RESEARCH
HILIGHTS
Segmentation of the Alaska subduction and volcanic activity - Alaska margin
Crustal modification during continental rifting - Hartford Basin
Uplift of cratonic mountains - Adirondack Mountains
Subsidence of cratonic basins - Illinois Basin
DETAILS
Ongoing:
Seismic imaging to examine the evolution of the North American craton
Imaging of the Alaska subduction zone with amphibious data
Investigating the seismic signature of groundwater variations
Seismic monitoring of active volcanoes
2019-2021: Characterization of seismic wavefields and the ground motion of future megathrust earthquakes on the Cascadia subduction zone.
This study investigates the earthquake ground motions along the Cascadia subduction margin. Using virtual earthquakes from seismic ambient noise cross-correlations, we found that the strongest ground motions occur mainly at the Seattle Basin, the Portland Basin, and the near coast offshore region along the accretionary wedge. Check here for details.
2018-2019: Crustal modification during terrane accretion at the eastern North American margin (southern New England).
The eastern North American margin has undergone two complete Wilson cycles of assembly and breakup of the supercontinents, together with terrane accretion, rifting, and post-rift evolution. It is not clear how the continental crust has been modified by these tectonic processes. Southern New England, which is composed of a few tectonic terranes, is an ideal place to study the crustal modification associated with the tectonic activities inferred from surface geology. The deployments of the EarthScope Transportable Array (~70-km spacing) in the northeastern United States and the SEISConn (Seismic Experiment for Imaging Structure beneath Connecticut) array (~ 10-km spacing) have provided unprecedently dense coverage of seismic data for southern New England. Using full-wave ambient noise tomography, we aim to address the following scientific questions:
What are the seismic characteristics associated with tectonic activities at the margin?
What is the mechanism of the Hartford basin and the implication on rift evolution at passive margins?
2017-2019: Subduction dynamics and volcanic activity along the Alaska margin.
The Alaska/Aleutian subduction margin is one of the most active convergent plate boundaries, with prominent variations of seismogenic properties, volcanic activities, and lithospheric deformations observed along strike and down dip. However, the detailed characteristics and origin of the segmentations are not well understood. For example:
How does the subducting slab vary along strike and dip?
What controls the segmentation of seismicity and magmatism?
What is the velocity property of the mantle wedge and how does it vary along the strike?
To address these questions, we have constructed a high-resolution shear velocity model for the crust and upper mantle in Alaska, using full-wave ambient noise tomography. We utilize an integrated data set from multiple seismic networks in Alaska, including the EarthScope Transportable Array and many permanent and flexible deployments.
2017-2018: Quality analysis of empirical Green's functions from Ocean Bottom Seismometers in Cascadia.
“How is the data quality of empirical Green’s functions from OBSs influenced by periods, stack duration, instruments, water depth, sediment thickness, and other physical properties?”
The deployment of ocean bottom seismic arrays has facilitated significant data sets for the science community in imaging the seismic velocity structures and understanding the lithosphere/mantle dynamics. Surface wave tomographic methods, using data from ambient noise cross-correlations, have been commonly implemented to image the crust and upper mantle structure. The quality of the data sets is fundamental to ensure reliable velocity images. In this study, we conduct a comprehensiveand quantitativeanalysis of the signal-to-noise ratio, as a proxy of data quality, of empirical Green’s functions, to understand how the waveform quality of ocean bottom seismometers is influenced by a few factors. Factors considered in this study include: site conditions and the types of instruments. We integrate all OBS data between 2011 to 2015 from the Cascadia Initiative, the Gorda Deformation Zone experiment, the Blanco Transform Fault experiment, the Neptune Canada array, and land stations. The result has been published on SRL (Jan 2019; https://doi.org/10.1785/0220180273).
2016-2017: Full-wave ambient noise tomography in the northeastern U.S. (New England)
The formation of the Adirondack Mountains in northeastern New York state has been mysterious to scientists. A few mechanisms have beenproposed, though deep structural constraints on evaluating those hypotheses are limited. Using an advanced seismic imaging method, we have constructed a detailed model of the lithosphere (the crust and mantle parts of the tectonic plates) in the northeastern United States, down to the depth of about 100 km. We discovered a pillow of anomalously low seismic speed with a diameter of ~70‐100 km, at the depths of 50‐85 km beneath the Adirondack Mountains. This low‐velocity column is connected, at greater depths, with the broader low‐velocity anomaly beneath the southern New England and eastern New York region. These low-speed features may have resulted from the rising of the asthenosphere, a weak layer beneath the lithosphere. The upward force of the upwelling asthenosphere flow, together with possible thermal expansion, may have provided the mechanisms that formed the Adirondack Mountains. Results from this study have been published in GRL in June 2018 (Yang and Gao, 2018).
Schematic illustration of the lithosphere structure in the northeastern United States, highlighting the formation (uplift) of the Adirondack Mountains. The thick red arrows depict the proposed asthenosphere flows based on our seismic tomographic imaging, illustrated by the red volume.
Animation caption. A 3-D view of the volume of low seismic shear-wave speed below 4.4 km/s in the northeastern United States in red color. The translucent color image shows the depth of the crust-mantle boundary in kilometers. Outline of the Adirondack Mountains (ADM; dotted red line) and the boundary (solid line) between the relatively stable old North American continent and the younger Appalachian accretionary terranes are shown for references. The triangles are locations of the seismic stations used to generate the velocity model. There is about 5 times vertical exaggeration.
2014-2016: Seismic imaging of crustal structures in Illinois Basin, Central U.S. using P-wave receiver functions
We present high-resolution imaging results of crustal and upper mantle velocity discontinuities across the Illinois Basin area using both common conversion point stacking and plane wave migration methods applied to P wave receiver functions from the EarthScope Ozark, Illinois, Indiana, and Kentucky experiment. The images reveal unusually thick crust (up to 62 km) throughout the central and southeastern Illinois Basin area. A significant Moho gradient underlies the NW trending Ste. Genevieve Fault Zone, which delineates the boundary between the Illinois Basin and Ozark Dome. Relatively thinner crust (<45 km) underlies most of the Precambrian highlands surrounding the Illinois Basin and beneath the rift-related structures of the Reelfoot Rift and the Rough Creek Graben. We consider four hypotheses to explain the presence of thick crust under the central and southeastern Illinois Basin. Crustal thickening may have been produced (1) prior to its accretion to North America around 1.55 Ga and is an inherited characteristic of this crustal province; (2) by underthrusting or shortening during Proterozoic convergent margin tectonics around 1.55–1.35 Ga; (3) by Late Precambrian magmatic underplating at the base of older crust, associated with the creation of the Eastern Granite-Rhyolite Province around 1.3 Ga; and (4) through crustal “relamination” during an episode of Proterozoic flat-slab subduction beneath the Illinois Basin, possibly associated with the Grenville Orogeny. Published on JGR (July 2017). See following animation slicing through the image volume.
Moho depth maps the NSF funded EarthScope OIINK (Ozark, Illinois, Indiana and Kentucky) project, showing the strongly varied crustal thicknesses in the south-central Illinois Basin region. (a) Plane-Wave Migration result using OIINK P-wave receiver functions. (b) Common Conversion Point stacking result using P-wave receiver functions recorded by other regional networks. See Figure 2 in the main text for names (abbreviations) and locations of major structures.
2014-2015: Receiver function quality control
An automated quality control method is developed, which consists of 13 procedures in three groups, based on deconvolution attributes, characteristics of an individual trace, and statistics of the station gather. The effectiveness of these procedures has been tested through applications to OIINK dataset for Illinois basin and USArray dataset for the contiguous United States. The quality control procedures have the advantages of generating reproducible and accurate results with high adaptability and efficiency. It benefits any seismic processing of teleseismic P-wave receiver function, especially the processing of large volume seismic data. The method is implemented as RFeditor, a C++ program with GUI option. Download the source code for this program at https://github.com/xtyangpsp/RFeditor. The method is published on BSSA in 2016.
2012-2014: Seismicity of the Ste. Genevieve Seismic Zone in southern Illinois Basin
The seismicity analysis focuses on local earthquake records from July 2011 to June 2012. The presence of the dense OIINK stations lowers the earthquake detection limit from magnitude 2.3 to about 1.8 within the Ste. Genevieve seismic zone. Earthquakes in the seismic zone typically occur from the surface to a depth of 22 km. This depth range is contrasting with that of the New Madrid seismic zone, where most earthquakes are located approximately between the depth of 5 km to 12 km. Focal mechanism analysis indicates that seismicity in the Ste. Genevieve seismic zone is a response to the NE-SW regional stress regime. The seismicity rate in the Ste. Genevieve seismic zone is a factor of 3 to 4 times lower than that of the New Madrid seismic zone and is comparable to that of the neighboring Wabash Valley seismic zone. The result is published on SRL in 2014.
