Single Fault Plane Research Articles

Three-dimensional structural modeling of an active fault zone based on complex outcrop and subsurface data: The Middle Durance Fault Zone inherited from polyphase Meso-Cenozoic tectonics (southeastern France)

The objective of this study was to realize a three-dimensional (3-D) geological model of the deep basin structure of the Middle Durance region (of folds and faults) by integration of geological and geophysical data, and to evaluate its fault geometry and tectonic history. All of the available geophysical and geological data were compiled in three dimensions using the gOcad geomodeler. The geological and geophysical data were used to build a 3-D geological model of the Middle Durance region. The data on the 3-D geometry of fault surfaces and stratigraphic horizons and the thickness maps of the main stratigraphic units are supported by the 3-D geological model. We show that the Middle Durance Fault cannot be interpreted as a single fault plane that affected the entire Meso-Cenozoic sedimentary layers and the Paleozoic basement but as a listric segmented faulting system in sedimentary layers, rooted in Triassic evaporites and a normal block faulting system in the basement. This decoupling level in the Triassic layers reveals thin-skin deformation, formed by strong mechanical decoupling between the Mesozoic sedimentary cover and the Paleozoic basement. This study also confirms that the Provence geological structure has resulted mainly from Pyrenean deformation, which was partly reactivated by Alpine deformation. We demonstrate that the Middle Durance Fault Zone is a transfer fault that accommodates deformation of the sedimentary filling of the South-East Basin through modified fold geometry over a zone of 7 km to 8 km around the main segment of the fault zone.

Construction of equivalent single planar fault model for strike-slip stepovers

Non-planarity is a common and natural characteristic of seismic faults that strongly affects the rupture dynamics and the associated seismic waves. Geometrically uniform planar fault model is widely used in studies of kinematic source inversion, seismic hazard estimation and rupture dynamic simulation because of the simplicity and computational efficiency of this method. The influence of the geometric heterogeneity of faults on rupture dynamics and associated seismic waves may under some circumstances be reasonably approximated by an appropriate heterogeneous distribution of shear strength of planar faults, particularly for the case with uniform initial stress distribution. An outstanding issue is how to approximate the dynamic behavior of a non-planar fault by means of heterogeneous properties on a planar fault. Thus, we propose the construction of an equivalent planar fault model for a given stepover fault through the introduction of a heterogeneous distribution of static friction coefficient. Our study shows that the distribution of static friction coefficient on the equivalent planar fault model is mainly controlled by the geometric irregularities of the stepovers regardless of the initial shear stress, thereby providing a way to quantify the effect of fault geometry, particularly the step width effect in seismic hazard estimation. Moreover, our results indicate that the influence of geometric discontinuity of stepovers on rupture dynamics is equivalent to a barrier of the planar fault model.

High-precision relocation and focal mechanism of the injection-induced seismicity at the Basel EGS

In early December 2006, a massive fluid injection was carried out at 5km depth below the city of Basel, Switzerland, for geothermal reservoir enhancement. During the six-day stimulation, approximately 13,000 induced microearthquakes were detected by a borehole network. The largest of the induced earthquakes, which had a magnitude of ML 3.4, was strongly felt in the Basel area and led to the termination of the project after only six days of stimulation. We analyzed the approximately 3500 locatable events of this induced earthquake sequence, which is one of the most densely monitored deep fluid-injections in the world. The seismic monitoring system consisted of six borehole seismometers at depths between 300m and 2700m near the injection well and of numerous surface stations in the Basel area. In this article, we report on the analysis of the sequence using exclusively data from the down-hole instruments. We show how a refinement of arrival time picks by cross-correlation techniques and subsequent high-precision relocations lead to significant improvements of the hypocenter locations compared to routinely applied manual procedures. We also analyze focal mechanisms determined from both first-motion polarities and amplitudes of signals recorded by the borehole sensors alone and compare the results to the focal mechanisms of the larger events recorded also by the surface networks. Our findings indicate that the induced sequence consists to a large part of repeating earthquakes that repeatedly rupture the same fault patches, and that the activated seismogenic structure is not a single fault plane but a heterogeneous fault zone.

Forest-Fire Analogy to Explain thebValue of the Gutenberg-Richter Law for Earthquakes

The Drössel-Schwabl model of forest fires can be interpreted in a coarse-grained sense as a model for the stress distribution in a single planar fault. Fires in the model are then translated to earthquakes. I show that when a second class of trees that propagate fire only after some finite time is introduced in the model, secondary fires (analogous to aftershocks) are generated, and the statistics of events becomes quantitatively compatible with the Gutenberg-Richter law for earthquakes, with a realistic value of the b exponent. The change in exponent is analytically demonstrated in a simplified percolation scenario. Experimental consequences of the proposed mechanism are indicated.

Fault ruptures and geothermal effects of the second earthquake, 29 May 2008, South Iceland Seismic Zone

The Reykjafjall N–S source fault ruptured during the Mw 6.3 earthquake doublet on May 29, 2008, in the South Iceland Seismic Zone (SISZ). The northern part of the fault intersects the easternmost high-temperature geothermal field of Hengill in an extinct volcano that began shifting away from the Western Rift Zone 120,000 years ago. Geophysical data indicate that the N–S fault ruptured over 20km length at depth. Our detailed field mapping along the fault length shows that the southern part of the fault did not rupture the surface mostly due to the greater depth of seismicity (down to 9km) there. The surface ruptures were dominantly found along the northern half of the fault in the highland within the high-temperature field where aftershocks were shallower than 5km. There, the surface ruptured on portions of six parallel N–S fault segments and not on a single fault plane. Hot springs are aligned on the two westernmost fault segments within the area of increased geothermal activity after the earthquake. Other Riedel shears (ENE, E–W, WNW and NNW) hosting geothermal activity also ruptured. On the N–S faults, surface ruptures and hot springs are organised in left-stepping arrays indicating dextral motion with offsets between 0.5 and 2m. On other Riedel shear sets ruptures have a right-stepping arrangement indicating sinistral motion with a maximum horizontal offset of a metre. All fresh ruptures had centimetre-scale normal-slip and openings <1m but up to 1.5m along atypical N–S structures.Our results fit with the lineations in the aftershocks and with fault plane solutions. They also show that the 2008 source fault ruptured within a 2-km wide deformation zone where previous earthquakes and changes to geothermal activity have occurred at least since 1706. This implies that major earthquakes in this area may have a common source fault at depth and a wider deformation zone at the surface. Our study demonstrates how transform faulting at the junction of a rift segment causes recurring medium–large earthquakes that control fault behavior and permeability in fractured geothermal fields.

Finite-Fault Source Inversion Using Teleseismic P Waves: Simple Parameterization and Rapid Analysis

We examine the ability of teleseismic P waves to provide a timely image of the rupture history for large earthquakes using a simple, 2D finite‐fault source parameterization. We analyze the broadband displacement waveforms recorded for the 2010 M w∼7 Darfield (New Zealand) and El Mayor‐Cucapah (Baja California) earthquakes using a single planar fault with a fixed rake. Both of these earthquakes were observed to have complicated fault geometries following detailed source studies conducted by other investigators using various data types. Our kinematic, finite‐fault analysis of the events yields rupture models that similarly identify the principal areas of large coseismic slip along the fault. The results also indicate that the amount of stabilization required to spatially smooth the slip across the fault and minimize the seismic moment is related to the amplitudes of the observed P waveforms and can be estimated from the absolute values of the elements of the coefficient matrix. This empirical relationship persists for earthquakes of different magnitudes and is consistent with the stabilization constraint obtained from the L‐curve in Tikhonov regularization. We use the relation to estimate the smoothing parameters for the 2011 M w 7.1 East Turkey, 2012 M w 8.6 Northern Sumatra, and 2011 M w 9.0 Tohoku, Japan, earthquakes and invert the teleseismic P waves in a single step to recover timely, preliminary slip models that identify the principal source features observed in finite‐fault solutions obtained by the U.S. Geological Survey National Earthquake Information Center (USGS/NEIC) from the analysis of body‐ and surface‐wave data. These results indicate that smoothing constraints can be estimated a priori to derive a preliminary, first‐order image of the coseismic slip using teleseismic records.

Source process of the 2011 off the Pacific coast of Tohoku Earthquake inferred from waveform inversion with long-period strong-motion records

We have investigated the rupture process of the 2011 off the Pacific coast of Tohoku Earthquake using a multi-time-window linear waveform inversion method using long-period strong-ground motion data. From the record section of the long-period motion of the 3 phases, it is indicated that the rupture process mainly consists of the 3 stages. We have assumed a single planar fault model of 468 km long in strike and 228 km wide in dip. The seismic moment of this earthquake was estimated to be 4.3 × 1022 N m (Mw 9.0). The inverted slip distribution shows a large asperity with a maximum slip of about 47 m which is located on the shallower part of the fault plane. The rupture process is divided into three stages: a first stage with moderate slip; a second stage with large and long-duration slip in the shallow part of the fault; and a third stage with relatively small and short-duration slip in the southern part of the fault. The feature of the ground motion suggested from the record section is well represented by the peak moment rate distributions, rather than the slip distributions. The rupture velocity is around 3–4 km/s in the first and third stages, while rupture progression was suspended for a while before the rupture of the asperity.

The August 17, 1999 Izmit, Turkey, earthquake: slip distribution from dislocation modeling of DInSAR and surface offset

We show the results of application of Differential SAR Interferometry to the MW 7.4, August 17, 1999, Izmit earthquake, Western Turkey. The differential interferogram is obtained using an interferometric ERS2 ascending pair with a time interval of 35 days (August 13th - September 17th). The fringe pattern clearly defines the coseismic displacement field extended in an area of about 100 km N-S and 120 km E-W. The analysis of the interferogram shows the right-lateral strike-slip movement on the activated section of the North Anatolian fault system. The maximum SAR-detected displacement ranges between 117.6 cm and 134.4 cm in the proximity of Golcuk. We invert SAR data for uniform dislocation on a single fault plane using a Montecarlo procedure, with the aim of testing a large set of a priori possible asperity distributions on the fault. We then use a forward modeling approach to evaluate the slip variability for the dislocation using additional constraints as surface offsets and seismicity distribution: in this case we allow unit cells to undergo different values of slip in order to refine the initial dislocation model. Misfits between SAR data and modeled slant range displacements are generally low for all our models (~ 12 cm). Our results indicate that slip is concentrated in the central-western part of the fault, in the upper 10-15 km, tapering to the fault tips. For the Izmit case, we note that a well constrained fault model can be obtained only integrating DInSAR data with additional observations. This is mainly due to an undersampling of the displacement field by DInSAR, caused by decorrelation and lack of image data.

Rupture Process of the 2008 Northern Iwate Intraslab Earthquake Derived from Strong-Motion Records

The rupture process of the 2008 northern Iwate, Japan, earthquake ( M 6.8) has been derived from strong-motion records obtained at 22 regional stations by the multi-time-window linear waveform inversion method. This event occurred in the lower plane of the double seismic zone in the subducting Pacific slab at a depth of 115 km. A preliminary inversion analysis revealed that a single planar fault plane model was insufficient to satisfactorily explain waveforms at stations in a specific azimuth. A two-segment fault plane model was then assumed in which the fault model consisted of the northern and southern segments with a 44° difference in strike direction, which respectively followed the source mechanisms determined by moment tensor inversion and P -wave polarity analysis. The source model derived from the two-segment fault plane model greatly improved the waveform fitting at stations for which there was a large discrepancy in the preliminary analysis with the single planar fault plane model. The largest asperity was located on the northern segment, which generated the main phase observed at most stations. The amount of slip on the southern segment was smaller but contributed significantly to the first of the two pulses observed at the stations to the south of the source area. The stress drop of the largest asperity for this intraslab earthquake was large in comparison with that predicted from empirical relations obtained for crustal earthquakes, which may be one of the causes of the relatively large accelerations recorded during this earthquake.

Faulting and deformation in chalk

In chalk, strong deformation at the grain level is associated with faulting. Within a 5–10 cm thick fringe on each side of normal faults, texture modifications were documented and analysed in samples from the Campanian White Chalk. We used SEM observations, image analysis, reconstruction of the porous media and physical measurements. Dissolution and cementation features, together with a reorganisation of the pore space, can be explained by massive fluid transfers. Faulting is interpreted as ductile shearing, which involves slip and dissolution first in shear deformation bands and then along a single fault plane.

Seismically and geodetically determined nondouble‐couple source mechanisms from the 2000 Miyakejima volcanic earthquake swarm

The volcanic eruption on Miyakejima, Japan, in 2000 was marked by the largest earthquake swarm ever recorded in Japan, a seismicity migration accompanying a dike intrusion as the dike propagated from Miyakejima to the northwest, and formation of a caldera on Mount Oyama on Miyakejima. In this study, we propose a seismic source model which can be used to model both seismic and geodetic displacements from volcanic earthquakes. Our model, the “crack + double‐couple” (CDC) model, combines tensile opening with shear slip along a single fault plane. We find that this model can fit both seismic and GPS data from the 1 July and 30 July earthquakes, the largest two in the Miyakejima sequence. The results of our GPS inversions for these two earthquakes are consistent with the seismic mechanisms and aftershock locations, and the GPS mechanisms successfully forward predict the observed regional seismograms. The 1 July earthquake, located near the northwest tip of the dike, has a large opening component and a geodetic moment about 5 times larger than that inferred from the seismic data alone. The source process for this event consists of tensile failure, which occurred quickly, and a much slower accumulation of shear slip. We apply the CDC model to 16 additional earthquakes from this sequence and find that the CDC model fits the seismic data for these earthquakes at least as well as established seismic moment tensor models.

Methodology for risking fault seal capacity: Implications of fault zone architecture

We introduce a methodology for quantifying the risk associated with a seal for fault-bounded prospects. Applying this methodology, the aspects of fault seal are confined within four main risk categories. The methodology allows comparable criteria to be applied in the risking procedure to reduce uncertainty in fault seal assessments. As a foundation for the methodology, we combine onshore and offshore data from large faults and demonstrate how architecture and the distribution of fault rocks may influence sealing capacity. Despite the variable and complex structure of fault zones, we have observed fault zone characteristics that appear in common to the faults investigated, and we consider these factors to be crucial in the risking of fault seal predictions. The fault zones in our database, typically bounded by external slip surfaces, represent two main categories: (1) a layer of shale smear entrained into the fault zone and derived from a thick shale source layer within the sequence offset by the fault and (2) fault zones characterized by internal slip surfaces, slivers of footwall and hanging-wall–derived material rotated along the fault zone and commonly enclosed in a matrix of shaly-silty fault gouge. This study highlights the disparity between the complexity of actual faults and the abrasion-type shale gouge ratio (SGR) algorithm currently used in the industry to estimate sealing capacity of faults, which assumes that the seismically derived throw is concentrated in a single fault plane. We discuss how this may influence sequence juxtaposition across a fault, the associated SGR values, and ultimately, the fault seal risking.

Kinematic and dynamic fault slip analyses: implications from the surface rupture of the 1999 Chi-Chi, Taiwan, earthquake

The 1999 Chi-Chi earthquake created a 100-km-long surface rupture on the Chelungpu thrust fault in Taiwan due to convergence between the Phillipine Sea and Eurasian plates. Fault slip measurements were made by several researchers from the entire length of the rupture directly following the earthquake and can thus be attributed to a single tectonic event. Conventional fault slip analyses are applied to these data and the results are compared with independent seismological and kinematic observations. Unlike many fault slip analyses, complications due to multiple deformations can be categorically excluded and the results can be evaluated from the seismological and plate movement data. Kinematic analyses of fault slip data that are weighted by displacement show sub-horizontal NW–SE shortening that is parallel to the plate convergence vector. A single fault plane solution satisfies almost all the data. Right dihedra and trihedra solutions also satisfy almost all the surface rupture measurements and give σ 1 in a NW–SE orientation that is similar to the results of stress inversion and to inversion from earthquakes in the Chi-Chi earthquake sequence. Despite criticisms of fault slip analysis methods, these results show that fault slip analyses from data collected along major faults, which have not witnessed multiple deformation events, can be valid. Homogeneous strain and stress states exist in the sense that kinematic and dynamic solutions can be found that fit essentially all the data, and these solutions have tectonic significance.

Small-scale spatial variation of the stress field in the back-arc Aegean area: Results from the seismotectonic study of the broader area of Mygdonia basin (N. Greece)

In the present work a detailed seismotectonic study of the broader area of the Mygdonia basin (N. Greece) is performed. Digital data for earthquakes which occurred in the broader Mygdonia basin and were recorded by the permanent telemetric network of the Geophysical Laboratory of the Aristotle University of Thessaloniki during the period 1989–1999 were collected and fault plane solutions for 50 earthquakes which occurred in the study area were calculated with a modified first motions approach which incorporates amplitude and radiation pattern information. Fault plane solutions for the 3 main shocks of Volvi (23/05/78, M W = 5.8 and 20/06/78, M W = 6.5) and Arnaia (04/05/95, M W = 5.8) events and the 1978 aftershock sequence were additionally used. Moreover, data from two local networks established in the Mygdonia basin were also incorporated in the final dataset. Determination of the stress field was realized by the use of the method of Gephart and Forsyth [Gephart, J.W., Forsyth, D.W., 1984. An improved method for determining the regional stress tensor using earthquake focal mechanism data: application to the San Fernando earthquake sequence: Jour. Geophys. Res., v.89, no. B11, p. 9305–9320] for the stress tensor inversion and the results were compared with independent estimates based on the calculation of the average moment tensor [Papazachos, C.B.,Kiratzi, A.A., 1992. A formulation for reliable estimation of active crustal deformation and its application to central Greece. Geophys. J. Int. 111, 424–432]. The obtained stress results show a relatively good agreement between the two approaches, with differences in the azimuth of the dominant extension axis of the order of 10°. Furthermore, comparison with independent information for the mean stress axes provided by the study of kinematics on neotectonic faults [Mountrakis, D., Kilias, A., Tranos, M., Thomaidou, E., Papazachos, C., Karakaisis, G., Scordilis, E., Chatzidimitriou, P., Papadimitriou, E., Vargemezis, G., Aidona, E., Karagianni, E., Vamvakaris, D. Skarlatoudis, A. 2003. Determination of the settings and the seismotectonic behavior of the main seismic-active faults of Northern Greece area using neotectonic and seismological data. Earthquake Planning and Protection Organisation (OASP) (in Greek)] shows a similar agreement with typical misfit of the order 10°. The stress inversion method was modified in order to select one or both nodal planes of the focal mechanism which corresponds to the “true” fault plane of the occurred earthquakes and was able to select a single fault plane in the majority of examined cases. Using this approach, the obtained fault plane rose diagrams are in agreement with results from various neotectonic studies. Moreover, several secondary active fault branches were identified, which are still not clearly observed in the field.

Low-temperature (

The paucity of Phanerozoic rock sequences in the Proterozoic Mt Isa and Murphy Inliers in northern Australia renders it difficult to determine their Phanerozoic tectonic histories. However, thermochronological methods provide a means to assess this problem. Apatite fission track ages vary from 390 to 218 Ma, and mean track lengths range between 11.1 and 13.6 μm. These results record a non-linear cooling history below about 110 ± 10°C. Forward modelling of the fission track data suggests that the first episode of relatively rapid cooling since the Early Carboniferous occurred between ca 350 and 250 Ma at rates of ∼0.9°C/million years and was synchronous with intracontinental deformation associated with the Alice Springs Orogeny and tectonics at the contemporaneous eastern margin of Australia. This event resulted in ≥ 2 km of exhumation across both inliers. Cooling rates also increased after 100 Ma, although estimates of the amount of cooling and exhumation across trhe inliers obtained by forward modelling of the fission track data exceed those determined by constraints provided by the 40Ar/39Ar ages of weathering profiles. This observation adds to the growing amount of evidence that the fission track annealing model employed overestimates cooling rates at low temperatures (<60 – 70°C). The spatial variation of apatite fission track data within the Mt Isa Inlier indicates the three major structural belts, the Western Fold Belt, Kalkadoon – Leichhardt Belt and the Eastern Fold Belt, which are separated by major north – south faults, experienced similar thermal histories on a regional scale. This suggests that the main faults bounding the belts have not experienced major reactivation in a vertical sense, as a single large-scale fault plane, since ca 350 Ma. However, adjacent smaller scale, fault-bounded blocks within the structural belts demonstrate variable cooling histories, suggesting that reactivation of favourably oriented minor faults in the inlier, including segments of the major faults, has occurred over this time interval. Variations in apatite fission track data show that up to 1.5 km of vertical displacement occurred along parts of the Mt Isa Fault Zone and the Pilgrim Fault between 350 and 250 Ma.

Surface displacements and source parameters of the 2003 Bam (Iran) earthquake from Envisat advanced synthetic aperture radar imagery

The Mw 6.6, 26 December 2003 Bam (Iran) earthquake was one of the first earthquakes for which Envisat advanced synthetic aperture radar (ASAR) data were available. Using interferograms and azimuth offsets from ascending and descending tracks, we construct a three‐dimensional displacement field of the deformation due to the earthquake. Elastic dislocation modeling shows that the observed deformation pattern cannot be explained by slip on a single planar fault, which significantly underestimates eastward and upward motions SE of Bam. We find that the deformation pattern observed can be best explained by slip on two subparallel faults. Eighty‐five percent of moment release occurred on a previously unknown strike‐slip fault running into the center of Bam, with peak slip of over 2 m occurring at a depth of ∼5 km. The remainder occurred as a combination of strike‐slip and thrusting motion on a southward extension of the previously mapped Bam Fault ∼5 km to the east.

The 2002 Molise earthquake sequence: What can we learn about the tectonics of southern Italy?

On October 31, 2002 a M L = 5.4 earthquake occurred in southern Italy, at the margin between the Apenninic thrust belt (to the west) and the Adriatic plate (to the east). In this area, neither historical event nor seismogenic fault is reported in the literature. In spite of its moderate magnitude, the earthquake caused severe damage in cities close to the epicenter and 27 people, out of a total of 29 casualties, were killed by the collapse of a primary school in S. Giuliano di Puglia. By inverting broadband regional waveforms, we computed moment tensor solutions for 15 events, as small as M L = 3.5 ( M w = 3.7). The obtained focal mechanisms show pure strike-slip geometry, mainly with focal planes oriented to NS (sinistral) and EW (dextral). In several solutions focal planes are rotated counterclockwise, in particular for later events, occurring west of the mainshock. From the relocated aftershock distribution, we found that the mainshock ruptured along an EW plane, and the fault mechanisms of some aftershocks were not consistent with the mainshock fault plane. The observed stress field, resulting from the stress tensor inversion, shows a maximum principal stress axis with an east–west trend (N83°W), whereas the minimum stress direction is almost N–S. Considering both the aftershock distribution and moment tensor solutions, it appears that several pre-existing faults were activated rather than a single planar fault associated with the mainshock. The finite fault analysis shows a very simple slip distribution with a slow rupture velocity of 1.1 km/s, that could explain the occurrence of a second mainshock about 30 h after. Finally, we attempt to interpret how the Molise sequence is related to the normal faulting system to the west (along the Apennines) and the dextral strike-slip Mattinata fault to the east.

Source analysis of the Mw 6.3 2004 Al Hoceima earthquake (Morocco) using regional apparent source time functions

We use small and moderate aftershocks of the 24 February 2004 Al Hoceima earthquake as empirical Greens' functions (EGFs) to retrieve the rupture history of the main event. The magnitude, depth, and geometry of faulting were estimated for the main shock and 20 small and moderate aftershocks (Mw between 3.9 and 5.2) computing moment tensor solutions. For the main shock, we obtain a moment magnitude of Mw = 6.3 and a nearly pure double‐couple source (4% compensated linear vector dipole) with predominately strike‐slip faulting. The preferred fault plane has a strike of N11°E, a dip of 72°, and a rake of −17°. Fourteen aftershocks, having depths (±4 km) and faulting geometries (±11° for the strike of the preferred plane) similar to the ones found for the main event, were used as EGFs. Apparent source time functions (ASTFs) of the Al Hoceima event were retrieved at twelve regional stations using an iterative time domain deconvolution on the P waveforms. At each station, time functions are enhanced by stacking of the deconvolved waveforms for different EGFs. ASTFs show total durations of 5 to 6 s, two consecutive subevents separated by about 3 s, and only a weak effect of directivity. We further investigate the source history by inverting the set of ASTFs for the distribution of fault slip and rupture propagation. Two different source models were selected for presentation, one involving one single fault plane and the other involving two parallel fault planes. The two‐fault model fits more accurately the observations and leads to a more plausible slip distribution. We estimate rupture dimensions of about 10 × 10 km2 and 8 × 8 km2 and moment magnitudes of Mw = 6.2 and 6.0 for the first and second subevent, respectively.

Locations of selected small earthquakes in the Zagros mountains

The Zagros mountains of southern Iran are marked by a zone of high seismicity and accommodate a significant portion of the convergence between Arabia and Eurasia. Due to the lack of dense local seismic or geodetic networks, the inferred kinematics of the collision in Iran is mainly based on catalogs of teleseismically determined earthquake locations. We surveyed all Mw &gt; 4.5 earthquakes in the Harvard Centroid Moment Tensor (HCMT) and International Seismological Centre (ISC) catalogs that occurred in the Zagros mountains during the period 1992–2002 and that were spanned by Interferometric Synthetic Aperture Radar (InSAR) images from the ERS 1 and 2 satellites. We invert the observed deformation for the best fitting point source, single fault plane, and distributed fault slip for four earthquakes and one unexplained deformation event. We find that we can precisely locate earthquakes that are too small to be well‐located by either the HCMT or ISC catalogs, allowing us to tie specific earthquakes to active geologic structures.

Fault geometry and slip distribution of the 2003 Tokachi-oki earthquake as deduced from teleseismic body waves

I have analyzed teleseismic body waves from the 2003 Tokachi-oki earthquake (M w 8.1), and inferred the slip distribution. Two simple fault models are assumed for estimating the effect of fault geometry on derived slip distributions. One is a single planar fault with a dip of 20° and the other is a compound fault having a shallow plane with a dip of 5° and deeper, landward plane with a dip of 20°. The compound-fault model is preferable because it explains the initial part of the observed P-waves better. It is found that the planar fault has one asperity (patch of large slip) near the hypocenter and the other asperity to the landward side of the hypocenter. The compound-fault model shares the landward asperity with the planar-fault model, but does not have the asperity near the hypocenter. The other asperity on the compound fault is found far from the hypocenter. This difference of the slip distributions suggests the importance of accurate modeling of the fault dip angle when deducing the slip distribution from teleseismic body waves.