Chunquan Yu

Characterization and Petrological Constraints of the Midlithospheric Discontinuity

Within continental lithosphere, widespread seismic evidence suggests a sharp discontinuous downward decrease in seismic velocity at 60–160 km depth. This midlithospheric discontinuity (MLD) may be due to anisotropy, melt, hydration, and/or mantle metasomatism. We survey global seismologic observations of the MLD, including observed depths, velocity contrasts, gradients, and locales across multiple seismic techniques. The MLD is primarily found in regions of thick continental lithosphere and is a decrease in seismic shear velocity (2–7% over 10–20 km) at 60–160 km depth, the majority of observations clustering at 80–100 km. Of xenoliths in online databases, 25% of amphibole-bearing xenoliths, 90% of phlogopite-bearing xenoliths, and none of carbonate-bearing xenoliths were formed at pressures associated with these depth (2–5 GPa). We used Perple_X modeling to evaluate the elastic moduli and densities of multiple petrologies to test if the MLD is a layer of crystallized melt. The fractional addition of 5–10% phlogopite, 10–15% carbonate, or 45–100% pyroxenite produce a 2–7% velocity decrease. We postulate this layer of crystallized melt would originate at active margins of continents and crystallize in place as the lithosphere cools. The concentration of mildly incompatible elements (Y, Ho, Er, Yb, and Lu) in xenoliths near the MLD is consistent with higher degrees of melting. Thus, we postulate that the MLD is the seismological signature of a chemical interface related to the paleointersection of a volatile-rich solidus and progressively cooling lithosphere. Furthermore, the MLD may represent a remnant chemical tracer of the lithosphere-asthenosphere boundary (LAB) from when the lithosphere was active and young.

Earthquake rupture below the brittle-ductile transition in continental lithospheric mantle

The slow and inefficient deep Wyoming earthquake ruptured in the ductile regime of the upper mantle. Earthquakes deep in the continental lithosphere are rare and hard to interpret in our current understanding of temperature control on brittle failure. The recent lithospheric mantle earthquake with a moment magnitude of 4.8 at a depth of ~75 km in the Wyoming Craton was exceptionally well recorded and thus enabled us to probe the cause of these unusual earthquakes. On the basis of complete earthquake energy balance estimates using broadband waveforms and temperature estimates using surface heat flow and shear wave velocities, we argue that this earthquake occurred in response to ductile deformation at temperatures above 750°C. The high stress drop, low rupture velocity, and low radiation efficiency are all consistent with a dissipative mechanism. Our results imply that earthquake nucleation in the lithospheric mantle is not exclusively limited to the brittle regime; weakening mechanisms in the ductile regime can allow earthquakes to initiate and propagate. This finding has significant implications for understanding deep earthquake rupture mechanics and rheology of the continental lithosphere.

Western US intermountain seismicity caused by changes in upper mantle flow

Understanding the causes of intraplate earthquakes is challenging, as it requires extending plate tectonic theory to the dynamics of continental deformation. Seismicity in the western United States away from the plate boundary is clustered along a meandering, north–south trending ‘intermountain’ belt. This zone coincides with a transition from thin, actively deforming to thicker, less tectonically active crust and lithosphere. Although such structural gradients have been invoked to explain seismicity localization, the underlying cause of seismicity remains unclear. Here we show results from improved mantle flow models that reveal a relationship between seismicity and the rate change of ‘dynamic topography’ (that is, vertical normal stress from mantle flow). The associated predictive skill is greater than that of any of the other forcings we examined. We suggest that active mantle flow is a major contributor to seismogenic intraplate deformation, while gravitational potential energy variations have a minor role. Seismicity localization should occur where convective changes in vertical normal stress are modulated by lithospheric strength heterogeneities. Our results on deformation processes appear consistent with findings from other mobile belts, and imply that mantle flow plays a significant and quantifiable part in shaping topography, tectonics, and seismic hazard within intraplate settings.

Crazyseismic: A MATLAB GUI‐Based Software Package for Passive Seismic Data Preprocessing

We introduce an open‐source MATLAB software package, named Crazyseismic, for passive seismic data preprocessing. Built‐in core functions such as seismic phase travel‐time calculation and multichannel cross correlation significantly improve the efficiency of data processing. Compared with conventional command‐line‐style toolboxes, all functions in Crazyseismic are embedded in one single graphic user interface (GUI). The human–machine interactive nature of GUI facilitates data quality control. The simplicity of the software allows users to process Seismic Analysis Code format seismic data with great ease and also provides a means by which users can tailor the software for their specific needs. We demonstrate the power of our software through two field examples: one for P‐wave arrival‐time picking and the other for receiver function calculation. The software can essentially be used for analyzing all major body‐wave phases in seismology.

Strong SH-to-Love wave scattering off the Southern California Continental Borderland

Seismic scattering is commonly observed and results from wave propagation in heterogeneous medium. Yet, deterministic characterization of scatterers associated with lateral heterogeneities remains challenging. In this study, we analyze broadband waveforms recorded by the Southern California Seismic Network and observe strongly scattered Love waves following the arrival of teleseismic SH wave. These scattered Love waves travel approximately in the same (azimuthal) direction as the incident SH wave at a dominant period of ~10 s but at an apparent velocity of ~3.6 km/s as compared to the ~11 km/s for the SH wave. Back-projection suggests that this strong scattering is associated with pronounced bathymetric relief in the Southern California Continental Borderland, in particular the Patton Escarpment. Finite-difference simulations using a simplified 2-D bathymetric and crustal model are able to predict the arrival times and amplitudes of major scatterers. The modeling suggests a relatively low shear wave velocity in the Continental Borderland.

Simultaneous Determination of Crustal Thickness and P Wavespeed by Virtual Deep Seismic Sounding (VDSS)

The method of virtual deep seismic sounding (VDSS) uses the large amplitude, postcritical reflection off the Moho, the SsPmp phase, to probe the Moho. In this study, we augment VDSS using the change in differential travel times between phases SsPmp and Ss ( T ) as a function of distance (moveout) to determine simultaneously both the thickness ( H ) and overall P wavespeed ( V P ) of the crust. Tests using synthetic data show that for typical uncertainties in measuring T (1 standard deviation of ±0.2  s), a minimum uncertainty of ∼±1.1  km and ±0.06  km/s can be reached for nominal values of H and V P , respectively. We then demonstrate its utility with field data recorded by two permanent stations in Australia.

Constraints on residual topography and crustal properties in the western United States from virtual deep seismic sounding

We use virtual deep seismic sounding (VDSS) and data from ~1,000 broadband seismic stations to provide high-resolution estimates of crustal structure in the western Cordillera of the United States (US). The most robust result is the geographic distribution of residual topography (that is, the difference between observed elevation and that expected from crustal buoyancy alone) and, by implication, thermal or petrologic anomalies in the mantle. Overall, residual topography of the western US Cordillera varies considerably; with contrasts of up to about 3 km across distances of 200 km or less. High residual topography, indicating large mantle effects, is evident along the periphery of the Colorado Plateau and the surroundings of the Great Basin. In contrast, the central Colorado Plateau and the Wyoming Basin show low residual topography, close to what is expected of a geologically stable lithosphere. Overall, in regions to the east of the Wasatch hinge line (the eastern limit of significant extension in the North American cratonic basement) patterns of high residual topography and anomalies of low seismic wave-speeds in the upper mantle are similar, suggestive of a common, thermal origin. In contrast, such a similarity is absent in regions to the west of the hinge line, suggesting substantial effects of petrological heterogeneities in the mantle. Finally, joint analyses of VDSS and conventional receiver functions reveal a wide range of crustal P-wave speeds, locally as high as 6.7 km/s, perhaps indicating magmatic modification of the crust.

Compositional heterogeneity near the base of the mantle transition zone beneath Hawaii

Global seismic discontinuities near 410 and 660 km depth in Earth’s mantle are expressions of solid-state phase transitions. These transitions modulate thermal and material fluxes across the mantle and variations in their depth are often attributed to temperature anomalies. Here we use novel seismic array analysis of SS waves reflecting off the 410 and 660 below the Hawaiian hotspot. We find amplitude–distance trends in reflectivity that imply lateral variations in wavespeed and density contrasts across 660 for which thermodynamic modeling precludes a thermal origin. No such variations are found along the 410. The inferred 660 contrasts can be explained by mantle composition varying from average (pyrolitic) mantle beneath Hawaii to a mixture with more melt-depleted harzburgite southeast of the hotspot. Such compositional segregation was predicted, from petrological and numerical convection studies, to occur near hot deep mantle upwellings like the one often invoked to cause volcanic activity on Hawaii.Seismic discontinuities near 410 and 660 km depth have often been used to map lateral variations in mantle temperature. Here, the authors apply array analysis to SS reflections off these discontinuities under Hawaii and find evidence of lateral variations in mantle composition at 660 km, but not at 410 km.

Mapping Mantle Transition Zone Discontinuities Beneath the Central Pacific With Array Processing of SS Precursors: Array Analysis of MTZ Discontinuities

We image mantle transition zone (MTZ) discontinuities beneath the Central Pacific using ~120,000 broadband SS waveforms. With a wave packet-based array processing technique (curvelet transform), we improve the signal-to-noise ratio of SS precursors and remove interfering phases, so that precursors can be identified and measured over a larger distance range. Removal of interfering phases reveals possible phase shifts in the underside reflection at the 660, that is, S^(660)S, which if ignored could lead to biased discontinuity depth estimates. The combination of data quantity and improved quality allows improved imaging and uncertainty estimation. Time to depth conversions after corrections for bathymetry, crustal thickness, and tomographically inferred mantle heterogeneity show that the mean depths of 410 and 660 beneath the Central Pacific are 420 ± 3 km and 659 ± 4 km, respectively. The mean MTZ thickness (239 ± 2 km) is close to global estimates and suggests an adiabatic mantle temperature of ~1,400°C for the Central Pacific. Depth variations of the 410 and 660 appear to be relatively small, with peak-to-peak amplitudes of the order of 10–15 km. The 410 and 660 are weakly anticorrelated, and MTZ is thinner beneath Hawaii and to the north and east of the hotspot and thicker southwest of it. The relatively small discontinuity topography argues against the presence of large-scale (more than 5° wide) thermal anomalies with excess temperatures over 200 K across the transition zone. The data used cannot exclude stronger thermal anomalies that are of more limited lateral extent or that are not continuous across the MTZ.