Tsunami Modeling of Caribbean Sources Affecting The North Coast of Puerto Rico

Abstract

The location of Puerto Rico makes it vulnerable to seismic activity due to the different sources that surround the Island. This is why the risk of a tsunami affecting the island increases as it did in 1918. The Puerto Rico Trench, located to the north of the island, is one of the most interesting sources since there are two escarpments with great potential to generate tsunamis that would directly affect the towns on the north coast of the island. In this study we can observe the possible effects and the extension of a tsunami, generated by submarine landslides in the two escarpments, on the north coast of the island, specifically in the municipalities of Toa Baja, Cataño and Bayamón called in summary and for purpose of this study Rio Hondo. Analyzing and comparing flooding on land caused by tsunamis generated due to underwater landslides can help determine the effectiveness of existing flood maps for the area. Generally, submarine landslides near the coast cause potentially catastrophic tsunamis because they are more locally concentrated and the arrival time of a wave is extremely short.

Introduction

After the Sumatra earthquake in 2004, which caused a mega tsunami affecting the coasts of seven countries around the Indian Ocean, many scientist considered the possibility of similar events due to the limits of the plates that may affect the coasts of the other regions such as; United States, Puerto Rico and the Virgin Islands of the United States. One of the areas of interest is the Puerto Rico Trench, which is one of the deepest parts of the Atlantic Ocean and is located where the North American Plate (NOAM) is subducted under the Caribbean plate (Ten Brink et al., 2004). This plate extends around 3200 km from north Central America to the northern end of the sub-area of the Lesser Antilles.

Due to the geological and tectonic configuration at the north of the Caribbean tsunamis of at least 12 m height may be generated. The evidence of its effects has extended to distances up to 2200 km (Grindlay et al., 2005). This is the reason an anomaly of gravity is present in the negative free air on Earth with -380 m Gal, located 50 km south of the Trench, where the water depth is 7950 m. Over time, the tsunami generating sources using these areas have been improved, such as the fall of the shelf platform along the Virginia-North Carolina margin and the fall of volcanic buildings in the Islands. Canary Islands.

Evidence of erosion and tectonic collapse exist on a large scale, related to the oblique subduction located on the south-east platform of the Bahamas, Puerto Rico and the Virgin Islands. For Grindlay (2005), the existence of something more alarming than the historical record of tsunamis is the presence of large-scale submarine landslide features, that may have produced huge prehistoric tsunamis on the northern margin of Puerto Rico. The recent concern in the scientific world is due to the rapid population increase in the northern Caribbean, which means that future tsunamis could be much more deadly or catastrophic compared to those that have occurred in the past.

In the last few years, two large amphitheater-shaped landslides have been mapped, known as Arecibo and Loiza, which are located on the southern slope of the Puerto Rico Trench, about 37 kilometers off the north coast of Puerto Rico. These escarpments are up to 55 km wide, have a crown in a water depth of approximately 3000 m, they extend to a water depth of about 6000 m and are excavated at the edge of a neogonium carbonate platform with an inclination towards the north at depths about 3km underwater. The ages of these escarpments are unknown (Ten Brink et al., 2004).

Initially, these escarpments or landslides were previously recognized using long-range (GLORIA) and far-reaching seismic lateral seismic survey and seismic reflection data. (Grindlay et al., 2005; Schwab et al., 1991). After this, a more detailed mapping was done using lateral sweeping bathymetry images and higher resolution sonar image. It is thought that although it is unlikely that the amphitheaters were formed during a single catastrophic event, however, the scientists observed that, the estimated total volume of material collapsed in one of the landslide areas is similar to the calculated volume of the event that occurred 7000 years ago in front of Norway and that generated a tsunami of 25 to 30 m height along the coast of Scotland. The images of bathymetry and lateral scanning show that the semicircular escarpments did not form themselves by a single catastrophic failure, as previously suggested, but that they have been formed by a continuous retrograde fall of smaller segments and fissures located near the edge of the carbonate platform indicating that the fall process is ongoing.

It is feared that in addition to an earthquake related shock, the flow of groundwater from Puerto Rico to the underwater springs near the cracking sites on the shelf may play a role in the eventual failure of the platform. These flows could slowly erode the carbonate platform and contribute to small landslides that could cause tsunamis.

In addition, in the margin of the platform, the slope of the seabed reaches 45 °, and the depth of the water increases by 4000 m. These characteristic potentially increases the risk of tsunami on the north coast of Puerto Rico due to faults in the submarine slope that can to be provoked spontaneously or by small earthquakes. According to Mercado (2002), rapid submarine slope failures can take many shapes: falls, slumps, slides, debris flows, grain flows, and turbidity currents, and the amphitheater-shaped scarp along the south slope of the Puerto Rico Trench is the result of a landslide with the characteristics of a debris flow. Underwater landslides also pose difficulties for tsunami warning systems, as they often occur on coastal margins near coasts, they can not be predicted and can occur within minutes of a moderate earthquake.

The Puerto Rico Seismic Network (PRSN) has developed tsunami inundation and evacuation maps, based on seismic events which can be found online in the Puerto Rico Seismic Network internet pages. These maps have been created simulating the possible events that could be generated from hundreds of faults around Puerto Rico in order to create a Maximum of Maximums (MOM). Nonetheless, these maps do not take into consideration any other potential tsunami source, such as submarine landslides, volcanic eruptions, among others.

The objetive of this study was to quantify the extent of a tsunami inundation along the municipalities of Toa Baja, Cataño and Bayamón, named as Rio Hondo in the proposal summary, on northern of Puerto Rico using two offshore submarine landslide sources, known as the landslide escarpment of Loiza amphitheater and the landslide escarpment of Arecibo amphitheater. The purpose of this, is to compare the floods between this two sources and then compared with the tsunami flood and evacuation maps provided by the Puerto Rico Seismic Network to quantify how many assembly points are inside the potential danger zone.

According to Mercado (2002), the modeling of landslide tsunamis is much more difficult than the modeling of earthquake tsunamis, since the time scale of the displacement of the bottom is much longer than those of the bottom displacement due to an earthquake. The generation of waves due to landslides depends mainly on the volume of material moved, the depth of immersion and the speed of the downhill movement. As the speed of the slide increases, and is closer in magnitude to the speed of the tsunami wave, the response of the sea surface increases.

Bathymetry grids were used for the modeling of landslides, paying more attention to the 1/3 arc seconds grids obtained from the National Geophysical Data Center (NGDC) of NOAA (www.ngdc.noaa .gov). Bathymetric grids of the landslide escarpments of the Loiza and Arecibo amphitheatres were used at a resolution of 5 arc seconds, scenarios used to produce the propagation of the wave amplitudes that were compared in the study. The squares of 1/3 second of arc and 5 seconds of arc were constructed, ordered and nested using the software SURFER v.10 described by López-Venegas (2008). The propagation of the tsunami wave and the flooding of this study were simulated using the numerical model algorithm Ocean WAVE non-hydrostatic evolution (NEOWAVE) and the tsunami solution using the Navier-Stokes algorithm with multiple interfaces (TSUNAMI3D) that are a three-dimensional model solves the Navier Stokes equations with two different phases of fluids, water and sliding material.

To achieve this study, a series of nested grids were created. These grids are increasing in resolution as time decreases so they go from lower resolution with 3 minute grids, to higher resolution with 1-3 arc-second grids.

Once the floods were correctly generated, the Maximun Inundation Grid data from each run was extracted to create the flood maps using the ArcMap program. With these, a detailed evaluation of the results was made using Geographic Information Systems (GIS) in order to compare the flood generated by the amphitheater of Loiza with the flood generated by the amphitheater of Arecibo in the municipalities of Toa Baja, Cataño and Bayamón. Once these floods were compared, we proceeded to compare each of them with the flood maps of the towns of Toa Baja and Cataño supplied by the Puerto Rico Seismic Network to determine the assembly points that would be within the flood zone if a tsunami by underwater landslide was generated in the Puerto Rico Trench.

Discussion/ Interpretation

Comparing the tide gauges located along the coastline of the municipalities of Toa Baja and Cataño, a great similarity is observed with respect to the arrival patterns of the wave fronts of the tsunami models generated by each amphitheater. In general, the arrival curves of the first wave front follow exactly the same pattern with a difference of 300 to 400 seconds of arrival and 200 to 400 centimeters height, with the exception of the eighth tide gauges who read a difference of approximately 1,460cm.

Comparing the models of the hypothetical flood extensions generated by the landslides in the amphitheater of Arecibo and Loiza and the flood exhaustion generated by seismic events created by the Seismic Network of Puerto Rico, can be said that a landslide generated by the escarpment of the amphitheater of Arecibo has the capacity to cause greater damages and cover an extension of earth greater than the flood generated by the escarpment of the amphitheater of Loiza and a seismic event to the north of Puerto Rico, due the amount of displaced material of 375km^3. Comparing the models of the tsunami inundation, caused by a hypothetical landslide in the Arecibo and Loiza Amphitheater we can see that the area of inundation generated by the Loiza Amphitheater is less comparing with the area of inundation generated by the Arecibo Amphitheater due to the amount of displaced material of de Loiza escarpement is of 168km^3.

The extension of land flooded by the escarpment of the amphitheater of Arecibo, was approximately 30.2 km^2, flooding an area of 11.7 km^2 of the municipality of Cataño, 13.4 km^2 of the municipality of Toa Baja, 4.9 km^2 of the municipality of Bayamón and 0.16 km^2 of the municipality of Guaynabo. This impacted a population of approximately 40,613 people through the affected municipalities. On the other hand, the extension of land flooded by the escarpment of the amphitheater of Loiza, was approximately 21.6 km^2, flooding an area of 9.8 km^2 of the municipality of Cataño, 10.9 km^2 of the municipality of Toa Baja , 0.81km^2 of the municipality of Bayamón and 0.13km^2 of the municipality of Guaynabo. This affected a population of approximately 29,047 people through the affected municipalities. Finally, the extension of land flooded by seismic event created by the Seismic Network of Puerto Rico, was approximately 18.6 km^2, flooding an area of 7.1km^2 of the municipality of Cataño, 11 km^2 of the municipality de Toa Baja, 0.53 km^2 from the municipality of Bayamón. This affected a population of approximately 25,013 people through the affected municipalities.

Comparing the flood model generated by the amphitheater of Arecibo with that of the Red Sismica, we observed that three assembly points determined by the Seismic Network of Puerto Rico, eleven schools, five hospitals and six interest rates as Palo Seco Power Plant in Toa Baja and the Industrial Zone and the Treatment Plant of the AAA of Cataño, would be inside the flood zone. On the other hand, when comparing the flood model generated by the amphitheater of Loiza with that of the Red Seismic, we observed that two assembly points determined by the Seismic Network of Puerto Rico, ten schools, four hospitals and three points of interest as Palo Seco power plant in Toa Baja and the Industrial Zone and the Treatment Plant of the AAA of Cataño, would be inside the flood zone

Through this study, the vulnerability of various types of public and private infrastructure and the vulnerability of the assembly areas determined by the Seismic Network of Puerto Rico were observed. The latter was due to the fact that to determine the zones of assemblies in Puerto Rico, tsunamis generated by other types of sources other than those generated by seismic events were not considered. This means that the evaluation of multiple scenarios with tsunamis from different sources is important because it could elucidate the accuracy of the evacuation zone maps since time could be a limiting factor in the evacuation process should an underwater landslide occur in the Puerto Rico Trench.

Conclusion/Summary

The Puerto Rico Trench located north of the coast of the island of Puerto Rico has a tsunami and seismic potential. The bathymetric studies provide evidence of large submarine landslides as a possible source of a tsunami that would immediately affect the north coast of the island. Through this study, hypothetical event models generated from the escarpments of Arecibo and Loiza amphitheaters, located in the Puerto Rico Trench, were compared with the tsunami flood model caused by earthquakes developed by the Puerto Rico Seismic Network.

It was found that the flood of the tsunami caused by a landslide generated by the amphitheater of Arecibo covered approximately 11.6 km^2 more than the model of the Seismic Network of Puerto Rico of the flood of the tsunami caused by the earthquakes and that the amphitheater of Loiza covered 3km^2 more than the model of the Seismic Network. This led to the demonstration that if a tsunami generated by some of the two aforementioned amphitheaters of three to four assembly points were to occur, they would be within the flood zone.

Through this study, the importance of creating evacuation maps considering other sources of tsunamis in addition to seismic events has been demonstrated, due to the extent of the flood of tsunamis generated by these sources could affect an area greater than that stipulated in the flood and evacuation maps developed.