Yuji Yagi

• PhD
• University of Tsukuba

On August 21, 2021 a large earthquake occurred in the South Sandwich subduction zone, and the associated tsunami was widely observed. In order to robustly analyse the detailed seismic source process of this long-source duration (over 200 s) event occurring in a convexly shaped subduction zone, we applied the Potency Density Tensor Inversion with a...

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...

A devastating earthquake with moment magnitude 7.5 occurred in the Noto Peninsula, central Japan. We estimate the rupture evolution of this earthquake from teleseismic P-wave data using the potency-density tensor inversion method, which can give spatiotemporal slip distribution including the information on fault orientations. The result shows a lon...

Seismic waveforms, including teleseismic body waves, contain information about the irregular behavior of rupture propagation, which is essential for understanding the evolution process of large earthquakes. Here, a high-degree-of-freedom source inversion is applied to the teleseismic P waves of the 2023 moment magnitude 6.8 Morocco earthquake to re...

A large non-double-couple component of a tectonic earthquake indicates that its rupture likely was complex and likely involved multiple faults. Detailed source models of such earthquakes can add to our understanding of earthquake source complexity. The 2007 Martinique earthquake in the Caribbean Sea is one of the largest recent earthquakes with a k...

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-unique...

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...

A large non-double-couple component of a tectonic earthquake indicates that its rupture likely was complex and likely involved multiple faults. Detailed source models of such earthquakes can add to our understanding of earthquake source complexity. The 2007 Martinique earthquake in the Caribbean Sea is one of the largest recent earthquakes with a k...

A devastating doublet of earthquakes with moment magnitude Mw 7.9 and Mw 7.6 earthquakes contiguously occurred in south-eastern Türkiye near the north-western 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 invo...

In September 2022, two destructive earthquakes of moment magnitude (Mw) 6.6 (foreshock) and 7.1 (mainshock) occurred in Taitung County, south-eastern Taiwan. To understand their complex rupture processes, we analysed these earthquakes using the Potency Density Tensor Inversion method, which can stably estimate the rupture propagation process, inclu...

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 T...

In September 2022, two destructive earthquakes of moment magnitude (Mw) 6.6 (foreshock) and 7.1 (mainshock) occurred in Taitung County, south-eastern Taiwan. To understand their complex rupture processes, we analysed these earthquakes using the Potency Density Tensor Inversion method, which can stably estimate the rupture propagation process, inclu...

On September 5, 2022, a strike-slip earthquake with a moment magnitude (Mw) 6.7 occurred along the Moxi segment of the Xianshuihe fault zone in Luding, Sichuan province, China. To estimate the rupture evolution of the 2022 Luding earthquake, we inverted teleseismic P-waves by applying the Potency Density Tensor Inversion, a novel method that can es...

We consider a Bayesian multi‐model fault slip estimation (BMMFSE), which incorporates many candidates of the underground structure (Earth structure and plate boundary geometry) model characterized by a prior probability density function (PDF). The technique is used to study long‐term slow slip events (L‐SSEs) that occurred beneath the Bungo Channel...

Large earthquakes are often accompanied by complex fault rupture, but it has been difficult to reliably estimate such a complex rupture process with conventional waveform analysis tools due to modelling errors originating from limited accuracy of the fault geometry. Recently, a potency density tensor inversion method has been developed to solve thi...

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...

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 th...

The 26 May 2019 MW 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 distrib...

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 earth...

We developed a flexible finite-fault inversion method for teleseismic P waveforms to obtain a detailed rupture process of a complex multiple-fault earthquake. We estimate the distribution of potency-rate density tensors on an assumed model plane to clarify rupture evolution processes, including variations of fault geometry. We applied our method to...

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 invers...

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,...

How an earthquake rupture propagates strongly influences the potentially destructive ground shaking. Complex ruptures often involve slip along multiple faults, which masks information on the frictional behaviour of fault zones. Geometrically smooth ocean transform fault plate boundaries offer a favourable environment to study fault dynamics, becaus...

The September 28 2018 Palu tsunami surprised the scientific community, as neither the earthquake magnitude nor its strike-slip mechanism were deemed capable of producing the wave heights that were observed. However, recent research has shown that the earthquake generated several landslides inside Palu bay. The authors conducted a post-disaster fiel...

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 invers...

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,...

Teleseismic waveforms contain information on fault slip evolution during an earthquake, as well as on the fault geometry. A linear finite-fault inversion method is a tool for solving the slip-rate function distribution under an assumption of fault geometry as a single or multiple- fault-plane model. An inappropriate assumption of fault geometry wou...

Rupture propagation of an earthquake strongly influences potentially destructive ground shaking. Variable rupture behaviour is often caused by complex fault geometries, masking information on fundamental frictional properties. Geometrically smoother ocean transform fault (OTF) plate boundaries offer a favourable environment to study fault zone dyna...

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, bu...

Teleseismic waveforms contain information on fault slip evolution during an earthquake, as well as on the fault geometry. A linear finite-fault inversion method is a tool for solving the slip-rate function distribution under an assumption of fault geometry as a single or multiple-fault-plane model. An inappropriate assumption of fault geometry woul...

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 foreshock...

The last earthquake that affected the city of Coquimbo took place in September 2015 and had a magnitude of Mw=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=8.2 to 8.5) and tsunami could occur i...

Waveform backprojection 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...

Geometric discontinuities within fault systems known as geometric barriers contribute to irregular rupture evolutions during earthquakes. We applied a hybrid backprojection method to high-frequency teleseismic P-waveforms to investigate the role of geometric barriers in the rupture propagation during the MW 7.9 2008 Wenchuan, China, earthquake. We...

Previous research revealed a dynamically triggered earthquake front propagating along Southwest Japan, in correlation with the arrival of surface waves from the 2011 M9.0 Tohoku-oki earthquake. However, some characteristics of the dynamically initiated seismic activation have not been investigated in detail. We analyse first the dynamic triggering...

A seismic source model for the MW 8.1 2017 Chiapas, Mexico, earthquake was constructed by kinematic waveform inversion using globally observed teleseismic waveforms, suggesting that the earthquake was a normal-faulting event on a steeply dipping plane, with the major slip concentrated around a relatively shallow depth of 28 km. The modeled rupture...

A seismic source model for the Mw 8.1 2017 Chiapas, Mexico, earthquake was constructed by kinematic waveform inversion using globally observed teleseismic waveforms, suggesting that the earthquake was a normal-faulting event on a steeply dipping plane, with the major slip concentrated around a relatively shallow depth of 28 km. The modeled rupture...

The last earthquake which affected Coquimbo city took place in September 2015, with localized damage observed in low areas of the city. In addition, another seismic gap north of the 2015 earthquake rupture area has been identified; therefore, a significant earthquake and tsunami could occur in the near future. The present paper develops the tsunami...

As high-resolution observational data become more common, the demand for numerical simulations of crustal deformation using 3-D high-fidelity modelling is increasing. To increase the efficiency of performing numerical simulations with high computation costs, we developed a fast solver using heterogeneous computing, with graphics processing units (G...

We constructed a seismic source model for the 2015 MW 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 distrib...

On September 16, 2015 a magnitude Mw 8.3 earthquake took place off the coast of the Coquimbo Region, Chile. Three tsunami survey teams covered approximately 700 km of the Pacific coast. The teams surveyed the area, recording 83 tsunami flow depth and runup measurements. The maximum runup was found to be 10.8 m at only one small bay, in front of the...

We discuss the theoretical estimation of co-seismic energy release with respect to a random spatial distribution of stress drop, which is an extension of conventional stress drop modelling. For slip distributions characterised by the von Kármán-type power spectral density, we examine the dependence of the estimated energy release on the upper limit...

We estimated moment tensor and centroid depth of eight small/moderate local earthquakes occurred between 2011 and 2013 in the northern Red Sea Triple Junction using a moment tensor inversion of near-source waveform data recorded by very broadband stations of the Egyptian National Seismic Network (ENSN). We also estimated the focal mechanism using t...

In 2013, a large deep earthquake occurred at the sea of Okhotsk. Some researchers have indicated the complex rupture process of this earthquake. However, there is still a lack of information about the rupture process, especially obtained by a high frequency analysis. Thus, we investigated the precise source process of the 2013 Sea of Okhotsk deep e...

The MJMA7.3 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...

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 volcan...

The temporal epidemic-type aftershock sequence (ETAS) model is widely used to describe seismicity. However, only a few programs for estimation of the temporal ETAS model parameters are publicly available, and it is difficult to routinely apply some of them to observed data, due to initial value dependence. A robust temporal ETAS estimation program...

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 texttimes 1019 Nm (Mw = 7.1), fault length = 40 km, an...

During the last decades, major tsunamigenic events as Mw 9.2 Sumatra – Andaman, 2004, Mw 8.8 Maule, Chile and Mw 9.0 Tohoku – Oki, Japan have provided valuable information to the scientific community about predominant processes in the generation, propagation and impact of a tsunami on far and near – field coastal zones. In this context, new insight...

Finite-fault earthquake source inversions infer the (time-dependent) displacement on the rupture surface from geophysical data. The resulting earthquake source models document the complexity of the rupture process. However, multiple source models for the same earthquake, obtained by different research teams, often exhibit remarkable dissimilarities...

In this study, an integrated multi-channel analysis of Surface Waves (MASW) technique is applied to explore the geotechnical parameters of subsurface layers at the Zafarana wind farm. Moreover, a seismic hazard procedure based on the extended deterministic technique is used to estimate the seismic hazard load for the investigated area. The study ar...

We constructed a seismic source model for the 2015 M W 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 distri...

On September 16, 2015 a magnitude Mw 8.3 earthquake took place off the coast of the Coquimbo Region, Chile. Three tsunami survey teams covered approximately 700 km of the Pacific coast. The teams surveyed the area, recording 83 tsunami flow depth and runup measurements. The maximum runup was found to be 10.8 m at only one small bay, in front of the...

The 2015 Mw 7.8 Nepal-Gorkha earthquake with casualties of over 9,000 people was the most devastating disaster to strike Nepal since the 1934 Nepal-Bihar earthquake. Its rupture process was imaged by teleseismic back-projections (BP) of seismograms recorded by three, large regional networks in Australia, North America and Europe. The source images...

Tsunami inundation maps are a powerful tool to design evacuation plans of coastal communities, additionally can be used as a guide to territorial planning and assessment of structural damages in port facilities and critical infrastructure (Borrero et al., 2003; Barberopoulou et al., 2011; Power et al., 2012; Mueller et al., 2015). The accuracy of i...

A few low-angle normal fault earthquakes at approximately the depth of the plate interface, with a strike nearly parallel to the trench axis, were detected immediately after the 2011 Tohoku-oki earthquake. After that, however, no such normal fault events have been observed until the occurrence of the 2014 MW 6.6 Fukushima-oki earthquake. Here we an...

We compared spatiotemporal slip-rate and high-frequency (around 1 Hz) radiation distributions from teleseismic P wave data to infer the seismic rupture process of the 2015 Gorkha, Nepal, earthquake. For these estimates, we applied a novel waveform inversion formulation that mitigates the effect of Green's functions uncertainty and a hybrid backproj...

Seismic Energy ES gives a minimum of strain energy drop defined as an inner product of spacial distribution of coseismic slip and stress change on a fault surface (Andrews 1978 JGR). Traditionally, ES has been obtained by multiplying mean stress drop and seismic moment divided by the rigidity by assuming the distribution of stress drop is constant...

On April 1, 2014 a great thrust earthquake (Mw 8.1) occurred off Iquique, northern Chile. A remarkable foreshock sequence was observed about 2 weeks before the mainshock. Seismic source models are essentially important to reveal the relationship between the mainshock and foreshock activity. However, seismic source models for the Iquique earthquake...

High-frequency seismic waves are generated by abrupt changes of rupture velocity and slip-rate during an earthquake. Therefore, analysis of high-frequency waves is crucial to understanding the dynamic rupture process. Here, we developed a hybrid back-projection method that considers variations in focal mechanisms by introducing a non-planar fault m...

The dynamic triggering of earthquakes is well documented, however the underlying physical mechanisms remain obscure. Here we analyze the seismicity in Northern Nagano, central Japan, following the Tohoku-oki quake, until the occurrence 13 h later of an Mw6.2 local earthquake. We use waveform-detection techniques to identify 17 times more earthquake...

The rupture process of the 2014 Iquique, Chile earthquake is inverted from teleseismic P-wave data applying a novel formulation that takes into account the uncertainty of Green's function, which has been a major error source in waveform inversion. The estimated seismic moment is 1.5 × 1021 Nm (Mw =8.1), associated with a 140 km long and 140 km wide...

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-on...

We clarified the theoretical relationship between the back-projection (BP) imaging and classical linear inverse solutions via the hybrid back-projection (HBP) imaging. In the HBP imaging, which is mathematically similar to the time-reversal source imaging, cross correlations of observed waveforms with the corresponding Green's functions are calcula...

The September 2, 2009 Tasikmalaya, West Java earthquake was a quite unique earthquake. The source mechanism of this event is not consistent with the characteristic of the tectonic stress in this region in which the strike direction in general parallels to the present-day trench. In fact, the strike of the September 2, 2009 Tasikmalaya earthquake is...

In order to enhance our understanding of the earthquake mechanism and seismotectonic characteristics of the September 2, 2009 Tasikmalaya earthquake, we have performed the joint inversion method to the near field data from the MCGA regional network and teleseismic body wave data from the IRIS-DMC seismic network. To make the stable inversion, we ad...

The 2011 Tohoku earthquake demonstrated that extremely large seismic slip can occur at shallow plate interfaces in subduction zones. The large slip area for the Tohoku earthquake included the source region of a tsunami earthquake. We performed dynamic rupture simulations using simplified fault models and the mechanism of thermal fluid pressurizatio...

The back-projection (BP) method has become a popular tool to image the rupture process of large earthquakes since the success of Ishii et al. (2005), while it has not been clear what the BP image represents physically. We clarified the theoretical background of the back-projection (BP) imaging and related it to classical inverse solutions via the h...

A sequence of M7-class interplate earthquakes (M6.7-7.3) occurred from 2003 to 2011 before the occurrence of the M9 2011 Tohoku-oki earthquake. To investigate the relationship between these previous events and the giant earthquake, we determined the source processes of these events by joint inversion method. We performed the inversions by using bot...

From the comparison of observed 1-Hz GPS data with simple forward computation that evaluates near and intermediate field terms, we put constraints on the early-stage rupture process of the 2011 Tohoku-oki earthquake. Mainly based on the time difference of the onset of large eastward displacements at stations along the northern coast of the source r...

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 obse...

The Tohoku-oki earthquake resulted in substantial slip relatively near the coast from Miyagi to Ibaraki prefectures as well as extremely large slip near the trench. The moment-rate function has a shoulder around s and a large peak about s. The former corresponds to a significant moment release in the source area of the Miyagi-oki earthquake, and th...

We have estimated the spatio-temporal slip distribution of the 12 May, 2008, Wenchuan earthquake from teleseismic P-wave data. Teleseismic body wave observations are useful geophysical observations for estimating the overall seismic source process immediately after a great earthquake. However, it is difficult to constrain the slip vectors of later...

We investigated the source process of the 2010 Mentawai earthquake by joint inversion method using near source and teleseismic body-wave data. To perform a stable inversion, we applied smoothing constraints and determined their relative weights on the observed data using ABIC criterion. The teleseismic waveforms were windowed for 150 sec, band-pass...

On 11 March 2011, the Tohoku-oki earthquake in eastern Japan and the devastating tsunami that followed it caused severe damage and numerous deaths. To clarify the rupture process of the earthquake, we inverted teleseismic P-wave data applying a novel formulation that takes into account the uncertainty of Green's function, which has been a major err...

The devastating 2011 Tohoku earthquake was observed by dense networks of geophysical instruments. A unified source model was constructed through joint inversion of teleseismic, strong motion, and geodetic datasets. The result indicates that the earthquake consists of three main ruptures. After small rupture in the initial 50 s, the first rupture ex...

We estimated the spatial and temporal slip distribution for the 27 February 2010 Maule earthquake from teleseismic body wave data. To obtain a stable inversion solution, we used the data covariance matrix from the observation and modeling errors, incorporated smoothing constraints and determined their optimal values by using the Akaike Bayesian Inf...

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...