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.     

Skew of mantle upwelling beneath the East Pacific Rise governs segmentation

Mantle upwelling is essential to the generation of new oceanic crust at mid-ocean ridges, and it is generally assumed that such upwelling is symmetric beneath active ridges. Here, however, we use seismic imaging to show that the isotropic and anisotropic structure of the mantle is rotated beneath the East Pacific Rise. The isotropic structure defines the pattern of magma delivery from the mantle to the crust. We find that the segmentation of the rise crest between transform faults correlates well with the distribution of mantle melt. The azimuth of seismic anisotropy constrains the direction of mantle flow, which is rotated nearly 10 degrees anticlockwise from the plate-spreading direction. The mismatch between the locus of mantle melt delivery and the morphologic ridge axis results in systematic differences between areas of on-axis and off-axis melt supply. We conclude that the skew of asthenospheric upwelling and transport governs segmentation of the East Pacific Rise and variations in the intensity of ridge crest processes.     

The influence of ridge migration on the magmatic segmentation of mid-ocean ridges

The Earth's mid-ocean ridges display systematic changes in depth and shape, which subdivide the ridges into discrete spreading segments bounded by transform faults and smaller non-transform offsets of the axis. These morphological changes have been attributed to spatial variations in the supply of magma from the mantle, although the origin of the variations is poorly understood. Here we show that magmatic segmentation of ridges with fast and intermediate spreading rates is directly related to the migration velocity of the spreading axis over the mantle. For over 9,500 km of mid-ocean ridge examined, leading ridge segments in the 'hotspot' reference frame coincide with the shallow magmatically robust segments across 86 per cent of all transform faults and 73 per cent of all second-order discontinuities. We attribute this relationship to asymmetric mantle upwelling and melt production due to ridge migration, with focusing of melt towards ridge segments across discontinuities. The model is consistent with variations in crustal structure across discontinuities of the East Pacific Rise, and may explain variations in depth of melting and the distribution of enriched lavas.     

Friction falls towards zero in quartz rock as slip velocity approaches seismic rates

An important unsolved problem in earthquake mechanics is to determine the resistance to slip on faults in the Earth's crust during earthquakes. Knowledge of coseismic slip resistance is critical for understanding the magnitude of shear-stress reduction and hence the near-fault acceleration that can occur during earthquakes, which affects the amount of damage that earthquakes are capable of causing. In particular, a long-unresolved problem is the apparently low strength of major faults, which may be caused by low coseismic frictional resistance. The frictional properties of rocks at slip velocities up to 3 mm s(-1) and for slip displacements characteristic of large earthquakes have been recently simulated under laboratory conditions. Here we report data on quartz rocks that indicate an extraordinary progressive decrease in frictional resistance with increasing slip velocity above 1 mm s(-1). This reduction extrapolates to zero friction at seismic slip rates of approximately 1 m s(-1), and appears to be due to the formation of a thin layer of silica gel on the fault surface: it may explain the low strength of major faults during earthquakes.     

Earthquake slip on oceanic transform faults

Oceanic transform faults are one of the main types of plate boundary, but the manner in which they slip remains poorly understood. Early studies suggested that relatively slow earthquake rupture might be common; moreover, it has been reported that very slow slip precedes some oceanic transform earthquakes, including the 1994 Romanche earthquake. The presence of such detectable precursors would have obvious implications for earthquake prediction. Here we model broadband seismograms of body waves to obtain well-resolved depths and rupture mechanisms for 14 earthquakes on the Romanche and Chain transform faults in the equatorial Atlantic Ocean. We found that earthquakes on the longer Romanche transform are systematically deeper than those on the neighbouring Chain transform. These depths indicate that the maximum depth of brittle failure is at a temperature of approximately 600 degrees C in oceanic lithosphere. We find that the body waves from the Romanche 1994 earthquake can be well modelled with relatively deep slip on a single fault, and we use the mechanism and depth of this earthquake to recalculate its source spectrum. The previously reported slow precursor can be explained as an artefact of uncertainties in the assumed model parameters.     

“311”东北日本M9.0大地震可预测性特征探索

 大地震不是任何地方都能发生的,大地震也不是任何时间都能发生的,大地震有其特定的时空结构。前期工作表明,条环交会、差异活动以及深震与“天外来客”事件等是制约发震时空窗口的主要因素。2011年3月11日发生在日本海沟俯冲带的M9.0大地震,其震前地震活动全面、清晰地展现出了其可预测性特征：(1) 1925年开始出现的板间地震长期平静区;(2) 2003年开始出现的位于平静区中部,起始于深震的海沟垂向地震活动条带;(3) 2009年3月(海沟垂向地震活动条带形成后)开始出现的广义前震发震地方时的非随机显著聚集特征;(4) 2011年3月9日(主震前2天)发生在平静区内的前震。主震发生在平静区边缘,海沟垂向地震活动条带与日本海沟板间地震带的交会处的地震活动差异性较大的地方,其断层面与海沟垂向地震活动条带内另外两个板间地震(2005-08-16地震和前震)的断层面共面。主震发生于广义前震发震地方时非随机聚集时段的中位时辰。日本大地震的可预测性特征逐一印证了我们的前期研究。     

Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip

Non-volcanic seismic tremor was discovered in the Nankai trough subduction zone in southwest Japan and subsequently identified in the Cascadia subduction zone. In both locations, tremor is observed to coincide temporally with large, slow slip events on the plate interface downdip of the seismogenic zone. The relationship between tremor and aseismic slip remains uncertain, however, largely owing to difficulty in constraining the source depth of tremor. In southwest Japan, a high quality borehole seismic network allows identification of coherent S-wave (and sometimes P-wave) arrivals within the tremor, whose sources are classified as low-frequency earthquakes. As low-frequency earthquakes comprise at least a portion of tremor, understanding their mechanism is critical to understanding tremor as a whole. Here, we provide strong evidence that these earthquakes occur on the plate interface, coincident with the inferred zone of slow slip. The locations and characteristics of these events suggest that they are generated by shear slip during otherwise aseismic transients, rather than by fluid flow. High pore-fluid pressure in the immediate vicinity, as implied by our estimates of seismic P- and S-wave speeds, may act to promote this transient mode of failure. Low-frequency earthquakes could potentially contribute to seismic hazard forecasting by providing a new means to monitor slow slip at depth.     

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。     

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.     

Interseismic strain accumulation and the earthquake potential on the southern San Andreas fault system

The San Andreas fault in California is a mature continental transform fault that accommodates a significant fraction of motion between the North American and Pacific plates. The two most recent great earthquakes on this fault ruptured its northern and central sections in 1906 and 1857, respectively. The southern section of the fault, however, has not produced a great earthquake in historic times (for at least 250 years). Assuming the average slip rate of a few centimetres per year, typical of the rest of the San Andreas fault, the minimum amount of slip deficit accrued on the southern section is of the order of 7-10 metres, comparable to the maximum co-seismic offset ever documented on the fault. Here I present high-resolution measurements of interseismic deformation across the southern San Andreas fault system using a well-populated catalogue of space-borne synthetic aperture radar data. The data reveal a nearly equal partitioning of deformation between the southern San Andreas and San Jacinto faults, with a pronounced asymmetry in strain accumulation with respect to the geologically mapped fault traces. The observed strain rates confirm that the southern section of the San Andreas fault may be approaching the end of the interseismic phase of the earthquake cycle.     

Decay of aftershock density with distance indicates triggering by dynamic stress

The majority of earthquakes are aftershocks, yet aftershock physics is not well understood. Many studies suggest that static stress changes trigger aftershocks, but recent work suggests that shaking (dynamic stresses) may also play a role. Here we measure the decay of aftershocks as a function of distance from magnitude 2-6 mainshocks in order to clarify the aftershock triggering process. We find that for short times after the mainshock, when low background seismicity rates allow for good aftershock detection, the decay is well fitted by a single inverse power law over distances of 0.2-50 km. The consistency of the trend indicates that the same triggering mechanism is working over the entire range. As static stress changes at the more distant aftershocks are negligible, this suggests that dynamic stresses may be triggering all of these aftershocks. We infer that the observed aftershock density is consistent with the probability of triggering aftershocks being nearly proportional to seismic wave amplitude. The data are not fitted well by models that combine static stress change with the evolution of frictionally locked faults.     

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.     

Seismic identification of along-axis hydrothermal flow on the East Pacific Rise

Hydrothermal circulation at the axis of mid-ocean ridges affects the chemistry of the lithosphere and overlying ocean, supports chemosynthetic biological communities and is responsible for significant heat transfer from the lithosphere to the ocean. It is commonly thought that flow in these systems is oriented across the ridge axis, with recharge occurring along off-axis faults, but the structure and scale of hydrothermal systems are usually inferred from thermal and geochemical models constrained by the geophysical setting, rather than direct observations. The presence of microearthquakes may shed light on hydrothermal pathways by revealing zones of thermal cracking where cold sea water extracts heat from hot crustal rocks, as well as regions where magmatic and tectonic stresses create fractures that increase porosity and permeability. Here we show that hypocentres beneath a well-studied hydrothermal vent field on the East Pacific Rise cluster in a vertical pipe-like zone near a small axial discontinuity, and in a band that lies directly above the axial magma chamber. The location of the shallow pipe-like cluster relative to the distribution and temperature of hydrothermal vents along this section of the ridge suggests that hydrothermal recharge may be concentrated there as a consequence of the permeability generated by tectonic fracturing. Furthermore, we interpret the band of seismicity above the magma chamber as a zone of hydrothermal cracking, which suggests that hydrothermal circulation may be strongly aligned along the ridge axis. We conclude that models that suggest that hydrothermal cells are oriented across-axis, with diffuse off-axis recharge zones, may not apply to the fast-spreading East Pacific Rise.     

Counter-rotating microplates at the Galapagos triple junction

An 'incipient' spreading centre east of (and orthogonal to) the East Pacific Rise at 2 degrees 40' N has been identified as forming a portion of the northern boundary of the Galapagos microplate. This spreading centre was described as a slowly diverging, westward propagating rift, tapering towards the East Pacific Rise. Here we present evidence that the 'incipient rift' has also rifted towards the east and opens anticlockwise about a pivot at its eastern end. The 'incipient rift' then bounds a second microplate, north of the clockwise-rotating Galapagos microplate. The Galapagos triple junction region, in the eastern equatorial Pacific Ocean, thus consists of two counter-rotating microplates partly separated by the Hess Deep rift. Our kinematic solution for microplate motion relative to the major plates indicates that the two counter-rotating microplates may be treated as rigid blocks driven by drag on the microplates' edges3.     

Remote triggering of deep earthquakes in the 2002 Tonga sequences

It is well established that an earthquake in the Earth's crust can trigger subsequent earthquakes, but such triggering has not been documented for deeper earthquakes. Models for shallow fault interactions suggest that static (permanent) stress changes can trigger nearby earthquakes, within a few fault lengths from the causative earthquake, whereas dynamic (transient) stresses carried by seismic waves may trigger earthquakes both nearby and at remote distances. Here we present a detailed analysis of the 19 August 2002 Tonga deep earthquake sequences and show evidence for both static and dynamic triggering. Seven minutes after a magnitude 7.6 earthquake occurred at a depth of 598 km, a magnitude 7.7 earthquake (664 km depth) occurred 300 km away, in a previously aseismic region. We found that nearby aftershocks of the first mainshock are preferentially located in regions where static stresses are predicted to have been enhanced by the mainshock. But the second mainshock and other triggered events are located at larger distances where static stress increases should be negligible, thus suggesting dynamic triggering. The origin times of the triggered events do not correspond to arrival times of the main seismic waves from the mainshocks and the dynamically triggered earthquakes frequently occur in aseismic regions below or adjacent to the seismic zone. We propose that these events are triggered by transient effects in regions near criticality, but where earthquakes have difficulty nucleating without external influences.     

Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface

At the northern Cascadia margin, the Juan de Fuca plate is underthrusting North America at about 45 mm x yr(-1) (ref. 1), resulting in the potential for destructive great earthquakes. The downdip extent of coupling between the two plates is difficult to determine because the most recent such earthquake (thought to have been in 1700) occurred before instrumental recording. Thermal and deformation studies indicate that, off southern Vancouver Island, the interplate interface is presently fully locked for a distance of approximately 60 km downdip from the deformation front. Great thrust earthquakes on this section of the interface (with magnitudes of up to 9) have been estimated to occur at an average interval of about 590 yr (ref. 3). Further downdip there is a transition from fully locked behaviour to aseismic sliding (where high temperatures allow ductile deformation), with the deep aseismic zone exhibiting slow-slip thrust events. Here we show that there is a change in the reflection character on seismic images from a thin sharp reflection where the subduction thrust is inferred to be locked, to a broad reflection band at greater depth where aseismic slip is thought to be occurring. This change in reflection character may provide a new technique to map the landward extent of rupture in great earthquakes and improve the characterization of seismic hazards in subduction zones.     

Analysing the 1811-1812 New Madrid earthquakes with recent instrumentally recorded aftershocks

Although dynamic stress changes associated with the passage of seismic waves are thought to trigger earthquakes at great distances, more than 60 per cent of all aftershocks appear to be triggered by static stress changes within two rupture lengths of a mainshock. The observed distribution of aftershocks may thus be used to infer details of mainshock rupture geometry. Aftershocks following large mid-continental earthquakes, where background stressing rates are low, are known to persist for centuries, and models based on rate-and-state friction laws provide theoretical support for this inference. Most past studies of the New Madrid earthquake sequence have indeed assumed ongoing microseismicity to be a continuing aftershock sequence. Here we use instrumentally recorded aftershock locations and models of elastic stress change to develop a kinematically consistent rupture scenario for three of the four largest earthquakes of the 1811-1812 New Madrid sequence. Our results suggest that these three events occurred on two contiguous faults, producing lobes of increased stress near fault intersections and end points, in areas where present-day microearthquakes have been hitherto interpreted as evidence of primary mainshock rupture. We infer that the remaining New Madrid mainshock may have occurred more than 200 km north of this region in the Wabash Valley of southern Indiana and Illinois--an area that contains abundant modern microseismicity, and where substantial liquefaction was documented by historic accounts. Our results suggest that future large mid-plate earthquake sequences may extend over a much broader region than previously suspected.     

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.     

日本Mw9.0级地震的库仑应力变化及对余震的触发

采用已公布的1个前震和3个主震震源可变滑动模型,计算了日本Mw7.3级前震、M_w9.0级主震产生的同震库仑应力变化,并采用哈佛Global CMT和日本F-net余震目录,在不同有效摩擦系数取值下计算了主震对余震的触发效应,分析了应力空间分布与余震活动空间分布的相关性。研究表明,日本地震前震对主震有触发作用,前震在主震处产生的正应力变化为0.404 bar,剪应力变化0.282 bar,同震库仑应力变化为0.517 bar,应力变化超过了应力触发阈值0.1 bar,达5倍之多,属于典型的地震应力触发,而非诱发事件。主震对后续余震有触发效应,最小触发率为57%,最大达到75%。余震多分布在主震产生的同震库仑应力红色加载区。同时,分析了可变滑动模型、有效摩擦系数、余震目录、震级及节面选取等对计算结果的影响。可变滑动模型、有效摩擦系数变化对计算结果影响不大,余震目录、震级及节面选取对结果有一定影响,尤其是当余震目录足够多、震级足够大时,计算结果更加可靠。为了深入讨论节面选取对计算结果的影响,对余震两个节面上库仑应力、剪切应力及正应力变化差值进行了频次统计。分析显示节面选取对正应力有一定影响,而对库仑应力及剪应力影响不大。     

Lithium isotope evidence for subduction-enriched mantle in the source of mid-ocean-ridge basalts

'Recycled' crustal materials, returned from the Earth's surface to the mantle by subduction, have long been invoked to explain compositional heterogeneity in the upper mantle. Yet increasingly, problems have been noted with this model. The debate can be definitively addressed using stable isotope ratios, which should only significantly vary in primitive, mantle-derived materials as a consequence of recycling. Here we present data showing a notable range in lithium isotope ratios in basalts from the East Pacific Rise, which correlate with traditional indices of mantle heterogeneity (for example, 143Nd/144Nd ratios). Such co-variations of stable and radiogenic isotopes in melts from a normal ridge segment provide critical evidence for the importance of recycled material in generating chemical heterogeneity in the upper mantle. Contrary to many models, however, the elevated lithium isotope ratios of the 'enriched' East Pacific Rise lavas imply that subducted ocean crust is not the agent of enrichment. Instead, we suggest that fluid-modified mantle, which is enriched during residency in a subduction zone, is mixed back into the upper mantle to cause compositional variability.