Slip Distribution and Seismic Moment of the 2010 and 1960 Chilean Earthquakes Inferred from Tsunami Waveforms and Coastal Geodetic Data

The slip distribution and seismic moment of the 2010 and 1960 Chilean earthquakes were estimated from tsunami and coastal geodetic data. These two earthquakes generated transoceanic tsunamis, and the waveforms were recorded around the Pacific Ocean. In addition, coseismic coastal uplift and subsidence were measured around the source areas. For the 27 February 2010 Maule earthquake, inversion of the tsunami waveforms recorded at nearby coastal tide gauge and Deep Ocean Assessment and Reporting of Tsunamis (DART) stations combined with coastal geodetic data suggest two asperities: a northern one beneath the coast of Constitucion and a southern one around the Arauco Peninsula. The total fault length is approximately 400 km with seismic moment of 1.7 × 10²² Nm (Mw 8.8). The offshore DART tsunami waveforms require fault slips beneath the coasts, but the exact locations are better estimated by coastal geodetic data. The 22 May 1960 earthquake produced very large, ~30 m, slip off Valdivia. Joint inversion of tsunami waveforms, at tide gauge stations in South America, with coastal geodetic and leveling data shows total fault length of ~800 km and seismic moment of 7.2 × 10²² Nm (Mw 9.2). The seismic moment estimated from tsunami or joint inversion is similar to previous estimates from geodetic data, but much smaller than the results from seismic data analysis.     

The 15 August 2007 Peru Earthquake and Tsunami: Influence of the Source Characteristics on the Tsunami Heights

The tsunami caused by the 2007 Peru earthquake (M_w 8.0) provoked less damage than by the seismic shaking itself (numerous casualties due to the earthquake in the vicinity of Pisco). However, it propagated across the Pacific Ocean and small waves were observed on one tide gauge in Taiohae Bay (Nuku Hiva, Marquesas, French Polynesia). We invert seismological data to recover the rupture pattern in two steps. The first step uses surface waves to find a solution for the moment tensor, and the second step uses body waves to compute the slip distribution in the source area. We find the slip distribution to consist of two main slip patches in the source area. The inversion of surface waves yields a scalar moment of 8.9 10²⁰ Nm, and body-wave inversion gives 1.4 10²¹ Nm. The inversion of tsunami data recorded on a single deep ocean sensor also can be used to compute a fault slip pattern (yielding a scalar moment of 1.1 10²¹ Nm). We then use these different sources to model the tsunami propagation across the Pacific Ocean, especially towards Nuku Hiva. While the source model taken from the body-wave inversion yields computed tsunami waves systematically too low with respect to observations (on the central Pacific Ocean DART buoy as on the Polynesian tide gauge), the source model established from the surface-wave inversion is more efficient to fit the observations, confirming that the tsunami is sensitive to the low frequency component of the source. Finally we also discuss the modeling of the late tsunami arrivals in Taiohae Bay using several friction coefficients for the sea bottom.     

Numerical Simulation of the 2011 Tohoku Tsunami Based on a New Transient FEM Co-seismic Source: Comparison to Far- and Near-Field Observations

In this work, we simulate the 2011 M9 Tohoku-Oki tsunami using new coseismic tsunami sources based on inverting onshore and offshore geodetic data, using 3D Finite Element Models (FEM). Such FEMs simulate elastic dislocations along the plate boundary interface separating the stiff subducting Pacific Plate from the relatively weak forearc and volcanic arc of the overriding Eurasian plate. Due in part to the simulated weak forearc materials, such sources produce significant shallow slip (several tens of meters) along the updip portion of the rupture near the trench. To assess the accuracy of the new approach, we compare observations and numerical simulations of the tsunami's far- and near-field coastal impact for: (i) one of the standard seismic inversion sources (UCSB; Shao et al. 2011); and (ii) the new FEM sources. Specifically, results of numerical simulations for both sources, performed using the fully nonlinear and dispersive Boussinesq wave model FUNWAVE-TVD, are compared to DART buoy, GPS tide gauge, and inundation/runup measurements. We use a series of nested model grids with varying resolution (down to 250 m nearshore) and size, and assess effects on model results of the latter and of model physics (such as when including dispersion or not). We also assess the effects of triggering the tsunami sources in the propagation model: (i) either at once as a hot start, or with the spatiotemporal sequence derived from seismic inversion; and (ii) as a specified surface elevation or as a more realistic time and space-varying bottom boundary condition (in the latter case, we compute the initial tsunami generation up to 300 s using the non-hydrostatic model NHWAVE). Although additional refinements are expected in the near future, results based on the current FEM sources better explain long wave near-field observations at DART and GPS buoys near Japan, and measured tsunami inundation, while they simulate observations at distant DART buoys as well or better than the UCSB source. None of the sources, however, are able to explain the largest runup and inundation measured between 39.5° and 40.25°N, which could be due to insufficient model resolution in this region (Sanriku/Ria) of complex bathymetry/topography, and/or to additional tsunami generation mechanisms not represented in the coseismic sources (e.g., splay faults, submarine mass failure). This will be the object of future work.     

Combined Effects of Tectonic and Landslide-Generated Tsunami Runup at Seward, Alaska During the M W 9.2 1964 Earthquake

We apply a recently developed and validated numerical model of tsunami propagation and runup to study the inundation of Resurrection Bay and the town of Seward by the 1964 Alaska tsunami. Seward was hit by both tectonic and landslide-generated tsunami waves during the $M_{\rm W}$ 9.2 1964 megathrust earthquake. The earthquake triggered a series of submarine mass failures around the fjord, which resulted in landsliding of part of the coastline into the water, along with the loss of the port facilities. These submarine mass failures generated local waves in the bay within 5?min of the beginning of strong ground motion. Recent studies estimate the total volume of underwater slide material that moved in Resurrection Bay to be about 211?million m³ (Haeussler et?al. in Submarine mass movements and their consequences, pp 269?C278, 2007). The first tectonic tsunami wave arrived in Resurrection Bay about 30?min after the main shock and was about the same height as the local landslide-generated waves. Our previous numerical study, which focused only on the local landslide-generated waves in Resurrection Bay, demonstrated that they were produced by a number of different slope failures, and estimated relative contributions of different submarine slide complexes into tsunami amplitudes (Suleimani et?al. in Pure Appl Geophys 166:131?C152, 2009). This work extends the previous study by calculating tsunami inundation in Resurrection Bay caused by the combined impact of landslide-generated waves and the tectonic tsunami, and comparing the composite inundation area with observations. To simulate landslide tsunami runup in Seward, we use a viscous slide model of Jiang and LeBlond (J Phys Oceanogr 24(3):559?C572, 1994) coupled with nonlinear shallow water equations. The input data set includes a high resolution multibeam bathymetry and LIDAR topography grid of Resurrection Bay, and an initial thickness of slide material based on pre- and post-earthquake bathymetry difference maps. For simulation of tectonic tsunami runup, we derive the 1964 coseismic deformations from detailed slip distribution in the rupture area, and use them as an initial condition for propagation of the tectonic tsunami. The numerical model employs nonlinear shallow water equations formulated for depth-averaged water fluxes, and calculates a temporal position of the shoreline using a free-surface moving boundary algorithm. We find that the calculated tsunami runup in Seward caused first by local submarine landslide-generated waves, and later by a tectonic tsunami, is in good agreement with observations of the inundation zone. The analysis of inundation caused by two different tsunami sources improves our understanding of their relative contributions, and supports tsunami risk mitigation in south-central Alaska. The record of the 1964 earthquake, tsunami, and submarine landslides, combined with the high-resolution topography and bathymetry of Resurrection Bay make it an ideal location for studying tectonic tsunamis in coastal regions susceptible to underwater landslides.     

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 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.     

Validation and Joint Inversion of Teleseismic Waveforms for Earthquake Source Models Using Deep Ocean Bottom Pressure Records: A Case Study of the 2006 Kuril Megathrust Earthquake

The importance of accurate tsunami simulation has increased since the 2004 Sumatra-Andaman earthquake and the Indian Ocean tsunami that followed it, because it is an important tool for inundation mapping and, potentially, tsunami warning. An important source of uncertainty in tsunami simulations is the source model, which is often estimated from some combination of seismic, geodetic or geological data. A magnitude 8.3 earthquake that occurred in the Kuril subduction zone on 15 November, 2006 resulted in the first teletsunami to be widely recorded by bottom pressure recorders deployed in the northern Pacific Ocean. Because these recordings were unaffected by shallow complicated bathymetry near the coast, this provides a unique opportunity to investigate whether seismic rupture models can be inferred from teleseismic waves with sufficient accuracy to be used to forecast teletsunami. In this study, we estimated the rupture model of the 2006 Kuril earthquake by inverting the teleseimic waves and used that to model the tsunami source. The tsunami propagation was then calculated by solving the linear long-wave equations. We found that the simulated 2006 Kuril tsunami compared very well to the ocean bottom recordings when simultaneously using P and long-period surface waves in the earthquake source process inversion.     

The October 4, 1994 Shikotan (Kurile Islands) Tsunamigenic Earthquake: An Open Problem on the Source Mechanism

—On October 4, 1994, an earthquake of magnitude M _w = 8.2 occurred in the western part of the Kurile Islands, generating a tsunami that has been well recorded along the entire coast of Japan. Previous works have shown that this earthquake does not represent a low angle thrust event, normally expected in a subduction zone, rather an intra-plate event rupturing through the slab. On the basis of the accepted mechanism, two fault models, representative of the nodal plane ambiguity, have been suggested. The goal of this work is to verify whether the tsunami simulations are able to rule out one of the two proposed fault models. Taking into account both fault models together with a heterogeneous slip along the fault, we have performed numerical simulations of the tsunami. All source models produce tide-gauge records in agreement with the observed ones. The limit of resolution of the performed simulations, estimated by means of a perturbed bathymetry, does not allow us to distinguish the best source model.     

海底地震有限断层破裂模型对近场海啸数值预报的影响

快速准确的海啸源模型是近场海啸精确预警的关键.尽管目前还没有办法直接对其进行正演定量计算,但是可以通过多源地震、海啸观测数据进行反演或联合反演推算.不同的海啸源可能导致不同的预警结论,了解不同类型海啸源适用性、评估海啸源特征差异对近场海啸的影响,无论对于海啸预警还是海啸模拟研究尤为重要.本文评估分析了6种不同同震断层模型对2011年3月11日日本东北地震海啸近场数值预报的影响,重点对比分析了有限断层模型与均一滑动场模型对近场海啸产生、传播、淹没特征的影响及各自的误差.研究表明:近场海啸波能量分布主要取决于海啸源分布特征,特别是走向角的差异对海啸能量分布影响较大;有限断层模型对海啸灾害最为严重的39°N以南沿岸地区的最大海啸爬坡高度明显优于均一滑动场模型结果;综合对比DART浮标、GPS浮标及近岸潮位站共32个站次的海啸波幅序列结果发现有限断层模型整体平均绝对/相对误差比均一滑动场模型平均误差要低,其中Fujii海啸源的平均绝对/相对误差最小,分别是0.56m和26.71%.UCSB海啸源的平均绝对/相对误差次之.3个均一滑动场模型中USGSCMT海啸源模拟精度最高.相对于深海、浅海观测站,有限断层模型比均一滑动场模型对近岸观测站计算精度更高.海啸源误差具有显著的方向性,可能与反演所采用的波形数据的代表性有关;谱分析结果表明Fujii海啸源对在12至60min主频波谱的模拟要优于UCSB海啸源.海啸源中很难真实反映海底地震破裂过程,然而通过联合反演海啸波形数据推算海啸源的方法可以快速确定海啸源,并且最大限度的降低地震破裂过程与海啸产生的不确定性带来的误差.     

Re-examination of the Source Mechanism of the 1998 Papua New Guinea Earthquake and Tsunami

— Simulation of tsunami propagation and runup of the 1998 Papua New Guinea (PNG) earthquake tsunami using the detailed bathymetry measured by JAMSTEC and adding bathymetric data at depths less than 60 m is carried out, reproducing the tsunami energy focus into Warapu and Arop along the Sissano Lagoon. However, the computed runup heights in the lagoon are still lower than those measured. Even if the error in estimating the fault parameters is taken into consideration, computational results are similar. Analysis by the wave ray method using several scenarios of the source size of the tsunami and location by the wave ray method suggests that a source characterized by small size in water 1,000-m deep approximately 25 km offshore the lagoon, best fits the arrival determined from the interviews with eyewitnesses. A two-layer numerical model simulating the interaction of the tsunami with a landslide is employed to study the behavior of a landslide-generated tsunami with different size sand depths of the initial slide just outside the lagoon. A landslide model with a volume of 4–8 × 10⁹ m³ is selected as the best in order to reproduce the distribution of the measured tsunami runup in the lagoon. The simulation of a tsunami generated in two stages, fault and landslide, could show good agreement with the runup heights and distribution of the arrival time, but a time gap of around 10 minutes remains, suggesting that a tsunami generated by the mainshock at 6:49 PM local time is too small for people to notice, and the following tsunami triggered by landslide or mass movement near the lagoon about ten minutes after the mainshock attacked the coast and caused the huge damage.     

Tsunami Probability in the Caribbean Region

We calculated tsunami runup probability (in excess of 0.5 m) at coastal sites throughout the Caribbean region. We applied a Poissonian probability model because of the variety of uncorrelated tsunami sources in the region. Coastlines were discretized into 20 km by 20 km cells, and the mean tsunami runup rate was determined for each cell. The remarkable ~500-year empirical record compiled by O’Loughlin and Lander (2003) was used to calculate an empirical tsunami probability map, the first of three constructed for this study. However, it is unclear whether the 500-year record is complete, so we conducted a seismic moment-balance exercise using a finite-element model of the Caribbean-North American plate boundaries and the earthquake catalog, and found that moment could be balanced if the seismic coupling coefficient is c = 0.32. Modeled moment release was therefore used to generate synthetic earthquake sequences to calculate 50 tsunami runup scenarios for 500-year periods. We made a second probability map from numerically-calculated runup rates in each cell. Differences between the first two probability maps based on empirical and numerical-modeled rates suggest that each captured different aspects of tsunami generation; the empirical model may be deficient in primary plate-boundary events, whereas numerical model rates lack backarc fault and landslide sources. We thus prepared a third probability map using Bayesian likelihood functions derived from the empirical and numerical rate models and their attendant uncertainty to weight a range of rates at each 20 km by 20 km coastal cell. Our best-estimate map gives a range of 30-year runup probability from 0–30% regionally.     

Tsunami generation of the 1993 Hokkaido Nansei-Oki earthquake

Heterogeneous fault motion of the 1993 Hokkaido Nansei-Oki earthquake is studied by using seismic, geodetic and tsunami data, and the tsunami generation from the fault model is examined. Seismological analyses indicate that the focal mechanism of the first 10 s, when about a third of the total moment was released, is different from the overall focal mechanism. A joint inversion of geodetic data on Okushiri Island and the tide gauge records in Japan and Korea indicates that the largest slip, about 6 m, occurred in a small area just south of the epicenter. This corresponds to the initial rupture on a fault plane dipping shallowly to the west. The slip on the northernmost subfault, which is dipping to the east, is about 2 m, while the slips on the southern subfaults, which are steeply dipping to the west, are more than 3 m. Tsunami heights around Okushiri Island are calculated from the heterogeneous fault model using different grid sizes. Computation on the smaller grids produces large tsunami height that are closer to the observed tsunami runup heights. Tsunami propagation in the nearly closed Japan Sea is examined as the free oscillation of the Japan Sea. The excitation of the free oscillation by this earthquake is smaller than that by the 1964 Niigata or 1983 Japan Sea earthquake.     

Co-seismic fault geometry and slip distribution of the 26 December 2004, giant Sumatra–Andaman earthquake constrained by GPS,coral reef,and remote sensing data

We analyze co-seismic displacement field of the 26 December 2004, giant Sumatra–Andaman earthquake derived from Global Position System observations,geological vertical measurement of coral head, and pivot line observed through remote sensing. Using the co-seismic displacement field and AK135 spherical layered Earth model, we invert co-seismic slip distribution along the seismic fault. We also search the best fault geometry model to fit the observed data. Assuming that the dip angle linearly increases in downward direction, the postfit residual variation of the inversed geometry model with dip angles linearly changing along fault strike are plotted. The geometry model with local minimum misfits is the one with dip angle linearly increasing along strike from 4.3oin top southernmost patch to 4.5oin top northernmost path and dip angle linearly increased. By using the fault shape and geodetic co-seismic data, we estimate the slip distribution on the curved fault. Our result shows that the earthquake ruptured *200-km width down to a depth of about 60 km.0.5–12.5 m of thrust slip is resolved with the largest slip centered around the central section of the rupture zone78N–108N in latitude. The estimated seismic moment is8.2 9 1022 N m, which is larger than estimation from the centroid moment magnitude(4.0 9 1022 N m), and smaller than estimation from normal-mode oscillation data modeling(1.0 9 1023 N m).     

GEODETIC AND TELESEISMIC CONSTRAINTS ON SLIP DISTRIBUTION OF 2015 MW6.4 PISHAN EARTHQUAKE

On July 3^rd, 2015, a M_W6.4 earthquake occurred on Pishan County, Xinjiang, located in the front of western Kunlun thrust belt, which is the largest earthquake(M_W6.0~7.0)in the past 40 years in this region. In this study, we collected both the near-filed geodetic coseismic deformation observations including 4 GPS sites and one high-resolution ALOS-2 InSAR imagery, and far-field teleseismic P waveforms from 25 stations provided by IRIS/USGS, to invert the fault parameters(strike and dip)and coseismic rupture model of 2015 M_W6.4 Pishan earthquake. Using the finite fault theory, a non-linear simulated annealing algorithm was employed to resolve our joint inversion problem. The strike (120°~130°) and dip angle(35°~40°)of optimal models are different from that of some previous studies, and the dip change is strongly constrained by combined data than that of strike. In fixing the geometric parameters of optimal fault model, we also considered data weight(5)(geodetic data/teleseismic P waveforms)and constrained weight from moment and smooth factor(2.5). Clearly, our results indicate that the slip distribution mainly concentrates in the depth range from 9 to 16km and a length range of 20km along the strike direction, which is similar to the spatial distribution of the relocated aftershocks. The maximum slip is~95cm. The seismic moment release is 5.45×10¹⁸N·m, corresponding to M_W6.42. Compared with the single data set, geodetic data or teleseismic waveform, our joint inversion model could simultaneously constrain the seismic moment and slip distribution well, thus avoiding effectively a lower-resolution rupture distribution determined by teleseismic-only inversion and a bias released moment estimated by the geodetic-only inversion. Importantly, we should consider both the near-field geodetic data and far-field teleseismic data in retrieving the rupture model for accurately describing the seismogenic structure of active fault in western Kunlun region.     

Linear and nonlinear computations of the 1992 Nicaragua earthquake tsunami

Numerical computations of tsunamis are made for the 1992 Nicaragua earthquake using different governing equations, bottom frictional values and bathymetry data. The results are compared with each other as well as with the observations, both tide gauge records and runup heights. Comparison of the observed and computed tsunami waveforms indicates that the use of detailed bathymetry data with a small grid size is more effective than to include nonlinear terms in tsunami computation. Linear computation overestimates the amplitude for the later phase than the first arrival, particularly when the amplitude becomes large. The computed amplitudes along the coast from nonlinear computation are much smaller than the observed tsunami runup heights; the average ratio, or the amplification factor, is estimated to be 3 in the present case when the grid size of 1 minute is used. The factor however may depend on the grid size for the computation.     

Field Survey of the 27 February 2010 Chile Tsunami

On 27 February 2010, a magnitude M _w?=?8.8 earthquake occurred off the coast of Chile??s Maule region causing substantial damage and loss of life. Ancestral tsunami knowledge from the 1960 event combined with education and evacuation exercises prompted most coastal residents to spontaneously evacuate after the earthquake. Many of the tsunami victims were tourists in coastal campgrounds. The international tsunami survey team (ITST) was deployed within days of the event and surveyed 800?km of coastline from Quintero to Mehuín and the Pacific Islands of Santa María, Mocha, Juan Fernández Archipelago, and Rapa Nui (Easter). The collected survey data include more than 400 tsunami flow depth, runup and coastal uplift measurements. The tsunami peaked with a localized runup of 29?m on a coastal bluff at Constitución. The observed runup distributions exhibit significant variations on local and regional scales. Observations from the 2010 and 1960 Chile tsunamis are compared.     

Modeling of the 2011 Japan Tsunami: Lessons for Near-Field Forecast

During the devastating 11 March 2011 Japanese tsunami, data from two tsunami detectors were used to determine the tsunami source within 1.5 h of earthquake origin time. For the first time, multiple near-field tsunami measurements of the 2011 Japanese tsunami were used to demonstrate the accuracy of the National Oceanic and Atmospheric Administration (NOAA) real-time flooding forecast system in the far field. To test the accuracy of the same forecast system in the near field, a total of 11 numerical models with grids telescoped to 2 arcsec (~60 m) were developed to hindcast the propagation and coastal inundation of the 2011 Japanese tsunami along the entire east coastline of Japan. Using the NOAA tsunami source computed in near real-time, the model results of tsunami propagation are validated with tsunami time series measured at different water depths offshore and near shore along Japan’s coastline. The computed tsunami runup height and spatial distribution are highly consistent with post-tsunami survey data collected along the Japanese coastline. The computed inundation penetration also agrees well with survey data, giving a modeling accuracy of 85.5 % for the inundation areas along 800 km of coastline between Ibaraki Prefecture (north of Kashima) and Aomori Prefecture (south of Rokkasho). The inundation model results highlighted the variability of tsunami impact in response to different offshore bathymetry and flooded terrain. Comparison of tsunami sources inferred from different indirect methods shows the crucial importance of deep-ocean tsunami measurements for real-time tsunami forecasts. The agreement between model results and observations along Japan’s coastline demonstrate the ability and potential of NOAA’s methodology for real-time near-field tsunami flooding forecasts. An accurate tsunami flooding forecast within 30 min may now be possible using the NOAA forecast methodology with carefully placed tsunameters and large-scale high-resolution inundation models with powerful computing capabilities.     

Near-Field Tsunami Edge Waves and Complex Earthquake Rupture

The effect of distributed coseismic slip on progressive, near-field edge waves is examined for continental shelf tsunamis. Detailed observations of edge waves are difficult to separate from the other tsunami phases that are observed on tide gauge records. In this study, analytic methods are used to compute tsunami edge waves distributed over a finite number of modes and for uniformly sloping bathymetry. Coseismic displacements from static elastic theory are introduced as initial conditions in calculating the evolution of progressive edge-waves. Both simple crack representations (constant stress drop) and stochastic slip models (heterogeneous stress drop) are tested on a fault with geometry similar to that of the M _w = 8.8 2010 Chile earthquake. Crack-like ruptures that are beneath or that span the shoreline result in similar longshore patterns of maximum edge-wave amplitude. Ruptures located farther offshore result in reduced edge-wave excitation, consistent with previous studies. Introduction of stress-drop heterogeneity by way of stochastic slip models results in significantly more variability in longshore edge-wave patterns compared to crack-like ruptures for the same offshore source position. In some cases, regions of high slip that are spatially distinct will yield sub-events, in terms of tsunami generation. Constructive interference of both non-trapped and trapped waves can yield significantly larger tsunamis than those that produced by simple earthquake characterizations.     

远震体波有限破裂过程反演敏感性测试及其在汶川地震上的应用

本文利用远震P和SH波反演得到2008年5月12日汶川大地震(M_W=7.9)的一系列有限破裂模型。使用一种基于小波变换的模拟退火非线性反演方法, 我们将主断层划分成若干个子断层, 在反演时同时确定每个子断层上的滑移量、 滑动角、 上升时间(rise time)以及平均破裂速度。我们首先根据一个假定的破裂模型生成理论地震图, 将该理论地震数据作为输入进行反演, 对该有限破裂反演方法进行了一系列测试, 以验证反演对断层倾角、 平均破裂速度、 最大破裂深度等参数的敏感性。然后我们采用4个不同倾角的断层面来对汶川地震远震体波记录进行反演。结果表明, 若对只在一个断层面上模拟该地震, 30°倾角是个较为合适的值。反演的结果还表明, 此次地震有两个主要的能量释放区域, 并且主断层面存在倾角变化的可能性。在将来的研究中, 可以结合GPS, InSAR测地学以及强震等数据, 来对强震的破裂过程做更细致的研究。     

Evaluation of the 15 July 2009 Fiordland,New Zealand Tsunami in the Source Region

We undertake detailed near-field numerical modelling of the tsunami generated by the 15 July 2009 earthquake (Mw 7.8) in Fiordland, New Zealand. High resolution bathymetry and topography data at Breaksea and Dusky Sounds, and Chalky and Preservation Inlets are derived mostly from digitised New Zealand nautical charts, Shuttle Radar Topographic Mission (SRTM) 3 arc-second data, and General Bathymetric Chart of the Ocean (GEBCO) 30 s data. A combination of continuous and campaign Global Positioning System (GPS), satellite radar (ALOS/PALSAR InSAR images) and seismology data are used to constrain the seafloor deformation for the initial tsunami condition. This source model, derived independently of DART observations, provides an excellent fit to observed tsunami elevations recorded by DART buoy 55015. The model results in the near field show maximum tsunami elevations in the range 0.5–2.0 m inside the sounds and inlets with maximum flow speeds of 3.0 m/s. Along the open coast, maximum tsunami elevations reach 2.0 m. The high flow speeds through the inlets may change the inlet stratifications and water mass inside the sounds. Media reports and field reconnaissance data show some tsunami evidence at Cormorant Cove, Duck and Goose Coves, and Passage Point.     

The Effect of the Great Barrier Reef on the Propagation of the 2007 Solomon Islands Tsunami Recorded in Northeastern Australia

The effect of offshore coral reefs on the impact from a tsunami remains controversial. For example, field surveys after the 2004 Indian Ocean tsunami indicate that the energy of the tsunami was reduced by natural coral reef barriers in Sri Lanka, but there was no indication that coral reefs off Banda Aceh, Indonesia had any effect on the tsunami. In this paper, we investigate whether the Great Barrier Reef (GBR) offshore Queensland, Australia, may have weakened the tsunami impact from the 2007 Solomon Islands earthquake. The fault slip distribution of the 2007 Solomon Islands earthquake was firstly obtained by teleseismic inversion. The tsunami was then propagated to shallow water just offshore the coast by solving the linear shallow water equations using a staggered grid finite-difference method. We used a relatively high resolution (approximately 250 m) bathymetric grid for the region just off the coast containing the reef. The tsunami waveforms recorded at tide gauge stations along the Australian coast were then compared to the results from the tsunami simulation when using both the realistic 250 m resolution bathymetry and with two grids having fictitious bathymetry: One in which the the GBR has been replaced by a smooth interpolation from depths outside the GBR to the coast (the “No GBR” grid), and one in which the GBR has been replaced by a flat plane at a depth equal to the mean water depth of the GBR (the “Average GBR” grid). From the comparison between the synthetic waveforms both with and without the Great Barrier Reef, we found that the Great Barrier Reef significantly weakened the tsunami impact. According to our model, the coral reefs delayed the tsunami arrival time by 5–10 minutes, decreased the amplitude of the first tsunami pulse to half or less, and lengthened the period of the tsunami.