Focal mechanisms of three earthquakes in Finland and their relation to surface faults

Focal mechanisms for three recent earthquakes in Finland are determined using P-wave polarities together with SV/P and SH/P phase amplitude ratios. The events occurred on May 11, 2000 in Toivakka, Central Finland (M_L=2.4), on September 15, 2000 in Kuusamo, northeastern Finland (M_L=3.5), and on May 2, 2001 in Kolari, western Finnish Lapland (M_L=2.9).In order to obtain reliable estimates of the source parameters, one-dimensional crust and upper mantle velocity models are derived for the epicenter areas from deep-seismic sounding results. The starting models are modified by one-dimensional ray tracing using the earthquake observations. The events are relocated by employing P- and S-phase arrival times from the nearest seismic stations and the final velocity models. Synthetic waveforms, calculated with the reflectivity method, are used to further constrain and verify the source and structural parameters.The Toivakka earthquake indicates thrust- or reverse-faulting mechanism at a depth of 5 km. After comparison with aeromagnetic and topographic data we suggest the eastward dipping nodal plane (358°/42°) was the fault plane. The best-fitting fault plane solution of the Kolari earthquake suggests pure thrust-faulting at a depth of 5 km. The nodal plane striking 035°/30° correlates well with surface observations of the postglacial, possibly listric fault systems in the source area. The Kuusamo earthquake (focal depth 14 km) has a normal-faulting mechanism with the nodal planes trending 133°/47° or 284°/47°. Preference is given to the SW-dipping nodal plane, as it seems to coincide with topographic and magnetic lineament directions that have been active after the last ice age.The three earthquakes have occurred in old Precambrian faults and shear zones, which have been reactivated. The reactivated faults are favourably oriented in the local stress field.     

Graben width controlling syn-rift sedimentation: the Palaeogene southern Upper Rhine Graben as an example

Eocene to Early Oligocene syn-rift deposits of the southern Upper Rhine Graben (URG) accumulated in restricted environments. Sedimentation was controlled by local clastic supply from the graben flanks, as well as by strong intra-basinal variations in accommodation space due to differential tectonic subsidence, that in turn led to pronounced lateral variations in depositional environment. Three large-scale cycles of intensified evaporite sedimentation were interrupted by temporary changes towards brackish or freshwater conditions. They form three major base level cycles that can be traced throughout the basin, each of them representing a stratigraphic sub-unit. A relatively constant amount of horizontal extension (ΔL) in the range of 4–5 km has been estimated for the URG from numerous cross-sections. The width of the rift (L _f ), however, varies between 35 and more than 60 km, resulting in a variable crustal stretching factor between the bounding masterfaults. Apart from block tilting, tectonic subsidence was, therefore, largely controlled by changes in the initial rift width (L ₀). The along-strike variations of the graben width are responsible for the development of a deep, trough-like evaporite basin (Potash Basin) in the narrowest part of the southern URG, adjacent to shallow areas in the wider parts of the rift such as the Colmar Swell in the north and the Rhine Bresse Transfer Zone that delimits the URG to the south. Under a constant amount of extension, the along-strike variation in rift width is the principal factor controlling depo-centre development in extensional basins.     

Tectonic structures across the East African Rift based on the source parameters of the 20 May 1990 M7.2 Sudan earthquake

Earthquakes in Kenya are common along the Kenya Rift Valley because of the slow divergent movement of the rift and hydrothermal processes in the geothermal fields. This implies slow but continuous radiation of seismic energy, which relieves stress in the subsurface rocks. On the contrary, the NW-SE trending rift/fault zones such as the Aswa-Nyangia fault zone and the Muglad-Anza-Lamu rift zone are the likely sites of major earthquakes in Kenya and the East African region. These rift/fault zones have been the sites of a number of strong earthquakes in the past such as the M _w = 7.2 southern Sudan earthquake of 20 May 1990 and aftershocks of M _w = 6.5 and 7.1 on 24 May 1990, the 1937 M _s = 6.1 earthquake north of Lake Turkana close to the Kenya-Ethiopian border, and the 1913 M _s = 6.0 Turkana earthquake, among others. Source parameters of the 20 May 1990 southern Sudan earthquake show that this earthquake consists of only one event on a fault having strike, dip, and rake of 315°, 84°, and ?3°. The fault plane is characterized by a left-lateral strike slip fault mechanism. The focal depth for this earthquake is 12.1 km, seismic moment M _o = 7.65 × 10¹⁹ Nm, and moment magnitude, M _w = 7.19 (?7.2). The fault rupture started 15 s earlier and lasted for 17 s along a fault plane having dimensions of ?60 km × 40 km. The average fault dislocation is 1.1 m, and the stress drop, ?σ, is 1.63 MPa. The distribution of historical earthquakes (M _w ≥ 5) from southern Sudan through central Kenya generally shows a NW-SE alignment of epicenters. On a local scale in Kenya, the NW–SE alignment of epicenters is characterized by earthquakes of local magnitude M _l ≤ 4.0, except the 1928 Subukia earthquake (M _s = 6.9) in central Kenya. This NW–SE alignment of epicenters is consistent with the trend of the Aswa-Nyangia Fault Zone, from southern Sudan through central Kenya and further southwards into the Indian Ocean. We therefore conclude that the NW–SE trending rift/fault zones are sites of strong earthquakes likely to pose the greatest earthquake hazard in Kenya and the East African region in general.     

Evaluation of deep crustal earthquakes in northern Germany – Possible tectonic causes

We present new evidence for seven deep crustal, intraplate earthquakes in northern Germany, a region regarded as an area of low seismicity. From 2000 to 2018, seven earthquakes with magnitudes of M_L 1.3–3.1, were detected at depths of 17.0–31.4 km. By placing the earthquake hypocentres in a geological three‐dimensional model, we can correlate two of the earthquakes with the Thor Suture, a major fault zone in this area. Five of the earthquakes group in the lower crust near the Moho, which implies that parts of the lower crust and the crust/mantle boundary in northern Germany act as a structural discontinuity on which deformation localizes. Numerical simulation implies that stress changes due to glacial isostatic adjustment most likely triggered these deep crustal earthquakes.     

The 2003 earthquake swarm in Anjalankoski, south-eastern Finland

During May 2003 a swarm of 16 earthquakes (M_L = 0.6–2.1) occurred at Anjalankoski, south-eastern Finland. The activity lasted for three weeks, but additional two events were observed at the same location in October 2004. A comparison of the waveforms indicated that the source mechanisms and the hypocentres of the events were nearly identical.A relative earthquake location method was applied to better define the geometry of the cluster and to identify the fault plane associated with the earthquakes. The relocated earthquakes aligned along an ENE–WSW trending zone, with a lateral extent of about 1.0 km by 0.8 km. The relative location and the waveform-modelling of depth sensitive surface wave (Rg) and S-to-P converted body wave (sP) phases indicated that the events were unusually shallow, most likely occurring within the first 2 km of the surface. The revised historical earthquake data confirm that shallow swarm-type seismicity is characteristic to the area.The focal mechanism obtained as a composite solution of the five strongest events corresponds to dip-slip motion along a nearly vertical fault plane (strike = 250°, dip = 80°, rake = 90°). The dip and strike of this nodal plane as well as the relocated hypocentres coincide with an internal intrusion boundary of the Vyborg rapakivi batholith.The events occur under a compressive local stress field, which is explained by large gravitational potential energy differences and ridge-push forces. Pore-pressure changes caused by intrusion of ground water and/or radon gas into the fracture zones are suggested to govern the swarm-type earthquake activity.     

Earthquakes in Switzerland and surrounding regions during 2004

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2004. During this period, 677 earthquakes and 96 quarry blasts were detected and located in the region under consideration. With 22 events with M_L≥2.5, the seismic activity in the year 2004 was close to the average over the last 30 years. As in previous years, most of the activity was concentrated in the Valais and in Graubünden. In addition, several moderate earthquakes occurred in the lower crust below the northern Alpine foreland. Unusual was that five earthquakes were sufficiently strong to cause ground shaking of intensity IV over large portions of the territory. Two were located in Switzerland (Liestal, M_L 3.8, and Brugg, M_L 4.0). The epicenters of the other three strong events were located outside Switzerland (Besan?on in the French Jura, M_L 4.8, Waldkirch in southern Germany, M_L 5.1, and Lago di Garda in northern Italy, M_L 5.3).     

Seismotectonics of the 26 November 2005 Jiujiang-Ruichang, Jiangxi, Ms?5.7 Earthquake

The 26 November 2005 Jiujiang-Ruichang, Jiangxi, Ms?5.7 earthquake occurred in a seismotectonic setting of moderate earthquake. The northwest-trending Xiangfan-Guangji fault (XFG) does not enter into the epicenter vicinity, but the northeast-trending Ruichang-Wuning fault (RWF) as a regional fault extends to the epicenter nearby, appearing as the Ruichang basin and its marginal faults. Tilting of the Ruichang Basin (RCB) in the Quaternary was controlled by the RCB southeast-marginal, buried fault (RSMBF). Shallow geophysical survey reveals that the RSMBF caused an offset of the reflection layers. Drill hole columnar section demonstrates that there are about 10–12?m displacement in the lower section of the middle-Pleistocene Series along the RSMBF, but no disruption is found in the upper section of the middle-Pleistocene Series. The RSMBF not only has activity in the Quaternary, but also coincides with the nodal plane I from the focal mechanism of the Jiujiang-Ruichang Ms?5.7 earthquake. This evidence, including aftershock distribution and isoseismic lines, strongly suggests that the RSMBF might be the seismogenic tectonics. The RWF is discontinuous at the surface, and consists of three en echelon Quaternary basins, which are the Ruichang, Fanzhen and Wuning basins. Three moderate earthquakes, the Fanzhen ML?4.9 earthquake, the Yejiapu ML?4.1 earthquake and the Jiujiang-Ruichang Ms?5.7 earthquake, have happened in the basins since 1995. The seismogenic tectonics of the Jiujiang-Ruichang Ms?5.7 earthquake is not isolated, but may be controlled by the RWF at depth, the slip of which causes the accumulation of energy for earthquake occurrence.     

The MW = 6.3 2003 Lefkada earthquake (Greece) and induced stress transfer changes

A large earthquake of magnitude M_W = 6.3 occurred on 14 August 2003 NW of the Lefkada Island, which is situated at the Ionian Sea (western Greece). The source parameters of this event are determined using body-wave modeling. The focal depth was found equal to 9 km, the constrained focal mechanism revealed dextral strike–slip motion (φ = 15°, Δ = 80° and λ = 170°), the duration of the source time function was 8 s and the seismic moment 2.9 × 10²⁵ dyn cm. The earthquake occurred close to the northern end of the Kefallinia transform fault, where the 1994 moderate event and its aftershock sequence were also located. The epicentral distribution of the 2003 aftershock sequence revealed the existence of two clusters. The first one is located close to the epicentral area of the mainshock, while the second southern, close to the northwestern coast of the Kefallinia Island. A gap of seismicity is observed between the two clusters. The length of the activated zone is approximately 60 km. The analysis of data revealed that the northern cluster is directly related to the mainshock, while the southern one was triggered by stress transfer caused by the main event.     

Tomographic image of crustal structures across the Chelungpu fault: Is the seismogenic layer structure- or depth-dependent?

In 1999, a large earthquake (M_w = 7.6) occurred along the Chelungpu fault in the fold-and-thrust belt of western Taiwan. To shed more light on the subsurface structures and the seismogenic layers, three-dimensional velocity structures were inverted by using the travel times of both P- and S-waves from 2391 aftershocks recorded by the Central Weather Bureau during the 15 months that followed. From tomography, a typical image of the large-scale thrusting structures in the upper crust across the Chelungpu fault was obtained. In general, high velocities beneath the Western Foothills and Central Ranges are separated from low velocities beneath the Coastal Plain by an east-dipping boundary that is roughly consistent with the Chelungpu fault on the surface. The contrast in velocity on either side of the Chelungpu fault is indicative of about a 7- to 9-km vertical offset in the upper crust. The relocated hypocenter for the Chi-Chi earthquake shifts by 2.2 km toward the northwest, and its focal depth decreases by 0.7 km. A plot of focal depths versus rock velocities where the aftershocks occurred shows earthquakes are more inclined to occur in rock with a velocity of around 5.6 km/s. This strongly suggests the seismogenic layer in the fold-and-thrust belt of Taiwan is more structure-dependent than depth-dependent.     

Source complexity of the 4 March 2010 Jiashian,Taiwan, Earthquake determined by joint inversion of teleseismic and near field data

The Jiashian earthquake (M_L 6.4) occurred on 4 March 2010. It was the largest inland event in southern Taiwan of 2010. The mainshock location was unexpected since it occurred in an area with relatively low background seismicity. In addition, reports of earthquake focal mechanisms do not fit with any known active fault geometry. In order to understand the origin of this earthquake, especially its rupture process, we perform a joint source inversion by using teleseismic body wave, GPS coseismic displacements and near field ground motion data. In this study, we considered a northwest–southeast trending fault with a northeast dip retrieved from GPS coseismic data and aftershocks distribution. To analyze the detailed slip distribution in space and time, we used near field 3D Green’s functions provided by spectral-element method and a full time–space inversion technique. We find a complex rupture process with several slip patches distributed inside two main asperities. The slip map reveals a mean slip of 12.9 cm for a maximum slip of 27.3 cm leading to a M_w 6.47 for this event. The rupture initiates in the deepest portion of the fault at 20 km depth, and propagated upward up to 2 km depth to form the two asperities. The source time function of this event revealed two pulses corresponding to the two asperities, for a total duration time of about 16 s. Most aftershocks occurred near the upper boundary of the deepest asperity while no aftershocks are located close to the shallowest one. We infer that the locations of these slip patches are related to the surrounding fault systems that may have restricted the rupture propagation during the earthquake.     

Usage of an early warning and information system Web-site for real-time seismicity in Iceland

Iceland has been subjected to destructive earthquakes and volcanic eruptions throughout history. Such events are often preceded by changes in earthquake activity over varying timescales. Although most seismicity is confined to micro-earthquakes, large earthquakes have occurred within populated regions. Following the most recent hazardous earthquakes in 2000, the Icelandic Meteorological Office (IMO) developed an early warning and information system (EWIS) Web-site for viewing near-real-time seismicity in Iceland. Here we assess Web-site usage data in relation to earthquake activity, as recorded by the South Iceland Lowland (SIL) seismic network. Between March 2005 and May 2006 the SIL seismic network recorded 12,583 earthquakes. During this period, the EWIS Web-site logged a daily median of 91 visits. The largest onshore event (M _L 4.2) struck 20 km from Reykjavík on 06 March 2006 and was followed by an immediate, upsurge in usage resulting in a total of 1,173 unique visits to the Web-site. The greatest cluster of large (≥M _L 3) events occurred 300 km offshore from Reykjavík in May 2005. Within this swarm, 9 earthquakes ≥M _L 3 were detected on 11 May 2005, resulting in the release of a media bulletin by IMO. During the swarm, and following the media bulletin, the EWIS Web-site logged 1,234 unique visits gradually throughout the day. In summary, the data reveal a spatial and temporal relationship between Web-site usage and earthquake activity. The EWIS Web-site is accessed immediately after the occurrence of a local earthquake, whereas distant, unfelt earthquakes generate gradual interest prompted by media bulletins and, possibly, other contributing factors. We conclude that the Internet is a useful tool for displaying seismic information in near-real-time, which has the capacity to help increase public awareness of natural hazards.     

Analysis of Izmit aftershocks 25 days before the November 12th 1999 Düzce earthquake, Turkey

We investigate spatial clustering of 2414 aftershocks along the Izmit M_w = 7.4 August 17, 1999 earthquake rupture zone. 25 days prior to the Düzce earthquake M_w = 7.2 (November 12, 1999), we analyze two spatial clusters, namely Sakarya (SC) and Karadere–Düzce (KDC). We determine the earthquake frequency–magnitude distribution (b-value) for both clusters. We find two high b-value zones in SC and one high b-value zone in KDC which are in agreement with large coseismic surface displacements along the Izmit rupture. The b-values are significantly lower at the eastern end of the Izmit rupture where the Düzce mainshock occurred. These low b-values at depth are correlated with low postseismic slip rate and positive Coloumb stress change along KDC. Since low b-values are hypothesized with high stress levels, we propose that at the depth of the Düzce hypocenter (12.5 km), earthquakes are triggered at higher stresses compared to shallower crustal earthquake. The decrease in b-value from the Karadere segment towards the Düzce Basin supports this low b-value high stress hypothesis at the eastern end of the Izmit rupture. Consequently, we detect three asperity regions which are correlated with high b-value zones along the Izmit rupture. According to aftershock distribution the half of the Düzce fault segment was active before the 12 November 1999 Düzce mainshock. This part is correlated with low b-values which mean high stress concentration in the Düzce Basin. This high density aftershock activity presumably helped to trigger the Düzce event (M_w = 7.2) after the Izmit M_w 7.4 mainshock.     

The earthquakes and related tsunamis of October 6, 1944 and March 7, 1867; NE Aegean Sea

During two distinct earthquakes occurred on March 7, 1867 and October 6, 1944, tsunami waves were also observed at some localities around the Gulf of Edremit, NE Aegean Sea. The first event (M _w = 6.8) mostly affected the city of Mitilini of Lesvos Island while the Gulf of Edremit-Ayvacık earthquake (M _S = 6.8) largely affected the northern and eastern coastal areas of the Gulf of Edremit. In 1944 earthquake, numerous surface cracks and water gushes were reported. The coastal neighborhoods of the town of Ayvalık in the east were flooded by tsunami waves. At the WSW extend of the main fault observed on land, which is parallel to the present-day slip vectors, some normal-oblique faults were observed close and subparallel to the northern coast. On the basis of historical documents, reports, interviews, geological setting, field observations and marine seismic reflection data, the 1944 earthquake was not triggered by one of the main fault segments but by a secondary fault or fault group which was described in this study. Depending on the distribution of tensional and compressional forces in the region, which rotates clockwise under the control of the middle strand of the North Anatolian fault, secondary fault groups become important. The moment tensor parameters of such small-size events have been determined and have obtained consistent results with the faults proposed in this study.     

Correlating recurrent radon precursors with local earthquake magnitude and crust strain near the Chihshang fault of eastern Taiwan

The active Chihshang fault in the southern segment of longitudinal valley of eastern Taiwan is part of the suture boundary between the Eurasia plate and the Philippine Sea plate. Radon anomalies in groundwater were recorded prior to three major earthquakes—(1) 2003 M _w = 6.8 Chengkung, (2) 2006 M _w = 6.1 Taitung, and (3) 2008 M _w = 5.4 Antung. The epicenters were located 24, 52, and 13 km, respectively, from the radon-monitoring well (D1) in the Antung hot spring about 3 km southeast of the Chihshang fault. Prior to the three major earthquakes, radon decreased from background levels of 787 ± 42, 762 ± 57, and 700 ± 57 pCi/L to minima of 326 ± 9, 371 ± 9, and 480 ± 43 pCi/L, respectively. Based on the radon volatilization model and the rock dilatancy model, this paper correlates the observed radon minima with local earthquake magnitude and crust strain. The correlation is a useful means of forecasting local disastrous earthquakes in the southern segment of longitudinal valley of eastern Taiwan.     

Source parameters of the ML 4.1 earthquake of November 08, 2006, southeast Beni-Suef, northern Egypt

In the early morning hours on Wednesday November 08, 2006 at 04:32:10(GMT) a small earthquake of M_L 4.1 has occurred at southeast Beni-Suef, approximately 160 km SEE of Cairo, northern Egypt. The quake has been felt as far as Cairo and its surroundings while no casualties were reported. The instrumental epicentre is located at 28.57°N and 31.55°E. Seismic moment is 1.76 E14 Nm, corresponding to a moment magnitude M_w 3.5. Following a Brune model, the source radius is 0.3 km with an average dislocation of 1.8 cm and a 2.4 MPa stress drop. The source mechanism from a first motion fault plane solution shows a left-lateral strike-slip mechanism with a minor dip-slip component along fault NNW striking at 161°, dipping 52° to the west and rake −5°. Trend and plunging of the maximum and minimum principle axes P/T are 125°, 28°, 21°, and 23°, respectively. A comparison with the mechanism of the October, 1999 event shows similarities in faulting type and orientation of nodal planes.Eight small earthquakes (3.0  M_L < 5.0) were also recorded by the Egyptian National Seismological Network (ENSN) from the same region. We estimate the source parameters and fault mechanism solutions (FMS) for these earthquakes using displacement spectra and P-wave polarities, respectively. The obtained source parameters including seismic moments of 4.9 × 10¹²–5.04 × 10¹⁵ Nm, stress drops of 0.2–4.9 MPa and relative displacement of 0.1–9.1 cm. The azimuths of T-axes determined from FMS are oriented in NNE–SSW direction. This direction is consistent with the present-day stress field in Egypt and the last phase of stress field changes in the Late Pleistocene, as well as with recent GPS measurements.     

Unravelling the complexity of Apenninic extensional fault systems: A review of the 2009 L'Aquila earthquake (Central Apennines,Italy)

The 2009 L'Aquila sequence activated a normal fault system 50 km long in the Central Apennines, composed of two main NW-trending faults 12–16 km long: the main high angle L'Aquila segment and the Campotosto listric fault.The M_W 6.1 L'Aquila mainshock nucleated on the Paganica fault at a depth of ∼8.6 km and cut through the upper crust producing coseismic surface slip of up to 10 cm observed along a strike length of ∼13 km. Analysis of historical seismicity and data collected in paleo-seismological trenches suggest that this event filled a >500-year gap. In contrast, the blind Campotosto listric fault is composed of different fault segments displaying abrupt changes in dip at a depth where major events nucleate suggesting a rheological and geometrical control on stress concentration.A foreshock sequence that started around 4 months before the L'Aquila mainshock activated the deepest portion of the Paganica fault and marked the onset of large variations in elastic properties of the crustal volume. The variations have been modelled in terms of dilatancy and diffusion processes, corroborating the hypothesis that fluids play a key role in the nucleation process of extensional faults in the crust.     

New constraints on the current stress field and seismic velocity structure of the eastern Yilgarn Craton from mechanisms of local earthquakes

The Yilgarn Craton has hosted some of the largest earthquakes within the Australian continent in the last 100 years. Earthquakes have mainly been studied in the western part of the craton, and are thought to result from the reactivation of Precambrian structures in an E–W compressive regional stress field imposed by plate-scale processes. Here we present moment tensor solutions for three recent moderate-sized earthquakes around the town of Kalgoorlie that are inconsistent with E–W compression, but instead suggest E–W extension in the eastern Yilgarn Craton. Waveforms of earthquakes at Boulder (M_W = 4.0, 20 April 2010), Kalgoorlie (M_W = 4.3, 26 February 2014) and Coolgardie (M_W = 3.9, 31 October 2014) were inverted for moment tensors. All three earthquakes were shallow (centroid depth ≤4 km) normal-faulting events that occurred along roughly N–S-striking planes, either with a steep westward or a relatively shallow eastward dip. The robustness of the retrieved mechanisms has been thoroughly tested, employing different earth models, assuming different locations for the earthquakes and using different period bands for the inversion. The fit of synthetic long-period waveforms to the observations was in all cases substantially improved by assuming a two-layered crust with high S wavespeeds (about 3.9–4 km/s) overlying substantially slower material. Since there is independent evidence from active source profiles for a P velocity increase between the upper and lower crust, a large difference in v_p/v_s ratio between upper and lower crust is the only way to explain both lines of evidence. This vertical contrast could represent a dominance of felsic material in the upper crust, and substantially more mafic material in the lower crust. Taken together, our results also appear to imply that the regional stress field is E–W extensive in the Kalgoorlie area, and possibly for the entire Kalgoorlie Terrane. This is contrary to current assumptions from continent-scale stress modelling. That the orientations of rupture planes roughly align with the regional structural grain could indicate that Archean structures are reactivated in response to the current stress field.     

Continental rifting as a function of lithosphere mantle strength

The role of the uppermost mantle strength in the pattern of lithosphere rifting is investigated using a thermo-mechanical finite-element code. In the lithosphere, the mantle/crust strength ratio (S_M/S_C) that decreases with increasing Moho temperature T_M allows two strength regimes to be defined: mantle dominated (S_M > S_C) and crust dominated (S_M < S_C). The transition between the two regimes corresponds to the disappearance of a high strength uppermost mantle for T_M > 700 °C. 2D numerical simulations for different values of S_M/S_C show how the uppermost mantle strength controls the style of continental rifting. A high strength mantle leads to strain localisation at lithosphere scale, with two main patterns of narrow rifting: “coupled crust–mantle” at the lowest T_M values and “deep crustal décollement” for increasing T_M values, typical of some continental rifts and non-volcanic passive margins. The absence of a high strength mantle leads to distributed deformations and wide rifting in the upper crust. These numerical results are compared and discussed in relation with series of classical rift examples.