Research interests

1. Observational seismology

Seismic data processing and analysis

2. Seismic imaging, migration and inversion

Crust-mantle boundary (Moho) 

Upper mantle transition zone discontinuities

3. Earthquake source characterization

Earthquake focal mechanism

Earthquake depth

 

Some research topics

1) Imaging mantle transition zone discontinuities with SS precursors

a. MTZ and SS precursors

The mantle transition zone (MTZ), bounded by discontinuities near 410 km and 660 km depth, plays a critical role in mantle dynamics. The presence of the 410 and 660 is generally explained by mineralogical phase transitions from olivine to wadsleyite and from ringwoodite to perovskite + ferropericlase, respectively. With knowledge of the Clapeyron slopes of these two phase transitions, which have opposite signs (positive for 410, negative for 660), lateral variations in depth to the 410 and 660 can be used to infer thermal anomalies in the mantle. Precursors to SS result from underside reflections at elasticity contrasts (roughly) beneath the midpoint of the earthquake source and the receiver. The differential travel time between the SS phase and its precursors has been widely used for mapping upper mantle discontinuities. 

b. Curvelet-based array analysis of SS precursors and its applications to Hawaii

However, SS precursors can be strongly contaminated by near-source or near-receiver multiples, especially at relatively small source-receiver distances (see panel A), risking phase misidentification and making precursor identification and measurement difficult without careful processing. 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.

For more details, please refer to:

Yu, C.Q., Day, E.A., de Hoop, M.V., Campillo, M., van der Hilst, R.D. (2017), Mapping mantle transition zone discontinuities beneath the Central Pacific with array processing of SS precursors. J. Geophys. Res.: Solid Earth, doi: 10.1002/2017JB014327.

 

 

c. SdS/SS amplitude ratios, reflectivity, and elastic contrasts across 410 and 660

The successful separation of SS precursors from interfering phases and noise also allows us to measure its amplitudes more precisely. We measure precursor amplitudes relative to the surface reflections (that is, S410S/SS and S660S/SS) and remove effects of geometrical spreading, intrinsic attenuation, mantle heterogeneity, and interface topography. By matching the corrected amplitude ratios as a function of distances with theoretical predictions (similar to AVO analysis in exploration seismology), we estimate the density and wavespeed contrasts across 410 and 660. 

For more details, please refer to:

Yu, C.Q., Day, E.A., de Hoop, M.V., Campillo, M., Goes S., Blythe R.A., van der Hilst, R.D. (2018), Compositional heterogeneity near the base of the mantle transition zone beneath Hawaii. Nat. Commun., 9(1), 1266, doi: 10.1038/s41467-018-03654-6

 

d. Lateral variation in composition at 660 beneath Hawaii

In the Hawaii region, we found that the amplitude–distance trends in the NW and SE bins are similar to one another for 410. In contrast, the 660 trends reveal remarkable regional differences, with the distance of zero reflection and the range of S660S/SS amplitudes substantially smaller in the region NW than that SE of Hawaii . We infer that the elasticity contrasts (Δρ660, Δβ660) increase from (4.8 ± 0.7, 4.7 ± 2.5%) NW of Hawaii (Fig. 5c) to (6.9 ± 1.3, 7.8 ± 4.6%) SE of it.

To assess what variations in temperature and/or composition might be responsible for the observed lateral variation in Δρ660 and Δβ660, we applied thermodynamic modeling to calculate density and velocity profiles along a range of mantle temperatures for several representative mantle compositions. We calculate profiles for pyrolite, commonly assumed to represent average background mantle (containing 60% olivine, the main mineral responsible for the global phase transitions), harzburgite (a melt-depleted end member composition containing 80% olivine), and a mechanical mixture of 80% harzburgite and 20% basalt, which has a similar overall composition as pyrolite.

For the 410, the inferred contrasts are consistent with a pyrolitic composition across the region.

For the 660, the inferred regional differences in Δρ and Δβ can be explained by lateral variations in composition. NW of Hawaii the inferred contrast is consistent with an average (pyrolitic) composition; SE of Hawaii the data require a more olivine-rich, i.e., more harzburgitic composition. We suggest that local harzburgite enrichment near the base of the MTZ could result from compositional segregation due density contrasts that result from differences in phase-transition depths in basaltic and harzburgitic material, which was predicted, from petrological and numerical convection studies, to occur near hot deep mantle upwellings.

For more details, please refer to:

Yu, C.Q., Day, E.A., de Hoop, M.V., Campillo, M., Goes S., Blythe R.A., van der Hilst, R.D. (2018), Compositional heterogeneity near the base of the mantle transition zone beneath Hawaii. Nat. Commun., 9(1), 1266, doi: 10.1038/s41467-018-03654-6

 

2) Virtual Deep Seismic Sounding (VDSS), Moho depth and crustal buoyancy

a. VDSS method

VDSS utilizes the prominent SsPmp phase, which undergoes S-to-P conversion at the free surface and P-to-P total reflection off the Moho. The differential timing between the SsPmp phase and the direct S phase, Ss, is closely related to the crustal thickness H and average crustal P-wave velocity VP.

TSsPmp-Ss=2H(VP-2-p2)1/2

where p is the incident S-wave ray parameter (horizontal slowness).

VDSS has several distinct advantages over other methods. First, VDSS returns a robust signal from the crust-mantle boundary even if the Moho is complicated or transitional in nature. Second, VDSS is not particularly susceptible to signal-generated noise such as scatterings from thick sediments or intracrustal discontinuities. Last and most important, as will be shown here, VDSS can be used to put tight constraints on crustal buoyancy in spite of an inherent trade-off between overall crustal thickness and P wave speed.

 

b. Application to the North China craton

We applied VDSS and conventional receiver function method to the North China craton. We found a thick crust beneath the eastern part of the Ordos plateau.

For more details, please refer to:

Yu, C.Q., Chen, W.-P., Ning, J.Y., Tao, K., Chen, Y.S., Teng, T.-L., Fang, X.D., and Van der Hilst, R.D. (2012), Thick crust beneath the Ordos Plateau: Implications for instability of the North China craton, Earth and Planet. Sci. Lett., 357, 366-375, 10.1016/j.epsl.2012.09.027

c. Application to the Western US

We provided tight constraints on the crustal buoyancy of western US.

Key points:

  1. Virtual deep seismic sounding (VDSS) tightly constrains crustal buoyancy and residual topography
  2. Residual topography in the western U.S. is significant and highly variable
  3. Joint analyses of VDSS and conventional receiver functions provide further constraints on crustal properties

For more details, please refer to:

Yu, C.Q., Chen, W.-P., Van der Hilst, R.D. (2016), Constraints on residual topography and crustal properties in the western United States from virtual deep seismic sounding, J. Geophys. Res.: Solid Earth 121, doi: 10.1002/2016JB013046.

 

d. VDSS source deconvolution and applications to the Hi-CLIMB seismic array

Original applications of VDSS rely on deep earthquakes as sources of illumination to circumvent strong, near-source scattering (e.g. depth phases) and are, therefore, limited by the uneven distribution of deep seismicity. To extend both the applicability and the quality of VDSS, we developed a method to effectively remove earthquake source signatures. It involves two steps. First, based on analyses of particle motion, we separate 'pseudo-P' and 'pseudo-S' wave trains from the vertical and the radial component of ground motion. The latter is then used as the appropriate reference time-series for the deconvolution of the vertical and the radial component of ground motion.

The method is verified from a series of synthetic tests, and is further validated using data recorded by the Hi-CLIMB array from both deep and shallow earthquakes. Since shallow earthquakes are much more abundant (and geographically distributed more widely) than deep seismicity, the approach presented here greatly extends the applicability of VDSS, including many geologically important regions where crustal isostasy and dynamic topography are yet to be constrained.

For more details, please refer to:

Yu, C., Chen, W.-P., & van der Hilst, R. D. (2013). Removing source-side scattering for virtual deep seismic sounding (VDSS). Geophysical Journal International, 195(3), 1932–1941. https://doi.org/10.1093/gji/ggt359

 

3) Earthquake source parameters

a. Earthquake focal depth determination

Key points:

  1. We determine earthquake depths of the 2010 El Mayor‐Cucapah sequence by modeling Pn depth phases and using a relative location method.
  2. The 2010 El Mayor‐Cucapah earthquake sequence is mainly confined in the depth range between 3 and 10 km.
  3. Most aftershocks are located outside or near the lower terminus of coseismic high-slip patches of the main shock.

Abstract: The 2010 MW 7.2 El Mayor-Cucapah earthquake ruptured a zone of ~120 km in length in northern Baja California. The geographic distribution of this earthquake sequence was well constrained by waveform relocation. The depth distribution, however, was poorly determined as it is near the edge of, or outside, the Southern California Seismic Network (SCSN). Here, we use two complementary methods to constrain focal depths of moderate-sized events (M³4.0) in this sequence. We first determine absolute earthquake depth by modeling regional depth phases at high frequencies (~1 Hz). We mainly focus on Pn and its depth phases pPn and sPn, which arrive early at regional distance and are less contaminated by crustal multiples. To facilitate depth phase identification and to improve signal-to-noise ratio, we take advantage of the dense SCSN and use array analysis to align and stack Pn waveforms. For events without clear depth phases, we further determine their relative depths with respect to those with known depths using differential travel times of the Pn, direct P and direct S phases recorded for event pairs. Focal depths of 93 out of 122 M³4.0 events are tightly constrained with absolute uncertainty of about 1 km. Aftershocks are clustered in the depth range of 3-10 km, suggesting a relatively shallow seismogenic zone, consistent with high surface heat flow in this region. Most aftershocks are located outside or near the lower terminus of coseismic high-slip patches of the main shock, which may be governed by residual strains, local stress concentration, or post-seismic slip.

For more details, please refer to:

Yu, C., Hauksson, E., Zhan, Z., Cochran, E. S., & Helmberger, D. V. (2019). Depth determination of the 2010 El Mayor‐Cucapah earthquake sequence (M≥4.0). Journal of Geophysical Research: Solid Earth, 124(7), 6801–6814. https://doi.org/10.1029/2018JB016982


 

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