Local Tsunami Research Articles

The 29th January 2014 submarine landslide at Statland, Norway—landslide dynamics, tsunami generation, and run-up

A coastal landslide occurred at Statland, Namdalseid county, mid-Norway, on January 29, 2014, and generated a local tsunami. Neither the landslide nor the tsunami resulted in severe casualties, but the landslide tsunami gave rise to a maximum run-up height of 10 m and caused local damage to the Statland village. The limited size of the landslide as well as the availability of both pre- and post-landslide bathymetry and tsunami run-up data enable insight into the joint landslide-tsunami process. We first present the results of the post-tsunami field survey, followed by a description of the combined modeling of the landslide dynamics, tsunami generation, and run-up. The modeling initially involved different hypotheses of the landslide dynamics. By comparing the simulated tsunami run-up for these different scenarios with on-site observations, it was possible to reconstruct the landslide dynamics and tsunami generation. Retrofitting of this event utilized both observations of the landslide deposits as well as sea surface withdrawal values.

How heterogeneous coseismic slip affects regional probabilistic tsunami hazard assessment: A case study in the South China Sea

AbstractRupture complexity, typically in the form of heterogeneous slip distribution pattern, significantly affects the local tsunami wave field. However, the effect of rupture complexity is not commonly considered in any form of tsunami hazard assessment. Taking rupture complexity into account significantly increases the computational load, particularly in regional‐scaled probabilistic tsunami hazard assessments (PTHAs) that usually require a large number of simulations based on synthetic scenarios. In this study, we investigate how the heterogeneous slip distribution affects the regional‐scaled PTHA by taking the South China Sea (SCS) as an example. By doing this, we update PTHA for the SCS by incorporating the best available information of seismic tsunamigenic sources along the Manila megathrust. We integrate a stochastic source model into a Monte Carlo‐type simulation, in which a broad range of slip distribution patterns is generated for large numbers of synthetic earthquake events. Green's function technique is employed to efficiently calculate the nearshore tsunami wave amplitude along the SCS coastlines. Our result suggests that for a relatively small and confined region like the SCS, the commonly used approach based on the uniform slip model significantly underestimates tsunami hazard not only in the near‐source region like west Luzon, as expected, but also in the relative far field, such as south China and central Vietnam. Additionally, our sensitivity test of the patch size effects suggests that large patch size is unable to adequately resolve the details of heterogeneous seafloor deformation, and such approaches considerably underestimate the potential tsunami hazard for the SCS coasts.

Intra-community implications of implementing multiple tsunami-evacuation zones in Alameda, California

Tsunami-evacuation planning in coastal communities is typically based on maximum evacuation zones for a single scenario or a composite of sources; however, this approach may over-evacuate a community and overly disrupt the local economy and strain emergency-service resources. To minimize the potential for future over-evacuations, multiple evacuation zones based on arrival time and inundation extent are being developed for California coastal communities. We use the coastal city of Alameda, California (USA), as a case study to explore population and evacuation implications associated with multiple tsunami-evacuation zones. We use geospatial analyses to estimate the number and type of people in each tsunami-evacuation zone and anisotropic pedestrian evacuation models to estimate pedestrian travel time out of each zone. Results demonstrate that there are tens of thousands of individuals in tsunami-evacuation zones on the two main islands of Alameda, but they will likely have sufficient time to evacuate before wave arrival. Quality of life could be impacted by the high number of government offices, schools, day-care centers, and medical offices in certain evacuation zones and by potentially high population density at one identified safe area after an evacuation. Multi-jurisdictional evacuation planning may be warranted, given that many at-risk individuals may need to evacuate to neighboring jurisdictions. The use of maximum evacuation zones for local tsunami sources may be warranted given the limited amount of available time to confidently recommend smaller zones which would result in fewer evacuees; however, this approach may also result in over-evacuation and the incorrect perception that successful evacuations are unlikely.

Pedestrian flow-path modeling to support tsunami evacuation and disaster relief planning in the U.S. Pacific Northwest

Successful evacuations are critical to saving lives from future tsunamis. Pedestrian-evacuation modeling related to tsunami hazards primarily has focused on identifying areas and the number of people in these areas where successful evacuations are unlikely. Less attention has been paid to identifying evacuation pathways and population demand at assembly areas for at-risk individuals that may have sufficient time to evacuate. We use the neighboring coastal communities of Hoquiam, Aberdeen, and Cosmopolis (Washington, USA) and the local tsunami threat posed by Cascadia subduction zone earthquakes as a case study to explore the use of geospatial, least-cost-distance evacuation modeling for supporting evacuation outreach, response, and relief planning. We demonstrate an approach that uses geospatial evacuation modeling to (a) map the minimum pedestrian travel speeds to safety, the most efficient paths, and collective evacuation basins, (b) estimate the total number and demographic description of evacuees at predetermined assembly areas, and (c) determine which paths may be compromised due to earthquake-induced ground failure. Results suggest a wide range in the magnitude and type of evacuees at predetermined assembly areas and highlight parts of the communities with no readily accessible assembly area. Earthquake-induced ground failures could obstruct access to some assembly areas, cause evacuees to reroute to get to other assembly areas, and isolate some evacuees from relief personnel. Evacuation-modeling methods and results discussed here have implications and application to tsunami-evacuation outreach, training, response procedures, mitigation, and long-term land use planning to increase community resilience.

Comparing source inversion techniques for GPS‐based local tsunami forecasting: A case study for the April 2014 M8.1 Iquique, Chile, earthquake

AbstractReal‐time GPS is nowadays considered as a valuable component of next generation near‐field tsunami early warning systems able to provide fast and reliable source parameters. Looking for optimal methodologies and assessing corresponding uncertainties becomes an important task. We take the opportunity and consider the 2014 Pisagua event as a case study to explore tsunami forecast uncertainty related to the GPS‐based source inversion. We intentionally neglect all other sources of uncertainty (observation set, signal processing, wave simulation, etc.) and exclusively assess the effect of inversion technique. In particular, we compare three end‐member methods: (1) point‐source fastCMT (centroid moment tensor), (2) distributed slip along predefined plate interface, and (3) unconstrained inversion into a single uniform slip finite fault. The three methods provide significantly different far‐field tsunami forecast but show surprisingly similar tsunami predictions in the near field.

Initialising landslide-generated tsunamis for probabilistic tsunami hazard assessment in Cook Strait

The Cook Strait Canyon is a submarine canyon which lies within 10 km of Wellington, the capital city of New Zealand. The canyon flanks are scarred with the evidence of past landslides that may have caused large local tsunamis. City planning and civil defence management require information on the magnitude and frequency of these tsunamis to adequately plan for them. Submarine-landslide-generated tsunamis are by nature local features. While they may be catastrophic in the near field, they are generally far smaller scales than co-seismic tsunamis and their energy does not travel very far. Including them within a comprehensive tsunami hazard assessment requires accounting for a large number of potential landslide sources. Unless we only use simple rules of thumb to approximate tsunami height, this requires considerable computing power. This article describes a technique for expanding two-dimensional vertical-slice tsunami generation by landslide modelling into a two-dimensional horizontal surface which can be used for tsunami propagation and inundation modelling. As such, it spans the gap between full three-dimensional modelling of the landslide and simple initialisation.

Evaluación de la peligrosidad del tsunami de Chile del 16 de setiembre del 2015 para Costa Rica

El 16 de setiembre del 2015 un sismo Mw 8.3 en Chile causó un tsunami que afectó únicamente a dicho país. Sin embargo, este sismo motivó la realización de un análisis de peligrosidad de tsunami por parte del Sistema Nacional de Monitoreo de Tsunamis (SINAMOT) para recomendar a la Comisión Nacional de Emergencias (CNE) las medidas a tomar al respecto para Costa Rica. Este evento representó un ejercicio de los procedimientos operativos estandarizados con los que cuenta el SINAMOT y la utilización por primera vez de herramientas locales de pronóstico de alturas y corrientes de tsunamis, además con resultados exitosos. En este artículo se detallan el desarrollo del evento y las herramientas usadas para el análisis de peligrosidad por tsunami, así como los registros que se dieron del tsunami en territorio nacional. Dicho tsunami fue registrado en Costa Rica continental únicamente en mareógrafos, tal y como había sido pronosticado por el SINAMOT. Este artículo tiene un enfoque operativo y muestra los resultados en la misma forma en que se usan durante el evento para la toma de decisiones.

Local tsunami warnings: Perspectives from recent large events

AbstractWe demonstrate a flexible strategy for local tsunami warning that relies on regional geodetic and seismic stations. Through retrospective analysis of four recent tsunamigenic events in Japan and Chile, we show that rapid earthquake source information, provided by methodologies developed for earthquake early warning, can be used to generate timely estimates of maximum expected tsunami amplitude with enough accuracy for tsunami warning. We validate the technique by comparing to detailed models of earthquake source and tsunami propagation as well as field surveys of tsunami inundation. Our approach does not require deployment of new geodetic and seismic instrumentation in many subduction zones and could be implemented rapidly by national monitoring and warning agencies. We illustrate the potential impact of our method with a detailed comparison to the actual timeline of events during the recent 2015 Mw8.3 Illapel, Chile, earthquake and tsunami that prompted the evacuation of 1 million people.

Slip segmentation and slow rupture to the trench during the 2015, Mw8.3 Illapel, Chile earthquake

AbstractThe 2015 Mw8.3 Illapel, Chile earthquake is the latest megathrust event on the central segment of that subduction zone. It generated strong ground motions and a large (up to 11 m runup) tsunami which prompted the evacuation of more than 1 million people in the first hours following the event. Observations during recent earthquakes suggest that these phenomena can be associated with rupture on different parts of the megathrust. The deep portion generates strong shaking while slow, large slip on the shallow fault is responsible for the tsunami. It is unclear whether all megathrusts can have shallow slip during coseismic rupture and what physical properties regulate this. Here we show that the Illapel event ruptured both deep and shallow segments with substantial slip. We resolve a kinematic slip model using regional geophysical observations and analyze it jointly with teleseismic backprojection. We find that the shallow and deep portions of the megathrust are segmented and have fundamentally different behavior. We forward calculate local tsunami propagation from the resolved slip and find good agreement with field measurements, independently validating the slip model. These results show that the central portion of the Chilean subduction zone has accumulated a significant shallow slip deficit and indicates that, given enough time, shallow slip might be possible everywhere along the subduction zone.

Household evacuation characteristics in American Samoa during the 2009 Samoa Islands tsunami.

Tsunamis represent significant threats to human life and development in coastal communities. This quantitative study examines the influence of household characteristics on evacuation actions taken by 211 respondents in American Samoa who were at their homes during the 29 September 2009 Mw 8.1 Samoa Islands earthquake and tsunami disaster. Multiple logistic regression analysis of survey data was used to examine the association between evacuation and various household factors. Findings show that increases in distance to shoreline were associated with a slightly decreased likelihood of evacuation, whereas households reporting higher income had an increased probability of evacuation. The response in American Samoa was an effective one, with only 34 fatalities in a tsunami that reached shore in as little as 15 minutes. Consequently, future research should implement more qualitative study designs to identify event and cultural specific determinants of household evacuation behaviour to local tsunamis.

DETERMINED SOIL CHARACTERISTIC OF PALU IN INDONESIA BY USING MICROTREMOR OBSERVATION

The Palu city, which is located in the active seismic zone of the Palu-Koro fault. Several powerful earthquakes have struck Palu city at previous years. One of the greatest earthquake event occurred in year 2006 which 6.2 Mw, about more a hundred buildings and houses were collapsed due to this earthquake. Palu-Koro fault also produced several tsunami events in year of 1927, 1968 and 1996. The latest earthquake struck Palu on 2012 (7.2 Mw) and it produced local tsunami. Since no soil profile map of Palu region available, we performed single observations of microtremors at 122 sites in Palu. Its results enabled us to estimate the site dependent amplification characteristics of earthquake ground-motion. We also conducted a 8-site microtremor array investigation to gain a representative determination of the soil condition of subsurface structures in Palu. From the dispersion curve of array observations, the central business district of Palu corresponds to relatively from soft soil to medium condition. Predominant periods due to horizontal vertical ratios (HVSRs) in Palu are in the range of 1.2 to 2.5 s

Geological and Sedimentological Evidence of a Large Tsunami Occurring ~1100 Year BP from a Small Coastal Lake along the Bay of La Paz in Baja California Sur, Mexico

The importance of small-scale seismic events in enclosed water bodies, which can result in large tsunami waves capable of affecting comprehensive damage over small, geographically-confined areas are generally overlooked, although recognizing the occurrence of such events is a necessary element in adequately assessing the risk of natural hazards at specific locations. Here we present evidence for a probable large localized tsunami that occurred within the Bay of La Paz, Baja California Sur, ~1100 year before present (BP), which resulted in the creation of a shelly ridge at an elevation of ~2 m above mean high water (MHW). This ridge consists of a continuous wedge of poorly mixed marine sands and shells ~50 cm in depth deposited along the entire seaward edge of the lake. The marine shells collected from terrestrial environments around the lake include species from a variety of environments, including offshore species with minimum preferred depths of >13 m. The evidence suggests that this material was likely deposited by a tsunami with a runup of 2–3.6 m above MHW, probably associated with the slumping of an island along the tectonically active eastern edge of the bay.

The 1755 Lisbon Tsunami at Vila do Bispo Municipality, Portugal

The numerical model of the 1755 Lisbon Tsunami was conducted at Vila do Bispo municipality. The tsunami source area of this historical event is assumed to be on the Gorringe Bank. Numerical model results of historical data have been validated by a combined analysis of eyewitnesses’ accounts, field surveys, geological records and archeological findings (conducted by previous authors). The coastal area has remained a mostly natural landscape since the 18thcentury, with popular beaches that increase the population potentially exposed to a tsunami, especially one occurring in summer. For these reasons, we use local tsunami hazard assessment as a criterion varying between moderate and critical. Results also show that the 16 low ground areas we surveyed were inundated between 15 and 30 minutes after the earthquake. For this reason, persons should act quickly in future to evacuate areas immediately after an earthquake and to move to higher ground. Results also show that safe, swift evacuation may be difficult in 50% of these areas, endangering special populations such as tourists. Tsunami information boards should therefore be put on beaches and evacuation exercises and drills should be implemented and practiced regularly.

Time Evolution of Man-Made Harbor Modifications in San Diego: Effects on Tsunamis

San Diego, one of the largest ports on the U.S. West Coast and home to the largest U.S. Navy base, is exposed to various local and distant tsunami sources. During the first half of the twentieth century, extensive modifications to the port included but were not limited to dredging, expansion of land near the airport and previous tidal flats, as well as creation of jetties. Using historical nautical charts and available Digital Elevation Models, this study gives an overview of changes to San Diego harbor in the last 150+ years due to human intervention and examines the effects of these changes on tsunamis. Two distant and two local scenarios were selected to demonstrate the impact of modified nearshore topography and bathymetry to incoming tsunamis. Inundation pattern, flow depths, and flooded localities vary greatly from year to year in the four scenarios. Specifically, flooded areas shift from the inner harbor to outer locations. Currents induced by the distant tsunamis intensify with modifications and shift from locations primarily outside the harbor to locations inside. A new characteristic in tsunami dynamics associated with port modifications is the introduction of high current spots. Numerical results also show that the introduction of high currents could threaten navigation, vessels, and facilities at narrow openings and also along the harbor “throat”—therefore, at an increased number of locations. Modifications in the port show that changes could have a negative but also a positive impact through constraint of flooding outside of the harbor and shifting of high currents to locations of minimal impact. The results of this study may be used as a first step toward future harbor design plans to reduce tsunami damages.

Rapidly Estimated Seismic Source Parameters for the 16 September 2015 Illapel, Chile M w 8.3 Earthquake

On 16 September 2015, a great (M w 8.3) interplate thrust earthquake ruptured offshore Illapel, Chile, producing a 4.7-m local tsunami. The last major rupture in the region was a 1943M S 7.9 event. Seismic methods for rapidly characterizing the source process, of value for tsunami warning, were applied. The source moment tensor could be obtained robustly by W-phase inversion both within minutes (Chilean researchers had a good solution using regional data within 5 min) and within an hour using broadband seismic data. Short-period teleseismic P wave back-projections indicate northward rupture expansion from the hypocenter at a modest rupture expansion velocity of 1.5–2.0 km/s. Finite-fault inversions of teleseismic P and SH waves using that range of rupture velocities and a range of dips from 16°, consistent with the local slab geometry and some moment tensor solutions, to 22°, consistent with long-period moment tensor inversions, indicate a 180- to 240-km bilateral along-strike rupture zone with larger slip northwest to north of the epicenter (with peak slip of 7–10 m). Using a shallower fault model dip shifts slip seaward toward the trench, while a steeper dip moves it closer to the coastline. Slip separates into two patches as assumed rupture velocity increases. In all cases, localized ~5 m slip extends down-dip below the coast north of the epicenter. The seismic moment estimates for the range of faulting parameters considered vary from 3.7 × 1021Nm(dip 16°) to 2.7 × 1021 Nm (dip 22°), the static stress drop estimates range from 2.6 to 3.5 MPa, and the radiated seismic energy, up to 1 Hz, is about 2.2–3.15 × 1016 J.

Numerical identification of tsunami boulders and estimation of local tsunami size at Ibaruma reef of Ishigaki Island, Japan

AbstractTsunami boulders deposited along the coast constitute important geological evidence for paleotsunami activity. However, boulders can also be deposited by large storm waves. Although several sedimentological and theoretical methods have been proposed to differentiate tsunami and storm wave affected boulders, no appropriate numerical method exists for their differentiation. Therefore, we developed a new numerical scheme to differentiate tsunami and storm wave boulders for coastal boulders on Ishigaki Island, Japan. In this area, tsunami and storm waves have emplaced numerous boulders on the reef and the coast. By conducting numerical calculations of storm waves in this region, we estimated the size of a storm wave that can explain the maximum clast size distribution of boulders on the reef. Consequently, we showed that a wave with a combination of 8 m in initial wave height and 10 s period can satisfy the above conditions when we assume mean sea level. In contrast to the boulders on the reef, all boulders deposited along the shore are heavier than the calculated possible maximum clast size distribution by the storm wave. Therefore, we confirmed these boulders as being of tsunami origin. Results of previous studies showed that they were most likely deposited or reworked by the 1771 Meiwa tsunami. Then, using the tsunami boulders, we numerically estimated the wave period and amplitude of the 1771 Meiwa tsunami, which should have had a 4–5 min period and 5.6–5.9, 6.3–7.0 m amplitude, respectively. Using the proposed scheme, it is possible to differentiate tsunami and storm wave boulders and estimate the size of past storm waves and tsunami waves, although it is noteworthy that there are exceptions for which the scheme cannot be applied.

Beat-the-wave evacuation mapping for tsunami hazards in Seaside, Oregon, USA

Previous pedestrian evacuation modeling for tsunamis has not considered variable wave arrival times or critical junctures (e.g., bridges), and did not effectively communicate multiple evacuee travel speeds. We summarize an approach that identifies evacuation corridors, recognizes variable wave arrival times, and produces a map of minimum pedestrian travel speeds to reach safety, termed a “beat-the-wave” (BTW) evacuation analysis. We demonstrate the improved approach by evaluating difficulty of pedestrian evacuation of Seaside, Oregon, for a local tsunami generated by a Cascadia subduction zone earthquake. We establish evacuation paths by calculating the least-cost distance (LCD) to safety for every grid cell in a tsunami hazard zone using geospatial, anisotropic path distance algorithms. Minimum BTW speed to safety on LCD paths is calculated for every grid cell by dividing surface distance from that cell to safety by the tsunami arrival time at safety. We evaluated three scenarios of evacuation difficulty: (1) all bridges are intact with a 5-min evacuation delay from the start of earthquake, (2) only retrofitted bridges are considered intact with a 5-min delay, and (3) only retrofitted bridges are considered intact with a 10-min delay. BTW maps also take into account critical evacuation points along complex shorelines (e.g., peninsulas, bridges over shore-parallel estuaries) where evacuees could be caught by tsunami waves. The BTW map is able to communicate multiple pedestrian travel speeds, which are typically visualized by multiple maps with current LCD-based mapping practices. Results demonstrate that evacuation of Seaside is problematic seaward of the shore-parallel waterways for those with any limitations on mobility. Tsunami vertical evacuation refuges or additional pedestrian bridges may be effective ways of reducing loss of life seaward of these waterways.

Tsunami potential from local seismic sources along the southern Middle America Trench

The Middle America convergent margin is capable of generating tsunamigenic earthquakes as portrayed through historical data. However, historical tsunamigenic sources appear to be unevenly distributed along the Middle America convergent margin. The last significant tsunami generated along the southern segments of this convergent margin occurred in 1992 causing 10-m run-up in Nicaragua. In particular, the Costa Rican Pacific coast has not clear evidences of significant wave heights or local tsunami events. Partly this could be explained by the lack of historical data since paleotsunami information is not available; thus, no tsunami data beyond 500 years could be found. Nevertheless, large rupture areas or shallow ruptures offshore Costa Rica cannot be neglected. We hereby present the results of numerical simulations aimed to assess the tsunamigenic potential of seismic sources offshore southern Central America. Our approach follows the deterministic method where earthquake maximum credible scenarios are assessed. We assumed hundreds of fault ruptures to obtain surface displacement and tsunami wave heights at the shoreline to assess the tsunami threat along south Nicaragua and Costa Rica. These fault scenarios are based on historical earthquakes as well as on the newest geodetic studies that provided slip deficit. Shallow part of locked segments in this region could release accumulated stress in the near future and may generate tsunamis similar to the 1992 Nicaragua M w 7.6 and 2012 El Salvador M w 7.4 tsunami earthquakes. The numerical simulations presented in this study show that maximum wave heights up to 2–6 m could affect the southern Pacific coast of Central America even when moderate-to-large M w 7.0–8.0 interplate or outer rise earthquakes occur.

Estimation of Cascadia Local Tsunami Loads on Pacific Northwest Bridge Superstructures

Abstract The time histories of Cascadia Subduction Zone (CSZ) local tsunami horizontal and vertical loads and the overturning moment on four selected Oregon coastal bridges are computed for different load scenarios. These scenarios, containing the tsunami-free surface elevation and flow horizontal velocity components, are developed using various CSZ rupture models. One of the bridges is selected to evaluate the effect of various bridge geometry characteristics on the resultant tsunami loads. Maximum tsunami horizontal and downward vertical loads are found to occur approximately simultaneously when the tsunami flow reaches the landward side of the bridge cross section and overtops the barrier. A comparison between tsunami loads on a deck-girder bridge and a box-girder bridge under identical tsunami flow condition reveals that on box-girder bridges, (1) maximum horizontal loads are slightly larger, (2) downward vertical loads are smaller, and (3) uplift loads are significantly larger. It is shown that bridg...

Landslides and Megathrust Splay Faults Captured by the Late Holocene Sediment Record of Eastern Prince William Sound, Alaska

Abstract We present new marine seismic‐reflection profiles and bathymetric maps to characterize Holocene depositional patterns, submarine landslides, and active faults beneath eastern and central Prince William Sound (PWS), Alaska, which is the eastern rupture patch of the 1964 M w 9.2 earthquake. We show evidence that submarine landslides, many of which are likely earthquake triggered, repeatedly released along the southern margin of Orca Bay in eastern PWS. We document motion on reverse faults during the 1964 Great Alaska earthquake and estimate late Holocene slip rates for these growth faults, which splay from the subduction zone megathrust. Regional bathymetric lineations help define the faults that extend 40–70 km in length, some of which show slip rates as great as 3.75 mm/yr. We infer that faults mapped below eastern PWS connect to faults mapped beneath central PWS and possibly onto the Alaska mainland via an en echelon style of faulting. Moderate ( M w >4) upper‐plate earthquakes since 1964 give rise to the possibility that these faults may rupture independently to potentially generate M w 7–8 earthquakes, and that these earthquakes could damage local infrastructure from ground shaking. Submarine landslides, regardless of the source of initiation, could generate local tsunamis to produce large run‐ups along nearby shorelines. In a more general sense, the PWS area shows that faults that splay from the underlying plate boundary present proximal, perhaps independent seismic sources within the accretionary prism, creating a broad zone of potential surface rupture that can extend inland 150 km or more from subduction zone trenches.