Regions Of High Slip Research Articles

Rupture Propagation of the 2004 Parkfield, California, Earthquake from Observations at the UPSAR

Using a short-baseline seismic array (U.S. Geological Survey Parkfield Dense Seismograph Array [upsar]) about 12 km west of the rupture initiation of the 28 September 2004 M 6.0 Parkfield, California, earthquake, we have observed the movement of the rupture front of this earthquake on the San Andreas fault. The sources of high-frequency arrivals at upsar, which we use to identify the rupture front, are mapped onto the San Andreas fault using their apparent velocity and back azimuth. Measurements of apparent velocity and back azimuth are calibrated using aftershocks, which have a compact source and known location. Aftershock back azimuths show considerable lateral refraction, consistent with a high-velocity ridge on the southwest side of the fault. We infer that the initial mainshock rupture velocity was approximately the Rayleigh speed (with respect to slower side of the fault), and the rupture then slowed to about 0.66 β near the town of Parkfield after 2 sec. The last well-correlated pulse, 4 sec after S, is the largest at upsar, and its source is near the region of large accelerations recorded by strong-motion accelerographs and close to northern extent of continuous surface fractures on the southwest fracture zone. Coincidence of sources with preshock and aftershock distributions suggests fault material properties control rupture behavior. High-frequency sources approximately correlate with the edges of asperities identified as regions of high slip derived from inversion of strong-motion waveforms.

Spatial relation between main earthquake slip and its aftershock distribution

We examine where aftershocks occur relative to the spatial distribution of the main shock slip using data from several recent large earthquakes. No universal relation between high‐ and low‐moment regions and high or low aftershock occurrence, or vice versa, is found. We generally find that few, and usually the smaller, aftershocks occur in the high‐slip regions of the fault, a notable exception to this being the great 1996 Biak, Indonesia, subduction zone earthquake. In all cases, aftershocks occur on favorably oriented planes of weakness in regions of increased postseismic stress. Generally, they are clustered at both ends of faults, but examples where aftershocks occur only at one end or where there is no clustering at the ends are found. Aftershock clusters are also found at the edge of unbroken barriers, and regions of rapid transition from high to low slip, within the main fault area. We identify examples of geometrical and inhomogeneous barriers and sharp and dull stress concentrations. Rupture in the main shock is generally found to nucleate in the region of low slip or at the edge of high‐slip regions, the 1996 Biak earthquake again being the only exception, nucleating in a very high slip region. Off‐fault aftershocks are found for all earthquakes in this study, and they sometimes rupture the nodal plane conjugate to the main shock fault plane.

The slip history of the 1994 Northridge, California, earthquake determined from strong-motion, teleseismic,GPS, and leveling data

AbstractWe present a rupture model of the Northridge earthquake, determined from the joint inversion of near-source strong ground motion recordings, P and SH teleseismic body waves, Global Positioning System (GPS) displacement vectors, and permanent uplift measured along leveling lines. The fault is defined to strike 122° and dip 40° to the south-southwest. The average rake vector is determined to be 101°, and average slip is 1.3 m; the peak slip reaches about 3 m. Our estimate of the seismic moment is 1.3 ± 0.2 × 1026 dyne-cm (potency of 0.4 km3). The rupture area is small relative to the overall aftershock dimensions and is approximately 15 km along strike, nearly 20 km in the dip direction, and there is no indication of slip shallower than about 5 to 6 km. The up-dip, strong-motion velocity waveforms are dominated by large S-wave pulses attributed to source directivity and are comprised of at least 2 to 3 distinct arrivals (a few seconds apart). Stations at southern azimuths indicate two main S-wave arrivals separated longer in time (about 4 to 5 sec). These observations are best modeled with a complex distribution of subevents: The initial S-wave arrival comes from an asperity that begins at the hypocenter and extends up-dip and to the north where a second, larger subevent is centered (about 12 km away). The secondary S arrivals at southern azimuths are best fit with additional energy radiation from another high slip region at a depth of 19 km, 8 km west of the hypocenter. The resolving power of the individual data sets is examined by predicting the geodetic (GPS and leveling) displacements with the dislocation model determined from the waveform data, and vice versa, and also by analyzing how well the teleseismic solution predicts the recorded strong motions. The general features of the geodetic displacements are not well predicted from the model determined independently from the strong-motion data; likewise, the slip model determined from geodetic data does not adequately reproduce the strong-motion characteristics. Whereas a particularly smooth slip pattern is sufficient to satisfy the geodetic data, the strong-motion and teleseismic data require a more heterogeneous slip distribution in order to reproduce the velocity amplitudes and frequency content. Although the teleseismic model can adequately reproduce the overall amplitude and frequency content of the strong-motion velocity recordings, it does a poor job of predicting the geodetic data. Consequently, a robust representation of the slip history and heterogeneity requires a combined analysis of these data sets.

Parkfield seismicity: Fluid‐driven?

Characteristics of microearthquake occurrence along the locked‐to‐creeping transition of the San Andreas fault at Parkfield in central California are reviewed for evidence that fault zone fluids play a critical role in slip dynamics there. Previous studies at Parkfield have defined a low‐velocity, anisotropic, attenuating fault zone and a high Vp/Vs ratio in the nucleation zone of the repeating M 6 earthquakes. Most of the ongoing seismicity is organized into a temporally‐evolving checkerboard pattern of alternating high and low seismic slip regions divided on the fault zone at 3–6 km spacings. Much of the seismicity is further confined to a few hundred small (20–30 m radius) cells of densely clustered microearthquakes that exhibit periodic recurrence. Space‐time development of small (tens to hundreds of meters), transient (minutes to days) earthquake sequences reveals diffusivelike outward spreading along the fault zone. There is some evidence for anomalous source mechanisms in sequence‐initiating events, possibly indicative of hydraulic fracturing. The data are consistent with a model in which some microearthquake dusters and confined sequences occur by the cyclic pressurization of fluid within localized patches of the fault zone. The consequent modulation of the effective normal stress leads to fluid‐driven slip manifested both as the highly periodic earthquake clusters and as localized earthquake sequences observed at Parkfield.

Frequency domain inversion of strong motions: Application to the 1992 Landers earthquake

We present a frequency domain inversion in which the observed earthquake strong ground motions are used to constrain the space‐time dependence of slip on a fault. Green's functions are numerically evaluated and the parameters describing the rupture are the local slip, rupture time and rise time. These parameters are simultaneously evaluated without additional constraints. This procedure allows for large variations in the local rupture velocity. The June 28, 1992 Landers earthquake (Mw = 7.3) offers an exceptional opportunity to apply this technique to a major strike‐slip event. We model the rupture evolution, including local differences in slip durations and variations in rupture velocity. Our final results are in good agreement with other inversion studies, geodetic and surface observations. The main discrepancies occurred at depth and at the end of the Johnson Valley fault. We show that a relatively low resolution could be an explanation for these differences. Rupture velocity and slip are extremely heterogeneous, both along strike and with depth. A moment of 0.90×1020 N m was found. The slip distribution shows that this event consists of a series of regions of high slip (subevents) separated by regions of relative low slip. Approximately 50% of the moment was released on the Homestead Valley fault; in this region of large slip, the rupture velocity inferred by our inversion is well constrained and is equal to 3.0 km/s at depth and 2.5 km/s near the surface. Our inversion favors the hypothesis that the duration of the slip at each point of the fault is of the order of the duration of rupture of each subevent.

Application of seismogram synthesis to the study of earthquake source from strong motion records

We present the results of an analysis of the Michoacan and Landers earthquakes to constrain the kinematic description of the rupture process. The example of the Michoacan earthquake shows that a crack type model is better a dislocation model to describe the displacement in the vicinity of the fault. We also show that this in contradiction with the fact that the faulting appears to be a complex process. We attribute this complexity to instabilities in crack growth. The June 28, Landers earthquake offers an exceptional opportunity to apply a new inversion technique to a major strike slip event. We model the rupture evolution including local differences in slip durations and variations in rupture velocity. The slip distribution shows that this event consists of a series of regions of high slip (sub-events) separated by regions of relative low slip. Our inversion favors the hypothesis that the duration of the slip at each point is of the order of the duration of the rupture of each sub-event and is consistent with a crack type process occurring during each sub-event. For such a large earthquake, the slip duration is however smaller than the total rupture duration.

Slip distribution of the 1992 Landers earthquake and its implications for earthquake source mechanics

We use near-source (10 to 164 km) displacement seismograms to image the slip distribution and rupture history of the 28 June 1992 Mw = 7.3 Landers, California, earthquake. Aftershock seismograms from similar distances are modeled to find the velocity model and frequency range (0.05 to 0.25 Hz) over which theoretical Green's functions are most accurate, and the measure of fit is used as an upper bound on theoretical error in the mainshock inversion. We represent the rupture surface with three planar segments divided into 3 by 3 km elements extending from the surface to 18-km depth, and solve for the slip distribution and the rupture time model that minimizes misfit to both recorded seismograms, and mapped surface displacement in a least-squares sense. We investigate two faulting models, one with a uniformly short (<3 sec) rise time everywhere on the fault surface and the other with a variable rise time (2 to 6 sec). We perform sensitivity tests using synthetic data from a hypothetical earthquake to evaluate model resolution and solution stability. In the sensitivity tests, both fault models recover similar slip and rupture features, but neither is capable of imaging details of the rise time. For the Landers earthquake, we find an average rupture velocity of 2.5 km/sec and use this average for a starting model in a linearized inversion for rupture time. In our solutions of slip-amplitude distribution, the southernmost, Johnson Valley fault segment has 20% of the total seismic moment (6 to 8 × 1019 N-m) with small displacements near the hypocenter; the Homestead Valley segment contributes half of the moment with the largest slip amplitudes 25 to 35 km northwest of the hypocenter at 4 to 12-km depth; and the Camp Rock-Emerson segment contributes the remaining moment with the largest slip amplitude 35 to 50 km northwest of the hypocenter in the shallow crust (<9 km). There is some evidence that the rupture front is delayed as it encounters high-slip regions, suggesting that prior to the mainshock these areas were further from failure owing either to greater strength or lower prestress.

Earthquake recurrence and risk assessment in circum-Pacific seismic gaps

THE development of the concept of seismic gaps, regions of low earthquake activity where large events are expected, has been one of the notable achievements of seismology and plate tectonics. Its application to long-term earthquake hazard assessment continues to be an active field of seismological research. Here I have surveyed well documented case histories of repeated rupture of the same segment of circum-Pacific plate boundary and characterized their general features. I find that variability in fault slip and spatial extent of great earthquakes rupturing the same plate boundary segment is typical rather than exceptional but sequences of major events fill identified seismic gaps with remarkable order. Earthquakes are concentrated late in the seismic cycle and occur with increasing size and magnitude. Furthermore, earthquake rup-ture starts near zones of concentrated moment release, suggesting that high-slip regions control the timing of recurrent events. The absence of major earthquakes early in the seismic cycle indicates a more complex behaviour for lower-slip regions, which may explain the observed cycle-to-cycle diversity of gap-filling sequences.

Frictional Instabilities in Ball Bearings

Computer modeling techniques are used to investigate instabilities in the motions of ball and cage in a ball bearing. As the friction at ball/race and ball/cage contacts increases, the cage mass center whirl orbit changes from circular to polygonal and then to a rather erratic shape under excessive friction. The corresponding variations in whirl velocities also increase to represent bearing squeal. It is shown that cage instabilities are directly dependent on the ball/race traction slope, under low slip velocities, and the friction coefficient at the cage interfaces. Under steep traction slopes, the variation in traction at higher slip rates is also significant in high-speed bearings. In particular, it is found that a negative traction slope, in the high-slip region, may produce appreciable ball skid which promotes excessive interaction in the cage pockets and, perhaps, the most damaging instability of the cage, where the mechanical interactions progressively increase to indicate significant potentials for cage failure. Under stable conditions of operation, the computer results also provide correlations between time-averaged wear rates of the cage and the frictional behavior.

Boundary layer flow of a dusty fluid over a semi-infinite flat plate

Both the drag force due to slip and the transverse force due to slip-shear have been considered in boundary layer equations. The solution has been found in a power series of non-dimensionalx, x being the distance in the down-stream direction. Solutions for high slip region and small slip region characterised byx≪1 andx≫1 respectively, have been found separately. In the high slip region the effect of increase in concentration parameter α of the dust particles is to increase the magnitude of the longitudinal fluid phase velocityu. Also the magnitude of the longitudinal particle slip velocityup-u is becoming maximum on the plate and decreasing along the plate withx. The transverse particle velocityvp is independent of α but it is directly proportional to β, the transverse force coefficient. An interesting result is thatvp is assuming small positive value on the plate. The transverse force has taken an important role in migration of particles away from the plate. In the small slip region the flow of dust particles is mainly governed by the fluid-phase. The effect of α on the flow field in this region is to decrease the boundary layer thickness. In this region the particles are having some tendency to accumulate near the plate. Lastly, it has been found that the shearing stress, skinfriction and the dimensionless drag-coefficient on the plate increase with increase of α.

The Effects of Elastohydrodynamic Traction Behavior on Cage Slip in Roller Bearings

A simple theoretical model of a bearing and measured traction curves are used to explain the different types of experimentally observed cage behavior which result from different lubricants. Traction curves with a pronounced peak are shown to produce unstable cage slip behavior, although bearings can operate steadily in the high slip region. The instability occurred in tests with a high traction fluid but not with ester-type gas turbine lubricants. A bench-top roller bearing rig of the type used in these experiments provides an effective method of assessing the tractive properties of lubricants, using a 25-ml sample.