Bilateral Rupture Propagation Research Articles

Along-Strike Variation of Rupture Characteristics and Aftershock Patterns of the 2023 Mw 7.8 Türkiye Earthquake Controlled by Fault Structure

Abstract On 6 February 2023, an Mw 7.8 earthquake occurred along the East Anatolian fault zone (EAFZ) in southeastern Türkiye, representing the strongest earthquake in the region in nearly 80 yr. We investigate rupture characteristics and aftershock patterns of the earthquake through focal mechanism calculation, backprojection analysis, and finite-fault inversion. The results show bilateral rupture propagation of the mainshock with transient supershear speed in the southwest portion of the EAFZ, as well as shallower coseismic slip and abundant normal-faulting aftershocks in the same portion. We attribute these earthquake behaviors to the along-strike variation of fault structure of the EAFZ, which features a more complex fault geometry accompanied by numerous short normal faults in the southwest portion. These results shed light on fault segmentation, rupture speed variation, and slip partitioning along the EAFZ, advancing our understanding of fault structural control on earthquake behaviors in a complex multisegment fault system.

Asymmetric Bilateral Rupture of the 2022 Ms 6.8 Luding Earthquake on a Continental Transform Fault, Tibetan Border, China

Abstract The Xianshuihe (XSH) fault on the eastern boundary of the Tibetan plateau is one of the most active faults in the world. However, the Moxi fault, the southernmost segment of the XSH fault, remained seismically quiescent for over 230 yr, with a large slip rate and high locking degree. Thus, this region is considered to be a seismic gap capable of hosting earthquakes of Mw 7.0 or greater. In September 2022, the Ms 6.8 Luding earthquake occurred on the Moxi fault, resulting in destructive landslide damage, with 93 people dead and 25 missing. Using regional and global seismic recordings, we integrate the multiple-point-source analysis, finite-fault inversions, and backprojection imaging to investigate in detail the rupture process associated with the 2022 Luding, China, earthquake. Our results show that this event is characterized by an asymmetric bilateral rupture with three episodes: (1) an initial bilateral rupture propagation mainly toward north-northwest; (2) major rupture propagating toward south-southeast and up-dip direction; (3) south-southeast-rupture propagation speed accelerates to ∼2 km/s. Over 70% of the seismic moment is released at shallow depths <12 km thus explaining significant damaging effects. Furthermore, the seismic potential of the remaining locked portions of the XSH and Anninghe faults is still high and positive increases of the Coulomb stress on these faults due to the Luding earthquake might bring them closer to future failure.

Characteristics of Strong Ground Motions from Four M S ≥ 5.0 Earthquakes in the 2021 Yangbi, Southwest China, Seismic Sequence

ABSTRACT A seismic sequence has struck the Yangbi county, west part of Yunnan Province in southwest China since May 18, 2021 . This region experienced the strongest ground shakings on the night of 21 May with the successive occurrence of four earthquakes with M S ≥5.0 in a short time and more small earthquakes. Large numbers of strong-motion recordings obtained in the four earthquakes with M S ≥5.0 (i.e. the M S 5.6 foreshock, the M S 6.4 mainshock, the M S 5.0 and M S 5.2 aftershocks) were analyzed in detail to characterize the source effects, distance decay, etc. The velocity pulse-like waveform at near-source station 53YBX and the significant asymmetry of distance decay along the SSE-NNW direction (i.e. much slower toward the SSE) in the M S 6.4 mainshock seem to hint the occurrence of the asymmetrical source rupture. The source rupture directivity for the four events were investigated by fitting the azimuth-dependent residuals of the PGAs. The M S 6.4 mainshock is characterized by the significantly asymmetrical bilateral rupture propagation, predominantly in the SSE direction with a rupture velocity of about 2.24 km/s. The PGA and PSAs from the four earthquakes were further compared with the predicted medians of the ASK14 and CY14 prediction models for global shallow crustal earthquakes. The negative inter-events residuals reflect the weaker source contributions of the Yangbi events to ground motions. The source contributions to short-period ground motions approximately show the dependence on earthquake type, that is, M S 5.6 foreshock > M S 6.4 mainshock > M S 5.2 aftershock, consistent with the dependence of stress drop on earthquake type. We further noted that the inter-events values for the M S 5.0 aftershock are far below those for the M S 5.2 aftershock. This may be attributed to the differences in the spatial and temporal distance to the mainshock, that is, weak source contributions from the close aftershocks to the mainshock. The values of adjustment coefficient Δc 3 are almost positive, indicating the weaker anelastic attenuations in the study region. Finally, we also found that the distance scaling of significant duration can be well described by both empirical models (AS16 model for predicting significant duration and the BT14 model for empirically illustrating the ground-motion path duration).

Laboratory earthquakes along faults with a low velocity zone: Directionality and pulse-like ruptures

Low velocity zone (LVZ) is a common structural feature in geological faults. To examine the effect of LVZ on faulting, a laboratory fault model considering the LVZ is constructed and loaded to different levels. Subsequently ruptures are triggered on one of its contact boundaries with the host rock. The bilateral rupture propagation along this laboratory fault that separates different materials shows distinct directionality in both rupture speeds and rupture modes. The rupture in the positive direction with respect to the fault is always crack-like growing at the Generalized Rayleigh (GR) wave speed. However, in the negative direction both stable and unstable slip pulses are observed. Higher load and thinner LVZ facilitate the unstable slip pulses. The trailing tip for both types of pulses propagates at the GR wave speed, the leading tip of the stable pulse propagates with the GR wave speed while that of the unstable pulse features a supershear speed.

Source Mechanism and Rupture Process of the 24 January 2020 Mw 6.7 Doğanyol–Sivrice Earthquake obtained from Seismological Waveform Analysis and Space Geodetic Observations on the East Anatolian Fault Zone (Turkey)

Here, we present the source mechanism and rupture process for the destructive 24 January 2020 Mw 6.7 Doğanyol–Sivrice earthquake at the East Anatolian Fault Zone (EAFZ, Turkey), obtained from seismological waveform analysis and space geodetic observations. Multi-data analyses and modelling in the present study provide fundamental data and strong constraints for retrieving complex source mechanism of an earthquake and its spatiotemporal slip characteristics along the ruptured segment of fault. The acquired slip model of this earthquake reveals heterogeneous slip distribution along strike N244°E of the fault plane dipping NW (68°) with duration of the source time function (STF) and low stress drop value (Δσ) of ~25 s and ~6 bars, respectively. Back-projection analysis validates fault length (L) stretching along strike for a distance of ~75 km and supports predominant south-westerly bilateral rupture propagation with a variable rupture velocity (Vr) of ~2.3–3.4 km/s along with two main patches, presumably a sequence of two asperities being ruptured following the surface trace of the EAFZ. The distribution of aftershocks based on the analysis of two months long data consistently confirms spreading of seismicity along the ruptured fault. The evaluation of Interferometric Synthetic Aperture Radar (InSAR) data reveals that left-lateral co-seismic slip and significant deformation extends for ~20 km on either side of the fault with evident post-seismic displacement. Yet, no significant vertical offsets were observed as GNSS stations detected only horizontal motions. Coda-wave analysis as an independent tool also confirms moment magnitude of Mw 6.7. Our results highlight a case of a damaging earthquake and enhance our understanding of earthquake mechanics, continental deformation and augmented earthquake risk on the EAFZ.

Directivity analysis of the 2017 December Kerman earthquakes in Eastern Iran

Using an empirical Green’s function (EGF) approach and data from local to regional distances we analyzed rupture propagation directivity in the three mainshocks (ML 6.0–6.1) and in six of the largest aftershocks (ML 5.0 – 5.5) of the 2017 Kerman, Iran, seismic sequence. The EGF procedure was based on data from smaller events (ML 4.0 – 4.8). Deconvolution was applied separately to P and S phases. Using the P-wave data, we calculated relative source-time functions and examined azimuthal variations in rupture duration. In the S-wave analysis, we investigated along strike rupture directivity of the mainshocks and the largest aftershocks by evaluating azimuthal variation of the amplitude spectra. Two of the mainshocks and four of the aftershocks clearly showed rupture propagation from the south-east toward the north-west. The third mainshock and one of the aftershocks suggested almost bilateral rupture propagation, and one aftershock showed rupture directivity to the southeast. It seems that the rupture propagation direction in the area is generally to the north-west and the events which have different propagation directions are located within the NW and SE ends of the faulting area. We suggest that the general rupture propagation direction in the area is steered by regional tectonic stress field regarding the faulting orientations which have been affected by stress redistribution around a restraining bend.

Bayesian Dynamic Finite‐Fault Inversion: 2. Application to the 2016 Mw 6.2 Amatrice, Italy, Earthquake

AbstractIn 2016 Central Italy was struck by a sequence of three normal‐faulting earthquakes with moment magnitude (Mw) larger than 6. The Mw 6.2 Amatrice event (24 August) was the first one, causing building collapse and about 300 casualties. The event was recorded by a uniquely dense network of seismic stations. Here we perform its dynamic source inversion to infer the fault friction parameters and stress conditions that controlled the earthquake rupture. We consider a linear slip‐weakening friction law with spatially variable parameters along the fault. The inversion uses a novel Bayesian framework developed in our companion paper, which combines efficient finite‐difference dynamic rupture simulations and the Parallel Tempering Monte Carlo algorithm to sample the posterior probability density function. The main advantage of such formulation is that by subsequent analysis of the posterior samples we can infer stable features of the result and their uncertainty. The inversion results in a million of visited models. The preferred model ensemble reveals intriguing dynamic features. The rupture exhibits a slow and irregular nucleation followed by bilateral rupture propagation through two asperities, accelerating toward the heavily damaged city of Amatrice. The stress drop reaches locally 10–15 MPa, with slip‐weighted mean of 4–4.5 MPa. The friction drop ranges from 0.1 to 0.4. The characteristic slip‐weakening distance is the most heterogeneously distributed dynamic parameter, with values of 0.2–0.8 m. The radiation efficiency was rather low, 0.2, suggesting that approximately 80% of the total available energy was spent in the fracture process, while just 20% was radiated by seismic waves.

Rupture imaging for the 2016 August 24, Mw=6.0 central Italy earthquake, from back-projection of strong-motion array data

<p class="western" align="justify"><span style="font-family: 'Bitstream Charter', serif;"><span>By extending the conventional Beam-Forming frequency-wavenumber power spectral estimate to the case of arbitrarily-shaped wavefronts, we obtained images of rupture propagation during the 2016 August 24, Mw=6.0 central Italy earthquake. Using a set of strong-motion accelerometers, we evaluate the beam power along the travel time curves associated with synthetic sources spanning a model fault surface. This allows deriving time-dependent images of the distribution of energy radiation throughout the fault plane. Results indicate bi-lateral rupture propagation toward SE and NW, in rough agreement with surface co-seismic displacement and surface damage pattern. To a first order, our results are also consistent with those obtained from full-waveform inversion of strong-motion data.</span></span></p>

The 24 May 2014 (Mw6.8) earthquake (North Aegean Trough): Spatiotemporal evolution, source and slip model from teleseismic data

We use teleseismic data to calculate the source model of the 24 May 2014 earthquake and regional catalogues to examine the spatial-temporal characteristics of the sequence. The sequence started in Saros Basin but almost simultaneously aftershocks spread along a ∼200 km zone, activating the entire North Aegean Trough. The aftershock sequence was rich in moderate (M < 4) size events, but very deficient in strong events – only two Mw4.9 aftershocks-a characteristic observed in previous sequences in the region. The teleseismic waveforms were best fit by two sub-events, which were lagged by 18 s in time and by a 50 km jump in space, along the same fault line. The centroid depth of the first sub-event is 22 km, at the base of the lower crust, and for the second is 14 km. The resolved total source time function is ∼30 s. The finite-fault slip model is characterized by an asymmetric bilateral rupture propagation, to the west and east of the hypocentre. The major slip is confined downdip from the hypocentre, within the deeper 12–25 km part. This deep slip migrated updip from the hypocentre to form the second slip patch, in the eastward segment. In all our models the maximum dislocation was of the order of 1 m. For our preferred model parametrization, the rupture speed is 3 km/s, and the scalar moment equal to 1.76 × 1019 Nm (Mw6.8). This earthquake highlighted the fact that strike-slip faulting in the North Aegean Sea, can attain large lengths and activate very wide zones, reaching densely populated regions.

High-frequency seismic radiation from Maule earthquake (Mw 8.8, 2010 February 27) inferred from high-resolution backprojection analysis

The Maule earthquake (2010 February 27, Mw 8.8, Chile) broke the subduction megathrust along a previously locked segment. Based on an international aftershock deployment, catalogues of precisely located aftershocks have become available. Using 23 well-located aftershocks, we calibrate the classic teleseismic backprojection procedure to map the high-frequency seismic radiation emitted during the earthquake. The calibration corrects traveltimes in a standard earth model both with a static term specific to each station, and a ‘dynamic’ term specific to each combination of grid point and station. The second term has been interpolated over the whole slipping area by kriging, and is about an order of magnitude smaller than the static term. This procedure ensures that the teleseismic images of rupture development are properly located with respect to aftershocks recorded with local networks and does not depend on accurate hypocentre location of the main shock. We track a bilateral rupture propagation lasting ∼160 s, with its dominant branch rupturing northeastwards at about 3 km s−1. The area of maximum energy emission is offset from the maximum coseismic slip but matches the zone where most plate interface aftershocks occur. Along dip, energy is preferentially released from two disconnected interface belts, and a distinct jump from the shallower belt to the deeper one is visible after about 20 s from the onset. However, both belts keep on being active until the end of the rupture. These belts approximately match the position of the interface aftershocks, which are split into two clusters of events at different depths, thus suggesting the existence of a repeated transition from stick-slip to creeping frictional regime.

CyberShake-derived ground-motion prediction models for the Los Angeles region with application to earthquake early warning

Real-time applications such as earthquake early warning (EEW) typically use empirical ground-motion prediction equations (GMPEs) along with event magnitude and source-to-site distances to estimate expected shaking levels. In this simplified approach, effects due to finite-fault geometry, directivity and site and basin response are often generalized, which may lead to a significant under- or overestimation of shaking from large earthquakes (M > 6.5) in some locations. For enhanced site-specific ground-motion predictions considering 3-D wave-propagation effects, we develop support vector regression (SVR) models from the SCEC CyberShake low-frequency ( 415 000 finite-fault rupture scenarios (6.5 ≤ M ≤ 8.5) for southern California defined in UCERF 2.0. We use CyberShake to demonstrate the application of synthetic waveform data to EEW as a ‘proof of concept’, being aware that these simulations are not yet fully validated and might not appropriately sample the range of rupture uncertainty. Our regression models predict the maximum and the temporal evolution of instrumental intensity (MMI) at 71 selected test sites using only the hypocentre, magnitude and rupture ratio, which characterizes uni- and bilateral rupture propagation. Our regression approach is completely data-driven (where here the CyberShake simulations are considered data) and does not enforce pre-defined functional forms or dependencies among input parameters. The models were established from a subset (∼20 per cent) of CyberShake simulations, but can explain MMI values of all >400 k rupture scenarios with a standard deviation of about 0.4 intensity units. We apply our models to determine threshold magnitudes (and warning times) for various active faults in southern California that earthquakes need to exceed to cause at least ‘moderate’, ‘strong’ or ‘very strong’ shaking in the Los Angeles (LA) basin. These thresholds are used to construct a simple and robust EEW algorithm: to declare a warning, the algorithm only needs to locate the earthquake and to verify that the corresponding magnitude threshold is exceeded. The models predict that a relatively moderate M6.5–7 earthquake along the Palos Verdes, Newport-Inglewood/Rose Canyon, Elsinore or San Jacinto faults with a rupture propagating towards LA could cause ‘very strong’ to ‘severe’ shaking in the LA basin; however, warning times for these events could exceed 30 s.

Incorporating fault zone head wave and direct wave secondary arrival times into seismic tomography: Application at Parkfield, California

We present a three‐dimensional (3D) P wave velocity (Vp) model of the Parkfield region that utilizes existing P wave arrival time data, including fault zone head waves (FZHWs), and data from direct wave secondary arrivals (DWSAs). The first‐arrival and DWSA travel times are obtained as the global‐ and local‐minimum travel time paths, respectively. The inclusion of FZHWs and DWSAs results in as much as a 5% and a 10% increase in the across‐fault velocity contrast, respectively, for the Vp model at Parkfield relative to that of Thurber et al. [2006]. Viewed along strike, three pronounced velocity contrast regions are observed: a pair of strong positive velocity contrasts (SW fast), one NW of the 1966 Parkfield earthquake hypocenter and the other SE of the 2004 Parkfield earthquake hypocenter, and a strong negative velocity contrast (NE fast) between the two hypocenters. The negative velocity contrast partially to entirely encompasses peak coseismic slip estimated in several slip models for the 2004 earthquake, suggesting that the negative velocity contrast played a part in defining the rupture patch of the 2004 Parkfield earthquake. Following Ampuero and Ben‐Zion (2008), the pattern of velocity contrasts is consistent with the observed bilateral rupture propagation for the 2004 Parkfield earthquake. Although the velocity contrasts also suggest bilateral rupture propagation for the 1966 Parkfield earthquake, the fault is creeping to the NW here, i.e., exhibiting velocity‐strengthening behavior. Thus, it is not surprising that rupture propagated only SE during this event.

Rupture process of the 2011 Tohoku-oki earthquake and absolute elastic strain release

[1] On 11 March 2011, the Tohoku-oki earthquake in eastern Japan and the devastating tsunami that followed it caused severe damage and numerous deaths. To clarify the rupture process of the earthquake, we inverted teleseismic P-wave data applying a novel formulation that takes into account the uncertainty of Green's function, which has been a major error source in waveform inversion. The estimated seismic moment is 5.7 × 1022 Nm (Mw = 9.1), associated with a fault rupture 440 km long and 180 km wide along the plate interface. The source process is characterized by asymmetric bilateral rupture propagation, but we also found continuous slips up-dip from the hypocenter, which led to a large maximum slip (50 m), long slip duration (90 s), and a large stress drop (20 MPa). The long slip duration, large stress drop, extensional (normal faulting) aftershocks in a previously compressional stress regime, and low-angle normal slips at approximately the depth of the plate interface suggest that the earthquake released roughly all of the accumulated elastic strain on the plate interface owing to exceptional weakening of the fault. The stress accumulated on the plate interface was about 20 MPa near the trench and 0–10 MPa in the down-dip source region.

Hybrid Broadband Ground-Motion Simulation Using Scattering Green's Functions: Application to Large-Magnitude Events

We calculate near-source broadband (0-10 Hz) seismograms by combin- ing low-frequency three-dimensional (3D) finite-difference seismograms (0-0.5 Hz) computed in a 3D velocity model using site-specific scattering Green's functions for random, isotropic scattering media. The scattering Green's functions are convolved with a slip-rate function to form local scattering operators (scatterograms), which constitute the high-frequency scattered wave field. The low-frequency and high- frequency scatterograms are then combined in the frequency domain to generate broadband waveforms. Our broadband method extends the Mai et al. (2010) approach by incorporating dynamically consistent source-time functions and account- ing for finite-fault effects in the computation of the high-frequency waveforms. We used the proposed method to generate broadband ground motions at 44 sites located 5-100 km from the fault, for Mw 7.7 earthquake scenarios (TeraShake) on the south- ern San Andreas fault, which include north-to-south, south-to-north, and bilateral rupture propagation from kinematic and spontaneous dynamic rupture models. The broadband ground motions computed with the new method are validated by com- paring peak ground acceleration, peak ground velocity, and spectral acceleration with recently proposed ground-motion prediction equations (GMPEs). Our simulated ground motions are consistent with the median ground motions predicted by the GMPEs. In addition, we examine overturning probabilities for 18 precariously balanced rock sites (PBR). Our broadband synthetics for the Mw 7.7 TeraShake sce- narios show no preferred rupture direction on the southern San Andreas fault but are inconsistent with the existence of PBRs at several of the sites analyzed.

Source characteristics of a moderate earthquake (M 4.9)using empirical Green ?s function technique

The rupture process of a moderate earthquake (M 4.9) on 28th January 1999 was analyzed using velocity records at local distances less than 80 km. The characterization of the rupture process was obtained from studying aftershocks distribution, azimuthal variations of Relative Source Time Functions (RSTFs), and a set of spatio-temporal slip models. RSTFs were retrieved by deconvolution of small aftershock records from those of the mainshock. In addition, velocity P-wave records of the respective event were inverted to recover slip distribution on the fault plane using the records of aftershocks as Empirical Green Functions (EGFs). The waveform inversion was adopted using three EGFs. In the inversion, the rupture propagation velocity was fixed and assumed to be eight-tenths of the local shear wave velocity. The total seismic moment was estimated to range from 0.011 E + 18 Nm (Mw = 4.6) to 0.017 E + 18 Nm (Mw = 4.8). The hypocentral distribution of the aftershocks, azimuthal variations of RSTFs, and the set of slip distribution models were exhibited bilateral rupture propagation along the strike and dip of the fault plane. The presence of two to three high slip patches on the fault plane suggested that a complex rupture pattern is detectable for a moderate size earthquake. However, the so-called nucleation phase was invisible in the present analysis.

Complex geometry and segmentation of the surface rupture associated with the 14 November 2001 great Kunlun earthquake, northern Tibet, China

The 14 November 2001 Kunlun, China, earthquake with a moment magnitude (M w) 7.8 occurred along the Kusai Lake–Kunlun Pass fault of the Kunlun fault system. We document the spatial distribution and geometry of surface rupture zone produced by this earthquake, based on high-resolution satellite (Landsat ETM, ASTER, SPOT and IKONOS) images combined with field measurements. Our results show that the surface rupture zone can be divided into five segments according to the geometry of surface rupture, including the Sun Lake, Buka Daban–Hongshui River, Kusai Lake, Hubei Peak and Kunlun Pass segments from west to east. These segments, each 55 to 130 km long, are separated by step-overs. The Sun Lake segment extends about 65 km with a strike of N45° ∼ 75°W (between 90°05′E ∼ 90°50′E) along the previously unrecognized West Sun Lake fault. A gap of about 30 km long exists between the Sun Lake and Buka Daban Peak where no obvious surface ruptures can be observed either from the satellite images or field observations. The Buka Daban–Hongshui River, Kusai Lake, Hubei Peak and Kunlun Pass segments run about 365 km striking N75° ∼ 85°W along the southern slope of the Kunlun Mountains (between 91°07′E ∼ 94°58′E). This segmentation of the surface rupture is well correlated with the pattern of slip distribution measured in the field. Detailed mapping suggest that these five first-order segments can be further separated into over 20 second-order segments with a length of 10–30 km, linked by smaller scale step-overs or bends. Our result also shows that the total coseismic surface rupture length produced by the 2001 Kunlun earthquake is about 430 km (excluding the 30-km-long gap), which is the longest coseismic surface rupture for an intracontinental earthquake ever recorded. Finally, we suggest a multiple bilateral rupture propagation model that shows the rupture process of the 2001 M w 7.8 earthquake is complex. It consists of westward and eastward rupture propagations and interaction of these bilateral rupture processes.

Step-Over and Bend Structures along the 1999 Duzce Earthquake Surface Rupture, North Anatolian Fault, Turkey

The 12 November 1999 Duzce earthquake (M w 7.1), which occurred on the Duzce fault of the North Anatolian Fault Zone, formed a 40-km-long surface rupture, which extended between Golyaka in the west and Kaynasli in the east. The earthquake nucleated close to the Beykoy step-over, a 0.8-km-wide and 1.8-km-long pressure ridge, with bilateral rupture propagation both to the west and the east. The Beykoy, Kaynasli, and Cevizlik step-overs, through which the 1999 Duzce rupture propagated, are less than 2.5 km wide. The Eften and Bakacak step-overs, where the rupture was arrested, are more than 4 km wide. These exceptional dimensions of step-overs associated with the 1999 Duzce rupture were investigated for their influ- ence on rupture propagation. The earthquake broke three distinct geometric segments, from west to east, the Eften, Dagdibi, and Kaynasli fault segments, separated by left-restraining step-overs at Beykoy and Kaynasli. To the west of the main shock, 3.5 m of average slip on the Eften fault segment propagated into the 2-km-wide Cevizlik releasing double bend, where it was largely arrested, with smaller (0.5 m) slip continuing westward on the Golyaka intervening segment, west of Lake Eften. This is similar to the ter- mination of slip observed on the Karadere fault segment in the 1999 Izmit earthquake, where 1.5 m of slip was largely terminated by a releasing double bend at the Golyaka intervening segment, with minor slip continuing eastward near Lake Eften. To the east of the main shock, 5 m of slip decreased along the Dagdibi fault segment to about 3.5 m at the 1-km-wide Kaynasli step-over. From here to the east, the 2.5 m of average slip on the Kaynasli fault segment continued to decrease, reaching zero at the releasing step-overs to the Elmalik fault. The terminations of the 1999 Duzce surface rupture are defined by distinct struc- tural complexities. The rupture was arrested in the west at the 4-km-wide Eften releasing double bend, where the eastern termination of the 1999 Izmit rupture also occurred. To the east, the rupture terminated at the 4- to 5-km-wide Bakacak releasing double step-overs. Both eastern and western ends of the Duzce rupture have short fault segments that experienced minor rupture during the 1999 Duzce earthquake. The Golyaka intervening segment experienced minor rupture, during both the 1999 Izmit and Duzce earthquakes. The Duzce rupture propagated across 0.8- to 2-km- wide step-overs, but it was arrested at step-overs 4 km wide or wider. These results agree with the observations on ratios between step-over width (in kilometers) and strike-slip displacement (in meters) collected along strike-slip faults worldwide (Let- tis et al., 2002).

Constraints on the location and mechanism of the 1511 Western-Slovenia earthquake from active tectonics and modeling of macroseismic data

The 1511 Western Slovenia earthquake ( M = 6.9) is the largest event occurred so far in the region of the Alps–Dinarides junction. Though it strongly influences the regional seismic hazard assessment, the epicenter and mechanism are still under debate. The complexity of the active tectonics of the Alps–Dinarides junction is reflected by the presence of both compressional and transpressional deformations. This complexity is witnessed by the recent occurrence of three main earthquake sequences, the 1976 Friuli thrust faulting events, the 1998 Bovec–Krn Mountain and the 2004 Kobarid strike-slip events. The epicenters of the 1998 and 2004 strike-slip earthquakes ( M s = 5.7 and M s = 4.9, respectively) lie only 50 km far from the 1976 thrust earthquake ( M s = 6.5). We use the available macroseismic data and recent active tectonics studies, to assess a possible epicenter and mechanism for the 1511 earthquake and causative fault. According with previous works reported in the literature, we analyze both a two-and a single-event case, defining several input fault models. We compute synthetic seismograms up to 1 Hz in an extended-source approximation, testing different rupture propagations and applying a uniform seismic moment distribution on the fault segments. We extract the maximum horizontal velocities from the synthetics and we convert them into intensities by means of an empirical relation. A rounded-to-integer misfit between observed and computed intensities is performed, considering both a minimized and a maximized databases, built to avoid the use of half-degree macroseismic intensity data points. Our results are consistent with a 6.9 magnitude single event rupturing 50 km of the Idrija right-lateral strike-slip fault with bilateral rupture propagation.

Influence of Releasing Step-Overs on Surface Fault Rupture and Fault Segmentation: Examples from the 17 August 1999 Izmit Earthquake on the North Anatolian Fault, Turkey

The 17 August 1999 Izmit earthquake broke four distinct structural segments of the North Anatolian fault, from east to west: the Karadere, Sakarya, Sapanca, and Golcuk fault segments. These segments are separated from one another and the adjoining Yalova and Duzce fault segments to the west and east, respectively, by distinct right-releasing step-over and/or gaps in the fault trace. These include, from east to west, the Eften Lake step-over (2-4 km wide), Akyazi gap (6-13 km long), Sapanca step-over (1-2 km wide), Golcuk step-over (1-2 km wide), and Karamursel step-over (4-5 km wide). The earthquake nucleated within or near the Golcuk step-over with bilateral rupture propagation to the west and east. To the west, about 4-5.5 m of surface rupture (up to 7 m of subsurface rupture) on the Golcuk segment was mostly, if not entirely, arrested by the 4- to 5-km-wide Karamursel step-over. To the east, about 3-4 m of surface rupture (about 4 m of subsurface rupture) propagated through the 1- to 2-km-wide Sapanca step-over, triggering up to 4.5-5.5 m of surface rupture (up to 5-6 m of subsurface rupture) on the Sakarya segment. Rupture on the Sakarya segment stepped across the Akyazi gap triggering 1-1.5 m of rupture on the Karadere segment. Rupture on the Karadere segment was arrested by the 2- to 4-km-wide Eften Lake step-over. Modeling of the transient stress field associated with the rupture propagation agrees well with the field observations. The modeled fault rupture of 5 m on the Golcuk segment appears to decrease the transient stress on the eastern tip of the adjoining Yalova segment across the 4- to 5-km-wide Karamursel step-over, even if we allow rupture on the Golcuk segment to extend along the entire southern margin of the step-over. Our model shows that regardless of the amount of slip, in a generic sense, it is difficult to increase stress on the tip of an adjoining fault segment across a 5-km-wide step-over. On the other hand, fault rupture of 3 m on the western tip of the Sapanca fault segment significantly increases the transient stress on the adjoining Sakarya segment across the 1- to 2-km-wide Sapanca step-over, allowing rupture propagation across the basin. An evaluation of 30 historical strike-slip earthquakes involving 59 step-over basins supports our field observations and stress modeling of the Izmit fault rupture. Historical strike-slip events with small to large displacements usually propagate through step-overs less than 1-2 km wide. With increasing displacement, larger and larger step-overs can be ruptured through. The empirical data generally show a ratio ranging from 2:1 to 1:1 between step-over width (in kilometers) and strike-slip displacement (in meters) for a through-going rupture. However, step-overs of 4- to 5-km width appear to always arrest fault rupture, regardless of the amount of displacement. These results, therefore, provide important constraints for evaluating the probability of multisegment ruptures on segmented strike-slip faults. Manuscript received 20 August 2000.

Source rupture process of the Kocaeli, Turkey, earthquake of August 17, 1999, obtained by joint inversion of near‐field data and teleseismic data

The rupture process of the 1999 Turkey earthquake is examined using both near‐field strong motion data and teleseismic body wave data. The derived source parameters are as follows: (strike, dip, slip) = (268°, 86°, 180°), nearly pure strike‐slip; the seismic moment, Mo = 1.7 × 1020 Nm (Mw = 7.4); the source duration = 20 sec; the fault length = 70 km; the fault width = 15 km. The rupture process is characterized by an asymmetric bilateral rupture propagation and smooth slip. It consists of two major fault segments, a rupture propagating to the west and a second rupture propagating to the east. The maximum dislocation and the maximum dislocation velocity are 6.3 m and 2.7 m/s, respectively, both found at the former segment. The average dislocation is about 4 m. The extent of the coseismic rupture suggests that a considerable part of the anticipated seismic gap remains unruptured.