Effects of an impermeable layer on pore pressure response to tsunami-like inundation

Tsunami hazards have been observed to cause soil instability resulting in substantial damage to coastal infrastructure. Studying this problem is difficult owing to tsunamis’ transient, non-uniform and large loading characteristics. To create realistic tsunami conditions in a laboratory environment, we control the body force using a centrifuge facility. With an apparatus specifically designed to mimic tsunami inundation in a scaled-down model, we examine the effects of an embedded impermeable layer on soil instability: the impermeable layer represents a man-made pavement, a building foundation, a clay layer and alike. The results reveal that the effective vertical soil stress is substantially reduced at the underside of the impermeable layer. During the sudden runup flow, this instability is caused by a combination of temporal dislocation of soil grains and an increase in pore pressure under the impermeable layer. The instability during the drawdown phase is caused by the development of excess pore-pressure gradients, and the presence of the impermeable layer substantially enhances the pressure gradients leading to greater soil instability. The laboratory results demonstrate that the presence of an impermeable layer plays an important role in weakening the soil resistance under tsunami-like rapid runup and drawdown processes.

NUMERICAL SIMULATION OF TSUNAMI HAZARDS IN SOUTH ATLANTIC COAST: CASE OF THE CITY OF AGADIR – MOROCCO: PRELIMINARY RESULT

The coastal zone is a highly complex area because of its location at the interface between land and sea and as a preferred location for many forms of development. A mega tsunami from the Canary Islands will hit not only the Atlantic coasts of Morocco, but also Spain, Portugal, Great Britain and even reach US shores.A slight earthquake or possible volcanic eruption can trigger one of the most violent natural disasters in history. Indeed, according to Steven Ward and Simon Day (2001) the west flank of the Cumbre Vieja volcano, located on the island of Palma is unstable and could, as a result of a future eruption, collapse into the ocean. It would be in the worst scenario of a huge piece of 25 km long, 15 wide and 1,400 meters thick that would come off, a total of 500 cubic kilometers of land and rocks. This wave could reach 50 meters of height, once arrived on the Moroccan coasts. In this study, a numerical inundation and vulnerability models are used to identify the location and nature of current and future hazards and risk on the Moroccan coast to better understand the tsunami hazard and vulnerability along the Moroccan coast. We have worked on the correction of coastlines from satellite imagery on Google Earth and the digitization of bathymetric and topographic maps, in order to create digital elevation models (DEM). We have also studied the vulnerability assessment of the buildings by using the BTV model (Building Tsunami Vulnerability) such as a combination of tsunami inundation numerical modelling, field survey data and geographic information system.

Geological evidence of an unreported historical Chilean tsunami reveals more frequent inundation

Assessing tsunami hazards commonly relies on historical accounts of past inundations, but such chronicles may be biased by temporal gaps due to historical circumstances. As a possible example, the lack of reports of tsunami inundation from the 1737 south-central Chile earthquake has been attributed to either civil unrest or a small tsunami due to deep fault slip below land. Here we conduct sedimentological and diatom analyses of tidal marsh sediments within the 1737 rupture area and find evidence for a locally-sourced tsunami consistent in age with this event. The evidence is a laterally-extensive sand sheet coincident with abrupt, decimetric subsidence. Coupled dislocation-tsunami models place the causative fault slip mostly offshore rather than below land. Whether associated or not with the 1737 earthquake, our findings reduce the average recurrence interval of tsunami inundation derived from historical records alone, highlighting the importance of combining geological and historical records in tsunami hazard assessment.

Exposures of Building and Population to Tsunami Hazard in Pangandaran Beach, Indonesia

Pangandaran and Pananjung villages are located in the southern coast of Java Island, prone to tsunami hazard originating from a megathrust earthquake off south of Java Island. Those villages experience a tsunami earthquake on 2006 from an M7.8 earthquake. The National Center for Earthquake Studies released a map of the sources and hazards of Indonesia’s earthquake in 2017 with a potential earthquake of magnitude 8.7-9.2 in the megathrust of Java Island. This research aims to estimate the potential number of buildings and the population affected by tsunami inundation from two scenario; first scenario is based on historical event of a M7.8 intraplate earthquake, and second scenario is based on a plausible M8.7 intraplate earthquake. The first scenario tsunami modeling resulted an inundation of 108.606 ha, while in the second scenario estimate an 867.351 ha of inundation area. Building data is obtained by digitizing aerial photographs taken in November 2021. The calculation of potential affected buildings is carried out by overlaying the tsunami inundation data with the existing building data in the study area. Meanwhile, the population data used is obtained from the local government in 2021. To obtain the number of the affected population, population distribution is first carried out in each class of land cover, overlaid with the tsunami inundation data. The estimated number of buildings and population affected by scenario 1 and 2 in Pangandaran Village is 1,040 buildings along with 2,765 people, and 4,216 buildings with 11,209 people respectively. While in Pananjung Village, it is estimated a total of 149 buildings with 350 people affected, and 4,039 buildings with 9,493 people affected respectively. This indicate that scenario 2 impact is potentially 4 times greater than scenario 1 in Pangandaran village, and 27 times greater in Pananjung village, implying a different strategy of tsunami risk reduction should be taken to save more lives. The results of this study can be used as a basis for policymaking by the government in carrying out a more effective tsunami disaster mitigation efforts in Pangandaran and Pananjung Villages. This study also calls for reevaluation of coastal villages tsunami risk based on each plausible scenario.