Interplate Coupling Research Articles

Slow slip events and strain accumulation in the Guerrero gap, Mexico

Global positioning system (GPS) time series in Guerrero (Mexico) reveal the existence of large slow slip events (SSEs) at the boundary between the Cocos and North American plates. In this study, we examined the last three SSEs that occurred in 2001/2002, 2006 and 2009/2010, and their impact on the strain accumulation along the Guerrero subduction margin. GPS displacements were inverted to retrieve the slip distribution during each SSE and the inter‐SSE coupling of the subduction interface. The three analyzed SSEs have equivalent moment magnitudes of between 7.50 and 7.65, their lateral extents are variable, and they all show significant slip in the Guerrero seismic gap. During the inter‐SSE epochs, the interplate coupling is high in the area where slow slip subsequently occurs. In the Guerrero gap, the shallow portion of the plate interface from the trench to the coast is weakly coupled. The average slip deficit accumulated in the Guerrero gap over a period of 12 years, which corresponds to three cycles of SSE, is only 1/4 of the slip deficit accumulated on both sides of the gap. Moreover, the regions of large slip deficit coincide with the rupture areas of recent large earthquakes. We conclude that the SSEs in the Guerrero gap release a significant part of the strain accumulated during the inter‐SSE period. If large subduction thrust earthquakes occur in the Guerrero gap, their recurrence time is probably increased compared to adjacent regions.

沈み込み帯の地震の発生機構

The mechanisms that generate the three main types of earthquake in subduction zones are discussed addressing their relations to geofluids. Studies on the spatial distribution of earthquakes and seismic velocity structure within the subducted slab provide evidence that strongly supports the dehydration embrittlement hypothesis for the generation of intermediate-depth intraslab earthquakes. Detailed imaging of the seismic velocity structure in and around plate boundary zones suggests that interplate coupling is mainly controlled by local fluid over-pressure. Seismic tomography studies show the existence of inclined sheet-like seismic low-velocity zones in the mantle wedge, not only in Tohoku but also in other areas in Japan, which perhaps correspond to the upwelling flow of the subduction-induced convection system. These upwelling flows reach the Moho directly beneath the volcanic areas, suggesting that those volcanic areas are formed by the upwelling flows. Aqueous fluids derived from the slab are probably transported up through the upwelling flows to the arc crust, where they might weaken the surrounding crustal rocks and finally cause shallow inland earthquakes. All of these observations suggest that geofluids expelled from the subducting slab play an important role in the generation of earthquakes in subduction zones.

釧路沖の中規模繰り返し地震活動

We identified 10 moderate sized repeating earthquakes from 1954 to 2010 off Kushiro region, eastern Hokkaido, Japan, by careful visual and quantitative inspections of waveform data. Relocated hypocenters calculated by reconfirmed arrival time data indicated that these earthquakes occurred at the same location on the interface between the subducting Pacific plate and the overriding plate. Reanalysis of focal mechanisms of recent three events showed approximately the same solution. These facts strongly suggested recurrent rupturing of an asperity. Average magnitude and average recurrence interval were M4.9 and 6.2±0.88 years, respectively. Estimated average slip rate was 6.5cm/year, which is equivalent to an interplate coupling ratio of 78-86%. The next earthquake is expected to occur in a period between May 2015 and July 2017 with a 70% probability based on the small sampling theory with a lognormal distribution model. Recurrent interval tended to shorten when large earthquakes occurred in the surrounding region. This might suggest recurrence interval is probably affected by the stress disturbance due to coseismic and postseismic slip on the plate interface.

Mapping the Tohoku forearc: Implications for the mechanism of the 2011 East Japan earthquake (Mw 9.0)

To investigate the generation mechanism of the 2011 great East Japan (GEJ) earthquake (M 9.0), we determined a three-dimensional (3-D) seismic model under the NE Japan forearc region using a large number of P and S wave arrival times from local earthquakes. Our results show that the GEJ main-shock occurred in a high-velocity (Vp, Vs) zone with higher Poisson's ratio (σ). The aftershocks relocated with a master-event location (MEL) procedure indicate that most of them are located at the corner of the mantle wedge along the upper boundary of the subducting Pacific slab, and their focal depths are slightly shallower than the background seismicity. The tomographic images and spatial distribution of the aftershocks imply strong interplate coupling (asperity) in the main-shock area and weak coupling in the surrounding areas of the megathrust zone. We think that fluids in the crust and uppermost mantle in the subduction zone could have affected the earthquake generation through the physical role of fluid pressure and a variety of chemical effects. We conclude that the 2011 GEJ earthquake initiation and the rupture processes of the aftershock sequence were influenced by fluid extrusion into the rupture zone due to the dehydration of the subducting Pacific slab. Fluid-bearing structural heterogeneities with thermal–petrologic variations in the megathrust zone played a key role in the initiation of the 2011 GEJ earthquake and its aftershocks.

Silica diagenesis and its effect on interplate seismicity in cold subduction zones

The diagenesis and deformation processes of ribbon cherts embedded in a Jurassic accretionary complex, central Japan, were investigated in detail to gain a better understanding of the mechanical behavior of the plate boundary at depth in cold subduction zones. The analyzed cherts record two stages of deformation: (1) map- to outcrop-scale ductile folding, and (2) subsequent brittle faulting. The ductile deformation was facilitated by silica dehydration–precipitation, and is represented by multiple phases of vein networks. The folds are cut by brittle faults, indicating lithification and the concurrent mechanical transition from ductile to brittle behavior. Slip zones along the faults are typically filled with brecciated chert in a chlorite matrix. Geothermometry analysis of the matrix chlorite suggests that faulting occurred following the completion of opal-CT to quartz transition reaction. This is also confirmed by the kinetic simulation of silica conversion reactions. The results suggest that ductile deformation of thick pelagic deposits with abundant fluids results in an aseismic plate boundary, whereas chemical diagenesis of the deposits, producing crystalline cherts, results in interplate coupling in cold subduction zones such as the Japan Trench.

Mantle wedge structure in the Miyagi Prefecture forearc region, central northeastern Japan arc, and its relation to corner-flow pattern and interplate coupling

[1] We applied three-dimensional (3-D) seismic tomography to both active-source data and local earthquake data recorded at onshore and offshore seismic stations in the Miyagi Prefecture forearc region of central northeastern Japan to obtain a detailed seismic velocity model of the mantle wedge. The velocity model showed a clear boundary that divides the mantle wedge into an oceanward high-velocity toe and a landward low-velocity region that extends trenchward from beneath currently active arc volcanoes. The top of the seaward dipping boundary is about 20–30 km landward from the coastline, and its bottom is approximately beneath the coastline. We consider that this velocity boundary represents the trenchward limit of corner flow and the high-velocity toe represents a stagnant mantle wedge. The high spatial resolution of the tomographic image we produced allowed us to identify a spatial correlation between velocity variations in the mantle wedge immediately above the plate boundary and the spatial extent of the rupture areas of the large interplate earthquakes that occur repeatedly in the region. Our results suggest that variations of the volume fraction of serpentinization in the mantle wedge cause the irregularity of rupture patterns of megathrust earthquake sequences. We also identified a swarm of active microseismicity in the mantle wedge, well above the seismogenic plate boundary. This activity may be related to dehydration of a detached seamount.

Spatial correlation of interseismic coupling and coseismic rupture extent of the 2011 MW= 9.0 Tohoku-oki earthquake

[1] Imaging the extent to which the rupture areas of great earthquakes coincide with regions of pre-seismic interplate coupling is central to understanding patterns of strain accumulation and release through the earthquake cycle. Both geodetic and seismic estimates of the coseismic rupture extent for the March 11, 2011 MW = 8.9–9.0 earthquake Tohoku-oki earthquake may be spatially correlated (0.26 ± 0.05 to 0.82 ± 0.05) with a region estimated to be partially to fully coupled in the interseismic period preceding the earthquake, though there is substantial variation in the estimated distribution and magnitude of coseismic slip. The ∼400 km-long region estimated to have slipped ≥4 m corresponds to an area of the subduction zone interface that was coupled at ≥30% of long-term plate convergence rate, with peak slip near a region coupled ≥80%. The northern termination of rupture is collocated with a region of relatively low (<20%) interseismic coupling near the epicenter of the 1994 MW = 7.6 Sanriku-oki earthquake, and near a region of potential long-term low coupling or ongoing slow slip. Slip on the subduction interface beneath the coastline (40–50 km depth) is best constrained by the land-based GPS data and least constrained on the shallowest portion of the plate interface due to the ∼230 km distance between geodetic observations and the Japan trench.

Coupling coefficient, hierarchical structure, and earthquake cycle for the source area of the 2011 off the Pacific coast of Tohoku earthquake inferred from small repeating earthquake data

We have estimated the spatial distribution of interplate coupling in and around the source area for the 2011 Tohoku earthquake from small repeating earthquake data. The source area of the 2011 Tohoku earthquake exhibited a relatively high coupling coefficient (>0.5) for the period from 1993 to 2007 and was surrounded by areas of low coupling (<0.5) at its western (down-dip), northern and southern extents. These low-coupling areas probably prevented further propagation of the mainshock rupture. A high coupling coefficient is estimated even near the trench that can act as the source of the large tsunami of the present earthquake. The averaged seismic coupling of 0.5–0.8 in the M 9 earthquake’s source area and the seismic moment of the earthquake suggest that the slip deficit for the M 9 earthquake was accumulated over a period of 260–880 years, consistent with the recurrence interval of such great earthquakes from tsunami deposit data. It also suggests that the several M ~ 7 earthquakes which have occurred in the source area since 1926 were minor events that released only a part of the accumulated strain energy. The hierarchical structure of asperities, as in the case of the Kamaishi sequence, may be the key to understanding huge earthquakes that encompass several smaller asperities.

Fore-arc mantle wedge seismicity under northeast New Zealand

Relocated upper mantle seismicity, using a 3D velocity model, delineates three tightly clustered (about 12 km diameter) zones of seismicity that occur in the mantle wedge under the fore-arc region of the northern Hikurangi margin, North Island, New Zealand. These clusters extend from the subducting plate at about 55 km depth up to about 35 km, and coincide with strong seismic reflectivity recorded active source seismic reflection measurements, suggesting a fluids association. They have maximum magnitudes of about Mw 4.5. The clusters correspond closely with a change from low to high Vp and Qp, and with average Vp/Vs that is interpreted to be caused by the transition from serpentinised to normal peridotite. The location of this transition appears to correspond to the locus of dehydration of peridotite, suggesting that the seismicity is caused by dehydration embrittlement. The release of these fluids where the serpentinised mantle wedge reaches the locus of dehydration may provide a mechanism for transferring additional fluids or fluid fluxed partial melt into the adjacent back-arc basin and facilitate the generation of voluminous magmas that are erupted there. The occurrence of serpentinite in the fore-arc mantle under northeastern North Island may influence the shallow extent of interplate coupling under the region. The location and spacing of the clusters is consistent with thermally driven diapiric or fluid upwellings, but they also show a spatial relationship with proposed lithospheric fractures and associated volcanic seamounts on the subducting Hikurangi plateau.

Crustal anisotropy structure related to lateral and down‐dip variations in interplate coupling beneath the Kii Peninsula, SW Japan

Anisotropy is an important feature of elastic wave propagation in the Earth, and can arise from a variety of ordered architectures such as fractures with preferential alignments or lattice‐preferred orientations. We studied regional variations in shear wave anisotropy beneath southern Kii Peninsula, which is located above the subducting Philippine Sea plate, using waveforms of local earthquakes observed with a dense seismic network. The fast directions of polarization are normal to the strike of the plate margin in the proximity of the locked portion of the underlying mega‐thrust fault, but they are subparallel to the strike of the margin along the fault's down‐dip extension. The distribution of the fast directions roughly coincides with that of the maximum horizontal compressive stress axis directions as inferred from earthquake focal mechanisms and borehole stress measurements. This indicates that the observed anisotropy is likely controlled by crustal stresses. Furthermore, the directions of the stress‐induced anisotropy correlate well with variations of the plate coupling ratio along the plate boundary as inferred from geodetic measurements. These results indicate the possibility that crustal anisotropy within an overlying plate can be used as an effective measure to monitor temporal variations of the plate coupling strength during the interseismic period.

Continental and oceanic crustal structure of the Pampean flat slab region, western Argentina, using receiver function analysis: new high-resolution results

SUMMARY The Pampean flat slab of central Chile and Argentina (30°–32°S) has strongly influenced Cenozoic tectonics in western Argentina, which contains both the thick-skinned, basement-cored uplifts of the Sierras Pampeanas and the thin-skinned Andean Precordillera fold and thrust belt. In this region of South America, the Nazca Plate is subducting nearly horizontally beneath the South American Plate at ∼100 km depth. To gain a better understanding of the deeper structure of this region, including the transition from flat to ‘normal’ subduction to the south, three IRIS-PASSCAL arrays of broad-band seismic stations have been deployed in central Argentina. Using the dense SIEMBRA array, combined with the broader CHARGE and ESP arrays, the flat slab is imaged for the first time in 3-D detail using receiver function (RF) analysis. A distinct pair of RF arrivals consisting of a negative pulse that marks the top of the oceanic crust, followed by a positive pulse, which indicates the base of the oceanic crust, can be used to map the slab's structure. Depths to Moho and oceanic crustal thicknesses estimated from RF results provide new, more detailed regional maps. An improved depth to continental Moho map shows depths of more than 70 km in the main Cordillera and ∼50 km in the western Sierras Pampeanas, that shallow to ∼35 km in the eastern Sierras Pampeanas. Depth to Moho contours roughly follow terrane boundaries. Offshore, the hotspot seamount chain of the Juan Fernandez Ridge (JFR) is thought to create overthickened oceanic crust, providing a mechanism for flat slab subduction. By comparing synthetic RFs, based on various structures, to the observed RF signal we determine that the thickness of the oceanic crust at the top of the slab averages at least ∼13–19 km, supporting the idea of a moderately overthickened crust to provide the additional buoyancy for the slab to remain flat. The overthickened region is broader than the area directly aligned with the path of the JFR, however, and indicates, along with the slab earthquake locations, that the flat slab area is wider than the JFR volcanic chain observed in the offshore bathymetry. Further, RFs indicate that the subducted oceanic crust in the region directly along the path of the subducted ridge is broken by trench-parallel faults. One explanation for these faults is that they are older structures within the oceanic crust that were created when the slab subducted. Alternatively, it is possible that faults formed recently from tectonic underplating caused by increased interplate coupling in the flat slab region.

Control of high oceanic features and subduction channel on earthquake ruptures along the Chile–Peru subduction zone

We discuss the earthquake rupture behavior along the Chile–Peru subduction zone in terms of the buoyancy of the subducting high oceanic features (HOF's), and the effect of the interplay between HOF and subduction channel thickness on the degree of interplate coupling. We show a strong relation between subduction of HOF's and earthquake rupture segments along the Chile–Peru margin, elucidating how these subducting features play a key role in seismic segmentation. Within this context, the extra increase of normal stress at the subduction interface is strongly controlled by the buoyancy of HOF's which is likely caused by crustal thickening and mantle serpentinization beneath hotspot ridges and fracture zones, respectively. Buoyancy of HOF's provide an increase in normal stress estimated to be as high as 10–50 MPa. This significant increase of normal stress will enhance seismic coupling across the subduction interface and hence will affect the seismicity. In particular, several large earthquakes ( M w ≥ 7.5) have occurred in regions characterized by subduction of HOF's including fracture zones (e.g., Nazca, Challenger and Mocha), hotspot ridges (e.g., Nazca, Iquique, and Juan Fernández) and the active Nazca-Antarctic spreading center. For instance, the giant 1960 earthquake ( M w = 9.5) is coincident with the linear projections of the Mocha Fracture Zone and the buoyant Chile Rise, while the active seismic gap of north Chile spatially correlates with the subduction of the Iquique Ridge. Further comparison of rupture characteristics of large underthrusting earthquakes and the locations of subducting features provide evidence that HOF's control earthquake rupture acting as both asperities and barriers. This dual behavior can be partially controlled by the subduction channel thickness. A thick subduction channel smooths the degree of coupling caused by the subducted HOF which allows lateral earthquake rupture propagation. This may explain why the 1960 rupture propagates through six major fracture zones, and ceased near the Mocha Fracture Zone in the north and at the Chile Rise in the south (regions characterized by a thin subduction channel). In addition, the thin subduction channel (north of the Juan Fernández Ridge) reflects a heterogeneous frictional behavior of the subduction interface which appears to be mainly controlled by the subduction of HOF's.

Structural evolution of the southwestern margin of the Ulleung Basin, East Sea (Japan Sea) and tectonic implications

Structural mapping and restoration of depth-converted seismic profiles from the southwestern Ulleung Basin in the East Sea (Japan Sea) back-arc basin reveal the structural evolution of the area since the Middle Miocene. The back-arc closure began in the earliest Middle Miocene (ca. 15 Ma) and caused compressional deformation that has propagated southwestward from the Dolgorae to the Gorae structures and further to the unnamed inversion structure immediately adjacent to SE Korea. The accelerated convergence of the young (< 20 Ma) and hot Shikoku Basin lithosphere into SW Japan at the culmination (15 Ma) of the regional plate reorganization induced flat subduction along the Nankai Trough. Strong interplate coupling due to the flat subduction probably led to tectonic switching from back-arc opening to back-arc closure in the Middle Miocene. The NE–SW trending Dolgorae structures indicate NW–SE or N–S compression, orthogonal to the direction of convergence along SW Japan, whereas the younger, NW–SE trending Gorae V structure indicates NE–SW or E–W compression. We propose that this apparently abrupt change in stress regime is due to the eastward-movement of the Amur plate that began in the Pliocene.

Interplate coupling and seismotectonics under the fore-arc regions of Japan

Three-dimensional P- and S-wave velocities (νP, νS), Poisson’s ratio (σ), crack-density (ɛ) and bulk-sound velocity (νϕ) structures along the slab upper boundary beneath the fore-arc regions were determined using a large number of high-quality P-wave and S-wave arrival times from both onshore and offshore earthquakes in Japan. The velocity and Poisson’s ratio images provide a compelling evidence for a highly hydrated and serpentinized fore-arc mantle and fluid-bearing anomalous low velocity and high Poisson’s ratio associated with slab dehydration under the fore-arc areas. Most great thrust earthquakes (M>7.5) occurred at or close to the high-velocity areas along the slab interface under the fore-arc areas, suggesting strong interplate coupling (asperities) with slab subduction. On the other hand, prominent low-velocity anomalies were revealed along the slab upper boundary, which may reflect weak coupled or decoupled patches (aseismicity) of the plates due to serpentinization of the fore-arc mantle wedge. The crack-density and bulk-sound velocity images, calculated from the corresponding velocity models, indicate that the interplate coupling in northeastern Japan is different from that under central and southwestern Japan owing to differences between the tectonic backgrounds of the subduction system, such as the geological age, thermal regime and dipping angle of the oceanic plates. A comparison between fluid-related anomalies of Japan, Cascadia, Chile, and Costa Rica subduction zones suggests that seismic mantle may be common in fore-arc settings and these reflect similar 3-D seismic structures relatively to fluid liberating processes. We consider that the fluid-bearing anomalies along the interface of the subducting slab, attributing to processes such as slab dehydration and serpentinization of the fore-arc mantle, are mainly contributed to the interplate coupling and the repeated generation of the great thrust earthquakes under the fore-arc regions in Japan.

Horizontal subduction zones, convergence velocity and the building of the Andes

We discuss the relationships between Andean shortening, plate velocities at the trench, and slab geometry beneath South America. Although some correlation exists between the convergence velocity and the westward motion of South America on the one hand, and the shortening of the continental plate on the other hand, plate kinematics neither gives a satisfactory explanation to the Andean segmentation in general, nor explains the development of the Bolivian orocline in Paleogene times. We discuss the Cenozoic history of horizontal slab segments below South America, arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive. We argue that the existence of horizontal slab segments, below the Central Andes during Eocene–Oligocene times, and below Peru and North-Central Chile since Pliocene, resulted (1) in the shortening of the continental plate interiors at a large distance from the trench, (2) in stronger interplate coupling and ultimately, (3) in a decrease of the trenchward velocity of the oceanic plate. Present-day horizontal slab segments may thus explain the diminution of the convergence velocity between the Nazca and South American plates since Late Miocene.

Dynamics of the Kuril-Kamchatka subduction zone from GPS data

The Kuril-Kamchatka subduction zone is the most mobile and seismically active region in Northeast Eurasia. The Kuril island arc is one of the few tectonically active regions, where until recently there had been no space geodetic network. The first GPS stations were installed on the Kamchatka Peninsula in 1997, and on the islands of the Kuril arc from Kamchatka to Hokkaido, in 2006. The collected geodetic data allowed us to reveal the geometry of the interplate coupling along the whole Kuril-Kamchatka arc, and also to estimate the source parameters and their features for a number of major earthquakes in this area.

Geodetic imaging of plate motions, slip rates, and partitioning of deformation in Japan

Interseismic deformation in Japan results from the combined effects of tectonic processes including rotation of crustal blocks and the earthquake cycle process of elastic strain accumulation about upper plate faults and subduction zone interfaces. We use spherical linear block theory constrained by geodetic observations from densely spaced Global Positioning System (GPS) stations to estimate plate motions, fault slip rates, and spatially variable interplate coupling on the Japan‐Kuril, Sagami, and Nankai subduction zones. The reference model developed in this paper consists of 20 blocks, produces a mean residual velocity magnitude of 1.84 mm/yr at 950 stations, and accounts for 96% of the observed interseismic deformation signal. We estimate fault slip rates in excess of 15 mm/yr along the Niigata‐Kobe Tectonic Zone and Itoigawa‐Shizuoka Tectonic Line through central Japan, confirming their hypothesized roles as major tectonic boundaries. Oblique convergence across the Nankai Trough is partitioned, with 3/4 of the ∼30 mm/yr of trench‐parallel motion accommodated by strike‐slip motion on the subduction interface and the remaining 1/4 accommodated by right‐lateral slip on the Median Tectonic Line. In contrast, our models suggest negligible slip partitioning in eastern Hokkaido, where oblique slip on the Japan‐Kuril subduction interface accommodates all of the trench‐parallel component of relative plate motion. Inferred spatial variations in the rake and magnitude of slip deficit on subduction zone interfaces reflect the influences of megathrust geometry and earthquake cycle processes such as enhanced elastic strain accumulation about seismic asperities and coseismic sense fault motion indicative of silent slip events or afterslip following large earthquakes.

日本列島下のスラブの三次元構造と地震活動

A dense nationwide seismic network recently constructed in Japan has been yielding large volumes of high-quality data that have made it possible to investigate the seismic structure in the Japanese subduction zone with unprecedented resolution. We introduce a precise configuration of the Philippine Sea and Pacific plates subducting beneath the Japanese Islands, which was recently obtained by seismic tomographic imaging, precise earthquake hypocenter determinations, and focal mechanism studies. Seismic tomographic studies show that the Philippine Sea plate subducting beneath southwest Japan is continuous throughout the entire region, from Kanto to Kyushu, without disruption or splitting even beneath the area north of the Izu Peninsula. The estimated geometry of the subducted Pacific and Philippine Sea slabs shows a broad contact zone between the two slabs located directly beneath the Kanto plain. It further shows the wavy configuration of the Philippine Sea slab subducting beneath the entire region of southwestern Japan. Contact between the Philippine Sea plate and the Pacific plate causes anomalously deep interplate and intraslab earthquake activity in Kanto. Moreover, the interplate coupling coefficient estimated from repeating earthquake data shows a distinct change across the northeastern edge of this slab contact zone, suggesting that the overlying plate controls large-scale interplate coupling. High-resolution studies of spatial variations of intraslab seismicity and the seismic velocity structure of the slab crust strongly support the dehydration embrittlement hypothesis for the generation of intraslab earthquakes.

Elucidation of Interplate Coupling Based on Geodetic Data

Elucidation of Interplate Coupling Based on Geodetic Data

Role of the overriding plate in the subduction process: Insights from numerical models

Active convergent margins are primarily shaped by the interplay among the subducting plate, overriding plate, and mantle. The effect of important forces, like far-field mantle flow, overriding plate motion, and inter-plate coupling, however, remains partially ambiguous. In a preliminary attempt to clarify their role, a self-consistent, viscoelastic, plane-strain, mechanical finite element model, in which subducting plate, overriding plate and mantle interact dynamically, is developed. In this quasi-static framework with a freely moving slab, trench, and inter-plate fault, the role of a compressive overriding plate on subduction zone kinematics, morphology and stress-state is characterized. A slab interacting solely with a semi-analytical three-dimensional mantle flow formulation shows that local non-induced mantle flow influences slab geometry and kinematics, adding an important dynamic term to the system. The impact of an overriding plate on this system is determined completely by overriding plate trench-ward motions and is only pertinent if the overriding plate actively advances the trench. A trench-ward moving overriding plate indents the slab and thereby enforces trench retreat and decreases slab dip. It also stimulates over-thrusting of the overriding plate onto the slab, and thereby permits mountain building within the overriding plate. Frictional resistance is observed to have a dominant local effect within the overriding plate as it is increasingly dragged down, thereby inhibiting the growth of overriding plate topography. A distinguishable effect on large-scale trench motions and deep slab dip is, however, absent for re-normalized friction coefficients ranging up to about 0.2. Minor additional effects include a decrease in plate motions of about 15% and slab bending stresses of about 10%.