Seismology Research Potential for USArray CPEP Sites

Kelly Liu

An Earth Science Program funded by the NSF and other agenciesto Explore the Structure and Evolution of the North American Continent and Understand Processes Controlling Earthquakes and Volcanoes.

EarthScope

San Andreas Fault Observatory at Depth (SAFOD)

Plate Boundary Observatory (PBO)

Seismic Observatory (USArray)

Source: http://www.earthscope.org

USArray

-A continental –scale seismic observatory

Transportable Array (“Bigfoot”): 400 broadband seismometers; 70-km gridFlexible Array: 291 broadband, 120 short period, and 1700 active-sourceReference Network: ANSS backbone (~70 seismic stations) Magnetotelluric Facility: 7 permanent, 20 portable sensors

Each phase of the “Bigfoot” lasts for two years, and the network will completely cover the US in a 10-year period.

USArray Current Status

Source: http://www.earthscope.org

At each USArray TA station

Sensor: 0.01 -40 Hz; STS-2

Data acquisition system

Telemetry

Power system

GPS

http://www.iris.edu

Existing Permanent Broadband Stations in the 4-state Region

Existing/existed Broadband Stations (with accessible data) in the 48 states

http://www.iris.edu (Incorporated Research Institutions for Seismology)

The major reason of the uneven distribution:

difference in seismic activities

All the mag >0 quakes in the ANSS catalog: 1800-2007

Yet USArray deals with much more than earthquake distribution …

The USArray component of the EarthScope experiment is a continental-scale seismic observatory designed to provide a foundation for integrated studies of continental lithosphere and deep Earth structure over a wide range of scales. USArray will provide new insight and new data to address fundamental questions in

earthquake physics, volcanic processes, core-mantle interactions, active deformation and tectonics, continental structure and evolution, geodynamics, and

crustal fluids

Bouguer gravity map and major tectonic features of the Central Plains and adjacent areas.

Blue lines separate crustal blocks with different ages, which become progressively younger toward the south.

The permanent seismic network would enable a significantly improved understanding about the formation, evolution, and structure of these tectonic features and their roles in the formation and distribution of earthquakes in the Central Plains. Major features to be studied by the

CPEP permanent seismic network:MCRBoundaries of tectonics unitsNMSZ

Expected major seismological products from the CPEP permanent stations: Receiver function studies

Fundamentals of Receiver Functions

The converted phases at the interface

Mostly based on http://eqseis.geosc.psu.edu/~cammon/HTML/RftnDocs/rftn01.html

Stacking of RFs

The converted phases are very weak. In order to enhance the signals, various stacking techniques have been used.

The current study uses the approach of Zhu and Kanamori (2000)

Basically, we search for the optimal combination of crustal thickness (H) and crustal Vp/Vs that gives rise to the maximum stacking amplitude.

To account for the differences in the angle of incidence of the seismic raypath, the RFs are “moveout-corrected” using the formulas on the right before stacking (formulas )

Nair, Gao, Liu, Silver, 2006 JGR

Crustal thickness and Poisson’s ratio beneath the stations (station averages)

Depth variation of major velocity discontinuities in the mantle (at 410, 520, and 660 km)

Preliminary result of crustal thickness and Vp/Vs ratio beneath station CCM in eastern Missouri

To study azimuthal variation of P-to-S converted phases from the Moho and major mantle discontinuities provides critical information about the geometry of the discontinuities

Azimuthal variation of receiver functions from about 350 high-quality teleseismic events over a 16-year period (about 20 per year), recorded by station CCM in eastern Missouri

Preliminary results of non-linear stacking of the receiver functions at CCM reveal azimuthal variation in crustal

thickness and depths to the 410 and 660

Many events are needed to enhance the weak P-to-S converted phases. Thus some stations should be converted to permanent.

Expected major seismological products from the CPEP permanent stations: Seismic tomography

Seismic Tomography

--uses seismic waves with different ray paths to map three-dimensional velocity structure in the earth.

http://www.earth.northwestern.edu/people/seth/202/lectures/Seismology/seistomo.htm

Most seismic tomography techniques require a large number of ray-paths from different azimuths

van der Lee and Frederiksen, 2005

Velocity structure of the mantle from seismic tomography

http://www.victoria.ac.nz/geo/research/tectonics/seismic-anisotropy/index.html

Two parameters used to describe shear-wave splitting:

Fast direction: the polarization direction of the fast wave

Splitting time: the arrival time difference between the fast and slow waves. Global average is 1 second.

Expected major seismological products from the CPEP permanent stations: Shear-wave splitting

1. Vertical coherent deformation: Fast directions are parallel to mountain belts

Possible causes of mantle anisotropy

2. Asthenospheric flow: Fast directions are parallel to flow direction

Modified from Silver, 1996, Annu. Rev. Earth Planet. Sci.

3.Vertical magmatic dikes: fast direction parallel to strike of dikes

SKS splitting

A two-year project funded by NSF(1/2008-12/2009)

Data from all the portable and permanent network stations for Mag >= 5.5 events and in the epicentral range of 810 – 1400 were obtained from the IRIS DMC.

The method of Silver and Chan (1991) was used to measure the splitting parameters by minimizing the energy on the corrected transverse component.

Original and corrected radial and transverse components, particle motion patterns, and error function contour.

http://cache.eb.com/eb/image?id=347&rendTypeId=4

Horizontal slices at three different depths showing azimuthal anisotropy in the NA upper mantle. Model was obtained by joint inversion of surface waveforms and SKS splitting measurements.

Marione and Romanowicz [2007, Nature]

A fundamental difference in anisotropy between the western NA orogenic zone and the “stable” interior of the NA craton:

Most part of the west has a single layer of anisotropy (or the fast directions hardly change in the top 300 km).

In the stable part of NA, the fast directions are mostly N-S in the top layer, and are NE-SW in the lower layer (which is parallel to the APM direction).

First-order features of mantle anisotropy

100

200

300

Shear-wave splitting parameters

Preliminary results of an ongoing effort to produce a coherent shear-wave splitting database for North America using broadband data sets

Funded by NSF Geophysics Program

Good azimuthal coverage (which requires a long recording time) is needed to resolve two-layer anisotropy

Marone and Romanowicz, Nature, 2007

Results at TX31 (western TX) using 5-years of data

For instance, more permanent stations may be able to confirm the possibility that there are active fault in SE MO and NE AK

Expected major seismological products from the CPEP permanent stations: Detecting small earthquakes

Where does the $$ come from?Successful Examples

Institutions in the State of Arizona (AGS, UofA, ASU) submitted a proposal to FEMA and received $1.5M to convert 20 or so TA stations to permanent (including paying the maintenance fee for 3 years)

UT-Austin and Baylor University have received donations from Oil and other companies to convert 20

Oregon has received multiple funding (mostly from the university) to convert 5-10

Summary

Many geo-aspects of the Central Plains are poorly studied

USArray will fill many gaps Given the limited time span (2-years) of USArray, many

fundamental questions related to the formation, evolution, and structure of the Earth’s continents cannot be addressed by USArray TA

Converting some of the USArray TA stations into permanent ones will eliminate the drawbacks of USArray

The CPEP permanent seismic network will be an excellent facility for long-term research, as well as for educations at all the levels and for public outreach.