Tsunami mapping in the Gulf of Guayaquil,Ecuador, due to local seismicity

The North-Andean subduction zone generates recurrent tsunamigenic earthquakes. The seismicity is usually considered to be segmented because of different specific morphological features of the Nazca Plate driving the subduction motion. Most of the recent powerful earthquakes in the margin were located in its northern part. To the south, the region of the Gulf of Guayaquil, only (undocumented) three events in 1901, 1933 and 1953 were possibly powerful and tsunamigenic. Here we are interested in the tsunami signature due to local seismicity. Two realistic earthquake scenarios (M_w = 7 and M_w = 7.5) taking into account the hypothesized segmentation of the area are proposed. Their return period is supposed to be intra-centenary. Then, a larger magnitude unsegmented M_w = 8 scenario is computed (half-millennium return period). The interior of the Gulf of Guayaquil as well as the Santa Elena Peninsula are sheltered areas including numerous coastal infrastructures and the city of Guayaquil. It is predicted that potential flooding would occur at high tide only for both segmented and unsegmented scenarios in (1) south of Playas with however only a few centimeters of wave height and (2) Chanduy (a few meters). Both are important zones of coastal farms.     

马尼拉俯冲带潜在地震海啸对华南沿海的影响

马尼拉俯冲带潜在地震海啸对我国南部沿海城市构成巨大威胁,利用情景式数值模拟技术重构灾害过程并评估危险等级有助于理解南海海啸传播规律并指导预警预报和防灾减灾工作。根据美国太平洋海洋环境研究中心(Pacific Marine Environmental Laboratory, PMEL)发布的马尼拉俯冲带断层参数设计M_w 7.5、M_w 8.1和M_w 8.5三个震级下共19个震源,应用非静压海啸数值模型(Non-hydrostatic Evolution of Ocean WAVE, NEOWAVE)模拟各震源激发海啸在南海海盆的传播过程,通过最大波辐和测点时间序列发现海啸波能量传输分布并评估代表区域危险等级。研究表明, M_w 7.5级地震海啸对我国南部沿海的影响较低,波幅一般不超过30 cm; M_w 8.1级地震海啸对华南沿海主要造成太平洋海啸预警中心定义的Ⅱ或Ⅲ级海啸危险等级,海啸影响范围和能量分布特征由震源位置决定; M_w 8.5级地震海啸主要对中国沿海构...     

Distinguishing features of the seismicity in South Kamchatka in the 20th century with regard to the Avacha Bay seismic gap

The distinguishing features of the seismicity throughout South Kamchatka and within the Avacha Bay seismic gap in the 20th century are considered. The evolution of the evaluation of the magnitudes of the strongest earthquakes for this gap from M = 7.25–7.5 in 1965–1980 to 7.75–8.0 after 1980 is discussed. On the basis of the method for studying the characteristic features of the seismicity within a seismic gap developed for the Central Kuriles, the seismicity of South Kamchatka is considered for depths of 0–100, 101–200, and more than 200 km according to the data from the New Catalog [6] for the period from 1901 to 1974 (M ≥ 6.1), the Special Catalog for North Eurasia [3] for the period from 1975 to 1993 (M ≥ 4.5), and additional data from the Kamchatka stations for the period from 1994 to 1997. It was found that the seismic process within the region of South Kamchatka is typical of the island arcs; i.e, most of the earthquakes considered and the maximum of the seismic energy released are concentrated in the lithosphere at depths of 0–100 km. The seismological situation in the zone of Avacha Bay is found to be similar to that within the second kind of the seismic gap during the precursory seismic quiescence of the 1978 Oaxac earthquake with M = 7.8 in Central Mexico. This allows us to consider the zone of Avacha Bay as a possible seismic gap of the second kind. Such a result can be considered as a suggestion of the possibility of the occurrence in Avacha Bay of an earthquake with M ～ 8 according to the long-term forecast for the region of the Kuriles and Kamchatka made by S.A. Fedotov.     

A real-time observation network of ocean-bottom-seismometers deployed at the Sagami trough subduction zone,central Japan

We installed a real-time operating regional observation network of Ocean-Bottom-Seismometers, connected to an electro-optical fiber communication cable, at the Sagami trough subduction zone, just south of the Tokyo metropolitan area, central Japan. The network, called ETMC, has six seismic observation sites at approximately 20 km spacing. In addition, there are three tsunami observation sites along the ETMC network to monitor the propagation process of tsunamis around the Sagami trough region.The on-line data from the ETMC has been improving the detection capability of smaller-magnitude earthquakes even at areas close to the margin of the trough. The ETMC data analyzing system, which has a function of real-time digital filtering for each seismic channel, can read the arrival times of P- and S-waves precisely, constraining well the automatic on-line hypocenter locations. The network has been providing useful information regarding the bending and downgoing process of the Philippine sea plate at the Sagami trough subduction zone.The pressure sensors of the installed network have a detection capability of tsunami wave trains with an amplitude of less than 1 cm. For example, the sensors recorded the full time history of tsunami wave trains, with mm order resolution, originating from a tsunami earthquake with 5.7 M_W and the tsunami magnitude of 7.5 occurred near Tori Shima (Tori Is.) of the Izu-Bonin Is. arc on September 4, 1996. The maximum amplitude of the tsunami signals on the trough-floor was approximately 1 cm (P-P), in contrast with approximately 20 cm (0-P) at a coastal site on Izu-Oshima, near the trough. Also, the pressure sensors observed tsunamis due to a large tsunami earthquake (7.1 M_W) at the northern New Guinea, on July 17, 1998.     

The Kuril Earthquakes and tsunamis of November 15, 2006, and January 13, 2007: Observations,analysis, and numerical modeling

Major earthquakes occurred in the region of the Central Kuril Islands on November 15, 2006 (M _w = 8.3) and January 13, 2007 (M _w = 8.1). These earthquakes generated strong tsunamis recorded throughout the entire Pacific Ocean. The first was the strongest trans-Pacific tsunami of the past 42 years (since the Alaska tsunami in 1964). The high probability of a strong earthquake (M _w ≥ 8.5) and associated destructive tsunami occurring in this region was predicted earlier. The most probable earthquake source region was investigated and possible scenarios for the tsunami generation were modeled. Investigations of the events that occurred on November 15, 2006, and January 13, 2007, enabled us to estimate the validity of the forecast and compare the parameters of the forecasted and observed earthquakes and tsunamis. In this paper, we discuss the concept of “seismic gaps,” which formed the basis for the forecast of these events, and put forward further assumptions about the expected seismic activity in the region. We investigate the efficiency of the tsunami warning services and estimate the statistical parameters for the observed tsunami waves that struck the Far Eastern coast of Russia and Northern Japan. The propagation and transformation of the 2006 and 2007 tsunamis are studied using numerical hydrodynamic modeling. The spatial characteristics of the two events are compared.     

南桑威奇俯冲带大地水准面异常及其地幔对流特征分析

以南桑威奇俯冲带为例,根据EGM2008超高阶地球重力场模型、卫星重力数据为基础,利用移去-恢复原理计算了研究区大地水准面,实现了研究区不同场源深度大地水准面异常信息的分离,根据Runcorn模型计算了研究区小尺度地幔对流应力场,并结合天然地震空间展布和前人研究成果,对俯冲带结构特征与地幔对流模式进行了探讨。结果表明:南桑威奇俯冲带具有俯冲倾角较大、地震震级较低、弧前侵蚀明显等典型的马里亚纳型俯冲带特征,俯冲带南北部俯冲深度存在明显差异,中段偏北俯冲深度可达500 km;受到软流圈与上地幔上部物质密度差异的控制,东斯科舍海脊下存在沿海脊轴向南流动强地幔流;俯冲带结构与小尺度地幔对流应力场具有很强的相关性。本研究对于搞清南桑威奇俯冲带深部构造特征,理解俯冲运动、地幔对流方向及其动力控制机制提供了新的研究思路和方法。     

Retrospective analysis of seismic regime features before the Taiwan earthquakes of 1999 and 2002

The features of the seismic regime before the strongest earthquakes of Taiwan in the late 20th (Chi-Chi on September 21, 1999, M_w = 7.6) and the early 21st century (March 31, 2002, M_w = 7.4) are analyzed. Based on 1990–1999 and 1994–2002 data, respectively, retrospective analysis of three seismic regime parameters are studied: the total annual number of earthquakes N_Σ in the range of M_L = 2.5–5.5 and M_w = 3.0–7.0; the total annual quantity of released seismic energy ΣE, J; and angular coefficient b of earthquake recurrence graphs. Two explicit subperiods are revealed in the course of the seismic regime: quiescence in 1990–1996 before the Chi-Chi earthquake and in 1994–1997 before the March 2002 earthquake; in 1997–1999 and 1998–2002, respectively, seismic activation is observed. Due to the predominance of weak earthquakes during the Chi-Chi earthquake preparation, factor b appeared relatively higher (–1.16 on average); in contrast, before the March 2002 earthquake, due to the occurrence of foreshocks with M_w = 6.8–7.0, the factor b values appeared relatively lower (–0.55 and–0.74 for the quiescence and activation subperiods, respectively). Despite the fundamental difference in the seismotectonic situation between the domains where two mainshocks occurred and significantly difference energy ranges of the initial seismic events, the analysis results are similar for both earthquakes. In both cases, the mainshock occurred at the peak of released energy, which can be considered a coincidence. Solid verification of this positive tendency requires the accumulation of seismological statistics.     

Slope instability near the shelf break,Western Gulf of Alaska*

Abstract

The uppermost continental slope in the western Gulf of Alaska, from southern Albatross Bank to Portlock Bank, includes two broad areas where large submarine landslides occur and one intervening area where they are absent. In the areas containing large slides, seismic reflection records show evidence for active nearsurface folding and consequent slope steepening, which is apparently the ultimate control on this sliding. Evidence is lacking for similar active steepening in the area containing no large slides, where slope gradients are relatively gentle. Relatively small, shallow slides, fundamentally different from the larger ones, occur in all three areas on slopes that are not necessarily actively steepening. These slides are probably stratigraphically controlled, with failure occurring along weak subsurface strata. Strong earthquakes and the related accelerations are probably responsible for the actual triggering of many of the large and small slides. As long as the tectonic setting remains as it is today, future large‐scale sliding should remain confined to the two broad areas in which it now exists. Relatively small‐scale and shallow sliding might occur in any of the three areas.     

Modeling the near-field effects of the worst-case tsunami in the Makran subduction zone

As a first step towards the development of inundation maps for the northwestern Indian Ocean, we simulated the near-field inundation of two large tsunami in the Makran subduction zone (MSZ). The tsunami scenarios were based on large historical earthquakes in the region. The first scenario included the rupture of about 500 km of the plate boundary in the eastern MSZ, featuring a moment magnitude of M_w 8.6. The second scenario involved the full rupture of the plate boundary resulting from a M_w 9 earthquake. For each scenario, the distribution of tsunami wave height along the coastlines of the region is presented. Also, detailed runup modeling was performed at four main coastal cities in the region for the second scenario. To investigate the possible effect of splay fault branching on tsunami wave height, a hypothetical splay fault was modeled which showed that it can locally increase the maximum wave height by a factor of 2. Our results showed that the two tsunami scenarios produce a runup height of 12-18 m and 24-30 m, respectively. For the second scenario, the modeled inundation distance was between 1 and 5 km.     

再论台湾岛及邻近海域强震前的中强地震条带现象及成因初探

台湾岛1999年9月21日在南投发生7.6级地震前有出现地震条带现象,在2005年工作的基础上继续用地震条带方法对2004年12月台东海外7.0级地震、2006年12月高雄海外7.2级地震和2016年2月6日高雄6.7级地震前的地震图像进行分析发现,这3次地震前也存在ML5.5级以上地震条带现象,说明近年来台湾岛及邻近海域强地震前都有地震条带出现.对这些条带形成的原因进行分析后认为:EN向地震条带是受菲律宾板块挤压形成的,沿台湾岛东部海岸排列的地震条带是受台湾岛东部的地震断层影响形成的,沿琉球海沟方向排列的地震条带是受琉球海沟断裂带影响形成的.研究结果对台湾岛及邻近海域的地震预报具有参考意义.     

Regional long-period magnitude scales and their capabilities for tsunami warning

The tsunami warning system in the Russian Far East employs the medium-period magnitude MS (BB) by Vaniek–Soloviev. However, its use may lead to inadequacies and underestimates for the tsunamigenic potential of an earthquake. Specifically, this can happen in the case of a so-called tsunami–earthquake. This kind of earthquakes with a nonstandard spectrum was revealed by H. Kanamori in 1972. This problem can be overcome by using a magnitude scale that deals with longer period seismic waves. This study develops a technique for determining the magnitudes at regional distances (from 70 to 4500 km) using the amplitudes of surface seismic waves of periods of 40 and 80 s. At distances of 70–250 km, the amplitude of the joint group of shear and surface waves is used. For the new magnitudes designated M _S(40) and M _S(80), experimental calibration curves are constructed using more than 1250 three-component records at 12 stations of the region. The magnitudes are calibrated so as to produce an unbiased estimate of the moment magnitude M _w in the critical range 7.5–8.8. The rms error of the single-station estimate M _w is around 0.27. At distances below 250 km and M _w ≥ 8.3, the estimate of M _w obtained by the proposed technique becomes saturated at the level of M _w ~ 8.3, which is acceptable for operative analysis because no missed alarms arise. The technique can be used in operational tsunami warning based on seismological data. This can markedly decrease the number of false alarms.     

三峡及邻区上限发震水平预测(Ⅱ)

根据三峡及邻区地质、地震资料、光弹性模拟试验、用含定性变量逐步回归方法、建立各潜在震源上限发露水平预测方程、对所研究区地震进行预测。     

卫星热红外亮温异常与深水海域天然气水合物藏区分布——以墨西哥湾地区为例

以墨西哥湾西北部陆坡区为研究区,利用NOAA/AVHRR热红外影像,通过分析1999年Central Mexico 7.0级地震、1999年Oaxaco 7.5级地震和2003年Colima 7.6级地震3次地震期间与墨西哥湾西北部陆坡区海底天然气水合物藏区对应的海表面上方卫星热红外亮温异常的变化,研究了卫星热红外亮温异常与深水海域天然气水合物藏区分布的关系.研究发现,与墨西哥湾西北部陆坡区海底天然气水合物藏区对应的海表面上方,临震前频繁出现孤立的、带状的、强度较大的卫星热红外亮温异常,该研究结果表明,同次和多次地震临震前,该地区频繁出现孤立、带状、强度较大的卫星热红外亮温异常可能与海底蕴藏着天然气水合物藏有关.     

Some seasonal water temperature patterns in the Hauraki gulf,New Zealand

The changing pattern of water temperature in the Hauraki Gulf at approximately two‐monthly intervals during one and one‐half seasonal cycles in 1965–66 was determined from sea surface temperatures and bathythermograph profiles.

Surface and bottom temperatures ranged from 22.0°c and 20.5°c respectively in March to 12.5°c and 13.0°c in July‐September. Seasonal temperature ranges and short‐term variations were greatest in the shallow south‐west Gulf.

In winter the Gulf water was coolest close to shore. It was typically isothermal in depth but a temperature inversion of approximately l°c frequently formed, probably because of the combination of strong winds and an increased outflow of cool, low salinity water from harbours and bays. A similar inversion in Colville Channel may have been caused by more complex tidal and/or ocean current conditions.

In spring and summer the Gulf became thermally stratified, with warmest temperatures in the shallow areas. Thermoclines were generally irregular in position and size, and probably represented solar heating and minor current boundaries rather than a distinct separation of major water masses. In late summer and autumn bottom temperatures increased and almost equalled the maximum surface temperature.

During autumn surface water temperatures close to land decreased rapidly to return the Gulf to its winter isothermal condition.

Local factors (wind, rainfall, tides, depth of water, and proximity to land) probably influence sea temperatures in the Gulf. Seawards of a line from Cape Rodney to Cape Colville oceanic conditions prevail; water temperatures are more constant and increase to seaward in both winter and summer.

Oceanic and Gulf waters meet and mix in the Rodney‐Colville area, and Gulf water is transported east through Colville Channel. The extent of oceanic water penetration into the Gulf at depth is unknown.     

Are mega earthquakes predictable?

In the course of the ongoing since 1992 Global Test of the intermediate-term middle-range earthquake forecast/predictions by the algorithms M8 and MSc place and time of each of the mega-earthquakes of 27 February 2010 in Chile and 11 March 2011 in Japan were recognized as in state of increased probability of such events in advance their occurrences. In conjunction with a retrospective analysis of seismic activity preceding the first of a series of mega earthquakes of the 21st century, i.e., 26 December 2004 in the Indian Ocean, these evidences give grounds for assuming that the algorithms of proven validated effectiveness in magnitude ranges M7.5+ and M8.0+ can be applied to predict the mega-earthquakes as well.     

On the problem of local tsunamis and possibilities of their warning

At present, the problem of predicting tsunamis with source earthquakes near the shoreline remains practically unresolved. It is shown that, in the Pacific region, 87% of tsunamigenous earthquake epicenters are located closer than 100 km to the shoreline and 67% are closer than 50 km. For a more detailed analysis, the area of the Pacific Ocean was divided into ten subregions: Kamchatka, the Kuril Islands, Japan, Indonesia, Australia and Oceania, South and Central America, Alaska, and the Aleutian Islands. Each subregion was analyzed individually. All the earthquakes from 1950 to 2003 with Ms >= 6.0 causing tsunamis with intensities I > 0 were processed. The ITDB/PAC 2004 database was used as the data source. For each subregion, mean and minimal travel times were calculated. The minimal travel times for all the regions except for a single one are less than 10 min. It is shown that, in the near earthquake zone, no tsunami alert based sea-level gauge data is possible. One probable solution could be based on detecting hydroacoustic signals that precede strong earthquakes in the near-shore zone.     

Relationship between the seismicity and geologic structure of the Blanco Transform Fault Zone

Morphologic studies of an oceanic transform, the Blanco Transform Fault Zone (BTFZ), have shown it to consist of a series of extensional basins that offset the major strike-slip faults. The largest of the extensional basins, the Cascadia Depression, effectively divides the transform into a northwest segment, composed of several relatively short strike-slip faults, and a southeast segment dominated by fewer, longer faults. The regional seismicity distribution (m _b 4.0) and frequency-magnitude relationships (b-values) of the BTFZ show that the largest magnitude events are located on the southeast segment. Furthermore, estimates of the cumulative seismic moment release and seismic moment release rate along the southeast segment are significantly greater than that of the northwest segment. These observations suggest that slip along the southeast segment is accommodated by a greater number of large magnitude earthquakes. Comparison of the seismic moment rate, derived from empirical estimates, with the seismic moment rate determined from plate motion constraints suggests a difference in the seismic coupling strength between the segments. This difference in coupling may partially explain the disparity in earthquake size distribution. However, the results appear to confirm the relation between earthquake size and fault length, observed along continental strike-slip faults, for this oceanic transform.     

Large-scale short-term seismicity activation prior to the strongest earthquakes of Japan and the Kurile Islands

The Reverse Tracing of Precursors (RTP) algorithm for the prediction of strong earthquakes has become known owing to the successful predictions of the Tokachi-oki earthquake near Hokkaido Island and the San Simeon earthquake of California in 2003, as well as to other well-documented predictions found on the Internet, some of which also proved to be successful. The RTP predictions with the use of the Japan Meteorological Agency (JMA) data for the zone from Honshu Island to the Middle Kurile Islands deserve special attention. None of the five predictions starting in the middle of 2003, including the last one formulated for the region where the catastrophic earthquake of March 11, 2011, with a magnitude of M = 9 occurred, was a false alarm. One distinctive feature of predictions for this region is the enormous size (about 1000 km) of alarm regions. At the same time, the relatively short alarm interval makes it possible to record a real number of earthquakes with a magnitude of 7.2 and higher during alarm periods, which is about five times larger than on average over the equivalent period, i.e., to reach a probability gain of about five.     

Correlation between the intensity of tsunami in the Black Sea and the magnitude of underwater earthquakes

We perform the analysis of tsunami waves in the shelf zone of the Crimean peninsula generated by underwater earthquakes whose epicentres are located near the lower boundary of the continental slope. For this purpose, we use a one-dimensional nonlinear dissipative numerical model of long waves. The investigated distributions of the depth of the basin correspond to four points of the south coast of the Crimean peninsula. We use the empirical dependences of parameters of the sources of tsunami waves on the magnitude of the earthquake obtained earlier for the Pacific Ocean. It is shown that the height, vertical climb, and duration of tsunami waves increase with the magnitude of the earthquake. For equal magnitudes of the earthquake, the highest tsunamis on the south coast of the Crimea are observed between Alushta and Yalta. We also deduced a generalized regression dependence of the height of tsunami waves near the coast on the magnitude of the earthquake. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

Numerical analysis of the propagation and amplification of tsunami waves on the northwest shelf of the Black Sea

We perform the numerical analysis of the process of propagation of long waves in the northwest part of the Black Sea and consider ten possible zones of the seismic generation of tsunamis. The numerical analysis is performed on a grid with steps of 500 m. It is shown that the location of the tsunami source significantly affects the distribution of the heights of waves along the coast. As a rule, the most intense waves are formed in the closest part of the coast. The earthquakes in the South-Coast seismic zone do not lead to the formation of tsunamis in the west part of the sea. Only strong earthquakes in the northwest part of the sea can be responsible for noticeable oscillations of the Black-Sea level. The period of tsunamis near Odessa is close to 1 h and depends on the magnitude of the earthquake. In the region of Sevastopol, this period is 2--3 times smaller. In the major part of the coastal points, the extreme elevations and lowerings of the sea level do not exceed (in modulus) the initial displacements of the sea surface at the source of tsunamis. An intensification of waves emitted from the zones of generation located in the deeper part of the investigated region was observed for some parts of the Romanian coast and the west coast of Crimea. As the magnitude of the earthquake increases, the intensification of waves near the coast becomes more pronounced.