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26W 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1W 1E 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26E

C I M A R R O NT E X A S B E A V E R

H A R P E R W O O D S

W O O D W A R D

E L L I S

R O G E R M I L L S

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103°102°

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103°

SCALE 1:750 0000 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

OPEN-FILE REPORT OF4-2015Preliminary Oklahoma Optimal Fault Orientations

OKLAHOMA GEOLOGICAL SURVEYDr. Jeremy Boak, Director

EXPLANATIONFault Orientation (Azimuth):

Moderately Optimal[15 1–45°, 60°–75°, 90°–105°, 20°–135°]°Sub -Optimal[0°–15°, 75°–90° and 150°–180°]

Optimal[45°–60°, 105°–120° and 135°–150°]

Summary This work is an extension of Holland (2013) and is consid e re d prelim ina ry as our focal m echanism d ataba se and fault d atabas e (Holland , 2015) continue to be upd ated as m ore inform ation becom es available. Six hund re d and eighty-eight focal m echanism s were calculated between 2010 and 2015 and use d to d eterm ine optim ally oriente d fault orientations within Oklahom a. Focal m echanism s cha racterize the d irection of slip and the orientation of a fault and are calculated from seism ic waveform s record e d on seism om eters within and nea r Oklahom a. Id entifying optim al fault orientations (those likely to have an ea rthquake within the contem pora ry stre s s field , N 85°E) is im portant for d eterm ining the potential ea rthquake haza rd of both naturally occurring and triggere d s eism icity. The m ajority of the focal m echanism solutions use d in this publication were com puted using earthquake s occurring in central and north-central Oklahom a, a region where the greatest num ber of ea rthquake s has occurre d . The focal m echanism s use d in this com pilation includ e Regional Mom ent Tensor solutions and first-m otion focal m echanism s (Holland , 2013; Darold et al., 2015). First-m otion focal m echanism s provid e two possible fault plane solutions for every ea rthquake. The se fault plane solutions are re fe rre d to a s nod al plane s. The probability d ensity functions (PDFs) for fault azim uths were calculated with 15° bins for both pos sible nod al plane s as sociated with the obse rve d focal m echanism s. The contem pora ry stre s s, m axim um horizontal stre s s (s m ax) orientation of N 85°E, was d eterm ine d from the orientation of the Pres sure-axis and Tension-axis (P- and hT-axes) of the focal m echanism s use d , following the m ethod of Z oback (1992). Focal m echanism s repre s e nt the stre s s orientation at the hypocenter of the ea rthquake and the stre s s orientation governs the d irection of slip on the fault. W e d eterm ine d a m ean s m ax of 83.2° with a stand a rd d eviation of 21.3° azim uth. The m ed ian hs m ax is 84.8° with 633 observations (Inset 1). h

The PDFs show the m ajority of focal m echanism s expre s s slip on steeply d ipping faults with d ips greater than 75° (Inset 2a). The d istribution of rake is consistent with thes e find ings and shows that strike-slip m otions are the prim a ry source m echanism s (Inset 2b). The PDFs a re d eterm ine d by d ivid ing the num be r of nod al plane orientations in each bin by the total num ber of nod al plane orientations and the num be r of d egree s in each bin. From the PDFs, it is possible to d e fine orientations of optim al, m od e rately optim al and sub-optim al fault strike s (Inset 2c). The focal m echanism d istribution is d om inated by strike-slip m otion on steeply d ipping faults and thus fault strike is re stricted to the range of 0° to 180° (Inset a). Optim al orientation range s between 45°-60°, 105°-120° and 135°-150° and repre s ent fault orientations m ost likely to have an ea rthquake. Mod e rately optim al orientation ranges between 15°-45°, 60°-75°, 90°-105° and 120°-135° and repre s ent fault orientations m od e rately likely to have an ea rthquake. All other orientations of fault strike a re sub-optim al orientation and have a low likelihood to have an ea rthquake. Thes e re sults d o not ind icate that earthquake s cannot occur on sub-optim al fault strike s, but suggest that they are les s likely. ReferencesDarold , A. P., A. A. Holland , J. K. Morris, and A. R. Gibson, 2015, Oklahom a Earthquake Sum m ary Report 2014: Oklahom a Geological Survey Open-File Report, OF1-2015.Holland , A. A., 2015, Prelim ina ry Fault Map of Oklahom a: Oklahom a Geological Survey Open File Report, OF3-2015.Holland , A.A., 2013, Optim al Fault Orientations within Oklahom a: Seism ological Res e a rch Letters, v.84, p.876-890.Z oback, M. L., 1992, First- and second -ord e r patterns of stre s s in the lithosphe re: The world stre s s m ap project, Journal of Geophysical Rese a rch. v.97, p.11,703–11,728. AcknowledgementsI would like to thank all thos e who have worke d , contributed and continue to work on the Oklahom a Fault Databas e includ ing Stephen Marsh, Stephen Holloway, Dr. Rand y Kelle r, Russell Stand rid ge, David Brown, Jacob Hernand ez, and the Oklahom a Ind epend ent Petroleum Association. Fund ing for this project is provid e d by RPSEA through the “Ultra-Deepwater and Unconventional N atural Gas and Other Petroleum Resource s” program authorize d by the U.S. Energy Policy Act of 2005. RPSEA (www.rpse a.org)is a nonprofit corporation whose m is sion is to provid e a steward ship role in ensuring the focuse d re s e a rch, d evelopm ent and d eploym ent of sa fe and environm entally re sponsible technology that can effectively d eliver hyd roca rbons from d om e stic re sources to the citizens of the United State s. RPSEA, operating as a consortium of prem ie r U.S. ene rgy re se a rch universities, ind ustry, and ind epend ent re s e a rch organizations, m anages the program und e r a contract with the U.S. Departm ent of Energy's N ational Energy Technology Laboratory.Open-file Report Disclaimer Open-File Reports are use d for the d is s em ination of inform ation that fills a public nee d and are intend e d to m ake the re sults of re s e a rch available at the ea rlie st pos sible d ate. Because of their nature and pos sibility of being supers e d e d , an Open-File Report is intend e d as a prelim ina ry report not as a final publication. Analyse s pre s ented in this article are base d on inform ation available to the author, and d o not neces s a rily repre s ent the views of the Oklahom a Geological Survey, the University of Oklahom a, their em ployee s, or the State of Oklahom a. The accuracy of the inform ation containe d he rein is not guarantee d and any m ention of tra d e nam e s are not an end ors e m ent by the author, the Oklahom a Geological Survey, or the University of Oklahom a.

Inset 1: Maximum horizontal stress orientations determined from orientation of P- and T-axes of 633 focal mechanisms, following the convention of Zoback (1992); red, normal faulting; green, strike-slip faulting; blue, thrust faulting.

(a)

(b)

(c)

Inset 2: Probability density functions for fault orientations for 688 focal mechanisms within Oklahoma with 15° bin intervals and a total of 1,376 nodal planes; (a) PDF of fault dip, (b) PDF of fault rake, (c) PDF of fault strike.

PRELIMINARY OKLAHOMA OPTIMAL FAULT ORIENTATIONS By

Amberlee P. Darold and Austin A. Holland2015