Defining megathrust tsunami source scenarios for northernmost Cascadia

Natural Hazards - Disaster insurance is an effective way in reducing and sharing natural disaster risk. In this paper, a special risk management model based on the cooperative insurance among the...     

Tsunami vulnerability assessment of Casablanca-Morocco using numerical modelling and GIS tools

Earthquakes and tsunamis along Morocco’s coasts have been reported since historical times. The threat posed by tsunamis must be included in coastal risk studies. This study focuses on the tsunami impact and vulnerability assessment of the Casablanca harbour and surrounding area using a combination of tsunami inundation numerical modelling, field survey data and geographic information system. The tsunami scenario used here is compatible with the 1755 Lisbon event that we considered to be the worst case tsunami scenario. Hydrodynamic modelling was performed with an adapted version of the Cornell Multigrid Coupled Tsunami Model from Cornell University. The simulation covers the eastern domain of the Azores-Gibraltar fracture zone corresponding to the largest tsunamigenic area in the North Atlantic. The proposed vulnerability model attempts to provide an insight into the tsunami vulnerability of building stock. Results in the form of a vulnerability map will be useful for decision makers and local authorities in preventing the community resiliency for tsunami hazards.     

Using numerical modelling to evaluate tsunami hazard near the Kuril Island

A sequence of computer experiments is used to study questions concerning the tsunami problem as a quantitative estimate of tsunami danger, detailed geographical tsunami classification, determination of the parameters of critical tsunami waves, and the conditions of their development. We call a wave critical, if its impact on the coast is most hazardous.Using the Middle Kuril Island as an example, we present the results of a computer experiment which includes determining the wavefields on the shelf and estimating the effects connected with the deep-water Bussol and Diana Straits.Numerical simulation of tsunami waves of different sources permits the assessment of the extent of tsunami danger in different areas of the coastal zone of Simushir Island, depending on the location of the focus zone and their geometry.The major singularities of the wavefield arise in the zones of the deep-water straits. The distribution of the amplification factors is determined by both the global parameters of the wavefields and the local properties of individual harbours. The results obtained for a particular harbour in the northern part of Simushir Island, formed the basis for the quantitative estimate of tsunami danger for this area.     

Expected tsunami amplitudes and currents along the North American coast for Cascadia subduction zone earthquakes

Tsunamis are numerically modeled using the nonlinear shallow-water equations for three hypothetical Cascadia subduction zone earthquakes. Maximum zero-to-peak tsunami amplitudes and currents are tabulated for 131 sites along the North American coast. Earthquake source parameters are chosen to satisfy known subduction zone configuration and thermal constraints. These source parameters are used as input to compute vertical sea-floor displacement. The three earthquakes modeled are moment magnitude 8.8, 8.5, and 7.8. Maximum zero-to-peak tsunami amplitude for theMw = 8.8 earthquake is near 6 m normal to the fault break and maximum current is near 3.5 m/s. Maximum amplitudes decrease by about one-half north and south of the fault break in the source region. Tsunami amplitudes vary along the Alaskan coast from less than 0.5 to 1.6 m. The modeled amplitudes for theMw = 8.8 quake decrease to less than 0.4 m south of Point Conception, CA. TheMw = 7.8 earthquake generates a tsunami with a maximum amplitude of less than 1 m normal to the source. North and south of the fault break the maximum amplitude again decreases by about one-half. In all the models, amplitudes and currents arc less than one-sixth of the outer coast value within Puget Sound.     

Numerical modelling of coastal inundation from Cascadia Subduction Zone tsunamis and implications for coastal communities on western Vancouver Island,Canada

Natural Hazards - In the present study, numerical simulations were conducted to estimate the spatio-temporal characteristics of tsunami inundation for municipalities on Vancouver Island, Canada, as...     

Tsunami hazard assessment of Canada

We present a preliminary probabilistic tsunami hazard assessment of Canadian coastlines from local and far-field, earthquake, and large submarine landslide sources. Analyses involve published historical, palaeotsunami and palaeoseismic data, modelling, and empirical relations between fault area, earthquake magnitude, and tsunami run-up. The cumulative estimated tsunami hazard for potentially damaging run-up (≥1.5 m) of the outer Pacific coastline is ~40–80 % in 50 years, respectively one and two orders of magnitude greater than the outer Atlantic (~1–15 %) and the Arctic (<1 %). For larger run-up with significant damage potential (≥3 m), Pacific hazard is ~10–30 % in 50 years, again much larger than both the Atlantic (~1–5 %) and Arctic (<1 %). For outer Pacific coastlines, the ≥1.5 m run-up hazard is dominated by far-field subduction zones, but the probability of run-up ≥3 m is highest for local megathrust sources, particularly the Cascadia subduction zone; thrust sources further north are also significant, as illustrated by the 2012 Haida Gwaii event. For Juan de Fuca and Georgia Straits, the Cascadia megathrust dominates the hazard at both levels. Tsunami hazard on the Atlantic coastline is dominated by poorly constrained far-field subduction sources; a lesser hazard is posed by near-field continental slope failures similar to the 1929 Grand Banks event. Tsunami hazard on the Arctic coastline is poorly constrained, but is likely dominated by continental slope failures; a hypothetical earthquake source beneath the Mackenzie delta requires further study. We highlight areas susceptible to locally damaging landslide-generated tsunamis, but do not quantify the hazard.     

Distribution and sedimentary characteristics of tsunami deposits along the Cascadia margin of western North America

Tsunami deposits have been found at more than 60 sites along the Cascadia margin of Western North America, and here we review and synthesize their distribution and sedimentary characteristics based on the published record. Cascadia tsunami deposits are best preserved, and most easily identified, in low-energy coastal environments such as tidal marshes, back-barrier marshes and coastal lakes where they occur as anomalous layers of sand within peat and mud. They extend up to a kilometer inland in open coastal settings and several kilometers up river valleys. They are distinguished from other sediments by a combination of sedimentary character and stratigraphic context. Recurrence intervals range from 300–1000 years with an average of 500–600 years. The tsunami deposits have been used to help evaluate and mitigate tsunami hazards in Cascadia. They show that the Cascadia subduction zone is prone to great earthquakes that generate large tsunamis. The inclusion of tsunami deposits on inundation maps, used in conjunction with results from inundation models, allows a more accurate assessment of areas subject to tsunami inundation. The application of sediment transport models can help estimate tsunami flow velocity and wave height, parameters which are necessary to help establish evacuation routes and plan development in tsunami prone areas.     

Tsunami Hazard and Risk in Canada

Tsunamis have occurred in Canada due to earthquakes, landslides, and a large chemical explosion. The Pacific coast is at greatest risk from tsunamis because of the high incidence of earthquakes and landslides in that region. The most destructive historical tsunamis, however, have been in Atlantic Canada – one in 1917 in Halifax Harbour, which was triggered by a catastrophic explosion on a munitions ship, and another in 1929 in Newfoundland, caused by an earthquake-triggered landslide at the edge of the Grand Banks. The tsunami risk along Canada's Arctic coast and along the shores of the Great Lakes is low in comparison to that of the Pacific and Atlantic coasts. Public awareness of tsunami hazard and risk in Canada is low because destructive tsunamis are rare events.     

Tsunami risk analysis for China

Historical data have been used in this paper, in particular that available on the tsunami history and the geological and seismological characteristics along the coasts of China. The nature and effects of both local tsunamis and tele-tsunamis on the coasts of China are analyzed. The coastal response of China to tsunamis is estimated theoretically, also. Finally, the tsunami risk for the coast of China is calculated and the zonation of preliminary tsunami hazard of China is mapped for three levels of hazardicity.     

Tsunami impact to Washington and northern Oregon from segment ruptures on the southern Cascadia subduction zone

This paper explores the size and arrival of tsunamis in Oregon and Washington from the most likely partial ruptures of the Cascadia subduction zone (CSZ) in order to determine (1) how quickly tsunami height declines away from sources, (2) evacuation time before significant inundation, and (3) extent of felt shaking that would trigger evacuation. According to interpretations of offshore turbidite deposits, the most frequent partial ruptures are of the southern CSZ. Combined recurrence of ruptures extending ~490 km from Cape Mendocino, California, to Waldport, Oregon (segment C) and ~320 km from Cape Mendocino to Cape Blanco, Oregon (segment D), is ~530 years. This recurrence is similar to frequency of full-margin ruptures on the CSZ inferred from paleoseismic data and to frequency of the largest distant tsunami sources threatening Washington and Oregon, ~M _w 9.2 earthquakes from the Gulf of Alaska. Simulated segment C and D ruptures produce relatively low-amplitude tsunamis north of source areas, even for extreme (20 m) peak slip on segment C. More than ~70 km north of segments C and D, the first tsunami arrival at the 10-m water depth has an amplitude of <1.9 m. The largest waves are trapped edge waves with amplitude ≤4.2 m that arrive ≥2 h after the earthquake. MM V–VI shaking could trigger evacuation of educated populaces as far north as Newport, Oregon for segment D events and Grays Harbor, Washington for segment C events. The NOAA and local warning systems will be the only warning at greater distances from sources.     

Analysis of the global tsunami data for vulnerability and risk assessment

For the assessment of tsunami risk and vulnerability, one has to make use of past tsunami observations. The most comprehensive tsunami databases for the world have been prepared by the National Geophysical Data Center of USA which are listed on their website for all the four oceans as well as the following marginal seas: Caribbean Sea, Mediterranean Sea, Black Sea, Red Sea and Gulf of Mexico. The dataset goes back as far as the first century AD and lists the events on a confidence rating scale of 0–4; 0 being an erroneous entry and 4 being a definite tsunami. Based on these various datasets for different geographical areas, a comprehensive global dataset was prepared in this study, which included only tsunami events with confidence rating of 3 and 4, meaning either probable or definite. In this composite and abridged global tsunami database there is no distinction either according to geography or tsunami strength as implied by its impact on the coast. A simple and straightforward statistical analysis suggests an almost complete randomness and no patterns that can be used for future tsunami predictions with a few minor exceptions.     

Tsunami vulnerability evaluation in the Mentawai islands based on the field survey of the 2010 tsunami

On October 25, 2010, a large earthquake occurred off the coast of the Mentawai islands in Indonesia, generating a tsunami that caused damage to the coastal area of North Pagai, South Pagai, and Sipora islands. Field surveys were conducted soon after the event by several international survey teams, including the authors’. These surveys clarified the tsunami height distribution, the damage that took place, and residents’ awareness of tsunamis in the affected islands. Heights of over 5 m were recorded on the coastal area of the Indian Ocean side of North and South Pagai islands and the south part of Sipora island. In some villages, it was difficult to evacuate immediately after the earthquake because of the lack of routes to higher ground or the presence of rivers. Residents in some villages had taken part in tsunami drills or education; however, not all villages shared awareness of tsunami threats. In the present paper, based on the results of these field surveys, the vulnerability of these islands with regards to future tsunami threats was analyzed. Three important aspects of this tsunami disaster, namely the geographic disadvantage of the islands, the resilience of buildings and other infrastructure, and people’s awareness of tsunamis, are discussed in detail, and corresponding tsunami mitigation strategies are explained.     

Strike-slip fault bridge fluid pumping mechanism: insights from field-based palaeostress analysis and numerical modelling

We present a finite-element study of stress perturbation in evolving compressive and extensional strike-slip fault bridges. The results are compared with a fracture study of a compressive bridge at St Donats, UK. Horizontally interbedded calcareous mudstone and bioclastic calcilutite at St Donats have a distinct vertical permeability anisotropy. This sedimentary sequence behaves as a set of horizontal aquifers. The fluid flow in these aquifers is sensitive to mean stress gradients. Paleostress analysis of field fracture data, verified by finite-element modelling, indicates a rotation of σ₁ towards parallelism with boundary faults inside the growing compressive bridge. Boundary faults and bridge faults recorded numerous fluid flow events. The modelled mean stress pattern shows a regional maximum within the bridge and local maxima/minima pairs at boundary fault tips.Finite-element modelling of an extensional bridge indicates that σ₃ rotates towards parallelism with boundary faults. The mean stress pattern is similar to the pattern in compressive bridge but with maxima and minima locations interchanged. The stress patterns are reestablished by each stress build-up preceding the rupturation of the boundary faults throughout the development stages of strike-slip fault bridges. Mean stress gradients developed pre-failure control the fluid flow in fractures of the strike-slip fault system at and after the end of each stress build-up and the fluid flow in boundary faults post-failure. Fracture reactivation and new fracture generation within an evolving bridge is a process consisting of multiple successive events that retain the storage capacity of the bridge. Rupture and sealing of the main bounding-faults is a step-wise process that opens and closes fluid conduits between areas with different pressures.     

A corrected 3-D SPH method for breaking tsunami wave modelling

A 3-D large eddy simulation model that was first transformed to smoothed particle hydrodynamics (LES-SPH)-based model was employed to study breaking tsunami waves in this paper. LES-SPH is a gridless (or mesh-free), purely Lagrangian particle approach which is capable of tracking the free surface of violent deformation with fragmentation in an easy and accurate way. The Smagorinsky closure model is used to simulate the turbulence due to its simplicity and effectiveness. The Sub-Particle Scale scheme, plus the link-list algorithm, is applied to reduce the demand of computational power. The computational results show that the 3-D LES-SPH model can capture well the breaking wave characteristics. Spatial evolution features of breaking wave are presented and visualized. The detailed mechanisms of turbulence transport and vorticity dynamics are demonstrated as well. This application also presents an example to validate the SPH model.     

Tsunami risk perception and preparedness on the east coast of New Zealand during the 2009 Samoan Tsunami warning

Several coastal communities on the North Island of New Zealand were evacuated during the 2009 Samoan Tsunami warning. This study aimed to explore the risk perception and preparedness levels of a small cohort of people living in an at-risk area. Qualitative data were collected using semi-structured interviews from fifteen residents evacuated from the town of Pauanui on the Coromandel Peninsula. Thematic content analysis showed common themes and gaps where emergency management systems were deficient. This study found that participants had inaccurate risk perception, a high reliance on warning systems, low levels of preparedness and lacked knowledge about natural warning signs of tsunamis. The themes identified are useful indicators of where current systems are failing people but need to be expanded to generalise results. The event, on which this study is based, provided a unique opportunity to explore people’s reactions to a predicted tsunami. The study confirmed the findings of prior studies that people in at-risk places are not necessarily well informed or prepared. The paper contributes further knowledge to inform the advance of public education and community engagement with respect to tsunami preparedness.     

Recurrence intervals of major paleotsunamis as calibrated by historic tsunami deposits in three localities: Port Alberni, Cannon Beach, and Crescent City, along the Cascadia margin, Canada and USA

We review geologic records of both historic and prehistoric tsunami inundations at three widely separated localities that experienced significant damage from the 1964 Alaskan tsunami along the Cascadia margin. The three localities are Port Alberni, Cannon Beach, and Crescent City, representing, respectively, the north, central, and south portions of the study area (1,000 km in length). The geologic records include anomalous sand sheets from marine surges that are hosted in supratidal peaty mud deposits. Paleotsunami sand sheets that exceed the thickness, continuity and/or extent of the 1964 historic tsunami are counted as major paleotsunami inundations. Major paleotsunamis (6–7 in number) at each locality during the last 3,000 years demonstrate mean recurrence intervals of 450–540 years, and within-cluster intervals (three events each) of 270–460 years. It has been 313 years since the last major paleotsunami from a great Cascadia earthquake in AD 1700. We compare the dated sequences of major paleotsunami inundations to the nearest regional records of coastal coseismic subsidence in Willapa Bay in the central margin, Waatch/Neah Bay in the northern margin, and Coquille in the southern margin. Similar numbers of events from both types of records suggest that the major paleotsunamis are locally derived (near-field) from ruptures of the Cascadia margin megathrust fault zone, rather than from transoceanic tsunamis (far-field) originating at other subduction zones around the Pacific Rim. Given the catastrophic hazard of the near-field Cascadia margin tsunamis, we propose a basic rule for reminding the general public of the need for self-initiated evacuation following a great earthquake at the Cascadia margin.     

ISPH numerical simulation of tsunami generation by submarine landslides

The present investigation focused on the numerical simulation of the gravity currents of non-Newtonian fluids by means of the incompressible smoothed particle hydrodynamics (ISPH) approach. Firstly, to solve the rheological properties of the non-Newtonian Bingham model by ISPH method, the multi-viscodensity approach has been introduced. Then, this methodology has been used to simulate the annular viscometer and landslide deformation test cases. Through simulating these test cases, the viscoplastic behavior of the non-Newtonian viscosity and propagation of tsunami waves due to underwater landslide movement have been observed. Numerical results were in good agreement with the theoretical and experimental studies and showed that this methodology can be used to investigate precisely the generation and propagation of tsunami surface waves.