Locked Section Research Articles

Influence of seafloor topography on the depositional behavior of bi-disperse turbidity currents: a three-dimensional, depth-resolved numerical investigation

We discuss the results of direct numerical simulations of bi-disperse turbidity currents interacting with a flat bottom wall and a Gaussian bump, respectively, with a focus on the final deposit profiles of the coarse and fine particles. We identify regions of reduced and enhanced deposition, as a result of the presence of the bump. Coarse particles are predominantly deposited towards the sides of the bump, as a result of the bi-section and lateral deflection of the current by the bump. In contrast, for fine particles the influence of the bump is felt more in its far wake. We furthermore employ Lagrangian markers in order to track the coarse and fine particles in the current, and to investigate their deposit locations as function of their location of origin. By comparing the final deposit maps, we observe that the bump has the strongest influence on those particles originating in the central lock sections.

Interpretation of interseismic deformations and the seismic cycle associated with large subduction earthquakes

The deformations of the overriding and subducting plates during the seismic cycle associated with large subduction earthquakes are modelled using 2D and 3D finite element techniques. A particular emphasis is put on the interseismic velocities and on the impact of the rheology of the asthenosphere.The distance over which the seismic cycle perturbs significantly the velocities depends upon the ratio of the viscosity in the asthenosphere to the period of the seismic cycle and can reach several thousand km for rheological parameters deduced from the first years of deformation after the Aceh earthquake. For a same early postseismic velocity, a Burger rheology of the asthenosphere implies a smaller duration of the postseismic phase and thus smaller interseismic velocities than a Maxwell rheology. A low viscosity wedge (LVW) modifies very significantly the predicted horizontal and vertical motions in the near and middle fields. In particular, with a LVW, the peak in vertical velocity at the end of the cycle is predicted to be no longer above the deep end of the locked section of the fault but further away, above the continentward limit of the LVW.The lateral viscosity variations linked to the presence at depth of the subducting slab affect substantially the results.The north–south interseismic compression predicted by this preliminary 2D model over more than 1500km within the Sunda block is in good agreement with the pre-2004 velocities with respect to South-China inferred from GPS observations in Thailand, Malaysia and Indonesia. In Japan, before the Tohoku earthquake, the eastern part of northern Honshu was subsiding while the western part was uplifting. This transition from subsidence to uplift so far away from the trench is well fitted by the predictions from our models involving a LVW.Most of the results obtained here in a 2D geometry are shown to provide a good estimate of the displacements for fault segments of finite lateral extent, with a 3D spherical geometry, as long as the displacements during the seismic cycle are normalised by the coseismic displacement.

Structural control on along-strike variation in the seismicity of the northwest Himalaya

An overview of seismicity along the northwest Himalaya exhibit along-strike segmentation intricately controlled by the subsurface topographic ridges on the underthrusting Indian Plate as well by rift and nappe structures in the overriding wedge of the Himalaya. The segmentation exists for thrust dominated large magnitude earthquakes (M>6) seated on the active detachment beneath the Outer and Lesser Himalaya. Segmentation also prevails for moderate and small magnitude earthquakes concentrated in a narrow Himalaya Seismic Belt (HSB) straddling northern Lesser Himalaya and southern Higher Himalaya. Numerical calculations of stress distribution favour that the degree of seismicity in the HSB is a good proxy to the presence of mid-crustal ramp connecting the locked section of active detachment to aseismically slipping detachment beneath Higher Himalaya. Further, gap or diffused pattern in the concentrated seismicity in the HSB, in agreement with mapped high electrical conductive structure, suggests ramp structure may be absent where underthrusting Delhi-Hardwar Ridge interacts with the Himalaya arc. In the nappe dominated tectonics, the accommodation of the accumulating strains on the listric thrust faults, produces increased frequency of moderate magnitude earthquakes and thereby possibly influences stress level on the detachment. This may explain relatively less frequent occurrences of large earthquake in the Kangra–Chamba region compared to that in the Garhwal Himalaya. The low level seismicity in the Simla region may also be influenced by the active Kaurik Chango Rift in the Higher Himalaya, as the former casts a stress shadow on the latter.

Performance comparison of track ropes in contemporary ropeway installations

Non-destructive testing is an important means for assessment of the performance of track ropes in a bi-cable zig-back ropeway installation. The visual method produces the possibility of inadequate inspection due to its subjectivity. Electromagnetic and visual wire rope inspections complement each other. Both are essential for safe operation. Periodic in-situ measurement of faults in ropes would help to determine the effects of various parameters on rope life. Track ropes are generally locked coil ropes. Full locked coil (FLC) ropes are made with a spiral strand as the main core and are finally covered with one or more layers of shaped wires. The final covering layer is made from full locked sections of wires which interlock with each other and present a smooth surface. Wire ropes wear out eventually, gradually losing work capability throughout their useful life, making periodic inspections, lubrication and tensioning necessary. Magnetic flaw detectors, which measure the loss of metallic area (LMA) and detect local faults (LF) such as broken wires and other faults, are used to determine rope wear. In this paper, an attempt has been made to describe the performance of track ropes in contemporary ropeway installations.

Spatiotemporal evolution of surface creep in the Parkfield region of the San Andreas Fault (1993–2004) from synthetic aperture radar

The Parkfield section of the San Andreas Fault (SAF) is defined as a transitional portion of the fault between slip-release behavior types in the creeping section of the SAF to the northwest and the apparently locked section to the southeast. The Parkfield section is characterized by complex frictional fault behavior because it represents a transition zone from aseismic creep to stick-slip regime. At least six historic earthquakes of M w ~ 6 have occurred in this area in 1881, 1901, 1922, 1934, 1966, and 2004. It was observed in the 2004 M w 6.0 Parkfield earthquake that ~ 70% of the total (coseismic and postseismic) moment release occurred aseismically. To understand the SAF behavior in this area, it is of particular interest to measure and analyze, not only the spatial evolution of the surface displacement in this area, but also its evolution over time. Using radar data acquired by the European Space Agency's European Remote Sensing (ERS1-2) satellites, we constructed descending interferograms and retrieved time series of surface displacements along the central SAF for the decade preceding the 2004 Parkfield earthquake. We focus on characterizing the space and time evolution of surface creep in the Parkfield and Cholame sections. The spatial pattern of the interseismic displacement rate indicates that tectonic strain was not uniformly distributed along the strike of the fault between 1993 and 2004. Our data indicate not only a decrease in the creep rate from the Parkfield section to south of Highway-46 from 1.4 ± 0.3 cm/yr to 0.6 ± 0.3 cm/yr, but also a small but significant creep-rate increase in the Cholame section to 0.2 ± 0.1 cm/yr. The evidence for episodic creep in the Cholame section of the SAF south-east of Parkfield is in contrast with previously published interpretations of GPS and trilateration data. The Cholame section of the SAF merits close monitoring because it was likely the nucleation site of the 1857 Fort Tejón earthquake and because it has shown recent evidence of deep slow slip as revealed by deep tremors.

Orogenic-wedge deformation and potential for great earthquakes in the central Andean backarc

Subduction of the Nazca plate beneath South America has driven the growth of the Andes Mountains. Subduction has routinely generated earthquakes larger than magnitude 8.0 along the western margin of the mountain belt 1 , but the potential size of less frequent earthquakes in the eastern, backarc margin is unknown. Continued support of the high Andean Plateau at the centre of the Andes can be explained only if deformation of the backarc margin is ongoing 2‐4 . Here we present GPS data that record surface motions in the Subandean ranges that are part of the backarc margin. We find that the velocity of surface movement decreases sharply from west to east across the Subandean ranges. We suggest that a subhorizontal fault underlying the ranges slips freely at depth in the west, but is locked for up to 100 km in shallower sections further east. Analysis of fault scarps formed where the subhorizontal fault intersects the surface indicates that the fault has generated repeated large earthquakes. We suggest that rupture of the entire locked section of the fault could generate an earthquake of magnitude 8.7‐8.9. We attribute the large seismic potential to the unusual width of the Subandean ranges, and suggest that deformation of the Subandean ranges, at a rate unmatched by erosion, causes the mountain range to widen. The central Andean Plateau (AP) is a bilaterally symmetric plateau of 4;000m average elevation (Fig. 1). Although AP uplift scenarios are currently debated 5,6 it is generally accepted that the backarc orogenic wedge on the AP’s east flank, comprising the Eastern Cordillera (EC), Inter-Andean zone (IAZ), and Subandes (SA), has contracted by more than 250km since 45Myr ago (refs 79). Wedge deformation has been facilitated by the presence

Scale‐invariant stress orientations and seismicity rates near the San Andreas Fault

We analyzed measurements of the direction of maximum horizontal compressive stress as a function of depth in two scientific research wells near the San Andreas Fault in central and southern California. We found that the stress orientations exhibit scale‐invariant fluctuations over intervals from tens of cm to several km. Similarity between the scaling of the stress orientation fluctuations and the scaling of earthquake frequency with fault size suggests that these fluctuations are controlled by stress perturbations caused by slip on faults of various sizes in the critically‐stressed crust adjacent to the fault. The apparent difference in stress scaling parameters between the two studies wells seem to correspond to differences in the earthquake magnitude‐frequency statistics for the creeping versus locked sections of the fault along which these two wells are located. This suggests that stress heterogeneity adjacent to active faults like the San Andreas may reflect variations in stresses and loading conditions along the fault.

Precise tremor source locations and amplitude variations along the lower‐crustal central San Andreas Fault

We precisely locate 88 tremor families along the central San Andreas Fault using a 3D velocity model and numerous P and S wave arrival times estimated from seismogram stacks of up to 400 events per tremor family. Maximum tremor amplitudes vary along the fault by at least a factor of 7, with by far the strongest sources along a 25 km section of the fault southeast of Parkfield. We also identify many weaker tremor families, which have largely escaped prior detection. Together, these sources extend 150 km along the fault, beneath creeping, transitional, and locked sections of the upper crustal fault. Depths are mostly between 18 and 28 km, in the lower crust. Epicenters are concentrated within 3 km of the surface trace, implying a nearly vertical fault. A prominent gap in detectible activity is located directly beneath the region of maximum slip in the 2004 magnitude 6.0 Parkfield earthquake.

Seismic tremor in subduction zones: Rock physics evidence

Episodic tremor and slip (ETS) have been correlated with rupture phenomena in subducting oceanic lithosphere at 30–45 km depth, where high VP/VS ratios, which suggest high‐fluid pressures, have been observed. ETS, by accommodating slip in the down‐dip portion of the subduction zone, may trigger megathrust earthquakes up‐dip in the locked section. During dehydration experiments on serpentinite (typical rock of the oceanic lithosphere) at temperatures found in nature at 30–45 km depth (400–550°C), we observe seismic signals in the form of acoustic emissions that closely resemble low frequency earthquakes, seismic tremor and regular earthquakes. Our findings support the concept that water released during dehydration reactions increases the pore pressures and can trigger ETS and regular earthquakes by reducing slip resistance.

Measurement of interseismic strain across the Haiyuan fault (Gansu, China), by InSAR

The Haiyuan fault is part of a major left-lateral fault system at the northeastern edge of the Tibet-Qinghai plateau. Two M8 earthquakes (1920 and 1927) occurred along the fault, bracketing an unbroken section of the fault identified as the Tianzhu seismic gap. We use interferometric synthetic aperture radar data from descending orbits of the ERS satellites, acquired between 1993 and 1998 along two adjacent tracks covering the gap, to measure the current surface movements and better understand the present day mechanical behavior of this fault section. The analysis of the radar data involves first the combined correction of orbital errors and errors associated with the phase delay through the troposphere. A subset of the data is then selected based on the analysis of the residual noise spectra for each pair of data. The selected interferograms are stacked and the average phase change rate is converted in fault-parallel velocity assuming that the ground movement is horizontal and parallel to the fault. Velocity maps from both tracks show a zone of high velocity gradient across the fault, a few kilometers wide, consistent with left-lateral slip on the Haiyuan fault. The average velocity field from the two tracks in their overlapping area is well fit with a single screw dislocation model in an elastic half-space. The derived fault slip rate at depth (4.2–8 mm/yr) is consistent with recent GPS results. The corresponding shallow apparent locking depth (0–4.2 km) can be explained by a current low stress accumulation on the fault due to creep almost on the entire fault plane. However, unless it is transient, this creep would be paradoxical with the occurrence of past large earthquakes along this fault section, as revealed by paleoseismology. An alternative model, implying both shallow creep in the brittle upper crust and deep aseismic slip beneath the seismogenic layer, separated by a locked section, would be consistent with InSAR observations and with the potential for large earthquakes on the fault as well. A two-dislocation model with slip at 5 mm/yr beneath 15 km and a transient creep rate of 11 mm/yr between 2 and 7 km fits the InSAR data. However, the width and creep rate of the shallow creeping zone and their possible along-strike variations are still poorly resolved with the present data set.

A seismic gap on the Anninghe fault in western Sichuan, China

3Earthquake Administration of Sichuan Province, Chengdu 610041, China Through integrated analyses of time-varying patterns of regional seismicity, occurrence background of strong and large historical earthquakes along active faults, and temporal-spatial distribution of accurately relocated hypocenters of modern small earthquakes, this paper analyzes and discusses the implication of a 30-year-lasting seismic quiescence in the region along and surrounding the Anninghe and Zemuhe faults in western Sichuan, China. It suggests that the seismic quiescence for ML≥4.0 events has been lasting in the studied region since January, 1977, along with the formation and evaluation of a seismic gap of the second kind, the Anninghe seismic gap. The Anninghe seismic gap has the background of a seismic gap of the first kind along the Anninghe fault, and has resulted from evident fault-locking and strain-accumulating along the fault during the last 30 years. Now, two fault sections either without or with less small earthquakes exist along the Anninghe fault within the Anninghe seismic gap. They indicate two linked and locked fault-sections, the northern Mianning section and the Mianning-Xichang section with lengths of 65 km and 75 km and elapsed time from the latest large earthquakes of 527 and 471 years, respectively. Along the Anninghe fault, characteristics of both the background of the first kind seismic gap and the seismicity patterns of the second seismic gap, as well as the hypocenter depth distribution of modern small earthquakes are comparable, respectively, to those appearing before the M=8.1 Hoh Xil earthquake of 2001 and to those emerging in the 20 years before the M=7.1 Loma Prieta, California, earthquake of 1989, suggesting that the Anninghe seismic gap is tending to become mature, and hence its mid- to long-term potential of large earthquakes should be noticeable. The probable maximum magnitudes of the potential earthquakes are estimated to be as large as 7.4 for both the two locked sections of the Anninghe fault.

Palynomorphs from the Penarth Group (Rhaetian, Late Triassic) at outcrop in north-west Lincolnshire, England

SUMMARY Palynomorphs have previously been documented from only two sections of the Penarth Group in Lincolnshire, in the Blyborough and Tetney Lock boreholes. These records are now augmented by the first from the group at crop in that county, from temporary exposures near Scotter and between Kettlethorpe and Thorney, north-west Lincolnshire. By comparison with those from the Blyborough and Tetney Lock sections and other records nearby in Yorkshire and Nottinghamshire, an assemblage from near Scotter is considered to be from a level low in the Westbury Formation, and those from a site between Kettlethorpe and Thorney to be from higher levels in the Penarth Group, within the Cotham Member of the Lilstock Formation. The assemblages include spores, pollen and organic-walled microplankton that are indicative of a Rhaetian (late Late Triassic) age. The organic-walled microplankton include acritarchs and dinoflagellate cysts, indicating a marine depositional environment.

Annual modulation of seismicity along the San Andreas Fault near Parkfield, CA

We analyze seismic data from the San Andreas Fault (SAF) near Parkfield, California, to test for annual modulation in seismicity rates. We use statistical analyses to show that seismicity is modulated with an annual period in the creeping section of the fault and a semiannual period in the locked section of the fault. Although the exact mechanism for seasonal triggering is undetermined, it appears that stresses associated with the hydrologic cycle are sufficient to fracture critically stressed rocks either through pore‐pressure diffusion or crustal loading/unloading. These results shed additional light on the state of stress along the SAF, indicating that hydrologically induced stress perturbations of ∼2 kPa may be sufficient to trigger earthquakes.

Interseismic deformation above the Sunda Megathrust recorded in coral microatolls of the Mentawai islands, West Sumatra

The geomorphology and internal stratigraphy of modern coral microatolls show that all the outer arc Mentawai islands of West Sumatra have been subsiding over the past several decades. These same islands rose as much as 3 m during the giant megathrust earthquakes of 1797 and 1833, and the current subsidence probably reflects strain accumulation that will lead to future large earthquakes. Average subsidence rates over the past half century vary from 2 to 14 mm yr−1 and increase southwestward, toward the subduction trench. The pattern is consistent with rates of subsidence measured by a sparse network of continuously recording Global Positioning System (cGPS) stations and with locking of a 400‐km‐long section of the underlying subduction megathrust, between about 1°S and 4°S. This record of subsidence and tilting, extending nearly a century into the past, implies that the region is advancing toward the occurrence of another giant earthquake. However, evidence of episodic rather than steady subsidence reflects a behavior that is more complex than simple elastic strain accumulation and relief. Most prominent of these episodes is an extensive emergence/subsidence couplet in about 1962, which may be the result of rapid, aseismic slip on the megathrust, between the islands and the trench. Lower subsidence rates recorded by the corals since about 1985 may reflect failure on many small patches within the locked section of the megathrust.

Fluid-controlled faulting process in the Asal Rift, Djibouti, from 8 yr of radar interferometry observations

The deformation in the Asal Rift (Djibouti) is characterized by magmatic inflation, diking, distributed extension, fissure opening, and normal faulting. An 8 yr time line of surface displacement maps covering the rift, constructed using radar interferometry data acquired by the Canadian satellite Radarsat between 1997 and 2005, reveals the aseismic behavior of faults and its relation with bursts of microseismicity. The observed ground movements show the asymmetric subsidence of the inner floor of the rift with respect to the bordering shoulders accommodated by slip on three of the main active faults. Fault slip occurs both as steady creep and during sudden slip events accompanied by an increase in the seismicity rate around the slipping fault and the Fieale volcanic center. Slip distribution along fault strike shows triangular sections, a pattern not explained by simple elastic dislocation theory. These observations suggest that the Asal Rift faults are in a critical failure state and respond instantly to small pressure changes in fluid-filled fractures connected to the faults, reducing the effective normal stress on their locked section at depth.

Frictional Properties on the San Andreas Fault near Parkfield, California, Inferred from Models of Afterslip following the 2004 Earthquake

The abundance of geodetic and seismic data recording postseismic deformation following the 2004 Parkfield earthquake provides an unprecedented opportunity to resolve frictional properties on the Parkfield section of the San Andreas fault. The Parkfield segment is a transition between the locked section to the southeast that last ruptured in the 1857 Fort Tejon earthquake and the creeping section to the northwest. We develop three-dimensional rate- and state-dependent friction models of afterslip following the 2004 earthquake to investigate the frictional behavior of the fault. It is assumed that the coseismic rupture occurred on an area of the fault surrounded by aseismic creep that accelerated after the earthquake. We estimate the distribution of coseismic slip, afterslip, and rate–state frictional parameters by inverting a two-step slip model. In the model we (1) estimate the coseismic slip distribution from 1-Hz Global Positioning System (gps) data and (2) use the corresponding coseismic shear stress change on the fault as input into a numerical afterslip model governed by rate–state friction. We find the rate–state frictional parameter A– B, an indicator of frictional stability, is in the range 10 −4 –10 −3 at 50 MPa normal stress, which is near the transition from potentially unstable (negative A–B) to nominally stable (positive A–B) friction. The estimate of A–B values falls within a wide range of experimental values reported for serpentinite, which crops out along the San Andreas fault zone. The critical slip distance, d c , which characterizes the distance over which strength breaks down during a slip event, is in the range 0.01–0.1 m, consistent with seismic estimates and a fault gouge thickness of 1–10 m. The afterslip model reproduces most features observed in the gps time-series data including high surface velocities in the first few months after the earthquake and lower rates at later times, as well as the cumulative postseismic displacement. The model tends to underpredict the displacement data at later times, suggesting that perhaps the modeled afterslip period ends too quickly or an unmodeled deformation process dominates the signal at later times.

Development and Testing of a New Platform for Retroflexed Flexible Transgastric Surgery: Cholecystectomy, Fundoplication, Gastric Restriction and Diaphragmatic Repair

Background: Transgastric surgery using instruments passed through flexible, unsupported, retroflexed endoscopes yields very little force transmission. Instruments in the scope channel limit retroflexion from 180° to 90°. Exposure and dissection of the gallbladder is difficult. Laparoscopy with current rigid scopes gives poor views of parts of the abdomen especially the diaphragm behind the liver. Methods: A new solution to these problems was developed and tested. The USGI TransPort™ (TP) is a multi-lumen surgical access system designed to enable translumenal therapy. The TP utilizes Shapelock® technology, which allows it to be locked into desired shapes. It is introduced in a flexible state and rigidized by squeezing the handle. The distal section can be steered freely to deliver therapy with efficiency and precision. The ability to steer and lock sections of the main body enables the user to access target locations which may be challenging to reach and provides a stable platform for performing therapeutic procedures. A new tissue grasping and retracting instrument, USGI g-Lix™ (gL) was also tested. Results: The TP platform was tested in anesthetised pigs. A sphincterotome incision of 2.5 cm was made in the anterior body of the stomach before advancing the TP into the insufflated peritoneal cavity. In biliary studies, the device was steered under the liver and locked while graspers and needle knives were used to manipulate and dissect the gallbladder, cystic and common bile duct. The liver could be effectively retracted when the device was locked but not when unlocked. Retroflexion behind the liver allowed access to parts of the diaphragm inaccessible with rigid laparoscopy. Conventional endoscopic graspers were unable to grasp the diaphragm while the gL was able to pull a fold of the diaphragm downward toward the TP. Sutures were placed in the diaphragm at sites inaccessible to the laparoscope, indicating that crural approximation was possible. These new devices allowed retroflexed transgastric retraction and suturing in the upper abdomen. Conclusion: These experiments demonstrated the potential for using the TransPort as a platform for improving retroflexed transgastric endoluminal procedures especially cholecystectomy, fundoplication, gastric restriction and diaphragm repair.

Three-Dimensional Compressional Wavespeed Model, Earthquake Relocations, and Focal Mechanisms for the Parkfield, California, Region

We present a new three-dimensional (3D) compressional wavespeed ( V p) model for the Parkfield region, taking advantage of the recent seismicity associated with the 2003 San Simeon and 2004 Parkfield earthquake sequences to provide increased model resolution compared to the work of Eberhart-Phillips and Michael (1993) (epm93). Taking the epm93 3D model as our starting model, we invert the arrival-time data from about 2100 earthquakes and 250 shots recorded on both permanent network and temporary stations in a region 130 km northeast–southwest by 120 km northwest–southeast. We include catalog picks and cross-correlation and catalog differential times in the inversion, using the double-difference tomography method of Zhang and Thurber (2003). The principal V p features reported by epm93 and Michelini and McEvilly (1991) are recovered, but with locally improved resolution along the San Andreas Fault (saf) and near the active-source profiles. We image the previously identified strong wavespeed contrast (faster on the southwest side) across most of the length of the saf, and we also improve the image of a high V p body on the northeast side of the fault reported by epm93. This narrow body is at about 5- to 12-km depth and extends approximately from the locked section of the saf to the town of Parkfield. The footwall of the thrust fault responsible for the 1983 Coalinga earthquake is imaged as a northeast-dipping high wavespeed body. In between, relatively low wavespeeds (<5 km/sec) extend to as much as 10-km depth. We use this model to derive absolute locations for about 16,000 earthquakes from 1966 to 2005 and high-precision double-difference locations for 9,000 earthquakes from 1984 to 2005, and also to determine focal mechanisms for 446 earthquakes. These earthquake locations and mechanisms show that the seismogenic fault is a simple planar structure. The aftershock sequence of the 2004 mainshock concentrates into the same structures defined by the pre-2004 seismicity, confirming earlier observations (Waldhauser et al. , 2004) that the seismicity pattern at Parkfield is long lived and persists through multiple cycles of mainshocks. Online material : 3D V p model and earthquake relocations.

Fault interactions and subduction tectonics: a re-examination of the Weber, New Zealand, earthquake sequence of 1990

Two moderate magnitude (Mw = 6.2 and 6.4) earthquakes in the Hikurangi subduction margin, North Island, New Zealand, occurred 3 months apart in 1990. The epicentres are nearly coincident, but the first (Weber 1, primarily normal faulting) occurred within the subducting Pacific Plate (depth about 28 km) and the second (Weber 2, a mix of thrusting and right-lateral motion) occurred within the overlying Australian Plate (depth about 13 km), the plate interface in between. The plate interface is interpreted to be locked trenchward (SE) from about the position of these events, with a transition to aseismic slip further down-dip to the NW. Several stress interaction questions are examined. First, to see whether Weber 1 triggered Weber 2, a range of possible mainshock parameters are used to calculate induced changes in the static Coulomb failure stress (δCFS). In most cases the results are consistent with triggering. Secondly, previous work showed that the rate of aftershock occurrence for Weber 1 decreased markedly about 35 days before Weber 2, recovering afterwards. To see whether aseismic pre-slip on the Weber 2 fault, as predicted by rate and state friction, could be the cause of the decrease, the same fault parameters have been used in reverse. The results are ambiguous, some fault parameters giving results consistent with the hypothesis and others not. The amount of pre-slip required for significant inhibition, however, is about equal to that in the mainshock and distributed over the entire fault plane. Thirdly, observations of episodic, aseismic slip down-dip from locked sections of other plate interfaces are becoming more common. Could such slip have triggered both Weber events? The induced changes in CFS for such slip are uniformly positive on the Weber 1 fault plane, and mostly positive on the Weber 2 fault plane, so the answer is yes. Although there is no independent evidence for aseismic slip prior to the Weber sequence, this case shows that such slip may trigger events on other nearby faults, besides loading the locked section of the plate interface. Static stress triggering considerations are thus likely to be important in subduction environments.

The relationship between the instantaneous velocity field and the rate of moment release in the lithosphere

SUMMARY Instantaneous velocity gradients within the continental lithosphere are often related to the tectonic driving forces. This relationship is direct if the forces are secular, as for the case of loading of a locked section of a subduction interface by the downgoing plate. If the forces are static, as for the case of lateral variations in gravitational potential energy, then velocity gradients can be produced only if the lithosphere has, on average, zero strength. The static force model may be related to the long-term velocity field but not the instantaneous velocity field (typically measured geodetically over a period of several years) because over short time intervals the upper lithosphere behaves elastically. In order to describe both the short- and long-term behaviour of an (elastic) lithosphere‐(viscoelastic) asthenosphere system in a selfconsistent manner, I construct a deformation model termed the expected interseismic velocity (EIV) model. Assuming that the lithosphere is populated with faults that rupture continually, each with a definite mean recurrence time, and that the Earth is well approximated as a linear elastic‐viscoelastic coupled system, I derive a simple relationship between the instantaneous velocity field and the average rate of moment release in the lithosphere. Examples with synthetic fault networks demonstrate that velocity gradients in actively deforming regions may to a large extent be the product of compounded viscoelastic relaxation from past earthquakes on hundreds of faults distributed over large ( >10 6 km 2 ) areas.