Spatiotemporal Slip Distribution Research Articles

Detection of a recent large Hyuga-nada long-term slow slip event and estimation of its spatiotemporal slip distributions

Using horizontal and vertical GNSS time series data from the GSI in sotheastern Kyushu from January 1, 2017 to June 30, 2022, we detected a recent long-term slow slip event (L-SSE) that occurred in the Hyuga-nada region, southwest Japan, and estimated its spatiotemporal slip distribution. We performed such analysis considering the piecewise evolution of slip over time with time windows of 0.8 years to ensure a good signal-to-noise ratio in the horizontal displacements in each time window. The results showed that a slip of more than 2 cm occurred in the central part of Miyazaki Prefecture from 2018.5 to 2019.3 (the units were 0.1 years = 36.5 days). Then, the amount of slip increased and expanded slightly to the southern part of Miyazaki Prefecture from 2019.3 to 2020.1, and the amount of slip reached a maximum of 3.9 cm in the subsequent period (2020.1–2020.9). A smaller slip occurred at almost the same location in the following 0.8 years. Therefore, the duration of the L-SSE was approximately 3.2 years from approximately 2018.5 to 2021.7. The annual average maximum slip rate was approximately 4.9 cm/yr (3.9 cm/0.8 yr) during the period from 2020.1 to 2020.9. The maximum total slip was estimated to be approximately 12.9 cm, and the equivalent release moment was 4.9 × 1019 Nm, corresponding to Mw7.1. Compared to previous L-SSEs that occurred in the Hyuga-nada region, the annual average maximum slip rate was relatively low, and the main total slip was estimated at almost the same location on the plate interface. On the other hand, the slip duration of this L-SSE was the longest, and the release moment and moment magnitude were the greatest. The total slip area of more than approximately 10 cm in the Hyuga-nada L-SSE estimated in this study almost overlapped with the afterslip area of the December 3, 1996 Hyuga-nada earthquake, with a depth range of approximately 30–40 km. Short-term SSEs also occurred in the same depth range. These coincidental same depth ranges of interplate seismic events stem from large thermal gradients. The large thermal gradients play an important role in narrowing the depth range of frictional parameters (a–b) at the plate interface. In addition, the pore pressure and normal stress are closely related to the critical stiffness. In particular, contrasting values of pore pressure and normal stress at the shallow and deep sides of the mantle wedge corner may also contribute to transient aseismic slip within the same depth range.

Insights into spatio-temporal slow slip events offshore the Boso Peninsula in central Japan during 2011–2019 using GPS data

Using Global Positioning System (GPS) coordinate time series, we detect three transient slow slip events (SSEs) offshore the Boso Peninsula in central Japan during 2011–2019. To extract the tiny SSE signals obscured by the significant post-seismic deformation after the 2011 MW9.0 Tohoku earthquake, we develop a new GPS coordinate time series processing software to obtain these SSE-induced deformations from high-noise GPS data. In addition, we apply the principal component analysis-based inversion method (PCAIM) to get the spatio-temporal slip distribution of the three SSEs. The spatio-temporal evolutions of these slips reveal that the nucleation styles are different. Compared to the 2011 and 2018 SSEs, the 2013–2014 SSE displays faster slip spatio-temporal variation, deeper slip, shorter slip duration, minor seismic moment, and lower maximum slip rate. The 2018 SSE exhibits the most significant seismic moment, the maximum slip, and the maximum slip rate of these three SSEs. The spatio-temporal variations of the 2011 SSE are the most complex, containing two acceleration and deceleration phases. The slip zone expanded along the eastern side of the Boso Peninsula in the acceleration phase and shrank back in the deceleration phase. Furthermore, the recurrence interval of SSEs spans from 2.2 to 4 years during 2011–2019, suggesting that the recurrence interval might become shorter and non-periodic due to the enormous earthquake. After the 2013–2014 SSE, the recurrence interval of the SSE gradually returns to normal. Thus, we can infer that the SSE may occur every 4–7 years after the 2018 SSE if there is no large earthquake.

Source characteristics of the March 16, 2014 Mw 6.7 earthquake and its implications for the Mw 8.2 Pisagua mainshock

The March 16, 2014 Mw 6.7 earthquake played an important role in the complex seismic sequence leading to the nucleation of the April 1st, 2014 Mw 8.2 Pisagua earthquake in northern Chile. The Mw 6.7, an upper plate reverse faulting event with nodal planes highly rotated counterclockwise with respect to the strike of the megathrust at its location, is analyzed. For each nodal plane of the regional W-phase centroid moment tensor solution, the spatiotemporal slip distribution is inverted using near-field records. The optimal space and time smoothing constraints are determined objectively based on the Akaike's Bayesian Information Criterion. We analyze seismicity on the region affected by the Mw 6.7 and Mw 8.2 Pisagua mainshock, with accurate hypocenter locations obtained using a 3D heterogeneous medium. We also performed regional moment tensor inversion of events (M≥4.0) occurred before the Mw 8.2. Our results suggest the sub-vertical fault plane dipping southward (dip∼70°) as being the causative fault of the March 16 Mw 6.7 earthquake. The rupture propagated to the northwest, lasted 15s, and yielded a total seismic moment of 1.36×1019Nm (Mw 6.7). Estimated slip is characterized by total rupture length of ∼22km, and suggest shallow slip distributed between ∼ 3km, down to 17km depth. A comparison between coastal tide gauge records and forward tsunami modeling show similarity of the time series, thus supporting the plausibility of our estimated slip models. Our interpretation suggests that the rupture of the Mw 6.7 intraplate could be explained by stress accommodation between two segments along-dip with different mechanical properties at the interplate boundary, and inside the upper plate.

Insights Into the Kinematic Rupture of the 2015 Mw 8.3 Illapel, Chile, Earthquake From Joint Analysis of Geodetic, Seismological, Tsunami, and Superconductive Gravimeter Observations

AbstractWe investigated the spatiotemporal slip distribution of the 2015 Illapel earthquake (Mw = 8.3) by joint analyses of geodetic, seismological, tsunami, and superconductive gravimeter observations. The coseismic rupture directly overlaps the interseismic coupling zone determined by onshore geodetic measurements. The main slip asperity is located north of the hypocenter with a peak slip of ~9.2 m, and the rupture spans ~200 km along strike and ~150 km along dip with an average rupture speed of ~2.0 km/s. Most aftershocks reveal a clear complementary distribution with the coseismic rupture; that is, aftershock clusters are located at the border of unbroken barriers and regions of sudden transition from high to low rupture area. The calculated Coulomb failure stress changes indicate that 80% of the aftershocks were triggered by the main event. The low‐frequency radiation corresponds to the large coseismic rupture at the shallow portion of the megathrust, while the high‐frequency radiation is associated with the edge of large slip asperity or an isolated patch at a deeper position. The large coseismic slip region correlates well with the high Vp/Vs ratio of the subduction interface, which implies that the Illapel earthquake might nucleate at a relatively weak patch within a strong coupled zone. Furthermore, we argue that the coseismic slip did reach to the trench from tsunami simulations, and historical earthquakes analyses indicate a complex strain accumulation and release in central Chile.

Joint inversion of GNSS and teleseismic data for the rupture process of the 2017 Mw6.5 Jiuzhaigou, China, earthquake

The spatio-temporal slip distribution of the earthquake that occurred on 8 August 2017 in Jiuzhaigou, China, was estimated from the teleseismic body wave and near-field Global Navigation Satellite System (GNSS) data (coseismic displacements and high-rate GPS data) based on a finite fault model. Compared with the inversion results from the teleseismic body waves, the near-field GNSS data can better restrain the rupture area, the maximum slip, the source time function, and the surface rupture. The results show that the maximum slip of the earthquake approaches 1.4 m, the scalar seismic moment is ~ 8.0 × 1018 N·m (Mw ≈ 6.5), and the centroid depth is ~ 15 km. The slip is mainly driven by the left-lateral strike-slip and it is initially inferred that the seismogenic fault occurs in the south branch of the Tazang fault or an undetectable fault, a NW-trending left-lateral strike-slip fault, and belongs to one of the tail structures at the easternmost end of the eastern Kunlun fault zone. The earthquake rupture is mainly concentrated at depths of 5–15 km, which results in the complete rupture of the seismic gap left by the previous four earthquakes with magnitudes > 6.0 in 1973 and 1976. Therefore, the possibility of a strong aftershock on the Huya fault is low. The source duration is ~ 30 s and there are two major ruptures. The main rupture occurs in the first 10 s, 4 s after the earthquake; the second rupture peak arrives in ~ 17 s. In addition, the Coulomb stress study shows that the epicenter of the earthquake is located in the area where the static Coulomb stress change increased because of the 12 May 2017 Mw7.9 Wenchuan, China, earthquake. Therefore, the Wenchuan earthquake promoted the occurrence of the 8 August 2017 Jiuzhaigou earthquake.

Analysis of Spatio-Temporal Tsunami Source Models for Reproducing Tsunami Inundation Features

A powerful tsunami triggered by the Mw 9.0 Tohoku earthquake struck the northern Pacific coast of Japan in 2011, destroying several coastal communities in Iwate Prefecture, Miyagi Prefecture, and Fukushima Prefecture. Here, we investigate a new spatio-temporal slip model (model B) developed by the Cabinet Office of the Government of Japan. This slip model was compared against the non-uniform slip model estimated using tsunami waveform data (model A). We focused our analysis on two areas that were destroyed by the tsunami. The town of Onagawa and the coastal Sendai Plain area were selected because they were located in front of the epicentre, where the most significant slips were registered. Our simulation results revealed that the spatio-temporal slip distribution better replicated the observed data. Regarding the tsunami waveforms from the coastal tide gauge station and offshore stations, the Cabinet Office’s slip model showed an approximately 30% better accuracy relative to the non-uniform slip model. Furthermore, by comparing the local inundation features at two locations with unique topographic and coastal morphological characteristics, we also found that model B better replicated the measured inundation depths. Finally, considering that tsunami-induced damage is a direct function of various inundation features such as the flow depth and flow velocity, this new slip model can generate more realistic damage scenarios for future tsunami assessments.

Simultaneous Estimation of the Spatio-Temporal Slip Distribution and Duration of the Slow Slip Event by a Switching Model

Simultaneous Estimation of the Spatio-Temporal Slip Distribution and Duration of the Slow Slip Event by a Switching Model

Rupture Process During the 2015 Illapel, Chile Earthquake: Zigzag-Along-Dip Rupture Episodes

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 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.

Automatic imaging of earthquake rupture processes by iterative deconvolution and stacking of high‐rate GPS and strong motion seismograms

AbstractBy combining the complementary advantages of conventional network inversion and backprojection methods, we have developed an iterative deconvolution and stacking (IDS) approach for imaging earthquake rupture processes with near‐field complete waveform data. This new approach does not need any manual adjustment of the physical (empirical) constraints, such as restricting the rupture time and duration, and smoothing the spatiotemporal slip distribution. Therefore, it has the ability to image complex multiple ruptures automatically. The advantages of the IDS method over traditional linear or nonlinear optimization algorithms are demonstrated by the case studies of the 2008 Wenchuan and 2011 Tohoku earthquakes. For such large earthquakes, the IDS method is considerably more stable and efficient than previous inversion methods. Additionally, the robustness of this method is demonstrated by comprehensive synthetic tests, indicating its potential contribution to tsunami and earthquake early warning and rapid response systems. It is also shown that the IDS method can be used for teleseismic waveform inversions. For the two major earthquakes discussed here, the IDS method can provide, without tuning any physical or empirical constraints, teleseismic rupture models consistent with those derived from the near‐field GPS and strong motion data.

Source process and tectonic implication of the January 20, 2007 Odaesan earthquake, South Korea

The source process for the 20th of January 2007, Mw 4.5 Odaesan earthquake in South Korea is investigated in the low- and high-frequency bands, using velocity and acceleration waveform data recorded by the Korea Meteorological Administration Seismographic Network at distances less than 70km from the epicenter. Synthetic Green functions are adopted for the low-frequency band of 0.1–0.3Hz by using the wave-number integration technique and the one dimensional velocity model beneath the epicentral area. An iterative technique was performed by a grid search across the strike, dip, rake, and focal depth of rupture nucleation parameters to find the best-fit double-couple mechanism. To resolve the nodal plane ambiguity, the spatiotemporal slip distribution on the fault surface was recovered using a non-negative least-square algorithm for each set of the grid-searched parameters. The focal depth of 10km was determined through the grid search for depths in the range of 6–14km. The best-fit double-couple mechanism obtained from the finite-source model indicates a vertical strike-slip faulting mechanism. The NW faulting plane gives comparatively smaller root-mean-squares (RMS) error than its auxiliary plane. Slip pattern event provides simple source process due to the effect of Low-frequency that acted as a point source model. Three empirical Green functions are adopted to investigate the source process in the high-frequency band. A set of slip models was recovered on both nodal planes of the focal mechanism with various rupture velocities in the range of 2.0–4.0km/s. Although there is a small difference between the RMS errors produced by the two orthogonal nodal planes, the SW dipping plane gives a smaller RMS error than its auxiliary plane. The slip distribution is relatively assessable by the oblique pattern recovered around the hypocenter in the high-frequency analysis; indicating a complex rupture scenario for such moderate-sized earthquake, similar to those reported for large earthquakes.

Source Process of the 2011 Tohoku Earthquake Estimated from the Joint Inversion of Teleseismic Body Waves and Geodetic Data Including Seafloor Observation Data: Source Model with Enhanced Reliability by Using Objectively Determined Inversion Settings

Research Article| May 01, 2013 Source Process of the 2011 Tohoku Earthquake Estimated from the Joint Inversion of Teleseismic Body Waves and Geodetic Data Including Seafloor Observation Data: Source Model with Enhanced Reliability by Using Objectively Determined Inversion Settings Hisahiko Kubo; Hisahiko Kubo Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611‐0011, Japankubo.hisahiko.26a@st.kyoto-u.ac.jp Search for other works by this author on: GSW Google Scholar Yasumaro Kakehi Yasumaro Kakehi Graduate School of Science, Kobe University, 1‐1 Rokkodai‐cho, Nada, Kobe, Hyogo 657‐8501, Japankakehi@kobe-u.ac.jp Search for other works by this author on: GSW Google Scholar Author and Article Information Hisahiko Kubo Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611‐0011, Japankubo.hisahiko.26a@st.kyoto-u.ac.jp Yasumaro Kakehi Graduate School of Science, Kobe University, 1‐1 Rokkodai‐cho, Nada, Kobe, Hyogo 657‐8501, Japankakehi@kobe-u.ac.jp Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1943-3573 Print ISSN: 0037-1106 Bulletin of the Seismological Society of America (2013) 103 (2B): 1195–1220. https://doi.org/10.1785/0120120113 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Hisahiko Kubo, Yasumaro Kakehi; Source Process of the 2011 Tohoku Earthquake Estimated from the Joint Inversion of Teleseismic Body Waves and Geodetic Data Including Seafloor Observation Data: Source Model with Enhanced Reliability by Using Objectively Determined Inversion Settings. Bulletin of the Seismological Society of America 2013;; 103 (2B): 1195–1220. doi: https://doi.org/10.1785/0120120113 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract The rupture process of the 2011 Tohoku earthquake is estimated from the joint inversion of teleseismic body waves and geodetic data using multiple time‐window analysis. To enhance the reliability of the inversion solution, two important inversion‐setting parameters that control the spatiotemporal slip distribution, the maximum slip duration of each subfault (Tsd) and the propagation velocity of the first time window (Vftw), are determined by objective criteria. By performing a semblance analysis of small‐aperture seismic array data of near‐source strong motion waveforms, Tsd is determined to be 100 s. Vftw is determined to be 2.0 km/s from the matching of the teleseismic and geodetic slip models. The spatial resolution of geodetic inversion is significantly improved by using seafloor crustal deformation data in the source area in addition to the terrestrial Global Positioning System network data. The estimated seismic moment and the maximum slip are 3.4×1022 N·m (Mw=9.0) and 43 m, respectively. The total rupture duration is ∼150 s. Large slip is seen in the shallow part of the fault surface, including the hypocenter, and its spatial extent along the trench axis is ∼300 km. This model is consistent with the results of bathymetric surveys and the slip models from tsunami data. The relationships among the fault parameters of the characterized source model of the 2011 Tohoku earthquake suggest the possibility that saturation of fault width occurs for this huge earthquake.Online Material: Text S1 denotes the explanation of how the rupture propagation in the dip direction affects the estimated value of maximum slip duration (Tsd). Table S1 denotes the detailed information of the teleseismic stations. Table S2 denotes the detailed information of the terrestrial geodetic stations. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Source process of the 12 May 2008 Wenchuan, China, earthquake determined by waveform inversion of teleseismic body waves with a data covariance matrix

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 sub-events during a large earthquake such as the 2008 Wenchuan earthquake because the modeling error due to the Green’s function uncertainty generally increases with time. In this study, we have applied a new waveform inversion method that incorporates the uncertainty of the Green’s function, and we have assumed a simple fault plane model to evaluate our inversion scheme for a rapid analysis of the seismic source process. The results show that the slip history of the Wenchuan earthquake include six distinct sub-events. The derived source parameters were a seismic moment M0 = 1.4 × 1021 Nm (MW 8.0) and a source duration = 125 s. The total slip vector distribution and segmentations obtained in our study are consistent with results obtained from InSAR data. The manner of the rupture propagation suggests that the two major source faults, the Beichuan fault and the Pengguan fault, intersected in the deep part of the seismogenic zone.

Rupture at the flank of the subducted Gagua ridge: The 18 December 2001 earthquake (Mw 6.8) offshore eastern Taiwan

The southern Ryukyus represents an area where different tectonic stress regimes result in high seismicity and increased seismic hazard for nearby areas such as Taiwan. On 18 December 2001 at 04:03 (GMT) a strong earthquake (Mw 6.8) occurred in the forearc area of the southern Ryukyu subduction zone. Revised moment tensor solutions published by GCMT and BATS groups show a normal faulting mechanism with some strike-slip component and also point to a shallow focal depth (∼12 km). We use arrival times picked at both Taiwanese and Japanese stations along with a 3D geo-realistic a priori velocity model in order to obtain accurate absolute locations for the mainshock and 153 of its aftershocks. Locations are derived by using the Maximum intersection (MAXI) algorithm which has been used in many previous seismicity studies in the southern Ryukyus. These improved locations indicate that the mainshock was caused by the failure of a NE–SW oriented fault that extends from the edge of the Nanao forearc sedimentary basin to the Ryukyu arc basement. Far-field P and SH waveforms of the mainshock recorded at stations surrounding the source and at distances 30–100°, were inverted for the purpose of investigating its rupture process. A non-negative least-squares inversion technique utilizing multiple time windows was used to derive the spatio-temporal slip distribution. The preferred slip distribution model shows that there is one large area of high slip (∼0.9 m) at 5–15 km depth that essentially represents the crystalline rocks of the Ryukyu arc basement. Another smaller area with lower slip (∼0.4 m) extends at 10–15 km depth beneath the Nanao basin. Most aftershocks are located in areas of low slip (<0.4 m) filling the regions of slip deficit. It is likely that the 18 December 2001 earthquake was caused by a stress field interaction generated by the oblique subduction of the Gagua ridge and the gravitational forces acting at its landward flank.

Rupture Process and Aftershocks Relocation of the 8 June 2008 Mw 6.4 Earthquake in Northwest Peloponnese, Western Greece

On 8 June 2008 at 12:25 (GMT) an M w 6.4 strong earthquake occurred in the area of northwest Peloponnese, western Greece, causing the death of two people and extensive damage to the surrounding area. The main event and its aftershocks were recorded by one nationwide and three regional networks equipped with three-component broadband seismometers. Initial locations of the earthquake sequence comprising 438 aftershocks showed a linear northeast–southwest trend and that the mainshock was located at 22 km depth. After the relocation using catalog and differential travel times, most events form three distinct clusters at depths 15–25 km. Moment tensor solutions for the main event and its largest aftershocks exhibited a pure strike-slip mechanism with one nodal plane orientated northeast–southwest in accordance with the relocated seismicity. A parallel, nonnegative least-squares inversion technique utilizing multiple-time windows was used to derive the spatiotemporal slip distribution of the main event. The resulting slip distribution model revealed a large slip patch (maximum slip ∼150 cm) between 10 and 20 km depth at the northeast part of the fault that also coincides with the area that suffered most of the damage. Another patch exhibiting smaller amounts of slip (20–50 cm) is located to the southwest direction at the same depth range, and smaller patches exist at 25–30 km depth. Most aftershocks are located in areas of low slip (<25 cm) filling the regions of slip deficit. The 8 June earthquake occurred at an area where no previous seismological or other observations indicated the existence of a seismogenic fault at that depth and with this strike. This, and the fact that the event nucleated in the middle to lower crust, may be interpreted as the reactivation of a fault structure that was inherited from previous tectonic phases.

Source process and tectonic implications of the 8 January 2006 ( Mw 6.7) Kythira earthquake, southern Greece

On 8 January 2006 at 11:34 (GMT) a strong ( M w 6.7) subcrustal earthquake occurred near Kythira island in southern Greece, causing damage to the surrounding area while the shock was also felt in most of the eastern Mediterranean. The event and its aftershocks were recorded by a regional network of 31 stations equipped with three-component broad-band seismometers that cover most parts of the Greek region. Initial locations of the earthquake sequence showed a diffuse pattern, therefore catalog and cross-correlation differential travel times were input to a shrinking grid-search relocation algorithm in order to obtain accurate relative locations. After this relocation most aftershocks form a tight cluster around the mainshock at depths between 44 and 53 km. Subsequently, waveforms from 12 stations giving the best possible azimuthal coverage around the Kythira event were inverted in order to investigate its source process. A parallel, non-negative least-squares inversion technique utilizing multiple time windows was used to derive the spatiotemporal slip distribution. The fault surface was parameterized by subfaults of 3 km × 3 km size covering a length and down-dip width of 91 and 100 km, respectively. Inversion results indicate that the fault plane is probably the one dipping towards SE. The slip distribution model showed that there are two areas of large slip (∼60 cm), the largest extending between 38 and 80 km depth and a smaller one at depths 65–82 km. The moment rate function consists of a large initial peak with a duration of about 11 s followed by a number of smaller peaks, probably due to coseismic slip on some of the subfaults. Furthermore, the hypocenters of the relocated aftershocks coincide with the areas of minimum slip (<10 cm) while the hypocenter of the mainshock is located near the edge of the larger slip patch. Sensitivity and resolution tests confirmed the adequacy of the inversion parameters used and the stability of the solution. The Kythira earthquake was probably caused by the contortion of the downgoing African slab that results in the development of along-strike compression forces in the Kythira strait.

A compact representation of spatio-temporal slip distribution on a rupturing fault

We propose a mathematical representation to qualitatively describe the spatio-temporal slip evolution during earthquake rupture in an efficient and easy-to-use manner for numerical simulations of strong ground motion. It is based on three basis functions and associated expansion coefficients. It is an extension of the approach of Ide and Takeo, (J Geophys Res, 102:27379–27391, 1997). We compare our approach and theirs using simple kinematic source models to illustrate differences between the two approaches, and show that our approach more accurately represents the spatio-temporal slip evolution. We also propose a technique based on our representation for extracting a spatio-temporal slip velocity function from a kinematic source model obtained by the conventional source inversion. We then demonstrate the feasibility of our procedure with application to an inverted source model of the 26 March 1997 Northwestern Kagoshima, Japan, earthquake (MW6.1). In the simulations for actual earthquakes, source models obtained from kinematic source inversions are commonly employed. Our scheme could be used as an interpolation method of slip time functions from relatively coarse finite-source models obtained by conventional kinematic source inversions.

Three‐dimensional dense strong motion waveform inversion for the rupture process of the 1999 Chi‐Chi, Taiwan, earthquake

We inverted the high‐resolution spatiotemporal slip distribution of the 21 September 1999 Chi‐Chi, Taiwan, earthquake utilizing data from densely distributed island‐wide strong motion stations for a three‐dimensional (3‐D) fault geometry, and 3‐D Green's functions calculations based upon parallel nonnegative least squares inversion. The 3‐D fault geometry, consistent with high‐resolution reflection profile, is determined from GPS inversion and aftershocks distribution. This 3‐D fault model presents the dip angle gradually becoming shallower from south to north along the fault and near flat at the deeper portion of the fault. The 3‐D Green's functions are calculated through numerical wavefield simulation from three‐dimensional heterogeneous velocity structure derived from tomography studies. The Green's functions show significant azimuthal variations and suggest the necessity of lateral heterogeneity in velocity structure. Considering complex fault geometry and Green's functions in full 3‐D scale, we invert the spatial/temporal slip distribution of the 1999 Chi‐Chi earthquake using the best available and most densely populated strong motion waveform data. We perform the inversion under a parallel environment utilizing multiple‐time window to manage the large data volume and source parameters. Results indicate that most slip occurred at the shallower portion of the fault above the decollement. Two major asperities are found, one in the middle of the fault and another one at the northern portion of the fault near the bend in the fault trace. The slip in the southern portion of the fault shows a relatively low slip rate with longer time duration, while the slip in the northern portion of the fault shows a large slip rate with shorter time duration. The synthetics explain the observations well for the island‐wide distributed strong motion stations. This comprehensive study emphasizes the importance of realistic fault geometry, 3‐D Green's functions, and parallel inversion technique in correctly accounting for both the detailed source rupture process and its relationship with the strong ground motion of this intense earthquake.

Source rupture process of the 2003 Tokachi-oki earthquake determined by joint inversion of teleseismic body wave and strong ground motion data

The spatio-temporal slip distribution of the 2003 Tokachi-oki, Japan, earthquake was estimated from teleseismic body wave and strong ground motion data. To perform stable inversion, we applied smoothing constraints to the slip distribution with respect to time and space, and determined the optimal weights of constraints using an optimized Akaike’s Bayesian Information Criterion (ABIC). We found that the rupture propagates mainly along the dip direction, and the length of the rupture area is shorter than its width. The mean rise time in the shallow asperity is significantly longer than that in the deep asperity, which might be attributed to variable frictional properties or lower strength of the plate interface at shallower depths. The average rupture velocity of deep asperity extends to the shear-wave velocity. The derived source parameters are as follows: seismic moment Mo = 1.7×1021 Nm (Mw 8.0); source duration = 50 sec. We also estimated the shear stress change due to the mainshock on and around the major fault zone. It appears that many aftershocks on the plate boundary took place in and adjacent to the zones of stress increase due to the rupture of the mainshock.

Rupture Process of the 1999 Kocaeli, Turkey, Earthquake Estimated from Strong-Motion Waveforms

The multiple time-window linear waveform inversion is applied to study the source process of the 1999 Kocaeli, Turkey, earthquake using strong-motion data. Two peculiar observations characterize this event. The first one is a conflicting observation of the long surface ruptures and short source duration resulting from the observed strong-motion waveforms. The second is an anomalously short S - P time observed at a station SKR, considering its hypocentral distance. A supershear rupture propagation (Ellsworth and Celebi, 1999) and the P -wave triggering of an asperity (Anderson et al., 2000) were both proposed to explain the record at SKR. Our waveform inversion study is aimed to examine these possibilities in order to clarify the relation between the spatial distribution of the surface ruptures and coseismic slips. We divide the assumed fault plane into three parts and look for the first time-window front propagation velocity (propagation velocity of triggering front of the first time window at each subfault) in each part by performing numerous inversions, changing the first time-window front propagation velocity separately in each part, and selecting the best first time-window propagation velocities that give largest variance reduction. At the same time, appropriate smoothness of spatiotemporal slip distribution is searched for by performing inversions with different strength of smoothing constraints against the observational equations and judging the relative appropriateness of inversion results in terms of the Akaike's Bayesian Information Criterion. The best source model is characterized by an asperity that is about 35 km east of the epicenter and near a segment boundary at Sapanca Lake, triggered by the first time-window front with 5.8 km/sec, close to P -wave velocity. The time progression of the rupture suggests a scenario where a P wave arriving from the hypocenter or somewhere around the hypocenter triggered the second largest asperity near the junction of the segments at Sapanca Lake, which is a preferable interpretation. The first time-window propagation velocities west of the hypocenter and east of SKR are expected to be 3.0 km/sec. Manuscript received 20 August 2000.

Determination of constitutive relations of fault slip based on seismic wave analysis

Constitutive laws define the boundary conditions on fault plane and govern many aspects of earthquake failure. Although several constitutive laws have been formulated based on laboratory rock experiments and applied to theoretical studies in various fields, no actual relation during a natural earthquake has been determined. The 1995 Kobe earthquake is suitable for detailed kinematic analysis, and this enables the first evaluation of constitutive relations for a natural earthquake. In this study, we determine spatiotemporal slip distribution on an assumed fault plane of the 1995 Kobe earthquake by waveform inversion and then solve elastodynamic equations using a finite difference method to determine the stress distribution and constitutive relations on the fault plane. An inversion method based on Bayes theorem is employed to obtain a spatiotemporal slip distribution, and enables us to ensure the objective uniqueness of the solution with numerous parameters and smoothing constraints. This slip distribution is then used as part of the boundary condition in the finite difference calculation. The time histories of slip and shear stress obtained then provide a constitutive relation at each point on the fault plane. They show slip weakening relations almost everywhere on the fault plane, while slip rate dependency is not clear. The slip weakening behavior has a clear depth dependency indicating that the slip weakening rate (dτ/du) is smaller in the shallow crust than that in the deep crust. This may be associated with the paucity of shallow seismicity observed in the source region of this earthquake as reported for many mature fault systems.