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Modeling the Combined Effects of Deterministic and Statistical Structure for Optimization of Regional Monitoring
Vernon F. Cormier; Christopher J. Sanborn; Michele Fitzpatrick; Steven Walsh; Nil Mistry Physics Department, University of Connecticut, Storrs, CT 06269
REFERENCES o Menke, W., Raytrace3d, www.iris.edu/software/downloads/plotting/ o Shearer, P. M., and P.S. Earle, in Advances in Geophysics, Volume 50:
Earth Heterogeneity and Scattering Effects on Seismic Waves, H. Sato and M.C. Fehler (ed.), 2008.
o Sato, H. and M.C. Fehler, 1998: Seismic wave propagation and scattering in the heterogeneous earth, AIP Press
o Kim, Won-Young, Paul G. Richards, Diane Baker, Howard Patton, and George Randall, Improvements to a Major Digital Archive of Seismic Waveforms from Nuclear Explosions, AFRL-RV-HA-TR-2010-1024, Final Report, 23 March 2010
o Bormann, et al. (figure) from: http://ebooks.gfz-potsdam.de/pubman/item/escidoc%3A4015/component/escidoc%3A4016/Chapter_3_rev1.pdf
o Hartse, H.E., Randall, G.E., Anderson, D.N., Phillips, W.S, and Arrowsmith, S.J. (2012). Regional Event Identification Research in Central Asia (LANL).
Verifying a ban on nuclear testing requires global monitoring systems capable of distinguishing between energy released by explosion events and earthquake events. In general, earthquakes create both compression (P) and shear (S) waves. Explosions generate a compression wave in all directions with little or no shear energy.
LOP NOR REGION
ABSTRACT
The differences between earthquakes and explosions are largest in the highest recordable frequency band. The scattering of seismic energy also increases at high frequencies. Numerical techniques for modeling high frequency seismograms in general 3-D structure exist, but are currently computationally expensive at ranges exceeding several 100 km.
DETERMINISTIC STRUCTURE Examples: • Changes in Moho depth • Lateral variation in seismic
velocity
INTRODUCTION
The differences between earthquakes and explosions are largest in the highest recordable frequency band. In this band, scattering of elastic energy by small-scale heterogeneity (less than a wavelength) can equilibrate energy on components of motion and stabilize the behavior of the Lg wave trapped in the Earth's crust. Larger-scale deterministic structure (greater than a wavelength) can still assume major control over the efficiency or blockage of the Lg and other regional/local seismic waves. This project proposes to model the combined effects of the large-scale (deterministic) and the small scale (statistical) structure to invert for improved structural models and to evaluate the performance of yield estimators and discriminants at selected IMS monitoring stations in Eurasia. This will be accomplished by synthesizing seismograms using a radiative transport technique to predict the high frequency coda (>5 Hz) of regional seismic phases at stations having known large-scale three-dimensional structure, combined with experiments to estimate the effects of multiple-scattering from unknown small-scale structure.
STATISTICAL STRUCTURE Example: • fine-scale variations of seismic
velocity, due to material in-homogeneity, small cracks and fissures, etc. This pattern of Gaussian heterogeneity can be parameterized by a scale-length and a strength parameter.
EARTH STRUCTURE EARTH STRUCTURE From a modeling standpoint, we divide Earth structure into two categories, based on the approach used in simulation:
SOFTWARE TOOL: RADIATIVE3D
TEST MODEL Seismic envelope synthetics were produced using Radiative3D and a hypothesized Earth model serving as a simplified representation of the Lop Nor region. The model was constructed of layers of uniform velocity, with interface planes separating the layers. Current functionality in Radiative3D allows these interface planes to take on arbitrary orientation. Depth profiles from CRUST2.0 and elevations and Moho depths at three locations (Lop Nor, MAK, and WUS) were used to locate and orient the planes.
Event Sources: We modeled two event types: an idealized explosion at a depth of 2.0 km and an idealized earthquake at a depth of 42.0 km.
Scattering Parameters:
FUNDED BY: AFRL/DOE Contract No. FA9453-12-C-0207, May 30, 2012 through May 29, 2015 Address correspondence to: [email protected] or [email protected]
Scattering is modeled statistically via a Monte Carlo algorithm based on the scattering amplitude formulation of Sato and Fehler, 1998:
ENVELOPE AND TRAVEL-TIME SYNTHETICS
Scattering amplitudes determine both the directionality of scattering as well as total scattering probability, which translates to a mean-free path between scattering events.
We are developing Radiative3D to be a next-generation tool for synthetics generation in models with complex deterministic and statistical structure. Features include:
Simulates realistic earthquake and explosion point-source radiation patterns, parameterized via moment tensor elements
Propagates rays in full 3D
Complex 3D model structure via tetrahedral grid
Radiative Transport technique tracks energy through model; Can collect energy at virtual seismometers to produce synthetic envelopes and travel time curves
Realistic scattering patterns in full 3D
Realistic reflection/transmission handled at discontinuous interfaces, including P-wave / S-wave conversion
The figure below shows the area we’re currently investigating. Stations MAK and WUS are approximately 8o from Lop Nor.
SCATTERING MODEL
Nu Eps A (km) Sediment Layer: 0.5 0.012 0.25
Crust Layers: 0.5 0.010 0.50 Mantle Layers: 0.5 0.008 1.00
Earthquake Time-Series:
Explosion Time-Series:
T=10.8
T=15.1T=7.0T=3.7T=0.8
T=10.8
T=15.1T=7.0T=3.7T=0.8
Envelope (below) and travel-time (right) synthetics were produced with Radiative3D for an earthquake source and an explosion source in our hypothesized Lop Nor model. Signal is proportional to amplitude-squared and represents phonon energy.
Frequency: 4.0 Hz
Phonons cast: 200 Million
Phonon collection and binning via two linear arrays of 160 simulated seismometers, one along the Lop-Nor to MAK path, the other along Lop Nor to WUS
Compute time: 15.3 hours (earthquake) 7.4 hours (explosion) single-core execution
Hardware: Intel Core i7 CPU at 2.93 GHz; 8 GB RAM
Earthquake Explosion
Earthquake Explosion
WAVEFRONT EVOLUTION
CONCLUSIONS
Radiative transport is a computationally efficient method of synthesizing the very high frequency (>2.0 Hz) seismic wave field where differences between explosion and earthquake sources are largest.
By incorporating both known large-scale and unknown small-scale 3-D structure, radiative transport can be used to predict the behavior of ratios of regional phases along specific paths, the homogenization of source radiation patterns with range, and uncertainties in travel-time picks.
FUTURE WORK:
Completion of planned Radiative3D features, including incorporation of intrinsic attenuation, spatial gradients in velocity, and anisotropy of heterogeneity scale lengths.
Use of Radiative3D to model chosen paths for refinement of attenuation and scattering models in regions of interest.
The time series below represents phonon propagation through a prototype model (not the Lop Nor model) and shows how the wave fronts evolve with time. Red markers represent P phonons and blue markers represent S phonons. Interface reflections, ray-bending, and coda development through scattering are all visible. Transitions between P and S polarization can happen via scattering or reflection/transmission.
