Toward the integration of seismic analysis and tsunami model for rapid inundation forecasting system

Aditya Riadi Gusman
Earthquake Research Institute, the University of Tokyo

1. INTRODUCTION

Great earthquakes in the subduction zones generated large tsunamis that devastated coastal areas. Huge tsunamis such as the 2004 Indian Ocean or the 2011 Tohoku tsunamis caused fatal damages in areas more than 1 km away from the coastlines. Unfortunately, existing tsunami early warning systems in the world do not have the capability to forecast tsunami inundation in the near field.

We developed a methodology for Near-field Tsunami Inundation Forecasting (NearTIF) that can produce high-resolution tsunami inundation forecast maps in around 3 minutes after the source of the tsunami has been determined (Gusman et al., 2014). This algorithm requires database of pre-computed tsunami waveforms and tsunami inundations.

The algorithm has been tested in a retrospective forecast of the 2011 Tohoku tsunami along the Sanriku Coast, Japan. Tsunami inundation maps were forecasted with earthquake source model that was estimated from the W phase of seismic data. We found that the resulting tsunami inundations were similar to the observed tsunami run-up heights.

We carried out tsunami evacuation drill in Kushiro city Hokkaido that used results from our algorithm to evaluate the effectiveness of tsunami inundation forecast maps.

2. NEAR-FIELD TSUNAMI INUNDATION FORECASTING (NearTIF)

Tsunami inundations from different scenarios can be similar to each other as long as the tsunami waveforms at near-shore points are also similar. Tsunami waveforms at near-shore point deeper than 50 m from low-resolution and high-resolution simulations are similar. Base on these assumptions, we developed a methodology for high-resolution tsunami inundation forecasting. This method required pre-computed tsunami waveforms and tsunami inundation database.

When an earthquake happens, tsunami waveforms at near-shore points can be simulated on low-resolution grid by solving the linear shallow water equations. This kind of simulation does not take long time and can be done using regular computer. A scenario in the database than produced tsunami waveform that is similar to the simulated one can be found by RMS analysis. Finally the corresponding tsunami inundation map from that best-fit scenario is selected as tsunami inundation forecast map. This process takes only approximately 3 minutes on a regular computer.

3. FAULT MODEL FOR THE 2011 TOHOKU TSUNAMI

W phase is a long period phase (200 to 1,000 s) that arrives before S phase, and can be used for rapid and robust determination of great earthquake’s source parameters with sufficient accuracy for tsunami early warning purposes (Kanamori and Rivera, 2008). About 5 minutes after the 2011 Tohoku earthquake Japanese F-net stations recorded seismic data that is enough to accurately estimate the seismic moment by W phase inversion. The estimated scalar moment for the 2011 Tohoku earthquake from the 5 min W phase data is 3.69 × 1022 Nm (Mw 9.0). The moment tensor solution was used to make a fault model for the 2011 Tohoku earthquake. The length (L) and width (W) of the fault model using scaling relation of Hanks and Bakun (2002) and simple relation of L = 2 × W are 246 km and 123 km, respectively (Gusman and Tanioka, 2013).

4. RETROSPECTIVE TSUNAMI INUNDATION FORECAST MAPS FOR THE 2011 TOHOKU TSUNAMI

The fault model based on the 5-min W phase solution was used as an input for NearTIF to produce tsunami inundation maps in 15 locations along the Sanriku Coast. The retrospective tsunami inundation forecast from this fault model can explain the observed limit of tsunami inundation and inland tsunami heights very well in all locations (Gusman et al., 2014). For example, tsunami inundation maps for Rikuzentakata, Japan produced by the NearTIF algorithm is shown in Fig. 1. The forecasted limit of inundation and inland tsunami heights are very similar with the observations (Fig. 1).

Fig. 1 Tsunami inundation forecast in Rikuzentakata produced by the NearTIF algorithm from the fault model of 5-min W phase solution. Green bars represent the measured tsunami heights, magenta dots represent the forecasted tsunami heights, blue lines represent the actual limit of inundation, and black dots are the measured positions. K value indicates the relative size of the observed and simulated tsunami heights, and κ (kappa) value indicates the precision of the simulated tsunami heights (Gusman et al., 2014).

Fig. 1 Tsunami inundation forecast in Rikuzentakata produced by the NearTIF algorithm from the fault model of 5-min W phase solution. Green bars represent the measured tsunami heights, magenta dots represent the forecasted tsunami heights, blue lines represent the actual limit of inundation, and black dots are the measured positions. K value indicates the relative size of the observed and simulated tsunami heights, and κ (kappa) value indicates the precision of the simulated tsunami heights (Gusman et al., 2014).

The tsunami inundation map produced by the NearTIF algorithm is comparable with that from numerical forward modeling. However, the numerical forward modeling required approximately 2 hours of CPU time to produce tsunami inundation map for a single location (Rikuzentakata). The required time is much longer if we simulate more locations. Based on performance test of NearTIF vs. numerical forward modeling in our previous study, the NearTIF algorithm is 800 times faster than numerical forward modeling to produce tsunami inundation maps in 15 locations (Gusman et al., 2014).

5. TSUNAMI EVACUATION DRILL IN KUSHIRO CITY, HOKKAIDO

From historical and pre-historical tsunamigenic earthquake studies (Hatori, 1984; Hirata et al., 2003; Satake et al., 2005; Tanioka et al., 2007; Ioki, 2013), Kushiro city has been identified as a tsunami prone area. Kushiro city office has provided to the public a tsunami hazard map that can increase the effectiveness of evacuation plans for the community.

We evaluated the effectiveness of the NearTIF algorithm in the real world by carrying out a tsunami evacuation drill in Kushiro city, Hokkaido, Japan, involving the residents. The drill started by an announcement of tsunami warning to evacuate the residents to the nearest evacuation building. About 10 minutes after the announcement, the participants used tablet computers to see the tsunami inundation forecast map that was uploaded to the Internet. It was easy for the participants to see their current location on the tsunami inundation forecast map because the tablet computer was equipped with GPS device. The participants found that the use of the tsunami inundation forecast map produced by NearTIF was effective to make a better decision with high confidence during the tsunami evacuation drill.

6. CONCLUSIONS

Tsunami inundation forecast on high-resolution topography can help to make a decision for evacuation during a tsunami event. Tsunami inundation can be simulated accurately by solving the nonlinear shallow water wave equations. However, high-resolution tsunami simulation is numerically expensive. To resolve this challenge, we developed a methodology for Near-field Tsunami Inundation Forecasting (NearTIF) that is equipped with a database of pre-computed tsunami waveforms and pre-computed tsunami inundation. The tsunami inundation forecasted by the NearTIF algorithm is similar to the result from numerical forward simulation and thus are a fast alternative to the slower numerical simulation.

W phase data can give a reliable earthquake magnitude estimate (Kanamori, 2008; Duputel et al., 2011; Gusman and Tanioka, 2013; Benavente and Cummins, 2013). Reliable centroid moment tensor solutions of the 2011 Tohoku earthquake can be estimated using 5 min of W phase data recorded at Japanese F-net stations (Gusman and Tanioka 2013; Gusman et al., 2014).

We evaluated the effectiveness of the NearTIF algorithm in the real world by carrying out a tsunami evacuation in Kushiro city, Hokkaido, Japan. The participants found that the use of the tsunami inundation forecast map was effective to make a better decision with high confidence during the tsunami evacuation process.
The NearTIF algorithm is recommended to be use as part of the reconstruction policy by local authorities to improve the evacuation efficiency particularly in tsunami prone areas. We recommend the use of the NearTIF method in developing future tsunami forecasting systems with a capability of providing tsunami inundation forecast maps for locations near the tsunami source area.

REFERENCES
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W phase inversion and tsunami inundation modeling for tsunami early warning: Case study for the 2011 Tohoku event

Aditya Riadi Gusman, Yuichiro Tanioka

Pure and Applied Geophysics, July 2014, Volume 171, Issue 7, pp 1409-1422, DOI: 10.1007/s00024-013-0680-z
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Abstract

Centroid moment tensor solutions for the 2011 Tohoku earthquake are determined by W phase inversions using 5 and 10 min data recorded by the Full Range Seismograph Network of Japan (F-net). By a scaling relation of moment magnitude to rupture area and an assumption of rigidity of 4 × 1010 N m−2, simple rectangular earthquake fault models are estimated from the solutions. Tsunami inundations in the Sendai Plain, Minamisanriku, Rikuzentakata, and Taro are simulated using the estimated fault models. Then the simulated tsunami inundation area and heights are compared with the observations. Even the simulated tsunami heights and inundations from the W phase solution that used only 5 min data are considerably similar to the observations. The results are improved when using 10 min of W phase data. These show that the W phase solutions are reliable to be used for tsunami inundation modeling. Furthermore, the technique that combines W phase inversion and tsunami inundation modeling can produce results that have sufficient accuracy for tsunami early warning purposes.

A methodology for near‐field tsunami inundation forecasting: Application to the 2011 Tohoku tsunami

Aditya Riadi Gusman, Yuichiro Tanioka, Breanyn T MacInnes, Hiroaki Tsushima

Journal of Geophysical Research: Solid Earth, Volume 119, Issue 11, pages 8186–8206, November 2014, DOI: 10.1002/2014JB010958
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Abstract

Existing tsunami early warning systems in the world can give either one or a combination of estimated tsunami arrival times, heights, or qualitative tsunami forecasts before the tsunami hits near-field coastlines. A future tsunami early warning system should be able to provide a reliable near-field tsunami inundation forecast on high-resolution topography within a short time period. Here we describe a new methodology for near-field tsunami inundation forecasting. In this method, a pre-computed tsunami inundation and pre-computed tsunami waveform database is required. After information about a tsunami source is estimated, tsunami waveforms at near-shore points can be simulated in real-time. A scenario that gives the most similar tsunami waveforms is selected as the site-specific best scenario and the tsunami inundation from that scenario is selected as the tsunami inundation forecast. To test the algorithm, tsunami inundation along the Sanriku Coast is forecasted by using source models for the 2011 Tohoku earthquake estimated from GPS, W phase, or offshore tsunami waveform data. The forecasting algorithm is capable of providing a tsunami inundation forecast that is similar to that obtained by numerical forward modeling, but with remarkably smaller CPU time. The time required to forecast tsunami inundation in coastal sites from the Sendai Plain to Miyako City is approximately 3 minutes after information about the tsunami source is obtained. We found that the tsunami inundation forecasts from the 5-min GPS, 5-min W phase, 10-min W phase fault models, and 35-min tsunami source model are all reliable for tsunami early warning purposes and quantitatively match the observations well, although the latter model gives tsunami forecasts with highest overall accuracy. The required times to obtain tsunami forecast from the above four models are 8 min, 9 min, 14 min, and 39 min after the earthquake, respectively, or in other words 3 minutes after receiving the source model. This method can be useful in developing future tsunami forecasting systems with a capability of providing tsunami inundation forecasts for locations near the tsunami source area.

Effect of the largest foreshock (Mw 7.3) on triggering the 2011 Tohoku earthquake (Mw 9.0)

Aditya Riadi Gusman, Mitsuteru Fukuoka, Yuichiro Tanioka, Shin’ichi Sakai

Geophysical Research Letters, Volume 40, Issue 3, pages 497–500, 16 February 2013, DOI: 10.1002/grl.50153
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Abstract

The slip distribution of the largest foreshock that occurred 2 days before the mainshock of the 2011 Tohoku earthquake is estimated by tsunami waveform inversion. The major slip region was located on the down-dip side of the hypocenter, and the slip amounts ranged from 0.6 to 1.5 m. By assuming the rigidity of 4 × 1010 N m-2, the seismic moment calculated from the slip distribution is 1.2 × 1020 N m (Mw 7.3). The slip distribution suggests that the largest foreshock did not rupture the plate interface where the dynamic rupture of the mainshock was initiated. The largest foreshock increased the Coulomb stress (1.6–4.5 bars) on the plate interface around the hypocenter of the mainshock. This indicates that the 2011 Tohoku earthquake was brought closer to failure by the largest foreshock.

Numerical experiment and a case study of sediment transport simulation of the 2004 Indian Ocean tsunami in Lhok Nga, Banda Aceh, Indonesia

Aditya Riadi Gusman, Yuichiro Tanioka, Tomoyuki Takahashi

Earth Planets Space, 64, 817–827, 2012, doi:10.5047/eps.2011.10.009

Abstract

We use a two-dimensional tsunami sediment transport model to study the source of the 2004 earthquake. To test the model behavior, numerical experiment on sediment deposition and erosion is performed using various hypothetical parameters of tsunami wavelength, topographic slope, and sediment supply. The numerical experiment results show that erosion and deposition are strongly influenced by the tsunami wavelength and the topographic slope. The model is used to compute the spatial distribution of tsunami deposit thickness produced by the 2004 Indian Ocean over an actual elevation datasets in the coastal area of Lhok Nga, Banda Aceh, Indonesia. The model produced simulated tsunami deposits that have similar thicknesses with the measured data along a surveyed transect. Then we estimate a simple fault model for the southern portion of the 2004 earthquake using tsunami sediment transport simulations. The simulated tsunami run-up from the fault model is very close to the measured run-up. This result indicates that a source process of a large earthquake that generates a large tsunami has a potential to be estimated using sediment deposit distribution data.

Fault slip distribution of the 2014 Iquique, Chile earthquake estimated from ocean-wide tsunami waveforms and GPS data

Aditya Riadi Gusman, Satoko Murotani, Kenji Satake, Mohammad Heidarzadeh, Endra Gunawan, Shingo Watada, and Bernd Schurr

Geophysical Research Letters, DOI: 10.1002/2014GL062604
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Abstract

We applied a new method to compute tsunami Green’s functions for slip inversion of the 1 April 2014 Iquique earthquake using both near-field and far-field tsunami waveforms. Inclusion of the effects of the elastic loading of seafloor, compressibility of seawater, and the geopotential variation in the computed Green’s functions reproduced the tsunami travel-time delay relative to long-wave simulation, and allowed us to use far-field records in tsunami waveform inversion. Multiple time window inversion was applied to tsunami waveforms iteratively until the result resembles the stable moment-rate function from teleseismic inversion. We also used GPS data for a joint inversion of tsunami waveforms and co-seismic crustal deformation. The major slip region with a size of 100 km × 40 km is located down-dip the epicenter at depth ~28 km, regardless of assumed rupture velocities. The total seismic moment estimated from the slip distribution is 1.24 × 1021 Nm (Mw 8.0).

Maximum tsunami amplitude distribution in the Pacific Ocean for the 2014 Iquique earthquake. Maximum tsunami amplitude and epicentral distance for each DART station are shown by the size of the circle and blue text, respectively.

Maximum tsunami amplitude distribution in the Pacific Ocean for the 2014 Iquique earthquake. Maximum tsunami amplitude and epicentral distance for each DART station are shown by the size of the circle and blue text, respectively.

Source model of the great 2011 Tohoku earthquake estimated from tsunami waveforms and crustal deformation data

Aditya Riadi Gusman, Yuichiro Tanioka, Shinichi Sakai, Hiroaki Tsushima

Earth and Planetary Science Letters, Volumes 341–344, August 2012, Pages 234–242, doi:10.1016/j.epsl.2012.06.006
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Abstract

The slip distribution of the 11 March 2011 Tohoku earthquake is inferred from tsunami waveforms, GPS data, and seafloor crustal deformation data. The major slip region extends all the way to the trench, and the large slip area extends 300 km long and 160 km wide. The largest slip of 44 m is located up-dip of the hypocenter. The large slip amount, about 41 m, ruptured the plate interface near the trench. The seismic moment calculated from the estimated slip distribution is 5.5×1022 N m (Mw 9.1). The large tsunami due to the 2011 Tohoku earthquake is generated from those large slip areas near the trench. The additional uplift at the sedimentary wedge as suggested for the 1896 Sanriku earthquake may have occurred during the 2011 Tohoku earthquake, too.

Tsunamigenic ionospheric hole

Yoshihiro Kakinami, Masashi Kamogawa, Yuichiro Tanioka, Shigeto Watanabe, Aditya Riadi Gusman, Jann‐Yenq Liu, Yasuyuki Watanabe, Toru Mogi

Geophysical Research Letters, Volume 39, Issue 13, July 2012, DOI: 10.1029/2011GL050159

Abstract

Traveling ionospheric disturbances generated by an epicentral ground/sea surface motion, ionospheric disturbances associated with Rayleigh-waves as well as post-seismic 4-minute monoperiodic atmospheric resonances and other-period atmospheric oscillations have been observed in large earthquakes. In addition, a giant tsunami after the subduction earthquake produces an ionospheric hole which is widely a sudden depletion of ionospheric total electron content (TEC) in the hundred kilometer scale and lasts for a few tens of minutes over the tsunami source area. The tsunamigenic ionospheric hole detected by the TEC measurement with Global Position System (GPS) was found in the 2011 M9.0 off the Pacific coast of Tohoku, the 2010 M8.8 Chile, and the 2004 M9.1 Sumatra earthquakes. This occurs because plasma is descending at the lower thermosphere where the recombination of ions and electrons is high through the meter-scale downwelling of sea surface at the tsunami source area, and is highly depleted due to the chemical processes.

Nationalwide post event survey and analysis of the 2011 Tohoku earthquake tsunami

Nobuhito Mori, Tomoyuki Takahashi, THE 2011 TOHOKU EARTHQUAKE TSUNAMI JOINT SURVEY GROUP

Coastal Engineering Journal 54, 1250001

Abstract

At 14:46 local time on March 11, 2011, a magnitude 9.0 earthquake occurred off the coast of northeast Japan. This earthquake generated a tsunami that struck Japan as well as various locations around the Pacific Ocean. With the participation of about 300 researchers from throughout Japan, joint research groups conducted a tsunami survey along a 2,000 km stretch of the Japanese coast. More than 5,200 locations have been surveyed to date, generating the largest tsunami survey dataset in the world. The inundation height and run-up height were surveyed by laser, GPS, and other instruments, and the tidal correction has been accurately adjusted using a tidal database and a numerical simulation for Tohoku, an area where tide gauges were destroyed by the tsunami. Based on the survey dataset, the regional and local scale analyses were conducted to understand the basic characteristics of this event. Maximum run-up heights greater than 10 m are distributed along 500 km of coast in direct distance. The affected area of this event was several times larger than historically recorded in Tohoku. The mean inundation height in the southern Sanriku region is 10–15 m and there are several peaks of inundation along the coast from the northern to middle part of Sanriku.

Tsunami Hazard Mitigation at Palabuhanratu, Indonesia

Yuichiro Tanioka, Hamzah Latief, Haris Sunendar, Aditya Riadi Gusman, Shunichi Koshimura

Journal of Disaster Research 7 (1), 19-25

Abstract

Several large earthquakes have recently occurred along the Sumatra-Java subduction zone, the 2004 great Sumatra-Andaman earthquake, the 2005 great Nias earthquake, the 2006 West Java tsunami earth- quake, the 2007 great Bengkulu earthquake, and the 2010 Mentawai tsunami earthquakes. Serious tsunami disasters were caused by the great underthrust earthquakes which ruptured the plate interface near the trench such as the 2004 Sumatra-Andaman, 2006 West Java, 2010 Mentawai earthquakes. At Palabuhanratu, maximum tsunami height distribution and inundation areas were computed from expected fault models located near the Java trench. The results shows that the most populated areas of Palabuhanratu would be severely damaged by the expected tsunami caused by the fault model of Mw 8.5. After discussing tsunami disaster mitigation measures with the local government, the result of tsunami inundation area in this study were used to decide tsunami evacuation areas and evacuation routes. The local government also installed tsunami evacuation sign boards near the coast.