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SCEC Community Thermal Model (CTM)

CTM WORKING GROUP
CXM Representative
Elizabeth Hearn
SCEC Software Team
Mei-Hui Su
Phil Maechling
 
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Figure 1: Southern California heat flow map, showing CTM heat flow regions (outlined in black), mean surface heat flow in each heat flow region (HFR), and point measurements from wells (Williams and DeAngelo, 2011). 

Overview

The SCEC CTM provides estimates of temperatures and thermal properties of the southern California lithosphere.  Heat flow data used in this study come from Williams and DeAngelo (2011).The preferred CTM, described by Thatcher and Chapman (2020), comprises a suite of geotherms for fourteen distinct heat flow regions (HFR’s).  Details of the calculations and assumptions are included in Thatcher and Chapman (2020). An alternative thermal model developed for southern California by Shinevar et al. (2018) is described below and also provided at the download link. Others will be made available as they are supplied by the research community. Details on this page pertain to the preferred model (hereafter referred to as “the CTM”), unless otherwise noted.

CTM data include longitude-latitude coordinates of the HFR boundaries, and within each HFR, temperatures as a function of depth at 1 km increments from the surface to 100 km. Values of parameters used in the CTM geotherm calculations (e.g. surface heat flux and LAB depth and radiogenic heat production) are also provided. CTM metadata includes descriptions of heat flow data, thermal and heat-generation parameters assumed in the modeling, and references. CTM software includes a Jupyter notebook query tool for the CTM that optionally smooths lateral discontinuities in temperature between adjoining HFR’s using Gaussian kernels, and queries the smoothed model for temperature given position (latitude, longitude, and depth in km). CTM data, metadata, tools, and documentation in the form of README files are available from the download link in the sidebar.

An unsmoothed version of the CTM and the Shinevar et al. (2018) temperatures resolved onto a pre-set grid are also available via the SCEC Geologic Framework query tool, accessible via a link in the sidebar.

Research Priorities

  1. Deliver a preliminary CTM that provides temperatures and uncertainties throughout the southern California lithosphere and asthenosphere consistent with radiogenic heat production. ​In coordination with SCEC and the CXM group leaders, s​hare the CTM and query tools via (this) web page linked to the SCEC CXM web page.
  2.  Improve/interpolate surface heat flow maps. Search for additional heat flow and thermal property data in areas with poor coverage. 
  3. Make an in-depth assessment of published seismic tomography results to constrain southern California mantle wave speed and temperature. 
  4. Add community-submitted models, community assessment of uncertainties, and testing and refinement of the CTM.

CTM: Thatcher and Chapman, 2020

As noted above, the CTM comprises a suite of geotherms for fourteen distinct heat flow regions (HFR’s, see figure at the top of this page). The HFR’s are subjectively defined as areas within which surface heat flow is nearly constant. Since vertical temperature gradients are typically much larger than horizontal gradients it is often useful to approximate a regional 3D thermal field with a single temperature-depth profile (geotherm). Accordingly, one geotherm is provided per HFR. 1D steady-state thermal conduction models are applied to downward continue the mean surface heat flow for each HFR and calculate geotherms (e.g. Chapman, 1986). The intersection of the geotherm with an assumed asthenosphere melting curve then determines the depth of the thermal lithosphere-asthenosphere boundary (tLAB). Below this depth the mantle is convecting and temperature increases modestly with depth along a mantle adiabat. Thermal conductivity and radiogenic heat generation are based on values given in Chapman (1986). As noted above, surface heat flow data are from Williams and DeAngelo (2011).

For six HFR’s with high mean surface heat flow values (WBR, MD, LA, WTR, CCR, iCB), CTM geotherms have been computed assuming a simple, steady-state conduction model with the seismic LAB depth equal to the thermal LAB depth. For six HFR’s with low mean surface heat flow values (40-68 mW/m2; SN, GV, VB, SG, PR and ETR), the surface heat flow and seismic LAB depth are inconsistent with steady-state 1D thermal conduction. In each of these regions there is independent evidence from seismic imaging of lower lithosphere detachment and transient heat flow calculations have been required to reconcile surface heat flow with lithosphere thickness and other observations. For two remaining provinces (oST, iST), lithosphere thinning with sedimentation, and magmatic underplating must be accounted for in the heat flow models.

Figure 2. CTM Temperatures at 25 km depth. Unsmoothed CTM temperatures are on the left and smoothed CTM temperatures are on the right. Lateral smoothing and other figure properties may be customized using the Jupyter notebook available at the download link.

Alternate CTM: Shinevar et al., 2018

Figure 3. Temperatures at 25 km depth from Shinevar et al. (2018).
A thermal model for the southern California crust was developed by Shinevar et al. (2018) as part of their study estimating lower crustal effective viscosity throughout southern California. This model assumes steady-state, 1D (vertical) heat conduction throughout the region, and relies on heat flow measurements from Williams and DeAngelo (2011) and the SMU Geothermal Database. A natural neighbor scheme is used to interpolate the heat flow data to the same grid as the SCEC CVM S4.26 seismic velocity model. Temperature as a function of depth is calculated at each grid point assuming a steady-state 1-D geotherm following the assumption that the heat production in the upper crust accounts for 40% of the measured surface heat flow (Pollack and Chapman, 1977). Heat production in the upper crust (<15 km depth) is assumed to be constant within this depth interval. (Shinevar et al. state that varying the distribution of heat production in the upper crust does not have a large impact on estimated lower crustal temperatures.) A constant lower crust heat production of 0.5 μWm-3 based on average values for exposed lower crustal rocks in the Sierra Nevadas (Brady et al., 2006). Thermal conductivity is taken to be a function of temperature (Durham et al., 1987, and EQ 8 in Shinevar et al., 2018).
 
To smooth horizontal temperature gradients, the temperature field is allowed to diffuse laterally for five million years, assuming constant boundary temperatures and a thermal diffusivity of 10-6 m2 s-1. Fig. 3d of Shinevar et al. (2018; shown above) displays the resulting temperature field at 25 km depth. 
 
The download link in the sidebar includes an ascii text file (Shinevar_2018_Temperature.txt) with region-wide crustal temperatures at the GFM viewer grid points. The region extending from 31 to 37 degrees latitude and -121 to -112 degrees longitude is discretized into a 100 by 101 matrix of equally-spaced points, and at 0.5 km depth intervals from 0 to 35 km depth (for a total of 717,101 points). The MATLAB code used to generate these temperatures is also included for users who want temperatures over a subregion and/or with different discretization. Please credit the Shinevar et al. (2018) reference below when making use of this temperature model. 

Data Products

Download SCEC CTM v20.8

CTM Data Product Details

Folder Name Contents
Components_and_metadata
  • lon-lat files defining HFR boundaries 
  • CTM geotherms for each HFR 
  • Mean surface heat flow and uncertainty for each HFR; mean LAB thickness and uncertainty for each HFR
  • Lithosphere thermal properties and other model parameters (also, see Thatcher and Chapman, 2020). 
Shinevar_T_info Shinevar et al. (2018) temperatures at uniform grid coordinates used for the SCEC GFM viewer, MATLAB code.
CTM_Jup_Notebook Notebook with python query and plotting code for the CTM (CTM_tools.ipynb) and data files (in subdirectory For_CTM_Notebook)

TBA: Community-supplied software tools, e.g. in other languages or with added capabilities. Contribute by contacting Liz Hearn and CC software@scec.org.

References

Selected Publications