Mentawai Earthquake Research Articles

NUMERICAL ANALYSES TO OBSERVE THE PERFORMANCE OF A MONUMENTAL BUILDING AT THE UNIVERSITY OF BENGKULU, INDONESIA

The Integrated Laboratory Building of Political and Social Science Faculty, located in a seismic-prone area, has been the subject of a comprehensive study of its performance using seismic response analysis. This research has yielded significant findings, gathering crucial secondary data, such as soil layers, bedrock depth, building structure, and earthquake wave information. The seismic response analysis, conducted by referencing the synthetic earthquake wave from the Bengkulu–Mentawai earthquake in 2007, with a magnitude of Mw 8.6, has provided a comprehensive overview of the earthquake waves at the foundation soil layers. The data, analysed using the finite element method to understand the structural response, revealed that the Peak Ground Acceleration (PGA) and amplification factor at the surface soil layer are 0.220g and 1.429, respectively. The most critical PGA and amplification factors at the foundation soil layer are 0.147g and 0.955. Structural analysis has revealed internal forces and beam elements experiencing over-strength, necessitating retrofitting the affected structural elements to reduce the impact. One practical and highly effective method of retrofitting involves increasing the beam dimensions by 53.12%. With retrofitting, the impact of structural deformation can be minimised, enhancing the building’s resilience in case of an earthquake of equal or greater magnitude. These findings underscore the importance of our research and highlight the significant role of engineers, architects, and researchers in ensuring the safety and longevity of structures in seismic-prone areas.

A review of pre-disaster public awareness activities on public readiness: The 2010 Mentawai tsunami

A qualitative study on impact of pre-disaster public awareness activities on public readiness was conducted in the North and South Pagai Islands after the 25 October 2010 Mentawai earthquake. Parameters of readiness are public response to tsunami and the number of casualties. The results show that public awareness activities that commenced from 2004 have effectively increased people's awareness of tsunamis as indicated by their knowledge of tsunami signals. Yet apart from failure of the official warning system to alert the public in remote areas, the response of people to the natural tsunami signals has been made on the basis of misperceptions of strong earthquake signals and tsunami lead-time. Tsunami lead-time of the Pagai Islands had been perceived as long as that of Sumatra or Java Islands and strong earthquakes have been perceived as merely strong ground shaking. These misperceptions came from invalid materials of public education and invalid translation of scientific information, and were confirmed by people's experiences in the 12 September 2007 Bengkulu earthquake. Hence, despite mitigation by relocating houses to the high ground that has saved many lives in Malakopa and Asahan, there is no evidence on the positive relation of pre-disaster awareness intervention with the reduction in loss of lives in the study sites. In order to not give invalid information on tsunami awareness in the future, instead of using a generic formula of regional tsunamis, public education on tsunami should use a specific formula based on local characteristics. Moreover, sensing earthquakes that may trigger tsunamis through the duration of ground shaking may be more effective than that of the strength of ground shaking. Because sensing time duration is difficult, particularly for communities that are not used to using modern timers, it is necessary to develop simple means of sensing time duration.

Coseismic Deformation Responses due to Geometrical Structure and Heterogeneity of the Accretionary Wedge: Study Case 2010 Mentawai Earthquake, West Sumatra, Indonesia

The assumption of a homogeneous elastic half-space model is widely used to model the earth’s deformation. However, the homogeneous assumption would not accurately reflect the complexity of the shallow crust. We performed a 3D coseismic deformation model using the finite element method and referred to the 2010 Mentawai earthquake. The 2010 tsunami earthquake was located at the Mentawai segment, which is a part of the accretionary wedge in the Sumatra subduction zone. This active accretionary wedge is identified as the most complicated structure on earth and lies along the Sumatra subduction zone, at which most destructive earthquakes happen in this region. We examined the impact of the accretionary wedge geometry and material properties by considering the wedge as a single different property separated from the continental plate. Various geometrical features, such as topography and wedge dimension, as well as physical properties, were simulated. Those features are then observed for their responses on the surface deformation. The topography affected the magnitude of the horizontal deformation up to 10% but only the pattern of the vertical deformation. The wedge dimension seems to have an insignificant influence on the surface deformation compared to the topography. Different physical properties of the accretionary wedge affect not only the magnitude of the horizontal deformation up to 40% but also the orientation. The direction of the lateral movement is seemingly affected by the material under the GPS station and by the source. On the other hand, the variations in the physical properties resulted in discrepancies of 0.5 meters in the vertical deformation near the source. These results indicated that regional physical property information and geometrical features are critical in estimating coseismic deformation, leading to more accurate slip inversion and earthquake and tsunami hazard prediction, particularly in regions with significant inhomogeneity.

Detection of Surface Displacement of Mentawai Earthquake Mw7.8, October 25, 2010, using High Rate (HR) 1Hz GNSS Data

Conventional seismic sensors, may have some problems, for example, seismometers are vulnerable to clipping and going off-scale when recording in the near-field of large earthquakes, that caused magnitude saturation. High-rate GNSS measurement can avoid the problems in conventional seismic records, in determining true size of large earthquake. The Mw7.8 Mentawai earthquake on October 25, 2010, generate significant tsunami run-up reached 9.3 m, five hundred more casualties, and raise economic loss of 200 bilions rupiahs. We utilize 1 Hz high-rate GNSS data from 11 GNSS stations of the Sumatran GPS Array (SuGAr) network operated by Earth Observatory of Singapore (EOS) associated with the October 25, 2010, Mw7.8 Mentawai earthquake. We generate displacement waveform in three components (east, north, up) and it can be seen that all GNSS stations can detect surface displacement caused by October 25, 2010, Mw7.8 Mentawai earthquake. We also display the displacement waveform based on the distance from the GNSS station to the earthquake hypocenter, and found that displacement amplitude decreases as the distance between the earthquake hypocenter and the location of the GNSS station increases.

Slow Fault Slip Signatures in Coseismic Ionospheric Disturbances

AbstractRise times of earthquake moment release influence the spectra of seismic waves. For example, slow fault movements in tsunami earthquakes excite larger tsunamis than expected from intensities of short‐period seismic waves. Here we compare amplitudes of two different atmospheric waves, long‐period internal gravity waves and short‐period acoustic waves, excited by coseismic vertical crustal movements. We observe them as coseismic ionospheric disturbances by measuring ionospheric electrons using global navigation satellite systems. Four regular megathrust earthquakes Mw 8.0–9.0 showed that the internal gravity waves become ten times stronger as the magnitude increases by one. We found that the 2010 Mentawai earthquake, a typical tsunami earthquake, excited internal gravity waves stronger than those expected by this empirical relationship. On the other hand, amplitudes of acoustic waves excited by tsunami earthquakes were normal. This suggests that slow fault ruptures excite long‐period atmospheric waves efficiently, leaving a slow earthquake signature in ionospheric disturbances.

Comparison of Deformation Vectors Due to Earthquake in Subduction Zone and Sumatran Fault for Each Phase of Earthquake Cycle

This study compares the deformation in West Sumatra due to the earthquakes in the subduction zone and the Sumatran Fault. The Mw6.0 Mentawai earthquake 2019 with a thrust fault mechanism and the Mw5.4 South Solok earthquake 2019 with a strike-slip fault mechanism were used as case studies for the subduction zone and Sumatran Fault, respectively. The deformation was observed using 12 SuGAr (Sumatra GPS Array) and 8 InaCORS (Indonesian Continuously Operating Reference Station) stations, which were processed using GAMIT/GLOBK software. There are differences in the deformation vectors of the two earthquakes. The Mentawai earthquake experienced larger energy accumulation than the South Solok earthquake. The coseismic phase of the Mentawai earthquake experienced the largest horizontal shift at the SLBU station, which was 15.48 mm in the direction of S29.96W, while the South Solok earthquake is found to horizontally shift the CSDH station at the size of 5.75 mm towards S11.45E. The postseismic phase of the Mentawai earthquake lasted 60 days, longer than the South Solok earthquake (20 days). The difference in deformation characteristic between these two earthquakes found in this study will be valuable information in modeling earthquakes in Sumatra.

Focal Mechanism Inversion on the Aftershock Sequence of the 25 October, 2010 Mw 7.8 Mentawai Earthquake: A Preliminary Result

An Mw 7.8 tsunami earthquake ruptured the shallow part of the Mentawai megathrust on 25 October 2010 and generated a large tsunami that hit the Mentawai Islands. The earthquake continued a partial rupture sequence of the 2007 Mw 8.5 and 7.9 Bengkulu earthquake and the 2008 Mw 7.2 North Pagai earthquake. The aftershock activity gives the opportunity to investigate the geometry of the subduction interface, which is critical to understanding the propagation and termination of a large earthquake. However, it is necessary to have a precise location and mechanism of the aftershock distribution to clearly image the fault geometry. In this preliminary study, we have applied the Cut-and-Paste (CAP) focal mechanism inversion method to 20 aftershocks (M ≥ 4.9) of the 2010 Mentawai earthquake. The seismic waveforms recorded by the BMKG broadband seismic network on the regional distance were used in the inversion. They successfully obtained ten focal mechanisms more than that provided in the GCMT catalog. The result exhibits a distinctly different thrust and normal faulting aftershocks activity. The thrust faulting aftershocks are mainly distributed in the downdip of the Mentawai 2010 mainshock rupture, indicating the stresses that have not been released during the mainshock. In the shallow region near the trench, the aftershocks are dominated by a normal faulting mechanism which shows the tensile stress caused by the bending of the subducting plate.

Spherical-earth finite element model of coseismic slip distribution during the 2010 Mentawai earthquake

The moment magnitude (MW) 7.8 earthquake occurred along the Mentawai seismic region of Sunda Trench on 25 October 2010, which is classified as the tsunami earthquake. The GPS inversion results under the assumption of simple horizontal layered medium show that the slip near the trench is not apparent, which is inconsistent with our understanding of tsunami earthquake. Here, we construct a spherical-earth finite element model (FEM) to investigate the coseismic slip distribution of the 2010 Mentawai earthquake by geometrically combining the subducting plate with a precise subduction interface. The FEM-based coseismic slip distribution shows that there are three major slip patches on the fault interface, one is located on the shallow region and the other two are located on the deeper part of the fault plane. The largest slip patch is located near the trench with the depth less than 10 km, and the maximum amplitude is about 12 m. This significant near-trench slip is consistent with the tsunami earthquake studies. A more realistic domain of FEM is used to change the coseismic slip distribution, thus significantly improving the fitting degree of the data model. These findings suggest that the FEM-derived Green's function is essential to image a more robust and realistic coseismic slip distribution of the large earthquake in the subduction zone.

Model system dynamics potential impact of disasters due to the re-period of the October 25th, 2010 earthquake in the Mentawai Islands

Geographically, the Mentawai Islands Regency has a territorial boundary with the sea. The north side is the Siberut Strait, the south side is bordered by the Indian Ocean, the east side is bordered by the Mentawai Strait, and the west side is bordered by the Indian Ocean. The Mentawai Islands Regency area has a high level of seismicity which makes it prone to earthquakes and tsunamis because it has an earthquake return period. Population and economic growth in the Mentawai Islands Regency continue to increase, resulting in a higher level of threat due to earthquakes and tsunamis to people and buildings. By using earthquake return period modeling based on secondary data and population and building growth modeling using system dynamics, Pentahelix can implement disaster risk mitigation in the Mentawai Islands Regency to reduce the risk of casualties and material losses. Based on the prediction that the Mentawai earthquake return period on October 25, 2010, will occur for 24 years to 57 years or around 2034 to 2067, and the results of system dynamics modeling with Powersim Studio 10 software, the number of vulnerable people affected is 24,764 people up to 42,944 people and potential losses. housing sector between 144.73 billion to 250.98 billion.

Reliability of Build Back Better at enhancing resilience of communities

PurposeThe purpose of this study is to analyse the reliability of Build Back Better slogan in the context of post-disaster housing reconstruction in developing countries, at enhancing disaster-resilience of housing and its occupants in the long term from socio-ecological systems resilience perspective.Design/methodology/approachA predominantly qualitative methodology and multi-disciplinary case study methodology is adopted to compare long-term outcomes of two post-disaster housing reconstruction interventions: post-2008 Bihar Kosi River floods in India and post-2010 Mentawai earthquake and tsunami in Indonesia.FindingsOut of the nine generalizable findings, two of the most significant findings include giving freedom of choice or human capabilities to the disaster survivors and sustaining capacity development during and beyond the completion of housing reconstruction. These two processes play a significant role in linking reconstruction to resilience in the long term, especially of those living at- risk and poverty.Originality/valueThis paper further advances the current scholarship on overarching long-term impacts of housing reconstruction efforts, based on longitudinal and empirical studies in India and Indonesia. While these findings represent a snapshot of diverse and complex disaster experiences in the developing-world context, the comparison offers insight into how to turn the rhetoric surrounding “owner-driven” or “built back better” into long-term resilience outcomes.

Afterslip following the 2007 Mw 8.4 Bengkulu earthquake in Sumatra loaded the 2010 Mw 7.8 Mentawai tsunami earthquake rupture zone

AbstractThe 12 September 2007 Mw 8.4 Bengkulu earthquake in Sumatra marked the first in a modern series of large earthquakes along the Mentawai section of the Sunda megathrust. Understanding the spatial distribution of coseismic slip and ensuing afterslip is important for assessing seismic hazard in neighboring unruptured regions of the megathrust. We reestimate the spatial distribution of coseismic slip during this earthquake with improved coseismic offsets from the Sumatran GPS Array (SuGAr) and estimate afterslip following this earthquake with SuGAr postseismic time series spanning ∼6.3 years after the earthquake. We invert for the spatiotemporal distribution of afterslip with the principal component analysis‐based inversion method (PCAIM), and we take into account viscoelastic deformation by incorporating into the inversion the estimation of strain within ductile deforming blocks located at asthenospheric depths. Our results suggest cumulative afterslip concentrated within, updip, and downdip of the 2007 coseismic rupture area and shallow afterslip that borders and overlaps the 2010 Mw 7.8 Mentawai earthquake rupture zone. The cumulative contribution of stress changes due to the coseismic event and the ensuing afterslip likely increased strain rates in the shallow portion of the megathrust adjacent to the Mentawai earthquake rupture area, potentially promoting its rupture in 2010.

Estimation of the 2010 Mentawai tsunami earthquake rupture process from joint inversion of teleseismic and strong ground motion data

Joint inversion of teleseismic body-wave data and strong ground motion waveforms was applied to determine the rupture process of the 2010 Mentawai earthquake. To obtain stable solutions, smoothing and non-negative constraints were introduced. A total of 33 teleseismic stations and 5 strong ground motion stations supplied data. The teleseismic and strong ground motion data were separately windowed for 150 s and 250 s and band-pass filtered with frequencies of 0.001–1.0 Hz and 0.005–0.5 Hz, respectively. The finite-fault model was established with length and width of 190 km and 70 km, and the initial seismic source parameters were set by referring to centroid moment tensor (CMT) solutions. Joint inversion results indicate that the focal mechanism of this earthquake is thrust fault type, and the strike, dip, and rake angles are generally in accordance with CMT results. The seismic moment was determined as 5.814 × 1020 Nm (Mw7.8) and source duration was about 102 s, which is greater than those of other earthquakes of similar magnitude. The rupture nucleated near the hypocenter and then propagated along the strike direction to the northwest, with a maximum slip of 3.9 m. Large uncertainties regarding the amount of slip retrieved using different inversion methods still exist; however, the conclusion that the majority of slip occurred far from the islands at very shallow depths was found to be robust. The 2010 Mentawai earthquake was categorized as a tsunami earthquake because of the long rupture duration and the generation of a tsunami much larger than was expected for an earthquake of its magnitude.

Rapid Moment Magnitude Estimation Using Strong Motion Derived Static Displacements

The static surface deformation can be recovered from strong motion records. Compared to satellite-based measurements such as GPS or InSAR, the advantage of strong motion records is that they have the potential to provide real-time coseismic static displacements. The use of these valuable data was optimized for the moment magnitude estimation. A centroid grid search method was introduced to calculate the moment magnitude by using1 model. The method to data sets was applied of the 2011, Mw 9.0 Tohoku earthquake, the 2004, Mw 9.0 Sumatra earthquake, the 2010, Mw 8.8 Maule earthquake, the 2003, Mw 8.3 Tokachi-Oki earthquake, the 2010, Mw 7.8 Mentawai earthquake and the 2007, Mw 7.7 Tocopilla earthquake. The method calculates reasonable moment magnitude using static surface displacement data even only from single station. This method can be done very rapidly within approximately 5minutes. It provides crucial information e.g. for making tsunami early warning decision.

Tsunami Source of the 2010 Mentawai, Indonesia Earthquake Inferred from Tsunami Field Survey and Waveform Modeling

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 × 1021 Nm, which corresponded to Mw 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.

Web-based Tsunami Early Warning System: a case study of the 2010 Kepulaunan Mentawai Earthquake and Tsunami

Abstract. This study analyzes the response of the Global Disasters Alerts and Coordination System (GDACS) in relation to a case study: the Kepulaunan Mentawai earthquake and related tsunami, which occurred on 25 October 2010. The GDACS, developed by the European Commission Joint Research Center, combines existing web-based disaster information management systems with the aim to alert the international community in case of major disasters. The tsunami simulation system is an integral part of the GDACS. In more detail, the study aims to assess the tsunami hazard on the Mentawai and Sumatra coasts: the tsunami heights and arrival times have been estimated employing three propagation models based on the long wave theory. The analysis was performed in three stages: (1) pre-calculated simulations by using the tsunami scenario database for that region, used by the GDACS system to estimate the alert level; (2) near-real-time simulated tsunami forecasts, automatically performed by the GDACS system whenever a new earthquake is detected by the seismological data providers; and (3) post-event tsunami calculations using GCMT (Global Centroid Moment Tensor) fault mechanism solutions proposed by US Geological Survey (USGS) for this event. The GDACS system estimates the alert level based on the first type of calculations and on that basis sends alert messages to its users; the second type of calculations is available within 30–40 min after the notification of the event but does not change the estimated alert level. The third type of calculations is performed to improve the initial estimations and to have a better understanding of the extent of the possible damage. The automatic alert level for the earthquake was given between Green and Orange Alert, which, in the logic of GDACS, means no need or moderate need of international humanitarian assistance; however, the earthquake generated 3 to 9 m tsunami run-up along southwestern coasts of the Pagai Islands where 431 people died. The post-event calculations indicated medium-high humanitarian impacts.

The 2010 Mw 7.8 Mentawai earthquake: Very shallow source of a rare tsunami earthquake determined from tsunami field survey and near‐field GPS data

The Mw 7.8 October 2010 Mentawai, Indonesia, earthquake was a “tsunami earthquake,” a rare type of earthquake that generates a tsunami much larger than expected based on the seismic magnitude. It produced a locally devastating tsunami, with runup commonly in excess of 6 m. We examine this event using a combination of high‐rate GPS data, from instruments located on the nearby islands, and a tsunami field survey. The GPS displacement time series are deficient in high‐frequency energy, and show small coseismic displacements (<22 cm horizontal and <4 cm subsidence). The field survey shows that maximum tsunami runup was >16 m. Our modeling results show that the combination of the small GPS displacements and large tsunami can only be explained by high fault slip at very shallow depths, far from the islands and close to the oceanic trench. Inelastic uplift of trench sediments likely contributed to the size of the tsunami. Recent results for the 2011 Mw 9.0 Tohoko‐Oki earthquake have also shown shallow fault slip, but the results from our study, which involves a smaller earthquake, provide much stronger constraints on how shallow the rupture can be, with the majority of slip for the Mentawai earthquake occurring at depths of <6 km. This result challenges the conventional wisdom that the shallow tips of subduction megathrusts are aseismic, and therefore raises important questions both about the mechanical properties of the shallow fault zone and the potential seismic and tsunami hazard of this shallow region.

2011年東北地方太平洋沖地震による津波解析結果から再検討する巨大津波の発生様式

Review of the previous tsunami analyses for the 2011 Tohoku-oki earthquake indicates that the large slip of more than 40m was estimated on the plate interface near the Japan Trench. The plate interface near the Japan Trench was previously ruptured by the tsunami earthquakes such as 1896 or 1611 Sanriku tsunami earthquakes. Along the subduction zone off Sumatra, the 2004 great Sumatra earthquake ruptured the plate interface near the trench with a large slip amount of 29m. There were also tsunami earthquakes such as the 1907 Sumatra earthquake or 2010 Mentawai earthquake which ruptured the plate interface close to the trench. These evidences suggest that occurrence of the large earthquakes which rupture the plate interface near trench such as tsunami earthquakes is more common than that we thought previously. Variability of the great earthquakes along the plate interface makes us difficult to evaluate future great interplate earthquakes along subduction zones.

ESTIMATION OF SUBSURFACE STRUCTURE BASED ON MICROTREMOR OBSERVATIONS AT PADANG, INDONESIA

In this study, we estimated a ground profile in Padang City, Indonesia, using microtremor observations. Indonesian is located on the boundaries of three major tectonic plates, namely the Indian-Australian, Pacific, and Eurasian plates. Consequently, subduction-related major earthquakes occur repeatedly, including, even in the 21st century, such large interplate earthquakes as the 2004 Sumatra earthquake, the 2006 Nias Island earthquake, the 2007 Bengkulu earthquake, and the 2010 Mentawai earthquake. The 2009 Padang earthquake was also one of the most destructive earthquakes in recent times, although it was an intra-plate based earthquake. This earthquake was of magnitude Mw7.6, producing severe tremors and causing extensive damage to buildings and casualties. To prevent loss of human lives and dwellings from a devastating earthquake, we investigated the shaking characteristics of the ground in Padang. Spectral ratios for horizontal and vertical motion, H/V, from single-station microtremor records were used to identify the predominant periods of the ground vibrations. Microtremor array observations were conducted to find the phase velocities of the Rayleigh wave. The ground profile was estimated by means of the dispersion curves obtained using the Spatial Autocorrelatio (SPAC) and Centerless Circular Array (CCA) methods. The Particle Swarm Optimization (PSO) technique was introduced to determine the dispersion curves. By compiling the soil profile model obtained at each site, a three-layered generalized model was proposed. The velocity of the first layer was less than Vs = 300 m/s, the second layer Vs = 300-3000 m/s and the third layer more than Vs = 3,000 m/s. The subsurface structure models constructed provide useful information for 2-D and 3-D simulations of ground motion in Padang.

Field Test on the Lodging and Uprooting Conditions of Casuarinas and Verification of Those through the 2010 Mentawai Earthquake Tsunami

Coastal tree failure conditions such as the lodging, uprooting and breaking are necessary to estimate tsunami energy reduction due to coastal forest. A field test on the lodging and uprooting conditions of Sea casuarinas is carried out on the west coast of Central Sumatra, Indonesia. Based on the field test results, empirical lodging and uprooting conditions of Sea casuarinas are presented. The validity of the conditions is checked through comparison with those data obtained through a field survey of the 2010 Mentawai Earthquake tsunami. It is confirmed that more than 40% of the data satisfy the conditions. Based on the geometric and dynamic characteristics of Sea casuarinas, its scale model in the hydraulic experiments is examined considering the similarity laws for trunk and branches.

Field Survey of 2010 Mentawai Earthquake Tsunami

A tsunami generated by an earthquake of Mw 7.7 hit North and South Pagai Islands and Sipola Island of Mentawai Islands lying off the west coast of Sumatra Island. This paper reports results of a joint field survey on the tsunami disasters. In the survey tsunami inundation and runup heights were measured in North and South Pagai Islands, and interview investigation was also conducted to understand people's evacuation action in the islands. Averaged tsunami height was about 6 m in the islands, resulting in many losses of human lives and houses. However, there was a good example of evacuation. A person who heard sound of tsunami approaching run to hill with transmitting information of the tsunami coming to other residents, and therefore all residents could evacuate to save their lives.