Aditya Gusman | The University of Tokyo (original) (raw)

Papers by Aditya Gusman

The great outer-rise earthquake (M w 8:3) occurred near the Sunda trench, Indonesia, on 19 August... more The great outer-rise earthquake (M w 8:3) occurred near the Sunda trench, Indonesia, on 19 August 1977. The earthquake has been previously studied using seismological data. The earthquake generated a large tsunami that caused severe damage in Sumbawa and Sumba Islands in Indonesia. The tsunami was also observed at tide gauges in Australia. We numerically computed a far-field tsunami, and we compared the observed tsunami waveforms on three tide gauges with the computed waveforms. We also numerically computed the tsunami inundation and compared the observed tsunami run-up of 8 m and tsunami inundation distance of 500 m in Lunyuk on Sumbawa Island with the computed ones. To explain the observed tsunami waveforms, tsunami run-up, and tsunami inundation distance, the slip amount is found to be 3 m on the assumed fault model (with a fault length of 200 km and fault width of 70 km). The rigidity is assumed to range between 6.0 and 6:8 × 10 10 N m 2 , and the range of the total seismic moment is calculated to be between 2.5 and 2:9 × 10 21 N m (M w 8:2), which is similar to those estimated by the previous seismological studies. Additionally, we calculated the ratio between the observed tsunami run-up and the computed maximum tsunami height along the coastline of Lunyuk. This ratio, called the amplification factor, may possibly be used to roughly estimate the tsunami run-up from a tsunami numerical calculation result on a coarse grid system.

Zisin (Journal of the Seismological Society of Japan. 2nd ser.), 2012

On September 12, 2007 at 11:10:26 UTC, an earthquake with moment magnitude of 8.4 occurred off th... more On September 12, 2007 at 11:10:26 UTC, an earthquake with moment magnitude of 8.4 occurred off the west coast of Sumatra. The epicenter of the earthquake located at 4.52°S- 101.374°E about 130 km southwest of Bengkulu. This earthquake located in Sumatra subduction zone where at least two previous major events of the 1833 (M8.5-9) and the 1797 have ruptured the same plate interface. In this study, we estimate the slip distribution of the 2007 earthquake using tsunami waveforms. By comparing the result with the rupture area of the previous two large earthquakes, the recurrence pattern of large earthquakes in this area can be understood in order to identify the source area of future tsunamigenic earthquake. The tsunami waves generated by the earthquake were recorded by tide gauge stations around Indian Ocean and one DART buoy (Thailand Meteorological Department) deployed in the deep ocean northwest Sumatra. We select tsunami waveforms recorded in Padang, Cocos Islands, and on the DART buoy. The synthetic tsunami waveforms at those three locations are calculated by solving the non linear shallow water equations. With observation data and synthetic waveforms we calculate the slip distribution using non linear inversion method by an iterative process. On the ruptured area, we create a fault segment area of 100 km width by 250 km length and divide it into 10 subfaults. We use single focal mechanism (strike= 327°, slip= 12°, rake= 144°) determined by Global CMT solution for each subfault. The tsunami waveform records can be well explained by a slip distribution with the largest slip amount of 9.4 m located at South West of Pagai Selatan Island on deeper part of the fault. Assuming the rigidity of 4 × 1010 Nm-2, the total seismic moment obtain from the slip amount is 3.65 × 1021 Nm (Mw=8.3) which is consistent with the Global CMT solution on the seismic moment determination of 5.05 × 1021 Nm.

The great earthquake, Mw 8.8, occurred in Chile on 27 February, 2010 at 06:34:14 UTC. The number ... more The great earthquake, Mw 8.8, occurred in Chile on 27 February, 2010 at 06:34:14 UTC. The number of casualties by this earthquake was reached 800, and more than 500 people among that were killed by tsunamis. The large tsunami was generated by the earthquake and propagated through Pacific and reached along the coast of Pacific include Hawaii, Japan, and Alaska. The maximum run-up height of the tsunami was 28 m in Chile. The tsunami was observed at DART real-time tsunami monitoring systems installed in the Pacific by NOAA-PMEL and also tide gauges around Pacific. In this paper, the tsunami waveforms observed at 9 DART stations, 32412, 51406, 51426, 54401, 43412, 46412, 46409, 46403, and 21413, are used to estimate the slip distribution of the 2010 Chile earthquake. The source area of 500km x 150km is divided into 30 subfaults of 50 km x 50 km. The Global CMT solution shows the focal mechanism of the earthquake, strike=18degree, dip=18degree, rake=112degree. Those fault parameters are assumed for all subfaults. The tsunami is numerically computed on actual bathymetry. The finite-difference computation for the linear long-wave equations are carried out in the whole Pacific. The grid size is 5 minutes, about 9km. Tsunami waveforms at 9 DART stations are computed from each subfault with a unit amount of slip, and used as the Green’s function for the inversion. The result of the tsunami inversion indicates that the large slip amount of more than 10m is estimated in the source area from about 150 km northeast of the epicenter to about 200 km southwest of the epicenter. The maximum slip amount is estimated to be 19 m at a subfault located at the southwest of the epicenter. The total length of the rupture length is found to be about 400-350 km. The result also indicates the bilateral rupture process of the great Chile earthquake. The total seismic moment calculated from the slip distribution is 2.6 x 10^{22} Nm (Mw 8.9) by assuming the rigidity of 4 x 10^{10} N/m^{2}. This seismic moment is consistent with those estimated by the other seismological analyses. The tsunami waveforms observed at DART stations located less than 10,000km away from the epicenters are well explained by the computed ones. However, those observed at stations located more than 10,000 km away, such as 21413, are not explained well by the computed one. It may be related to be the accuracy of the tsunami numerical computation.

A two-dimensional tsunami sediment transport model is developed based on exchange rate method pro... more A two-dimensional tsunami sediment transport model is developed based on exchange rate method proposed by Takahashi et al. (2000). To test the model behavior, numerical experiment on sediment deposition and erosion is conducted using variable 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 sediment transport model is used to compute the spatial distribution of the 2004 Indian Ocean tsunami deposit thickness in the coastal area of Lhoknga, Indonesia. Then we compare the computed bed changes with the observed sediment thickness data along a transect line measured by Moore et al. (2006). The results of tsunami sediment transport model along the transect show that the simulated tsunami deposits have similar thickness and distribution with the observed tsunami deposits. The result of numerical simulation also shows that the sand layer is dominated by suspended sediment which is consistent with evidences observed in the sediment samples. We also numerically computed sediment thickness along the transect by using an assumed fault model with length of 300 km and width of 150 km. The slip amounts that can rather explain the observed sediment thickness are ranging from 10 to 15 m on the assumed fault model. This result indicates that there is possibility in estimating earthquake magnitude using sediment deposits data.

Linear and nonlinear responses of ten well-type tide gauge stations on the Japan Sea coast of cen... more Linear and nonlinear responses of ten well-type tide gauge stations on the Japan Sea coast of central Japan were estimated by in situ measurements. We poured water into the well or drained water from the well by using a pump to make an artificial water level difference between the outer sea and the well, then measured the recovery of water level in the well. At three tide gauge stations. Awashima, Iwafune, and Himekawa, the sea-level change of the outer sea is transmitted to the tide well instantaneously. However, at seven tide gauge stations. Nezugaseki, Ryotsu, Ogi, Teradomari, Banjin, Kujiranami, and Naoetsu, the sea-level change of the outer sea is not always transmitted to the tide well instantaneously. At these stations, the recorded tsunami waveforms are not assured to follow the actual tsunami waveforms. Tsunami waveforms from the Niigataken Chuetsu-oki Earthquake in 2007 recorded at these stations are corrected by using the measured tide gauge responses. The corrected amplitudes of the first and second waves were larger than the uncorrected ones, and the corrected peaks are a few minutes earlier than the uncorrected ones at Banjin, Kujiranami, and Ogi. At Banjin, the correction was significant; the corrected amplitudes of the first and second upward motion are +103 cm and +114cm, respectively, while the uncorrected amplitudes were +96 cm and +88cm. At other tide gauge stations, the differences between the uncorrected and corrected tsunami waveforms were insignificant.

Geophysical Research Letters, 2007

magnitude M w 7.8 earthquake off the south coast of western Java, Indonesia, generated a tsunami ... more magnitude M w 7.8 earthquake off the south coast of western Java, Indonesia, generated a tsunami that effected over 300 km of coastline and killed more than 600 people, with locally focused runup heights exceeding 20 m. This slow earthquake was hardly felt on Java, and wind waves breaking masked any preceding withdrawal of the water from the shoreline, making this tsunami difficult to detect before impact. An International Tsunami Survey Team was deployed within one week and the investigation covered more than 600 km of coastline. Measured tsunami heights and run-up distributions were uniform at 5 to 7 m along 200 km of coast; however there was a pronounced peak on the south coast of Nusa Kambangan, where the tsunami impact carved a sharp trimline in a forest at elevations up to 21 m and 1 km inland. Local flow depth exceeded 8 m along the elevated coastal plain between the beach and the hill slope. We infer that the focused tsunami and runup heights on the island suggest a possible local submarine slump or mass movement.

Pure and Applied Geophysics, 2011

The 2006 western Java tsunami deposited a discontinuous sheet of sand up to 20 cm thick, flooded ... more The 2006 western Java tsunami deposited a discontinuous sheet of sand up to 20 cm thick, flooded coastal southern Java to a depth of at least 8 m and inundated up to 1 km inland. In most places the primarily heavy mineral sand sheet is normally graded, and in some it contains complex internal stratigraphy. Structures within the sand sheet probably record the passage of up to two individual waves, a point noted in eyewitness accounts. We studied the 2006 tsunami deposits in detail along a flow parallel transect about 750 m long, 15 km east of Cilacap. The tsunami deposit first becomes discernable from the underlying sediment 70 m from the shoreline. From 75 to 300 m inland the deposit has been laid down in rice paddies, and maintains a thickness of 10-20 cm. Landward of 300 m the deposit thins dramatically, reaching 1 mm by 450 m inland. From 450 m to the edge of deposition (around 700 m inland) the deposit remains \1 mm thick. Deposition generally attended inundation-along the transect, the tsunami deposited sand to within about 40 m of the inundation limit. The thicker part of the deposit contains primarily sand indistinguishable from that found on the beach 3 weeks after the event, but after about 450 m (and roughly coinciding with the decrease in thickness) the tsunami sediment shifts to become more like the underlying paddy soil than the beach sand. Grain sizes within the deposit tend to fine upward and landward, although overall upward fining takes place in two discrete pulses, with an initial section of inverse grading followed by a section of normal grading. The two inversely graded sections are also density graded, with denser grains at the base, and less dense grains at the top. The two normally graded sections show no trends in density. The inversely graded sections show high density sediment to the base and become less dense upward and represents traction carpet flows at the base of the tsunami. These are suggestive of high shear rates in the flow. Because of the grain sorting in the traction carpet, the landward-fining trends usually seen in tsunami deposits are masked, although lateral changes of mean sediment grain size along the transect do show overall landward fining, with more variation as the deposit tapers off. The deposit is also thicker in the more seaward portions than would be produced by tsunamis lacking traction carpets.

Journal of Geophysical Research: Solid Earth, 2014

Existing tsunami early warning systems in the world can give either one or a combination of estim... more 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 precomputed tsunami inundation and precomputed tsunami waveform database is required. After information about a tsunami source is estimated, tsunami waveforms at nearshore 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 min 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 min 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.

Pure and Applied Geophysics, 2014

We studied the tsunami generated by the 1 April 2014 M w 8.2 Iquique (Chile) earthquake using 20 ... more We studied the tsunami generated by the 1 April 2014 M w 8.2 Iquique (Chile) earthquake using 20 Deep-ocean Assessment and Reporting of Tsunamis (DART) records and applying Fourier and wavelet analyses as well as performing numerical simulations. Deep-water tsunami wave heights were in the range of 0.8-35.0 cm. For the stations located more than 2,200 km from the source, the average wave height was 1.7 ± 1.1 cm. The observed tsunami arrivals were delayed by 1-17 min relative to the simulated ones based on the linear long wave equations, and the delays were proportional to the tsunami travel distances. A small initial depression was observed at DART stations located at distances [10,000 km from the source whereas, traditionally, an initial elevation is expected at stations located seaward of subduction zones. Fourier analyses showed tsunami governing periods of 21.1 ± 1.7 and 14.7 ± 0.7 min, corresponding to a fault length of 60-70 km and a fault width of 40-45 km. While the two 21-min and 15-min signals appeared in most DART stations during *0.5 h following the conventional arrival times, the 15-min signal was delayed at some far-field stations. Distribution of maximum DART wave heights across the Pacific Ocean did not show a meaningful relation between maximum DART wave heights and directivity or distance from the source.

Advances in Natural and Technological Hazards Research, 2014

An algorithm called NearTIF, designed to produce tsunami inundation maps of near-fi eld sites bef... more An algorithm called NearTIF, designed to produce tsunami inundation maps of near-fi eld sites before the actual tsunami hits the shore, was previously developed by the authors. This algorithm relies on a database of precomputed tsunami waveforms at several near-shore locations and tsunami inundation maps from various earthquake fault models. In the event of a great earthquake, tsunami waveforms at the above mentioned near-shore locations are computed on the basis of real-time observation data by use of linear long-wave equations. Simulating these tsunami waveforms takes only 1-3 min on a common personal computer, so the realistic offshore tsunami waveforms can be forecasted. The offshore real-time simulated tsunami waveforms are then compared with precomputed tsunami waveforms in a database to select the site-specifi c best fault model and the corresponding tsunami inundation map. The best tsunami inundation map is then used as the tsunami inundation forecast. We evaluated the effectiveness of this algorithm in the real world by carrying out a tsunami evacuation drill in Kushiro City, Hokkaido, Japan, involving the city residents. The drill started with the announcement of a tsunami warning, to evacuate the residents to the nearest evacuation building. Approximately 10 min after the announcement, the tsunami inundation forecast map was given to the participants in the drill. The participants found that the use of the tsunami inundation forecast map produced by NearTIF was effective in helping them make better decisions with high confi dence during the tsunami evacuation drill. The NearTIF algorithm is recommended for use as part of the reconstruction policy by local authorities to improve the evacuation effi ciency, particularly in tsunami-prone areas.

Journal of Geophysical Research, 2010

Bulletin of The Seismological Society of America, 2009

Geophysical Research Letters, 2015

We applied a new method to compute tsunami Green's functions for slip inversion of the 1 April 20... more 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 traveltime 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 coseismic crustal deformation. The major slip region with a size of 100 km × 40 km is located downdip the epicenter at depth~28 km, regardless of assumed rupture velocities. The total seismic moment estimated from the slip distribution is 1.24 × 10 21 N m (M w 8.0).

Pure and Applied Geophysics, 2009

Linear and nonlinear responses of ten well-type tide gauge stations on the Japan Sea coast of cen... more Linear and nonlinear responses of ten well-type tide gauge stations on the Japan Sea coast of central Japan were estimated by in situ measurements. We poured water into the well or drained water from the well by using a pump to make an artificial water level difference between the outer sea and the well, then measured the recovery of water level in the well. At three tide gauge stations, Awashima, Iwafune, and Himekawa, the sea-level change of the outer sea is transmitted to the tide well instantaneously. However, at seven tide gauge stations, Nezugaseki, Ryotsu, Ogi, Teradomari, Banjin, Kujiranami, and Naoetsu, the sea-level change of the outer sea is not always transmitted to the tide well instantaneously. At these stations, the recorded tsunami waveforms are not assured to follow the actual tsunami waveforms. Tsunami waveforms from the Niigataken Chuetsu-oki Earthquake in 2007 recorded at these stations were corrected by using the measured tide gauge responses. The corrected amplitudes of the first and second waves were larger than the uncorrected ones, and the corrected peaks are a few minutes earlier than the uncorrected ones at Banjin, Kujiranami, and Ogi. At Banjin, the correction was significant; the corrected amplitudes of the first and second upward motion are ?103 cm and ?114 cm, respectively, while the uncorrected amplitudes were ?96 cm and ?88 cm. At other tide gauge stations, the differences between the uncorrected and corrected tsunami waveforms were insignificant.

Pure and Applied Geophysics, 2013

The 2010 Mentawai earthquake (magnitude 7.7) generated a destructive tsunami that caused more tha... more The 2010 Mentawai earthquake (magnitude 7.7) generated a destructive tsunami that caused more than 500 casualties in the Mentawai Islands, west of Sumatra, Indonesia. Seismological analyses indicate that this earthquake was an unusual ''tsunami earthquake,'' which produces much larger tsunamis than expected from the seismic magnitude. We carried out a field survey to measure tsunami heights and inundation distances, an inversion of tsunami waveforms to estimate the slip distribution on the fault, and inundation modeling to compare the measured and simulated tsunami heights. The measured tsunami heights at eight locations on the west coasts of North and South Pagai Island ranged from 2.5 to 9.3 m, but were mostly in the 4-7 m range. At three villages, the tsunami inundation extended more than 300 m. Interviews of local residents indicated that the earthquake ground shaking was less intense than during previous large earthquakes and did not cause any damage. Inversion of tsunami waveforms recorded at nine coastal tide gauges, a nearby GPS buoy, and a DART station indicated a large slip (maximum 6.1 m) on a shallower part of the fault near the trench axis, a distribution similar to other tsunami earthquakes. The total seismic moment estimated from tsunami waveform inversion was 1.0 9 10 21 Nm, which corresponded to M w 7.9. Computed coastal tsunami heights from this tsunami source model using linear equations are similar to the measured tsunami heights. The inundation heights computed by using detailed bathymetry and topography data and nonlinear equations including inundation were smaller than the measured ones. This may have been partly due to the limited resolution and accuracy of publically available bathymetry and topography data. One-dimensional run-up computations using our surveyed topography profiles showed that the computed heights were roughly similar to the measured ones.

Pure and Applied Geophysics, 2013

Centroid moment tensor solutions for the 2011 Tohoku earthquake are determined by W phase inversi... more 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 9 10 10 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.

Journal of Geophysical Research, 2010

Pure and Applied Geophysics, 2000

Geophysical Research Letters, 2013

Zisin (Journal of the Seismological Society of Japan. 2nd ser.), 2012

Geophysical Research Letters, 2007

Pure and Applied Geophysics, 2011

Journal of Geophysical Research: Solid Earth, 2014

Pure and Applied Geophysics, 2014

Advances in Natural and Technological Hazards Research, 2014

Journal of Geophysical Research, 2010

Bulletin of The Seismological Society of America, 2009

Geophysical Research Letters, 2015

Pure and Applied Geophysics, 2009

Linear and nonlinear responses of ten well-type tide gauge stations on the Japan Sea coast of cen... more Linear and nonlinear responses of ten well-type tide gauge stations on the Japan Sea coast of central Japan were estimated by in situ measurements. We poured water into the well or drained water from the well by using a pump to make an artificial water level difference between the outer sea and the well, then measured the recovery of water level in the well. At three tide gauge stations, Awashima, Iwafune, and Himekawa, the sea-level change of the outer sea is transmitted to the tide well instantaneously. However, at seven tide gauge stations, Nezugaseki, Ryotsu, Ogi, Teradomari, Banjin, Kujiranami, and Naoetsu, the sea-level change of the outer sea is not always transmitted to the tide well instantaneously. At these stations, the recorded tsunami waveforms are not assured to follow the actual tsunami waveforms. Tsunami waveforms from the Niigataken Chuetsu-oki Earthquake in 2007 recorded at these stations were corrected by using the measured tide gauge responses. The corrected amplitudes of the first and second waves were larger than the uncorrected ones, and the corrected peaks are a few minutes earlier than the uncorrected ones at Banjin, Kujiranami, and Ogi. At Banjin, the correction was significant; the corrected amplitudes of the first and second upward motion are ?103 cm and ?114 cm, respectively, while the uncorrected amplitudes were ?96 cm and ?88 cm. At other tide gauge stations, the differences between the uncorrected and corrected tsunami waveforms were insignificant.

Pure and Applied Geophysics, 2013

Journal of Geophysical Research, 2010

Pure and Applied Geophysics, 2000

Geophysical Research Letters, 2013