Andaman-Sumatra island arc: 1. Spatiotemporal manifestations and focal mechanisms of the earthquakes

The spatiotemporal manifestations of seismicity in the Andaman-Sumatra island arc are studied using the instrumental data for 1900–2010. The data on the largest tsunamigenic earthquakes of the 18th–19th centuries were also taken into account. The epicenters of the earthquakes are established to cluster in some areas; their possible relation to the structural features of the island arc is considered. A distinctive feature of seismicity in the region of the Andaman Sea is the presence of compact swarms of numerous earthquakes occurring during short intervals of time. The distribution of the earthquakes by the depth of their hypocenters in different segments of the island arc is investigated. The focal mechanisms of the earthquakes are analyzed using the centroid-moment-tensor (CMT) determinations over the period of 1980–2004, and the characteristic features of their parameters in different segments of the Andaman-Sumatra island arc are formulated. The focal parameters of the earthquakes determined by CMT and the moment-tensor-solution (MTS) are compared; the possible uncertainty in the estimates of the focal mechanisms is assessed. The pattern of the spatiotemporal manifestations of the Andaman-Sumatra earthquakes and their focal mechanisms are compared to the data on the Kuril-Kamchatka and the Aleutian island arcs previously studied by the authors. The results of analyzing the long-term seismicity and focal mechanisms in the Andaman-Sumatra island arc provide a necessary basis for the further thorough investigation of the geological conditions and source parameters of the major Sumatra earthquakes of 2000–2010.     

Regional earthquake catalogues of Russia

Some aspects concerning collection and analysis of primary materials, on whose basis the Tambov (1954) and Tyumen’ (1926) earthquakes have appeared in the “New Catalogue...” [Novyi..., 1977] are discussed. Moreover, the regional catalogues of the Urals (1914–2002) and Kamchatka Region (1737–1899, 1900–1952) have been compiled.     

The earthquake history of the Koryak Upland and the aftershock process of the <Emphasis Type="Italic">M</Emphasis><Subscript><Emphasis Type="Italic">W</Emphasis></Subscript> 7.6 April 20(21), 2006 Olyutorskii earthquake

We investigated the seismicity in the aftershock area of a great earthquake occurring on April 20, 2006 (21:04 LT) in the area of the Koryak Autonomous Okrug. This analysis of the aftershock process was based on a complete catalog of the earthquakes that were recorded during April–May 2006 by the regional network of seismic stations operated by the Kamchatka Branch of the Geophysical Service of the Russian Academy of Sciences. We also made use of a catalog of low magnitude events for May 2–17 as recorded by a mobile seismic network deployed in the rupture zone. We provide a review of seismicity for the Koryak Upland for the period of instrumental observations in the 20th and early 21st centuries.     

A geological study of the epicentral area of the April 20(21), 2006 Olyutorskii earthquake

Primary and secondary earthquake effects were investigated (surface breakage and felt effects in the villages of Tilichiki, Korf, and Khailino) in the epicentral area of a large earthquake occurring in northern Kamchatka Krai. The primary effects include an extended surface break that can be followed for a length of about 140 km. The secondary (gravitational and vibrational) effects include soil slides and minor rockfalls, snow flows on slopes, resonant cracks, soil liquefaction phenomena consisting of mud cones, mud gryphons, and discharges of soil from cracks. Detailed maps showing the main types of earthquake surface effects have been made. Paleoseismic data show traces of several large earthquakes that have occurred in the Olyutorskii earthquake zone. Radio carbon dating of soil samples from these paleoseismic scarps revealed a history of great earthquakes occurring in Koryakia. In all, four seismic events with magnitudes about that of the 2006 Olyutorskii earthquake have occurred there during 8000 years, the return period of such events being 1000–2500 years.     

Variations in short-period shear-wave attenuation in the Baikal Rift Zone and their relationship to seismicity

This paper presents results from a study of variations in short-period shear-wave attenuation in the lithosphere of the Baikal Rift Zone (BRZ). We used earthquake records made at the Ulan-Bator station (ULN) at epicentral distances of ～400–1300 km. The ratios of maximum amplitudes in the Sn and Pn waves were considered. We show that these ratios are on the whole considerably higher than those in other areas of Central Asia. It was found that zones of low shear wave attenuation coincide with the rupture zones of large earthquakes that occurred during the 19th and 20th centuries. We identified zones of high attenuation where no large (M～ > 7.0) seismic events have occurred during at least 180 years. The hypothesis we propose is that precursory processes before future large earthquakes may be occurring in these zones. We discuss the question of whether wave attenuation characteristics may be related to seismicity.     

On the seismicity of central Kamchatka before the 1997, M = 7.9 Kronotskii earthquake

We report results from a detailed study of seismicity in central Kamchatka for the period from 1960 to 1997 using a modified traditional approach. The basic elements of this approach include (a) segmentation of the seismic region concerned (the Kronotskii and Shipunskii geoblocks, the continental slope and offshore blocks), (b) studying the variation in the rate of M = 4.5–7.0 earthquakes and in the amount of seismic energy release over time, (c) studying the seismicity variations, (d) separate estimates of earthquake recurrence for depths of 0–50 and 50–100 km. As a result, besides corroborating the fact that a quiescence occurred before the December 5, 1997, M = 7.9 Kronotskii earthquake, we also found a relationship between the start of the quiescence and the position of the seismic zone with respect to the rupture initiation. The earliest date of the quiescence (decreasing seismicity rate and seismic energy release) was due to the M = 4.5–7.0 earthquakes at depths of 0–100 km in the Kronotskii geoblock (8–9 years prior to the earthquake). The intermediate start of the quiescence was due to distant seismic zones of the Shipunskii geoblock and the circular zone using the RTL method, combining the Shipunskii and Kronotskii geoblocks (6 years). Based on the low magnitude seismicity (M≥2.6) at depths of 0–70 km in the southwestern part of the epicentral zone (50–100 km from the mainshock epicenter), the quiescence was inferred to have occurred a little over 3 years (40 months) before the mainshock time and a little over 2 years (25 months) in the immediate vicinity of the epicenter (0–50 km). These results enable a more reliable identification of other types of geophysical precursors during seismic quiescences before disastrous earthquakes.     

The possible changes of tectonic stress fields in North China in 16–17th centuries

The ancient tectonic stress field in North China during 16–17th centuries were recovered by studying seismogenic faults of four great earthquakes. Three of them are dip slip events, the direction of the maximum principal stress is vertical, and that of the minimum principal stress is in the NW—SE direction and nearly horizontal tensile stress. Another earthquake is a complicated one which includes thrust and dip-slip activities. The above-stated character is quite different from that of present stress field, it is inferred that the stress field has significantly changed since 16th century, the main stress axis revolved an angle of 90 degrees nearly. The mantle upheaval in the faulted basins in North China during 16–17th centuries may cause the tensile stress field and a series of great earthquakes. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, 144–148, 1992.     

The effects of cyclones on seismicity

This study is concerned with the effects of powerful Pacific cyclones on the seismicity of Kamchatka, Japan, and the Philippines. We used complete seismological catalogs for these regions that span a few tens of years. It was found that the cyclones that originate from the western Pacific generally do not exert a significant triggering effect on the seismicity of these three regions, at least in the short term, during a few weeks. The ground motion generated by cyclones, which might be treated as a trigger, does not have amplitudes above those in the motion due to numerous local earthquakes of moderate magnitudes (∼4–5) and larger teleseismic earthquakes.     

Source study of two small earthquakes of Delhi, India, and estimation of ground motion from future moderate, local events

We study source characteristics of two small, local earthquakes which occurred in Delhi on 28 April 2001 (Mw3.4) and 18 March 2004 (Mw2.6). Both earthquakes were located in the heart of New Delhi, and were recorded in the epicentral region by digital accelerographs. The depths of the events are 15 km and 8 km, respectively. First motions and waveform modeling yield a normal-faulting mechanism with large strike-slip component. The strike of one of the nodal planes roughly agrees with NE–SW orientation of faults and lineaments mapped in the region. We use the recordings of the 2004 event as empirical Green’s functions to synthesize expected ground motions in the epicentral region of a Mw5.0 earthquake in Delhi. It is possible that such a local event may control the hazard in Delhi. Our computations show that a Mw5.0 earthquake would give rise to PGA of ~200 to 450 gal, the smaller values occurring at hard sites. The estimate of corresponding PGV is ~6 to 15 cm/s. The recommended response spectra, Sa, 5% damping, for Delhi, which falls in zone IV of the Indian seismic zoning map, may not be conservative enough at soft sites for a postulated Mw5.0 local earthquake.     

Simulation of Ground Motion from Strong Earthquakes of Kamchatka Region (1992–1993) at Rock and Soil Sites

To estimate the parameters of ground motion in future strong earthquakes, characteristics of radiation and propagation of seismic waves in the Kamchatka region were studied. Regional parameters of radiation and propagation of seismic waves were estimated by comparing simulations of earthquake records with data recorded by stations of the Kamchatka Strong Motion Network. Acceleration time histories of strong earthquakes (M _w = 6.8–7.5, depths 45–55 km) that occurred near the eastern coast of Kamchatka in 1992–1993 were simulated at rock and soil stations located at epicentral distances of 67–195 km. In these calculations, the source spectra and the estimates of frequency-dependent attenuation and geometrical spreading obtained earlier for Kamchatka were used. The local seismic-wave amplification was estimated based on shallow geophysical site investigations and deep crustal seismic explorations, and parameters defining the shapes of the waveforms, the duration, etc. were selected, showing the best-fit to the observations. The estimated parameters of radiation and propagation of seismic waves describe all the studied earthquakes well. Based on the waveforms of the acceleration time histories, models of slip distribution over the fault planes were constructed for the studied earthquakes. Station PET can be considered as a reference rock station having the minimum site effects. The intensity of ground motion at the other studied stations was higher than at PET due to the soil response or other effects, primarily topographic ones. At soil stations INS, AER, and DCH the parameters of soil profiles (homogeneous pyroclastic deposits) were estimated, and nonlinear models of their behavior in the strong motion were constructed. The obtained parameters of radiation and propagation of seismic waves and models of soil behavior can be used for forecasting ground motion in future strong earthquakes in Kamchatka.     

Experimental evidence of the correlation between possible precursors of earthquakes in near-surface quasistatic electric fields and in the ionosphere

We consider data obtained when the parameters of the ionospheric E_s and F2 layers and the vertical gradient of the electric potential in the surface atmosphere were simultaneously measured during the preparatory period of crustal earthquakes with M = 5.0–6.2 in the Kamchatka region. The appearance of anomalously high E_s, accompanied by an increase in frequency parameters of the sporadic layer and the regular F2 layer, was detected on days when possible earthquake precursors, as determined earlier, occurred in atmospheric electric fields. The presumed earthquake precursors in the ionosphere are divided into two groups with different earthquake lead times ranging from several hours to two weeks. Empirical dependences are presented that connect the lead time of an earthquake (from the moment of the appropriate anomaly’s occurrence in the ionosphere or in the atmospheric electric field to the moment of the shock) and the epicentral distance to the observation point with the earthquake magnitude. These dependences are different for the two groups of presumed earthquake precursors, but they are close inside each group of possible precursors selected on the basis of quasistatic electric field measurements and revealed in ionospheric parameter variations.     

Intermediate-term precursors of large (<Emphasis Type="Italic">M</Emphasis> ≥ 6.6) Kamchatka earthquakes for the period from 1987 to 2004: A retrospective assessment of their information content for prediction

Data from the literature were used to systematize intermediate-term (with advance times of 1 month to ∼2.5 years) precursors to the M ≥ 6.6 Kamchatka earthquakes of 1987–2004. The precursors were observed as changes in seismological, geodetic, geophysical, water-level, and hydrochemical parameters. Retrospective assessment of the information content in these intermediate-term precursors for earthquake prediction is in progress. The focus was on estimating the occurrence times of various precursors as functions of earthquake parameters (magnitude M, hypocentral distance R, and epicenter location). In the conditions of the Kamchatka observing network, precursors can be identified by a combination of methods, mostly before M ∼ 7 earthquakes or greater south of the Kronotskii Peninsula, for which M/logR ≥ 3. It is shown that the relative proportion of earthquakes for which precursors have been identified in the observations considered here is 0.43–0.86.     

On the possibility of using the tidal modulation of seismic waves for forecasting earthquakes

The earthquake forecasting technique based on the effect of the modulation of high frequency seismic noise (HFSN) by the Earth’s tides is discussed and the experience of its practical application in Kamchatka is described based on the extensive measurement data for 1996–2013. The empirical development of the lower magnitude threshold on the epicentral distance is refined. The reliability and validity of the precursor are assessed. The efficiency of the precursor is estimated in two ways. It is shown that the results of applying the prognostic procedure statistically significantly differ from a random guess. The presented estimates are based on the catalog containing 68 earthquakes of 1996–2013 in the predicted magnitude interval M ≥ 5.0 at the epicentral distances that are linked to the magnitude by functional dependence.     

Problematic historical earthquakes in the Baikal region

Macroseismic data on strong historical earthquakes of the 18th–19th centuries are presented that are unknown to a wide range of seismologists and therefore missing in earthquake catalogs. The data are related to such earthquakes as the 1715 (1716) Nerchinsk and 1727 Kirensk, and ones occurred in the southern Baikal region in 1809 and in the region near cape of Svyatoi Nos in 1862. Additional information about the earthquake of 1771 is given that considerably refines the parameters of this event. The earthquakes of 1725, 1742, 1772, and 1885 and interpretations of evidence doubted by various researchers are also considered.     

华北地区烈度衰减模型建立及其用于震中区域和震级的定量估算.

利用地震烈度资料定量估算历史地震震中区域和震级的方法是由Bakun和Wentworth于1997年首先提出的．该方法定量程度高,对烈度数据较少或发生在近海的历史地震的定位和震级估算尤为有效．按照Baku和Wentworth给出的思路,笔者尝试了对我国华北地区的历史和近代地震的震中和震级进行初步分析．首先,选取20世纪以来发生在该地区的10次有仪器记录的地震（5.3le;MSle;7.8）,对该地区的烈度-震级-震中距衰减关系进行标定并给出了烈度衰减模型,表明华北地区烈度随震中距增大而衰减的速率明显小于美国加州地区（约50%）．在此衰减模型的基础上,提出了确定震级和震中区域的网格搜索试算方法（GSTSL）,并给出了适用于华北地区的圈定震中区域和烈度震级的等值线置信值．最后,讨论了计算震中区域等值线时所引进的权因子Wi及其中参数b对震中区域等值线圈闭性的影响．利用该方法,对发生在1668年莒县——郯城地震,1679年三河——平谷地震,以及1966年隆尧地震和1969年渤海地震进行了分析．需要指出的是,该方法也可推广应用于我国其它历史地震资料丰富的地区．      

Histortical seismicity of the offshore Fujian-Guangdong region

The earthquakes offshore Fujian and Guangdong Provinces concentrated along the two segments near Nan’ao in the south and Quanzhou in the north of the off coast fault, which is very active since the late Pleistocene. In 1918 and 1906, two earthquakes with magnitudes 7.3 and 6.1 respectively occurred in the south and the north regions. With the instrumentally determined seismic parameters of these two earthquakes as standards, the author evaluated the parameters of the historical earthquakes by comparing their macroseismic materials with consideration of the geological background. As a result, chronological tables of historical earthquakes of the south and the north regions were compiled. The seismic activity of the two regions synchronized basically, and their strongest recorded earthquakes were both aroundM _s 7.3. Seismic activity usually intensified before the occurrence of strong events. Aftershocks were frequent, but strong aftershocks usually occurred one to several years after the main shock. Two high tides of seismic activity occurred since the late 15th century. Around 1600, eight earthquakes each with magnitudes over 4.3 occurred in both of the two regions. The magnitude of the strongest shock in the south region is 6.7, that in the north region is 7.5. The second high tide occurred at the early 20th century. Among the 18 earthquakes occurred in the south region, one was of magnitude 7.3; whilst only two earthquakes with magnitudes 6.1 and 5.5 respectively occurred in the north region. Further, medium to strong earthquakes never occurred since 1942. Whether this is the “mitigation effect” of strong shocks, or a big earthquake is brewing in the north region is worth intensive study. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 505–515, 1991. This work is supported by Chinese Joint Seismological Science Foundation.     

Evidence for ground-rupturing earthquakes on the Northern Wadi Araba fault at the archaeological site of Qasr Tilah, Dead Sea Transform fault system, Jordan

The archaeological site of Qasr Tilah, in the Wadi Araba, Jordan is located on the northern Wadi Araba fault segment of the Dead Sea Transform. The site contains a Roman-period fort, a late Byzantine–Early Umayyad birkeh (water reservoir) and aqueduct, and agricultural fields. The birkeh and aqueduct are left-laterally offset by coseismic slip across the northern Wadi Araba fault. Using paleoseismic and archaeological evidence collected from a trench excavated across the fault zone, we identified evidence for four ground-rupturing earthquakes. Radiocarbon dating from key stratigraphic horizons and relative dating using potsherds constrains the dates of the four earthquakes from the sixth to the nineteenth centuries. Individual earthquakes were dated to the seventh, ninth and eleventh centuries. The fault strand that slipped during the most recent event (MRE) extends to just below the modern ground surface and juxtaposes alluvial-fan sediments that lack in datable material with the modern ground surface, thus preventing us from dating the MRE except to constrain the event to post-eleventh century. These data suggest that the historical earthquakes of 634 or 659/660, 873, 1068, and 1546 probably ruptured this fault segment.     

Subduction of the Pacific plate under Kamchatka: Seismic velocity of underthrust

The results of studying the process of underthrusting in the Pacific plate under Kamchatka are presented, and the phenomena associated with this process are described. The seismic component of the velocity of the plate underthrust is estimated on the basis of (1) data from the CMT (Centroid Moment Tensor) catalog and (2) the sequence of the strongest Kamchatka earthquakes. A flat site with a strike azimuth of 217° and a dip angle of 25° located at depths of 30–70 km is assumed to be the interface between the plunging plate and Kamchatka. From CMT focal mechanisms, the underthrust velocity is estimated at V = 0.7 cm/yr for the southern zone (located south of Cape Shipunski) and at V = 1.1 cm/yr for the central zone (from Cape Shipunski to Cape Kronotski). From pairs of the strongest earthquakes that have occurred successively since 1737, the underthrust velocity for the southern zone is estimated at V = 6.6–7.1 cm/yr (from two pairs) and for the central zone, at V = 6.6 cm/yr. The creep portion of the underthrust amounts to 5–15% of the total velocity (the velocity of motion of the Pacific plate is 8 cm/yr).     

The present-day seismicity variations in the Kuril-Kamchatka seismic zone

This study is concerned with seismicity variations in Kamchatka and the Kuril Islands for the period 1962–2009; the effects of large earthquakes on the seismicity of adjacent areas are taken into account. The 1997 Kronotskii earthquake was followed by seismicity decreases in most areas over Kamchatka, which is presumably related to decreased tectonic stresses. After the 2007 Simushir earthquake synchronization and periodicities in seismicity were identified, indicating increased instabilities and the likelihood of a large event in Kamchatka in the near future. The instability of seismic regions is discussed within the framework of the theory of nonequilibrium dynamical systems. We suggest successive phases in the occurrence of seismological precursors.     

A comprehensive analysis of Kamchatka seismicity for the period 2005–2007 using the regional catalog

We characterize the Kamchatka seismicity for the period 2005–2007. Regional catalogs of Kamchatka earthquakes were used to develop 2D distributions of parameters of background seismicity. The characteristics we consider include the activity A ₁₀, the slope of the recurrence curve γ, the parameters involved in the methods RTL, ΔS, and the “Z-function”, as well as the control of earthquake clustering. We have detected the space-time agreement between the anomalies exhibited by several parameters.