Anomalously large seismic amplifications in the seafloor area off the Kii peninsula

Seismic wave amplifications were investigated using strong-motion data obtained from the ground’s surface (K-net) on the Kii peninsula (southwestern Japan) and from the network of twenty seismic stations on the seafloor (DONET) located off the peninsula near the Nankai trough. Observed seismograms show that seismic signals at DONET stations are significantly larger than those at K-net stations, independent of epicentral distances. In order to investigate the cause of such amplifications, seismic wavefields for local events were simulated using the finite-difference method, in which a realistic 3D velocity structure in and around the peninsula was incorporated. Our simulation results demonstrate that seismic waves are significantly amplified at DONET stations in relation to the presence of underlying low-velocity sediment layers with a total thickness of up to 10 km. Our simulations also show considerable variations in the degree of amplification among DONET stations, which is attributed to differences in the thickness of the sediment layers. The degree of amplification is relatively low at stations above thin sediment layers near the trough axis, but seismic signals are much more amplified at stations closer to the Kii peninsula, where sediment layers are thicker than those at the trough axis. Simulation results are consistent with observations. This study, based on seafloor observations and simulations, indicates that because seismic signals are amplified due to the ocean-specific structures, the magnitude of earthquakes would be overestimated if procedures applied to data observed at land stations are used without corrections.     

Near-field tsunami amplification factors in the Kii Peninsula,Japan for Dense Oceanfloor Network for Earthquakes and Tsunamis (DONET)

We investigated the correlation between coastal and offshore tsunami heights by using data from the Dense Oceanfloor Network for Earthquakes and Tsunamis (DONET) observational array of ocean-bottom pressure gauges in the Nankai trough off the Kii Peninsula, Japan. For near-field earthquakes, hydrostatic pressure changes may not accurately indicate sea surface fluctuations, because ocean-bottom pressure gauges are simultaneously displaced by crustal deformation due to faulting. To avoid this problem, we focused on the average waveform of the absolute value of the hydrostatic pressure changes recorded at all the DONET stations during a tsunami. We conducted a Monte Carlo tsunami simulation that revealed a clear relationship between the average waveforms of DONET and tsunami heights at the coast. This result indicates the possibility of accurate real-time prediction of tsunamis by use of arrays of ocean-bottom pressure gauges.     

A nonlinear parametric model based on a power law relationship for predicting the coastal tsunami height

When a subduction-zone earthquake occurs, the tsunami height must be predicted to cope with the damage generated by the tsunami. Therefore, tsunami height prediction methods have been studied using simulation data acquired by large-scale calculations. In this research, we consider the existence of a nonlinear power law relationship between the water pressure gauge data observed by the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) and the coastal tsunami height. Using this relationship, we propose a nonlinear parametric model and conduct a prediction experiment to compare the accuracy of the proposed method with those of previous methods and implement particular improvements to the extrapolation accuracy.

     

Possible triggering relationship of six M_w>6 earthquakes in 2018–2019 at Philippine archipelago

Philippine archipelago(PA) has strong background seismicity, but there is no systematic study of earthquake triggering in this region. There are six earthquakes(M_w 6) occurred between 2018/12/29 and 2019/09/29 in PA,which provides an excellent opportunity to investigate the triggering relationship among these events. We calculate the static Coulomb stress changes of the first five events, and find that the local seismicity after the2018/12/29 M_w 7.0 earthquake is mostly associated with positive Coulomb stress changes, including the2019/05/31 M_w 6.1 event, suggesting a possible triggering relationship. However, we cannot rule out the dynamic triggering mechanism, due to increased microseismicity in both positive and negative stress change regions, and an incomplete local catalog, especially right after the first M_w 7.0 mainshock. The dynamic stresses from these M_w 6 events are large enough(from 5 kPa to 3 532 kPa) to trigger subsequent events, but a lack of seismicity and waveform evidence does not support delayed dynamic triggering among these events, even the shortest time interval is less than 24 hours. In the past 45 years, the released seismic energy shows certain peaks every 5–10 years. However, earthquakes with M_w 6.0 were relatively infrequent between 2004 and 2018 at PA. Hence, it is possible that several regions are relatively late in their earthquake cycles, which would enhance their susceptibility of being triggered by earthquakes at nearby and regional distances.     

台湾海峡及其西边地区正常地震动态及危险性特征

本文从地震的时，空分布特征，区域应力场动态，能量释放方式，ｂ值及震群特征６个方面研究了台湾海峡及其西边地区地震活动的正常动态及异常特征。结果表明，具有前兆意义的变化模式表现为区域地震活动在时间，空间及功能方面的有序性变化，即：（ａ）地震空间分布由分散转为集中，形成条带或空区；（ｂ）断裂活动由多组转为单一，应力场趋向一致；（display status     

Historical Gorda Ridge t-phase swarms: relationships to ridge structure and the tectonic and volcanic state of the ridge during 1964–1966

The U.S. Navy’s Sound Surveillance System (SOSUS) hydrophone arrays are extemely efficient receptors of a high-frequency earthquake energy phase known as the t(ertiary)-wave, or t-phase (Fox et al., 1994). After a nearly 30-year hiatus in such studies, SOSUS arrays are again being utilized to detect t-phases and to locate seismic and volcanic events occurring along the Gorda seafloor spreading center (Fox et al., 1995; Fox and Dziak, 1998). Earlier, Northrop et al. (1968) also used other military arrays to infer tectonic structure along the Gorda Ridge. From October 1964 through December 1966, over 600 low-magnitude earthquakes occurred along the Gorda Ridge. Nearly all of these events had magnitudes below the detection thresholds of land-based seismic networks. Northrop et al. (1968) interpreted the geographic distribution of these events as evidence for a nascent fracture zone near the midpoint of the ridge. In the present study, the spatial distributions of these older data and, for the first time, their temporal distributions as well, were examined with respect to detailed bathymetry of the ridge that was acquired in the early 1980s. This analysis, of 570 on-axis and 74 off-axis events, led to the following observations: (1) nearly all of the Gorda Ridge t-phase events occurred in discreet swarms centered about the ridge axis, (2) most of the events within each of 8 (of 9) observed swarms occurred mainly along single ridge segments, and, (3) reconfirming the earlier Northrop et al. (1968) conclusion, most of the events originated in the region of a major change in the strike of the ridge axis. During the 27-month interval that the ridge was observed, relatively few t-phase events took place along the northernmost segment of the Gorda Ridge where the 1996 eruption occurred. However, a unique sequence of small events which visually resemble the events associated with a Juan de Fuca Ridge eruption in 1993 (Fox et al., 1995) and a Gorda Ridge eruption in 1996 (Fox and Dziak, 1998) may have been associated with an eruption on the ridge during 1965.     

黄河口邻近海域海冰变化特征及机制分析

黄河口邻近海域海冰是渤海海冰的一部分,为了解其独特的变化特征及机制,本研究基于北海预报中心提供的黄河口周边海洋台站观测数据以及CMEMS (Copernicus Marine Environment Monitoring Service)全球海冰密集度再分析数据,使用统计分析和两种滑动相关分析,结合小波相干方法及大气过程的影响,得到长期变化分析的结果。黄河口冰情在1979—2020年间整体呈减轻趋势(–0.25%/a),显然其直接因素为局地温度整体升高;海冰密集度与黄河径流量呈明显正相关,相关系数为0.46,其原因为径流增大导致盐度降低,海冰增加;与北极涛动指数(AOI,Arctic Oscillation Index)呈明显负相关,相关系数为–0.44,因为当北极涛动为正位相时,东亚大槽强度减弱,北极冷空气南侵受阻隔,冬季黄河口的整体气温升高,导致海冰减少;1997年和2016年左右与北极涛动的相关性都出现了显著正异常,其原因为两次强厄尔尼诺事件的影响,同时海冰密集度在1985年左右的跃变可能与AOI和黄河径流量的突变有关。短期变化分析的结果显示:从2010年和2020年冬季逐日的典型寒潮过程与海冰密集度的变化分析可知,海冰与前6 d负积温的相关性最大,平均相关系数为–0.77,寒潮的出现时间、强度及间隔,控制海冰的生成,而整体气温的低频变化控制海冰的维持和发展。     

有限断层模型在2015年9月16日智利Mw8.3级地震海啸数值模拟中的应用与评估

2015年9月16日22时54分（当地时间）智利中部近岸发生M_w8.3级地震,震源深度25 km。同时,强震的破裂区长200 km,宽100 km,随之产生了中等强度的越洋海啸。海啸影响了智利沿岸近700 km的区域,局部地区监测到近5 m的海啸波幅和超过13 m的海啸爬坡高度。太平洋区域的40多个海啸浮标及200多个近岸潮位观测站详细记录了此次海啸的越洋传播过程,为详细研究此次海啸近场及远场传播及演化规律提供了珍贵的数据。本文选择有限断层模型和自适应网格海啸数值模型建立了既可以兼顾越洋海啸的计算效率又可以实现近场海啸精细化模拟的高分辨率海啸模型。模拟对比分析了海啸的越洋传播特征,结果表明采用所建立的模型可以较好地再现远场及近场海啸特征,特别是对近场海啸的模拟结果非常理想。表明有限断层可以较好地约束近场、特别是局部区域的破裂特征,可为海啸预警提供更加精确的震源信息,结合高分辨率的海啸数值预报模式实现海啸传播特征的精细化预报。本文结合观测数据与数值模拟结果初步分析了海啸波的频散特征及其对模型结果的影响。同时对观测中典型的海啸波特征进行的简要的总结。谱分析结果表明海啸波的能量主要分布在10~50 min周期域内。这些波特征提取是现行海啸预警信息中未涉及,但又十分重要的预警参数。进一步对这些波动特征的详细研究将为海啸预警信息及预警产品的完善提供技术支撑。     

Numerical assessment of bathymetric changes caused by the 2004 Indian Ocean tsunami at Kirinda Fishery Harbor,Sri Lanka

Thus far various numerical models have been developed and improved to aid understanding of the sediment transport process due to tsunamis. However, the applicability of these models for the field-scale bathymetric change remains a major issue due to the scarcity of measured bathymetric data immediately before and after tsunamis. This study focuses on assessing the applicability of the sediment transport model by comparing the model results with measured bathymetry data obtained one month before and two months after the 2004 Indian Ocean tsunami at Kirinda Fishery Harbor, Sri Lanka. Obtained model results were compared with measured data along four different transects. In particular, similar to the measured data, the model reproduced the bed level change at the harbor mouth well, although it shows some discrepancy on bathymetric change along the shoreline, which is directly affected by littoral drift. Therefore, it is noted that the divergence of reproducing the local bathymetry change is due to the normal wind wave effect on measured data and the model limitations. Hence we included the wind wave effect in modeled data and the discrepancy between measured and modeled data was reduced. Furthermore, the modeled bed level change indicates a dynamic behavior in terms of the net variation during the tsunami flow, such that deposition dominates in the inflow and erosion dominates in the backflow. Both bed level variation and the suspended load concentration reveal that the large amount of eroded sediment attributable to tsunami waves was in suspended form and was deposited in the nearshore area after the water fluctuation had abated. The model results further indicate that eroded sediment at the initial depth deeper than 11 m might be brought by the incoming tsunami waves and deposited in the nearshore area where the depth is shallower than 7 m.     

2017年9月8日墨西哥沿岸Mw8.2级地震海啸观测数据分析与模拟

2017年9月8日4时49分（UTC）,墨西哥瓦哈卡州沿岸海域（15.21°N,93.64°W）发生M_w8.2级地震,震源深度30 km。强震在该海域引发海啸,海啸对震源附近数百千米范围内造成了严重影响。位于太平洋上的多个海啸监测网络捕捉到了海啸信号并详细记录了此次海啸的传播过程。本文选用了近场2个DART浮标和6个验潮站的水位数据,通过潮汐调和分析和滤波分离出海啸信号,对近场海啸特征值进行了统计分析,并采用小波变换分析方法进一步分析了海啸的波频特征。基于Okada弹性位错理论断层模型计算得到了强震引发的海底形变分布,并采用MOST海啸模式对本次海啸事件近场传播特征进行了模拟,模拟结果与观测吻合较好。最后,基于实测和模拟结果,详细分析了此次地震海啸的近场分布特征,发现除受海啸源的强度和几何分布特征影响外,近岸海啸波还主要受地形特征控制,在与特定地形相互作用后波幅产生放大效应,会进一步加剧海啸造成的灾害。     

Vertical zonation and aggregated distribution of the Manila clam on subtidal sand flats in a coastal brackish lagoon along the Sea of Japan

The Manila clam, Ruditapes philippinarum, has maintained small‐sized populations in a semi‐enclosed brackish lake along the Sea of Japan, the Honjo area of Lake Nakaumi, although the environment and biota of this area have changed dramatically due to a large‐scale reclamation project. There should be underlying processes that enable the restoration of this species from small‐sized populations, such as the existence of source (i.e. reproductive) populations in other areas and depth zones of the lake. However, there has been no robust, properly designed evaluation of the distribution of the Manila clam in the subtidal sand flats. In order to elucidate the possible mechanisms that allow for the persistence of populations of the Manila clam, we examined the spatiotemporal and vertical variation in distributions of 0‐age clams in the subtidal zone of sand flats. Seasonal effects on population variations showed erratic changes among depth zones without a decreasing trend along the depth gradient. Further, many local populations became extinct even in the shallower zones due to seasonal (summer) hypoxia at deeper zones and hypoxia by the accumulation of key benthic species (Asian mussel and decaying macroalgae) in mats at shallower zones. A few surviving local populations were stable with a spatial‐fragmental (patchy) distribution, associated with fragmented accumulations of Asian mussels and macroalgae. Efforts to maintain stable populations and to restore this species in the subtidal area may depend on these small, restricted, patchy local populations. These findings suggest that high fertility and productivity of the Manila clam as well as patchy distribution of small populations may contribute to the maintenance of the population and the avoidance of extinction (by spatially diffusing the risk of extinction) in harsh environments resulting from the reclamation project.     

北极夏季海冰反照率的观测和数值模拟试验

在中国第3次北极科学考察浮冰站开展了积雪/海冰反照率观测.本文对观测结果进行了分析,并结合一维高分辨雪/冰模式(HIGHTSI)对3个常用的反照率参数化方案在天气尺度的表现进行了评估.观测期间测站反照率变化范围0.75～0.85,其天气尺度变化同天气和表面冰、雪状况紧密相关,降雪和吹雪过程可改变表面积雪厚度及水平分布,...     

盐度对变化2014年东北太平洋“暖泡”的作用

A significant strong, warm "Blob"(a large circular water body with a positive ocean temperature anomaly)appeared in the Northeast Pacific(NEP) in the boreal winter of 2013–2014, which induced many extreme climate events in the US and Canada. In this study, analyses of the temperature and salinity anomaly variations from the Array for Real-time Geostrophic Oceanography(Argo) data provided insights into the formation of the warm"Blob" over the NEP. The early negative salinity anomaly dominantly contributed to the shallower mixed layer depth(MLD) in the NEP during the period of 2012–2013. Then, the shallower mixed layer trapped more heat in the upper water column and resulted in a warmer sea surface temperature(SST), which enhanced the warm"Blob". The salinity variability contributed to approximately 60% of the shallowing MLD related to the warm"Blob". The salinity anomaly in the warm "Blob" region resulted from a combination of both local and nonlocal effects. The freshened water at the surface played a local role in the MLD anomaly. Interestingly, the MLD anomaly was more dependent on the local subsurface salinity anomaly in the 100–150 m depth range in the NEP.The salinity anomaly in the 50–100 m depth range may be linked to the anomaly in the 100–150 m depth range by vertical advection or mixing. The salinity anomaly in the 100–150 m depth range resulted from the eastward transportation of a subducted water mass that was freshened west of the dateline, which played a nonlocal role.The results suggest that the early salinity anomaly in the NEP related to the warm "Blob" may be a precursor signal of interannual and interdecadal variabilities.     

中国沿海验潮站及其邻近地区陆地垂直运动分析

利用中国沿岸验潮站GNSS和邻近地区陆态网络GNSS基准站观测数据,结合卫星高度计和验潮站海平面观测数据分析了中国沿海验潮站及其邻近地区陆地垂直运动特征。中国沿海海平面观测以及验潮站和陆态网GNSS基准站观测结果显示,中国沿海省区市及沿海验潮站陆地垂直运动总体表现为：辽宁至江苏沿海上升、上海至福建泉州沿海沉降、福建厦门至广西沿海升降交替的格局,局部滨海平原地区如华北平原天津南部、河北平原的沧县则表现出显著的沉降特征。验潮站陆地的抬升与沉降是沿海相对海平面变化的重要组成部分,准确掌握验潮站及其邻近区域的陆地垂直运动特征,可为沿海相对海平面变化分析、海平面变化影响评估以及未来海平面上升预测提供依据。     

Sediment transport and morphodynamics of the Douro River estuary

A combination of dredging data, hydrographic surveys and numerical modelling has been used to assess morphological change and sediment transport in the Douro River estuary. The system is dominated by sand- and gravel-sized sediments and confined by resistant rock types. The evolution of the bed in the last 20 years has been strongly influenced by the opening of a navigation channel. According to the data available to date, the average maintenance dredging volume has been of the order of 0.4 × 10⁶ m³ year⁻¹. Comparisons of hydrographic surveys reveal a rate of volume loss of the same magnitude. Apparently, maintenance dredging mainly involves local material, transported into the channel from shallower areas of the estuary. The results of numerical modelling indicate that the sediment transport capacity due to tidal currents is very limited. River flood events increase the transport capacity by several orders of magnitude, thus playing a critical role in sediment redistribution and supply to the coast. The average sediment transport capacity is estimated to be of the order of 0.1 × 10⁶ m³ year⁻¹ in most of the estuary and 0.5 × 10⁶ m³ year⁻¹ at the inlet, with a large uncertainty. It is concluded that, if morphological stability is set as an environmental objective, the dredged material should not be removed from the system but rather be used to nourish the estuarine beaches and the barrier spit.     

Pliocene calcium carbonate sedimentation patterns of the Ontong Java Plateau: ODP Sites 804 and 806

Two deep-sea cores from the northeastern slope of the Ontong Java Plateau, Ocean Drilling Program Site 806 (2520 mbsl) and Site 804 (3861 mbsl), has been studied with the purpose of investigating Pliocene (Site 806, 2.4–3.8 Ma; Site 804, 1.8–4.4 Ma) sub- and supralysoclinal changes in calcium carbonate accumulation, based on CaCO₃, coarse-fraction and faunal data. At the shallower site, Site 806, there is a slight decrease in carbonate content and a distinct long-term trend in decreasing accumulation rate throughout the studied interval. Small-scale changes in density, hence accumulation rate, are related to grain-size changes due to winnowing at this site, but the overall pattern of decreasing carbonate accumulation seems to be the result of both winnowing and a decrease in productivity. The deeper site, Site 804, exhibits a general increase in carbonate content after about 3.15 Ma as well as a long-term increase in mass accumulation rate throughout the Pliocene interval studied. Detailed comparison of the records of density and carbonate content emphasizes the strong relationship between carbonate content and accumulation rate at this site. The observed pattern of decreasing accumulation rate at shallow depth parallel to enhanced preservation at greater depths may be explained by a decrease in the production of North Atlantic Deep Water (NADW). A reduction of the volume of NADW pumped into the Pacific would reduce the supply of nutrients and deep-water CO₂. This could account for both the decrease in accumulation rate at the shallower site, due to decreased productivity, and the enhanced preservation at the deeper site, due to a decrease in the corrosiveness of deep waters.     

Ring-Shaped Seismicity Structures in the Areas of Sarez and Nurek Water Reservoirs (Tajikistan): Lithosphere Adaptation to Additional Loading

Seismicity characteristics in the areas of Sarez Lake and the Nurek water reservoir are studied. Ring-shaped seismicity structures in two depth ranges (0–33 and 34–70 km) formed prior to the Pamir earthquake of December 7, 2015 (M _w = 7.2). Seismicity rings cross each other near the Usoi Dam, which formed after the strong earthquake in 1911 and led to the formation of Sarez Lake, and near the epicenter of the Pamir earthquake. In addition, three out of the four strongest events (М ≥ 6.0) recorded in the Pamir region at depths of more than 70 km since 1950 have occurred near Sarez Lake. An aggregate of the data allows us to conclude that the Pamir earthquake, despite its very large energy, refers to events related to induced seismicity. Ring-shaped seismicity structures in two depth ranges also formed in the Nurek water reservoir area. It is supposed that the formation of ring-shaped structures is related to the self-organization processes of a geological system, which result in the ascent of deep-seated fluids. In this respect, the lithosphere is gradually adapting to the additional load related to the filling of the water reservoir. The difference between Nurek Dam (and many other hydroelectric power stations as well) and Usoi Dam is the permanent vibration in the former case due to water falling from a height of more than 200 m. Such an effect can lead to gradual stress dissipation, resulting in the occurrence of much weaker events when compared to the Pamir earthquake of December 7, 2015, in the areas of artificial water reservoirs.     

Earthquake activity in the Southern Vanuatu arc recorded by the Texas digital OBS

We have reconfigured the Texas digital ocean bottom seismograph (OBS) to operate in a triggered mode and record regional earthquake signals. This paper reports the results of a deployment program designed to test these digital OBS, by moni toring earthquake activity in and near the trench in southern Vanuatu (formerly, the New Hebrides). We successfully recorded hundreds of earthquakes, including 133 located regional earth quakes recorded by three or more stations. We also report J-B residuals for 21 earthquakes reported and located by the ISC. Fourier analysis of seismograms from regional earthquakes suggest that the frequencies of spectral peaks at any station were nearly the same for large, small, nearby, and distant events. However, we obtained very disparate frequencies when we analyzed seismograms for the same earthquake recorded at different stations. The most plausible interpretation is that spectral peaks do not depend on the characteristics of the earthquake source, but instead on site characteristics, or, more specifically, on the coupling of the instrument to the seafloor. To record reliable spectral data, we need to overcome this problem.     

跨海高程传递三角高程数据处理及质量分析

对跨河水准测量和跨海高程传递三角高程测量的观测方法和数据处理进行了比较.简单介绍了4处跨海高程传递测区的概况,并提出一种针对“光段差互差”的评估方法.对4处跨海点三角高程实测数据进行质量分析和平差计算,结果表明:除实验区外,跨海三角高程测量的偶然中误差可以达到二等水准测量的精度;数据质量分析结果表明,满足二等水准限差的外业观测数据的有效率为50％左右;满足三等水准限差的数据有效率为90％左右.