Changes in Earthquake Source Properties across a Shallow Subduction Zone: Kamchatka Peninsula

—This paper studies the source properties of earthquakes originating within the shallow subduction zone near Kamchatka Peninsula. We use the regional catalog of 1962–1993 Kamchatkan earthquakes completed by the Institute of Volcanology, Russia. Our previous investigations (Zobin, 1990, 1996a) and this study allow us to show a gradual change in source properties of earthquakes from trench to coast.¶It was demonstrated that the swarm sequences change to the mainshock–aftershock sequences from trench to coast. The source area of aftershock sequences is generally smaller than the swarm areas for the same magnitude M _s of the mainshock or clue event of the swarm. Study of the M _s –K _s relation, where K _s is the energy class for Kamchatka earthquakes, reveals that the events radiate relatively higher frequencies from trench to coast.     

Simulations of Seismic Hazard for the Pacific Northwest of the United States from Earthquakes Associated with the Cascadia Subduction Zone

-- We investigate the impact of different rupture and attenuation models for the Cascadia subduction zone by simulating seismic hazard models for the Pacific Northwest of the U.S. at 2% probability of exceedance in 50 years. We calculate the sensitivity of hazard (probabilistic ground motions) to the source parameters and the attenuation relations for both intraslab and interface earthquakes and present these in the framework of the standard USGS hazard model that includes crustal earthquakes. Our results indicate that allowing the deep intraslab earthquakes to occur anywhere along the subduction zone increases the peak ground acceleration hazard near Portland, Oregon by about 20%. Alternative attenuation relations for deep earthquakes can result in ground motions that differ by a factor of two. The hazard uncertainty for the plate interface and intraslab earthquakes is analyzed through a Monte-Carlo logic tree approach and indicates a seismic hazard exceeding 1 g (0.2 s spectral acceleration) consistent with the U.S. National Seismic Hazard Maps in western Washington, Oregon, and California and an overall coefficient of variation that ranges from 0.1 to 0.4. Sensitivity studies indicate that the paleoseismic chronology and the magnitude of great plate interface earthquakes contribute significantly to the hazard uncertainty estimates for this region. Paleoseismic data indicate that the mean earthquake recurrence interval for great earthquakes is about 500 years and that it has been 300 years since the last great earthquake. We calculate the probability of such a great earthquake along the Cascadia plate interface to be about 14% when considering a time-dependent model and about 10% when considering a time-independent Poisson model during the next 50-year interval.     

Small-scale mantle convection system and stress field under the Pacific plate

The existence of nonhydrostatic high-degree harmonics in the gravitational field of the earth has recently been determined using satellite and gravity observations. In this paper, we have applied the Goddard Space Flight Center GEM-8 gravity field model to calculate the small-scale mantle flow system under the Pacific plate. The resulting tectonic forces or stresses exerted by the flow currents show tensional forces under the Hawaiian Island chain and a system of latitudinal convection rolls under the East Pacific plate and are in agreement with geophysical theories.     

Subduction and retreating of the western Pacific plate resulted in lithospheric mantle replacement and coupled basin-mountain respond in the North China Craton

The North China Craton (NCC) witnessed Mesozoic vigorous tectono-thermal activities and transition in the nature of deep lithosphere. These processes took place in three periods: (1) Late Paleozoic to Early Jurassic (～170 Ma); (2) Middle Jurassic to Early Cretaceous (160–140 Ma); (3) Early Cretaceous to Cenozoic (140 Ma to present). The last two stages saw the lithospheric mantle replacement and coupled basin-mountain response within the North China Craton due to subduction and retreating of the Paleo-Pacific plate, and is the emphasis in this paper. In the first period, the subduction and closure of the Paleo- Asian Ocean triggered the back-arc extension, syn-collisional compression and then post-collisional extension accompanied by ubiquitous magmatism along the northern margin of the NCC. Similar processes happened in the southern margin of the craton as the subduction of the Paleo-Tethys ocean and collision with the South China Block. These processes had caused the chemical modification and mechanical destruction of the cratonic margins. The margins could serve as conduits for the asthenosphere upwelling and had the priority for magmatism and deformation. The second period saw the closure of the Mongol-Okhotsk ocean and the shear deformation and magmatism induced by the drifting of the Paleo-Pacific slab. The former led to two pulse of N-S trending compression (Episodes A and B of the Yanshan Movement) and thus the pre-existing continental marginal basins were disintegrated into sporadically basin and range province by the Mesozoic magmatic plutons and NE-SW trending faults. With the anticlockwise rotation of the Paleo-Pacific moving direction, the subduction-related magmatism migrated into the inner part of the craton and the Tanlu fault became normal fault from a sinistral one. The NCC thus turned into a back-arc extension setting at the end of this period. In the third period, the refractory subcontinental lithospheric mantle (SCLM) was firstly remarkably eroded and thinned by the subduction-induced asthenospheric upwelling, especially those beneath the weak zones (i.e., cratonic margins and the lithospheric Tanlu fault zone). Then a slightly lithospheric thickening occurred when the upwelled asthenosphere got cool and transformed to be lithospheric mantle accreted (～125 Ma) beneath the thinned SCLM. Besides, the magmatism continuously moved southeastward and the extensional deformations preferentially developed in weak zones, which include the Early Cenozoic normal fault transformed from the Jurassic thrust in the Trans-North Orogenic Belt, the crustal detachment and the subsidence of Bohai basin caused by the continuous normal strike slip of the Tanlu fault, the Cenozoic graben basins originated from the fault depression in the Trans-North Orogenic Belt, the Bohai Basin and the Sulu Orogenic belt. With small block size, inner lithospheric weak zones and the surrounding subductions/collisions, the Mesozoic NCC was characterized by (1) lithospheric thinning and crustal detachment triggered by the subduction-induced asthenospheric upwelling. Local crustal contraction and orogenesis appeared in the Trans-North Orogenic Belt coupled with the crustal detachment; (2) then upwelled asthenosphere got cool to be newly-accreted lithospheric mantle and crustal grabens and basin subsidence happened, as a result of the subduction zone retreating. Therefore, the subduction and retreating of the western Pacific plate is the outside dynamics which resulted in mantle replacement and coupled basin-mountain respond within the North China Craton. We consider that the Mesozoic decratonization of the North China Craton, or the Yanshan Movement, is a comprehensive consequence of complex geological processes proceeding surrounding and within craton, involving both the deep lithospheric mantle and shallow continental crust.     

Seismic investigation of the roots of the Kliuchevskaya group volcanoes,Kamchatka

Earthquake data from Δ<2° and of indermediate depth of focus in the upper mantle near the Kliuchevskaya volcanic region show a screening effect of longitudinal and transverse waves. The indicated anomalous zones are identified with the region of magma localization. The anomalous zones are at 35–110 km depths.     

Increment of Macroseismic Intensity at Seismic Stations of the Kamchatka Region Relative to the Petropavlovsk Seismic Station

The paper presents the method and results of calculating the increment of macroseismic intensity at seismic stations of Kamchatka. Calculation is based on measurement of the relative level of maximum accelerations of intense earth vibrations in the phase of S-waves of comparatively strong regional earthquakes and the root-mean-square deviation of acceleration in the phase of P-waves of a strong distant earthquake. In the latter case, records of an earthquake with a magnitude of 9.1, which occurred in Japan on March 11, 2011, were used. The Petropavlovsk seismic station was used as the reference station. At the foundation of this station rests on rocky soil composed of siliceous shales. An estimate of the increment for the majority of digital stations is presented. Anomalously high intensity values were noted at a number of stations. The data obtained are used to assess the properties of soils in the investigated area. At several stations, the intensity of the horizontal component of soil vibrations above the intensity of the vertical component is much greater than the corresponding design value, which is probably due to the presence of resonant soil layers under these stations. The discrepancy in the incremental intensity estimates from records of intense oscillations from regional earthquakes and from records of a very strong remote earthquake obtained from sensors located in basements of heavy-frame concrete structures is revealed. To avoid distortion in recording ground vibrations, it is desirable to place seismic instruments far from such structures. The results obtained in the study can be used for seismic microzoning of construction sites in the investigated territory.     

Potential radionuclide transport pathways from seafloor dumpsites: Kamchatka region of the North Pacific Ocean

Encapsulated nuclear waste materials, dumped by Russia, are present at two deepwater seafloor locations in the offshore north-west Pacific Ocean, south-east of the Kamchatka Peninsula. This paper assesses potential pathways by which these wastes might, if released from their containers, disperse away from the dumpsites and through the surrounding ocean. A review of large-scale ocean circulation theory and of field and model results suggests that mean abyssal currents are north-eastward to eastward from the dumpsite locations and would advect leaking materials toward the north-eastern Pacific. Results of advective and diffusive horizontal plume transport models are consistent with this sense of flow. Trajectory speeds are, however, subject to considerable uncertainty. Our results suggest that as little as 5 years or as long as 100 years might be required for material to be transported from the dump sites to the north-east Pacific. Dilution by 4 or 5 orders of magnitude is predicted during this transit. Vertical mixing or upwelling are necessary in order to transport contaminants upward from north-east Pacific abyssal waters to the near-surface layers before they can potentially impact productive coastal regions, such as those off Alaska. Information concerning such upwelling mechanisms is inadequate for estimation of rates or to identify geographical areas that might be at risk.     

Probabilistic Estimates of Hypocenters from the Data of Kamchatka Seismic Network Stations

Izvestiya, Physics of the Solid Earth - A new approach is proposed for determining earthquake hypocenters aimed at a more comprehensive characterization of its uncertainty and ambiguity....     

菲律宾群岛深部速度结构成像与双向俯冲板片构造特征

菲律宾群岛受到欧亚板块、菲律宾海板块和印度-澳大利亚板块的碰撞作用,地质环境复杂,构造因素多样.尽管近几年来已经有了少数关于该区域层析成像的研究,但这些研究的区域主要集中在马尼拉海沟、吕宋岛及中菲律宾地区,而关于群岛周围其他海沟和南菲律宾地区的讨论相对较少.到目前为止,还没有同时获得过关于菲律宾群岛深部纵、横波速度结构的研究,本次研究通过反演155779条P波震相和59642条S波震相,同时获得了菲律宾群岛从地表至150 km深度的纵、横波速度结构.地震层析成像结果表明该地区的壳幔速度结构具有较强的不均一性,地壳内部存在着广泛的低速异常,而表征俯冲板块的高速异常则沿着群岛周边的海沟展布.南海块体在马尼拉海沟中段的俯冲角度和俯冲活动性比南段小;菲律宾海板块在东吕宋海槽南段微弱的俯冲作用很有可能同本哈姆海台的碰撞有关.菲律宾群岛大部分M_W＞6.0的强震沿着各个板块的边界发生,体现出菲律宾海板块同欧亚板块之间的强耦合作用,群岛西侧的南海块体在马尼拉海沟16°N-20°N之间呈现出的弱耦合状态可能跟北吕宋地区的拉张应力环境有关,南海块体在16°N以南的地区同上覆块体之间的耦合作用较强;群岛东侧的菲律宾海板块在14°N以北的地区没有强震发生,它与菲律宾群岛之间的耦合程度从北向南逐渐增强,在12°N以南的地区要强于12°N以北的地区;此外苏禄海盆和菲律宾构造带之间也存在着强耦合关系.

     

Seismic gaps and plate tectonics: Seismic potential for major boundaries

The theory of plate tectonics provides a basic framework for evaluating the potential for future great earthquakes to occur along major plate boundaries. Along most of the transform and convergent plate boundaries considered in this paper, the majority of seismic slip occurs during large earthquakes, i.e., those of magnitude 7 or greater. The concepts that rupture zones, as delineated by aftershocks, tend to abut rather than overlap, and large events occur in regions with histories of both long- and short-term seismic quiescence are used in this paper to delineate major seismic gaps.In detail, however, the distribution of large shallow earthquakes along convergent plate margins is not always consistent with a simple model derived from plate tectonics. Certain plate boundaries, for example, appear in the long term to be nearly aseismic with respect to large earthquakes. The identification of specific tectonic regimes, as defined by dip of the inclined seismic zone, the presence or absence of aseismic ridges and seamounts on the downgoing lithospheric plate, the age contrast between the overthrust and underthrust plates, and the presence or absence of back-arc spreading, have led to a refinement in the application of plate tectonic theory to the evaluation of seismic potential.The term seismic gap is taken to refer to any region along an active plate boundary that has not experienced a large thrust or strike-slip earthquake for more than 30 years. A region of high seismic potential is a seismic gap that, for historic or tectonic reasons, is considered likely to produce a large shock during the next few decades. The seismic gap technique provides estimates of the location, size of future events and origin time to within a few tens of years at best.The accompanying map summarizes six categories of seismic potential for major plate boundaries in and around the margins of the Pacific Ocean and the Caribbean, South Sandwich and Sunda (Indonesia) regions for the next few decades. These categories range from what we consider high to low potential for being the site of large earthquakes during that period of time. Categories 1, 2 and 6 define a time-dependent potential based on the amount of time elapsed since the last large earthquake. The remaining categories, 3, 4, and 5, are used for areas that have ambiguous histories for large earthquakes; their seismic potential is inferred from various tectonic criteria. These six categories are meant to be interpreted as forecasts of the location and size of future large shocks and should not be considered to be predictions in which a precise estimate of the time of occurrence is specified.Several of the segments of major plate boundaries that are assigned the highest potential, i.e., category 1, are located along continental margins, adjacent to centers of population. Some of them are hundreds of kilometers long. High priority should be given to instrumenting and studying several of these major seismic gaps since many are now poorly instrumented. The categories of potential assigned here provide a rationale for assigning prorities for instrumentation, for future studies aimed at predicting large earthquakes and for making estimates of tsunami potential.Lamont-Doherty Geological Observatory Contribution No. 2906.     

用远震接收函数研究西安地震台台基的一维速度模型

以西安地震台1992年至2000年记录到的853个远震宽频带数字地震记录为基础,利用远震接收函数的方法反演了西安地震台台基的一维速度结构及莫霍面深度,结果表明,西安地震台下的莫霍面深度为39.7 km,P波速度6.59 km/s,P波与S波速度比为1.65.在反演台下一维速度模型时,对接收函数分别进行了算术平均和4次方根的叠加,对这两种不同的叠加得出了相应的反演结果.由于反演结果的非唯一性,在计算的过程中,加入了平滑参数来约束反演结果,并选择了0.2,0.3,0.4等3个平滑约束值,获得了比较理想的台基一维速度模型.     

Seismic evidence for a metastable olivine wedge in the subducting Pacific slab under Japan Sea

We apply a forward-modeling approach to high-quality arrival time data from 23 deep earthquakes greater than 400 km depth to investigate the detailed structure of the subducting Pacific slab beneath the Japan Sea. Our results show that a finger-like anomaly exists within the subducting Pacific slab below 400 km depth, which has a P-wave velocity 5% lower than the surrounding slab velocity (or 3% lower than that of the normal mantle), suggesting the existence of a metastable olivine wedge (MOW) in the slab. The MOW top and bottom depths are 400 and 560 km, respectively. The MOW is estimated to be about 50 km wide at 400 km depth and close to the slab upper boundary. At 560 km depth the MOW is located at about 25 km below the slab upper boundary. Most of the deep earthquakes are located in the MOW. Our results favor transformational faulting as the mechanism for deep earthquakes.     

A study of the velocity structure for the Benioff zone of Kamchatka: The Avacha Bay-Kronotskii Bay Segment

We present computations of 3D P-wave velocity field for the segment of the Benioff zone of Kamchatka between 51.5° N to Avacha Bay to Cape Shipunskii to Kronotskii Bay. P-wave travel times for regional earthquakes were used to compute and construct 3D models of the velocity field for this segment of the Benioff zone. We examined the velocity structure to interpret it in conjunction with neotectonic morphostructures. The computations showed a complex structure of the field and the presence of inhomogeneities, both along and across the Benioff zone. Of special interest are the results for the Avacha and Kronotskii basins, where high velocity masses are unusually shallow beneath the low velocity layers in the top of the earth section. This creates high-gradient zones that can possibly generate large earthquakes.     

基于广义帕累托分布的马尼拉海沟俯冲带地震危险性估计

选取马尼拉海沟俯冲带作为潜源区, 基于广义帕累托分布, 通过对一定时段内超过某一阈值的震级数据进行拟合, 建立该潜源区地震危险性估计模型, 估计强震重现水平和震级上限, 并对估计结果的不确定性进行了分析, 得到马尼拉海沟俯冲带震级上限为9.0级, 10 a、 50 a、 100 a、 200 a马尼拉海沟俯冲带的震级重现水平期望值分别为7.1级、 7.6级、 7.7级、 7.9级。     

Determining the Double Couple Moment Tensor for Kamchatka Earthquakes from Regional Seismic Waveforms

Izvestiya, Physics of the Solid Earth - Abstract—The digital seismic network in Kamchatka deployed in 2006–2010 provided a fundamental possibility for calculating seismic moment tensor...     

利用中国地震台网（CSN）记录到的PKP波研究中太平洋下地幔底部的小尺度P波速度变化

PKP震相包含了下地幔底部P波速度结构的重要信息.中国地震台网（CSN）台站记录到的南美洲地震的PKP波的射线,对中太平洋下的D″层有很好的采样.本研究采用这些PKP波的AB和DF两个分支的走时差,研究了中太平洋下地幔底部P波速度的小尺度变化.AB-DF的走时差减小了上地幔横向不均匀性的影响,而对下地幔底部P波速度的横向变化十分敏感.与此同时,AB-DF的走时差也减小了地震定位误差的影响,消除地震发震时间测定误差的影响.本研究的结果表明,在中太平洋的地幔底部存在着大范围的AB-DF走时正残差,也即低速异常区,这可能是太平洋下超大地幔热柱的源处.观测到的P波速度异常的空间分布总体上与Grand通过层析成像得到的CMB的S波的速度异常相一致,并在变化的幅度上很好地相关,P波速度的扰动值（在D″层大约为2%）是Grand 速度模型中的S波异常的36%.这一结果有助于太平洋下超大地幔热柱的结构和性质的进一步研究.     

太平洋板块俯冲对长白山天池火山的影响

长白山天池火山是我国最具潜在喷发危险的活火山之一,属于远离海沟的陆内火山.太平洋板块平卧于长白山天池火山的下方,其俯冲过程对长白山天池火山形成及活动具有重要意义.本文通过二维热力学耦合的数值模型对海洋板块俯冲的动力学过程进行了模拟,分析了海洋板块俯冲对远离海沟的陆内火山的深部温压条件、速度场、岩浆补给量等的影响.依据模拟结果探讨了太平洋板块对长白山天池火山活动影响的可能性及方式.

     

Sediment Yield of Kamchatka Krai Rivers into the Pacific Ocean and the Seas of Bering and Okhotsk

Sediment yield of Kamchatka Krai rivers into the Pacific Ocean and the seas of Bering and Okhotsk, is evaluated, including that from the drainage basins that are not covered by regular observations. Regression dependences of specific suspended sediment yield (SSSY) on factors determining it were constructed for erosion regions identified in Kamchatka Krai. The total average many-year yield of suspended sediments of Kamchatka Krai rivers into the Pacific is 11.4 × 10⁶ t/year, 73.9% of which is transported by rivers of the eastern coast and 26.1%, by those of the western coast. Among other factors, such distribution is attributed to the location of the majority of volcanoes on Kamchatka eastern coast. About one third of all sediments transported from Kamchatka Krai territory is discharged by the two largest rivers in the region, i.e., the Penzhina and Kamchatka.     

俯冲带中的洋壳在中国东北上地幔底部分离的地震学证据

中国东北西太平洋俯冲带位于欧亚板块与西太平洋板块的交界部位,西太平洋板块的俯冲运动从日本海沟开始一直延伸到了中国东北地区.层析成像(Gudmundsson & Sambridge,1998;Huang & zhao,2006)结果表明,西太平洋板块和该区域'660'的相互作用,导致了该区域上地幔结构的复杂性.