Geodetically determined strain across the southern end of the Tonga-Kermadec-Hikurangi subduction zone

Summary. A method of simultaneous reduction is presented for determining strain rates from multiple triangulation surveys where common triangulation stations have been used, but the angles of the old survey have not necessarily been reobserved. This method is applied to triangulation in the northern South Island, at the southern end of the Tonga-Kermadec-Hikurangi subduction zone. From a profile of shear strain across the Indian-Pacific plate boundary, the displacement of the Indian plate relative to the Pacific is calculated to be 54 ± 9 mm yr⁻¹ at an azimuth of 84°± 10°, in remarkable agreement with the motion predicted by global plate tectonic models. Most of this motion occurs within a 150 km wide zone bounded on the east by the Hikurangi Trough. Within this zone the motion is partitioned: near the Hikurangi Trough no slip is occurring at the upper surface of the subducting Pacific plate (the subduction thrust) and motion is predominantly thrusting normal to the trough axis: to the west is a region of predominantly dextral strike slip faulting. This pattern is consistent with Fitch's model of oblique subduction. To the south of the profile, a change is observed in the azimuth of the faulting along a line which marks the southern extent of the subduction slab, indicating the end of the partitioned motion.     

Focal mechanisms of recent earthquakes in the Southern Korean Peninsula

We evaluate the stress field in and around the southern Korean Peninsula with focal mechanism solutions, using the data collected from 71 earthquakes ( M_L = 1.9–5.2) between 1999 and 2004. For this, the hypocentres were relocated and well-constrained fault plane solutions were obtained from the data set of 1270 clear P -wave polarities and 46 SH / P amplitude ratios. The focal mechanism solutions indicate that the prevailing faulting types in South Korea are strike-slip-dominant-oblique-slip faultings with minor reverse-slip component. The maximum principal stresses (σ₁) estimated from fault-slip inversion analysis of the focal mechanism solutions show a similar orientation with E–W trend (269°–275°) and low-angle plunge (10°–25°) for all tectonic provinces in South Korea, consistent with the E–W trending maximum horizontal stress (σ_Hmax) of the Amurian microplate reported from in situ stress measurements and earthquake focal mechanisms. The directions of the intermediate (σ₂) and minimum (σ₃) principal stresses of the Gyeongsang Basin are, however, about 90 deg off from those of the other tectonic provinces on a common σ₂–σ₃ plane, suggesting a permutation of σ₂ and σ₃. Our results incorporated with those from the kinematic studies of the Quaternary faults imply that NNW- to NE-striking faults (dextral strike-slip or oblique-slip with a reverse-slip component) are highly likely to generate earthquakes in South Korea.     

The 2004 May 28 Baladeh earthquake (Mw 6.2) in the Alborz, Iran: overthrusting the South Caspian Basin margin, partitioning of oblique convergence and the seismic hazard of Tehran

We use teleseismic waveform analysis and locally recorded aftershock data to investigate the source processes of the 2004 Baladeh earthquake, which is the only substantial earthquake to have occurred in the central Alborz mountains of Iran in the modern instrumental era. The earthquake involved slip at 10–30 km depth, with a south-dipping aftershock zone also restricted to the range 10–30 km, which is unusually deep for Iran. These observations are consistent with co-seismic slip on a south-dipping thrust that projects to the surface at the sharp topographic front on the north side of the Alborz. This line is often called the Khazar Fault, and is assumed to be a south-dipping thrust which bounds the north side of the Alborz range and the south side of the South Caspian Basin, though its actual structure and significance are not well understood. The lack of shallower aftershocks may be due to the thick pile of saturated, overpressured sediments in the South Caspian basin that are being overthrust by the Alborz. A well-determined earthquake slip vector, in a direction different from the overall shortening direction across the range determined by GPS, confirms a spatial separation ('partitioning') of left-lateral strike-slip and thrust faulting in the Alborz. These strike-slip and thrust fault systems do not intersect within the seismogenic layer on the north side, though they may do so on the south. The earthquake affected the capital, Tehran, and reveals a seismic threat posed by earthquakes north of the Alborz, located on south-dipping thrusts, as well as by earthquakes on the south side of the range, closer to the city.     

Aseismic fault movement before the 1995 Kobe earthquake detected by a GPS survey: implication for preseismic stress localization?

By inversion analysis of the baseline changes and horizontal displacements observed with GPS (Global Positioning System) during 1990–1994, a high-angle reverse fault was detected in the Shikoku-Kinki region, southwest Japan. The active blind fault is characterized by reverse dip-slip (0.7±0.2  m yr⁻¹ within a layer 17–26  km deep) with a length of 208±5  km, a (down-dip) width of 9±2  km, a dip-angle of 51°±2° and a strike direction of 40°±2° (NE). Evidence from the geological investigation of subfaults close to the southwestern portion of the fault, two historical earthquakes ( M _L=7.0, 1789 and 6.4, 1955) near the centre of the fault, and an additional inversion analysis of the baseline changes recorded by the nationwide permanent GPS array from 18 January to 31 December 1995 partially demonstrates the existence of the fault, and suggests that it might be a reactivation of a pre-existing fault in this region. The fact that hardly any earthquakes ( M _L>2.0) occurred at depth on the inferred fault plane suggests that the fault activity was largely aseismic. Based on the parameters of the blind fault estimated in this study, we evaluated stress changes in this region. It is found that shear stress concentrated and increased by up to 2.1 bar yr⁻¹ at a depth of about 20  km around the epicentral area of the 1995 January 17  Kobe earthquake ( M _L=7.2, Japan), and that the earthquake hypocentre received a Coulomb failure stress of about 5.6 bar yr⁻¹ during 1990–1994. The results suggest that the 1995  Kobe earthquake could have been induced or triggered by aseismic fault movement.     

Seismicity within a zone of plate convergence—the Wellington region, New Zealand

Summary. A telemetered network of sensitive seismographs is being used to study the seismicity of the Wellington region, within the broad shear belt through New Zealand that marks the convergence of the Pacific and Indian plates. On average about 4 event/day are detected within 75 km of the network centre. Of the events located during the first two years of operation, the majority define a band of relatively intense activity, at depths from 20 to 40 km, dipping gently to the north-west and marking some surface near the top of the underthrust Pacific lithosphere. There is less intense and more diffuse activity both above and below this zone. The shallower activity does not correlate with major surface faults and reflects the widespread nature of shallow deformation throughout the shear belt. The mechanism of shallow events well above the band of high activity appears to be a mixture of strike-slip and thrust faulting, while that of at least part of the activity in the band is normal faulting. The rate of activity and b value, as functions of time, show significant variations, some of which may be related to the occurrence of the largest shock of the period, M _L= 6.1.     

Focal mechanisms of small earthquakes in the southeastern Brazilian shield: a test of stress models of the South American plate

Focal mechanisms of small earthquakes with magnitudes of about 3 in the SE Brazilian shield are calculated using S / P amplitude ratios. Low attenuation ( Q _p from 400 to 800) in the shield upper-crustal layers allowed sharp S arrivals to be recorded up to distances of 100 km. Besides P -wave polarities, SH -wave first motions were also used to constrain the nodal-plane orientations. Normal and reverse faulting mechanisms with strike-slip components were found. The inversion of four mechanisms to estimate the stress tensor indicated a strike-slip stress regime with roughly E–W-orientated σ ₁ and N–S σ ₃. Both the orientations and the shape factor ( φ =0.7) of the inverted stress are in excellent agreement with theoretical predictions for that part of Brazil from the driving-force model of Coblentz & Richardson (1996) . Good agreement with the nature of the stress, as well as its orientation, was also found for the model of Meijer (1995) . Both of these theoretical models include spreading stresses along the continent/ocean lithospheric transition. Because the earthquakes are more than 300 km from the continental shelf they should not be affected by the local flexural forces caused by sediment load in the marginal basins. The agreement between observed and theoretical stresses then confirms the importance of continental spreading forces in modelling intraplate stresses.     

On the consistency of earthquake moment release and space geodetic strain rates: Europe

In this paper, approximately 100 VLBI/SLR/GPS velocities map European strain rates from <0.09 × 10⁻⁸ to >9.0 × 10⁻⁸ yr⁻¹ with regional uncertainties of 20 to 40 per cent. Kostrov's formula translates these strain-rate values into regional geodetic moment rates M¯˙ _geodetic . Two other moment rates, M¯˙ _seismic , extracted from a 100-year historical catalogue and M¯˙ _plate , taken from plate-tectonic models, contrast the geodetic rates. In Mediterranean Europe, the ratios of M¯˙ _seismic to M¯˙ _geodetic are between 0.50 and 0.71. In Turkey the ratio falls to 0.22. Although aseismic deformation may contribute to the earthquake deficit ( M¯˙ _seismic values less than M¯˙ _geodetic ), the evidence is not compelling because the magnitudes of the observed shortfalls coincide with the random variations expected in a 100-year catalogue. If the lack of aseismic deformation inferred from the 100-year catalogue holds true for longer periods, then much of Europe's strain budget would have to be accommodated by more frequent or larger earthquakes than have been experienced this century to raise the ratios of M¯˙ _seismic to M¯˙ _geodetic to unity. Improved geological fault data bases, longer historical earthquake catalogues, and densification of the continent's space geodetic network will clarify the roles of aseismic deformation versus statistical quiescence.     

2001 August earthquake swarm at Shadwan Island, Gulf of Suez, Egypt

The earthquake swarm that struck Shadwan Island at the entrance of the Gulf of Suez in 2001 August included 408 events. Almost all of these events (94 per cent) were microearthquakes and only 6 per cent had small measurable magnitudes  (5.0 > M _L≥ 3.0)  . Most of the earthquakes were weak and followed each other so closely in time that they could not be identified at more distant stations. The fault plane solutions of the strongest events of the swarm show almost identical focal mechanisms, predominately normal faulting with a significant sinistral strike-slip component for nodal planes trending NW–SE. A comparison with the mechanisms of the 1969 and 1972 events which took place 20 km north of the swarm epicentral region shows similarities in faulting type and orientation of nodal planes. The azimuths of T -axes determined from focal mechanisms in this study are oriented in the NNE–SSW direction. This direction is consistent with the present-day stress field derived from borehole breakouts in the southern Gulf of Suez and the last phase of stress field changes in the Late Pleistocene, as well as with recent GPS results.
The source parameters of the largest  ( M _L≥ 3.0)  events of the 2001 August Shadwan swarm have been estimated from the P -wave spectra of the Egyptian National Seismograph Network (ENSN). Averaging of the values obtained at different stations shows relatively similar source parameters, including a fault length of  0.65 ≤ L ≤ 2 km  , a seismic moment of  7.1 × 10¹²≤ Mo ≤ 3.0 × 10¹⁴ N m  and a stress drop of  0.4 ≤Δσ≤ 10  bar.     

Dynamic stress field of a kinematic earthquake source model with k-squared slip distribution

Source models such as the k -squared stochastic source model with k -dependent rise time are able to reproduce source complexity commonly observed in earthquake slip inversions. An analysis of the dynamic stress field associated with the slip history prescribed in these kinematic models can indicate possible inconsistencies with physics of faulting. The static stress drop, the strength excess, the breakdown stress drop and critical slip weakening distance D _c distributions are determined in this study for the kinematic k -squared source model with k -dependent rise time. Several studied k -squared models are found to be consistent with the slip weakening friction law along a substantial part of the fault. A new quantity, the stress delay, is introduced to map areas where the yielding criterion of the slip weakening friction is violated. Hisada's slip velocity function is found to be more consistent with the source dynamics than Boxcar, Brune's and Dirac's slip velocity functions. Constant rupture velocities close to the Rayleigh velocity are inconsistent with the k -squared model, because they break the yielding criterion of the slip weakening friction law. The bimodal character of D _c / D _tot frequency–magnitude distribution was found. D _c approaches the final slip D _tot near the edge of both the fault and asperity. We emphasize that both filtering and smoothing routinely applied in slip inversions may have a strong effect on the space–time pattern of the inferred stress field, leading potentially to an oversimplified view of earthquake source dynamics.     

Tectonics of the Marmara Sea region of Turkey: new evidence from microearthquake fault plane solutions

Summary. Composite and single-event fault plane solutions for microearthquakes in the Izmit Bay area of the Marmara Sea indicate right-lateral strike-slip motion and tension on this extension of the North Anatolian Fault. This interpretation is consistent with teleseismically determined fault-plane solutions obtained for large earthquakes on the Marmara Sea seismic lineation. Consideration of the microplate geometry of north-western Turkey, inferred from seismicity as well as earthquake mechanisms, suggests that the region comprises two seismotectonic units with differing styles of deformation. The (Anatolid) structures of south- and central-western Anatolia are undergoing major extension, whereas the (Pontid) structures of the Marmara Sea region are being sheared, resulting in a mixed regime of both strike-slip and extensional faulting.     

Source process and tectonic implications of the great 1975 North Atlantic earthquake

Summary. The Atlantic segment of the Africa–Europe plate boundary has usually been interpreted as a transform boundary on the basis of the bathymetric expression of the Gloria fault and dextral strike-slip first-motion mechanisms aligned along the Azores–Gibraltar line of seismicity. The 1975 May 26 earthquake ( M _s=7.9) was assumed to fit into this framework because it occurred in the general area of this line and has a similar first-motion focal mechanism (strike=288°, dip=72°, slip angle=184°). However, several anomalies cast doubt on this picture: the event is abnormally large for an oceanic transform event; a sizeable tsunami was excited; the aftershock area is unusually small for such a large event; and most significantly, the epicentre is 200 km south of the presumed plate boundary. The Rayleigh wave radiation pattern indicates a change in focal mechanism to one with a significant dip-slip component. The short duration of the source time history (20 s, as deconvolved from long-period P -waves), the lack of directivity in the Rayleigh waves, and the small one-day aftershock area suggest a fault length less than 80 km. One nodal plane of the earthquake is approximately aligned with the trace of an ancient fracture zone.
We have compared the Pasadena 1-90 record of the 1975 earthquake to that of the 1941 North Atlantic strike-slip earthquake (200 km to the NNW) and confirmed the large size of the 1941 event ( M =8.2). The non-colinear relationship of the 1975 and 1941 events suggests that there is no well-defined plate boundary between the Azores and Gibraltar. This interpretation is supported by the intraplate nature of both the 1975 event and the large 1969 thrust event 650 km to the east. This study also implies that the largest oceanic strike-slip earthquakes occur in old lithosphere in a transitional tectonic regime.     

Multimode surface waveform tomography of the Pacific Ocean: a closer look at the lithospheric cooling signature

We present a regional surface waveform tomography of the Pacific upper mantle, obtained using an automated multimode surface waveform inversion technique on fundamental and higher mode Rayleigh waves, to constrain the   V_SV   structure down to ∼400 km depth. We have improved on previous implementations of this technique by robustly accounting for the effects of uncertainties in earthquake source parameters in the tomographic inversion. We have furthermore improved path coverage in the South Pacific region by including Rayleigh wave observations from the French Polynesian Pacific Lithosphere and Upper Mantle Experiment deployment. This improvement has led to imaging of vertical low-velocity structures associated with hotspots within the South Pacific Super-Swell region. We have produced an age-dependent average cross-section for the Pacific Ocean lithosphere and found that the increase in   V_SV   with age is broadly compatible with a half-space cooling model of oceanic lithosphere formation. We cannot confirm evidence for a Pacific-wide reheating event. Our synthetic tests show that detailed interpretation of average   V_SV   trends across the Pacific Ocean may be misleading unless lateral resolution and amplitude recovery are uniform across the region, a condition that is difficult to achieve in such a large oceanic basin with current seismic stations.     

New techniques for the analysis of earthquake sources from local array data with an application to the 1993 Scotts Mills, Oregon, aftershock sequence

We analysed aftershocks recorded by a temporary digital seismic network following the moderate M _w = 5.5 1993, Scotts Mills, Oregon, earthquake. A technique to retrieve source moment tensors from local waveforms was developed, tested, and applied to 41 small earthquakes ( M _w ranging from 1.6 to 3.2). The derived focal mechanisms, although well resolved, are highly variable and do not share a common nodal plane. In contrast, the majority of the events, relocated with a joint hypocentre determination algorithm, collapse to a well-focused plane. The incompatibility of the nodal planes of most events with the plane defined by their locations suggests that the aftershocks did not occur on the fault plane, but tightly around it, outlining the rupture area rather than defining it. Furthermore, the moment tensors reveal stable P -axes, whereas T  -axes plunges are highly dispersed. We detect a rotation of average T  -axis plunge with depth, indicating a change from shallower, predominantly dip-slip mechanisms to deeper strike-slip mechanisms. These characteristics are difficult to explain by remnant stress concentrations on the main-shock rupture plane or asperity- and barrier-type models. We suggest that the aftershocks occurred under the ambient regional stress, triggered by a sudden weakening of the region surrounding the main-shock slip, rather than from a shear stress increase due to the main shock.     

ScSp observed on North Island, New Zealand: implications for subducting plate structure

Seismic phase conversions provide important constraints on the layered nature of subduction zone structures. Recordings from digital stations in North Island, New Zealand, have been examined for converted ScS ‐to‐ p ( ScSp ) arrivals from deep (>150 km) Tonga–Kermadec earthquakes to image layering in the underlying Hikurangi subduction zone. Consistent P ‐wave energy prior to ScS has been identified from stations in eastern and southern North Island, where the subducted plate interface is at a depth of between 15 and 30 km. Two ScS precursors are observed. Ray tracing indicates that the initial precursor ( ScSp ₁) corresponds to conversion from the base of an 11–14 km thick subducting Pacific crust. The second precursor is interpreted as a conversion from the top of the subducting plate. The amplitude ratio, ScSp ₁: ScS , increases from 0.10 to 0.19 from northern to southern North Island. This is within the range expected from a simple first‐order velocity discontinuity at an oceanic Moho. A 1–2 km thick layer of low‐velocity sediment at the top of the subducting plate is required to explain the remaining ScSp waveform. Our results imply that the abnormally thick Hikurangi–Chatham Plateau has been subducting beneath New Zealand for at least 2.9 Myr, thus explaining the high uplift rates observed across eastern North Island.     

Source history of the 1905 great Mongolian earthquakes (Tsetserleg, Bolnay)

Two great Mongolian earthquakes, Tsetserleg and Bolnay, occurred on 1905 July 9 and 23. We determined the source history of these events using body waveform inversion. The Tsetserleg rupture (azimuth N60°) correspond to a N60° oriented branch of the long EW oriented Bolnay fault.
Historical seismograms recorded by Wiechert instruments are digitized and corrected for the geometrical deformation due to the recording system. We use predictive filters to recover the signals lost at the minute marks.
The total rupture length for the Tsetserleg earthquake may reach up to 190 km, in order to explain the width of the recorded body waves. This implies adding 60 km to the previously mapped fault. The rupture propagation is mainly eastward. It starts at the southwest of the central subsegment, showing a left lateral strike-slip with a reverse component. The total duration of the modelled source function is 65 s. The seismic moment deduced from the inversion is 10²¹ N m, giving a magnitude   M _w = 8  .
The nucleation of the Bolnay earthquake was at the intersection between the main fault (375 km left lateral strike-slip) and the Teregtiin fault (N160°, 80 km long right lateral strike-slip with a vertical component near the main fault). The rupture was bilateral along the main fault: 100 km to the west and 275 km to east. It also propagated 80 km to the southeast along the Teregtiin fault. The source duration was 115 s. The moment magnitude Mw varies between 8.3 and 8.5.
The nucleation and rupture depths remain uncertain. We tested three cases: (1) nucleation and rupture depth limited to the seismogenic zone; (2) nucleation in the seismogenic zone and rupture propagation going to the base of the crust and (3) nucleation within the crust–upper mantle interface and rupture propagation within the upper mantle.     

Earthquake processes of the Himalayan collision zone in eastern Nepal and the southern Tibetan Plateau

Focal mechanisms determined from moment tensor inversion and first motion polarities of the Himalayan Nepal Tibet Seismic Experiment (HIMNT) coupled with previously published solutions show the Himalayan continental collision zone near eastern Nepal is deforming by a variety of styles of deformation. These styles include strike-slip, thrust and normal faulting in the upper and lower crust, but mostly strike-slip faulting near or below the crust–mantle boundary (Moho). One normal faulting earthquake from this experiment accommodates east–west extension beneath the Main Himalayan Thrust of the Lesser Himalaya while three upper crustal normal events on the southern Tibetan Plateau are consistent with east–west extension of the Tibetan crust. Strike-slip earthquakes near the Himalayan Moho at depths >60 km also absorb this continental collision. Shallow plunging P -axes and shallow plunging EW trending T -axes, proxies for the predominant strain orientations, show active shearing at focal depths ∼60–90 km beneath the High Himalaya and southern Tibetan Plateau. Beneath the southern Tibetan Plateau the plunge of the P -axes shift from vertical in the upper crust to mostly horizontal near the crust–mantle boundary, indicating that body forces may play larger role at shallower depths than at deeper depths where plate boundary forces may dominate.     

Coseismic slip model of the 2007 August Pisco earthquake (Peru) as constrained by Wide Swath radar observations

The Pisco earthquake ( M _w 8.0; 2007 August 15) occurred offshore of Peru's southern coast at the subduction interface between the Nazca and South American plates. It ruptured a previously identified seismic gap along the Peruvian margin. We use Wide Swath InSAR observations acquired by the Envisat satellite in descending and ascending orbits to constrain coseismic slip distribution of this subduction earthquake. The data show movement of the coastal regions by as much as 85 cm in the line-of-sight of the satellite. Distributed-slip model indicates that the coseismic slip reaches values of about 5.5 m at a depth of ∼18–20 km. The slip is confined to less than 40 km depth, with most of the moment release located on the shallow parts of the interface above 30 km depth. The region with maximum coseismic slip in the InSAR model is located offshore, close to the seismic moment centroid location. The geodetic estimate of seismic moment is 1.23 × 10²¹ Nm ( M _w 8.06), consistent with seismic estimates. The slip model inferred from the InSAR observations suggests that the Pisco earthquake ruptured only a portion of the seismic gap zone in Peru between 13.5° S and 14.5° S, hence there is still a significant seismic gap to the south of the 2007 event that has not experienced a large earthquake since at least 1687.     

Quaternary normal faulting in southeastern Sicily (Italy):a seismic source for the 1693 large earthquake

We present geological and morphological data, combined with an analysis of seismic reflection lines across the Ionian offshore zone and information on historical earthquakes, in order to yield new constraints on active faulting in southeastern Sicily. This region, one of the most seismically active of the Mediterranean, is affected by WNW–ESE regional extension producing normal faulting of the southern edge of the Siculo–Calabrian rift zone. Our data describe two systems of Quaternary normal faults, characterized by different ages and related to distinct tectonic processes. The older NW–SE-trending normal fault segments developed up to ≈400  kyr ago and, striking perpendicular to the main front of the Maghrebian thrust belt, bound the small basins occurring along the eastern coast of the Hyblean Plateau. The younger fault system is represented by prominent NNW–SSE-trending normal fault segments and extends along the Ionian offshore zone following the NE–SW-trending Avola and Rosolini–Ispica normal faults. These faults are characterized by vertical slip rates of 0.7–3.3  mm  yr ⁻¹ and might be associated with the large seismic events of January 1693. We suggest that the main shock of the January 1693 earthquakes ( M ~ 7) could be related to a 45  km long normal fault with a right-lateral component of motion. A long-term net slip rate of about 3.7  mm  yr ⁻¹ is calculated, and a recurrence interval of about 550 ± 50  yr is proposed for large events similar to that of January 1693.     

Dynamic rupture and stress change in a normal faulting earthquake in the subducting Cocos plate

A large nearly vertical, normal faulting earthquake ( M _w = 7.1) took place in 1997 in the Cocos plate, just beneath the ruptured fault zone of the great 1985 Michoacan thrust event ( M _w = 8.1). Dynamic rupture and resultant stress change during the 1997 earthquake have been investigated on the basis of near-source strong-motion records together with a 3-D dynamic model.
Dynamically consistent waveform inversion reveals a highly heterogeneous distribution of stress drop, including patch-like asperities and negative stress-drop zones. Zones of high stress drop are mainly confined to the deeper, southeastern section of the vertical fault, where the maximum dynamic stress drop reaches 280 bars (28 MPa). The dynamically generated source time function varies with location on the fault, and yields a short slip duration, which is caused by a short scalelength of stress-drop heterogeneities. The synthetic seismograms calculated from the dynamic model are generally consistent with the strong-motion velocity records in the frequency range lower than 0.5 Hz.
The pattern of stress-drop distribution appears, in some sense, to be consistent with that of coseismic changes in shear stress resulting from the 1985 thrust event. This consistency suggests that the stress transfer from the 1985 event to the subducting plate could be one of the possible mechanisms that increased the chance of the occurrence of the 1997 earthquake.     

Evidence for different scaling of earthquake source parameters for large earthquakes depending on faulting mechanism

Scaling relationships between seismic moment, rupture length, and rupture width have been examined. For this purpose, the data from several previous studies have been merged into a database containing more than 550 events. For large earthquakes, a dependence of scaling on faulting mechanism has been found. Whereas small and large dip-slip earthquakes scale in the same way, the self-similarity of earthquakes breaks down for large strike-slip events. Furthermore, no significant differences in scaling could be found between normal and reverse earthquakes and between earthquakes from different regions. Since the thickness of the seismogenic layer limits fault widths, most strike-slip earthquakes are limited to rupture widths of between 15 and 30 km while the rupture length is not limited. The aspect ratio of dip-slip earthquakes is similar for all earthquake sizes. Hence, the limitation in rupture width seems to control the maximum possible rupture length for these events. The different behaviour of strike-slip and dip-slip earthquakes can be explained by rupture dynamics and geological fault growth. If faults are segmented, with the thickness of the seismogenic layer controlling the length of each segment, strike-slip earthquakes might rupture connected segments more easily than dip-slip events, and thus could produce longer ruptures than dip-slip events of the same width