H. Lyon-Caen

Convergence rate across the Nepal Himalaya and interseismic coupling on the Main Himalayan Thrust: Implications for seismic hazard

We document geodetic strain across the Nepal Himalaya using GPS times series from 30 stations in Nepal and southern Tibet, in addition to previously published campaign GPS points and leveling data and determine the pattern of interseismic coupling on the Main Himalayan Thrust fault (MHT). The noise on the daily GPS positions is modeled as a combination of white and colored noise, in order to infer secular velocities at the stations with consistent uncertainties. We then locate the pole of rotation of the Indian plate in the ITRF 2005 reference frame at longitude = − 1.34° ± 3.31°, latitude = 51.4° ± 0.3° with an angular velocity of Ω = 0.5029 ± 0.0072°/Myr. The pattern of coupling on the MHT is computed on a fault dipping 10° to the north and whose strike roughly follows the arcuate shape of the Himalaya. The model indicates that the MHT is locked from the surface to a distance of approximately 100 km down dip, corresponding to a depth of 15 to 20 km. In map view, the transition zone between the locked portion of the MHT and the portion which is creeping at the long term slip rate seems to be at the most a few tens of kilometers wide and coincides with the belt of midcrustal microseismicity underneath the Himalaya. According to a previous study based on thermokinematic modeling of thermochronological and thermobarometric data, this transition seems to happen in a zone where the temperature reaches 350°C. The convergence between India and South Tibet proceeds at a rate of 17.8 ± 0.5 mm/yr in central and eastern Nepal and 20.5 ± 1 mm/yr in western Nepal. The moment deficit due to locking of the MHT in the interseismic period accrues at a rate of 6.6 ± 0.4 × 10^(19) Nm/yr on the MHT underneath Nepal. For comparison, the moment released by the seismicity over the past 500 years, including 14 M_W ≥ 7 earthquakes with moment magnitudes up to 8.5, amounts to only 0.9 × 10^(19) Nm/yr, indicating a large deficit of seismic slip over that period or very infrequent large slow slip events. No large slow slip event has been observed however over the 20 years covered by geodetic measurements in the Nepal Himalaya. We discuss the magnitude and return period of M > 8 earthquakes required to balance the long term slip budget on the MHT.

Active deformation of the Corinth rift, Greece : Results from repeated Global Positioning System surveys between 1990 and 1995

Between 1990 and 1995, we carried out seven Global Positioning System (GPS) campaigns in the Corinth rift area in order to constrain the spatial and temporal crustal deformation of this active zone. The network, 193 points over ∼10,000 km2, samples most of the active faults. In order to estimate the deformation over a longer period, 159 of those points are also Greek triangulation pillars previously measured between 1966 and 1972. Two earthquakes of magnitude 6.2 and 5.9 have occurred in the network since it was installed. The extension rate deduced from the analysis of the different GPS data sets is 14±2 mm/yr oriented N9° in the west, 13±3 mm/yr oriented S-N in the center, and 10±4 mm/yr oriented N19°W in the east of the gulf. The comparison between GPS and triangulation gives higher rates and less angular divergence (25±7 mm/yr, N4°E; 22±7 mm/yr, S-N; 20±7 mm/yr, N15°W, respectively). Both sets of data indicate that the deforming zone is very narrow (10–15 km) in the west, might be wider in the center (15–20 km), and is more diffuse in the east. The analysis of the displacements observed after the Ms = 6.2, June 15, 1995, and the Ms = S.9, November 18, 1992, earthquakes, both located in the west of the gulf, together with seismological and tectonic observations shows that these two earthquakes occurred on low-angle (≤35°) north dipping normal faults located between 4.5 and 10 km depth in the inner part of the rift. Assuming that the deformation is concentrated in relatively narrow deforming zones, we use a simple model of a dislocation in an elastic half-space to study the implication of the localization. Using the geometry of the known seismogenic faults, our observations imply continuous aseismic deformation in the uppermost crust of the inner rift. This model predicts geodetic strain rates close to seismic strain rates in opposition to previous estimates. This is because our model takes into account the activity on low-angle normal faults in the inner rift and an effective seismogenic layer of 6–7 km, about half that usually assumed.

The Ms = 6.2, June 15, 1995 Aigion earthquake (Greece): evidence for low angle normal faulting in the Corinth rift

We present the results of a multidisciplinary study of the Ms = 6.2, 1995, June 15, Aigion earthquake (Gulf of Corinth, Greece). In order to constrain the rupture geometry, we used all available data from seismology (local, regional and teleseismic records of the mainshock and of aftershocks), geodesy (GPS and SAR interferometry), and tectonics. Part of these data were obtained during a postseismic field study consisting of the surveying of 24 GPS points, the temporary installation of 20 digital seismometers, and a detailed field investigation for surface fault break. The Aigion fault was the only fault onland which showed detectable breaks (< 4 cm). We relocated the mainshock hypocenter at 10 km in depth, 38 ° 21.7 ′ N, 22 ° 12.0 ′ E, about 15 km NNE to the damaged city of Aigion. The modeling of teleseismic P and SH waves provides a seismic moment Mo = 3.4 1018 N.m, a well constrained focal mechanism (strike 277 °, dip 33 °, rake − 77°), at a centroidal depth of 7.2 km, consistent with the NEIC and the revised Harvard determinations. It thus involved almost pure normal faulting in agreement with the tectonics of the Gulf. The horizontal GPS displacements corrected for the opening of the gulf (1.5 cm/year) show a well-resolved 7 cm northward motion above the hypocenter, which eliminates the possibility of a steep, south-dipping fault plane. Fitting the S-wave polarization at SERG, 10 km from the epicenter, with a 33° northward dipping plane implies a hypocentral depth greater than 10 km. The north dipping fault plane provides a poor fit to the GPS data at the southern points when a homogeneous elastic half-space is considered: the best fit geodetic model is obtained for a fault shallower by 2 km, assuming the same dip. We show with a two-dimensional model that this depth difference is probably due to the distorting effect of the shallow, low-rigidity sediments of the gulf and of its edges. The best-fit fault model, with dimensions 9 km E–W and 15 km along dip, and a 0.87 m uniform slip, fits InSAR data covering the time of the earthquake. The fault is located about 10 km east-northeast to the Aigion fault, whose surface breaks thus appears as secondary features. The rupture lasted 4 to 5 s, propagating southward and upward on a fault probably outcropping offshore, near the southern edge of the gulf. In the shallowest 4 km, the slip – if any – has not exceeded about 30 cm. This geometry implies a large directivity effect in Aigion, in agreement with the accelerogram aig which shows a short duration (2 s) and a large amplitude (0.5 g) of the direct S acceleration. This unusual low-angle normal faulting may have been favoured by a low-friction, high pore pressure fault zone, or by a rotation of the stress directions due to the possible dip towards the south of the brittle-ductile transition zone. This fault cannot be responsible for the long term topography of the rift, which is controlled by larger normal faults with larger dip angles, implying either a seldom, or a more recently started activity of such low angle faults in the central part of the rift.

East-west extension and Holocene normal-fault scarps in the Hellenic arc

Examination of surface fault traces with Spot images and in the field corroborates the inference that the active tectonics of southern Peloponnesus and Crete are dominated by approximately north-south normal faulting and approximately east-west extension. The heights of Holocene normal-fault scarps yield first-order regional estimates of fault slip rates between 0.1 and 2-3 mm/yr. Most of the surface scarps probably ruptured during past earthquakes, such as that which destroyed Sparta in 464 B.C. On the Sparta fault the Holocene average slip rate and the recurrence time of large earthquakes may be ∼1 mm/yr and 3000 yr, respectively. The regional pattern of Quaternary faulting suggests that the east-west extension near the Hellenic subduction zone is fast (about 5%-10%/m.y.). The change from north-south to east-west extension in the late Pliocene (∼2-4 Ma) implies that the Aegean is starting to collide with the northern margin of Africa.

Gravity anomalies and flexure of the Brazilian Shield beneath the Bolivian Andes

Abstract Seismological and geological data suggest that the Brazilian Shield is now underthrusting beneath the sub-Andes. In order to constrain the minimum amount of shortening that results from this underthrusting, we analyzed Bouguer gravity anomalies over the shield and the Bolivian Andes, assuming that they are due primarily to variations in crustal thickness that in turn are due to the flexure of an effectively elastic plate (Brazilian Shield) loaded by the weight of the Andean mountain range. Gravity anomalies show that Airy-type isostatic equilibrium does not exist; the sub-Andes are overcompensated and the Chaco Plain to the east is undercompensated. The flexure of the plate can help explain both the overcompensation and the undercompensation if the Brazilian Shield behaves as an essentially elastic plate that extends at least 150 km, and probably 200 km, beneath the sub-Andes and the Cordillera Oriental and if the flexural rigidity of the plate is between 0.1 and 2 × 10 24 N m. The fit of observed and calculated anomalies, however, is far from perfect, and additional sources of short-wavelength anomalies appear to be present. Assuming that the sediments and sedimentary rocks in the sub-Andes and Cordillera Oriental were deposited on the Brazilian Shield and that later they were overthrust eastward onto one another causing the present topography and the deflection of the plate, we computed a family of estimates for the minimum amount of crustal shortening. These estimates depend strongly on the amount of sedimentary rock deposited before deformation, which does not seem to be well constrained by published geologic mapping. Nevertheless, the calculations suggest that at least 100 km of crustal shortening have occurred, and probably more, implying that the Andes may have been built mostly by crustal shortening.

The MW=8.1 Antofagasta (North Chile) Earthquake of July 30, 1995: First results from teleseismic and geodetic data

A strong (Mw = 8.1) subduction earthquake occurred on July 30, 1995 in Antofagasta (northern Chile). This is one of the largest events during this century in the region. It ruptured the southernmost portion of a seismic gap between 18°S and 25°S. In 1992 we had used GPS to survey a network with about 50 benchmarks covering a region nearly 500 km long (N-S) and 200 km wide (E-W). Part of these marks were re-surveyed with GPS after the 1995 earthquake. Comparison with 1992 positions indicate relative horizontal displacement towards the trench reaching 0.7 m. The inland subsided several decimeters. The Mejillones Peninsula was uplifted by more than 15 cm. Teleseismic body-wave modelling of VBB records gives a subduction focal mechanism and source time function with three distinct episodes of moment release and southward directivity. Modelling the displacement field using a dislocation with uniform slip in elastic half-space suggests a rupture zone with 19°–24° eastward dip extending to a depth no greater than 50 km with N-S length of 180 km and an average slip of about 5 m. The component of right-lateral slip inferred both from the teleseismic and geodetic data does not require slip partitioning at the plate boundary. That the well-constrained northern end of the 1995 rupture zone lies under the southern part of the Mejillones Peninsula increases the probability for a next rupture in the gap north of it.

Seismic slip on a low angle normal fault in the Gulf of Corinth: Evidence from high‐resolution cluster analysis of microearthquakes

The Gulf of Corinth in Western Greece is one of the most active extensional zones in the Aegean region. It is still an open question whether extension can be actively accommodated on low angle faulting or if those faults as seen in geological records have been rotated. Whilst numerous fault plane solutions obtained from a dense temporary network deployed in the western part of the gulf in July-August of 1991 showed one of the nodal planes as a subhorizomal plane, slip on the high-angle conjugate plane is equally probable from the focal mechanism data (Rigo et al. 1996). Since part of the activity occurred in spatial clusters with similar focal mechanisms, we used a high resolution cluster analysis to determine the most likely active plane. Exploiting the waveform similarity of these events, relative onset times of P and S waves could be determined at subsample accuracy (less than 0.01 s). The cluster analyzed here contains 12 evems, among these 8 have a well con- strained normal faulting fault plane solution with a shal- low (12-20 o) north dipping plane and a steeply south dipping plane. A master evem relocation shows that the relocated 12 hypocenter cemroids are aligned along the low angle plane showing clear evidence for active low angle normal faulting.

Gravity anomalies, the deep structure, and dynamic processes beneath the Tien Shan

Bouguer gravity anomalies over the Tien Shan and Pamir are large and negative, and therefore consistent with crustal thickening beneath both mountain ranges. Gravity anomalies over the western Tien Shan are within about 10 mgal of those expected for local Airy isostatic equilibrium, but those over the eastern Tien Shan are up to 50 mgal too large and negative. Thus they require static or dynamic support of an uncompensated mass deficit. These gravity anomalies cannot be matched using regional compensation of the weight of the range on a continuous elastic plate. A simple model of two separate elastic plates extending north and south from the axis of the Tien Shan can allow an adequate fit of observed and calculated anomalies. The flexural rigidity of the northern plate must be quite small (D < 1023 N m, corresponding to an equivalent effective elastic thicknessTE < 25 km), but that of the southern plate must be larger (D ≈ 1024 N m, TE ≈ 50–60 km). The data for the eastern Tien Shan cannot be fit without a force per unit length of roughly 1–4 × 1012 N, or by a bending moment, applied to the end of the southern plate. Thus either a deep mass per unit length of about 1–4 × 1011 kg/m must underlie the eastern part of the range, or dynamic (deviatoric tensional) normal stresses of up to about 5 MPa, due to flow in the asthenosphere, must pull the Tien Shan down. Probably a combination of them, resulting from the formation and sinking of a thickened lithospheric root, causes the deflection of the Moho and the deviations from local isostatic equilibrium.

A seismological study of the 1835 seismic gap in south-central Chile

We study the possible seismic gap in the Concepcion–Constitucion region of south-central Chile and the nature of the M = 7.8 earthquake of January 1939. From 1 March to 31 May 1996 a seismic network of 26 short period digital instruments was deployed in this area. We located 379 hypocenters with rms travel time residuals of less than 0.50 s using an approximate velocity distribution. Using the VELEST program, we improved the velocity model and located 240 high precision hypocenters with residuals less than 0.2 s. The large majority of earthquakes occurred along the Wadati–Benioff zone along the upper part of the downgoing slab under central Chile. A few shallow events were recorded near the chain of active volcanos on the Andes; these events are similar to those of Las Melozas near Santiago. A few events took place at the boundary between the coastal ranges and the central valley. Well constrained fault plane solutions could be computed for 32 of the 240 well located events. Most of the earthquakes located on the Wadati–Benioff zone had “slab-pull” fault mechanism due to tensional stresses sub-parallel to the downgoing slab. This “slab-pull” mechanism is the same as that of eight earthquakes of magnitude around 6 that are listed in the CMT catalog of Harvard University for the period 1980–1998. This is also the mechanism inferred for the large 1939 Chilean earthquake. A very small number of events in the Benioff zone had “slab-push” mechanisms, that is events whose pressureaxis is aligned with the slab. These events are found in double layered Wadati–Benioff zones, such as in northern Chile or Japan. Our spatial resolution is not good enough to detect the presence of a double layer, but we suspect there may be one.

Shear wave anisotropy in the upper mantle beneath the Aegean related to internal deformation

Seismic anisotropy, deduced from SKS splitting measured at 25 stations installed in the Aegean, does not show a homogeneous pattern. It is not restricted to the North Anatolian Fault but is distributed over a region several hundreds kilometers wide. Little anisotropy is observed in continental Greece or along the Hellenic arc; however, significant anisotropy is observed in the north Aegean Sea. Large values of delay times suggest that anisotropy is due to a long path within the upper mantle and to strong intrinsic anisotropy. Our results, both in fast polarization directions and in values of delay time, do not support the idea that anisotropy is associated with inherited tectonic fabric nor are they consistent with the present-day Aegean motion relative to an absolute frame. In contrast, the direction of fast polarization and the magnitude of delay times correlate well with the present-day strain rate observed at the surface deduced from both geodetic measurements and seismicity. This anisotropy is not horizontally restricted to major surface faults but is spread over a wide region.