Source mechanism of the tsunamis of 19 and 21 September 1985, in Mexico

The major earthquake measuring 8.1 on the Richter scale which struck the west coast of Mexico on Thursday 19 September 1985, generated a small tsunami. A major aftershock on 21 September, with a magnitude of 7.5 also produced a small tsunami. Both tsunamis propagated across the Pacific and were recorded by several tide stations in Central America, Colombia, Ecuador, French Polynesia, Samoa, and Hawaii. No reports of damage were received from any of the stations, and only minor damage due to the first tsunami was reported from the source region.A survey was made by the International Tsunami Information Center (ITIC) of the coastal area affected, from Manzanillo to Zihuatanejo. Tsunami runup measurements were taken and interviews with local residents in the coastal areas were conducted.A source mechanism study of the tsunamis was undertaken using seismic and geologic data and empirical relationships. Earthquake and tsunami energies were estimated and the tsunami genertion areas defined.The earthquake energies were estimated to be 5.61 × 10²⁴ erg for the 19 September event and 9.9 × 10²³ erg for the 21 September event. Tsunami energies were estimated to be 0.7 × 10²⁰ erg for the first event and 0.56 × 10²⁰ erg for the second event. The source area of the first tsunami was determined to be approximately one-half of the earthquake source area, or approximately 7500 km², while the source area of the second tsunami was estimated to be equal to the earthquake area.The relatively small tsunamis generated by these large earthquakes are attributed to the shallow angle of subduction of the Cocos plate underneath the North American plate for this particular region, and to the small vertical component of crustal displacements. However, the angle of subduction increases further south and local earthquakes from that area have the potential of producing large tsunamis on the west coast of Mexico.This paper was presented at the 4th International Symposium on Natural and Man-made Coastal Hazards held in Ensenada, Mexico, August 1988.     

Possible mechanism of rockbursts in coal mines

One of the important questions of rockburst prevention is the understanding of the mechanism of rockburst source. This question can be effectively studied by direct seismic observations in the rockburst regions. For this purpose, the distribution ofP-wave onset signs and the inversion of first motion amplitudes were utilized as the basic method. In such a way the coal mine regions in Poland (Upper Silesia) and in Czechoslovakia (Kladno coal mine district) were studied as part of the Polish-Czechoslovak rockburst investigation project. More than 250 rockburst events were recorded here in the decade 1977–1983, and analyzed. The results of the statistical analysis of these data allow us to formulate and introduce a model of the rockburst source with an implosion component. The suitability of this conception was verified by laboratory simulation conditions; it was confirmed that the seismoactive displacements with a clear implosive component were recorded in the neighbourhood of a stress concentrator weakened by holes. The results of both the field observations and laboratory tests were in good agreement with the theoretically derived radiation patterns for a combined shear-implosive source and also with the theoretical conception of such a source based on real geometrical configurations of mine excavations and tectonic dislocations.The rockbursts treated exhibit a dominant shear component, the magnitude of the additional implosive component not exceeding 10 percent of the shear component.     

The Event of 26th of December 2004 – The Biggest Earthquake in the World in the Last 40 years

A great earthquake occurred at 00:58:49 (UTC) on Sunday, December 26, 2004 off the northwest coast of Sumatra, Indonesia. Its revised moment magnitude was M 9.3 making it in the top four largest earthquakes in the world since 1900 and the largest since the Alaskan 1964 event. The earthquake caused tsunami waves which killed more than 300,000 people in Southern Asia and Africa. There were 31 earthquakes with magnitudes between 5.5 and 7.3 in the 48-h period after the main event, and it seemed that seismicity migrated northwards along the 1200 km fault (http: //www.ga.gov.au). Similar size events occurred in that location off Sumatra in the 19th century, but no evidence of written records of their tsunami effects in Australia is found. The devastating megathrust earthquake of 26 December 2004 occurred on the interface of the Indo-Australian and Euro-Asian plates where the first plate subducts beneath the overriding second plate and the Indo-Australian plate begins its descent into the mantle. In the epicentral region, the Indo-Australian plate moves toward the northeast at a rate of about 7 cm/year relative to the Euro-Asian plate resulting in oblique convergence and partitioning into thrust-faulting. From the size of the earthquake, it is likely that the displacement on the fault plane was up to fifteen meters. As with the recent event, megathrust earthquakes often generate large tsunamis that cause damage over a much wider area than is directly affected by ground shaking near the earthquake’s rupture. The subduction zone continues further south of the Indonesian archipelago and that area is also a potential risk of producing a megathrust event that may affect coastal parts of northwest Australia. The tragic events of Boxing Day 2004 highlighted the importance of establishing a tsunami warning system for the Indian Ocean like the one for the Pacific. Issues like more and better instrumentation, and a long-term program to educate people in the region about the dangers of tsunamis, were identified as priorities. Of particular interest is the time for identifying and issuing alerts for such devastating earthquakes with possibility to reduce it in future for warning purposes.     

The landslides and tsunamis of the 30th of December 2002 in Stromboli analysed through numerical simulations

On the 30th of December 2002 two tsunamis were generated only 7 min apart in Stromboli, southern Tyrrhenian Sea, Italy. They represented the peak of a volcanic crisis that started 2 days before with a large emission of lava flows from a lateral vent that opened some hundreds of meters below the summit craters. Both tsunamis were produced by landslides that detached from the Sciara del Fuoco. This is a morphological scar and is the result of the last collapse of the northwestern flank of the volcanic edifice, that occurred less than 5 ka b.p. The first tsunami was due to a submarine mass movement that started very close to the coastline and that involved about 20×10⁶ m³ of material. The second tsunami was engendered by a subaerial landslide that detached at about 500 m above sea level and that involved a volume estimated at 4–9×10⁶ m³. The latter landslide can be seen as the retrogressive continuation of the first failure. The tsunamis were not perceived as distinct events by most people. They attacked all the coasts of Stromboli within a few minutes and arrived at the neighbouring island of Panarea, 20 km SSW of Stromboli, in less than 5 min. The tsunamis caused severe damage at Stromboli.In this work, the two tsunamis are studied by means of numerical simulations that use two distinct models, one for the landslides and one for the water waves. The motion of the sliding bodies is computed by means of a Lagrangian approach that partitions the mass into a set of blocks: we use both one-dimensional and two-dimensional schemes. The landslide model calculates the instantaneous rate of the vertical displacement of the sea surface caused by the motion of the underwater slide. This is included in the governing equations of the tsunami, which are solved by means of a finite-element (FE) technique. The tsunami is computed on two different grids formed by triangular elements, one covering the near-field around Stromboli and the other also including the island of Panarea.The simulations show that the main tsunamigenic potential of the slides is restricted to the first tens of seconds of their motion when they interact with the shallow-water coastal area, and that it diminishes drastically in deep water. The simulations explain how the tsunamis that are generated in the Sciara del Fuoco area, are able to attack the entire coastline of Stromboli with larger effects on the northern coast than on the southern. Strong refraction and bending of the tsunami fronts is due to the large near-shore bathymetric gradient, which is also responsible for the trapping of the waves and for the persistence of the oscillations. Further, the first tsunami produces large waves and runup heights comparable with the observations. The simulated second tsunami is only slightly smaller, though it was induced by a mass that is approximately one third of the first. The arrival of the first tsunami is negative, in accordance with most eyewitness reports. Conversely, the leading wave of the second tsunami is positive.     

The Tsunami of 26 December, 2004: Numerical Modeling and Energy Considerations

A numerical model for the global tsunamis computation constructed by Kowalik et al. (2005), is applied to the tsunami of 26 December, 2004 in the World Ocean from 80°S to 69°N with spatial resolution of one minute. Because the computational domain includes close to 200 million grid points, a parallel version of the code was developed and run on a Cray X1 supercomputer. An energy flux function is used to investigate energy transfer from the tsunami source to the Atlantic and Pacific Oceans. Although the first energy input into the Pacific Ocean was the primary (direct) wave, reflections from the Sri Lankan and eastern shores of Maldives were a larger source. The tsunami traveled from Indonesia, around New Zealand, and into the Pacific Ocean by various routes. The direct path through the deep ocean to North America carried miniscule energy, while the stronger signal traveled a considerably longer distance via South Pacific ridges as these bathymetric features amplified the energy flux vectors. Travel times for these amplified energy fluxes are much longer than the arrival of the first wave. These large fluxes are organized in the wave-like form when propagating between Australia and Antarctica. The sources for the larger fluxes are multiple reflections from the Seychelles, Maldives and a slower direct signal from the Bay of Bengal. The energy flux into the Atlantic Ocean shows a different pattern since the energy is pumped into this domain through the directional properties of the source function. The energy flow into the Pacific Ocean is approximately 75% of the total flow to the Atlantic Ocean. In many locations along the Pacific and Atlantic coasts, the first arriving signal, or forerunner, has lower amplitude than the main signal which often is much delayed. Understanding this temporal distribution is important for an application to tsunami warning and prediction.     

Traces of historical tropical cyclones and tsunamis in the Ashburton Delta (north‐west Australia)

Although the north‐western coast of Western Australia is highly vulnerable to tropical cyclones and tsunamis, little is known about the geological imprint of historic and prehistoric extreme wave events in this particular area. Despite a number of site‐specific difficulties such as post‐depositional changes and the preservation potential of event deposits, both tropical cyclones and tsunamis may be inferred from the geomorphology and the stratigraphy of beach ridge sequences, washover fans and coastal lagoons or marshes. A further challenge is the differentiation between tsunami and storm deposits in the geological record, particularly where modern deposits and/or historical reports on the event are not available. This study presents a high‐resolution sedimentary record of washover events from the Ashburton River delta (Western Australia) spanning approximately the last 150 years. A detailed characterization of event deposits is provided, and a robust chronostratigraphy for the investigated washover sequence is established based on multi‐proxy sediment analyses and optically stimulated luminescence dating. Combining sedimentological, geochemical and high‐resolution optically stimulated luminescence data, event layers are assigned to known historical events and tropical cyclone deposits are separated from tsunami deposits. For the first time, the 1883 Krakatoa and 1977 Sumba tsunamis are inferred from sedimentary records of the north‐western part of Western Australia. It is demonstrated that optically stimulated luminescence applied in coastal sedimentary archives with favourable luminescence characteristics can provide accurate chronostratigraphies even on a decadal timescale. The results contribute to the data pool of tropical cyclone and tsunami deposits in Holocene stratigraphies; however, they also demonstrate how short‐lived sediment archives may be in dynamic sedimentary environments.     

南海定量海啸预警系统

我国面临着来自于马尼拉海沟、台湾岛以及琉球海沟等海域可能形成的局地和区域海啸风险,及时、准确地预警海啸是一个难题。2004年印度洋海啸之后,基于海量数据库和GIS技术,国家海洋环境预报中心开发了南海定量海啸预警系统,可以对潜在海啸进行快速定量化的预警,并利用GIS软件进行预警结果可视化。利用该系统对2006年台湾南部地震海啸进行模拟预报,预报结果与实测结果基本吻合。     

Tsunami deposits: Present knowledge and future challenges

Tsunami deposits are the primary source of information on (past) large tsunami events and thereby are crucial for accurate hazard assessments. Tsunami deposits studies have developed over the last three decades, but this is still a young geoscience discipline. Following the 5^th International Tsunami Field Symposium in 2017 an opportunity arose to publish a Special Issue focusing on present knowledge and future research challenges. This paper aims to briefly review current state-of-the-art research, summarizing major findings and gathering relevant works that describe the progress achieved over the last three decades. In this paper the relevance of tsunami deposits, their peculiar sedimentary characteristics and their differentiation from other high energy events are presented. Especially over the last decade an incredibly high number of studies have been published on tsunami deposits, many of which are of a high quality and provide detailed literature reviews. Some of these studies represent the current progress discussed here. Challenges are also introduced, to spur a discussion on future scientific questions that can and should be addressed by tsunami geoscientists. Coupling onshore–offshore records is an area where tsunami geoscience faces some of its major challenges. Moreover, the application of non-destructive high-resolution techniques to study the internal structure and composition of tsunami deposits can also provide an opportunity to further examine deposits, and from this derive physical parameters of the forcing mechanism. Another topic is better understanding of the erosional signature of tsunami events and a continuation of the effort to better incorporate age-estimation methods by developing more accurate dating methodology. Finally, there is also the need for the improvement of empirical, forward and regressive numerical models to better contribute to the characterization of tsunami events.     

Organic geochemical investigation of far-field tsunami deposits of the Kahana Valley,O‘ahu,Hawai‘i

Far-field tsunami deposits observed in the Kahana Valley, O‘ahu, Hawai‘i (USA), were investigated for their organic-geochemical content. During short high-energy events, (tsunamis and storms) organic and chemical components are transported with sediment from marine to terrestrial areas. This study investigates the use of anthropogenic based organic geochemical compounds (such as polycyclic aromatic hydrocarbons, pesticides and organochlorides) as a means to identify tsunami deposits. Samples were processed by solid–liquid extraction and analyzed using gas chromatography–mass spectrometry. A total of 21 anthropogenic marker compounds were identified, of which 11 compounds were selected for detailed analysis. Although the tsunami deposits pre-date industrial activity in Hawai‘i by several hundred years, distinct changes were found in the concentrations of anthropogenic marker compounds between sandy tsunami deposits and the surrounding mud/peat layers, which may help in identifying tsunami deposits within cores. As expected, low overall concentrations of anthropogenic markers and pollutants were observed due to the lack of industrial input-sources and little anthropogenic environmental impact at the study site. This geochemical characterization of tsunami deposits shows that anthropogenic markers have significant potential as another high-resolution, multi-proxy method for identifying tsunamis in the sedimentary record.