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California Transverse Mercator projection (CATM) for Building Gridded Seismic Velocity Volumes for Seismic Wave Propagation Simulations

David A. Okaya, Yao-Yi Chiang, Philip J. Maechling, & Mei-Hui Su

Published August 15, 2018, SCEC Contribution #8808, 2018 SCEC Annual Meeting Poster #288

Numerical computation of seismic wave propagation is a major tool within SCEC community research. From CME's high-end CyberShake and Broadband Platforms down to individual researcher needs for synthetic seismograms, all of these require inputs of 3D seismic velocity earth volumes representing regions within California. A standard procedure to create a gridded velocity volume is to use UTM-based coordinates to define uniformly-spaced rectilinear points. This grid is then used to extract from latitude/longitude-based community velocity models such as SCEC's CVM for southern California. A UTM zone is an independent metric rectilinear coordinate frame that is only 6 longitude degrees wide due to transverse Mercator (TM) projection stretch. Zone 11 (centered at -117W longitude) and Zone 10 (-123W) are widely used for southern and northern CA gridding, respectively.

Central California straddles Zones 10 and 11, and statewide California at -114W to -124W spans both zones. In practice, gridding of these cases is performed using one zone's coordinate frame with results subject to projection stretch. We describe a modified, California-centric TM zone that offers gridding that minimizes projection stretch. Defining a local TM zone needs just four parameters: the centering longitude (central meridian, mandatory) and optional reference latitude and false easting/northing. The simplest version of the California zone ("CATM") is to re-center the zone at -120W (zone "10.5"). An advanced version is to move the central meridian and reference latitude to inside California, such as to -120W, 37N ("CATM-Madera"). Other variations are possible. Other regions that straddle UTM zones and have implemented their own zones include Wisconsin (WTM), Idaho (IDTM83), New Zealand (MZTM2000), and Taiwan (TM2).

In our presentation, we review the selection of UTM zones used in recent key SCEC velocity studies in California, provide background information on the UTM system, and determine projection stretch outside normal zone boundaries. We describe how to define the above CATM versions and how it reduces projection distortion compared to using Zones 10 or 11 for central California and state-wide volumes. We show how to easily implement this projection using Proj, a widely-used projection software library. Community discussion can decide if to use a CATM and if so its specific definition. Alternative projection methods exist such as California Albers and may be equivalent or superior.

Key Words
CVM, velocity grids, numerical seismic propagation

Okaya, D. A., Chiang, Y., Maechling, P. J., & Su, M. (2018, 08). California Transverse Mercator projection (CATM) for Building Gridded Seismic Velocity Volumes for Seismic Wave Propagation Simulations. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Community Modeling Environment (CME)