Slip Area Research Articles

Shocks in a box: An analogue model of subduction earthquake cycles with application to seismotectonic forearc evolution

We introduce and test an experimental approach to simulate elastoplastic megathrust earthquake cycles using an analogue model and apply it to study the seismotectonic evolution of subduction zones. The quasi‐two‐dimensional analogue model features rate‐ and state‐dependent elastic‐frictional plastic and viscoelastic material properties and is scaled for gravity, inertia, elasticity, friction, and viscosity. The experiments are monitored with a high‐resolution strain analysis tool based on digital image correlation (particle imaging velocimetry, PIV), providing deformation time series comparable to seismologic, geodetic, and geologic observations. In order to separate elastic and nonelastic effects inherent the experimental deformation patterns, we integrate elastic dislocation modeling (EDM) into a hybrid approach: we use the analogue earthquake slip and interseismic locking distribution as EDM dislocation input and forward model the coseismic and interseismic elastic response. The residual, which remains when the EDM prediction is subtracted from the experimental deformation pattern, highlights the accumulation of permanent deformation in the model. The setup generates analogue earthquake sequences with realistic source mechanisms and elastic forearc response and recurrence patterns and reproduces principal earthquake scaling relations. By applying the model to an accretionary‐type plate margin, we demonstrate how strain localization at the rupture peripheries may lead to a seismotectonically segmented forearc, including a tectonically stable shelf and coastal high (<20% plate convergence accommodated by internal shortening) overlying the area of large megathrust earthquake slip. Fifty to 75% of plate convergence is accommodated by internal shortening in the slope region where earthquake slip tapers out toward the trench. The inner forearc region remains undeformed and represents a basin.

Spatial heterogeneity of the mantle wedge structure and interplate coupling in the NE Japan forearc region

We performed 3‐D seismic tomography in the forearc region of the northeastern Japan subduction zone using both onshore and offshore seismic station data. We obtained the Vp, Vs, and Vp/Vs structures around the plate boundary with high spatial resolution. The position of the plate boundary as defined by relocated hypocenters coincides with the sharp velocity boundary between the oceanic crust and the mantle wedge. The mantle wedge above the coseismic slip area of the 1978 and 2005 off‐Miyagi interplate earthquakes (M > 7) is characterized by high Vp and Vs, but low Vp/Vs. Off Fukushima, however, where large earthquakes rarely occur, we found a high Vp/Vs anomaly at the tip of the mantle wedge. The spatial distribution of serpentinized mantle wedge limits the spatial extent of the strongly coupled area on the plate boundary, and thus can explain the difference in seismic activity between the off‐Miyagi and off‐Fukushima regions.

Finite-Fault Analysis of the 2004 Parkfield, California, Earthquake Using Pnl Waveforms

We apply a kinematic finite-fault inversion scheme to Pnl displacement waveforms recorded at 14 regional stations (Δ < 2°) to recover the distribution of coseismic slip for the 2004 Parkfield earthquake using both synthetic Green's func- tions (SGFs) calculated for one-dimensional (1D) crustal-velocity models and empiri- cal Green's functions (EGFs) based on the recordings of a single Mw 5.0 aftershock. Slip is modeled on a rectangular fault subdivided into 2 × 2 km subfaults assuming a constant rupture velocity and a 0.5 sec rise time. A passband filter of 0.1-0.5 Hz is applied to both data and subfault responses prior to waveform inversion. The SGF inversions are performed such that the final seismic moment is consistent with the known magnitude (Mw 6.0) of the earthquake. For these runs, it is difficult to repro- duce the entire Pnl waveform due to inaccuracies in the assumed crustal structure. Also, the misfit between observed and predicted vertical waveforms is similar in char- acter for different rupture velocities, indicating that neither the rupture velocity nor the exact position of slip sources along the fault can be uniquely identified. The pattern of coseismic slip, however, compares well with independent source models derived using other data types, indicating that the SGF inversion procedure provides a general first- order estimate of the 2004 Parkfield rupture using the vertical Pnl records. The best- constrained slip model is obtained using the single-aftershock EGF approach. In this case, the waveforms are very well reproduced for both vertical and horizontal com- ponents, suggesting that the method provides a powerful tool for estimating the dis- tribution of coseismic slip using the regional Pnl waveforms. The inferred slip model shows a localized patch of high slip (55 cm peak) near the hypocenter and a larger slip area (∼50 cm peak) extending between 6 and 20 km to the northwest.

Characterization of the 2007 Noto Hanto, Japan, earthquake

AbstractCharacterization of the 2007 Noto Hanto, Japan, earthquake (MS6.9), a moderate-size crustal event, was performed. The rupture process was firstly inferred from strong motion data. Inversion analysis revealed that the overall rupture finished within 6 s, and the seismic moment of this earthquake was estimated to be 1.1×1019N m (Mw6.6). Two areas of large slip and stress drop (asperities) were inferred on the fault plane. The maximum static stress drop calculated from the derived slip distribution exceeded 16 MPa for the major asperity, and the minor asperity has a similar maximum value. An area of negative stress drop corresponding to the distribution of small slip exists between the two asperities. This strongly suggests that the fault is segmented. A boundary between surface faults was located above the major asperity, but an area of negative stress drop appeared between the asperity and fault boundary. This suggests that the configuration of the surface faults reflects only a shallow part of the causative fault. The ratio of radiated energy to seismic moment was also estimated for the purpose of estimation of fault activity. Comparison of the derived value with those of other Japanese earthquakes suggests that the causative fault of the Noto Hanto earthquake is not very active.

Rupture process of the 2007 Notohanto earthquake by using an isochrones back-projection method and K-NET/KiK-net data

Abstract We estimated the source process of the 2007/3/25 Notohanto earthquake using a new method for source imaging based on an “isochrones-backprojection” of observed seismograms in the source region (IBM). The IBM differs from conventional earthquake source modeling approaches in that no inversion procedures are required. The idea of IBM is to directly back-project amplitudes of seismogram envelopes around the source into a space image of the earthquake rupture. The method requires the calculation of isochrones times at every station used for source imaging, for a set of grids points distributed within the source fault plane. Total grid “brightness” is calculated by adding all observed waveform envelope amplitudes at every station, for every isochrone line crossing the grid, in order to produce an image of the total fault plane brightness distribution. Our source imaging results of the Notohanto earthquake show two large brightness regions; the first region is located 10 km above the hypocenter, and the second region is located at the bottom of the northern end of the fault plane. These regions approximately correspond to large slip areas obtained by a conventional inversion approach. Our method has the capability to quickly map asperities of large earthquakes using observed strong motion data.

Postseismic displacement of the 1999 Athens earthquake retrieved by the Differential Interferometry by Synthetic Aperture Radar time series

In September 1999, a moderate (Mw = 5.9) earthquake struck the Attica plain, causing unexpected and extensive damage to Athens and its population. In this work, we exploit the potential of multitemporal Differential Interferometry by Synthetic Aperture Radar (DInSAR) analysis, using about a hundred European Remote Sensing (ERS) 1/2 images to calculate the displacement time series from 1992 to 2002. This analysis allows us to clearly separate a strictly coseismic signal from a postseismic gradual subsidence, reaching a maximum value of about 3 cm in the following 2.5 years. We model this signal in terms of afterslip on the seismogenic fault. The afterslip distribution, retrieved by linear inversion, reflects the coseismic slip distribution and occurs mainly downdip of the area that ruptured during the main shock. The analysis of the static stress transfer suggests that the afterslip was triggered by the main shock, then it propagated aseismically through the fault plane. A partial overlap between the coseismic and aseismic slip area at the hypocentral region indicates that the 1999 rupture surface was not “healed” at least until the date of the last postseismic image (April 2002). The results obtained with a time series approach for this moderate magnitude earthquake suggest that multitemporal DInSAR analysis should become an important methodology for the study of large earthquake ruptures.

Source Rupture Processes of 3 Jiashi Ms6 Events (1998~2003) and Its Correlation with the Aftershock Activity

AbstractBy using far field body wave data, the temporal and spatial rupture processes of 3 Ms6 events with different focal mechanisms occurred in Jiashi, Xinjiang autonomous region, China, from 1998 to 2003 were studied. The rupture pattern of Aug. 2, 1998 reveals that the rupture zone is small, the rupture process is simple and lasted about 8 s with the maximum slip 12 cm. The rupture scenario of Aug. 27, 1998 demonstrates a bilateral rupture characteristic, the process finished within 13 s with the maximum slip of 63 cm. The rupture process of Ms6.8 on Feb. 24 2003 is the most complicated among them. Its source time function is composed by 2 rise periods, and totally lasted about 32 s, with the maximum slip displacement of 30 cm. Our study reveals that aftershocks following the corresponding main shock occurred along the edge of the large slip area, or on the area of slip rapidly decreased. Aftershocks following the main shocks within the first 10 days exhibit focal mechanisms coincidental with the direction of the slip vector on the fault plane of the main shock, indicating that the location and mechanisms of aftershocks following the main shock are related with the stress redistribution resulted by the main shock

Numerical simulation of recurrence of asperity rupture in the Sanriku region, northeastern Japan

A numerical simulation study is presented for the recurrence of great earthquakes and aseismic sliding in the Sanriku region, northeastern Japan, where the Pacific Plate subducts beneath northern Honshu and M8 class earthquakes have occurred approximately every 100 years. The friction on the plate interface is assumed to obey a rate‐ and state‐dependent friction law, and the spatial distribution of constitutive parameters of the friction law is estimated, so that the simulated slip histories mimic those estimated from seismic and geodetic observations. By introducing two large asperities that have velocity‐weakening frictional properties, earthquakes similar to the 1968 Tokachi‐oki earthquake (Mw = 8.2) and the 1994 Sanriku‐oki earthquake (Mw = 7.7) can be modeled. The former broke both the large asperities, and the latter broke one of them. The ruptures of the two simulated large earthquakes started from almost the same location, and their nucleation processes are similar to each other. Two smaller asperities were introduced to represent smaller earthquakes. One of the small asperities is for the largest aftershock (Mw = 6.9) of the 1994 Sanriku‐oki earthquake, which was located westward from the 1994 main shock area and took place 10 d after the main shock. The present simulation suggests that the largest aftershock was triggered by stress caused by propagation of postseismic sliding. In the simulation, the area of greatest intensity of postseismic sliding is located in the region between the main shock slip area and the largest aftershock area, which is consistent with the estimation from GPS data analyses for the postseismic sliding of the 1994 Sanriku‐oki earthquake. Some important characteristics of seismic and aseismic processes, as estimated from observations, are reproduced in the model. This suggests that spatially nonuniform frictional properties on the plate interface can be estimated to some extent by comparison between model results and observed data.

Subduction process of the Philippine Sea Plate off the Kanto district, central Japan, as revealed by plate structure and repeating earthquakes

The structure of the Philippine Sea Plate in the offshore region adjacent to the Boso Peninsula in central Japan is resolved by comparison of recent oceanic seismic survey data with natural repeating earthquake data. This region is characterized by a range of seismic and aseismic phenomena associated with subduction motion of the Philippine Sea Plate, including slow slip events (SSEs), large backslip events, and repeating earthquakes. This offshore region is also the site of the largest aftershock of the 1923 Kanto earthquake (M7.9). The detailed structure of the Philippine Sea Plate is successfully resolved, and the upper boundary of plate is clearly traceable to depths of 20 km. The reflection intensity of the upper boundary varies spatially in accordance with the depth of the reflector. The upper boundary continues smoothly with a line of repeating earthquake hypocenters regarded as an indicator of subduction shear. A continuous model extending into the offshore region can thus be proposed. The variation in reflection intensity on the upper boundary of the Philippine Sea Plate also correlates well with variation in event types. A weak reflection zone overlaps the area of large backslip, while a moderately strong reflection zone encroaches into the large slip area of the Boso SSE. The reflectivity of the upper boundary is thus considered to be dependent on the physical properties and conditions of the reflector, which in turn affect the event types on the plate boundary. A source fault model is proposed for the largest aftershock of the 1923 Kanto earthquake. Based on geodetic data, the source fault is located eastward of the mainshock, with dimensions of 40 × 50 km and moment magnitude of 7.5. The source fault lies within the region of large backslip and the large slip area of the Boso SSE.

A study of the 2006 and 2007 earthquake sequence of Pisco, Peru, with InSAR and teleseismic data

We combine interferometric synthetic aperture radar (InSAR) and teleseismic body waves to study the largest earthquake (Mw 8.1) in a sequence of events on the subduction megathrust near Pisco, Peru. Our analysis includes some of the first InSAR data from the ALOS satellite and wide swath data from the Envisat satellite. The teleseismic data indicate the slip maximum occurred 60–90 seconds after the mainshock started. The InSAR data constrain the main slip patch to be about 70 km from the hypocenter, suggesting an extremely low rupture velocity (&lt;1.5 km/s) or long slip rise time. No large earthquake has occurred in the 2007 rupture area since at least 1746 and possibly 1687, suggesting significant aseismic deformation in the area. The slip deficit apparently cannot be filled with rapid after‐slip. In addition, the area where the Nazca Ridge is subducting appears to be either a seismic gap or a persistent area of aseismic slip.

Modeling the frictional boundary condition in friction stir welding

Material flow and frictional heating in friction stir welding are investigated using a three-dimensional numerical model. Two mechanical boundary conditions are investigated, including a sticking constant velocity and a slipping variable shear stress model. For the constant velocity model, material in contact with the tool is set at a velocity equal to some fraction of the tool rotational speed. The variable shear stress model is formulated such that it degenerates to Coulomb friction under low forces, but as forces at the tool increase, shear stresses at the tool/material boundary are limited according to Tresca friction. The variable shear condition model permits areas of significant slip, while shear stresses at other regions approach the flow stress of the material. The boundary models are compared with experimental data from flow visualization experiments and thermocouple measurements from plasticine welds. Results show that the variable shear model is superior to a sticking condition. With the variable shear model, the peak temperature in the weld compares well with thermocouple measurements. The variable shear model predicts that material simply extrudes around the tool, while the constant velocity model shows a region of material that rotates with the tool. Maximum velocities with the variable shear stress model are only 9% of the rotational speed of the tool and agree well with experimental findings. Additionally, the variable shear model shows a region of diminishing shear stress, velocity, and pressure at the back advancing side of the pin, suggesting formation of an internal void. The limited deformation, low velocities, and suggestion of void formation agree well with flow visualization studies using plasticine under identical operating parameters.

A dislocation dynamics study of the strength of stacking fault tetrahedra. Part I: interactions with screw dislocations

We present a comprehensive dislocation dynamics (DD) study of the strength of stacking fault tetrahedra (SFT) to screw dislocation glide in fcc Cu. Our methodology explicitly accounts for partial dislocation reactions in fcc crystals, which allows us to provide more detailed insights into the dislocation–SFT processes than previous DD studies. The resistance due to stacking fault surfaces to dislocation cutting has been computed using atomistic simulations and added in the form of a point stress to our DD methodology. We obtain a value of 1658.9 MPa, which translates into an extra force resolved on the glide plane that dislocations must overcome before they can penetrate SFTs. In fact, we see they do not, leading to two well differentiated regimes: (i) partial dislocation reactions, resulting in partial SFT damage, and (ii) impenetrable SFT resulting in the creation of Orowan loops. We obtain SFT strength maps as a function of dislocation glide plane-SFT intersection height, interaction orientation, and dislocation line length. In general SFTs are weaker obstacles the smaller the encountered triangular area is, which has allowed us to derive simple scaling laws with the slipped area as the only variable. These laws suffice to explain all strength curves and are used to derive a simple model of dislocation–SFT strength. The stresses required to break through obstacles in the 2.5–4.8-nm size range have been computed to be 100–300 MPa, in good agreement with some experimental estimations and molecular dynamics calculations.

Fault model of the 2007 Noto Hanto earthquake estimated from PALSAR radar interferometry and GPS data

Abstract The 2007 Noto Hanto earthquake (M w 6.6) is the first major earthquake in inland Japan since the launch of the Japanese ALOS satellite. Interferometric synthetic aperture radar successfully mapped the coseismic displacements from both ascending and descending orbits in and around the earthquake epicentral region. The line-of-sight displacements toward the satellite from the ascending and descending orbits amount to about 50 and 20 cm, respectively. The interferograms as well as the GPS data of the GEONET network are inverted in order to determine the slip distribution on the source fault. The dip and strike angles of the preferred model are 48° and N51°E, respectively, and its fault slip area reaches the seafloor. The dip angle is not well constrained in our tested range between 40° and 60°. The dominant slip area is located just above the hypocenter, at depths between 4 and 10 km. Many aftershocks seem to have occurred on and around the area of dominant slip.

Crustal deformation associated with the Noto Hanto Earthquake in 2007 in Japan

Abstract The Global Positioning System (GPS) network in Japan detected coseismic deformation from the 2007 Noto Hanto earthquake (M w = 6.9). The result indicates a 21-cm southwestward displacement and a 7-cm upheaval at the GPS site near the epicenter. Synthetic Aperture Interferometry (InSAR) shows an approximately 50-cm movement toward the “Daichi” satellite, in ascending orbit, near the epicenter. The estimated fault slip distribution based on GPS and InSAR results shows a large slip area ranging up to 2 m near and northeast of the hypocenter. The slip area beneath the Sea of Japan southwest of the hypocenter also shows large slippage of around 2 m. Aftershocks are distributed in the asperity area and its vicinity, demonstrating that the asperity hypothesis clearly does not hold completely for this earthquake. Computed vertical displacements using the model are consistent with the geomorphological evidence of long-term uplift.

Dynamic modeling of the 2004 Mw 6.0 Parkfield, California, earthquake

We present two spontaneous rupture models of the 2004 Mw 6.0 Parkfield earthquake constrained by near‐source ground motions. We start with a stress drop distribution calculated from a kinematic slip distribution. Using a linear slip‐weakening friction law, we utilize trial and error to obtain both the stress conditions and frictional parameters on the fault that produce synthetics consistent with records. The material contrast across the San Andreas Fault is incorporated using different one‐dimensional velocity structures on each side of the fault. An approximately constant S parameter of 0.3 and a uniform slip‐weakening distance of 0.15 m are used in the dynamic models. In our preferred dynamic model, consistent with the ground motion and GPS, the slip is bounded by seismicity streaks at 5 and 10 km depths, confirming a locked zone at depth. The stress drop is approximately 10 MPa in the hypocentral region and about 2 MPa elsewhere. The material contrast across the fault causes significant normal stress variations (∼1 MPa), leading to a larger strength drop to the southeast than to the northwest. The main rupture front propagates at nearly a constant subshear rupture velocity ∼3 km/s in both directions. The total radiated energy determined from the preferred dynamic model is 1.1 × 1013 J, seismic moment is 1.0 × 1018 Nm, and fracture energy is 3.0 × 1013 J. The limited number of aftershocks in the slipped area suggests the important role of stress on the distribution of seismicity in the locked zone.

Occurrence of quasi-periodic slow-slip off the east coast of the Boso peninsula, Central Japan

Abstract An analysis of Global Positioning System (GPS) data revealed south-southeastward transient deformation caused by the expected slow-slip near the Boso peninsula, central Japan, approximately 5 years after a similar event in 2002. An area of aseismic slip with a moment magnitude of 6.6 was estimated off the Boso peninsula, adjacent to the area of associated seismic activity. The 2007 aseismic slip started from around August 10, expanded slightly to the north, and gradually ceased activity over a period of about 10 days. This rupture process is different from those of the last two events in which the slow-slip area moved from north to south. However, the three slow-slip events detected by the GPS array verify the hypothesis that the Boso slow-slips occur quasi-periodically in a certain area, accompanied by seismic swarms. The factors causing the slight differences in the rupture process, magnitude, recurrence time, and slip area among the observed slow-slip events remain unclear.

Debris flow affecting the Cubatão Oil Refinery, Brazil

On February 6, 1994, a large debris flow developed because of intense rains in a 800-m-high mountain range called Serra do Cubatao, the local name for the Serra do Mar, located along the coast of the state of Sao Paulo, Brazil. It affected the Presidente Bernardes Refinery, owned by Petrobras, in Cubatao. The damages amounted to about US $40 million because of the muck cleaning, repairs, and 3-week interruption of the operations. This prompted Petrobras to conduct studies, carried out by the authors, to develop protection works, which were done at a cost of approximately US $12 million. The paper describes the studies conducted on debris flow mechanics. A new criteria to define rainfall intensities that trigger debris flows is presented, as well as a correlation of slipped area with soil porosity and rain intensity. Also presented are (a) an actual grain size distribution of a deposited material, determined by laboratory and a large-scale field test, and (b) the size distribution of large boulders along the river bed. Based on theory, empirical experience and back-analysis of the events, the main parameters as the front velocity, the peak discharge and the volume of the transported sediments were determined in a rational basis for the design of the protection works. Finally, the paper describes the set of the protection works built, emphasizing their concept and function. They also included some low-cost innovative works.

Source model of the 2005 Miyagi-Oki, Japan, earthquake estimated from broadband strong motions

AbstractWe estimate the source model of the 2005 Miyagi-Oki earthquake, which consists of two source patches called “strong motion generation areas (SMGAs)”, to explain the broadband strong motions. The location of the first SMGA is determined from the arrival time differences between the first motion and the first pulse in theP-wave portion. Based on the broadband waveform modeling obtained using the empirical Green’s function method, we estimate the parameters of the two SMGAs. The estimated location of the two SMGAs coincides with the two major large slip areas inferred from the inversion of strong-motion and teleseismic waveforms, which implies that the broadband strong motion of the 2005 Miyagi-Oki earthquake mainly radiates from the asperities. The deeper SMGA has a larger stress drop (34.1 MPa) than the shallower one (17.6 MPa). Comparison of the locations of SMGAs between the 2005 and 1978 Miyagi-Oki earthquakes suggests that they do not overlap with each other. The interplate earthquakes, including this event, were found to have smaller SMGAs than those predicted from the relationship of crustal earthquakes for a given seismic moment. This implies that the stress drop of the SMGAs for interplate earthquakes is larger than that for crustal earthquakes.

Source parameters of a M4.8 and its accompanying repeating earthquakes off Kamaishi, NE Japan: Implications for the hierarchical structure of asperities and earthquake cycle

We determine the source parameters of a M4.9 ± 0.1 ‘characteristic earthquake’ sequence and its accompanying microearthquakes at ∼50 km depth on the subduction plate boundary offshore of Kamaishi, NE Japan. The microearthquakes tend to occur more frequently in the latter half of the recurrence intervals of the M4.9 ± 0.1 events. Our results show that the microearthquakes are repeating events and they are located not only around but also within the slip area for the 2001 M4.8 event. From the hierarchical structure of slip areas and smaller stress drops for the microearthquakes compared to the M4.8 event, we infer the small repeating earthquakes rupture relatively weak patches in and around the slip area for the M4.8 event and their activity reflects a stress concentration process and/or change in frictional property (healing) at the area. We also infer the patches for the M4.9 ± 0.1 and other repeating earthquakes undergo aseismic slip during their interseismic period.

New approach in the kinematick−2source model for generating physical slip velocity functions

SUMMARY In an attempt to improve the ground motion modelling, the characteristics of the slip velocity functions (SVF) generated using the kinematic k−2 source are investigated and compared to the dynamic solutions proposed in the literature. Several numerical simulations were performed to test the influence of the model parameters on the SVF modelling. Overall, the shapes of SVF are very complex and exhibit a large variability in time and space. However, we found out that the mean SVF is a simple boxcar with duration equal to the largest rise time value. In the areas of weak slip, the SVFs are characterized by the existence of negative values, whereas in large slip areas, the SVF is more impulsive. Overall, on the examples investigated, the SVFs modelled with this k−2 source model are different from a typical Kostrov's solution. The critical analysis of the kinematic k−2 source led us to identify the Fourier decomposition of the slip to be responsible for these difficulties, and to propose a new recombination scheme. It consists of adding a positive correction to the Fourier slip components. The slip is described as the sum of positive contributions at various scales. The SVFs modelled using this new scheme are greatly improved. Moreover, through several parametrical analyses performed to qualify this new approach, we show that the SVF are corrected while preserving the essential quality of the k−2 modelling, that is, the ω2 spectral shape and Cd apparent directivity of the synthetic accelerograms. Strong ground motion modelling in the near-fault region was made and numerical ground motion parameters were compared to the empirical relationships. We show that predicted peak ground motion is consistent with near-source attenuation laws.