Oona Scotti

New Evidence on the State of Stress of the San Andreas Fault System

Contemporary in situ tectonic stress indicators along the San Andreas fault system in central California show northeast-directed horizontal compression that is nearly perpendicular to the strike of the fault. Such compression explains recent uplift of the Coast Ranges and the numerous active reverse faults and folds that trend nearly parallel to the San Andreas and that are otherwise unexplainable in terms of strike-slip deformation. Fault-normal crustal compression in central California is proposed to result from the extremely low shear strength of the San Andreas and the slightly convergent relative motion between the Pacific and North American plates. Preliminary in situ stress data from the Cajon Pass scientific drill hole (located 3.6 kilometers northeast of the San Andreas in southern California near San Bernardino, California) are also consistent with a weak fault, as they show no right-lateral shear stress at ∼2-kilometer depth on planes parallel to the San Andreas fault.

Fault mechanics and the kinematics of block rotations

In many strike-slip tectonic settings, large rotations (up to 100°) of crustal blocks have been inferred from paleomagnetic data. These blocks are bounded by sets of parallel faults, which accommodate the relative motion between the blocks as regional deformation progresses. Simple geometrical considerations require that the faults must also rotate. In this paper we show that on the basis of mechanical considerations, the amount of fault rotation permissible under a stationary stress field is limited to 20° to 45°. Consequently, block rotations that are larger than 40° or 45° require more than one set of accompanying faults to accommodate the block rotation. Examples of such multiple sets with 40° to 45° between them, as predicted by the model, were recognized in Sistan, Iran; in Yerington, the Lake Mead area, Nevada; and in southern California.

Dynamic versus static stress triggering and friction parameters: Inferences from the November 23, 1980, Irpinia earthquake

This paper concentrates on the problem of fault interaction and earthquake triggering through the 1980 Irpinia, Italy, sequence. More specifically, this paper deals with the problem of the triggering of the second subevent by the mainshock. The interaction between the two segments is modeled through a dynamic Coulomb failure function. The aims of this paper are, first, to discriminate between the dynamic and the static stress effects on the triggering, if these effects exist, second, to estimate the fault strength relative to the initial state of stress, third, to determine the parameters of a slip-dependent friction law that lead to the observed delay of 20 s. Numerical simulations show that the critical slip Dc may range from 0.03 m up to 1.7 m, and that the initial slope of the friction law μ′(0) must be lower than 0.04 m−1. We show that the relative magnitude of the fault strength and the initial state of stress govern the existence and value of a Dc lower threshold under which the fault always ruptures before 13 s. A close to failure fault is not consistent with a critical slip Dc less than 0.8 m, whereas small values of Dc , typically 0.03 m, imply a far from failure fault. General results concern the effect of a dynamic stress pulse. We show that an event can be triggered by a transient stress pulse and that in this case the event can have an initiation duration much longer than the pulse duration. We show that it is possible to explain both the triggering and the time delay only with the effect of the transient stress pulse. This may explain aftershock triggering even in regions of negative Coulomb failure function or long distance triggering of earthquakes by propagating waves.

Mechanics of Distributed Fault and Block Rotation

Most of the earth’s crust is broken by dense sets of subparallel faults which are organized in domains. Joint analysis of structural and paleomagnetic data (Ron et al., 1984) demonstrate that these domains when subject to tectonic deform by distributed fault slip and block rotations, rather than by uniform straining. Many such domains have been recognized in the Western U.S., and in California and Nevada in particular. Precise mechanical considerations of stress at failure, strength and friction by Nur et al. (1986) reveal under what conditions new fault sets must form when these rotations are sufficiently large (25°–45°) leading to domains of multiple sets. Several domains of multiple fault sets have by now been recognized in California and Nevada.

Distributed deformation and block rotation in three dimensions

The focus of this paper is to understand distributed deformation, in particular the relationship between fault slip and rotation of faults and blocks in a three dimensional stress field. Regions of distributed deformation, such as Southern California, are organized in complex arrays of contemporaneously active block-faulted domains. We believe that the present day orientation of faults in many domains is due to the contemporaneous slip and rotation of the faults and of the blocks they bound. Traditional friction models cannot explain active unfavorably oriented faults and do not consider how faults become unfavorably oriented. To solve this problem, we propose a three dimensional block rotation model that tracks the orientation of blocks and their bounding faults during rotation. Mechanically, we consider Coulomb criteria for rock fracture as an upper bound, and slippage on a frictional surface as a lower bound. The key parameter in our model is the value of ϕ = (σ2 – σ3)/(σ1 – σ3). Principal stress directions are assumed irrotational through time. This model predicts up to 75° of vertical axis rotation along a single set of faults. During rotation, fault slip may change, sometimes dramatically, giving rise to mixed vertical as well as horizontal axis of rotation of blocks and faults. For very unfavorably oriented faults the model predicts rotations about a vertical axis in both the normal and reverse stress regimes, and about a horizontal axis in the strike-slip stress regime. Therefore paleomagnetically inferred rotations may not always be directly related back to a specific stress regime. Combining frictional constraints of the block rotation model with paleomagnetic, structural and geological data, we show how only one set of faults, preexisting and rotating in an irrotational strike-slip stress field, can account for the three major phases of deformation observed in the Western Transverse Range domain, Southern California: preexisting north-northeast faults were reactivated as normal faults, rotated and became strike-slip, and subsequent rotations of faults resulted in their present east-west high angle reverse orientation. This example demonstrates that it is not necessary to invoke complex regional and local changes in the stress regime or erratic changes in plate motion to account for alternate periods of compression and extension.

REGAL; reseau GPS permanent dans les Alpes occidentales; configuration et premiers resultats

The kinematics of the present-day deformation in the western Alps is still poorly known, mostly because of a lack of direct measurements of block motion and internal deformation. Geodetic measurements have the potential to provide quantitative estimates of crustal strain and block motion in the Alps, but the low expected rates, close to the accuracy of the geodetic techniques, make such measurements challenging. Indeed, an analysis of 2.5 years of continuous GPS data at Torino (Italy), Grasse (France), and Zimmerwald (Switzerland), showed that the present-day differential motion across the western Alps does not exceed 3 mm/yr [Calais, 1999]. Continuous measurements performed at permanent GPS stations provide unique data sets for rigorously assessing crustal deformation in regions of low strain rates by reducing the amount of time necessary to detect a significant strain signal, minimizing systematic errors, providing continuous position time series, and possibly capturing co- and post-seismic motion. In 1997, we started the implementation of a network of permanent GPS stations in the western Alps and their surroundings (REGAL network). The REGAL network mostly operates dual frequency Ashtech Z12 CGRS GPS stations with choke-ring antennae. In most cases, the GPS antenna is installed on top of a 1.5 to 2.5 m high concrete pilar directly anchored into the bedrock. The data are currently downloaded once daily and sent to a data center located at Geosciences Azur, Sophia Antipolis where they are converted into RINEX format, quality checked, archived, and made available to users. Data are freely available in raw and RINEX format at http://kreiz.unice.fr/regal/. The GPS data from the REGAL network are routinely processed with the GAMIT software, together with 10 global IGS stations (KOSG, WZTR, NOTO, MATE, GRAZ, EBRE, VILL, CAGL, MEDI, UPAD) that serve as ties with the ITRF97. We also include the stations ZIMM, TORI, GRAS, TOUL, GENO, HFLK, OBER because of their tectonic interest. We obtain long term repeatabilities on the order of 2-3 mm for the horizontal components, 8-10 mm for the vertical component. Using a noise model that combines white and coloured noise (flicker noise, spectral index 1), we find uncertainties on the velocities ranging from 1 mm/yr for the oldest stations (ZIMM, GRAS, TOUL, TORI, SJDV) to 4-5 mm/yr for the most recently installed (CHAT, MTPL). Station velocities obtained in ITRF97 are rotated into a Eurasian reference by substracting the rigid rotation computed from ITRF97 velocities at 11 central European sites located away from major active tectonic structures (GOPE, JOZE, BOR1, LAMA, ZWEN, POTS, WETT, GRAZ, PENC, Effelsberg, ONSA). The resulting velocity field shows residual motions with respect to Eurasia lower than 3 mm/yr. We obtain at TORI, in the Po plain, a residual velocity of 2.3+ or -0.8 mm/yr to the SSW and a velocity of 1.9+ or -1.1 mm/yr at SJDV, on the Alpine foreland. These results indicate that the current kinematic boundary conditions across the western Alps are extensional, as also shown by the SJDV-TORI baseline time series. We obtain at MODA (internal zones) a residual velocity of 1.2+ or -1.2 mm/yr to the SSE. The MODA-FCLZ baseline show lengthening at a rate of 1.6+ or -0.8 mm/yr. These results are still marginally significant but suggest that the current deformation regime along the Lyon-Torino transect is extension, as also indicated by from recent seismotectonic data. It is in qualitative agreement with local geodetic measurements in the internal zones (Briancon area) but excludes more than 2.4 mm/yr of extension (FCLZ-MODA baseline, upper uncertainty limit at 95% confidence). Our results indicate a different tectonic regime in the southern part of the western Alps and Provence, with NW-SE to N-S compression. The GRAS-TORI baseline, for instance, shows shortening at a rate of 1.4+ or -1.0 mm/an. This result is consistent with seismotectonic data and local geodetic measurements in these areas. The Middle Durance fault zone, one of the main active faults in this area, is crossed by the GINA-MICH baseline, which shows shortening at a rate of 1.0+ or -0.8 mm/an. This result is only marginally significant, but confirms the upper bound of 2 mm/yr obtained from triangulation-GPS comparisons. The REGAL permanent GPS network has been operating since the end of 1997 for the oldest stations and will continue to be densified. Although they are still close to or within their associated uncertainties, preliminary results provide, for the first time, a direct estimate of crustal deformation across and within the western Alps.