Geologic and structural controls on rupture zone ...

Research Paper

GEOSPHERE GEOSPHERE; v. 11, no. 3 doi:10.1130/GES01078.1 17 figures; 1 table; 5 supplemental files

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Geologic and structural controls on rupture zone fabric: A fieldbased study of the 2010 Mw 7.2 El Mayor–Cucapah earthquake surface rupture Orlando J. Teran1, John M. Fletcher1, Michael E. Oskin2, Thomas K. Rockwell3, Kenneth W. Hudnut4, Ronald M. Spelz5, Sinan O. Akciz6, Ana Paula Hernandez-Flores1, and Alexander E. Morelan 2 Departamento de Geologia, Centro de Investigacion Cientifica y de Educacion Superior de Ensenada, Carretera Tijuana-Ensenada No. 3918, Zona Playitas, Ensenada, Baja California 22860, México Department of Earth and Planetary Sciences, University of California, Davis, One Shields Avenue, Davis, California 95616-8605, USA Department of Geological Sciences, San Diego State University, 5500 Campanile Drive, San Diego, California 92182, USA 4 U.S. Geological Survey, 525 & 535 S. Wilson Street, Pasadena, California 91106-3212, USA 5 Universidad Autónoma de Baja California, Facultad de Ciencias Marinas, Carretera Tijuana-Ensenada No. 3917, Zona Playitas, Ensenada, Baja California 22860, México 6 Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, 595 Charles Young Drive East, Los Angeles, California 90095, USA 1 2

CORRESPONDENCE: [email protected] CITATION: Teran, O.J., Fletcher, J.M., Oskin, M.E., Rockwell, T.K., Hudnut, K.W., Spelz, R.M., Akciz, S.O., Hernandez-Flores, A.P., and Morelan, A.E., 2015, Geologic and structural controls on rupture zone fabric: A field-based study of the 2010 Mw 7.2 El Mayor– Cucapah earthquake surface rupture: Geosphere, v. 11, no. 3, p. 899–920, doi:10.1130​/GES01078.1. Received 24 May 2014 Revision received 27 January 2015 Accepted 20 April 2015 Published online 13 May 2015

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ABSTRACT

INTRODUCTION

We systematically mapped (scales >1:500) the surface rupture of the 4 April 2010 Mw (moment magnitude) 7.2 El Mayor-Cucapah earthquake through the Sierra Cucapah (Baja California, northwestern Mexico) to understand how faults with similar structural and lithologic characteristics control rupture zone fabric, which is here defined by the thickness, distribution, and internal configuration of shearing in a rupture zone. Fault zone thickness and master fault dip are strongly correlated with many parameters of rupture zone fabric. Wider fault zones produce progressively wider rupture zones and both of these parameters increase systematically with decreasing dip of master faults, which varies from 20° to 90° in our dataset. Principal scarps that accommodate more than 90% of the total coseismic slip in a given transect are only observed in fault sections with narrow rupture zones (0.5m) EMC Minor (90% of total coseismic slip was typically accommodated by a single principal displacement scarp (yellow arrows). Total coseismic slip of 2.14 m (dextral slip of 1.89 m and vertical slip of 0.93 m) was measured at the red arrow (Fletcher et al., 2014). Photo azimuth is ~186°. (B) The moderately dipping sections of the Borrego fault show a dramatic change in rupture zone fabric, where 2010 EMC slip is widely distributed among numerous subparallel and/or anastomosing fault scarps (yellow arrows) throughout its damage zone (Dz; >130 m wide). A principal displacement scarp that accommodated >60% of 2010 EMC slip was generally not observed, and total coseismic slip (lateral:vertical slip ratio of ~1:1) of as much as 3 m was accommodated in this section (Fletcher et al., 2014). PSD—Paso Superior detachment. Photo azimuth ~265°. (C) Field photograph of the Paso Superior detachment showing the 2010 EMC rupture (yellow arrows) distributed throughout its wide fault zone (~130 m) and hanging-wall Tertiary fanglomerates (Tf). This section of the Paso Superior detachment dips ~37° (Fletcher et al., 2014), and its damage zone is composed of layered metasedimentary rocks derived entirely from its footwall protolith. Upper and lower limits of fault zone are mapped as black lines with single bar and double bar ornamentations, respectively. In the 2010 EMC event, this section accommodated as much as 1.3 m of total coseismic slip with a lateral:vertical ratio of ~1:2 (Fletcher et al., 2014). Photo azimuth is 330°. Photos A, B, and C taken by John M. Fletcher on 11 August 2010, 6 April 2010, and 14 August 2010, respectively. See Supplemental File 55 for photo information in Google Earth KMZ format. Photo locations shown in Figure 2.

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SUPPLEMENTAL FILE 3. TABLE OF FAULT SECTION GEOMETRY AND RUPTURE ZONE WIDTH. Segment*

Fault

Section

Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior Paso Inferior

Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Paso Superior Unnamed Unnamed Unnamed Unnamed Unnamed Unnamed Unnamed Unnamed Unnamed Unnamed Unnamed Unnamed Unnamed Unnamed Unnamed Unnamed Unnamed Borrego Borrego Borrego

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

Lithologic Class† bs bs bs bs bs bs bs bs bs bs ss ss ss ss ss ss ss ss ss ss ss ss ss ss ss ss ss ss ss ss ss ss ss bs bb bb ss

Strike

Dip

Area (m2)

348 334 347 330 301 313 329 307 329 319 332 351 333 338 334 342 22 145 200 174 144 159 170 298 129 151 151 177 164 184 326 300 317 311 352 322 5

37 55 41 20 21 37 32 39 36 32 44 37 39 40 40 40 43 65 65 65 66 63 58 65 65 65 65 65 65 65 45 48 48 48 75 75 75

310758 53370 62916 229202 754093 96947 69993 14672 18686 60173 43121 62712 219441 138065 180360 171071 86880 79131 14934 9714 117116 56177 45324 5563 74432 83893 38816 26889 18196 21697 913318 23580 19795 55312 23691 8555 18594

Max. RZ FZ Length RZ Map No. of Thickness Thickness (m) Width (m) Scarps (m) (m) 978 1047 765 700 1099 512 485 542 597 543 301 276 587 371 442 582 506 777 316 634 1110 735 797 424 728 550 268 686 524 581 2659 400 477 516 455 639 660

7 4 12 10 10 12 8 8 7 8 6 14 10 8 12 4 3 6 4 4 9 10 8 11 12 6 9 7 4 2 11 8 4 3 1 2 3

318 51 82 328 686 189 144 27 31 111 143 228 374 372 408 294 172 102 47 15 105 76 57 13 102 153 145 39 35 37 343 59 42 107 52 13 28

132 30 42 129 N.R.§ 98 26 13 21 64 97 121 223 235 262 192 105 91 42 13 93 66 47 12 91 135 127 35 31 33 N.R.§ 45 33 84 N.R.§ 13 27

159 157 155 154 127 116 110 110 110 110

MidPt_X

MidPt_Y

618981.4093 619316.6214 619633.988 619893.9239 620541.2224 621200.8308 621511.39 621850.673 622222.026 622555.7365 622805.1441 622898.2576 623052.4838 623253.306 623418.9124 623605.4981 622987.057 624730.5744 624900.583 624880.0469 625075.4267 625698.8546 625896.0898 625752.5644 626245.397 626662.6782 626937.5532 626814.1687 626901.1919 625652.6501 626695.951 627372.7377 627707.4527 628063.0635 627846.6357 628072.9479 628020.3087

3607604.267 3606656.59 3605814.14 3605137.91 3604554.202 3604098.454 3603714.356 3603341.185 3602922.057 3602462.193 3602124.841 3601856.615 3601458.503 3601024.066 3600653.523 3600178.46 3602322.981 3599787.048 3599322.664 3598859.341 3600880.975 3599968.253 3599231.792 3598948.384 3598609.149 3598139.836 3597812.502 3597557.674 3596962.452 3598572.475 3596231.582 3595600.285 3595324.864 3594979.372 3597168.782 3596690.136 3597600.502

Supplemental File 3. Table of fault section geometry and rupture zone dimensions defined in this study. Please visit http://dx.doi.org/10.1130/GES01078.S3 or the article on www​.gsapubs​.org to view Supplemental File 3. 3 

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Rupture Zone Thickness and Distribution of Coseismic Slip One of the most important parameters that is correlated with rupture zone thickness is the distribution of coseismic slip, which we measured as the number of fault scarps across a given rupture zone as well as the relative magnitude of slip that each accommodated. In order to systematically characterize the distribution of coseismic slip, all scarps were classified into four categories based on relative magnitude of total coseismic slip as reported by Fletcher et al. (2014; Supplemental File 1 [see footnote 1]): these include >90%, 60%–90%, 30%–60%, 90%

Largest Displacement Scarp (% of total coseismic Slip)

Figure 6. (A) Variations in rupture zone thickness versus the maximum (Max.) number of scarps. Enveloping surfaces show general positive trend for the different fault classes. (B) Variations in rupture zone thickness versus largest displacement scarps measured across individual fault sections. The radii of symbols are proportional to fault section length and are classified based on the rock types juxtaposed at the surface. Data are in Supplemental File 3 (see footnote 3).

ied section of the Paso Superior detachment (fault section 16; Fig. 2B), where much of the coseismic surface deformation is accommodated by penetrative shear and warping of the surface instead of discrete scarps (discussed herein). In general, fault sections of the basement-basement fault class are clustered at the low end of the range of observed values for the number of scarps in a given transect (Fig. 6A), and with few exceptions basement-basement faults typically do not exceed 50 m in width. Although the fields of basement-sediment and sediment-sediment rupture zones overlap significantly, there is a crude separation such that for the same number of scarps, sediment-sediment rupture zones are thicker than those found in basement-sediment fault sections (Fig. 6A). The expected tendency of focused coseismic slip occurring in narrow rupture zones is clearly documented by the fact that single principal scarps that accommodate >90% of coseismic slip are only observed in the narrowest rupture zones (~25 m wide; Fig. 6B). These rupture zones would be considered by most to have an extremely localized slip distribution (Heermance et al., 2003; Rockwell and Ben-Zion, 2007). As the rupture zone widens the largest scarps systematically accommodate a relatively smaller amount of the total coseismic slip measured in any given transect (Fig. 6B).

Rupture Zone Thickness and Master Fault Dip Our data clearly demonstrate that rupture zone thickness systematically increases with decreasing dip of the master fault, and faults with shallower dips have more variation in rupture zone thickness (Fig. 7A). Both fault dip and rupture zone thickness vary systematically with lithologic class of the master faults in the Sierra Cucapah. In general, basement-basement faults are steeply

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Supplemental File 4. Google Earth KMZ file containing the geologic mapping of the master fault zones that ruptured in the 2010 El Mayor–Cucapah earthquake. The mapped extents were used to derive the fault zone thickness. The KMZ file can be viewed in Google Earth. Please visit http://dx.doi.org/10.1130​ /GES01078.S4 or the article on www.gsapubs.org to view Supplemental File 4. 4

Downloaded from geosphere.gsapubs.org on June 14, 2015 dipping (>70°), whereas basement-sediment faults generally dip 90% or 60%–90% classes (Figs. 5A, 6B, and 8A). Master faults like the Paso Superior fault and sections of the Borrego fault have wide, complex fault zones composed of multiple zones of high shear strain, and these faults also typically have more broadly distributed coseismic slip through wider rupture zones (Figs. 5B, 5C, and 8A). Nonetheless, our data demonstrate that the thickness of the fault zone in any given section is systematically greater than

A

Lithologic Classes Basement Basement/Sed Sediments 1 km

B

fault zone > rupture zone

200 150 100 50 0

0

50 150 200 100 Rupture Zone Thickness (m)

250

0

10

20

30

40 50 60 Fault Dip (°)

70

80

90

Figure 8. Radii and classification of symbols are as in Figure 6. (A) Plot of fault zone thickness versus master fault dip. (B) Plot of fault zone thickness versus rupture zone thickness. Vertical bar symbols indicate error in fault zone thicknesses where the master fault has been cut by adjacent faults, such as observed along the Paso Superior detachment. Data from Supplemental File 3 (see footnote 3). See also Supplemental File 4 (see footnote 4) for a KML file containing the mapped extent of fault zones.

the thickness of the surface rupture zone (Fig. 8A). Owing to the strong correlation of fault zone and rupture zone thickness, we find that both of these parameters are inversely correlated with the dip of the master fault (Figs. 7A and 8B). The structural complexity and thickness of fault zones are commonly thought to be related to fault rock rheology as well as the amount of finite displacement the fault has accommodated (Chester and Logan, 1986; Scholz,

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Downloaded from geosphere.gsapubs.org on June 14, 2015 1987; Ben-Zion and Sammis, 2003; Faulkner et al., 2003, 2008). Our data show that there are also systematic variations in fault zone architecture with fault orientation. Even if all faults were subjected to the same regional stress state, individually they would have undergone tractions that would have been very different in both overall magnitude as well as the key ratio of shear stress to normal stress, which is known to control seismogenic failure (e.g., Wallace, 1951; Bott, 1959). It is beyond the scope of this paper to document the detailed structural relations required to evaluate the relative influence of rheology, finite displacement, and tectonic loading for each fault section, but the data set compiled for this study clearly establishes the strong correlation between fault zone thickness and rupture zone thickness.

RUPTURE ZONE FABRIC IN BURIED FAULTS Sediment Thickness and Presence of Fluids The most significant along-strike variation in the map-view width of rupture zones coincides with the boundary between the Sierra Cucapah and Colorado River delta (Fig. 1). In the Sierra Cucapah, rupture zones are generally confined to single master faults and have finite widths 2 m of oblique coseismic slip and as much as half is partitioned onto secondary fault scarps and zones of penetrative off-fault shearing that are distinct from the throughgoing principal scarp (Fig. 11). We observed that regardless of the stage of formation of a principal scarp, secondary faults generally accommodate 40%–60% of the total coseismic slip (Fig. 11). This strongly suggests that secondary faults must continue to be active after the formation of a throughgoing principal scarp. In this example the principal scarps are dominated by dip-slip displacement, and thus it is possible that secondary faults remained active because they were required to accommodate the overall oblique shear sense. In contrast, in the laboratory experiments of Tchalenko (1970), secondary and principal scarps have nearly identical kinematics and only one set is required to accommodate the net slip.

Shear Sense Variations and Types of Secondary Fractures

N

Figure 9. Field photograph of fracture arrays composed of synthetic P shear fractures and T extensional fractures. The corners defined by intersecting P and T fractures make excellent markers for measuring total slip, which commonly has both lateral and vertical components. In this photo, corners highlighted in red record vertical and dextral offsets of ~9 and ~6 cm, respectively (Fletcher et al., 2014). Yellow arrows in background indicate prominent scarps in the rupture array along the buried Paso Superior detachment. Photo azimuth is ~325° (taken by John M. Fletcher on 16 April 2010). See Supplemental File 5 (see footnote 5) for photo information in Google Earth KMZ format.

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In addition to the changes observed with increasing slip magnitude, rupture zone fabric shows marked variations with overall kinematics of seismogenic slip. Riedel shears are the most common secondary fractures and they take on different orientations relative to the principal scarps depending on overall kinematics (Fig. 12). If finite slip has a significant component of strike slip, secondary fractures dominantly have an en echelon configuration and the strike is oblique to the principal scarp (Fig. 12A). In contrast, if finite slip is dominated by normal dip-slip, then the Riedel shears generally strike parallel to the principal scarp, but dip more steeply (Fig. 12B). Among the subset of steeply dipping buried faults, an order-of-magnitude of increase in rupture zone thickness (12–135 m) is strongly correlated with variations in strike (Figs. 13A, 13B, and 14A). In Fletcher et al. (2010), it was demonstrated that the rake of slip changes systematically with fault orientation, indicating that rupture is controlled by regional stress with high phi values (>0.8) such that s1 is close in magnitude to s2, the axes of which are oriented in a vertical plane perpendicular to a subhorizontal s3 that trends S80W (Fig. 14B). Combining these results with the analysis of rupture zone fabric demonstrates that the rupture zone thickness is narrowest in fault sections dominated by either pure dip-slip or pure strike-slip kinematics, and

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A

N

Figure 10. Sequence of oblique aerial photographs (taken by John M. Fletcher on 14 April 2010) along the easternmost fault in the Paso Inferior accommodation zone that show along-strike variations in rupture zone fabric with magnitude of coseismic slip. See Supplemental File 5 (see footnote 5) for photo information in Google Earth KMZ format. Photo locations are shown in Figures 2B and 11. (A) At 60 cm of vertical coseismic displacement (red arrow points to measurement location), we observed a continuous and relatively straight principal slip surface (black arrows) from which numerous obliquely oriented fractures intersect. Photo azimuth ~102°.

N

B

N

C

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Downloaded from geosphere.gsapubs.org on June 14, 2015 Figure 11. Detailed rupture traces and lidar (light detection and ranging) difference map showing the progressive coalescence of secondary fractures into a throughgoing fault with increasing coseismic slip toward the north. This unnamed fault is located in the Paso Inferior accommodation zone (map of fault sections 27 and 28 in Fig. 2B). Total elevation (Elev.) changes measured across the surface rupture (plotted at lower end of transect lines) and field measurements of the principal fault scarp are plotted at upper end of line (from Fletcher et al., 2014), which indicates that ~40%–60% of total coseismic slip is accommodated by secondary faulting. White arrows indicate location and view direction of oblique photos shown in Figure 10. See Supplemental File 1 (see footnote 1) for detailed rupture traces in Google Earth KMZ format.

Δ Elev. (m)

-2.5

100 m

NORTH

0

10A

10C

the fivefold increase in thickness is associated with oblique-slip kinematics characterized by subequal amounts of dip slip and strike slip (Fig. 14). One possible explanation for the correlation of rupture zone thickness with kinematics involves the partitioning of slip into different sets of secondary fractures. Secondary fracture sets associated with steeply dipping buried faults can be classified into either subparallel and/or anastomosing, or en echelon arrays (Figs. 13C–13E). Several studies have documented that the same families of secondary fractures form on both normal faults and strike-slip faults, but are rotated 90° from each other (e.g., Petit, 1987; Davis et al., 2000). Therefore, it is likely that the en echelon rupture fabric develops in fault sections dominated by strike slip (Figs. 13E and 15A), whereas the rupture fabric of a normal-slip dominant fault appears subparallel and/or anastomosing in map view (Figs. 13C and 15C). We observed that only one set of secondary fractures is present along faults with orientations that coincide with either of these two end-member kinematic cases. In contrast, both sets of en echelon and subparallel and/or anastomosing secondary fractures are present along faults with orientations consistent with oblique slip (Figs. 13D and 15B). Therefore, the partitioning of slip in multiple sets of kinematically discrete secondary fractures may simply affect a greater volume of rock above oblique-slip master faults as compared to those that accommodate pure strike slip or pure dip slip.

Coseismic Slip Transfer One of the most complex classes of rupture zone fabric in the EMC surface rupture is associated with the buried sections of the Paso Superior detachment (Fig. 2B, sections 10–16). This is largely due to the structural adjustments related to the transfer of slip from the low-angle master fault to an array of more steeply dipping faults cutting sediment. Furthermore, this section of the Paso Superior detachment is located near the active margin of the Laguna Salada basin, where it is buried together with the crosscutting Laguna Salada fault beneath as much

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10B nt c la eme Increasing vertical disp

as several hundred meters of sedimentary cover. Thus, rupture zone fabric is affected by the propagation of coseismic slip through not only a wide complex fault zone, but also through a relatively thick section of sediments. Along the buried section of the Paso Superior detachment, fault scarps generated in the EMC rupture follow the base of a series of low-lying hills of uplifted basin fill (Fig. 16), which forms the southward continuation of the narrow horst block of crystalline basement that exists between well-exposed sections of the Laguna Salada fault and Paso Superior detachment (Fig. 2B). Coseismic displacement averaged ~2.5 m along the buried section of the Paso Superior detachment and was dominated by dip slip (Fletcher et al., 2014). The amount of lateral coseismic slip is generally low in magnitude and its sense changes along strike from predominately dextral to locally sinistral (Fletcher et al., 2014). In general, the lateral component of slip was accommodated on strands with the greatest vertical offset. The EMC rupture zones developed above the buried Paso Superior detachment are consistently wide (97–262 m thick), and most coseismic slip was accommodated by an array of moderately dipping (generally 300 m, which likely is the depth of burial of the Paso Superior detachment along these sections (Figs. 17C, 17E). Paleoscarps are poorly preserved in the fine-grained surficial deposits above the buried Paso Superior detachment. However, we have identified both synthetic and antithetic paleoscarps that coincide well with the traces of EMC fault scarps (Fig. 16A), permitting the hypothesis that patterns of rupture observed in the EMC earthquake had been replicated in previous events.

C

B U

D

U

Sedimentary Cover Crystalline Basement

Pure Strike Slip

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Pure Dip Slip

Oblique Slip

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Figure 15. Block diagrams schematically showing secondary fractures developed above steeply dipping buried fault zones with variations in kinematics. (A) Block showing a fault with pure strike slip and one set of Riedel fractures that form en echelon arrays in map view (yellow traces). (B) Block showing a fault that is pure dip slip and one set of Riedel fractures that form anastomosing subparallel arrays at the surface (blue traces). (C) Block showing a fault with oblique slip kinematics and two sets of Riedel fractures that accommodate the partitioned dip and strike slip in a larger volume of rock making thicker fault zones. U–upthrown; D–downthrown. Faults with pure strike slip (A) or pure dip slip (B) result in narrower rupture zones because slip can be accommodated with only one set of secondary fractures.

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A

Figure 16. Oblique aerial photographs of a buried section of the Paso Superior detachment (photo locations shown in Fig. 2B). See Supplemental File 5 (see footnote 5) for photo information in Google Earth KMZ format. (A) View looking southeast (azimuth of ~160°) and down strike of the characteristically wide rupture zones (to 460 m). In the 2010 El Mayor–Cucapah (EMC) event, coseismic slip was partitioned between synthetic and antithetic fault scarps, which respectively control the northeast and southwest margins of the rupture zone. The partitioning of deformation observed here is similar to that reported for buried normal faults that dip