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A devastating earthquake with moment magnitude 7.5 occurred in the Noto Peninsula in central Japan on 1 January 2024. We estimate the rupture evolution of this earthquake from teleseismic P‐wave data using the potency‐density tensor inversion method, which provides information on the spatiotemporal slip distribution including fault orientations. The results show a long and quiet initial rupture phase that overlaps with regions of preceding earthquake swarms and associated aseismic deformation. The following three major rupture episodes evolve on segmented, differently oriented faults bounded by the initial rupture region. The irregular initial rupture process followed by the multi‐scale rupture growth is considered to be controlled by the preceding seismic and aseismic processes and the geometric complexity of the fault system. Such a discrete rupture scenario, including the triggering of an isolated fault rupture, adds critical inputs on the assessment of strong ground motion and associated damages for future earthquakes. Plain Language Summary: On 1 January 2024, a moment magnitude 7.5 earthquake occurred in the northern Noto Peninsula, Japan. The strong ground motion and tsunami associated with the earthquake caused severe damage to buildings and infrastructure, resulting in at least 245 causalities in the affected areas. The Noto Peninsula is affected by northwest‐southeast compression, and active reverse faults are known along the northern coast of the peninsula and its offshore region. Before the 2024 earthquake, the source region experienced long‐lasting earthquake swarm activity, which is a set of seismic events without an obvious mainshock‐aftershock pattern. Our seismological analysis found that there was a 10‐s‐long initial rupture episode around the hypocenter that overlapped with the earthquake swarm region. The initial rupture was followed by a series of three different rupture episodes on differently oriented fault segments. This earthquake highlights a multi‐scale rupture growth across a segmented fault network after a very quiet initial rupture process that was controlled by the preceding earthquake swarms and associated aseismic deformation related to fluid injection from depth. The rupture process advances our understanding of earthquake source physics and can lead to a better assessment of future earthquake hazards. Key Points: The 2024 Mw 7.5 Noto Peninsula earthquake involves a multi‐segmented rupture sequence on differently oriented faultsThe long and quiet initial rupture domain coincides with the preceding earthquake swarm regionFluid‐induced earthquake swarms and a segmented fault network control the complex earthquake rupture growth [ABSTRACT FROM AUTHOR]

The 2016 Kaikoura earthquake, New Zealand, ruptured more than a dozen faults, making it difficult to prescribe a model fault for analyzing the event by inversion. To model this earthquake from teleseismic records, we used a potency density tensor inversion, which projects multiple fault slips onto a single model fault plane, reducing the non-uniqueness due to the uncertainty in selecting the faults' orientations. The resulting distribution of potency-rate density tensors is consistent with observed surface ruptures. In its initial stage, the rupture propagated northeastward primarily at shallow depths. Later, the rupture propagated northeastward at greater depths beneath a gap in reported surface ruptures. The main rupture phase started in the northeastern part of the Kekerengu fault after 50 s and propagated bilaterally to the northeast and southwest. The non-double-couple component grew to a large fraction of the source elements as the rupture went through the junction of the Jordan Thrust and the Papatea fault, which suggests that the rupture branched into both faults as it back-propagated toward the southwest. The potency density tensor inversion sheds new light on the irregular evolution of this earthquake, which produced a fault rupture pattern of unprecedented complexity. Our source model of the 2016 Kaikoura earthquake (e.g., back-rupture propagation) could prompt research to determine a more realistic model with segmented faults using near-field data. [ABSTRACT FROM AUTHOR]

A devastating doublet of earthquakes with moment magnitude MW 7.9 and MW 7.6 earthquakes contiguously occurred in SE Türkiye near the NW border of Syria. Here we perform a potency‐density tensor inversion to simultaneously estimate rupture evolution and fault geometry for the doublet. We find the initial MW 7.9 earthquake involved discrete episodes of supershear rupture and back‐rupture propagation, and was triggered by initial rupture along a bifurcated splay of the East Anatolian Fault. The second MW 7.6 event was triggered by the earlier MW 7.9 event, and it involved more extensive supershear rupture along a favorably curved fault, and was likely stopped by geometric barriers at the fault ends. Our results highlight the multi‐scale cascading rupture growth across the complex fault network that affects the diverse rupture geometries of the 2023 Türkiye earthquake doublet, contributing to the strong ground shaking and associated devastation. Plain Language Summary: On 6 February 2023, devastating dual earthquakes; moment magnitude 7.9 and 7.6 events struck southern Türkiye near the northern border of Syria. The two earthquakes were only separated ∼90 km and ∼9 hr apart. The strong shaking from the two earthquakes caused significant damage to the buildings and people, having caused over 50,000 fatalities in Türkiye and Syria. The source region is where the Anatolian, Arabian and African plates meet, developing the network of faults that hosted the large devastating earthquakes. Seismological analyses using observed seismic waveforms are effective for rapidly estimating how the rupture of the two earthquakes evolves over such distinctively oriented and possibly segmented faults. We use the globally observed seismic records to simultaneously estimate rupture evolution and fault geometry of the earthquake doublet. We find the sequence of both earthquakes involves curved and segmented fault ruptures, including the back‐propagating rupture for the initial earthquake, which is facilitated by the complex active fault network. The 2023 earthquake doublet displays the irregular rupture evolution and diverse triggering behaviors both in a single event and across the earthquake sequence, which provide critical inputs in both our understanding of earthquake‐rupture dynamics and better assessment of future damaging earthquakes. Key Points: An earthquake doublet of MW 7.9 and MW 7.6 ruptured multiple segments and curved faultsInitial splay fault rupture triggered a large MW 7.9 rupture involving pulses of back‐propagating supershear ruptureMulti‐scale rupture growth in a complex fault network may facilitate diverse rupture behaviors and triggering interactions in the doublet [ABSTRACT FROM AUTHOR]

A moment magnitude 6.2 crustal earthquake occurred in northern Thailand on May 5, 2014, and its aftershocks exhibit several lineaments with conjugate pattern, involving geometric complexity in a multi-segmented fault system of the Phayao Fault Zone. However, a relationship between those geometric complexities and the rupture evolution of the 2014 Thailand earthquake is still elusive, which is critical to understand complex nature of the earthquake physics and to assess the hazard. Here, we elaborated the newly developed potency density tensor inversion method, used it to invert the globally observed teleseismic P waveforms, and estimated the spatiotemporal distribution of both the slip and the fault geometry. We found the complex rupture evolution consisting of two rupture episodes along a conjugated strike-slip fault system that comprises two distinct fault planes. The first episode originated at the hypocenter and the rupture propagated south along the north–northeast to south–southwest fault plane. The second episode was triggered at around 5 km north from the epicenter, and the rupture propagated along the east–northeast to west–southwest fault plane and terminated at the west end of the source area at 5 s hypocentral time. Our work demonstrates that our potency density tensor inversion can be applied to the smaller-scale magnitude-6 class earthquakes, and it resolves the complex rupture process controlled by the underlying geometric complexity in the fault system. [ABSTRACT FROM AUTHOR]

The 2010 MW 7.2 El Mayor-Cucapah, Mexico, earthquake ruptured multiple faults with different faulting mechanisms. Resolving the earthquake rupture process and its relation to the geometric fault complexities is critical to our understanding of the earthquake source physics, but doing so by conventional finite-fault inversion is challenging because modelling errors due to inappropriate assumptions about the fault geometry distort the solution and make robust interpretation difficult. Here, using a potency density tensor approach to finite-fault inversion, we inverted the observed teleseismic P waveforms of the 2010 El Mayor-Cucapah earthquake to simultaneously estimate the rupture process and the fault geometry. We found that the earthquake consisted of an initial normal faulting rupture, which was followed by a strike-slip bilateral rupture towards the southeast and northwest that originated on the northwest side of the epicentre. The southeastern rupture propagated back through the initial rupture area, but with strike-slip faulting. Although the northwestern rupture propagated across the left step in the Puerta fault-accommodation zone, the rupture was temporarily stalled by the associated change of the fault geometry. These results highlight the irregular rupture process, which involved a back-propagating rupture and fluctuating rupture propagation controlled the complexity of the fault system. [ABSTRACT FROM AUTHOR]

The state‐of‐stress within subducting oceanic plates controls rupture processes of deep intraslab earthquakes. However, little is known about how the large‐scale plate geometry and the stress regime relate to the physical nature of the deep intraslab earthquakes. Here we find, by using globally and locally observed seismic records, that the moment magnitude 7.3 2021 East Cape, New Zealand earthquake was driven by a combination of shallow trench‐normal extension and unexpectedly, deep trench‐parallel compression. We find multiple rupture episodes comprising a mixture of reverse, strike‐slip, and normal faulting. Reverse faulting due to the trench‐parallel compression is unexpected given the apparent subduction direction, so we require a differential buoyancy‐driven stress rotation, which contorts the slab near the edge of the Hikurangi plateau. Our finding highlights that buoyant features in subducting plates may cause diverse rupture behavior of intraslab earthquakes due to the resulting heterogeneous stress state within slabs. Plain Language Summary: A key type of tectonic boundary is where two plates collide with one sinking into the mantle beneath. These subduction zones generate the world's largest earthquakes. Quantifying stress in the subducting plate ("slab") is important because slabs drive the global plate‐tectonic system, and large earthquakes can occur within them. These earthquakes can cause strong shaking, and when occurring near cities, can lead to damage. However, mapping stress is challenging as we cannot directly "see" inside deep slabs. Our best indications of slab stress come from earthquakes themselves. A magnitude 7.3 earthquake north of New Zealand in 2021 generated a distinct pattern of seismic waveforms at seismometers installed worldwide. We used these seismic records to probe the earthquake, providing a new view of stress in subduction zones. We found the earthquake generated both vertical and horizontal motions along faults, driven by compressional and extensional stresses deep within the slab. The compressional part is oriented 90 degrees from the subduction direction, which is opposite to the usual compression in subduction zones. This unusual direction of compression can be explained by subduction of a thickened and buoyant part of the Pacific plate, known as the Hikurangi plateau. Key Points: A moment magnitude 7.3 2021 East Cape, New Zealand intraslab earthquake comprised multiple rupture episodes with different faulting stylesThe complex rupture comprises components of shallow trench‐normal extension and unexpectedly, deep trench‐parallel compression in slabThe trench‐parallel compression likely reflects stress rotation at a buoyancy contrast that drives slab contortion [ABSTRACT FROM AUTHOR]

The 26 May 2019 M W 8.0 Peru intraslab earthquake ruptured the subducting Nazca Plate where the dip angle of the slab increases sharply and the strike angle rotates clockwise from the epicentre to north. To obtain a detailed seismic source model of the 2019 Peru earthquake, including not only the rupture evolution but also the spatiotemporal distribution of focal mechanisms, we performed comprehensive seismic waveform analyses using both a newly developed flexible finite-fault teleseismic waveform inversion method and a back-projection method. The source model revealed a complex rupture process involving a back-propagating rupture. The initial rupture propagated downdip from the hypocentre, then unilaterally northward along the strike of the slab. Following a large slip occurring ∼50–100 km north of the hypocentre, the rupture propagated bilaterally both further northward and back southward. The spatial distribution of focal mechanisms shows that the direction of T -axis azimuth gradually rotated clockwise from the epicentre northward, corresponding to the clockwise rotation of the strike of the subducting Nazca Plate, and the large-slip area corresponds to the high-curvature area of the slab iso-depth lines. Our results show that the complex rupture process, including the focal-mechanism transition, of the Peru earthquake was related to the slab geometry of the subducting Nazca Plate. [ABSTRACT FROM AUTHOR]

The 2011 magnitude (M) 9.0 Tohoku-oki earthquake was followed by seismicity activation in inland areas throughout Japan. An outstanding case is the M6.2 Northern Nagano earthquake, central Japan, occurred 13-h after the megathrust event, approximately 400 km away from its epicenter. The physical processes relating the occurrence of megathrust earthquakes and subsequent activation of relatively large inland earthquakes are not well understood. Here we use waveform data of a dense local seismic network to reveal with an unprecedented resolution the complex mechanisms leading to the occurrence of the M6.2 earthquake. We show that previously undetected small earthquakes initiated along the Nagano earthquake source fault at relatively short times after the Tohoku-oki megathrust earthquake, and the local seismicity continued intermittently until the occurrence of the M6.2 event, being likely 'modulated' by the arrival of surface waves from large, remote aftershocks off-shore Tohoku. About 1-h before the Nagano earthquake, there was an acceleration of micro-seismicity migrating towards its hypocenter. Migration speeds indicate potential localized slow-slip, culminating with the occurrence of the large inland earthquake, with fluids playing a seismicity-activation role at a regional scale. [ABSTRACT FROM AUTHOR]

Conventional seismic source inversion estimates the earthquake rupture process on an assumed fault plane that is determined a priori. It has been a difficult challenge to obtain the fault geometry together with the rupture process by seismic source inversion because of the nonlinearity of the inversion technique. In this study, we propose an inversion method to estimate the fault geometry and the rupture process of an earthquake from teleseismic P waveform data, through an elaboration of our previously published finite-fault inversion analysis (Shimizu et al. 2020). That method differs from conventional methods by representing slip on a fault plane with five basis double-couple components, expressed by potency density tensors, instead of two double-couple components compatible with the fault direction. Because the slip direction obtained from the potency density tensors should be compatible with the fault direction, we can obtain the fault geometry consistent with the rupture process. In practice we rely on an iterative process, first assuming a flat fault plane and then updating the fault geometry by using the information included in the obtained potency density tensors. In constructing a non-planar model-fault surface, we assume for simplicity that the fault direction changes only in either the strike or the dip direction. After checking the validity of the proposed method through synthetic tests, we applied it to the M W 7.7 2013 Balochistan, Pakistan, and M W 7.9 2015 Gorkha, Nepal, earthquakes, which occurred along geometrically complex fault systems. The modelled fault for the Balochistan earthquake is a curved strike-slip fault convex to the south-east, which is consistent with the observed surface ruptures. The modelled fault for the Gorkha earthquake is a reverse fault with a ramp-flat-ramp structure, which is also consistent with the fault geometry derived from geodetic and geological data. These results exhibit that the proposed method works well for constraining fault geometry of an earthquake. [ABSTRACT FROM AUTHOR]

A large strike‐slip earthquake occurred in the Caribbean Sea on January 28, 2020. We inverted teleseismic P waveforms from the earthquake to construct a finite‐fault model by a new method of inversion that simultaneously resolves the spatiotemporal evolution of fault geometry and slip. The model showed almost‐unilateral rupture propagation westward from the epicenter along a 300 km section of the Oriente transform fault with two episodes of rupture at speeds exceeding the local shear‐wave velocity. Our modeling indicated that the 2020 Caribbean earthquake rupture encountered a bend in the fault system associated with a bathymetric feature near the source region. The geometric complexity of the fault system triggered multiple rupture episodes and a complex rupture evolution. Our analysis of the earthquake revealed complexity of rupture process and fault geometry previously unrecognized for an oceanic transform fault that was thought to be part of a simple linear transform fault system. Plain Language Summary: On 28 January 2020, a large earthquake occurred on the Oriente fault, an oceanic transform fault in the Caribbean Sea between Jamaica and Cuba. The Oriente fault forms the boundary between the North America and Caribbean tectonic plates. The 2020 Caribbean earthquake was caused by horizontal sliding between the two plates. We used waveforms of the earthquake that were recorded around the world to build a model of the earthquake‐rupture process. The model showed that rupture during the earthquake was complex, featuring multiple rupture episodes with various rupture speeds and in various directions. Our model suggests that a bend in the fault was responsible for the changes of rupture speed and direction and the triggering of successive rupture episodes. Our analysis of the 2020 Caribbean earthquake has revealed complexity of both fault geometry and rupture process that were previously unknown in oceanic transform fault earthquakes. Key Points: We built a kinematic source model of the 2020 Caribbean earthquake to analyze the spatiotemporal evolution of fault geometry and slipA fault bend disturbed supershear rupture along the linear fault section and triggered subsequent ruptureOceanic transform faults can have geometric complexity that controls rupture evolution [ABSTRACT FROM AUTHOR]

Waveform backprojection (BP) is a key technique of earthquake-source imaging, which has been widely used for extracting information of earthquake source evolution that cannot be obtained by kinematic source inversion. The technique enjoys considerable popularity, owing to the simplicity of its implementation and the robustness of its processing, but the physical meaning of BP images has remained elusive. In this study, we reviewed the mathematical representation of BP and hybrid BP (HBP) methods, following the pioneering work of Fukahata et al. (2014), to clarify the physical implications of BP images. We found that signal intensity in BP and HBP images is scaled with the amplitude of the Green's function that corresponds to a unit-step slip, which results in the signal intensity being depth dependent. We propose variants of BP and HBP, which we call kinematic BP and HBP, respectively, to relate the BP signal intensity to slip motion of an earthquake by modifying the normalizing factors used in the original BP and HBP methods. The original BP and HBP images remain useful for assessing the spatiotemporal strength of the wave radiation, which scales with the amplitude of the Green's function, whereas the kinematic BP and HBP methods are suitable for imaging the slip motion that is responsible for the high-frequency radiation produced during the source-rupture process. [ABSTRACT FROM AUTHOR]

The last earthquake that affected the city of Coquimbo took place in September 2015 and had a magnitude of Mw D 8:3, resulting in localized damage in low-lying areas of the city. In addition, another seismic gap north of the 2015 earthquake rupture area has been identified; therefore, a significant earthquake (Mw D 8:2 to 8.5) and tsunami could occur in the near future. The present paper develops a tsunami fragility curve for the city of Coquimbo based on field survey data and tsunami numerical simulations. The inundation depth of the 2015 Chile tsunami in Coquimbo was estimated by means of numerical simulation with the Non-hydrostatic Evolution of Ocean WAVEs (NEOWAVE) model and five nested grids with a maximum grid resolution of 10 m. The fragility curve exhibited behavior similar to that of other curves in flat areas in Japan, where little damage was observed at relatively high inundation depths. In addition, it was observed that Coquimbo experienced less damage than Dichato (Chile); in fact, at an inundation depth of 2 m, Dichato had a ~ 75% probability of damage, while Coquimbo proved to have only a 20% probability. The new fragility curve was used to estimate the damage by possible future tsunamis in the area. The damage assessment showed that 50% of the structures in the low-lying area of Coquimbo have a high probability of damage in the case of a tsunami generated off the coast of the study area if the city is rebuilt with the same types of structures. [ABSTRACT FROM AUTHOR]

We resolve the density structure of a possible magma reservoir beneath Aso, an active volcano on Kyushu Island, Japan, by inverting gravity data. In the context of the resolved structure, we discuss the relationship between the fault rupture of the 2016 Kumamoto earthquake and Aso volcano. Low-density bodies were resolved beneath central Aso volcano using a three-dimensional inversion with an assumed density contrast of ±0.3 g/cm. The resultant location of the southern low-density body is consistent with a magma reservoir reported in previous studies. No Kumamoto aftershocks occur in the southern low-density body; this aseismic anomaly may indicate a ductile feature due to high temperatures and/or the presence of partial melt. Comparisons of the location of the southern low-density body with rupture models of the mainshock, obtained from teleseismic waveform and InSAR data, suggest that the rupture terminus overlaps the southern low-density body. The ductile features of a magma reservoir could have terminated rupture propagation. On the other hand, a northern low-density body is resolved in the Asodani area, where evidence of current volcanic activity is scarce and aftershock activity is high. The northern low-density body might, therefore, be derived from a thick caldera fill in the Asodani area, or correspond to mush magma or a high-crystallinity magma reservoir that could be the remnant of an ancient intrusion. [ABSTRACT FROM AUTHOR]

The M7.3 Kumamoto earthquake occurred on April 16, 2016, in the western part of Kyushu, at a depth of 12 km, on an active strike-slip fault. Here, we report on a relatively widespread activation of small remote earthquakes, which occurred as far as Hokkaido, detected by analyzing the continuous waveform data recorded at seismic stations all over Japan. Such relatively widespread remote seismicity activation, following a large inland earthquake, has not been reported before for Japan. Our analysis demonstrates that the remote events were triggered dynamically, by the passage of the surface waves from the Kumamoto earthquake. Most of the remotely triggered events in the Tohoku and Hokkaido regions, as well as close to Izu Peninsula, occur at or close to volcanoes, which suggests that the excitation of crustal fluids, by the passage of Rayleigh waves, played an important triggering role. Nevertheless, remote activation in other regions, like Noto Peninsula, occurred away from volcanoes. The relatively large-amplitude Love waves, enhanced by a source directivity effect during the Kumamoto earthquake, may have triggered seismicity on local active faults. The dynamic stresses in the areas where remote activation has been observed range from several kPa to tens of kPa, the thresholds being lower than in previous dynamic triggering cases for Japan; this might relate to a change in the crustal conditions following the 2011 M9.0 Tohoku-oki earthquake, in particular at volcanoes in NE Japan. [ABSTRACT FROM AUTHOR]

We constructed the rupture process model for the 2016 Kumamoto, Japan, earthquake from broadband teleseismic body waveforms (P-waves) by using a novel waveform inversion method that takes into account the uncertainty of Green's function. The estimated source parameters are: seismic moment = 5.1 × 10 Nm (Mw = 7.1), fault length = 40 km, and fault width = 15 km. The mainshock rupture mainly propagated northeastward from the epicenter, for about 30 km, along an active strike-slip fault. The rupture propagation of the mainshock decelerated and terminated near the southwest side of the Aso volcano; the aftershock activity was low around the northeastern edge of the major slip area. Our results suggest that the rupture process of the mainshock and the distribution of aftershocks were influenced by the high-temperature area around the magma chamber of Mt. Aso. [ABSTRACT FROM AUTHOR]

We constructed a seismic source model for the 2015 M 8.3 Illapel, Chile earthquake, which was carried out with the kinematic waveform inversion method adopting a novel inversion formulation that takes into account the uncertainty in the Green's function, together with the hybrid backprojection method enabling us to track the spatiotemporal distribution of high-frequency (0.3-2.0 Hz) sources at high resolution by using globally observed teleseismic P-waveforms. A maximum slip amounted to 10.4 m in the shallow part of the seismic source region centered 72 km northwest of the epicenter and generated a following tsunami inundated along the coast. In a gross sense, the rupture front propagated almost unilaterally to northward from the hypocenter at <2 km/s, however, in detail the spatiotemporal slip distribution also showed a complex rupture propagation pattern: two up-dip rupture propagation episodes, and a secondary rupture episode may have been triggered by the strong high-frequency radiation event at the down-dip edge of the seismic source region. High-frequency sources tends to be distributed at deeper parts of the slip area, a pattern also documented in other subduction zone megathrust earthquakes that may reflect the heterogeneous distribution of fracture energy or stress drop along the fault. The weak excitation of high-frequency radiation at the termination of rupture may represent the gradual deceleration of rupture velocity at the transition zone of frictional property or stress state between the megathrust rupture zone and the swarm area. [ABSTRACT FROM AUTHOR]

Geodetic imaging data and seismic waveform data have complementary strengths when considering the modelling of earthquakes. The former, particularly modern space geodetic techniques such as Interferometric Synthetic Aperture Radar (InSAR), permit high spatial density of observation and thus fine resolution of the spatial pattern of fault slip; the latter provide precise and accurate timing information, and thus the ability to resolve how that fault slip varies over time. In order to harness these complementary strengths, we propose a method through which the two data types can be combined in a joint inverse model for the evolution of slip on a specified fault geometry. We present here a derivation of Akaike's Bayesian Information Criterion (ABIC) for the joint inversion of multiple data sets that explicitly deals with the problem of objectively estimating the relative weighting between data sets, as well as the optimal influence of model smoothness constraints in space and time.We demonstrate our ABIC inversion scheme by inverting InSAR displacements and teleseismic waveform data for the 1997 Manyi, Tibet, earthquake. We test, using a simplified fault geometry, three cases—InSAR data inverted alone, vertical component teleseismic broad-band waveform data inverted alone and a joint inversion of both data sets. The InSAR-only model and seismic-only model differ significantly in the distribution of slip on the fault plane that they predict. The joint-inversion model, however, has not only a similar distribution of slip and fit to the InSAR data in the InSAR-only model, suggesting that those data provide the stronger control on the pattern of slip, but is also able to fit the seismic data at a minimal degradation of fit when compared with the seismic-only model. The rupture history of the preferred, joint-inversion model, indicates bilateral rupture for the first 20 s of the earthquake, followed by a further 25 s of westward unilateral rupture afterwards, with slip peaking at 7 m in the upper 6 km of the fault. This joint-inversion approach is thus shown to be a viable method for the study of large shallow continental earthquakes, and may be of particular benefit in cases where near-field seismic observations are not available. [ABSTRACT FROM PUBLISHER]

Abstract: We developed a new back-projection method that uses teleseismic P-waveforms to integrate the direct P-phase with reflected phases from structural discontinuities near the source and used it to estimate the spatiotemporal distribution of the seismic energy release of the 2011 Tohoku-oki earthquake. We projected a normalized cross-correlation of observed waveforms with corresponding Green''s functions onto the seismic source region to obtain a high-resolution image of the seismic energy release. Applying this method to teleseismic P-waveform data of the 2011 Tohoku-oki earthquake, we obtained spatiotemporal distributions of seismic energy release for two frequency bands, a low-frequency dataset and a high-frequency dataset. We showed that the energy radiated in the dip direction was strongly frequency dependent. The area of major high-frequency seismic radiation extended only downdip from the hypocenter, whereas the area of major low-frequency seismic radiation propagated both downdip and updip from the hypocenter. We detected a large release of seismic energy near the Japan Trench in the area of maximum slip, which was also the source area of the gigantic tsunami, when we used only the low-frequency dataset. The timing of this large seismic energy release corresponded to an episode of smooth and rapid slip near the Japan Trench, and reflects the strong dependence of the seismic energy distribution obtained on the frequency band of the input waveform dataset. The episode of smooth and rapid slip may have been the trigger for a release of roughly all of the accumulated elastic strain in the seismic source region of the 2011 Tohoku-oki earthquake. [Copyright &y& Elsevier]

SUMMARY In principle, we can never know the true Green's function, which is a major error source in seismic waveform inversion. So far, many studies have devoted their efforts to obtain a Green's function as precise as possible. In this study, we propose a new strategy to cope with this problem. That is to say, we introduce uncertainty of Green's function into waveform inversion analyses. Due to the propagation law of errors, the uncertainty of Green's function results in a data covariance matrix with significant off-diagonal components, which naturally reduce the weight of observed data in later phases. Because the data covariance matrix depends on the model parameters that express slip distribution, the inverse problem to be solved becomes non-linear. Applying the developed inverse method to the teleseismic P-wave data of the 2006 Java, Indonesia, tsunami earthquake, we obtained a reasonable slip-rate distribution and moment-rate function without the non-negative slip constraint. The solution was independent of the initial values of the model parameters. If we neglect the modelling errors due to Green's function as in the conventional formulation, the total slip distribution is much rougher with significant opposite slip components, whereas the moment-rate function is much smoother. If we use a stronger smoothing constraint, more plausible slip distribution can be obtained, but then the moment-rate function becomes even smoother. By comparing the observed waveforms with the synthetic waveforms, we found that high-frequency components were well reproduced only by the new formulation. The modelling errors are essentially important in waveform inversion analyses, although they have been commonly neglected. [ABSTRACT FROM AUTHOR]

Nominally continuous data in space and/or time is obtained in various observations in geophysics. Due to an enhanced technology of computers, we can now invert such observed data with a very high sampling rate. Densely sampled observed data are usually not completely independent of each other, and so we must take this effect into account. As for seismic waveform data, they have at least temporal correlation due to the effect of inelastic attenuation of the Earth. Taking the data covariance into account, we have developed a method of seismic source inversion and applied it to teleseismic P-wave data of the 2003 Boumerdes-Zemmouri, Algeria earthquake. From the comparison of the final slip distributions inverted with and without the covariance components, we found that the effect of covariance components is crucial for a data set of higher sampling rates (≥5 Hz). If we neglect the covariance components, the inverted results become unstable due to overestimation of the information from observed data. So far, it has been widely believed that we can obtain a finer image of seismic source processes, by inverting waveform data with a higher sampling rate. However, the covariance components of observed data originated from inelastic effect of the Earth give a limitation on the resolution of inverted seismic source models. [ABSTRACT FROM AUTHOR]

The spatial and temporal slip distribution of the Tecomán, Colima, Mexico earthquake is estimated from near-source strong-motion and teleseismic body-wave data. To perform a stable inversion, we applied smoothing constraints and determined their optimal relative weights on the observed data using an optimized Akaike's Bayesian information criterion (ABIC). The source parameters are as follows: (strike, dip, slip) = (300°, 20°, 93°), seismic moment M0 = 2.3 x 1020 N m; source duration = 30 sec; along-strike distance = 35 km; along-dip distance = 70 km. We found that the rupture process can be divided into three stages: the rupture nucleated near the hypocenter (stage I), then it broke the first asperity centering about 15 km southwest from the epicenter (stage II); and the rupture propagated to the northeast and the second asperity was broken (stage III). We also estimated the shear-stress change due to the rupture process of the mainshock on and around the major fault zone. It appears that one cluster of aftershocks for the first 5 days, which took place in and adjacent to the zones of stress, increased due to the fault rupture during the mainshock, but overall correlation between the aftershock location and the stress pattern is not clear. [ABSTRACT FROM AUTHOR]

We estimate the critical slip-weakening distance on in-slab earthquake faults in a subduction zone, by applying a recent approach proposed by us. This approach is to find a relation between the breakdown time of shear stress , the time of peak slip velocity , and the slip-weakening distance from the time histories of shear stress, slip and slip velocity at each point on the fault. The previous results show that at can be well approximated by at for faults even with a heterogeneous stress drop distribution, except at locations near barriers and fault edges. We apply the above method to three large in-slab, normal-faulting earthquakes in the Mexican subduction zone. To do this, we calculate the spatial distribution of slip-velocity functions and final slip from kinematic waveform inversion of strong-motion and teleseismic records, and the stress history and final stress change from dynamic rupture calculations. By integrating the slip-velocity functions obtained from the inversion, from the rupture arrival time to the time of peak slip velocity, we obtain slip at and then correct it for at through dynamic calculations. We also estimate the lowest resolvable limit and probable errors of from the slip-velocity functions, and its upper bound from a theoretical constraint between the dynamic stress drop and . We found that the slip-weakening distance estimated in the frequency band between 0.05 and 0.5 Hz ranges between 40 and 120 cm on the in-slab fault of the 1999 Oaxaca earthquake . The largest is detected in the central fault and in part of the deeper sections, and in the zone around the hypocentre ranges between 50 and 70 cm. The estimated values appear to be less depth-dependent but are rather more dependent on the local maximum slip. This possible slip dependence might be interpreted by the degree of fault roughness, in addition to stress heterogeneities. The fracture energy G in the central section and in the hypocentral zone are roughly estimated to be of the order of 10–15 and 5–8 MJ m−2, respectively. Both of the estimated and G values are somewhat larger than those on the vertical fault of two recent, shallow strike-slip earthquakes in western Japan. [ABSTRACT FROM AUTHOR]

We investigated the foreshock activity characteristics using the Japan Meteorological Agency Unified Earthquake Catalog for the last 20 years. Using the nearest-neighbor distance approach, we systematically and objectively classified the earthquakes into clustered and background seismicity. We further categorized the clustered events into foreshocks, mainshocks, and aftershocks and analyzed their statistical features such as the b-value of the frequency-magnitude distribution. We found that the b-values of the foreshocks are lower than those of the aftershocks. This b-value difference suggested that not only the stochastic cascade effect but also the stress changes/aseismic processes may contribute to the mainshock-triggering process. However, forecasting the mainshock based on b-value analysis may be difficult. In addition, the rate of foreshock occurrence in all clusters (with two or more events) was nearly constant (30-40%) over a wide magnitude range. The difference in the magnitude, time, and epicentral distance between the mainshock and largest foreshock followed a power law. We inferred that the distinctive characteristics of foreshocks can be better revealed using the improved catalog, which includes the micro-earthquake information. [ABSTRACT FROM AUTHOR]

• Detailed kinematic source model was constructed for 2018 Palu earthquake. • Slip and fault geometry were simultaneously resolved by teleseismic potency-density inversion. • Transient slip acceleration and deceleration across fault bends sustained supershear rupture. How does fault slip follow an earthquake rupture front propagating faster than the local shear-wave velocity (i.e., at supershear speed)? How does a supershear rupture front pass through a geometrically complex fault system? Resolving the evolution of such complex earthquake ruptures is fundamental to our understanding of earthquake-source physics, but these events have not been well captured by conventional waveform inversions of observational data. We applied a new framework of finite-fault inversion to globally observed teleseismic waveforms and resolved both the spatiotemporal evolution of slip and the fault geometry of the 2018 Palu earthquake (moment magnitude 7.6) in Sulawesi, Indonesia. We show that supershear rupture propagation for this event was sustained by transient slip stagnation and advancement as the rupture front passed through the geometrically complex fault system. This peculiar inchworm-like slip evolution was caused by the rupture front encountering fault bends with favorable and unfavorable orientations for rupture propagation. Our analysis also identified the possible existence of a fault junction beneath Palu Bay connecting an unmapped primary fault in northern Sulawesi with the Palu-Koro fault in the south. [ABSTRACT FROM AUTHOR]