Seismic Constraints and 

Coulomb Stress Changes of 

a Blind Thrust Fault 

System, 1: Coalinga and 

Kettleman Hills, California 

By Jian Lin1 and Ross S. Stein2 

2006 

Open-File Report 2006–1149 

1 Woods Hole Oceanographic Institution, Woods Hole, Mass. 

2 U.S. Geological Survey, Menlo Park, Calif. 

U.S. Department of the Interior 

U.S. Geological Survey 
2

U.S. Department of the Interior 

Dirk Kempthorne, Secretary 

U.S. Geological Survey 

P. Patrick Leahy, Acting Director 

U.S. Geological Survey, Reston, Virginia 2006

Revised and reprinted: 2006 

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Suggested citation:

Lin, J., and Stein, R.S., 2006, Seismic constraints and Coulomb stress changes of a blind 

thrust fault system, 1: Coalinga and Kettleman Hills, California: U.S. Geological Survey 

Open-File Report 2006-1149 17 p [available on the World Wide Web at URL 

http://pubs.usgs.gov/of/2006/1149/ ]. 

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3

Summary 

This report reviews the seismicity and surface ruptures associated with the 1982-1985 

earthquake sequence in the Coalinga region in California, and the role of Coulomb stress in 

triggering the mainshock sequence and aftershocks. The 1982-1985 New Idria, Coalinga, and 

Kettleman Hills earthquakes struck on a series of west-dipping, en echelon blind thrust faults. 

Each earthquake was accompanied by uplift of a Quaternary anticline atop the fault, and each 

was accompanied by a vigorous aftershock sequence. Aftershocks were widely dispersed, and 

are seen above and below the thrust fault, as well as along the up-dip and down-dip projection 

of the main thrust fault. For the Coalinga and Kettleman Hills earthquakes, high-angle reverse 

faults in the core of the anticlines are evident in seismic reflection profiles, and many of these 

faults are associated with small aftershocks. The shallowest aftershocks extended to within 3-4 

km of the ground surface. There is no compelling evidence for aftershocks associated with 

flexural slip faulting. No secondary surface rupture was found on any of the anticlines. In 

contrast, the 1983 Nuñez rupture struck on a high-angle reverse fault 10 km west of the Coalinga 

epicenter, and over a 40-80-day period, up to 1 m of oblique surface slip occurred. The slip on 

this Holocene fault likely extended from the ground surface to a depth of 8-10 km. We argue that 

both the Nuñez and Kettleman earthquakes were triggered by stresses imparted by the Coalinga 

mainshock, which was the largest of the four events in the sequence. 

1. Analysis of relationship of seismicity to the fold and fault structure 

The 1983 M=6.7 Coalinga earthquake is the largest in a sequence of four reverse faulting events 

that struck the eastern margin of the California Coast Ranges fold belt during 1982-1985 (Fig. 1). 

The three blind thrust events all occurred beneath growing anticlines, and all exhibited diffuse 

aftershocks (Fig. 2). The fold belt accommodates several millimeters per year of crustal 

shortening normal to the strike of the San Andreas fault, and the fold belt has been active for 

2-3 Ma [Namson and Davis, 1988; Stein and King, 1984; Stein and Yeats, 1989; Rymer and Ellsworth, 

1990; Wentworth and Zoback, 1990]. 

When the main fault ruptures, aftershocks, and geological structure interpreted from oil well 

and seismic reflection profiling are viewed together, all are seen to show highly diffuse 

seismicity that extends into the core of the anticline atop the thrust fault, and most show rootless 

high-angle reverse faults in the anticline as well, with some of these secondary faults being the 

site of aftershock activity. This is particularly clear for the 1983 Coalinga (Fig. 3b) and 1985 

Kettleman Hills (Fig. 3c) earthquakes. 
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All of the folds were uplifted by the blind thrust earthquakes, indicating that the anticlines grow 

through repeated earthquake events. Thus it is clear that the thrust faults have cumulative slip of 

1-4 km based on the fold geometry (Fig. 3) [Ekström et al., 1992; Stein and Ekström, 1992]. A similar 

relation is seen for the 1987 M=6.0 Whittier Narrows earthquake beneath a Quaternary fold [Lin 

and Stein, 1989]. Close inspection of recently reprocessed seismic reflection profiles across the 

Coalinga anticline reveal gently folded strata with only minor high-angle reverse faulting in the 

anticlinal core (Fig. 5). 

Relocated seismicity and reanalyzed focal mechanisms associated with the 1985 M=6.0 

Kettleman Hills earthquake indicate that reverse faulting is dominant, and that some of the 

back-thrusts and secondary faults extend from 10 km depth to as shallow as 3 km. On the basis 

of seismicity alignments, it is likely that some slip occurred on these high-angle structures 

during the 1985 earthquake sequence (Fig. 6). In the case of the 1983 Coalinga earthquake, few of 

the shallow aftershocks can be confidently ascribed to the high-angle reverse faults in the 

anticlinal core (Fig. 7). Regardless of the true orientation and extent of the thrust fault, it is 

evident that seismicity extends both above and below the blind thrust, and is widely dispersed. 

2. The role of Coulomb stress in triggering the mainshock sequence and aftershocks 

Recently, we analyzed the 1982-1985 Coalinga sequence to understand how the earthquakes 

progressed to the southeast and to seek an explanation for the distribution of aftershocks [Lin 

and Stein, 2004]. The sequence included not only the three blind faults but also the 

surface-cutting Nuñez rupture that occurred about a month after the Coalinga earthquake. 

Although stress transferred by the New Idria event to the Coalinga shock is negligible (Fig. 8a), 

the stress imparted by the Coalinga event to the Nuñez rupture plane is large (Fig. 9), 

suggesting that the Coalinga shock promoted the Nuñez earthquake. The Coulomb stress 

increase at the base of the Nuñez fault is about 4–10 bars (Fig. 9a, b), and is unclamped by 20 

bars, because of its proximity to the Coalinga source (Fig. 9d). These stresses should modestly 

increase and propagate outward with time, as a result of the viscoelastic relaxation of the 

asthenosphere [Freed and Lin, 1998]. This means that 2 years after the Coalinga earthquake, the 

stress at the future Nuñez and Kettleman Hills hypocenters was probably higher than it was 

immediately after the Coalinga mainshock. 

Aftershocks of the New Idria and Coalinga events are concentrated in regions of calculated 

Coulomb stress increase (Fig. 8a, b), although this is less true for the Kettleman Hills aftershocks 

(Fig. 8c). In cross section, the distributed pattern of Coalinga aftershocks in the epicentral area 

(Fig. 10a) and at the future epicenter of the Kettleman Hills shock (Fig. 10b) are also in rough 
5

accord with the calculated Coulomb stress change. This furnishes additional support that 

aftershocks are the product of the coseismic stress changes, rather than being a continuation of 

the background seismicity. 

3. Surface rupture associated with the 1983 Coalinga earthquake 

The New Idria, Coalinga, and Kettleman Hills folds and surrounding basins were carefully 

checked for surface faulting, and none was found [Clark et al., 1983; Rymer et al., 1990a; Rymer 

and Sharp, oral commun., 1985]. The sole observation of surface rupture accompanying any of 

the 1982-1985 earthquakes come from the 1983 MW=5.2 and MW=6.0 Nuñez shocks [Rymer et al., 

1990a] (Fig. 11). Both the June 11 and July 22 events caused surface rupture and the July 22 

event provided another slip pulse with afterslip [Rymer et al., 1990b]. 

The Nuñez is a high-angle reverse fault that lies 10 km west of the Coalinga mainshock and the 

Coalinga anticline, and its strike differs from the Coalinga shock by 30° (Fig. 9a). The Nuñez 

surface rupture was first seen 40 days after the Coalinga main shock, at the time of a shallow 

M=5.2 shock on the Nuñez fault; it probably did not slip at the time of the Coalinga mainshock. 

The largest Mw=6.0 shock on the Nuñez fault did not occur until 41 days later, or 81 days after 

the Coalinga mainshock [Rymer et al., 1990a] (Fig. 11). The Nuñez fault was mapped and 

trenched after the 1983 rupture, and it was found to be an active feature with past Holocene slip 

[Rymer et al., 1990a]. The fact that the surface slip preceded, rather than followed, the M=6.0 

mainshock is highly unusual and indicates more swarm-like behavior. The Nuñez fault 

displacements were also significant, at almost 1 m, and, based on the alignment of seismicity 

with the surface slip, it is likely that the slip extends from the surface to a depth of 8-10 km 

[Rymer et al., 1990b]. 

Coulomb stress calculations suggest that the Nuñez rupture was promoted by strong 

unclamping stresses at the base of the fault, where it is located only 5 km west of the Coalinga 

thrust fault (Fig. 9d). Perhaps the unclamping caused the base of the fault to creep, with the 

creep propagating upwards over the 40 days between the Coalinga mainshock and when the 

surface rupture was first seen. Why the creep was followed by the M=6.0 shock remains 

mysterious. 

Conclusions The 1982-1985 New Idria, Coalinga, and Kettleman earthquakes provide a 

beautiful example of blind thrust faults beneath growing en echelon anticlines that ruptured in a 

north-to-south sequence. In each case, the anticlines are simple Quaternary folds that are 

expressed in the surficial geology, topography, and geomorphology. All three earthquakes 
6

reveal the diagnostic features of blind thrust earthquakes: diffuse aftershock zones, high-angle 

reverse faults slipping in the anticlinal cores, but no surface rupture on the anticlines themselves. 

Our stress analysis suggests that the 1983 Nuñez and 1985 Kettleman Hills ruptures were likely 

promoted by Coulomb stress changes imparted by the Coalinga earthquake, but the Nuñez and 

Kettleman faults are quite different in character, the first being a high-angle reverse fault that 

ruptured to the surface, and the second being a deep blind thrust fault. 

Acknowledgments. We thank Michael Rymer and James Lienkaemper for constructive 

reviews and Dan Scheirer for providing photo of the Kettleman Hills North Dome. This article is 

based on a project report to the Central Research Institute of Electric Power Industry (CRIEPI), 

Japan, Sub-Contract Order Number 10504206.

References 

Ekström, G., R. S. Stein, J. P. Eaton, and D. Eberhart-Phillips (1992), Seismicity and geometry of 

a 110-km long blind thrust fault, 1, The 1985 Kettleman Hills, California, earthquake, J. 

Geophys. Res., 97, 4843-4864. 

Freed, A. M., and J. Lin (1998), Time-dependent changes in failure stress following thrust 

earthquakes, J. Geophys. Res., 103, 24,393-324,409. 

Guzofski, C., and J. H. Shaw (2003), Coalinga anticline, San Joaquin basin, California, U.S.A, in 

Seismic interpretation of contractional fault-related folds: An AAPG seismic atlas, edited by J. H. 

Shaw, et al., pp. 138-140, Amer. Assn. Petrol. Geol., Tulsa. 

Lin, J., and R. S. Stein (1989), Coseismic folding, earthquake recurrence, and the 1987 source 

mechanism at Whittier Narrows, Los Angeles Basin, California, J. Geophys. Res., 94, 

9614-9632. 

Lin, J., and R. S. Stein (2004), Stress triggering in thrust and subduction earthquakes, and stress 

interaction between the southern San Andreas and nearby thrust and strike-slip faults, J. 

Geophys. Res., 109, B02303, doi:10.1029/2003JB002607. 

Namson, J. S., and T. L. Davis (1988), Seismically active fold and thrust belt in the San Joaquin 

Valley, central California, Geological Society of America Bulletin, 100, 257-273. 

Rymer, M. J., and W. L. Ellsworth (1990), edited, The Coalinga, California, Earthquake of May 2, 

1983, U.S. Geological Surv. Prof. pap. 1487. 

Rymer, M. J., K. J. Kendrick, J. J. Lienkaemper, and M. M. Clark (1990a), The Nuñez fault and its 

surface rupture during the Coalinga earthquake sequence, in Rymer, M. J. and W. L. 

Ellsworth, ed., The Coalinga, California, Earthquake of May 2, 1983, U.S. Geol. Surv. Prof.. pap. 

1487, pp. 299-318. 
7

Rymer, M. J., J. J. Lienkaemper, and B. D. Brown (1990b), Distribution and timing of slip along 

the Nuñez fault after June 11, 1983, in Rymer, M. J. and W. L. Ellsworth, ed., The Coalinga, 

California, Earthquake of May 2, 1983, U.S. Geol. Surv. Prof.. pap. 1487, pp. 319-334. 

Stein, R. S., and G. E. Ekström (1992), Seismicity and geometry of a 110-km long blind thrust 

fault, 2, Synthesis of the 1982-85 California earthquake sequence, J. Geophys. Res., 97, 

4865-4883. 

Stein, R. S., and G. C. P. King (1984), Seismic potential revealed by surface folding: The 1983 

Coalinga, California, earthquake, Science, 224, 869-872. 

Stein, R. S., and R. S. Yeats (1989), Hidden Earthquakes, Sci. Am., 260, 48-57, June. 

Wentworth, C. M., and M. D., Zoback (1990), Structure of the Coalinga area and thrust origin of 

the earthquake, in Rymer, M. J. and W. L. Ellsworth, ed., The Coalinga, California, Earthquake 

of May 2, 1983, U.S. Geol. Surv. Prof.. pap. 1487, pp. 41-68. 
8

Figure 1. Active faults and folds in central and southern California modified from Lin and Stein

[2004]. Shown are locations of the 1982 New Idria, 1983 Nuñez and Coalinga, and 1985 

Kettleman Hills earthquakes. Ket. Mid., Kettleman Middle Dome; Ket. So., Kettleman South 

Dome. Quaternary folds are surrogates for blind thrust faults. They extend parallel to the San 

Andreas fault nearly as far south as the White Wolf reverse fault. 
9

Figure 2. Relocated seismicity of the Coalinga-Kettleman Hills region [Stein and Ekström, 1992]. 

(a) Relocated aftershock sequence of the 1982 New Idria (shaded hexagons, northwest), 1983 

Coalinga (open hexagons), 1985 Kettleman Hills (shaded hexagons, southeast), and the 1976 

Polvadero (solid hexagons) earthquakes. (b) Map showing focal mechanism solutions of the 1982 

New Idria, 1983 Coalinga, and 1985 Kettleman Hills main shocks together with the seismic 

stations of the CalNet network from which the data are used to locate the earthquakes. Station 

symbols: permanent (squares), temporary stations in 1983 (triangles), and temporary stations in 

1985 (stars). (c) Relocated pre-1983 seismicity 10-60 km northeast of the San Andreas fault during 

1969 to September 1982 (location error ≤ 1.5 km). 
10

Figure 3. Structural and interpreted seismic cross sections of the Coalinga-Kettleman Hills 

region [Stein and Ekström, 1992]. The cross sections are projected approximately normal to the 

fold axis at (a) New Idria, (b) Coalinga, (c) Kettleman Hills North Dome, (d) Kettleman Hills 

South Dome, and (e) Lost Hills. In panels a-c, a subset of aftershocks are projected together with 

back-hemisphere projections of the main shocks. PK, Parkfield; CH, Cholame. Panel (a) is a 

balanced section constructed from oil well logs by Namson and Davis [1988], while the other 

panels are based on interpretation of seismic reflection and refraction data supplemented by oil 

well logs. 
11

Figure 4. Photo of the Kettleman Hills North Dome, looking in the southeast direction 

(courtesy of Dan Scheirer, U.S. Geological Survey). 

Figure 5. Migrated seismic profile across the Coalinga anticline [Guzofski and Shaw, 2003]. With 

the possible exception of the anticlinal axis at 2-3 km depth, ~1 km SW of the Leavitt-Hintze #1 

well, there is remarkably little evidence for reverse faulting anywhere in this section. 
12

Figure 6. Cross sections showing the subsurface structure and seismicity beneath the Kettleman 

Hills North Dome region [Ekström et al., 1992]. (a) A cross section beneath a W-E oriented seismic 

reflection line immediately to the north of the Kettleman North Dome with seismicity 

superimposed. Small circles show earthquake hypocenters that fall within 3.0 km horizontal 

distance from the cross section. The foreshocks and the main shock are shown with solid 

hexagons. Bedding is shown as thin lines, and faults are shown as thicker lines. The topography 

is shown above the cross section. Three deep wells used to constrain the interpretation of the 

section are shown as vertical lines. Only events with vertical and horizontal location 

uncertainties of less than 1.5 km are included. (b) Same as in panel a, except both structure and 

hypocenters have been projected onto a plane perpendicular to the strike of the 

Coalinga-Kettleman fold axis. The structure to the west is shown as dashed lines, since it may 

not be representative of the Kettleman North Dome cross section. First motion focal mechanisms 

are shown in back-hemisphere projection. The unlabeled focal mechanism is the preferred 

solution for the main shock (event 3), obtained by waveform inversion. (c) Same as panel b, but 

for the southeastern section of the seismicity in the Kettleman North Dome. 
13

Figure 7. Cross sections of representative dislocation models that satisfy the geodetic data 

collected in the Coalinga region, together with the back-hemisphere projection of the Coalinga 

main shock [Stein and Ekström, 1992]. Best-fitting solutions are shown for coseismic 

displacements (bold lines) and postseismic displacements (dotted lines). The coseismic and 

postseismic data were collected for the periods February 1972 - June 1983 and June 1983 - July 

1987, respectively. The cross sections are projected along the azimuth N55°E, approximately 

perpendicular to the Coalinga fold axis. Folded strata and small fault offsets are from seismic 

reflection profile interpretation. (a) and (b) SW-dipping thrust fault models. (c) and (d) 

NE-dipping reverse fault models. 
14

Figure 8. Relationship between the 

1983-1985 earthquake sequence and 

the calculated changes in Coulomb 

stress changes [Lin and Stein, 2004]. All 

stress calculations assume receiver 

faults are optimally oriented for the 

regional stress with directions shown 

in panel a. Stresses are shown for a 

sampling depth of 9 km. (a) Stress 

from the 25 Oct 1982 MW=5.8 New 

Idria earthquake, with aftershocks (25 

Oct 1982-31 Dec 1982, M≥1.0). (b) 

Stress from the New Idria and 2 May 

1983 MW=6.7 Coalinga earthquake, 

with aftershocks of the Coalinga and 

Nuñez events (2 May 1983-24 April 

1984, M≥2.0). The Coalinga shock 

brings both the 22 July 1983 MW=6.0 

Nuñez and 4 Aug 1985 MW=6.0 

Kettleman Hills faults closer to failure. 

(c) Stress from the New Idria, 

Coalinga, Nuñez and Kettleman Hills 

earthquakes, with aftershocks of the 

Kettleman Hills event (3 Aug 1985-15 

Sept 1985, M≥0.7). 
15

Figure 9. Stress changes caused by the 

1982 MW=5.8 New Idria and 1983 MW=6.7 

Coalinga source faults resolved onto the 

plane of the subsequent 1983 MW=6.0 

Nuñez rupture [Lin and Stein, 2004]. (a) 

Map view of Coulomb stress changes at 9 

km depth. (b) Coulomb stress changes, (c) 

shear stress changes, and (d) unclamping 

in cross-section resolved on the Nuñez 

rupture plane. The resultant stress 

components are shown graphically in the 

upper right hand corners. 
16

Figure 10. Stress changes caused by the 1983 MW=6.7 Coalinga and MW=6.0 Nuñez ruptures 

[Lin and Stein, 2004]. The Coalinga source includes postseismic slip that occurred before the 1985 

MW=6.0 Kettleman Hills rupture. (a) Coulomb stress changes along a cross section cutting 

through the center of the Coalinga rupture zone, together with aftershocks along an 8-km-wide 

band. (b) Coulomb stress changes along a cross section within 5 km north of the future 

Kettleman Hills epicenter, together with aftershocks along an 8-km-wide band. Stress is 

imparted by the Coalinga earthquake, resolved onto the future Kettleman Hills rupture plane. 
17

 

Figure 11. Surface rupture of the Nuñez fault from Rymer et al [1990b]. Left panel: (A) 

Post-seismic slip; (B), right-lateral slip; (C) vertical slip; (D) map of surface rupture. Right panel: 

Relationship between Nuñez surface rupture with M≥5 earthquakes, and with the Coalinga 

mainshock (inset). The Nuñez fault dips steeply to the east.