Total Slip Research Articles

Quantifying seismicity associated with slow slip events in the Hikurangi margin, New Zealand

ABSTRACTTo investigate the relationship between seismicity and slow slip events (SSEs) in the Hikurangi margin we calculate seismicity rates during SSEs using the raw GeoNet earthquake catalogue and a derived sequence catalogue from 2002 through 2011. Most regions have more seismicity during SSEs times; the increase is significant for six out of nine regions. Seismicity regionally accompanying SSEs in the Manawatu and Kapiti regions is part of a cluster of normal faulting earthquakes correlated to a convergence-parallel SHmax stress direction. Seismicity accompanies the smallest published SSE in 2009, which suggests that the occurrence of seismicity is independent of the total slip. The Manawatu–Kapiti and East Coast regions have, respectively, 1.3 and 1.5 times the average rate of sequences during SSEs. The link between the rate of earthquakes and the timing of SSEs indicates that understanding the connection between the two is important for future hazard analysis and earthquake forecasts.

Analysis of increasing torque with recurrent slip in interference-fits

Previous research associated with interference-fitted assemblies has shown that as recurring slip occurs (i.e. load to total slip, unloading and reload to total slip) there is an observed increase in the holding torque after each loading cycle. The aim of this work was to identify the reasons for this ‘torque strengthening’ phenomenon. The work also has industrial relevance in the optimum design of interference-fitted rolls used for the hot rolling of steel sections. Previous work has shown that the major contributors to the overall holding torque were the interface pressure, material properties and the coefficient of friction between component materials. In this work, neutron diffraction tests and crack compliance tests showed no correlation between the interface pressure and increased holding torque. Meanwhile, experimental holding torque tests on sample interference-fits showed that for each recurring holding torque failure (slip) in a test cycle, the holding torque increased. Subsequent wear investigations showed that the wear of the surfaces increased throughout the testing and once a specific type of wear had occurred through a ‘ploughing’ mechanism, significant damage could be done to the more expensive shaft component. These observations suggest that an effective increase in the coefficient of friction between shaft and hub is responsible for the increase in holding torque, while the same level of interface pressure is maintained throughout slipping. The research provides a basis for the optimisation of interference-fit design in order that the working lives of expensive shafts, which are prone to damage through ploughing, and brittle hubs, which are prone to sudden fracture, are maximised when experiencing recurrent slipping.

Evolution and progressive geomorphic manifestation of surface faulting: A comparison of the Wairau and Awatere faults, South Island, New Zealand

Field mapping and lidar analysis of surface faulting patterns expressed in flights of geologically similar fluvial terraces at the well-known Branch River and Saxton River sites along the Wairau (Alpine) and Awatere strike-slip faults, South Island, New Zealand, reveal that fault-related deformation patterns expressed in the topography at these sites are markedly less structurally complex along the higher-displacement (hundreds of kilometers), structurally mature Wairau fault than along the Awatere fault (∼13–20 km total slip). These differences, which are generally representative of the surface traces of these faults, provide direct evidence that surface faulting becomes structurally simpler with increasing cumulative fault offset. We also examine the degree to which off-fault deformation (OFD) is expressed in the landscape at the Saxton River site along the less structurally mature Awatere fault. Significantly greater amounts of OFD are discernible as a wide damage zone (∼460 m fault-perpendicular width) in older (ca. 15 ka), more-displaced (64–74 m) fluvial terraces than in younger (ca. 1–7 ka), less-displaced (<55 m) terraces; no OFD is discernible in the lidar data on the least-displaced (<35 m) terraces. From this, we infer that OFD becomes progressively more geomorphically apparent with accumulating displacement. These observations imply that (1) the processes that accommodate OFD are active during each earthquake, but may not be evident in deposits that have experienced relatively small displacements; (2) structures accommodating OFD will become progressively geomorphically clearer with increasing displacement; (3) geomorphic measurements of overall fault zone width taken in deposits that have experienced small displacements will be underestimates; and (4) fault slip rates based on geomorphic surface offsets will be underestimates for immature faults if based solely on measurements along the high-strain fault core.

Recent geodynamics of major strike-slip zones

The subject of this study is strike-slip fault zones, where temporal variations of accumulation in strike-slip deformation complicate the standard process of deformation accumulation and release during strong earthquakes. These temporal variations are expressed in the El Ghab segment of the Dead Sea Transform zone (DST, Eastern Mediterranean) and in the Talas-Fergana fault zone (Central Asia). According to Global Positioning System (GPS) data, the strike-slip deformations within these zones are not now accumulating or are accumulating at a rate that is significantly less than their average rate during the Holocene and Quaternary or the Pliocene–Quaternary. Simultaneously, weak transverse shortening has been measured in both zones by GPS. In both of these zones, strong earthquakes have not registered within the XX century, yet epochs of intensified seismicity (strong earthquakes) took place throughout history. In the southern and central parts of the El Ghab zone, there is evidence of 30 strong historical earthquakes of Ms ≥ 5.7; however, no instrumental earthquakes of Ms ≥ 5 have been identified. The temporal distribution of seismic energy released by these earthquakes demonstrates a 350 ± 50-year cycle. Values for the seismic energies released during the peak phases of these cycles are approximated by a sinusoid that suggests the possibility of a ≥1800-year cycle (“hyper-cycle”), which began around the 3rd century, reached its maximum in the 12th century, and has continued until now. A combination of geological, archaeoseismological, and geodetic data show that the rate of sinistral strike-slip deformation varied in the fault zone, probably in conformity with the variation of seismicity during the “hyper-cycle.” In the Talas-Fergana fault zone, trenching and 14C dating that was correlated with right lateral offsets, gave a possible preliminary estimate of the average rates of the Late Holocene strike slip of about 10 mm per year, with a decrease in the SE direction to 4 mm–4.5 mm per year. These studies also showed that the slip in the Talas-Fergana fault zone was realized mainly during strong earthquakes. New trenching and 14C dating of paleoearthquake records identified the epoch of seismicity intensification dating to the XIV–XVII centuries. These paleoearthquakes could produce a total dextral slip at several meters. Therefore, consideration of these epochs was necessary to determine a calculated average slip rate during the Late Holocene.The main shock and the strongest aftershocks of the Altai earthquake of September 27, 2003, with Ms = 7.0 demonstrated a strike-slip focal mechanism with an NW-trending plane of the right lateral slip. An approximately 65 km-long NW-trending seismic rupture with a right lateral slip of up to 2 m, formed during the earthquake. The aftershock activity significantly decreased in 2004–2005 when reverse and rarer normal focal mechanisms became dominant. In the Palmyrides and the southern Aleppo block (NW Syria), strong earthquakes in 1994 (Mw = 5.3) and 1996 (Mw = 5.5) had strike-slip focal mechanisms, while only weak (magnitudes 1.1 to 3.3) earthquakes occurred in 2009–2011; the overwhelming majority of these weak earthquakes had normal and reverse mechanisms.In all of the cases mentioned above, strike-slip deformation was expressed only or mainly during strong earthquakes. At other times, the rate of its accumulation was small and the dominant stress conditions led to transverse shortening, rarely resulting in local lengthening of the tectonic zone. These variations are caused by the tectonic peculiarities of these zones. The sinistral component of the deformation is related to the shift of the Arabian Plate relative to the African one, but also the transverse component is related to the continental slope and is expressed by the Coastal range shortening that exists in the El Ghab segment zone. There is not only a dextral deformation component, but also a transverse component, expressed by shortening of the Fergana and Talas ranges existing in the Talas-Fergana fault zone. In both zones, the shortening component became appreciable or dominant when the strike-slip deformation rate decreased. Similar, but more local, relationships were expressed in the epicentral area of the 2003 Altai earthquake and in the Western Palmyrides.

Steadily propagating slip pulses driven by thermal decomposition

AbstractGeophysical observations suggest that mature faults weaken significantly at seismic slip rates. Thermal pressurization and thermal decomposition are two mechanisms commonly used to explain this dynamic weakening. Both rely on pore fluid pressurization with thermal pressurization achieving this through thermal expansion of native solids and pore fluid and thermal decomposition by releasing additional pore fluid during a reaction. Several recent papers have looked at the role thermal pressurization plays during a dynamically propagating earthquake, but no previous models have studied the role of thermal decomposition. In this paper we present the first solutions accounting for thermal decomposition during dynamic rupture, solving for steady state self‐healing slip pulses propagating at a constant rupture velocity. First, we show that thermal decomposition leads to longer slip durations, larger total slips, and a distinctive along–fault slip rate profile. Next, we show that accounting for more than one weakening mechanism allows multiple steady slip pulses to exist at a given background stress, with some solutions corresponding to different balances between thermal pressurization and thermal decomposition, and others corresponding to activating a single reaction multiple times. Finally, we study how the rupture properties depend on the fault properties and show that the impact of thermal decomposition is largely controlled by the ratio of the hydraulic and thermal diffusivities χ = αhy/αth and the ratio of pore pressure generated to temperature rise buffered by the reaction Pr/Er.

Paleoseismology of the Mount Narryer fault zone, Western Australia: A multistrand intraplate fault system

Our paleoseismological study of faults and fault-related folds comprising the Mount Narryer fault zone reveals a mid- to late Quaternary history of repeated morphogenic earthquakes that have influenced the planform and course of the Murchison, Roderick, and Sanford Rivers, Western Australia. The dominant style of deformation involves folding of near-surface sediments overlying discrete basement faults. Carbon-14, optically stimulated luminescence, and in situ–produced 10 Be constrain the timing of the events and late Quaternary slip rates associated with fault propagation folds in tectonically uplifted and deformed alluvial channel deposits. A flight of five inset fluvial terraces is preserved where the Murchison River flows across the Roderick River fault. These terraces, which we infer to be coseismic, are consistent with at least four late Quaternary seismic events on the order of moment magnitude (M w ) 7.1 within the last ~240 k.y. Secondary shears expressed on the folds indicate a component of dextral strike-slip displacement. Quaternary slip rates on the underlying faults range from 0.01 to 0.07 mm yr –1 , with a total slip rate for the zone between 0.04 and 0.11 mm yr –1 . These rates are intermediate to those in the adjacent Mesozoic basin (>0.1 mm yr –1 ) and Precambrian craton ( –1 ) and so provide insight into how tectonic strain is partitioned and transferred across a craton margin.

Crystallographic preferred orientations may develop in nanocrystalline materials on fault planes due to surface energy interactions

AbstractA layer of substantially noncrystalline material, composed of partially annealed nanopowder with local melt, was experimentally generated by comminution during ∼1.5 mm total slip at ∼2.5 × 10−6 m s−1, Pconf ∼ 0.5 GPa, and 450°C or 600°C, on saw cut surfaces in novaculite. The partially annealed nanopowder comprises angular grains mostly 5–200 nm diameter in a variably dense packing arrangement. A sharp transition from wall rock to partially annealed nanopowder illustrates that the nanopowder effectively localizes shear, consistent with generation of nanoparticles during initial fragmentation, not by progressive grain size reduction. Dislocation densities in nanopowder grains or immediate wall rock are not significantly high, but there are planar plastic defects spaced at 5–200 nm parallel to the host quartz grain's basal plane. We propose these plastic defects developed into through‐going fractures to generate nanocrystals. The partially annealed nanopowder has a crystallographic preferred orientation (CPO) that we hypothesize developed due to surface energy interactions to maximize coincident site lattices (CSL) during annealing. This mechanism may also have generated CPOs recently described in micro/nanocrystalline calcite fault gouges.

Holocene geologic slip rate for the Banning strand of the southern San Andreas Fault, southern California

AbstractNorthwest directed slip from the southern San Andreas Fault is transferred to the Mission Creek, Banning, and Garnet Hill fault strands in the northwestern Coachella Valley. How slip is partitioned between these three faults is critical to southern California seismic hazard estimates but is poorly understood. In this paper, we report the first slip rate measured for the Banning fault strand. We constrain the depositional age of an alluvial fan offset 25 ± 5 m from its source by the Banning strand to between 5.1 ± 0.4 ka (95% confidence interval (CI)) and 6.4 + 3.7/−2.1 ka (95% CI) using U‐series dating of pedogenic carbonate clast coatings and 10Be cosmogenic nuclide exposure dating of surface clasts. We calculate a Holocene geologic slip rate for the Banning strand of 3.9 + 2.3/−1.6 mm/yr (median, 95% CI) to 4.9 + 1.0/−0.9 mm/yr (median, 95% CI). This rate represents only 25–35% of the total slip accommodated by this section of the southern San Andreas Fault, suggesting a model in which slip is less concentrated on the Banning strand than previously thought. In rejecting the possibility that the Banning strand is the dominant structure, our results highlight an even greater need for slip rate and paleoseismic measurements along faults in the northwestern Coachella Valley in order to test the validity of current earthquake hazard models. In addition, our comparison of ages measured with U‐series and 10Be exposure dating demonstrates the importance of using multiple geochronometers when estimating the depositional age of alluvial landforms.

Thermal evolution of the Hengshan extensional dome in central South China and its tectonic implications: New insights into low-angle detachment formation

The Hengshan massif is an exhumed, mid-crustal, plutonic–metamorphic dome formed during Cretaceous crustal extension in the Jiangnan orogenic belt, central South China. Multiple thermochronometers (mica 40Ar/39Ar, apatite fission track and zircon (U–Th)/He) are applied to its footwall along a slip-parallel transect to quantify its thermal history and cooling rate, and the slip magnitude, rate, initial geometry and kinematic evolution of the low-angle Hengshan detachment fault. Our thermochronological data, in conjunction with previous ages, indicate that (1) footwall rocks cooled from ~700°C to ~60°C in less than 60Myr (136–80Ma) at variable rates ranging from ~50°C/Myr to ~13°C/Myr, (2) the Hengshan detachment fault accommodated ~8–12km of total slip at variable slip rates from 0.14 to 1mm/yr during tectonic exhumation, (3) the footwall has been tilted ~26°–50° to the east since slip began, indicating that the low-angle Hengshan detachment fault initiated at a steep dip and was passively rotated to a more gentle orientation during subsequent normal slip. This study provides compelling evidence supporting that the low-angle detachment fault in the extensional dome can be generated by the reactivation and passive rotation of an initially steep reverse fault during normal slip. In addition, our thermochronological data constrain the time of extension in the Hengshan dome between 136 and 80Ma, which implies that the back-arc extension within South China associated with the rollback of the Paleo-Pacific slab might have lasted until at least 80Ma.

Broadband Ground Motions from Dynamic Models of Rupture on the Northern San Jacinto Fault, and Comparison with Precariously Balanced Rocks

The Southern California San Jacinto fault is geometrically complex, consisting of several major strands with smaller scale complexity within each strand. The two northernmost strands, the Claremont and the Casa Loma–Clark, are separated by a 25‐km‐long extensional stepover with an average of 4 km separation between the strands. We use a combined modeling method to assess probable rupture and ground‐motion behaviors for this stepover. First, dynamic rupture modeling on geometrically complex fault strands embedded in a state‐of‐the‐art 3D crustal velocity model is used to generate a series of scenario earthquakes. We then use the resulting near‐fault low‐frequency (<1 Hz) ground‐motion time histories to generate broadband synthetic seismograms with a hybrid approach. These synthetics are then compared with a distribution of precariously balanced rocks (PBRs) near the fault to constrain our results and assess shaking hazard for the region surrounding the fault. Our dynamic models produce sources between M w 5.4 and 6.9, with rupture limits imposed by sharp contrasts in fault stress or by geometrical barriers. The main stepover serves as a primary barrier to rupture in our model, producing event sizes that are consistent with the historical behavior of the San Jacinto fault. The largest broadband synthetics are a good match to leading ground‐motion prediction equations and are generally consistent with the distribution of PBRs, none of which experience accelerations that produce toppling probabilities significantly higher than zero. Thus, although the PBRs do not rule out any of our model scenarios, they confirm that our models produce realistic rupture extents and shaking. Online Material: Figures of total slip for additional rupture models, low‐frequency intensity plots, synthetic seismograms, and comparison with ground‐motion prediction equations.

Active flexural‐slip faulting: A study from the Pamir‐Tian Shan convergent zone, NW China

AbstractThe flexural‐slip fault (FSF), a type of secondary fault generated by bed‐parallel slip, occurs commonly and plays an important role in accommodating fold growth. Although the kinematics and mechanics of FSFs are well studied, relatively few field observations or geometric models explore its geomorphic expression. In the Pamir‐Tian Shan convergent zone, NW China, suites of well‐preserved FSF scarps displace fluvial terraces in the Mingyaole and Wulagen folds. Integrating interpretations of Google Earth images, detailed geologic and geomorphic mapping, and differential GPS measurements of terrace surfaces, we summarize geomorphic features that typify these faults and create kinematic models of active flexural‐slip faulting. Our study indicates the following: (i) FSF scarps commonly occur near synclinal hinges, irrespective of whether (a) the dip direction of beds on either side of the hinge is unidirectional or in opposite directions, (b) the hinge is migrating or fixed, or (c) the hinge shape is narrow and angular or wide and curved. (ii) Active FSFs are likely to produce higher scarps on steeper beds, whereas lower or no topographic scarps typify gentler beds. (iii) Tilt angles of the terrace surface displaced above FSFs progressively decrease farther away from the hinge, with abrupt changes in slope coinciding with FSF scarps; the changes in tilt angle and scarp height have a predictable geometric relationship. (iv) Active FSFs can accommodate a significant fraction of total slip and play a significant role in folding deformation. (v) Active FSFs may be used to assess seismic hazards associated with active folds and associated blind thrusts.

Geologic and structural controls on rupture zone fabric: A field-based study of the 2010 Mw 7.2 El Mayor-Cucapah earthquake surface rupture

We systematically mapped (scales >1:500) the surface rupture of the 4 April 2010 Mw (moment magnitude) 7.2 El Mayor-Cucapah earthquake through the Sierra Cucapah (Baja California, northwestern Mexico) to understand how faults with similar structural and lithologic characteristics control rupture zone fabric, which is here defined by the thickness, distribution, and internal configuration of shearing in a rupture zone. Fault zone thickness and master fault dip are strongly correlated with many parameters of rupture zone fabric. Wider fault zones produce progressively wider rupture zones and both of these parameters increase systematically with decreasing dip of master faults, which varies from 20° to 90° in our dataset. Principal scarps that accommodate more than 90% of the total coseismic slip in a given transect are only observed in fault sections with narrow rupture zones (

A Multiscale Study on the Wall Slip Effect in a Ceramic–Steel Contact With Nanometer-Thick Lubricant Film by a Nano-to-Elastohydrodynamic Lubrication Approach

A novel nano-to-elastohydrodynamic lubrication (EHL) multiscale approach, developed to integrate molecular-scale phenomena into macroscopic lubrication models based on the continuum hypothesis, is applied to a lubricated contact problem with a ceramic–steel interface and a nanometric film thickness. Molecular dynamics (MD) simulations are used to quantify wall slip occurring under severe confinement. Its dependence on the sliding velocity, film thickness, pressure, and different wall materials is described through representative analytical laws. These are then coupled to a modified Reynolds equation, where a no-slip condition applies to the ceramic surface and slip occurring on the steel wall is described through a Navier-type boundary condition. The results of this nano-to-EHL approach can contradict the well-established lubrication theory for thin films. In fact, slip can occur over the whole contact length, leading to a significant modification of the lubricant flow and consequently of the film thickness. If both walls move at the same velocity, the flow is reduced at the contact inlet and the film thickness decreases. If the nonslipping wall entrains the fluid, this one is accelerated resulting in a larger mass flow; nevertheless, the surface separation is reduced as the lubricant flows even faster in the contact center. The opposite effect occurs if the slipping surface entrains the fluid, causing a lower mass flow but higher film thickness. Finally, friction is generally smaller compared to the classical no-slip case and becomes independent of the sliding velocity as total slip is approached.

Simulation of texture formation of body-centered-cubic metals by three kinds of intersections of two {110} slip planes

Based on the rotational symmehy of the principal axes of X[100], Y[010] and Z[001], 72 possible combinations of the five slips on {110} planes based on Taylor's formidable restriction rule of the five slips are calculated among three kinds of intersections of two {110} planes on 〈111〉 direction in bcc metal. Crystal rotation is carried out by only one solution among the 72 by the minimum total slip at every strain and simulates properly lengthy of accumulated researcher's experimental results such as the three stable orientations of bcc metal in rolling {112}〈110〉, {11 11 8}〈4 4 11〉 and {100}〈011〉.

현장타설 고강도콘크리트와 유리섬유 FRP 판 사이의 부착슬립관계에 관한 연구

Recently it has been actively studied that the use of hybrid GFRP-concrete structure with dual purpose of both a permanent forwork and main tensile reinforcement of GFRP plank. In applying general analysis and design technique to evaluate the performance of hybrid structures with cast-in-place high strength concrete and GFRP plank, it is essential that the characteristics of the bond slip model is identified. In this study a simplified bilinear bond slip model for hybrid structure with GFRP plank and cast-in-place high strength concrete is proposed. Maximum average bond stress of simple bond slip relationship that has been proposed in this study is 3.29MPa, initial slope is 35.66MPa/mm, the total slip is 0.23mm and interfacial fracture energy is 0.37kN/m.

Basal icequakes recorded beneath an Alpine glacier (Glacier d'Argentière, Mont Blanc, France): Evidence for stick‐slip motion?

AbstractWhile basal icequakes associated with glacier motion have been detected under Antarctica for several decades, there remains very little evidence of stick‐slip motion for Alpine glaciers. Here we analyzed 2357 basal icequakes that were recorded at Glacier d'Argentière (Mont‐Blanc Massif) between February and November of 2012 and that are likely to be associated with basal sliding. These events have been classified into 18 multiplets, based on their waveforms. The strong similarity of the waveforms within each multiplet suggests an isolated repeating source. Despite this similarity, the peak amplitude within each multiplet varies gradually in time, by up to a factor of 18. The distribution of these events in time is relatively complex. For long time scales, we observe progressive variations in the amplitudes of events within each multiplet. For intermediate time scales (hours), the events occur regularly in time, with typical return times of several minutes up to several hours. For short time scales (from 0.01 to 100 s), the largest multiplet shows clustering in time, with a power law distribution of the interevent times. The location of these events and their focal mechanisms are not well constrained, because most of these events were detected by a single seismometer. Nevertheless, the locations can be estimated with an accuracy of a few tens of meters using a polarization analysis. The estimated average depth of the basal events is 179 m, which is in good agreement with the estimated glacier thickness. The relative changes in distance between the source and the sensor can be measured accurately by correlating separately the P wave and S wave parts of the seismograms of each event with the template waveforms, which are obtained by averaging the signals within each multiplet. We observed small variations in the times between the P wave and the S wave of up to 0.6 ms over 50 days. These variations cannot be explained by displacement of the sensor with respect to the glacier but might be due to small changes in the seismic wave velocities with time. Finally, we found using numerical simulations that the observed signals are better explained by a horizontal shear fault with slip parallel to the glacier flow than by a tensile fault. These results suggest that the basal events are associated with stick‐slip motion of the glacier over rough bedrock. The rupture length and the slip are difficult to estimate. Nonetheless, the rupture length is likely to be of the order of meters, and the total seismic slip accumulated over one day might be as large as the glacier motion during the most active bursts.

Effect of water phase transition on dynamic ruptures with thermal pressurization: Numerical simulations with changes in physical properties of water

AbstractPhase transitions of pore water have never been considered in dynamic rupture simulations with thermal pressurization (TP), although they may control TP. From numerical simulations of dynamic rupture propagation including TP, in the absence of any water phase transition process, we predict that frictional heating and TP are likely to change liquid pore water into supercritical water for a strike‐slip fault under depth‐dependent stress. This phase transition causes changes of a few orders of magnitude in viscosity, compressibility, and thermal expansion among physical properties of water, thus affecting the diffusion of pore pressure. Accordingly, we perform numerical simulations of dynamic ruptures with TP, considering physical properties that vary with the pressure and temperature of pore water on a fault. To observe the effects of the phase transition, we assume uniform initial stress and no fault‐normal variations in fluid density and viscosity. The results suggest that the varying physical properties decrease the total slip in cases with high stress at depth and small shear zone thickness. When fault‐normal variations in fluid density and viscosity are included in the diffusion equation, they activate TP much earlier than the phase transition. As a consequence, the total slip becomes greater than that in the case with constant physical properties, eradicating the phase transition effect. Varying physical properties do not affect the rupture velocity, irrespective of the fault‐normal variations. Thus, the phase transition of pore water has little effect on dynamic ruptures. Fault‐normal variations in fluid density and viscosity may play a more significant role.

Late Quaternary slip history of the Mill Creek strand of the San Andreas fault in San Gorgonio Pass, southern California: The role of a subsidiary left-lateral fault in strand switching

The fault history of the Mill Creek strand of the San Andreas fault (SAF) in the San Gorgonio Pass region, along with the reconstructed geomorphology surrounding this fault strand, reveals the important role of the left-lateral Pinto Mountain fault in the regional fault strand switching. The Mill Creek strand has 7.1–8.7 km total slip. Following this displacement, the Pinto Mountain fault offset the Mill Creek strand 1–1.25 km, as SAF slip transferred to the San Bernardino, Banning, and Garnet Hill strands. An alluvial complex within the Mission Creek watershed can be linked to palinspastic reconstruction of drainage segments to constrain slip history of the Mill Creek strand. We investigated surface remnants through detailed geologic mapping, morphometric and stratigraphic analysis, geochronology, and pedogenic analysis. The degree of soil development constrains the duration of surface stability when correlated to other regional, independently dated pedons. This correlation indicates that the oldest surfaces are significantly older than 500 ka. Luminescence dates of 106 ka and 95 ka from (respectively) 5 and 4 m beneath a younger fan surface are consistent with age estimates based on soil-profile development. Offset of the Mill Creek strand by the Pinto Mountain fault suggests a short-term slip rate of ~10–12.5 mm/yr for the Pinto Mountain fault, and a lower long-term slip rate. Uplift of the Yucaipa Ridge block during the period of Mill Creek strand activity is consistent with thermochronologic modeled uplift estimates.

Testing the Bannock detachment breakaway: Negative results support moderate- to high-angle splay system and domino-style fault block rotation along the Valley fault, southern Portneuf Range, southeastern Idaho, U.S.A.

New geologic mapping, kinematic analysis, and tephrochronologic age correlations were completed in the southern Portneuf Range of southeast Idaho to characterize the Valley fault, a large-offset normal fault interpreted as the breakaway for the regionally extensive Bannock detachment system. The Valley fault separates ∼11.8–7.57 Ma Salt Lake Formation from underlying Neoproterozoic to Cambrian Brigham Group and Cambrian rocks. Three-point problems indicate that the Valley fault strikes north–northwest (NNW) and dips an average of 9° west–southwest (WSW), in stark contrast to previous work, which interpreted the fault as steeply west-dipping. Footwall strata strike NNW and dip 37 ± 11° east–northeast (ENE). Measured bedding-to-fault cutoff angles are, therefore, ∼46 ± 11°. Hanging-wall strata strike NNW and dip 20 ± 5° ENE adjacent to the fault and steepen progressively down -section to ∼70 ± 10° ENE due to the Valley fault's minor listricity and multiple hanging-wall splays. Beneath the Miocene strata, Cambrian hanging-wall bedrock strike NNW and dip 55 ± 10° ENE. Adding a 9° Valley fault dip to these bedding dips yields cutoff angles of 29° to 80° for Miocene strata and 64° for hanging-wall bedrock. Top-to-the-west, dip-slip offset across the Valley fault and its hanging-wall splays—using the Miocene unconformity as a marker—is 13.7 ± 1 km. The older, subhorizontal Mine Hollow fault has 1,000 ± 300 m top-to-the-west normal offset. The initial dip of the Valley fault is interpreted to have been steeper than its present 9° dip. If footwall strata were subhorizontal prior to extension, as indicated by reconstruction of the Miocene unconformity, then the Valley fault had an initial dip of 46 ± 11° WSW based on measured bedding-to-fault angles. Combined with the modern low dip of the Valley fault, these data are interpreted to indicate that the Salt Lake Formation accumulated as growth strata adjacent to the active Valley fault, while the fault as well as footwall and hanging-wall strata tilted northeast through time. The Valley fault's mildly listric shape, the progressive domino-style tilting, and the large amount of total slip suggest that the Valley fault is a Basin-Range normal fault rather than a low-angle breakaway fault for the Bannock detachment system. The preferred model for extension in the region is ∼7 km of uplift and exhumation during two phases of moderate- to high-angle domino-style fault block rotation. First, the Valley fault splays and Mine Hollow fault accommodated 5 km of extension between 11.8 to 9.16 Ma; then the Valley fault accommodated 10 km offset between 9.16 to <7.57 Ma (likely as recent as 5–4 Ma). Our regional cross section and restoration suggest that both the Clifton fault to the west (Bannock detachment) and the Valley fault are two separate faults, each with 13–15 km offset. Reconstruction of these Miocene faults shows that the Mesozoic Paris thrust fault has a preextensional footwall flat length of 30 km, similar to that predicted by other workers.

Slow and delayed deformation and uplift of the outermost subduction prism following ETS and seismogenic slip events beneath Nicoya Peninsula, Costa Rica

Two ODP CORK (Ocean Drilling Program circulation obviation retrofit kit) borehole hydrologic observatory sites deployed in 2002 at the toe of the subduction prism off Nicoya Peninsula, Costa Rica were visited in December 2013. The five years of seafloor and formation fluid pressure data collected since the previous visit include clear signals associated with an episodic tremor and slip (ETS) event off the coast of Nicoya Peninsula in 2009, and a Mw 7.6 subduction thrust earthquake beneath the Peninsula in 2012. Formation pressure anomalies associated with the ETS event are similar to ones observed following ETS events observed previously here, as well as ones following very low frequency earthquake swarms within the Nankai accretionary prism off southwestern Japan. Positive and negative impulsive transients in the hanging wall and foot wall of the subduction thrust, respectively, suggest contractional and dilatational strain generated by local slip propagating up the thrust fault beneath the outermost prism. In the case of the 2009 event, the transients occurred roughly two weeks after the initiation of slip observed at GPS sites along the adjacent coast. At the same time, a decrease in seafloor pressure at the prism site relative to the subducting plate was observed, indicating concurrent uplift of the prism of 1.2 cm. Other events at the prism toe following ETS events closer to the coast are seen in 2006, 2007, 2008, 2010, and 2011. The time between the initiation of ETS slip constrained by GPS and the onset of the prism toe transients suggest up-dip “rupture” propagation along the seaward part of the subduction thrust at rates of a few km/day. In the case of the 2009 event, the slip at the prism toe (c. 11 cm), estimated from the 1.2 cm uplift and the local dip on the decollement (6°), is roughly a factor of 5 greater than the slip further landward estimated from GPS data by Dixon et al. (in press). In other cases, slip at the toe is less or unresolvable. Similar uplift and formation fluid pressure transients occurred two days after the 2012 Nicoya thrust earthquake, then episodically (unrelated to aftershocks) every 1–3 weeks over the next few months. Slip rates at the prism toe, estimated from the magnitude and duration of these and ETS-related uplift events, are roughly 5 cm/day. Such rates are much too slow for tsunamigenesis or seismogenesis. Total slip of the outermost prism after the 2012 earthquake, determined from the uplift accumulated until the end of the observation period and the estimated local fault dip, is roughly 75 cm, a significant fraction of the seismogenic slip of the Mw 7.6 earthquake itself (a maximum of over 3 m). Negative steps in formation pressure are observed at both the prism and subducting plate sites simultaneous with the 2012 thrust earthquake. The sign of these steps is inconsistent with the contractional deformation predicted by an elastic fault-dislocation model with slip limited to the seismogenic zone. Highly compliant (low shear modulus) sediment below the decollement, or very low-stress deformation along splay faults, may be responsible for the co-seismic dilatation observed in the outermost prism and incoming plate.