Some subduction zones experience earthquake supercycles. Numerical simulations show that successive megathrust earthquakes may load neighbouring parts of the fault, causing it to eventually fail in a giant earthquake that completes a supercycle.

Earthquake supercycle in subduction zones controlled by the width of the seismogenic zone

Systemic discrepancies between observed and modeled tsunami wave speeds were previously identified for two recent major tsunamis: the 2010 Maule and 2011 Tohoku events. To account for these discrepancies, we developed a numerical tsunami propagation code solving the shallow water equation and including the effects of elastic loading of the seafloor by the tsunami as well as a linear density profile in the seawater column. We show here that both effects are important to explain the commonly observed difference between observations and simulations. We conclude that the density variation in the seawater column affects the wave speed without changing the waveform, whereas the loading effect has an effect on the wave speed and the waveform showing a negative phase before the main arrival due to the depression of the seafloor surrounding the tsunami wave. The combination of both effects is needed to achieve a better match between observations and simulations.

Numerical Tsunami Simulation Including Elastic Loading and Sea Water Density Stratification

The great 2011 Tohoku earthquake with M w 9.0 generated strong shaking and a destructive tsunami along the northeastern Japan coasts. We utilize a finite‐fault model obtained from seismic P ‐wave inversion to characterize the time‐dependent fault displacements and seafloor motions. A nonhydrostatic long‐wave model describes the resulting tsunami for investigation of the generation mechanism in terms of the rupture process and the ocean wave dynamics over the continental margin. The computed near‐field tsunami, which evolves from two dominant wave components generated by seafloor uplift near the epicenter and trench, is consistent with the recorded water‐level data around the source. Spectral analysis of the computed ocean surface elevation reveals energetic, standing edge waves with periods 32–115 min along the continental margin from Chiba to Hokkaido. While superposition of the two dominant wave components exacerbates the impact along the coasts fronting the rupture, constructive interference of standing edge waves accounts for the belated arrivals of the largest waves on the adjacent coasts and persistent wave activities in the aftermath.

Modeling of the 2011 Tohoku Near-field Tsunami from Finite-fault Inversion of Seismic Waves

On 8 September 2017, a great (M_w 8.2) normal faulting earthquake ruptured within the subducting Cocos Plate ~70 km landward from the Middle American Trench beneath the Tehuantepec gap. Iterative inversion and modeling of teleseismic and tsunami data and prediction of GPS displacements indicate that the steeply dipping rupture extended ~180 km to the northwest along strike toward the Oaxaca coast and from ~30 to 70 km in depth, with peak slip of ~13 m. The rupture likely broke through the entire lithosphere of the young subducted slab in response to downdip slab pull. The plate boundary region between the trench and the fault intersection with the megathrust appears to be frictionally coupled, influencing location of the detachment. Comparisons of the broadband body wave magnitude (m_B) and moment-scaled radiated energy (E_R/M_0) establish that intraslab earthquakes tend to be more energetic than interplate events, accounting for strong ground shaking observed for the 2017 event.

The 2017 M W 8.2 Chiapas, Mexico Earthquake: Energetic Slab Detachment

This paper presents a framework for a probabilistic hazard assessment for the multi-hazard seismic and tsunami phenomena (PSTHA). For this work, we consider a full-rupture event along the Cascadia Subduction Zone and apply the methodology to the study area of Seaside, Oregon, along the US Pacific Northwest coast. In this work, we show that the annual exceedance probabilities (AEP) of the tsunami intensity measures (IM) are qualitatively dissimilar to the IMs of the seismic ground motion in the study area. Specifically, the spatial gradients for the tsunami IM are much stronger across the length scale of the city owing to the physical differences of energy dissipation of the two mechanisms. Example results of probabilistic seismic hazard analysis (PSHA) and probabilistic tsunami hazard analysis (PTHA) are shown for three observation points in the study area of Seaside. For the seismic hazard, the joint mean annual rate of exceedance shows similar trends for the three observation points, even though there is a large scatter between and . For the tsunami hazard, the joint AEP of hmax and (MF)max shows a high correlation between the two IMs in the study area. The joint AEP at each of the three observation points follows a particular Froude number (Fr) due to the local site-specific conditions locally rather than the distributions of fault slips. The joint probability distribution of hmax and (MF)max throughout the study region falls between 0.1 ≤ Fr < 1.0 (i.e., the flow is subcritical) regardless of return interval (500-, 1,000-, and 2,500-yr). However, the peak of the joint probability distribution with respect to hmax and (MF)max varies with the return interval, and the largest values of hmax and (MF)max were observed with the highest return intervals (2,500 yr) as would be expected. The results of the PSTHA can be the basis for a probabilistic multi-hazard damage assessment and help to understand the uncertainties of the multi-hazard assessments.

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Probabilistic Seismic and Tsunami Hazard Analysis Conditioned on a Megathrust Rupture of the Cascadia Subduction Zone

The eruption and collapse of the Anak Krakatau volcano generated tsunamis in the Sunda Strait on December 22, 2018, leading to damage and casualties. In this paper, we use the two-layer model and nonlinear shallow equation model to study the triggering mechanism of the tsunami event. We first simulate the tsunami generated by volcano eruption and landslide, respectively. The tsunami source is analyzed by comparing with gauge measurements. It indicates that the volume of partial collapse for the landslide is 0.2–0.3 km3. The comparison between the numerical results of landslide and tide gauge measurements presents well-fitted results, especially for the leading tsunami waves and arrival time. Computed maximum tsunami amplitude distribution points out that the most hazardous area is located at the south of the Sunda Strait (Pandeglang), which suffered the most casualties.

Numerical study of the triggering mechanism of the 2018 Anak Krakatau tsunami: eruption or collapsed landslide?

On September 16, 2015, an earthquake with magnitude of Mw 8.3 occurred 46 km offshore from Illapel, Chile, generating a 4.4-m local tsunami measured at Coquimbo. In this study, the characteristics of tsunami are presented by a combination of analysis of observations and numerical simulation based on sources of USGS and NOAA. The records of 16 DART buoys in deep water, ten tidal gauges along coasts of near-field, and ten coastal gauges in the far-field are studied by applying Fourier analyses. The numerical simulation based on nonlinear shallow water equations and nested grids is carried out to provide overall tsunami propagation scenarios, and the results match well with the observations in deep water and but not well in coasts closed to the epicenter. Due to the short distance to the epicenter and the shelf resonance of southern Peru and Chile, the maximum amplitude ranged from 0.1 m to 2 m, except for Coquimbo. In deep water, the maximum amplitude of buoys decayed from 9.8 cm to 0.8 cm, suggesting a centimeter-scale Pacific-wide tsunami, while the governing period was 13–17 min and 32 min. Whereas in the far-field coastal region, the tsunami wave amplified to be around 0.2 m to 0.8 m, mostly as a result of run-up effect and resonance from coast reflection. Although the tsunami was relatively moderate in deep water, it still produced non-negligible tsunami hazards in local region and the coasts of farfield.

The September 16, 2015 M w 8.3 Illapel, Chile Earthquake: characteristics of tsunami wave from near-field to far-field

Tsunami resonance and standing waves in Hangzhou Bay

At 14:34 Beijing time on February 27, 2010, an earthquake of M
 w 8.8 rocked the central coast of Chile. A teletsunami was triggered by the huge earthquake and then propagated across the entire Pacific in less than 24 h. The initial tsunami wave arrived at the southeast coast of China at 16:20 on February 28 and was recorded in real time by a number of tidal gauges off mainland China and Taiwan Province. The teletsunami waves have never been recorded with such a broad geographical scope in China before this event. The aim of the present work is to report the first description of sea-level oscillations generated by the Chile tsunami, and evaluate the potential hazards that teletsunami poses to the coast of China. The results reveal that the east coast of Taiwan Province, Zhejiang Province and the north coast of Fujian Province are vulnerable to transoceanic tsunami. In addition, spectral and wavelet analysis of the recorded sea-level oscillations shows that “seiches” are easily excited by the resonance resulting from the interaction between the initial tsunami waves and local geometry (such as semi-enclosed bays and ports). The superimposition of the two oscillations would amplify the wave height further.

Evaluation of potential hazards from teletsunami in China: Tidal observations of a teletsunami generated by the Chile 8.8 M w earthquake

The South China Sea (SCS) and its adjacent small basins including Sulu Sea and Celebes Sea are commonly identified as tsunami-prone region by its historical records on seismicity and tsunamis. However, quantification of tsunami hazard in the SCS region remained an intractable issue due to highly complex tectonic setting and multiple seismic sources within and surrounding this area. Probabilistic Tsunami Hazard Assessment (PTHA) is performed in the present study to evaluate tsunami hazard in the SCS region based on a brief review on seismological and tsunami records. 5 regional and local potential tsunami sources are tentatively identified, and earthquake catalogs are generated using Monte Carlo simulation following the Tapered Gutenberg–Richter relationship for each zone. Considering a lack of consensus on magnitude upper bound on each seismic source, as well as its critical role in PTHA, the major concern of the present study is to define the upper and lower limits of tsunami hazard in the SCS region comprehensively by adopting different corner magnitudes that could be derived by multiple principles and approaches, including TGR regression of historical catalog, fault-length scaling, tectonic and seismic moment balance, and repetition of historical largest event. The results show that tsunami hazard in the SCS and adjoining basins is subject to large variations when adopting different corner magnitudes, with the upper bounds 2–6 times of the lower. The probabilistic tsunami hazard maps for specified return periods reveal much higher threat from Cotabato Trench and Sulawesi Trench in the Celebes Sea, whereas tsunami hazard received by the coasts of the SCS and Sulu Sea is relatively moderate, yet non-negligible. By combining empirical method with numerical study of historical tsunami events, the present PTHA results are tentatively validated. The correspondence lends confidence to our study. Considering the proximity of major sources to population-laden cities around the SCS region, the tsunami hazard and risk should be further highlighted in the future.

Peitao Wang

Papers

Numerical study of the triggering mechanism of the 2018 Anak Krakatau tsunami: eruption or collapsed landslide?

The September 16, 2015 M w 8.3 Illapel, Chile Earthquake: characteristics of tsunami wave from near-field to far-field

Tsunami resonance and standing waves in Hangzhou Bay

Evaluation of potential hazards from teletsunami in China: Tidal observations of a teletsunami generated by the Chile 8.8 M w earthquake

The Dependency of Probabilistic Tsunami Hazard Assessment on Magnitude Limits of Seismic Sources in the South China Sea and Adjoining Basins