Fault lubrication during earthquakes

The determination of rock friction at seismic slip rates (about 1?m?s(-1)) is of paramount importance in earthquake mechanics, as fault friction controls the stress drop, the mechanical work and the frictional heat generated during slip. Given the difficulty in determining friction by seismological methods, elucidating constraints are derived from experimental studies. Here we review a large set of published and unpublished experiments (～300) performed in rotary shear apparatus at slip rates of 0.1-2.6?m?s(-1). The experiments indicate a significant decrease in friction (of up to one order of magnitude), which we term fault lubrication, both for cohesive (silicate-built, quartz-built and carbonate-built) rocks and non-cohesive rocks (clay-rich, anhydrite, gypsum and dolomite gouges) typical of crustal seismogenic sources. The available mechanical work and the associated temperature rise in the slipping zone trigger a number of physicochemical processes (gelification, decarbonation and dehydration reactions, melting and so on) whose products are responsible for fault lubrication. The similarity between (1) experimental and natural fault products and (2) mechanical work measures resulting from these laboratory experiments and seismological estimates suggests that it is reasonable to extrapolate experimental data to conditions typical of earthquake nucleation depths (7-15?km). It seems that faults are lubricated during earthquakes, irrespective of the fault rock composition and of the specific weakening mechanism involved.     

Talc-bearing serpentinite and the creeping section of the San Andreas fault

The section of the San Andreas fault located between Cholame Valley and San Juan Bautista in central California creeps at a rate as high as 28 mm yr(-1) (ref. 1), and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust. Serpentinized ultramafic rocks have been associated with creeping faults in central and northern California, and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the San Andreas fault. However, the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength, and these minerals also have the potential for unstable slip under some conditions. Here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the San Andreas fault that was intersected during drilling of the San Andreas Fault Observatory at Depth (SAFOD) main hole in 2005. We infer that the talc is forming as a result of the reaction of serpentine minerals with silica-saturated hydrothermal fluids that migrate up the fault zone, and the talc commonly occurs in sheared serpentinite. This discovery is significant, as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault, and its inherently stable sliding behaviour is consistent with fault creep. Talc may therefore provide the connection between serpentinite and creep in the San Andreas fault, if shear at depth can become localized along a talc-rich principal-slip surface within serpentinite entrained in the fault zone.     

Frictional ageing from interfacial bonding and the origins of rate and state friction

Earthquakes have long been recognized as being the result of stick-slip frictional instabilities. Over the past few decades, laboratory studies of rock friction have elucidated many aspects of tectonic fault zone processes and earthquake phenomena. Typically, the static friction of rocks grows logarithmically with time when they are held in stationary contact, but the mechanism responsible for this strengthening is not understood. This time-dependent increase of frictional strength, or frictional ageing, is one manifestation of the 'evolution effect' in rate and state friction theory. A prevailing view is that the time dependence of rock friction results from increases in contact area caused by creep of contacting asperities. Here we present the results of atomic force microscopy experiments that instead show that frictional ageing arises from the formation of interfacial chemical bonds, and the large magnitude of ageing at the nanometre scale is quantitatively consistent with what is required to explain observations in macroscopic rock friction experiments. The relative magnitude of the evolution effect compared with that of the 'direct effect'--the dependence of friction on instantaneous changes in slip velocity--determine whether unstable slip, leading to earthquakes, is possible. Understanding the mechanism underlying the evolution effect would enable us to formulate physically based frictional constitutive laws, rather than the current empirically based 'laws', allowing more confident extrapolation to natural faults.     

Three-dimensional deformation caused by the Bam, Iran, earthquake and the origin of shallow slip deficit

Our understanding of the earthquake process requires detailed insights into how the tectonic stresses are accumulated and released on seismogenic faults. We derive the full vector displacement field due to the Bam, Iran, earthquake of moment magnitude 6.5 using radar data from the Envisat satellite of the European Space Agency. Analysis of surface deformation indicates that most of the seismic moment release along the 20-km-long strike-slip rupture occurred at a shallow depth of 4-5 km, yet the rupture did not break the surface. The Bam event may therefore represent an end-member case of the 'shallow slip deficit' model, which postulates that coseismic slip in the uppermost crust is systematically less than that at seismogenic depths (4-10 km). The InSAR-derived surface displacement data from the Bam and other large shallow earthquakes suggest that the uppermost section of the seismogenic crust around young and developing faults may undergo a distributed failure in the interseismic period, thereby accumulating little elastic strain.     

'Melt welt' mechanism of extreme weakening of gabbro at seismic slip rates

Laboratory studies of frictional properties of rocks at slip velocities approaching the seismic range (～0.1-1?m?s(-1)), and at moderate normal stresses (1-10?MPa), have revealed a complex evolution of the dynamic shear strength, with at least two phases of weakening separated by strengthening at the onset of wholesale melting. The second post-melting weakening phase is governed by viscous properties of the melt layer and is reasonably well understood. The initial phase of extreme weakening, however, remains a subject of much debate. Here we show that the initial weakening of gabbro is associated with the formation of hotspots and macroscopic streaks of melt ('melt welts'), which partially unload the rest of the slip interface. Melt welts begin to form when the average rate of frictional heating exceeds 0.1-0.4?MW?m(-2), while the average temperature of the shear zone is well below the solidus (250-450?°C). Similar heterogeneities in stress and temperature are likely to occur on natural fault surfaces during rapid slip, and to be important for earthquake rupture dynamics.     

Stress changes from the 2008 Wenchuan earthquake and increased hazard in the Sichuan basin

On 12 May 2008, the devastating magnitude 7.9 (Wenchuan) earthquake struck the eastern edge of the Tibetan plateau, collapsing buildings and killing thousands in major cities aligned along the western Sichuan basin in China. After such a large-magnitude earthquake, rearrangement of stresses in the crust commonly leads to subsequent damaging earthquakes. The mainshock of the 12 May earthquake ruptured with as much as 9 m of slip along the boundary between the Longmen Shan and Sichuan basin, and demonstrated the complex strike-slip and thrust motion that characterizes the region. The Sichuan basin and surroundings are also crossed by other active strike-slip and thrust faults. Here we present calculations of the coseismic stress changes that resulted from the 12 May event using models of those faults, and show that many indicate significant stress increases. Rapid mapping of such stress changes can help to locate fault sections with relatively higher odds of producing large aftershocks.     

Tectonic contraction across Los Angeles after removal of groundwater pumping effects

After the 1987 Whittier Narrows and 1994 Northridge earthquakes revealed that blind thrust faults represent a significant threat to metropolitan Los Angeles, a network of 250 continuously recording global positioning system (GPS) stations was deployed to monitor displacements associated with deep slip on both blind and surface faults. Here we augment this GPS data with interferometric synthetic aperture radar imagery to take into account the deformation associated with groundwater pumping and strike-slip faulting. After removing these non-tectonic signals, we are left with 4.4 mm yr-1 of uniaxial contraction across the Los Angeles basin, oriented N 36 degrees E (perpendicular to the major strike-slip faults in the area). This indicates that the contraction is primarily accommodated on thrust faults rather than on the northeast-trending strike-slip faults. We have found that widespread groundwater and oil pumping obscures and in some cases mimics the tectonic signals expected from the blind thrust faults. In the 40-km-long Santa Ana basin, groundwater withdrawal and re-injection produces 12 mm yr-1 of long-term subsidence, accompanied by an unprecedented seasonal oscillation of 55 mm in the vertical direction and 7 mm horizontally.     

Coseismic and postseismic slip of the 2011 magnitude-9 Tohoku-Oki earthquake

Most large earthquakes occur along an oceanic trench, where an oceanic plate subducts beneath a continental plate. Massive earthquakes with a moment magnitude, M(w), of nine have been known to occur in only a few areas, including Chile, Alaska, Kamchatka and Sumatra. No historical records exist of a M(w) = 9 earthquake along the Japan trench, where the Pacific plate subducts beneath the Okhotsk plate, with the possible exception of the ad 869 Jogan earthquake, the magnitude of which has not been well constrained. However, the strain accumulation rate estimated there from recent geodetic observations is much higher than the average strain rate released in previous interplate earthquakes. This finding raises the question of how such areas release the accumulated strain. A megathrust earthquake with M(w) = 9.0 (hereafter referred to as the Tohoku-Oki earthquake) occurred on 11 March 2011, rupturing the plate boundary off the Pacific coast of northeastern Japan. Here we report the distributions of the coseismic slip and postseismic slip as determined from ground displacement detected using a network based on the Global Positioning System. The coseismic slip area extends approximately 400?km along the Japan trench, matching the area of the pre-seismic locked zone. The afterslip has begun to overlap the coseismic slip area and extends into the surrounding region. In particular, the afterslip area reached a depth of approximately 100?km, with M(w) = 8.3, on 25 March 2011. Because the Tohoku-Oki earthquake released the strain accumulated for several hundred years, the paradox of the strain budget imbalance may be partly resolved. This earthquake reminds us of the potential for M(w)?≈?9 earthquakes to occur along other trench systems, even if no past evidence of such events exists. Therefore, it is imperative that strain accumulation be monitored using a space geodetic technique to assess earthquake potential.     

Quantitative study of tectonic geomorphology along Haiyuan fault based on airborne LiDAR

High-precision and high-resolution topography are the fundamental data for active fault research.Light detection and ranging(LiDAR)presents a new approach to build detailed digital elevation models effectively.We take the Haiyuan fault in Gansu Province as an example of how LiDAR data may be used to improve the study of active faults and the risk assessment of related hazards.In the eastern segment of the Haiyuan fault,the Shaomayin site has been comprehensively investigated in previous research because of its exemplary tectonic topographic features.Based on unprecedented LiDAR data,the horizontal and vertical coseismic offsets at the Shaomayin site are described.The measured horizontal value is about 8.6 m,and the vertical value is about 0.8 m.Using prior dating ages sampled from the same location,we estimate the horizontal slip rate as4.0±1.0 mm/a with high confidence and define that the lower bound of the vertical slip rate is 0.4±0.1 mm/a since the Holocene.LiDAR data can repeat the measurements of field work on quantifying offsets of tectonic landform features quite well.The offset landforms are visualized on an office computer workstation easily,and specialized software may be used to obtain displacement quantitatively.By combining precious chronological results,the fundamentallink between fault activity and large earthquakes is better recognized,as well as the potential risk for future earthquake hazards.     

Non-volcanic tremor and low-frequency earthquake swarms

Non-volcanic tremor is a weak, extended duration seismic signal observed episodically on some major faults, often in conjunction with slow slip events. Such tremor may hold the key to understanding fundamental processes at the deep roots of faults, and could signal times of accelerated slip and hence increased seismic hazard. The mechanism underlying the generation of tremor and its relationship to aseismic slip are, however, as yet unresolved. Here we demonstrate that tremor beneath Shikoku, Japan, can be explained as a swarm of small, low-frequency earthquakes, each of which occurs as shear faulting on the subduction-zone plate interface. This suggests that tremor and slow slip are different manifestations of a single process.     

Lushan M S7.0 earthquake: A special earthquake occurs on curved fault

Relocation result shows that the aftershocks of the Lushan M _S7.0 earthquake spatially distribute in a shape like “half bowl”, indicating that the rupture structure of the mainshock is a highly curved surface. Kinematic analysis reveals that a laterally varied dislocation pattern occurs on this curved fault even though a single relative horizontal movement controls slip on this fault. Reverse slip prevails on curved fault. However, significant normal slip is predicted near the edge of north flank. Moreover, the north flank features left-lateral slip while the south flank contrarily features right-lateral slip. The relative scope of aftershock distribution implies inadequate breaking of the curved fault during the mainshock, calling for the attention to potential earthquake risk on the neighboring portions of the coseismic rupture due to significant increase of the coseismic Coulomb stress. Coseismic stress modeling also reveals that it is unnecessary for the stress on ruptured part to be unloaded following the earthquakes on the curved fault. The coseismic stress loading on ruptured elements unveils the specialty of faulting for the Lushan earthquake and we conclude that this specialty is due to the highly curved fault geometry.     

A periodic shear-heating mechanism for intermediate-depth earthquakes in the mantle

Intermediate-depth earthquakes, at depths of 50-300 km in subduction zones, occur below the brittle-ductile transition, where high pressures render frictional failure unlikely. Their location approximately coincides with 600 to 800 degrees C isotherms in thermal models, suggesting a thermally activated mechanism for their origin. Some earthquakes may occur by frictional failure owing to high pore pressure that might result from metamorphic dehydration. Because some intermediate-depth earthquakes occur approximately 30 to 50 km below the palaeo-sea floor, however, the hydrous minerals required for the dehydration mechanism may not be present. Here we present an alternative mechanism to explain such earthquakes, involving the onset of highly localized viscous creep in pre-existing, fine-grained shear zones. Our numerical model uses olivine flow laws for a fine-grained, viscous shear zone in a coarse-grained, elastic half space, with initial temperatures from 600-800 degrees C and background strain rates of 10(-12) to 10(-15) s(-1). When shear heating becomes important, strain rate and temperature increase rapidly to over 1 s(-1) and 1,400 degrees C. The stress then drops dramatically, followed by low strain rates and cooling. Continued far-field deformation produces a quasi-periodic series of such instabilities.     

A great earthquake doublet and seismic stress transfer cycle in the central Kuril islands

Temporal variations of the frictional resistance on subduction-zone plate boundary faults associated with the stick-slip cycle of large interplate earthquakes are thought to modulate the stress regime and earthquake activity within the subducting oceanic plate. Here we report on two great earthquakes that occurred near the Kuril islands, which shed light on this process and demonstrate the enhanced seismic hazard accompanying triggered faulting. On 15 November 2006, an event of moment magnitude 8.3 ruptured the shallow-dipping plate boundary along which the Pacific plate descends beneath the central Kuril arc. The thrust ruptured a seismic gap that previously had uncertain seismogenic potential, although the earlier occurrence of outer-rise compressional events had suggested the presence of frictional resistance. Within minutes of this large underthrusting event, intraplate extensional earthquakes commenced in the outer rise region seaward of the Kuril trench, and on 13 January 2007, an event of moment magnitude 8.1 ruptured a normal fault extending through the upper portion of the Pacific plate, producing one of the largest recorded shallow extensional earthquakes. This energetic earthquake sequence demonstrates the stress transfer process within the subducting lithosphere, and the distinct rupture characteristics of these great earthquakes illuminate differences in seismogenic properties and seismic hazard of such interplate and intraplate faults.     

Earthquakes triggered by silent slip events on Kīlauea volcano, Hawaii

Slow-slip events, or 'silent earthquakes', have recently been discovered in a number of subduction zones including the Nankai trough in Japan, Cascadia, and Guerrero in Mexico, but the depths of these events have been difficult to determine from surface deformation measurements. Although it is assumed that these silent earthquakes are located along the plate megathrust, this has not been proved. Slow slip in some subduction zones is associated with non-volcanic tremor, but tremor is difficult to locate and may be distributed over a broad depth range. Except for some events on the San Andreas fault, slow-slip events have not yet been associated with high-frequency earthquakes, which are easily located. Here we report on swarms of high-frequency earthquakes that accompany otherwise silent slips on Kīlauea volcano, Hawaii. For the most energetic event, in January 2005, the slow slip began before the increase in seismicity. The temporal evolution of earthquakes is well explained by increased stressing caused by slow slip, implying that the earthquakes are triggered. The earthquakes, located at depths of 7-8 km, constrain the slow slip to be at comparable depths, because they must fall in zones of positive Coulomb stress change. Triggered earthquakes accompanying slow-slip events elsewhere might go undetected if background seismicity rates are low. Detection of such events would help constrain the depth of slow slip, and could lead to a method for quantifying the increased hazard during slow-slip events, because triggered events have the potential to grow into destructive earthquakes.     

潮汐应力与云南及邻区的地震

提出了潮汐应力对发震断层的力学模式,描述了附加潮汐应力对发震断层的促滑作用方式,并将该模式应用于云南及邻区的地震,计算了173个地震震源处沿主压应力P轴和主张应力T轴方向的附加潮汐应力分量,分析了这睦量对发震断层的作用方式以及受潮汐应力促滑作用的发震断层类型,结果表明,所研究的云南及邻区发生的173个地震中,64％的地震发震断层受到潮汐应力的促滑作用,其中,受减压型促滑作用的发震断层数比例略大于受增压型促滑作用的发动层数比例;在受到潮汐应力促滑作用的111个发震断层中,走滑型发震断层占67％,倾滑斜滑型发震断层占33％,说明云南及邻区的走滑型地震较易受到潮汐应力的触发作用。     

Foreshock sequences and short-term earthquake predictability on East Pacific Rise transform faults

East Pacific Rise transform faults are characterized by high slip rates (more than ten centimetres a year), predominantly aseismic slip and maximum earthquake magnitudes of about 6.5. Using recordings from a hydroacoustic array deployed by the National Oceanic and Atmospheric Administration, we show here that East Pacific Rise transform faults also have a low number of aftershocks and high foreshock rates compared to continental strike-slip faults. The high ratio of foreshocks to aftershocks implies that such transform-fault seismicity cannot be explained by seismic triggering models in which there is no fundamental distinction between foreshocks, mainshocks and aftershocks. The foreshock sequences on East Pacific Rise transform faults can be used to predict (retrospectively) earthquakes of magnitude 5.4 or greater, in narrow spatial and temporal windows and with a high probability gain. The predictability of such transform earthquakes is consistent with a model in which slow slip transients trigger earthquakes, enrich their low-frequency radiation and accommodate much of the aseismic plate motion.     

Slip rate variations on normal faults during glacial-interglacial changes in surface loads

Geologic and palaeoseismological data document a marked increase in the slip rates of the Wasatch fault and three adjacent normal faults in the Basin and Range Province during the Late Pleistocene/Early Holocene epochs. The cause of this synchronous acceleration of fault slip and the subsequent clustering of earthquakes during the Holocene has remained enigmatic, although it has been suggested that the coincidence between the acceleration of slip and the shrinkage of Lake Bonneville after the Last Glacial Maximum may indicate a causal relationship. Here we use finite-element models of a discrete normal fault within a rheologically layered lithosphere to evaluate the relative importance of two competing processes that affect fault slip: postglacial unloading (the removal of mass), which decreases the slip rate, and lithospheric rebound, which promotes faster slip. We show that lithospheric rebound caused by regression of Lake Bonneville and deglaciation of adjacent mountain ranges provides a feasible mechanism for the high Holocene rates of faulting in the Wasatch region. Our analysis implies that climate-controlled changes in loads applied to Earth's surface may exert a fundamental control on the slip history of individual normal faults.     

沂沭断裂带北段大盛-马站早白垩世盆地成因分析

为了分析大盛 -马站盆地的成因类型 ,重点调查了盆地内沉积相带的空间展布特征及盆地沉积中心迁移的规律 .盆地内同生构造显示盆地边缘主干断裂具有左行走滑特点 .盆地内生物化石及火山岩年代学特征表明盆地形成于早白垩世 .盆地性质为左旋走滑拉分盆地 ,说明郯庐断裂在早白垩世晚期发生过左旋走滑活动     

天堂山水库坝区断裂构造基本特征及其活动性

野外调查与室内分析表明,影响大坝基础稳定性的北西向断裂属不活动断层;压碎岩占80％以上,强度较高,有利于坝基工程基础处理;引水隧道及峒室工程必需考虑现今构造应力场主压应力近南北向来布置,以减少构造应力对围岩稳定性带来的不利影响。     

Earthquakes as beacons of stress change

Aftershocks occurring on faults in the far-field of a large earthquake rupture can generally be accounted for by changes in static stress on these faults caused by the rupture. This implies that faults interact, and that the timing of an earthquake can be affected by previous nearby ruptures. Here we explore the potential of small earthquakes to act as 'beacons' for the mechanical state of the crust. We investigate the static-stress changes resulting from the 1992 Landers earthquake in southern California which occurred in an area of high seismic activity stemming from many faults. We first gauge the response of the regional seismicity to the Landers event with a new technique, and then apply the same method to the inverse problem of determining the slip distribution on the main rupture from the seismicity. Assuming justifiable parameters, we derive credible matches to slip profiles obtained directly from the Landers mainshock. Our results provide a way to monitor mechanical conditions in the upper crust, and to investigate processes leading to fault failure.