preliminary oklahoma optimal fault orientationswichita.ogs.ou.edu/documents/of4-2015/of4-2015... ·...

6N

5

4

3

2

1N

21

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

D E W E Y

C U S T E R

B E C K H A M W A S H I T A

G R E E R

H A R M O N

J A C K S O N

K I O W A

T I L L M A N

C O T T O N

C O M A N C H E

S T E P H E N S

J E F F E R S O N

L O V E

C A R T E R

M A R S H A L L

J O H N S T O N

A T O K A

B R Y A N

C H O C T A W

P U S H M A T A H A

M C C U R T A I N

L E F L O R E

L A T I M E RP I T T S B U R G

C O A L

P O N T O T O C

M U R R A Y

G A R V I N

C A D D O

G R A D Y

C A N A D I A N

B L A I N E

K I N G F I S H E R

M A J O R

A L F A L F A

G R A N T

G A R F I E L D

K A Y

N O B L E

O S A G E

P A W N E E

WA

SH

IN

GT

ON

N O W A T AC R A I G

O T T A W A

D E L A W A R E

M A Y E S

A D A I RC H E R O K E E

M U S K O G E E

S E Q U O Y A H

H A S K E L L

R O G E R S

T U L S A

W A G O N E R

O K M U L G E E

M C I N T O S H

H U G H E S

S E M I N O L E

O K F U S K E E

C R E E K

C L E V E L A N D

M C C L A I N

PO

TT

AW

AT

OM

IE

O K L A H O M A

L I N C O L N

L O G A N

P A Y N E

95°96°97°

98°

34°

99°

35°

100°

36°

37°

103°102°

101°

7

15

16

17

21201918

1S

10S

9

8

6

5

4

3

2

262524232214131211

27E

1E

27

2829N

28E

1E

29N

1N

1W8

19

8

2

1N

11

29N

35°

4

95°

16

17

9

18

23

2

10

14

6

13

22

14

5

17

5

2

4

16

8

11

1816

7

617

11

19

11

10

9

100°

13

25

1224

10

11

22

20

37°

4

18

525W 23

6

27

10

3

4

3

10

17

19

9

15

9

4

19

7

25

18

7

7

102°

14

2

3

16

23

6

22

25W

20

1E

8

19

26

8

3

8

3

10

5

22

21

23

3

18

17 28

1N

27

27

24

21

11

9

7

18

15

24E

23

14

36°

24

22

99°

16 5 217

23

6

96°

24

7

2

17

4

21

12

9

26

22

20

20

3

12

13

14

15

2

21

3

13 1W 4

5

12

34°

101°

6

19

7

98°

15

15

6

1S

14

5

26

7

428

11

25

5

26

1215

2

2

22

18

20

12

13

21

13

13

20

9

16

97°

8

6

24 9

8

14

7

15

20 10

4

5

23

21

19

3

10

6

25

16

24 12

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