Tectonic Seismicity Research Articles

Turbidites associated with bentonites in the Upper Cretaceous of the Campos Basin, offshore Brazil

More than fifteen bentonite beds have been identified in the Santonian/Campanian section of the Campos Basin, offshore Brazil. These range in thickness up to 1 m and are mainly composed of interstratified illite/smectite. Relict textural features can still be observed, identifying them as former volcanic ash layers. They also display gradational or sharp contacts with the associated lithologies. In the Carapeba Field, `pure' bentonite beds, formed mainly by expandable clay minerals, occur intercalated with pelitic rocks. Bentonite beds with up to 20% terrigenous silty material (micas, quartz and feldspars), here classified as `impure', are found directly overlying siliciclastic turbidite sandstones. These bentonite beds incorporate constituents of turbiditic dust and are interpreted as equivalent to the Bouma E division of the underlying turbidite layers. This fact suggests a contemporaneity of the turbidite and the volcaniclastic processes. Therefore, the volcanism and associated tectonic seismicity are considered a potential mechanism for the onset of turbidite sedimentation.

Coda- Q distribution in the Canary Islands

The Canary Archipelago, near the African plate between 27° and 30° N and 13° and 19° W, is included in the oldest region (> 135 m.y.) of the eastern Atlantic Ocean. The tectonic seismicity is moderate, with low magnitudes, and it is almost totally related to an active fault located between the islands of Tenerife and Gran Canaria. The purpose of this paper is to provide a clear idea of the anelasticity of coda waves in the region. In order to compute master dispersion curves as a function of the quality factor, Q, we have applied inversion methods to obtain analytical magnification curves for all the seismographic stations used. f p- t data, corresponding to the coda wave dispersion of 52 local and regional seismic events recorded at 6 seismographic stations, have been obtained. The fit of the data to the master curves determined yields estimates of coda- Q values and their frequency dependence. Most of the Q 0 values determined, at 1 Hz, vary between 100 and 300, showing a strong frequency dependence in the range 0.2–2 Hz. The characteristic v value (degree of frequency dependence of Q) in the region, given by the law Q = Q 0 f v is v = 0.9. By applying regionalization techniques an iso- Q 0 map, for a reference frequency of 1 Hz, has been obtained for the Canary Archipelago. This map shows important lateral variations in Q 0, reflecting the tectonics and the volcanism of the region. The lowest Q 0 values are found along the trend of the fault and beneath the islands.

Poroelastic stressing and induced seismicity near the Lacq gas field, southwestern France

Hundreds of shallow, small to moderate earthquakes have occurred near the Lacq deep gas field in southwestern France since 1969. These earthquakes are clearly separated from tectonic seismicity occurring in the Pyrenees, 25 km to the southwest. The induced seismicity began when the reservoir pressure had declined by ∼30 MPa. Repeated leveling over the field shows localized subsidence reaching a maximum of 60 mm in 1989. Segall (1989) suggested that poroelastic stressing, associated with volumetric contraction of the reservoir rocks, is responsible for induced seismicity associated with fluid extraction. To test this model, we compare the observed subsidence and hypocentral distributions with the predicted displacement and stress fields. We find that the relationship between average reservoir pressure drop and subsidence is remarkably linear, lending support to the linear poroelastic model. Displacements and stresses are computed based on a priori knowledge of the reservoir geometry, material properties, and reservoir pressure changes. The computed vertical displacements are found to be in excellent agreement with the subsidence observed from leveling. Stress perturbations accompanying gas extraction, computed using the same parameters, are found to be ∼0.2 MPa or less. Changes in Coulomb failure stress are computed assuming that slip occurs on optimally oriented planes. The predicted failure zones correlate very well with the spatial distribution of earthquakes if the perturbing stresses are small in comparison to the ambient regional deviatoric stresses and if the minimum regional compressive stress axis is vertical. Accurate determination of focal mechanisms of the induced events would allow a more rigorous test of the poroelastic model and could lead to important inferences about the crustal stress state.

Contemporary tectonics, seismicity, and potential earthquake sources in the white mountains seismic gap, west-central Nevada and east-central California, USA

Abstract The White Mountains seismic gap (WMSG) is a broad area between the 1872 Owens Valley earthquake and the 1932 Cedar Mountain earthquake that has a complicated tectonic setting and seismicity patterns, and is considered to have the potential for a strong or larger-magnitude earthquake in the near future. We take the Sierra Nevada block as the western boundary of the WMSG, the Pancake Range lineament and the southernmost 1932 earthquake ruptures as its approximate northern boundary, a change in structure, tectonic rates and seismicity level as the eastern boundary, and the northernmost 1872 earthquake ruptures as the southern boundary. Seismicity within the WMSG, especially in the southern part, has been at a very high level since 1978. This activity has included six events of magnitude ⩾ 6 and their associated aftershocks. In addition many earthquake swarms have occurred throughout the WMSG, some of which are distinctly located at the ends of fault zones or near changes in structural orientation. Focal mechanisms show a predominance of strike-slip solutions for both small and large earthquakes, with NW-trending right-lateral and NE-trending left-lateral solutions for over half of the mechanisms. These are similar to the sense of displacements and orientations of the larger faults in the WMSG. Thus seismic strain is consistent with the faulting pattern in the WMSG and is accommodating contemporary deformation through a conjugate set of strike-slip and normal-oblique-slip faults. Examination and analysis of eighteen of the larger faults in the WMSG leads to the estimation of characteristic earthquakes ranging from magnitude 6.8 to 7.4. Given the size of the seismic gap (100 km) and the pronounced change in structure near the middle of the seismic gap, it seems likely that it will require two or more magnitude ~ M 7 + events to “fill” the gap.

Mining-related and tectonic seismicity in the East Mountain area Wasatch Plateau, Utah, U.S.A.

As part of a larger multi-institutional seismic monitoring experiment during June–August 1984 in the eastern Wasatch Plateau, Utah, data from a subarray of 20 portable seismographs were used to investigate seismicity in the East Mountain area, an area of active underground coal mining and intense microseismicity. Eight stations of the subarray were concentrated on top of East Mountain, about 600 m above mine level, at an average spacing of 2 to 3 km. The primary objective was the accurate resolution of hypocenters and focal mechanisms for seismic events originating at submine levels. Data from high-resolution seismic reflection profiles and drill-hole sonic logs yielded a detailed velocity model. This model features a strong velocity gradient in the uppermost 1 km, which has a significant effect on takeoff angles for first-arrivingP-waves from shallow seismic events. Two hundred epicenters located with a precision of ±500 m cluster within an area about 5 km in diameter and show an evident spatial association with four sites of longwall mining during the study period. A special set of foci rigorously tested for focal-depth reliability indicates submine seismicity predominating within 500 m of mine level and extending at least to 1 km, and perhaps to 2 km, below mine level. Continuous monitoring for a 61-day period (June 15–August 15) bracketed a 16-day mining shutdown (July 7–22) during which significant seismicity, comparable to that observed before the shutdown, was observed. Ten focal mechanisms for seismic events originating at or down to 2 km below mine level nearly all imply reverse faulting, consistent with previous results and the inferred tectonic stress field. Enigmatic events recorded with all dilatational first motions can be fit with double-couple normal-faulting solutions if they in fact occurabove mine level, perhaps reflecting overburden subsidence. If these events are constrained to occur at mine level, their first-motion distributions are incompatible with a double-couple source mechanism.

Seismicity in the vicinity of Ross Island, Antarctica

Earthquakes in the Ross Sea, Antarctica, are of two types: volcanic, and those which appear to be of tectonic origin. Volcanic events in the vicinity of Ross Island are associated exclusively with Mount Erebus, Ross Island; this volcano erupts regularly, generating several earthquakes per day whose characteristics are quite distinct from non-volcanic events. These nonvolcanic earthquakes are recognizable by their distinct P- and S-wave arrivals, and a lack of the high frequency, often monochromatic character typical of Erebus events. One hundred fifty-seven tectonic microearthquakes (M < 2.0) were recorded in 1983 and 1984 by the ten station network on Ross Island; these events were located using the least-squares routine, HYPOELLIPSE. Of these events, 106 have RMS residual traveltime errors of less than or equal to 0.6 seconds; they are clustered in the vicinity of Ross Island, but are not restricted to it. There is a linear trend of epicenters cutting across the island and continuing northward. Most activity seems to center beneath Mount Terra Nova, between Mount Erebus and Mount Terror. Mean depth for events is 8.2 km; however, depths are rather evenly distributed over a range of 0 to 25 km. Modelling based on Bouger gravity anomalies and seismic refraction studies indicates a depth to the Moho of about 40 km beneath the continent, shallowing to 27 km beneath the Ross Sea. This 27 km depth is approximately equal to the lower limit of the tectonic seismicity detected by the Erebus network; hence, events are of crustal origin. These data suggest, with the rift-type geochemistry of Erebus' magma, that the Ross Sea is a site of active crustal extension and rifting.

Part 6. Present plate boundary seismic, volcanic and kinematic processes: Seismicity and earthquake focal mechanisms in the New Guinea Solomon Islands region

There is general agreement between plate tectonic theory and seismicity for the New Guinea Solomon Islands region. Two minor plates, the Solomon Sea and South Bismarck plates, are sandwiched within the collision zone between the continental fronts of the Pacific and India plates, and derive their relative motions from the collision. The azimuth of collision is approximately east?northeast.