The Role of Frontal Thrusts in Tsunami Earthquake Generation

ABSTRACT The frontal sections of subduction zones are the source of a poorly understood hazard: “tsunami earthquakes,” which generate larger-than-expected tsunamis given their seismic shaking. Slip on frontal thrusts is considered to be the cause of increased wave heights in these earthquakes, but the impact of this mechanism has thus far not been quantified. Here, we explore how frontal thrust slip can contribute to tsunami wave generation by modeling the resulting seafloor deformation using fault-bend folding theory. We then quantify wave heights in 2D and expected tsunami energies in 3D for both thrust splays (using fault-bend folding) and down-dip décollement ruptures (modeled as elastic). We present an analytical solution for the damping effect of the water column and show that, because the narrow band of seafloor uplift produced by frontal thrust slip is damped, initial tsunami heights and resulting energies are relatively low. Although the geometry of the thrust can modify seafloor deformation, water damping reduces these differences; tsunami energy is generally insensitive to thrust ramp parameters, such as fault dip, geological evolution, sedimentation, and erosion. Tsunami energy depends primarily on three features: décollement depth below the seafloor, water depth, and coseismic slip. Because frontal ruptures of subduction zones include slip on both the frontal thrust and the down-dip décollement, we compare their tsunami energies. We find that thrust ramps generate significantly lower energies than the paired slip on the décollement. Using a case study of the 25 October 2010 Mw 7.8 Mentawai tsunami earthquake, we show that although slip on the décollement and frontal thrust together can generate the required tsunami energy, <10% was contributed by the frontal thrust. Overall, our results demonstrate that the wider, lower amplitude uplift produced by décollement slip must play a dominant role in the tsunami generation process for tsunami earthquakes.

Marine forearc structure of eastern Java and its role in the 1994 Java tsunami earthquake

We resolve a previously unrecognized shallow subducting seamount from a re-processed multichannel seismic profile crossing the 1994 Mw 7.8 Java tsunami earthquake rupture area. Seamount subduction occurs where the overriding plate experiences uplift by lateral shortening and vertical thickening. Pronounced back-thrusting at the landward slope of the forearc high and the formation of splay faults branching off the landward flank of the subducting seamount are observed. The location of the seamount in relation to the 1994 earthquake hypocentre and its co-seismic slip model suggests that the seamount acted as a seismic barrier to the up-dip co-seismic rupture propagation of this moderate-size earthquake.

Large slip, long duration, and moderate shaking of the Nicaragua 1992 tsunami earthquake caused by low near-trench rock rigidity

Large earthquake ruptures propagating up to areas close to subduction trenches are infrequent, but when they occur, they heavily displace the ocean seafloor originating destructive tsunamis. The current paradigm is that the large seafloor deformation is caused by local factors reducing friction and increasing megathrust fault slip, or prompting the activation of ancillary faults or energy sources. As alternative to site-specific models, it has been proposed that large shallow slip could result from depth-dependent rock rigidity variations. To confront both hypotheses, here, we map elastic rock properties across the rupture zone of the MS7.0-MW7.7 1992 Nicaragua tsunami earthquake to estimate a property-compatible finite fault solution. The obtained self-consistent model accounts for trenchward increasing slip, constrains stress drop, and explains key tsunami earthquake characteristics such as long duration, high-frequency depletion, and magnitude discrepancy. The confirmation that these characteristics are all intrinsic attributes of shallow rupture opens new possibilities to improve tsunami hazard assessment.

What Is Tsunami Earthquake?

A tsunami earthquake is defined as an earthquake which induces abnormally strong tsunami waves compared with its seismic magnitude (Kanamori 1972; Kanamori and Anderson 1975; Tanioka and Seno 2001). We investigate the possibility that the surface waves (Rayleigh, Love, and tsunami waves) in tsunami earthquakes are amplified by secondly submarine landslides, induced by the liquefaction of the sea floor due to the strong vibrations of the earthquakes. As pointed by Kanamori (2004), tsunami earthquakes are significantly stronger in longer waves than 100 s and low in radiation efficiencies of seismic waves by one or two order of magnitudes. These natures are in favor of a significant contribution of landslides. The landslides can generate seismic waves with longer period with lower efficiency than the tectonic fault motions (Kanamori et al 1980; Eissler and Kanamori 1987; Hasegawa and Kanamori 1987). We further investigate the distribution of the tsunami earthquakes and found that most of their epicenters are located at the steep slopes in the landward side of the trenches or around volcanic islands, where the soft sediments layers from the landmass are nearly critical against slope failures. This distribution suggests that the secondly landslides may contribute to the tsunami earthquakes. In the present paper, we will investigate the rapture processes determined by the inversion analysis of seismic surface waves of tsunami earthquakes can be explained by massive landslides, simultaneously triggered by earthquakes in the tsunami earthquakes which took place near the trenches.

Tsunami damage to ports: cataloguing damage to create fragility functions from the 2011 Tohoku event

Modern tsunami events have highlighted the vulnerability of port structures to these high-impact but infrequent occurrences. However, port planning rarely includes adaptation measures to address tsunami hazards. The 2011 Tohoku tsunami presented us with an opportunity to characterise the vulnerability of port industries to tsunami impacts. Here, we provide a spatial assessment and photographic interpretation of freely available data sources. Approximately 5000 port structures were assessed for damage and stored in a database. Using the newly developed damage database, tsunami damage is quantified statistically for the first time, through the development of damage fragility functions for eight common port industries. In contrast to tsunami damage fragility functions produced for buildings from an existing damage database, our fragility functions showed higher prediction accuracies (up to 75 % accuracy). Pre-tsunami earthquake damage was also assessed in this study and was found to influence overall damage assessment. The damage database and fragility functions for port industries can inform structural improvements and mitigation plans for ports against future events.

Marine forearc structure of eastern Java and its role in the 1994 Java tsunami earthquake

We resolve a previously unrecognized shallow subducting seamount from a re-processed multichannel seismic depth image crossing the 1994 M7.8 Java tsunami earthquake slip area. Seamount subduction is related to the uplift of the overriding plate by lateral shortening and vertical thickening, causing pronounced back-thrusting at the landward slope of the forearc high and the formation of splay faults branching off the landward flank of the subducting seamount. The location of the seamount in relation to the 1994 earthquake hypocentre and its co-seismic slip model suggests that the seamount acted as a seismic barrier to the up-dip co-seismic rupture propagation of this moderate size earthquake. The wrapping of the co-seismic slip contours around the seamount indicates that it diverted rupture propagation, documenting the control of forearc structures on seismic rupture.

Rapid estimation of tsunami earthquake magnitudes at local distance

A tsunami earthquake is an earthquake event that generates abnormally high tsunami waves considering the amplitude of the seismic waves. These abnormally high waves relative to the seismic wave amplitude are related to the longer rupture duration of such earthquakes compared with typical events. Rapid magnitude estimation is essential for the timely issuance of effective tsunami warnings for tsunami earthquakes. For local events, event magnitude estimated from the observed displacement amplitudes of the seismic waves, which can be obtained before estimation of the seismic moment, is often used for the first tsunami warning. However, because the observed displacement amplitude is approximately proportional to the moment rate, conventional magnitudes of tsunami earthquakes estimated based on the seismic wave amplitude tend to underestimate the event size. To overcome this problem, we investigated several methods of magnitude estimation, including magnitudes based on long-period displacement, integrated displacement, and multiband amplitude distribution. We tested the methods using synthetic waveforms calculated from finite fault models of tsunami earthquakes. We found that methods based on observed amplitudes could not estimate magnitude properly, but the method based on the multiband amplitude distribution gave values close to the moment magnitude for many tsunami earthquakes. In this method, peak amplitudes of bandpass filtered waveforms are compared with those of synthetic records for an assumed source duration and fault mechanism. We applied the multiband amplitude distribution method to the records of events that occurred around the Japanese Islands and to those of tsunami earthquakes, and confirmed that this method could be used to estimate event magnitudes close to the moment magnitudes.

The causal link of large shallow slip, long duration and moderate shaking of the Nicaragua 1992 tsunami earthquake

<p>Earthquakes rupturing up to close to subduction trenches have produced some of the largest tsunamis in history. Models indicate that the generation of these tsunamis require extraordinarily large near-trench sea-bottom displacement, but the underlying causes are disputed. They have been attributed to a wealth of factors prompting large shallow slip at the low-angle megathrust fault, the activation of steeper faults requiring smaller slip, or the triggering of ancillary energy sources. Although the postulated mechanisms are manifold, all of them coincide on the fact that the proposed causes and constraining factors are not universal but site-specific. As alternative to this local view, it has recently been proposed that the large near-trench slip could result from systematic upper-plate rock rigidity variations observed in worldwide subduction zones. Here we use a set of available controlled-source seismic data in the Middle America margin to obtain a model of upper-plate elastic rock properties across the rupture zone of the Ms7.0-Mw7.7 1992 Nicaragua tsunami earthquake. In combination with seismological data, our model shows that not only the required large shallow slip to generate the tsunami despite the moderate magnitude, but also the observed slow rupture propagation, long duration, high-frequency depletion, and magnitude discrepancy of this event, are all intrinsic physical attributes of near-trench rupture. The existence of a causal link between shallow slip and seismic record characteristics opens up new possibilities for tsunami early warning.</p>

Lombok’s Tsunami and Stock Abnormal Returns

Natural disaster often brings damage to the economy, including the decrease of stock’s market value. For this reason, this study aims to determine the effect of the tsunami earthquakes in Lombok in 2018 on abnormal returns and cumulative abnormal returns of insurance companies. This study used the event study approach, with three days window period after the three tsunami earthquakes from July to August 2018. The sample of this study is the stock price of 14 insurance companies listed on the Indonesia Stock Exchange. To test whether abnormal return exists, a one-sample t-test was used on the average abnormal and cumulative returns. The results show that the tsunami earthquake disasters in Lombok in 2018 have a significant effect on cumulative abnormal returns of insurance companies stocks, and this effect even bigger on the third tsunami. This finding shows that the market reacts to continuous disaster by considering the earthquake as negative information and thus decrease the stock price. This study implies that investors may buy the stocks after the disaster to get a cheaper price or hold the stocks to avoid loss. Keywords: abnormal return; event study; Lombok tsunami earthquake; signaling theory