Average Dip Angle Research Articles

Seismicity and stress distribution in the Copiapo, northern Chile subduction zone using combined on- and off-shore seismic observations

Abstract Historic, as well as teleseismically, and locally recorded seismicity of the subduction margin of Copiapo, northern Chile is analysed. An on- and off-shore temporary network was deployed during 2 months in the region in order to obtain a detailed definition of the Wadati–Benioff zone. An average dip angle of 20° along the subduction thrust interplate contact was determined; this value is similar to that observed along the Chilean margin, suggesting that there are no variations in the dip angle of the Nazca plate for depths less than 50–60 km. Locally recorded data do not provide seismic evidence to determine whether subhorizontal subduction exists north of latitude 28°S. Teleseismically recorded events exhibit a low rate of occurrence of inland earthquakes, with a tendency for epicentres to be located off-shore; in contrast, locally recorded seismicity reveals a homogeneous epicentral distribution, without a predominance of off-shore events. Along the subduction thrust contact there is a reverse stress regime with the greatest compression axis (σ1) horizontal and oriented perpendicular to the strike of the trench. Events located above the main decollement are in a normal stress regime with the minimum compressive stress axis σ3 horizontally oriented parallel to the convergence direction. The events located below the coupled interplate contact also lie in a normal faulting regime but the σ3 axis plunges almost parallel to the subducting plate. The intermediate depth fault plane solutions exhibit a substantial scatter, suggesting a complicated faulting regime similar to that observed in previous studies of northern Chile. They are associated with a normal faulting stress regime in which σ3 is oriented in a N14°E direction parallel to the strike of the trench. High resolution bathymetric data and seismicity located above the thrust contact suggest that seamounts are being subducting.

Regional Difference in Scaling Laws for Large Earthquakes and its Tectonic Implication

We compiled 67 large earthquakes which occurred at and around plate boundaries for the last 140 yrs, and classified them into four groups; interplate strike-slip events, intraplate strike-slip events, underthrust events at island-arc subduction zones, and underthrust events at continental-margin subduction zones. For each group of earthquakes we examined relations between seismic moment M 0, fault length L, fault width W and average fault slip D, and found the following scaling laws. In the case of interplate strike-slip events, the well-known L-cubed dependence of seismic moment breaks down when L exceeds 30 km, because the extent of the seismogenic zone is limited in depth (≤ 12 km). For large events (L ≥ 30 km), D and M 0 increase with L as \( D = \overline {\Delta \tau } L/\mu \left( {\alpha L + \beta } \right) \) and \( {{M}_{0}} = \overline {\Delta \tau W} {{L}^{2}}/\left( {\alpha L + \beta } \right) \) , respectively, where the mean fault width \( \bar{W} \) is 12 km and the mean stress drop \( \overline {\Delta \tau } \) is 1.8 MPa. Here μ, α and β are structural parameters. For intraplate strike-slip events we obtained nearly the same relations, except for significantly higher stress drop (3.1 MPa). The difference in stress drop between interplate and intraplate events may be ascribed to the difference in stress accumulation rates and thus the recurrence time of earthquakes. In the case of underthrust events at island-arc subduction zones we also found the saturation of fault width \( (\bar{W} = 120km) \) and the breakaway from the L-cubed dependence of M 0 for events larger than L = 200 km. If we consider the average dip-angle of plate boundaries at island-arc subduction zones to be 20–30°, this indicates that the extent of the seismogenic zone in depth is limited to 40–60 km. In the case of continental-margin subduction zones, on the other hand, we could not find the saturation of fault width nor the breakaway from the L-cubed dependence of M 0 from the analysis of the present data set (W ≤ 200 km, L ≥ 1000 km). For sufficiently large earthquakes, in general, the downward rupture growth is limited to a certain depth due to the existence of a ductile unstressed region which extends under the brittle seismogenic zone. Since the brittle-ductile transition occurs at 300–400°C, the difference in the lower limit of the seismogenic zones between tectonically different regions may be attributed to the difference in thermal state there.

Differential compaction and basinward tilting of the prograding capitan reef complex, Permian, west Texas and southeast New Mexico, USA

The Capitan Limestone (Permian) of southeast New Mexico and west Texas, U.S.A., contains a classic shelf-margin carbonate system which progrades into the Delaware basin. Geopetal structures were studied in Capitan reefal cavities in the Guadalupe Mountains to determine the effect of postdepositional tilting on stratal geometries at and near the shelf margin. Geopetal structures in the reefal part of the middle Capitan have basinward dips (average dip angle = 10° to the southeast, n = 28) which indicate that the Capitan reef was tilted basinward by approximately 10° after deposition. Backreef strata immediately above and lagoonward of the Capitan reef have basinward dips of approximately 8–9° to the southeast, similar to geopetal surfaces in the Capitan. However, backreef strata 100–150 m above the Capitan reef are nearly horizontal. Differential compaction of slope and basinal carbonate muds apparently tilted Capitan reef and lowest backreef beds shortly after deposition when dense lithified reef and shelfal carbonates prograded over highly compactable slope and basinal sediments. Differential compaction and down-to-the-basin tilting has probably increased the slope angle of many other prograding carbonate systems shortly after deposition.

On the stress system that formed the Laramide Wind River Mountains, Wyoming

A seismic reflection profile obtained by the Consortium for Continental Reflection Profiling [COCORP] has delineated the Wind River thrust fault which underlies the Wind River Mountains, Wyoming. This thrust fault dips at an average angle of 30° ‐ 40° to a depth of 24 km and possibly to 36 km. In this paper we associate the approximately constant angle of dip with the Anderson theory of faulting. With an average dip angle of 35° the derived coefficient of fraction is 0.36. We consider that the Anderson theory of faulting predicts the angle of dip of activated fractures in a rock mass pervaded by fractures in random directions.