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15 pages, 10 figures, 3 tables, supplementary material https://doi.org/10.1016/j.margeo.2022.106749.-- Data availability: The data (3D complex mesh of the ARFS and rake values, and the resulting grid files of the tsunami simulations containing the maximum wave amplitude) are archived at PANGAEA repository (https://doi.pangaea.de/10.1594/PANGAEA.941092).-- The EMODnet bathymetry is available at https://www.emodnet-bathymetry.eu/. The stochastic slip distributions have been produced by the code ANTI-FASc (https://github.com/antonioscalaunina/ANTI-FASc) a platform partially based on the code k223d (Herrero and Murphy, 2018 available at https://github.com/s-murfy/k223d), in turn based on the slipk2 (available at https://github.com/andherit/slipk2) and the trilateration codes (available at https://github.com/andherit/trilateration), The westernmost Mediterranean hosts part of the plate boundary between the European and African tectonic plates. Based on the scattered instrumental seismicity, this boundary has been traditionally interpreted as a wide zone of diffuse deformation. However, recent seismic images and seafloor mapping studies support that most of the plate convergence may be accommodated in a few tectonic structures, rather than in a broad region. Historical earthquakes with magnitudes Mw > 6 and historical tsunamis support that the low-to-moderate instrumental seismicity might also have led to underestimation of the seismogenic and tsunamigenic potential of the area. We evaluate the largest active faults of the westernmost Mediterranean: the reverse Alboran Ridge, and the strike-slip Carboneras, Yusuf and Al-Idrissi fault systems. For the first time, we use a dense grid of modern seismic data to characterize the entire dimensions of the main fault systems, accurately describe the geometry of these structures and estimate their seismic source parameters. Tsunami scenarios have been tested based on 3D-surfaces and seismic source parameters, using both uniform and heterogeneous slip distributions. The comparison of our results with previous studies, based on limited information on the fault geometry and kinematics, indicates that accurate fault geometries and heterogeneous slip distributions are needed to properly assess the seismic and tsunamigenic potential in this area. Based on fault scaling relations, the four fault systems have a large seismogenic potential, being able to generate earthquakes with Mw > 7. The reverse Alboran Ridge Fault System has the largest tsunamigenic potential, being able to generate a tsunami wave amplitude greater than 3 m in front of the coasts of Southern Spain and Northern Africa, This work is supported by the Cluster of Excellence “The Future Ocean”, within the framework of the Excellence Initiative by the Deutsche Forschungsgemeinschaft (DFG) on behalf of the German federal and state governments. This study benefited from an EU Marie Skłodowska-Curie Individual Fellowship to LGP (H2020-MSCA-IF-2017 796013). LGP, CS, FM and RB acknowledge the resources made available by the SISMOLAB-3D at INGV. This work has been carried out in collaboration with the Grup de Recerca Consolidat de la Generalitat de Catalunya “Barcelona Center for Subsurface Imaging” (2017 SGR 1662), and acknowledges the ICM “Severo Ochoa Centre of Excellence” accreditation (CEX2019-000928-S)

SUMMARYWe investigated the kinematic rupture model of the 2018 Mw 6.8 Zakynthos, Ionian Sea (Greece), earthquake by using a non-linear joint inversion of strong motion data, high-rate GPS time-series and static coseismic GPS displacements. We also tested inversion results against tide-gauge recordings of the small tsunami generated in the Ionian Sea. In order to constrain the fault geometry, we performed several preliminary kinematic inversions by assuming the parameter values resulting from different published moment tensor solutions. The lowest cost function values were obtained by using the geometry derived from the United States Geological Survey (USGS) focal solution. Between the two conjugate USGS planes, the rupture model which better fits the data is the one with the N9°E-striking 39°ESE-dipping plane. The rupture history of this model is characterized by a bilateral propagation, featuring two asperities; a main slip patch extending between 14 and 28 km in depth, 9 km northeast from the nucleation and a slightly shallower small patch located 27 km southwest from the nucleation. The maximum energy release occurs between 8 and 12 s, when both patches are breaking simultaneously. The maximum slip is 1.8 m and the total seismic moment is 2.4 × 1019 Nm, corresponding to a Mw value of 6.8. The slip angle shows a dominant right-lateral strike-slip mechanism, with a minor reverse component that increases on the deeper region of the fault. This result, in addition to the observed possibility of similar mechanisms for previous earthquakes occurred in 1959 and 1997, suggests that the tectonic deformation between the Cephalonia Transform Fault Zone and the northern tip of the Hellenic Arc Subduction zone may be accommodated by prevailing right lateral low-dipping faults, occurring on re-activated structures previously experiencing (until Pliocene) compressional regime. Comparison of predicted and observed tsunami data suggests the need of a better characterization of local harbour response for this type of relatively short-wavelength events, which is important in the context of tsunami early warning. However, the suggested dominantly strike-slip character would in turn imply a reduced tsunami hazard as compared to a dominant thrust faulting regime from this source region.

Seismological, tsunami and geodetic observations have shown that subduction zones are complex systems where the properties of earthquake rupture vary with depth as a result of different pre-stress and frictional conditions. A wealth of earthquakes of different sizes and different source features (e.g. rupture duration) can be generated in subduction zones, including tsunami earthquakes, some of which can produce extreme tsunamigenic events. Here, we offer a geological perspective principally accounting for depth-dependent frictional conditions, while adopting a simplified distribution of on-fault tectonic pre-stress. We combine a lithology-controlled, depth-dependent experimental friction law with 2D elastodynamic rupture simulations for a Tohoku-like subduction zone cross-section. Subduction zone fault rocks are dominantly incohesive and clay-rich near the surface, transitioning to cohesive and more crystalline at depth. By randomly shifting along fault dip the location of the high shear stress regions (“asperities”), moderate to great thrust earthquakes and tsunami earthquakes are produced that are quite consistent with seismological, geodetic, and tsunami observations. As an effect of depth-dependent friction in our model, slip is confined to the high stress asperity at depth; near the surface rupture is impeded by the rock-clay transition constraining slip to the clay-rich layer. However, when the high stress asperity is located in the clay-to-crystalline rock transition, great thrust earthquakes can be generated similar to the Mw 9 Tohoku (2011) earthquake.

We propose a procedure for uncertainty quantification in Probabilistic Tsunami Hazard Analysis (PTHA), with a special emphasis on the uncertainty related to statistical modelling of the earthquake source in Seismic PTHA (SPTHA), and on the separate treatment of subduction and crustal earthquakes (treated as background seismicity). An event tree approach and ensemble modelling are used in spite of more classical approaches, such as the hazard integral and the logic tree. This procedure consists of four steps: (1) exploration of aleatory uncertainty through an event tree, with alternative implementations for exploring epistemic uncertainty; (2) numerical computation of tsunami generation and propagation up to a given offshore isobath; (3) (optional) site-specific quantification of inundation; (4) simultaneous quantification of aleatory and epistemic uncertainty through ensemble modelling. The proposed procedure is general and independent of the kind of tsunami source considered; however, we implement step 1, the event tree, specifically for SPTHA, focusing on seismic source uncertainty. To exemplify the procedure, we develop a case study considering seismic sources in the Ionian Sea (central-eastern Mediterranean Sea), using the coasts of Southern Italy as a target zone. The results show that an efficient and complete quantification of all the uncertainties is feasible even when treating a large number of potential sources and a large set of alternative model formulations. We also find that (i) treating separately subduction and background (crustal) earthquakes allows for optimal use of available information and for avoiding significant biases; (ii) both subduction interface and crustal faults contribute to the SPTHA, with different proportions that depend on source-target position and tsunami intensity; (iii) the proposed framework allows sensitivity and deaggregation analyses, demonstrating the applicability of the method for operational assessments.

SUMMARY In this study, we investigate the rupture history of the 2009 April 6 (Mw 6.1) L’Aquila normal faulting earthquake by using a non-linear inversion of strong motion, GPS and DInSAR data. Both the separate and joint inversion solutions reveal a complex rupture process and a heterogeneous slip distribution. Slip is concentrated in two main asperities: a smaller shallow patch of slip located updip from the hypocentre and a second deeper and larger asperity located southeastwards along-strike direction. The key feature of the source process emerging from our inverted models concerns the rupture history, which is characterized by two distinct stages. The first stage begins with rupture nucleation and with updip propagation at relatively high (∼4.0 km s−1), but still subshear, rupture velocity. The second stage starts nearly 2.0–2.5 s after nucleation and it is characterized by the along-strike rupture propagation. The largest and deeper asperity fails during this stage of the rupture process. The rupture velocity is larger in the updip than in the along-strike direction. The updip and along-strike rupture propagation are separated in time and associated with a Mode II and a Mode III crack, respectively. The comparison between the source models inferred in this study with the Poisson ratio anomalies in the crustal volume containing the fault plane allows the interpretation of the delay in along-strike rupture propagation in terms of a structural control of the rupture history. Our results show that the L’Aquila earthquake featured a very complex rupture, with strong spatial and temporal heterogeneities suggesting a strong frictional and/or structural control of the rupture process.

SUMMARY In this study, we aim to understand the dependence of the critical slip weakening distance (D c) on the final slip (D tot) during the propagation of a dynamic rupture and the consistency of their inferred correlation. To achieve this goal we have performed a series of numerical tests suitably designed to validate the adopted numerical procedure and to verify the actual capability in measuring D c. We have retrieved two kinematic rupture histories from spontaneous dynamic rupture models governed by a slip weakening law in which a constant D c distribution on the fault plane as well as a constant D c/D tot ratio are assumed, respectively. The slip velocity and the shear traction time histories represent the synthetic ‘real’ target data which we aim to reproduce. We use a 3-D traction-at-split nodes numerical procedure to image the dynamic traction evolution by assuming our modelled slip velocity as a boundary condition on the fault plane. We assume a regularized Yoffe function as source time function in our modelling attempts and we measure the critical slip weakening distance from the inferred traction versus slip curves at each point on the fault. We compare the inferred values with those of the target dynamic models. Our numerical tests show that fitting the slip velocity functions of the target models at each point on the fault plane is not enough to retrieve good traction evolution curves and to obtain reliable measures of D c. We find that the estimation of D c is very sensitive to any small variation of the slip velocity function. An artificial correlation between D c/D tot is obtained when a fixed shape of slip velocity is assumed on the fault (i.e. constant rise time and constant time for positive acceleration) which differs from that of the target model. We point out that the estimation of fracture energy (breakdown work) on the fault is not affected by biases in measuring D c.

We calculated the expected impact on the Italian coast of the Adriatic Sea of a large set of tsunamis resulting from potential earthquakes generated by major fault zones. Our approach merges updated knowledge on the regional tectonics and scenario-like calculations of expected tsunami impact. We selected six elongated potential source zones. For each of them we determined a Maximum Credible Earthquake and the associated Typical Fault, described by its size, geometry and kinematics. We then let the Typical Fault float along strike of its parent source zone and simulated all tsunamis it could generate. Simulations are based on the solution of the nonlinear shallow water equations through a finite-difference technique. For each run we calculated the wavefields at specified simulation times and the maximum water height field (above mean sea level), then generated travel-time maps and maximum water height profiles along the target coastline. Maxima were also classified in a three-level code of expected tsunami threat. We found that the southern portion of Apulia facing Albania and the Gargano promontory are especially prone to the tsunami threat. We also found that some bathymetric features are crucial in determining the focalization-defocalization of tsunami energy. We suggest that our results be taken into account in the design of early-warning strategies.

The 18 April 1906 M ∼8 California earthquake generated a small local tsunami that was recorded in the near field by the Presidio, San Francisco tide gauge located near the Golden Gate. We investigate the causative, tsunamigenic, seismic source by forward modeling and nonlinear inversion of the Presidio marigram. We use existing seismological and geological observations to fix the fault system geometry and the surface slip on the onland portions of the San Andreas fault (SAF). We perform synthetic inversions to show that the single, near-field marigram constrains the main features of the rupture on the portion of the SAF system offshore of the Golden Gate. Finally, we perform nonlinear inversions for the slip distribution and the timing of the rupture of the 1906 earthquake. Our results, in agreement with previous studies, identify a dilatational stepover and show a bilateral rupture, possibly originating or propagated through the stepover region. We find that little or no coseismic slip on normal faults in the stepover region is required to fit the marigram, and we obtain adequate fits when allowing delays in the source initiation times of up to 3 min on the various fault segments. We constrain slip to be of about 5–6 m for the onshore portion of the SAF to the northwest of the Golden Gate, in agreement with 1906 surface observations of fault offset. Our results favor the hypothesis of a vertical dip for a currently aseismic SAF to the southeast of the Golden Gate, under the San Francisco peninsula.

Several independent indicators imply a high probability of a great ( M > 8) earthquake rupture of the subduction megathrust under the Mentawai Islands of West Sumatra. The human consequences of such an event depend crucially on its tsunamigenic potential, which in turn depends on unpredictable details of slip distribution on the megathrust and how resulting seafloor movements and the propagating tsunami waves interact with bathymetry. Here we address the forward problem by modelling about 1000 possible complex earthquake ruptures and calculating the seafloor displacements and tsunami wave height distributions that would result from the most likely 100 or so, as judged by reference to paleogeodetic data. Additionally we carry out a systematic study of the importance of the location of maximum slip with respect to the morphology of the fore-arc complex. Our results indicate a generally smaller regional tsunami hazard than was realised in Aceh during the December 2004 event, though more than 20% of simulations result in tsunami wave heights of more than 5 m for the southern Sumatran cities of Padang and Bengkulu. The extreme events in these simulations produce results which are consistent with recent deterministic studies. The study confirms the sensitivity of predicted wave heights to the distribution of slip even for events with similar moment and reproduces Plafker's rule of thumb. Additionally we show that the maximum wave height observed at a single location scales with the magnitude though data for all magnitudes exhibit extreme variability. Finally, we show that for any coastal location in the near field of the earthquake, despite the complexity of the earthquake rupture simulations and the large range of magnitudes modelled, the timing of inundation is constant to first order and the maximum height of the modelled waves is directly proportional to the vertical coseismic displacement experienced at that point. These results may assist in developing tsunami preparedness strategies around the Indian Ocean and in particular along the coasts of western Sumatra.

SUMMARY We introduce a rapid and robust, energy-duration procedure, based on the Haskell, extended-source model, to obtain an earthquake moment and a moment magnitude, MED. Using seismograms at teleseismic distances (30°–90°), this procedure combines radiated seismic energy measures on the P to S interval of broadband signals and source duration measures on high-frequency, P-wave signals. The MED energy-duration magnitude is scaled to correspond to the Global Centroid-Moment Tensor (CMT) moment-magnitude, MCMTw, and can be calculated within about 20 min or less after origin time (OT). The measured energy and duration values also provide the energy-to-moment ratio, Θ, used for identification of tsunami earthquakes. The MED magnitudes for a set of recent, large earthquakes match closely MCMTw, even for the largest, great earthquakes; these results imply that the MED measure is accurate and does not saturate. After the 2004 December 26 Sumatra-Andaman mega-thrust earthquake, magnitude estimates available within 1 hr of OT ranged from M= 8.0 to 8.5, the CMT magnitude, available about 3 hr after OT, was MCMTw= 9.0, and, several months after the event, Mw= 9.1–9.3 was obtained from analysis of the earth normal modes. The energy-duration magnitude for this event is MED= 9.2, a measure that is potentially available within 20 min after OT. After the 2006 July 17, Java earthquake, the magnitude was evaluated at M= 7.2 at 17 min after OT, the CMT magnitude, available about 1 hr after OT, was MCMTw= 7.7; the energy-duration results for this event give MED= 7.8, with a very long source duration of about 160 s, and a very low Θ value, indicating a possible tsunami earthquake.

The aim of this work is to infer the slip distribution and rupture velocity along the rupture zone of the 26 December 2004 Sumatra–Andaman earthquake from available tide gage records of the tsunami. We selected waveforms from 14 stations, distributed along the coast of the Indian Ocean. Then we subdivided the fault plane into 16 subfaults (both along strike and downdip) following the geometry and mechanism proposed by Banerjee et al. (2005) and computed the corresponding Green’s functions by numerical solution of the shallow-water equations through a finite- difference method. The slip distribution and rupture velocity were determined simultaneously by means of a simulated annealing technique. We compared the recorded and synthetic waveforms in the time domain, using a cost function that is a trade-off between the L1 and L2 norms. Preliminary tests on a synthetic dataset, together with a posteriori statistical analysis of the model ensemble enabled us to assess the effectiveness of the method and to quantify the model uncertainty. The main finding is that the best source model features a nonuniform distribution of coseismic slip, with high slip values concentrated into three main patches: the first is located in the southern part of the fault, off the coast of the Aceh Province; the second between 6.5° N and 11° N; and the third at depth, between 11° N and 14° N. Furthermore, we estimated that the rupture propagated at an average speed of 2.0 km/sec.

We propose a new source-time function, to be used in kinematic modeling of ground-motion time histories, which is consistent with dynamic propagation of earthquake ruptures and makes feasible the dynamic interpretation of kinematic slip models. This function is derived from a source-time function first proposed by Yoffe (1951), which yields a traction evolution showing a slip-weakening behavior. In order to remove its singularity, we apply a convolution with a triangular function and obtain a regularized source-time function called the regularized Yoffe function. We propose a parameterization of this slip-velocity time function through the final slip, its duration, and the duration of the positive slip acceleration ( Tacc ). Using this analytical function, we examined the relation between kinematic parameters, such as peak slip velocity and slip duration, and dynamic parameters, such as slip-weakening distance and breakdown-stress drop. The obtained scaling relations are consistent with those proposed by Ohnaka and Yamashita (1989) from laboratory experiments. This shows that the proposed source-time function suitably represents dynamic rupture propagation with finite slip-weakening distances.

Summary This paper presents a study of the tsunami following the disastrous earthquake that occurred on the 8th of September 1905 in Calabria, southern Italy. The shock caused devastation in many towns and villages leading to more than 500 victims. According to coeval sources the tsunami was not catastrophic, but was large enough to inundate low lying lands in several coastal segments and to affect boats. No specific study was ever devoted to this tsunami in the literature. Our analysis is carried out by means of numerical modelling and aims at filling this gap. Since the source fault of the earthquake has not yet been identified, we study three tsunamis produced by likely potential sources, according to macroseismic data and the seismotectonic knowledge of the region, namely the Capo Vaticano (CV) fault, the Vibo Valentia (VV) fault and the Lamezia (LA) fault. We compute tsunami waves under the hypotheses that the sea floor deformation is caused by a shear double-couple dislocation taking place over a rectangular fault and that sea waves propagate according to the shallow water approximation. The three tsunamis cause either a pure initial depression (VV and LA) or a predominant initial depression (CV) of the sea surface in the gulf of St. Eufemia. One very relevant finding is that tsunami energy is trapped within a narrow channel along the Calabrian coast to the north of the source with the consequence that wave amplitudes computed here are much larger than the values expected in the case of an ordinary energy decay law with distance; energy trapping along southern coastal segments is less efficient. Only a small fraction of tsunami energy penetrates the Straits of Messina to the south. Propagation to the west is affected by the Aeolian islands that produce a quasi-shadow zone on the side opposite to the tsunami impact. The LA tsunami is found to be too weak to be compatible with observations. A sensitivity analysis concerning some parameters of the faults (namely length, width and slip magnitude) has been performed in order to explore their influence on the tsunami features. The main result of this paper is that we cannot discriminate between the CV and VV sources: both are in agreement with some historical observations, whilst they do not match others. We believe that a better tuning of the source mechanism over these faults can improve the agreement between model computations and historical observations.

The islands in the Marquesas archipelago are most exposed to far-field tsunamis in French Polynesia. Four recent earthquakes located at the Pacific rim (Kurile 1994, Chile 1995, Mexico 1995 and Peru 1996 earthquakes) generated trans-Pacific tsunamis and caused contrasting inundations in several inhabited Marquesian bays. The aim of this study is to better understand the amplification phenomena observed in the Marquesas Islands and to improve the risk assessment. We present here numerical simulations of these four tsunamis by means of a finite difference model solving the equations of tsunami propagation at different scales. The water heights computed in the bays are in excellent agreement with the available observations and thus validate our numerical method. These numerical results allow us to determine (1) which propagation azimuths are dangerous for the whole Marquesas archipelago and (2) the behavior of each studied bay (amplitude, frequency) in response to tsunami waves. We observe that the seismogenic subduction zone along South America is likely to define one of the most dangerous tsunami generation areas for the Marquesas archipelago, owing to the source geometries and to the trapping of the tsunami energy by bathymetric features in the southern Pacific Ocean (fracture zones, seamount chains).

Summary Deep and large submarine slumps may generate tsunamis as disastrous as tsunamis of tectonic origin. Such a landslide is likely to be the origin of the 1998 July 17 tsunami of Papua New Guinea, the deadliest tsunami in the last 50 years. Water waves devastated a 20 km stretch of coastline, wiping out three villages and killing more than 2200 people. A numerical model has been developed to study the efficiency of deep slumps in producing tsunamis and has been applied to the Papua New Guinea event. The landslide is treated as the flow of a homogeneous gravity-driven continuum governed by a rheological law. Water waves are generated by sea-bottom displacements induced by the landslide. The shallow-water approximation is adopted for both the landslide and the associated water waves. The resulting differential equations are solved by a finite difference method based on shock-capturing. The shallow-water hypothesis is tested by comparison with a model solving Navier–Stokes equations for a mixture of water and sediments. Sensitivity tests carried out for a 2-D simplified geometry show that the water surface profile depends strongly on the constitutive law of the landslide. The 1998 event is simulated numerically by the shallow-water model, testing different friction laws. The observed inundation height distribution is well reproduced by the model for a volume of 4 km3, with its top located at a water depth of 550 m, and sliding with a Coulomb-type friction law over a distance of 5 km.

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.

We present what is to our knowledge the first direct observation of the deformation of the surface of the ocean upon passage of a tsunami wave, on the high seas, far from the influence of shorelines and continental shelves. We use satellite altimetry data from the ERS-1 and TOPEX/POSEIDON programs, complemented by spectrogram techniques and synthetic maregrams to examine the case of seven recent tsunamigenic earthquakes. We make a positive identification of the tsunami wave field in the case of the 1992 Nicaraguan tsunami, which we detect at 15°S, 106°W, five hours after origin time. We model the observed spectrogram by injecting a synthetic of variable amplitude into the signal of a repetitive cycle of the satellite along the same track, concluding that the Nicaraguan tsunami had a zero-to-peak amplitude of 8 cm in that region. In the case of the 1995 Chilean tsunami, a large scatter in the spectral properties of the reference tracks renders the detection tentative. We fail to detect the tsunamis of five other large events, including the 1996 Biak and 1996 Peru earthquakes, primarily on account of unfavorable source directivity in the geometry of existing satellite tracks, and of the strong and incoherent noise produced by large current systems, such as the Kuroshio in the Northwest Pacific.

The 1693 earthquake was a disastrous event affecting eastern Sicily, southern Italy, where it caused over 60,000 victims and total destruction of several villages and towns in the districts of Siracusa, Ragusa, and Catania. The earthquake was followed by a tsunami that struck the Ionian coasts of Sicily and the Messina Strait and was probably observed even in the Aeolian Islands. Historical documents on sea waves and flooding, though not abundant, allow us to form a picture of the tsunami first attack and inundation. The first water movement all along the Sicilian coastline was a strong sea withdrawal, followed by a violent sea return and coastal flooding. The main purpose of this research is to put constraints on the focal mechanism of this earthquake on the basis of the available documents on the tsunami (1) by simulating tsunami from different possible sources via numerical modeling based on finite element technique and shallow water approximation and (2) by choosing the source best fitting the tsunami data as the most plausible cause for this tsunami. The relevance of this study should also be evaluated in light of the circumstance that no certain indication on the earthquake fault can be deduced from the available macroseismic data alone. Solving the source problem for this event, one of the largest occurring in southeastern Sicily in historical times, is a significant contribution to understanding the seismotectonic regime of the region and to assessing the related earthquake and tsunami hazard/risk with implications on mitigation policies.

[1] We study the 2003 Mw 8.1 Tokachi-oki earthquake, a great interplate event that occurred along the southwestern Kuril Trench and generated a significant tsunami. To determine the earthquake slip distribution, we perform the first joint inversion of tsunami waveforms measured by tide gauges and of coseismic displacement measured both by GPS stations and three ocean bottom pressure gauges (PG) for this event. The resolution of the different data sets on the slip distribution is assessed by means of several checkerboard tests. Results show that tsunami data constrain the slip distribution offshore, whereas GPS data constrain the slip distribution in the onshore zone. The three PG data only coarsely constrain the offshore slip, indicating that denser networks should be installed close to subduction zones. Combining the three data sets significantly improves the inversion results. Joint inversion of the 2003 Tokachi-oki earthquake data leads to maximum slip values (∼6 m) confined at depths greater than ∼25 km, between 30 and 80 km northwest of the hypocenter, with a patch of slip (3 m) in the deepest part of the source (∼50 km depth). Slip values are very low (≤1 m) updip from the hypocenter. Furthermore, the rupture does not extend on the plate interface off Akkeshi. As a significant back slip amount (∼4 m) has accumulated there since the last 1952 earthquake, this segment could rupture during the next large interplate event along the Kuril Trench.

[1] We (re)analyzed the source of the 26 December 2004 Sumatra-Andaman earthquake and tsunami through a nonlinear joint inversion of an inhomogeneous data set made up of tide gauges, satellite altimetry, and far-field GPS recordings. The purpose is twofold: (1) the retrieval of the main kinematics rupture parameters (slip, rake, and rupture velocity) and (2) the inference of the rigidity of the source zone. We independently estimate the slip from tsunami data and the seismic moment from geodetic data to derive the rigidity. Our results confirm that the source of the 2004 Sumatra-Andaman earthquake has a complex geometry, constituted by three main slip patches, with slip peaking at ∼30 m in the southern part of the source. The rake direction rotates counterclockwise at the northern part of the source, according to the direction of convergence along the trench. The rupture velocity is higher in the deeper than in the shallower part of the source, consistent with the expected increase of rigidity with depth. It is also lower in the northern part, consistent with known variations of the incoming plate properties and shear velocity. Our model features a rigidity (20–30 GPa) that is lower than the preliminary reference Earth model (PREM) average for the seismogenic volume. The source rigidity is one of the factors controlling the tsunami genesis: for a given seismic moment, the lower the rigidity, the higher the induced seafloor displacement. The general consistence between our source model and previous studies supports the effectiveness of our approach to the joint inversion of geodetic and tsunami data for the rigidity estimation.

Italian Civil Defense; Project “Development of new technologies for the protection of the Italian territory from natural hazards” funded by the Italian Ministry of University and Research

[1] We present a two-stage nonlinear technique to invert strong motions records and geodetic data to retrieve the rupture history of an earthquake on a finite fault. To account for the actual rupture complexity, the fault parameters are spatially variable peak slip velocity, slip direction, rupture time and risetime. The unknown parameters are given at the nodes of the subfaults, whereas the parameters within a subfault are allowed to vary through a bilinear interpolation of the nodal values. The forward modeling is performed with a discrete wave number technique, whose Green's functions include the complete response of the vertically varying Earth structure. During the first stage, an algorithm based on the heat-bath simulated annealing generates an ensemble of models that efficiently sample the good data-fitting regions of parameter space. In the second stage (appraisal), the algorithm performs a statistical analysis of the model ensemble and computes a weighted mean model and its standard deviation. This technique, rather than simply looking at the best model, extracts the most stable features of the earthquake rupture that are consistent with the data and gives an estimate of the variability of each model parameter. We present some synthetic tests to show the effectiveness of the method and its robustness to uncertainty of the adopted crustal model. Finally, we apply this inverse technique to the well recorded 2000 western Tottori, Japan, earthquake (Mw 6.6); we confirm that the rupture process is characterized by large slip (3-4 m) at very shallow depths but, differently from previous studies, we imaged a new slip patch (2-2.5 m) located deeper, between 14 and 18 km depth.

A numerical model for the propagation of tsunamis generated by earthquakes is applied to study the impact of the water waves against the coast of a circular island. The hydraulic source is modeled by means of the static deformation of the seafloor produced by an offshore seismic fault. The initial sea-surface disturbance is assumed to equal the seabed displacement that is computed analytically through the classical dislocation theory of plane faults with uniform slip. The wave propagation is described by means of the shallow-water approximation of the Navier-Stokes equations and is computed in a radially symmetric domain for which a quasi-analytical solution is available. The governing equations are solved numerically by means of a finite-element (FE) method making use of the Galerkin procedure, the mesh consisting of triangular elements of variable size. After a preliminary test of the numerical model against the analytical results, several simulations are performed by varying the fault location, dimension, and dip as well as by varying the radial profiles of the basin bathymetry. Most attention is devoted to studying the amplification of tsunami waves impacting the island coast. What is seen is that, depending on the source characteristics (namely, fault length, dip, strike, and distance from the island) and on the basin bathymetry, high amplifications (in the range 1 to 5) are normally obtained not only in the front of the island as is expected but also on the lee side of the island, because of positive interference of waves traveling around the island in both directions. Under particular conditions (downwardly concave bathymetry), the largest amplifications are found neither on the front side nor on the lee side, but at intermediate places along the island coast.