Application of geophysics for marine geohazard assessment

Tariq Rahiman

May 23, 2019

Nearshore earthquakes and tsunamis are two naturally occurring geological processes which pose a risk to people and infrastructure, especially in tectonically active regions of the world. Engineers require accurate information on the location, magnitude and where possible frequency of such events to incorporate induced loads and impacts in the design of nearshore and coastal infrastructure. A case study is presented where high-resolution marine geophysical imaging techniques were used to locate and assess earthquake and tsunami hazards near Suva, the capital city of Fiji. Fiji is located in a seismically active and complex plate boundary zone between the Pacific and Australian tectonic plates in the southwest Pacific. The coast along Suva is a highly developed and populated part of Fiji and is vulnerable to the effects of large earthquakes that are expected to occur both onshore and offshore. High resolution SeaBAT 8160 multibeam bathymetry data and Bubble-Pulser seismic reflection data show in unprecedented detail the morphology and structure of the seabed beyond the coast of Suva, including active fault earthquake source structures and submarine slide tsunami sources. The faults show evidence for historical coseismic surface fault rupture, displacement of Quaternary geological or geomorphological features, and geographic association of the faults with strong earthquakes and well located epicentral data on micro to small earthquakes. A deterministic seismic hazard analysis shows the maximum credible earthquakes capable on these structures range from MW 6.8 to 7.6 and the estimated horizontal peak ground acceleration (PGA) value estimated for Suva range from 0.40g to 0.62g. Earthquake activity is primarily responsible for submarine slides that occur on the submarine slopes near Suva. The source of the destructive 1953 Suva tsunami that followed the Ms 6.75 Suva earthquake was a 60 million cubic metre submarine slide at the head of the Suva Canyon 4 km to the WSW of Suva City. A test simulation of this tsunami using the Geowave tsunami generation, propagation and inundation model, successfully reproduces the observed wave heights and arrival times at near-field localities. A predictive simulation using Geowave, based on an incipient failure in the 1953 source area and on a potentially worse-case scenario condition, is used to show a maximum vertical run up of at least 4 m and a maximum horizontal inundation level of at least 400 m at the Suva coast. Risk mitigations strategies that have resulted from this work have included raising public awareness, development of evacuation maps and warning systems, land use zoning and building codes for the inundation zone.