Static Stress Drop Research Articles

Dependence of earthquake stress drop on critical slip‐weakening distance

Numerical simulations are conducted to investigate the dependence of the static stress drop of earthquakes on the critical slip‐weakening distance. A fault model in a two‐dimensional elastic medium is used, in which a locked zone (asperity) is assumed to exist between creeping zones. The presence of such a zone is commonly assumed for plate boundaries that have relatively low seismic coupling. Shear stress is concentrated at the edges of the locked zone and seismic rupture occurs when the strain energy release by rupture extension becomes sufficiently large to overcome the fracture energy. The static stress drop is calculated by taking the ratio of the average seismic slip to the length of seismic slip zone for the simulated earthquakes in the model, using various values for the normal stress applied to the fault and the characteristic slip distance of a rate‐ and state‐dependent friction law. The simulation results indicate that the static stress drop is proportional to the square root of the product of the critical slip‐weakening distance and the normal stress, when the seismic rupture starts near an edge of the locked zone. In contrast, when the seismic rupture starts near the center of the locked zone, the static stress drop increases linearly with normal stress and is independent of the critical slip‐weakening distance. These simulation results may explain the relatively small depth dependence, as well as the large scatter, in observations of the static stress drop.

Estimating the Radiated Seismic Energy of the 2010 ML 6.4 JiaSian, Taiwan, Earthquake Using Multiple-Event Analysis

Through a proposed multiple-event analysis of teleseismic P-waves, this study investigated the radiated seismic energy and rupture process of the 2010 JiaSian earthquake. Results showed that the earthquake comprised at least two sub-events. The first sub-event was followed by the second sub-event, ~1.7 s later. The entire source duration was 5.4 s. Let the two sub-events occur on the same fault plane with a strike of 304° and a dip of 28°; the first sub-event had a relatively smaller seismic moment (M(subscript 0)) and larger radiated seismic energy (E(subscript S)) than the second sub-event, and this leads to the E(subscript S)/M(subscript 0) of the first sub-event larger than that of the second sub-event thus. This feature implies that the first sub-event probably had a higher static stress drop during faulting. The total M(subscript 0) was estimated to be 2.17 × 10^18 Nm, corresponding to M(subscript W) = 6.15, and the total E(subscript S) was ~2.91 × 10^13 Nm, larger than that estimated only from a single source. Subsequently, the E(subscript S)/M(subscript 0) was approximately 1.3 × 10^(-5), lower than ordinary earthquakes. The low static stress drop was probably responsible for the low E(subscript S)/M(subscript 0). Overall, the 2010 JiaSian earthquake was characterized by a relatively low E(subscript S)/M(subscript 0) and low static stress drop, and then the partial stress drop model would be relatively appropriate to interpret its rupture process.

Earthquake Repeat Time, Stress Drop, Type of Slip and Earthquake Magnitude

The stress field along a plate margin is one of the basic tools for forecasting the magnitude of future earthquakes. Tectonic features like faulting, folding, fracturing and volcanoes are the results of stresses that build up along different fault lines across the world. Stress drop is a fundamental parameter of earthquakes. It is important to know how much portion of stress energy is released by an earthquake. Another important source parameter of earthquakes is repeat time, or the reoccurrence period of earthquakes. The reoccurrence period or repeat time of earthquakes is controlled by the rate of tectonic loading which is the long term slip rate, stress accumulation and stress drop. Earthquakes with long reoccurrence period have a higher average stress drop than those with short repeat time. The probability of reoccurrence of earthquakes is time dependent; which defines that some period is required to recharge stresses before they are released by a large earthquake. Time dependency of reoccurrence of a large earthquake is the base of earthquake forecasting. In this project earthquake repeat time and stress drop is determine for different fault types. Results of this study provide a method to estimate important parameters of active faults including static and dynamic stress drop, rupture length, width and earthquake magnitudes. These parameters can be used for future earthquake forecasting and prediction.

Observed source parameters for dynamic rupture with non-uniform initial stress and relatively high fracture energy

We have conducted dynamic rupture propagation experiments to establish the relations between in-source stress drop, fracture energy and the resulting particle velocity during slip of an unconfined 2 m long laboratory fault at normal stresses between 4 and 8 MPa. To produce high fracture energy in the source we use a rough fault that has a large slip weakening distance. An artifact of the high fracture energy is that the nucleation zone is large such that precursory slip reduces fault strength over a large fraction of the total fault length prior to dynamic rupture, making the initial stress non-uniform. Shear stress, particle velocity, fault slip and acceleration were recorded coseismically at multiple locations along strike and at small fault-normal distances. Stress drop increases weakly with normal stress. Average slip rate depends linearly on the fault strength loss and on static stress drop, both with a nonzero intercept. A minimum fracture energy of 1.8 J/m2 and a linear slip weakening distance of 33 μm are inferred from the intercept. The large slip weakening distance also affects the average slip rate which is reduced by in-source energy dissipation from on-fault fracture energy.Because of the low normal stress and small per event slip (∼86 μm), no thermal weakening such as melting or pore fluid pressurization occurs in these experiments. Despite the relatively high fracture energy, and the very low heat production, energy partitioning during these laboratory earthquakes is very similar to typical earthquake source properties. The product of fracture energy and fault area is larger than the radiated energy. Seismic efficiency is low at ∼2%. The ratio of apparent stress to static stress drop is ∼27%, consistent with measured overshoot. The fracture efficiency is ∼33%. The static and dynamic stress drops when extrapolated to crustal stresses are 2–7.3 MPa and in the range of typical earthquake stress drops. As the relatively high fracture energy reduces the slip velocities in these experiments, the extrapolated average particle velocities for crustal stresses are 0.18–0.6 m/s. That these experiments are consistent with typical earthquake source properties suggests, albeit indirectly, that thermal weakening mechanisms such as thermal pressurization and melting which lead to near complete stress drops, dominate earthquake source properties only for exceptional events unless crustal stresses are low.

Sensitivity of broad-band ground-motion simulations to earthquake source and Earth structure variations: an application to the Messina Straits (Italy)

In this paper, we investigate ground-motion variability due to different faulting approximations and crustal-model parametrizations in the Messina Straits area (Southern Italy). Considering three 1-D velocity models proposed for this region and a total of 72 different source realizations, we compute broad-band (0-10 Hz) synthetics for Mw 7.0 events using a fault plane geometry recently proposed. We explore source complexity in terms of classic kinematic (constant rise-time and rupture speed) and pseudo-dynamic models (variable rise-time and rupture speed). Heterogeneous slip distributions are generated using a Von Karman autocorrelation function. Rise-time variability is related to slip, whereas rupture speed variations are connected to static stress drop. Boxcar, triangle and modified Yoffe are the adopted source time functions. We find that ground-motion variability associated to differences in crustal models is constant and becomes important at intermediate and long periods. On the other hand, source-induced ground-motion variability is negligible at long periods and strong at intermediate-short periods. Using our source-modelling approach and the three different 1-D structural models, we investigate shaking levels for the 1908 Mw 7.1 Messina earthquake adopting a recently proposed model for fault geometry and final slip. Our simulations suggest that peak levels in Messina and Reggio Calabria must have reached 0.6-0.7 g during this earthquake.

Surface rupture during the 2010 Mw 7.1 Darfield (Canterbury) earthquake: Implications for fault rupture dynamics and seismic-hazard analysis

Research Article| January 01, 2012 Surface rupture during the 2010 Mw 7.1 Darfield (Canterbury) earthquake: Implications for fault rupture dynamics and seismic-hazard analysis M. Quigley; M. Quigley 1Department of Geological Sciences, University of Canterbury, 8140 Christchurch, New Zealand Search for other works by this author on: GSW Google Scholar R. Van Dissen; R. Van Dissen 2GNS Science, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar N. Litchfield; N. Litchfield 2GNS Science, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar P. Villamor; P. Villamor 2GNS Science, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar B. Duffy; B. Duffy 1Department of Geological Sciences, University of Canterbury, 8140 Christchurch, New Zealand Search for other works by this author on: GSW Google Scholar D. Barrell; D. Barrell 3GNS Science, Dunedin 9016, New Zealand Search for other works by this author on: GSW Google Scholar K. Furlong; K. Furlong 4Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar T. Stahl; T. Stahl 1Department of Geological Sciences, University of Canterbury, 8140 Christchurch, New Zealand Search for other works by this author on: GSW Google Scholar E. Bilderback; E. Bilderback 1Department of Geological Sciences, University of Canterbury, 8140 Christchurch, New Zealand Search for other works by this author on: GSW Google Scholar D. Noble D. Noble 1Department of Geological Sciences, University of Canterbury, 8140 Christchurch, New Zealand Search for other works by this author on: GSW Google Scholar Geology (2012) 40 (1): 55–58. https://doi.org/10.1130/G32528.1 Article history received: 03 Jun 2011 rev-recd: 06 Aug 2011 accepted: 24 Aug 2011 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. Quigley, R. Van Dissen, N. Litchfield, P. Villamor, B. Duffy, D. Barrell, K. Furlong, T. Stahl, E. Bilderback, D. Noble; Surface rupture during the 2010 Mw 7.1 Darfield (Canterbury) earthquake: Implications for fault rupture dynamics and seismic-hazard analysis. Geology 2012;; 40 (1): 55–58. doi: https://doi.org/10.1130/G32528.1 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 SocietyGeology Search Advanced Search Abstract The September 2010 Mw 7.1 Darfield (Canterbury) earthquake in New Zealand is one of the best-recorded earthquakes of this magnitude. The earthquake occurred on a previously unidentified fault system and generated a 29.5 ± 0.5-km-long surface rupture across a low-relief agricultural landscape. High-accuracy measurements of coseismic displacements were obtained at over 100 localities along the Greendale fault. Maximum net displacement (Dmax) (5.3 ± 0.5 m) and average net displacement (Davg) (2.5 ± 0.1 m) are anomalously large for an earthquake of this Mw. Dmax / surface rupture length (SRL) and Davg/SRL ratios are among the largest ever recorded for a continental strike-slip earthquake. “Geologically derived” estimates of moment magnitude (MwG) are less than the seismologically derived Mw, derived using widely employed SRL-Mw scaling regressions. MwG is greater than Mw using Dmax- and Davg-Mw regressions. The “geologically derived” static stress drop of 13.9 ± 3.7 MPa provides a context with which to compare this earthquake rupture to interplate and intraplate ruptures of similar Mw. This data set provides fundamental information on fault rupture processes relevant to seismic-hazard modeling in this region and analogous settings globally. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Stress Drop during Earthquakes: Effect of Fault Roughness Scaling

We propose that a controlling parameter of static stress drop during an earthquake is related to the scaling properties of the fault-surface topography. Using high resolution laser distance meters, we have accurately measured the roughness scaling properties of two fault surfaces in different geological settings (the French Alps and Nevada). The data show that fault-surface topography is scale dependent and may be accurately described by a self-affine geometry with a slight anisotropy characterized by two extreme roughness exponents ( H R ), H ||=0.6 in the direction of slip and H ⊥=0.8 perpendicular to slip. Disregarding plastic processes like rock fragmentation and focusing on elastic deformation of the topography, which is the dominant mode at large scales, the stress drop is proportional to the deformation, which is a spatial derivative of the slip. The evolution of stress-drop fluctuations on the fault plane can be derived directly from the self-affine property of the fault surface, with the length scale ( λ ) as std Δσ ( λ )∝ λ H R -1. Assuming no characteristic length scale in fault roughness and a rupture cascade model, we show that as the rupture grows, the average stress drop, and its variability should decrease with increasing source dimension. That is for the average stress drop Δσ ( r )∝ r H R -1, where r is the radius of a circular rupture. This result is a direct consequence of the elastic squeeze of fault asperities that induces the largest spatial fluctuations of the shear strength before and after the earthquake at local (small) scales with peculiar spatial correlations.

Rupture Process of the 1964 MJMA 7.5 Niigata Earthquake Estimated from Regional Strong-Motion Records

The source rupture process of the 1964 Niigata earthquake ( M JMA 7.5), Japan, is inferred in detail from regional strong-motion data recorded with low-gain mechanical displacement seismographs. A fault-plane model with high-angle dipping to the west is assumed as a result of reinvestigating the aftershock distribution, and the thickness of the fault zone is determined from recent high-resolution seismicity in the source region. To develop appropriate Green’s functions, a one-dimensional velocity structure model for each station is constructed by modeling the waveforms from recent small earthquakes in the source region. A multiple-time-window technique is applied to invert the observed waveforms for the spatial and temporal slip distribution. The obtained source model indicates large slips mainly in the northern part of the fault plane and secondary slips in the vicinity of the hypocenter and slightly expanding to the south. The total moment release reached ![Graphic][1] ( M w 7.6) and the rupture velocity of the first time-window sweep was 1.5 km/s. The static stress drop was estimated to be 8.9 MPa by assuming a circular fault model. The crustal deformation estimated from the slip distribution agrees well with the tsunami-generating area, the sea-bottom movement, and the uplift measurement at Awashima Island, located above the fault plane. [1]: /embed/inline-graphic-1.gif

The ShakeOut Earthquake Source and Ground Motion Simulations

The ShakeOut Scenario is premised upon the detailed description of a hypothetical Mw 7.8 earthquake on the southern San Andreas Fault and the associated simulated ground motions. The main features of the scenario, such as its endpoints, magnitude, and gross slip distribution, were defined through expert opinion and incorporated information from many previous studies. Slip at smaller length scales, rupture speed, and rise time were constrained using empirical relationships and experience gained from previous strong-motion modeling. Using this rupture description and a 3-D model of the crust, broadband ground motions were computed over a large region of Southern California. The largest simulated peak ground acceleration (PGA) and peak ground velocity (PGV) generally range from 0.5 to 1.0 g and 100 to 250 cm/s, respectively, with the waveforms exhibiting strong directivity and basin effects. Use of a slip-predictable model results in a high static stress drop event and produces ground motions somewhat higher than median level predictions from NGA ground motion prediction equations (GMPEs).

Comparison of Ground Motions from Hybrid Simulations to NGA Prediction Equations

We compare simulated motions for a Mw 7.8 rupture scenario on the San Andreas Fault known as the ShakeOut event, two permutations with different hypocenter locations, and a Mw 7.15 Puente Hills blind thrust scenario, to median and dispersion predictions from empirical NGA ground motion prediction equations. We find the simulated motions attenuate faster with distance than is predicted by the NGA models for periods less than about 5.0 s After removing this distance attenuation bias, the average residuals of the simulated events (i.e., event terms) are generally within the scatter of empirical event terms, although the ShakeOut simulation appears to be a high static stress drop event. The intra-event dispersion in the simulations is lower than NGA values at short periods and abruptly increases at 1.0 s due to different simulation procedures at short and long periods. The simulated motions have a depth-dependent basin response similar to the NGA models, and also show complex effects in which stronger basin response occurs when the fault rupture transmits energy into a basin at low angle, which is not predicted by the NGA models. Rupture directivity effects are found to scale with the isochrone parameter.

Quantifying Seismic Source Parameter Uncertainties

We use data from a small aperture array in southern California to quantify variations in source parameter estimates at closely spaced stations (distances ranging from ∼7 to 350 m) to provide constraints on parameter uncertainties. Many studies do not consider uncertainties in these estimates even though they can be significant and haveimportantimplicationsforstudiesofearthquakesourcephysics.Here,weestimate seismic source parameters in the frequency domain using empirical Green's function (EGF) methods to remove effects of the travel paths between earthquakes and their recordingstations.Weexamineuncertainties inourestimatesbyquantifyingtheresult- ing distributions over all stations in the array. For coseismic stress drop estimates, we find that minimum uncertainties of ∼30% of the estimate can be expected. To test the robustness of our results, we explorevariations of the dataset using different groupings of stations, different source regions, and different EGF earthquakes. Although these differences affect our absolute estimates of stress drop, they do not greatly influence thespreadinourresultingestimates.Thesesensitivitytestsshowthatstationselectionis not the primary contribution to the uncertainties in our parameter estimates for single stations. We conclude that establishing reliable methods of estimating uncertainties in sourceparameterestimates(includingcornerfrequencies,sourcedurations,andcoseis- mic static stress drops) is essential, particularly when the results are used in the com- parisons among different studies over a range of earthquake magnitudes and locations.

Static stress drop as determined from geodetic strain rates and statistical seismicity

[1] Two critical items in the energetic budget of a seismic province are the strain rate, which is measured geodetically on the Earth's surface, and the yearly number of earthquakes exceeding a given magnitude. Our study is based on one of the most complete and recent seismic catalogs of Italian earthquakes and on the strain rate map implied by a multiyear velocity solution for permanent GPS stations. For each of 36 homogeneous seismic zones we use the appropriate Gutenberg-Richter relation, which is based on the seismicity catalog, to estimate a seismic strain rate, which is the strain rate associated with the mechanical work due to a coseismic displacement. We show that for each seismic zone, the volume storing most of the elastic energy associated with the long-term deformation, and hence the seismic strain rate, is inversely proportional to the static stress drop. The GPS-derived strain rate for each seismic zone limits the corresponding seismic strain rate, and an upper bound for the average stress drop is estimated. We show that the implied regional static stress drop varies from 0.1 to 5.7 MPa for catalog earthquakes in the moment magnitude range [4.5–7.3]. The stress drop results are independent of the regional a and b parameters and heat flow but are very sensitive to the assumed maximum magnitude of a seismic province. The data do not rule out the hypothesis that the stress drop positively correlates with the time elapsed after the largest earthquake recorded in each seismic zone.

Mechanical constraints on inversion of coseismic geodetic data for fault slip and geometry: Example from InSAR observation of the 6 October 2008Mw6.3 Dangxiong-Yangyi (Tibet) earthquake

[1] Modern geodetic techniques, such as the global positioning system (GPS) and Interferometric Synthetic Aperture Radar (InSAR), provide high-precision deformation measurements of earthquakes. Through elastic models and mathematical optimization methods, the observations can be related to a slip distribution model. The classic linear, kinematic, and static slip inversion problem requires specification of a smoothing norm of slip parameters and a residual norm of the data and a choice about the relative weight between the two norms. Inversions for unknown fault geometry are nonlinear and, therefore, the fault geometry is often assumed to be known for the slip inversion problem. We present a new method to invert simultaneously for fault slip and fault geometry assuming a uniform stress drop over the slipping area of the fault. The method uses a full Bayesian inference method as an engine to estimate the posterior probability distribution of stress drop, fault geometry parameters, and fault slip. We validate the method with a synthetic data set and apply the method to InSAR observations of a moderate-sized normal faulting event, the 6 October 2008 Mw 6.3 Dangxiong-Yangyi (Tibet) earthquake. The results show a 45.0 ± 0.2° west dipping fault with a maximum net slip of ∼1.13 m, and the static stress drop and rake angle are estimated as ∼5.43 MPa and ∼92.5°, respectively. The stress drop estimate falls within the typical range of earthquake stress drops known from previous studies.

Normal-Faulting Slip Maxima and Stress-Drop Variability: A Geological Perspective

Abstract We present an empirical estimate of maximum slip in continental normal-faulting earthquakes and present evidence that stress drop in intraplate extensional environments is dependent on fault maturity. A survey of reported slip in historical earthquakes globally and in latest Quaternary paleoearthquakes in the Western Cordillera of the United States indicates maximum vertical displacements as large as 6–6.5 m. A difference in the ratio of maximum-to-mean displacements between data sets of prehistoric and historical earthquakes, together with constraints on bias in estimates of mean paleodisplacement, suggest that applying a correction factor of 1.4±0.3 to the largest observed displacement along a paleorupture may provide a reasonable estimate of the maximum displacement. Adjusting the largest paleodisplacements in our regional data set (∼6 m) by a factor of 1.4 yields a possible upper-bound vertical displacement for the Western Cordillera of about 8.4 m, although a smaller correction factor may be more appropriate for the longest ruptures. Because maximum slip is highly localized along strike, if such large displacements occur, they are extremely rare. Static stress drop in surface-rupturing earthquakes in the Western Cordillera, as represented by maximum reported displacement as a fraction of modeled rupture length, appears to be larger on normal faults with low cumulative geologic displacement (

Kīholo Bay, Hawai‘i, earthquake sequence of 2006: Relationship of the main shock slip with locations and source parameters of aftershocks

We study the source process of the Kīholo Bay earthquake (MW 6.7), which occurred beneath the northwest part of the Island of Hawai‘i on 15 October 2006, and static stress drops of small earthquakes that occurred in 2006 and 2007 around the main shock including aftershocks. We relocate the aftershocks to determine the fault plane from the two nodal planes. The relocated aftershocks define an E‐W trending plane that dips to the south, in good agreement with one of the nodal planes given by the Global Centroid Moment Tensor solution. Waveform inversion is performed with multiple time windows to investigate the rupture speed and the slip distribution of the main shock. Waveforms of an aftershock with MW 5.2 are used to calculate empirical Green's functions. Our results indicate that the rupture propagated unilaterally to the west with a rupture speed greater than 3.0 km/s (63% of the shear wave velocity). This westward rupture is consistent with the fact that aftershocks are distributed predominantly to the west of the main shock epicenter. Most aftershocks are located on the edge of patches with a large slip, or asperities and some also occur inside the patches. We also estimate static stress drops of 39 earthquakes (2.5 < ML < 3.5) that occurred in 2006 and 2007 near the source region of the Kīholo Bay earthquake. Static stress drops range from 0.12 to 8.6 MPa and aftershocks around large slip patches of the main shock likely to have larger stress drops.

Bounding of near-fault ground motion based on radiated seismic energy with a consideration of fault frictional mechanisms

The energy radiated as seismic waves strongly depends on the fault rupture process associated with rupture speed and dynamic frictional mechanisms involved in the fault slip motion. Following McGarr and Fletcher approach, we derived a physics-based relationship of the weighted average fault slip velocity vs apparent stress, rupture speed and static stress drop based on a dynamic circular fault model. The resultant function can be approximately used to bound near-fault ground motion and seismic energy associated with near-fault coseismic deformation. Fault frictional overshoot and undershoot mechanisms governed by a simple slip-weakening constitutive relation are included in our consideration by using dynamic rupture models named as M- and D-models and proposed by Madariaga (1976) and Boatwright. We applied the above function to the 2008 great Wenchuan earthquake and the 1999 Jiji (Chi-Chi) earthquake to infer the near-fault ground motion called slip weighted average particle velocity and obtained that such model-dependent prediction of weighted average ground velocities is consistent to the results derived from the near-fault strong motion observations. Moreover, we compared our results with the results by McGarr and Fletcher approach, and we found that the values of the weighted average particle velocities we obtained for these two earthquakes are generally smaller and closer to the values by direct integration of strong motion recordings of the near-fault particle velocity waveform data. In other words, if this result comes to be true, it would be a straightforward way used to constrain the near-fault ground motion or to estimate source parameters such as rupture speed, static and dynamic stress drops.

The 24 May 2009 Mw5.2 earthquake sequence near Lake Doirani (FYROM-Greek borders): Focal mechanisms and slip model using empirical source time functions inversion

On 24 May 2009 (GMT 16:17) a moderate size (Mw5.2) earthquake sequence burst north of Lake Doirani along the Greek-FYROM borders and ruptured a N282°E striking normal fault of ∼ 6 km in length dipping at an angle of ∼ 60° to the north. The focal mechanisms for 25 of the stronger events, obtained through time-domain moment tensor inversion, reveal the prevalence of normal faulting along ∼ WNW–ESE striking planes, sometimes dip slip motions are combined with a considerable strike-slip component. Broad band waveforms from the Hellenic Unified Seismological Network (HUSN) and neighbouring networks were used to invert source time functions (STFs) and calculate the slip model for the strongest event (GMT 16:17). STFs were obtained through an empirical Green's function approach. No directivity could be clearly identified for this moderate magnitude event. Slip was released in a single patch of dimensions 6 km × 8 km, maximum slip is estimated at 15 cm, while average slip on the ruptured area (area of subfaults of non-zero slip) was 3 cm. One characteristic of the slip model was the large rise time, for a moderate size event, which was estimated to be 1.0 s, implying a slow slip-rate function. Brune's static stress drop is calculated ∼ 5 bars. It must be noted that the north dipping plane is identified as the fault plane, (slip vector trending N332°E) based on the model variance of the STFs inversion.

Characterization of Stress Drops on Asperities Estimated from the Heterogeneous Kinematic Slip Model for Strong Motion Prediction for Inland Crustal Earthquakes in Japan

Dense strong motion observation networks provided us with valuable data for studying strong motion generation from large earthquakes. From kinematic waveform inversion of seismic data, the slip distribution on the fault surface of large earthquakes is known to be spatially heterogeneous. Because heterogeneities in the slip and stress drop distributions control the generation of near-source ground motion, it is important to characterize these heterogeneities for past earthquakes in constructing a source model for reliable prediction of strong ground motion. The stress changes during large earthquakes on the faults recently occurring in Japan are estimated from the detailed slip models obtained by the kinematic waveform inversion. The stress drops on and off asperities are summarized on the basis of the stress change distributions obtained here. In this paper, we define the asperity to be a rectangular area whose slip is 1.5 or more times larger than the average slip over the fault according to the previous study for inland crustal earthquakes. The average static stress drops on the asperities of the earthquakes studied here are in the range 6–23 MPa, whereas those off the asperities are below 3 MPa. We compiled the stress drop on the asperities together with a data set from previous studies of other inland earthquakes in Japan and elsewhere. The static stress drop on the asperity depends on its depth, and we obtained an empirical relationship between the static stress drop and the asperity’s depth. Moreover, surface-breaking asperities seemed to have smaller stress drops than buried asperities. Simple ground motion simulations using the characterized asperity source models reveal that deep asperities generate larger ground motion than shallow asperities, because of the different stress drops of the asperities. These characteristics can be used for advanced source modeling in strong ground motion prediction for inland crustal earthquakes.

Asperity and Barriers of the 2004 Mid-Niigata Prefecture Earthquake Revealed by Highly Dense Seismic Observations

Abstract A highly resolved velocity structure on the mainshock fault of the 2004 Mid-Niigata prefecture earthquake was elucidated using arrival times from aftershocks observed by an extremely dense network of temporary seismic stations. We evaluated the spatial relationship between the velocity structure and distributions of the slip, stress change, and aftershocks. Distributions of static stress drop on the fault were calculated from a kinematic slip model, incorporating the 3D crustal structure. A high-velocity body with low aftershock activity was in proximity to the mainshock hypocenter and extended to the northeast side of the fault. This high-velocity body roughly coincided with an asperity, where the amounts of coseismic slip and static stress drop were larger than those in the surrounding areas. In contrast, a zone of negative stress drop was observed on the shallow periphery of the high-velocity body, where the sediment thickness steeply increased southwestward and the aftershock activity was high. We suggest that the structural heterogeneities of the host rocks surrounding the fault damaged zone have significant potentials to control dynamic rupture processes of the mainshock fault.

Source Parameters of the 2005 Mw 7.2 Miyagi-Oki, Japan, Earthquake as Inferred from Teleseismic P-Waves

We investigate the fault parameters of the 2005 Miyagi-Oki (Japan) earthquake using duration variations of teleseismic P-waves. The results show that the earthquake has a thrust-type mechanism and a seismic moment of 4.46 × 10 19 Nm. Rupture directivity analysis suggests that the earthquake occurred as a result of a bilateral faulting on the fault plane with a strike of 247°, a dip of 17° and a slip of 125°. The optimal rupture azimuth, measured counterclockwise from the strike on the fault plane, is 170° (or 350°). The rupture length and average source duration are estimated to be 73.4 km and 14.5 sec, respectively. Thus the rupture velocity is 2.53 km sec -1 (~0.57 times the value of S-wave velocity), which is lower than the value for other similarly sized earthquakes. This implies that the 2005 Miyagi-Oki earthquake was probably a slow event. Consequently, there may have been less release of high-frequency seismic energy, leading to lower radiated seismic energy and radiation efficiency (~0.32 - 0.48). In other words, relatively larger fracture energy occurred during earthquake faulting in addition to the heat due to friction. The ratio of the static stress drop to the apparent stress (> 2.0) also suggests that the earthquake can be modeled as a frictional overshoot in a stress model, which implies the transformation of a lower percentage of strain energy into seismic-wave energy during the process of earthquake rupturing.