Fenglin Niu

Hemispherical variations in seismic velocity at the top of the Earth's inner core.

Knowledge of the seismic velocity structure at the top of the Earths inner core is important for deciphering the physical processes responsible for inner-core growth. Previous global seismic studies have focused on structures found 100 km or deeper within the inner core, with results for the uppermost 100 km available for only isolated regions. Here we present constraints on seismic velocity variations just beneath the inner-core boundary, determined from the difference in travel time between waves reflected at the inner-core boundary and those transmitted through the inner core. We found that these travel-time residuals—observed on both global seismograph stations and several regional seismic networks—are systematically larger, by about 0.8 s, for waves that sample the ‘eastern hemisphere’ of the inner core (40° E to 180° E) compared to those that sample the ‘western hemisphere’ (180° W to 40° E). These residuals show no correlation with the angle at which the waves traverse the inner core; this indicates that seismic anisotropy is not strong in this region and that the isotropic seismic velocity of the eastern hemisphere is about 0.8% higher than that of the western hemisphere.

Fine structure of the lowermost crust beneath the Kaapvaal craton and its implications for crustal formation and evolution

High quality data from a dense seismic array covering an area of approximately 60×40 km2 are used to obtain tight quantitative estimates of the fine-scale velocity and density structure of the lowermost crust and the crust–mantle boundary (Moho) beneath the Kaapvaal craton in the vicinity of Kimberley, South Africa. Results based on a modified receiver function waveform analysis of Moho conversions and crustal reverberation phases show that the crust beneath the array is thin (35.4 km) with an average Poisson’s ratio of 0.254. The minimum S-wave velocity contrast across the Moho is 17.3% while the contrast in density is 15.4%. The density contrast across the Moho is particularly diagnostic. For an assumed uppermost mantle density beneath Kimberley of 3.3 gm/cc as determined from mantle xenoliths, the density of the lowermost crust is 2.86 gm/cc, indicating rocks of felsic to intermediate composition. Analysis of waveform broadening of the crustal reverberation phases relative to that of the direct P-wave shows the thickness of the Moho transition zone to be less than 0.5 km and the maximum variation in crustal thickness over the region of the array to be less than 1 km. The flat and almost perfectly sharp Moho, together with the absence of a mafic lower crust, suggests large-scale crustal reworking in the period between crustal formation and the time of cratonic stabilization.

Preseismic velocity changes observed from active source monitoring at the Parkfield SAFOD drill site

Measuring stress changes within seismically active fault zones has been a long-sought goal of seismology. One approach is to exploit the stress dependence of seismic wave velocity, and we have investigated this in an active source cross-well experiment at the San Andreas Fault Observatory at Depth (SAFOD) drill site. Here we show that stress changes are indeed measurable using this technique. Over a two-month period, we observed an excellent anti-correlation between changes in the time required for a shear wave to travel through the rock along a fixed pathway (a few microseconds) and variations in barometric pressure. We also observed two large excursions in the travel-time data that are coincident with two earthquakes that are among those predicted to produce the largest coseismic stress changes at SAFOD. The two excursions started approximately 10 and 2 hours before the events, respectively, suggesting that they may be related to pre-rupture stress induced changes in crack properties, as observed in early laboratory studies.

Depth variation of the mid‐mantle seismic discontinuity

Short-period array seismograms of deep events that occurred in the Indonesia, Japan and Izu-Bonin arcs are stacked and beam-formed to identify the near-source S-P converted waves that result from the mantle transition discontinuities. Most of the resulting images reveal the existence of a mid-mantle seismic discontinuity (“920 km discontinuity”) in these regions. Of the 15 events analyzed, three that occurred at the western end of the Indonesia arc show clear S-P arrivals observable even in individual seismograms. The mid-mantle discontinuity is characterized by large depth variation (900 ∼ 1080 km) and velocity contrast variation in different subduction zones. Especially, the depth variation of the mid-mantle discontinuity beneath the Indonesia arc, where the discontinuity deepens from 940 km at the eastern end to 1080 km at the western end, appears to be well correlated with the location of the high-velocity anomalies in recent tomographic models. However, the mid-mantle discontinuity cannot be simply coincided with the bottom of the high-velocity anomalies, because a velocity increase at the discontinuity is observed from the waveform analysis.

Migration of seismic scatterers associated with the 1993 Parkfield aseismic transient event.

The time-varying deformation field within a fault zone, particularly at depths where earthquakes occur, is important for understanding fault behaviour and its relation to earthquake occurrence. But detection of this temporal variation has been extremely difficult, although laboratory studies have long suggested that certain structural changes, such as the properties of crustal fractures, should be seismically detectable. Here we present evidence that such structural changes are indeed observable. In particular, we find a systematic temporal variation in the seismograms of repeat microearthquakes that occurred on the Parkfield segment of the San Andreas fault over the decade 1987–97. Our analysis reveals a change of the order of 10 m in the location of scatterers which plausibly lie within the fault zone at a depth of ∼3 km. The motion of the scatterers is coincident, in space and time, with the onset of a well documented aseismic transient (deformation event). We speculate that this structural change is the result of a stress-induced redistribution of fluids in fluid-filled fractures caused by the transient event.

Direct evidence for the undulation of the 660-km discontinuity beneath Tonga: Comparison of Japan and California array data

Short-period seismograms of Tonga deep earthquakes recorded by Japanese and Californian seismic networks are stacked to identify the S-P converted wave associated with the 660-km discontinuity. The travel-time difference between this S-P converted wave and the direct P wave is used to constrain the depth of the 660-km discontinuity. Analysis of a total of 29 events produced a detailed topographical map of the discontinuity beneath the Tonga subduction zone. Two events which exhibit clear S-P conversions in both Japan and California data are selected to show directly the depth variations of the 660-km discontinuity adjacent to the subducting slab. The S-P conversion points on the ray paths to Japan are observed to be approximately 10 to 30 km deeper than the conversion points on those to California, which represents direct evidence for a slab-induced depression of the 660-km discontinuity.

Remote triggering of fault-strength changes on the San Andreas fault at Parkfield

Fault strength is a fundamental property of seismogenic zones, and its temporal changes can increase or decrease the likelihood of failure and the ultimate triggering of seismic events. Although changes in fault strength have been suggested to explain various phenomena, such as the remote triggering of seismicity, there has been no means of actually monitoring this important property in situ. Here we argue that ∼20 years of observation (1987–2008) of the Parkfield area at the San Andreas fault have revealed a means of monitoring fault strength. We have identified two occasions where long-term changes in fault strength have been most probably induced remotely by large seismic events, namely the 2004 magnitude (M) 9.1 Sumatra–Andaman earthquake and the earlier 1992 M = 7.3 Landers earthquake. In both cases, the change possessed two manifestations: temporal variations in the properties of seismic scatterers—probably reflecting the stress-induced migration of fluids—and systematic temporal variations in the characteristics of repeating-earthquake sequences that are most consistent with changes in fault strength. In the case of the 1992 Landers earthquake, a period of reduced strength probably triggered the 1993 Parkfield aseismic transient as well as the accompanying cluster of four M > 4 earthquakes at Parkfield. The fault-strength changes produced by the distant 2004 Sumatra–Andaman earthquake are especially important, as they suggest that the very largest earthquakes may have a global influence on the strength of the Earth’s fault systems. As such a perturbation would bring many fault zones closer to failure, it should lead to temporal clustering of global seismicity. This hypothesis seems to be supported by the unusually high number of M ≥ 8 earthquakes occurring in the few years following the 2004 Sumatra–Andaman earthquake.

Mantle transition-zone structure beneath the South Pacific Superswell and evidence for a mantle plume underlying the Society hotspot

Abstract Underside reflections of shear waves from the discontinuities at 410 and 660 km depth are used to map lateral variations in the thickness of the mantle transition zone beneath the South Pacific Superswell and surrounding regions. Differential travel times of reflected waves indicate that the transition zone is about 25 km thinner than normal, and thus hotter than normal, over an area 500 km or less in diameter beneath the Society hotspot. There is no general difference, however, in transition-zone thickness between the Superswell area and its surroundings. Our observations support the inference that the thermal flux from lower to upper mantle beneath the South Pacific Superswell occur on the lateral scale of a mantle plume (several hundred kilometers) rather than that of a superplume (several thousand kilometers).

Upper mantle structure beneath the Caribbean‐South American plate boundary from surface wave tomography

[1] We have measured shear wave velocity structure of the crust and upper mantle of the Caribbean-South American boundary region by analysis of fundamental mode Rayleigh waves in the 20- to 100-s period band recorded at the BOLIVAR/GEODINOS stations from 2003 to 2005. The model shows lateral variations that primarily correspond to tectonic provinces and boundaries. A clear linear velocity change parallels the plate bounding dextral strike-slip fault system along the northern coast of Venezuela, illustrating the differences between the South American continental lithosphere, the Venezuelan archipelago, and the Caribbean oceanic lithosphere. At depths up to 120 km beneath the Venezuelan Andes and the Maracaibo block, there is evidence of underthrusting of the Caribbean plate, but there is no other evidence of subduction of the Caribbean plate beneath the South American plate. In eastern Venezuela, linear crustal low velocities are associated with the fold and thrust belts whereas as higher crustal velocities are imaged in the Guayana shield lithosphere. The subducting oceanic part of the South American plate is imaged beneath the Antilles arc. The surface wave images combined with seismicity data suggest shear tearing of the oceanic lithosphere away from the buoyant continental South American plate offshore of northeastern Venezuela. The continental lithosphere south of the slab tear is bent down toward the plate boundary in response to the propagating tear in the lithosphere. We interpret a nearly vertical low-velocity ‘‘column’’ west of the tear centered beneath the Cariaco Basin, with three-dimensional asthenospheric flow around the southern edge of the subducting oceanic lithosphere, with the asthenosphere escaping from beneath continental South America and rising into the plate boundary zone. The complex plate boundary structure is best examined in three dimensions. We discuss the new surface wave tomographic inversion in the context of results from other researchers including local seismicity, teleseismic shear wave splits, and interpretations from active source profiling.

Strong seismic scatterers near the core-mantle boundary west of Mexico

PKPphasesobservedinJapan showstrongpre- cursors with clear onsets for earthquakes that occurred in south America. The onset times of the PKP precursors can allbeexplainedbyexistenceofseismicscatterersinthelow- ermost 100 kmof themantlewest of Mexico, withinan area of 200 km300 km. Forward modeling of PKP precursor amplitudes indicates that a P-wave velocity variation of at least 6% is required to explain the large amplitudes of the observed PKP precursors, suggesting that the core-mantle boundary region beneath west of Mexico is a highly anoma- lous region characterized by strong seismic scatterers.