these subzones is practically aseismic. The reality of this defined structure is confirmed by the distribution of aftershocks of the earthquake that occurred on November 15, 2006 (M = 8.3). These seismic events served as foreshocks for the subsequent strong earthquake of January 13, 2007 (M = 8.1) with the hypocenter located beyond the trench under the ocean bottom. Such a structure of this zone within the central Kurile Arc segment is unique, having no analogues either in the flanks of the Kurile-Kamchatka Arc or other arcs. The results of detailed seismological observations obtained two months before the first of the catastrophic Central Kurile earthquakes appeared to be typical for the period of foreshocks (the lower seismic activity of the Simushir block, which hosted the hypocenter of the earthquake that occurred on November 15, 2006, particularly at depths of 0–50 km, the gentler incline of the recurrence plot, and other features).     

ETAS Model Analysis on the Chang Island Earthquake Swarm in Shandong Province,China

Influenced by the layout of seismic network and the location of earthquakes, earthquake catalogs are often incomplete; such incompleteness of earthquake catalogue directly affects the analysis of sequence activity characteristics. In this paper, the GPU-acceleration-based g template matching method is used to scan the continuous waveforms of Chang Island earthquake swarm in Shandong Province from February 9 to August 20, 2017. In total, 15,286 earthquakes events were detected, which was more than 6 times compared with those in network catalogue and thus reduced the magnitude of completeness from 1.0 to 0.5. Based on the intergrated catalogue of earthquakes, the characteristics of Chang Island earthquake swarm were then analyzed using the Epidemic Type Aftershock Sequences (ETAS) model. The stochastic components in the ETAS model are used as a proxy for possible earthquake triggered by external forces (fluids). The results show that the proportion of earthquakes triggered by external forces of Chang Island swarm increases gradually (from 31.9% to 63.5%) and then decreases. The latter stage of swarm development is mainly affected by the self-excitation of earthquakes,suggesting that the fluids play an important role in the development of the Chang Island swarm. However, the triggering intensity of fluids to microseismicity is divergent in different periods, which may be related to the process of fluid permeation.     

Demarcation and Scaling of Long-term Seismogenesis

A space-time envelope of minor seismicity related to major shallow earthquakes is identified from observations of the long-term Precursory Scale Increase () phenomenon, which quantifies the three-stage faulting model of seismogenesis. The envelope, which includes the source area of the major earthquake, is here demarcated for 47 earthquakes from four regions, with tectonic regimes ranging from subduction to continental collision and continental transform. The earthquakes range in magnitude from 5.8 to 8.2, and include the 24 most recent mainshocks of magnitude 6.4 and larger in the San Andreas system of California, the Hellenic Arc region of Greece, and the New Zealand region, together with the six most recent mainshocks of magnitude 7.4 and larger in the Pacific Arc region of Japan. Also included are the destructive earthquakes that occurred at Kobe, Japan (1995, magnitude 7.2), Izmit, Turkey (1999, magnitude 7.4), and W.Tottori, Japan (2000, magnitude 7.3). The space (A _P ) in the space-time envelope is optimised with respect to the scale increase, while the time (T _P ) is the interval between the onset of the scale increase and the occurrence of the earthquake. A strong correlation is found between the envelope A _P T _P and the magnitude of the earthquake; hence the conclusion that the set of precursory earthquakes contained in the envelope is intrinsic to the seismogenic process. Yet A _P and T _P are correlated only weakly with each other, suggesting that A _P is affected by differences in statical conditions, such as geological structure and lithology, and T _P by differences in dynamical conditions, such as plate velocity. Among other scaling relations, predictive regressions are found between, on the one hand, the magnitude level of the precursory seismicity, and on the other hand, both T _P and the major earthquake magnitude. Hence the method, as here applied to retrospective analysis, is potentially adaptable to long-range forecasting of the place, time and magnitude of major earthquakes.     

Updating the Probabilistic Seismic Hazard Values of Northern Algeria with the 21 May 2003 <Emphasis Type="Italic">M</Emphasis> 6.8 Algiers Earthquake Included

The occurrence of the Algiers earthquake (M 6.8) of May 21, 2003, has motivated the necessity to reassess the probabilistic seismic hazard of northern Algeria. The fact that this destructive earthquake took place in an area where there was no evidence of previous significant earthquakes, neither instrumental nor historical, strongly encourages us to review the seismic hazard map of this region. Recently, the probabilistic seismic hazard of northern Algeria was computed using the spatially smoothed seismicity methodology. The catalog used in the previous computation was updated for this review, and not only includes information until June 2003, but also considers a recent re-evaluation of several historical earthquakes. In this paper, the same methodology and seismicity models are utilized in an effort to compare this methodology against an improved and updated seismic catalog. The largest mean peak ground acceleration (PGA) values are obtained in northernmost Algeria, specifically in the central area of the Tell Atlas. These values are of the order of 0.48 g for a return period of 475 years. In the City of Algiers, the capital of Algeria, and approximately 50 km from the reported epicenter of this latest destructive earthquake, a new mean PGA value of 0.23 g is obtained for the same return period. This value is 0.07 g greater than that obtained in the previous computation. In general, we receive greater seismic hazard results in the surrounding area of Algiers, especially to the southwest. The main reason is not this recent earthquake by itself, but the significant increase in the m_max magnitude in the seismic source where the city and the epicenter are included.     

Evidence for tidal triggering on the earthquakes of the Hellenic Arc,Greece

In this paper we investigate the tidal triggering evidence on the earthquakes of the seismic area of the Hellenic Arc using the Hist(ogram)Cum(mulation) method. We analyze the series of the earthquakes occurred in the area which is confined by the longitudes 22° and 28°E and latitudes 34° and 36°N in the time period from 1964 to 2012. In this time period 16,137 shallow and of intermediate depth earthquakes with M_L up to 6.0 and 1,482 deep earthquakes with M_L up to 6.2 occurred. The result of the this analysis indicate that the monthly variation of the frequencies of earthquake occurrence is in accordance with the period of the tidal lunar monthly variations, and the same happens with the corresponding daily variations of the frequencies of earthquake occurrence with the diurnal luni-solar (K1) and semidiurnal solar (S2) tidal variations. These results are in favor of a tidal triggering process on earthquakes when the stress in the focal area is near the critical level.     

辽宁地区1509年4月21日和1552年10月27日地震考证

辽宁是历史地震资料比较缺乏的地区,深入研究历史疑难地震,尤其是确认一些破坏性地震,对补充已有的地震目录和开展地震预测及工程场地地震安全性评价有重要作用。在历史地震研究中认真考查,论证已有的地震史料是最主要的手段和方法。但是若能在此基础上,利用现代地震资料,按"地震重复"发生理论反推一些历史上可能存在的破坏性地震,也可以认为是对历史地震研究方法的一种补充。按照这一思路,本文在进一步分析地震史料的基础上,通过与现代地震活动图像和大地震震害现象的对比,确认辽宁地区1509年4月21日的地震是一次破坏性地震,并对1552年10月27日的地震做了讨论。     

Seismicity of Western Crete and the destruction of the town of Kisamos at AD 365: Archaeological evidence

The study of numerous archaeological excavations permits us to concludethat shortly after AD 355–361, the wealthy Roman town of Kisamos inwestern Crete was affected by a devastating earthquake (minimum intensityXI), which left many of the town's inhabitants buried under the ruins. Thisearthquake can be related to the July 21, 365 earthquake (M > 8),which was associated with a great tsunami, and was probably responsible forup to 9 m uplift of Western Crete; it probably resulted from thereactivation of a major thrust fault along the Hellenic Arc. Thearchaeological stratigraphy of Kisamos between circa AD 50 and AD650 testifies to two other, small-scale stratigraphic discontinuities that maybe related to two other smaller earthquakes, which produced 10–20 cmof coastal subsidence in AD 46–66 and in circa AD 270. There isevidence for a tsunami associated with the AD 46–66 earthquake, whichagrees with sedimentological data from the nearby ancient harbour ofPhalasarna, which was uplifted about 6.5 m in AD 365.The following evidence indicates that Western Crete is not seismicallyquiescent, as previously believed on the basis of historical data, but that ithas been affected by very strong, AD 365-type earthquakes followed byrelatively quiescence periods, that were at least several thousand years long:(1) Major earthquakes of Western Crete produced coastal uplift orsubsidence, while coastal stability indicates the absence of strongearthquakes; such stability characterizes this part of the island after AD365; (2) No evidence of earthquakes exist in the archaeological record ofKisamos between AD 365 and circa AD 650 and of Chania since AD1400/1500.     

东汉早期的地震活动

东汉早期(公元25—127年)的地震史料十分宝贵, 它反映了从远古零散、 不系统的地震记载转入连续记载并逐渐认识地震的过程. 这时的地震事件较多, 但研究不充分, 地震参数基本呈空白状. 史料的记载方式以“郡国地震”和“京师地震”为主; 时间上, 基本在汉和帝以后; 地点上, 以大华北为主. 本文具体分析了东汉早期的33次地震事件, 利用新的地震烈度衰减关系, 采用郡国平均地理面积和有感面积的概念, 比照地震范例的标准, 按《中国地震目录》的烈度震级表进行参数估算. 研究期间最主要的地震事件是4次震级≥6 1/₂ 的强震, 即公元46年10月和119年3月南阳地震、 121年10月冀南-鲁西地震和123年5月汉阳地震, 震后都有持续1—2年的余震活动. 公元118年前的地震主要分布于大华北, 震中至京师洛阳的距离多在200 km以外. 除两次5 1/₂ —6级地震外, 均属5—5 1/₂ 级中等强度地震, 少数小于4 3/₄ 级. 京师洛阳虽经历过多次地震, 但其受影响程度并不强, 基本为有感或强有感的水平, 没有遭受过破坏性或中等强度的震害.      

The seismic catastrophe of December 26, 2004, in the Indian Ocean as a tsunamigenic earthquake in island arc structures

An interpretation of the occurrence conditions and source parameters is proposed for the catastrophic earthquake of December 26, 2004, in the northwestern part of the Sunda island arc. The interpretation is based on the analysis of spatial distributions of aftershock epicenters and regions subjected to destructive tsunamis, seismicity manifestations in the NW part of the Sunda island arc in the past century, and locations of large tsunami sources of historical earthquakes off the Sumatra Island coast. The source parameters of the December 26, 2004, earthquake are compared with the reliably established main characteristics of sources of the largest tsunamigenic earthquakes in island arcs of the Pacific Ocean. According to the proposed interpretation, the December 26, 2004, earthquake source is a steep reverse fault striking NW and dipping toward the Indian Ocean. The source, ～450 km long, is located in front of the NW termination of Sumatra Island, in the southern part of the Nicobar Islands. Possible positions and sizes of large potential seismic sources in the NW part of the Sunda island arc are suggested.     

Seismicity and Seismic Hazard in Alexandria (Egypt) and its Surroundings

— Alexandria City has suffered great damage due to earthquakes from near and distant sources, both in historical and recent times. Sometimes the source of such damages is not well known. Seismogenic zones such as the Red Sea, Gulf of Aqaba-Dead Sea Hellenic Arc, Suez-Cairo-Alexandria, Eastern-Mediterranean-Cairo-Faiyoum and the Egyptian costal area are located in the vicinity of this city. The Egyptian coastal zone has the lowest seismicity, and therefore, its tectonic setting is not well known. The 1998 Egyptian costal zone earthquake is a moderate complex source. It is composed of two subevents separated by 4 sec. The first subevent initiated at a depth of 28 km and caused a rupture of strike (347°), dip (29°) and slip (125°). The second subevent occurred at a shallower depth (24 km) and has a relatively different focal parameter (strike 334°, dip 60° and slip 60°). The available focal mechanisms strongly support the manifestation of a complex stress regime from the Hellenic Arc into the Alexandria offshore area. In the present study a numerical modeling technique is applied to estimate quantitative seismic hazard in Alexandria. In terms of seismic hazard, both local and remote earthquakes have a tremendous affect on this city. A local earthquake with magnitude M_s = 6.7 at the offshore area gives peak ground acceleration up to 300 cm/sec². The total duration of shaking expected from such an earthquake is about three seconds. The Fourier amplitude spectra of the ground acceleration reveals that the maximum energy is carried by the low frequency (1–3 Hz), part of the seismic waves. The largest response spectra at Alexandria city is within this frequency band. The computed ground accelerations due to strong earthquakes in the Hellenic Arc, Red Sea and Gulf of Aqaba are very small (less than 10 cm/sec²) although with long duration (up to 3 minutes).     

Relationship between global seismicity and solar activities

The relations between sunspot numbers and earthquakes (M≧6), solar 10.7 cm radio flux and earthquakes, solar proton events and earthquakes have been analyzed in this paper. It has been found that: (1) Earthquakes occur frequently around the minimum years of solar activity. Generally, the earthquake activities are relatively less during the peak value years of solar activity, some say, around the period when magnetic polarity in the solar polar regions is reversed. (2) the earthquake frequency in the minimum period of solar activity is closely related to the maximum annual means of sunspot numbers, the maximum annual means of solar 10.7 cm radio flux and solar proton events of a whole solar cycle, and the relation between earthquake and solar proton events is closer than others. (3) As judged by above interrelationship, the period from 1995 to 1997 will be the years while earthquake activities are frequent. In the paper, the simple physical discussion has been carried out. These results supported the exploration and studies of some researchers to a certain extent. This work is supported by Foundation of the Chinese Academy of Sciences (major item).     

Expulsion of geopressured hydrothermal system along active faults and its relation to the occurrence of earthquakes in the Shinanogawa seismic belt,Japan

The 1995 Northern Niigata Earthquake (M 6.0) occurred at a shallow depth in the Niigata seismic gap. The anomaly areas in temperature, electrical conductivity and Cl- concentration of groundwater trend northeast as linear distribution in the epicentral area and are approximately coincident with the area of the seismic intensity 6 (JMA scale). The distributions of seismic intensity 6 and groundwater anomalies convincingly imaged the presence of a buried active fault beneath the epicentral area. The occurrence of this earthquake and the anomalies of groundwater were related to the expulsion of geopressured hydrothermal system (GHS). All epicenters of the destructive earthquakes along the Shinanogawa seismic belt are actually located in the buried active fault zones characterized by the areas of temperature and geochemical anomalies of groundwater. These earthquakes might have been triggered by the activity of GHS. The expulsion of GHS along an active fault in combination with the thermal softening of fault     

广东揭汕潮地区城市群地震灾害及地震易损性分析

从历史最大地震烈度分布图分析,可知揭阳、汕头、潮州三市位于Ⅶ～Ⅷ度高地震烈度区。该城市群地震灾害有以下特点:市区发生破坏性地震的潜在危险性大;地震时市区地震烈度高,地震灾害大,损失严重。采用宏观经济易损性的地震损失分析方法,重演该区的历史破坏性地震。分析表明:抓好该区的防震减灾工作是保持社会和经济建设持续发展的一项重要工作。     

云南地区地震受灾人口与经济损失评估

地震灾害损失评估标准实施以来,云南地区共发生过46次破坏性地震。以46次破坏性地震的灾害损失评估资料为基础,分析了受灾人口与地震灾害经济损失的相关性,结果为两者呈非线性关系。提出利用受灾人口数据进行地震灾害损失快速预估的简便方法,并应用所提的方法对2001年云南地区7组地震的灾害损失进行了分析研究。     

Tsunami Source of the 2010 Mentawai, Indonesia Earthquake Inferred from Tsunami Field Survey and Waveform Modeling

The 2010 Mentawai earthquake (magnitude 7.7) generated a destructive tsunami that caused more than 500 casualties in the Mentawai Islands, west of Sumatra, Indonesia. Seismological analyses indicate that this earthquake was an unusual “tsunami earthquake,” which produces much larger tsunamis than expected from the seismic magnitude. We carried out a field survey to measure tsunami heights and inundation distances, an inversion of tsunami waveforms to estimate the slip distribution on the fault, and inundation modeling to compare the measured and simulated tsunami heights. The measured tsunami heights at eight locations on the west coasts of North and South Pagai Island ranged from 2.5 to 9.3 m, but were mostly in the 4–7 m range. At three villages, the tsunami inundation extended more than 300 m. Interviews of local residents indicated that the earthquake ground shaking was less intense than during previous large earthquakes and did not cause any damage. Inversion of tsunami waveforms recorded at nine coastal tide gauges, a nearby GPS buoy, and a DART station indicated a large slip (maximum 6.1 m) on a shallower part of the fault near the trench axis, a distribution similar to other tsunami earthquakes. The total seismic moment estimated from tsunami waveform inversion was 1.0 × 10²¹ Nm, which corresponded to M_w 7.9. Computed coastal tsunami heights from this tsunami source model using linear equations are similar to the measured tsunami heights. The inundation heights computed by using detailed bathymetry and topography data and nonlinear equations including inundation were smaller than the measured ones. This may have been partly due to the limited resolution and accuracy of publically available bathymetry and topography data. One-dimensional run-up computations using our surveyed topography profiles showed that the computed heights were roughly similar to the measured ones.