Seismic and geodetic evidence for extensive, long-lived fault damage zones (original) (raw)
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The missing sinks: Slip localization in faults, damage zones, and the seismic energy budget
Geophysical Monograph Series, 2006
The majority of work done during an earthquake may be consumed by dissipative processes that occur within geometrically and mechanically complex fault zones, rather than radiated as seismic waves. Many processes are likely to act as dissipative energy sinks in a three-dimensional faulted volume: slip along the principal slip zone is likely to be accompanied by deformation in the surrounding damage zone volume. Examination of exhumed fault zones in granite, which are meters to tens of kilometers long, shows thin principal slip zones developing on the smallest faults. As slip is accumulated on progressively larger faults, the principal slip zones remain less than 0 cm wide, but there is increasing complexity in the damage zone of the larger faults. By considering the amount of energy required to crush fine-grained gouge in the principal slip zone, and the grain size of gouge reported from other (seismogenic) faults, we conclude that principal slip zones must remain relatively thin for all sizes of rupture. Any energy consumed by dissipative processes in the damage zone will therefore tend to make the principal slip zone thinner, and will reduce the energy available to propagate the rupture.
Journal of Geophysical Research, 2007
1] Long-term (10 5 years) fault slip rates test the scale of discrepancy between infrequent paleoseismicity and relatively rapid geodetic rates of dextral shear in the Eastern California Shear Zone (ECSZ). The Calico fault is one of a family of dextral faults that traverse the Mojave Desert portion of the ECSZ. Its slip rate is determined from matching and dating incised Pleistocene alluvial fan deposits and surfaces displaced by fault slip. A high-resolution topographic base acquired via airborne laser swath mapping aids in identification and mapping of deformed geomorphic features. The oldest geomorphically preserved alluvial fan, unit B, is displaced 900 ± 200 m from its source at Sheep Springs Wash in the northern Rodman Mountains. This fan deposit contains the first preserved occurrence of basalt clasts derived from the Pipkin lava field and overlies Quaternary conglomerate deposits lacking these clasts. The 40 Ar/ 39 Ar dating of two flows from this field yields consistent ages of 770 ± 40 ka and 735 ± 9 ka. An age of 650 ± 100 ka is assigned to this fan deposit based on these ages and on the oldest cosmogenic 3 He exposure date of 653 ± 20 ka on a basalt boulder from the surface of unit B. This assigned age and offset together yield a mid-Pleistocene to present average slip rate of 1.4 ± 0.4 mm/yr. A younger fan surface, unit K, records 100 ± 10 m of dextral displacement and preserves original depositional morphology of its surface. Granitic boulders and pavement samples from this surface yield an average age of 56.4 ± 7.7 ka after taking into account minimal cosmogenic inheritance of granitic clasts. The displaced and dated K fans yield a slip rate of 1.8 ± 0.3 mm/yr. Distributed deformation of the region surrounding the fault trace, if active, could increase the overall displacement rate to 2.1 ± 0.5 mm/yr. Acceleration of slip rate from an average of 1.4 mm/yr prior to 50kato1.8mm/yrsince50 ka to 1.8 mm/yr since 50kato1.8mm/yrsince50 ka is possible, though a single time-averaged slip rate of 1.6 ± 0.2 mm/yr satisfies the data. These rates are faster than any other paleoseismic or long-term slip rate yet determined for other dextral faults in the Mojave Desert and imply that fault slip rates and earthquake productivity are heterogeneous across this portion of the ECSZ. Total displacement across the Calico fault diminishes northward as shear is distributed into folding and sinistral faults in the Calico Mountains. This pattern is consistent with an approximately threefold drop in geologic slip rate as the Calico fault steps over onto the Blackwater fault and demonstrates the significance of fault interaction for understanding the pattern of present-day strain accumulation in the ECSZ. Citation: Oskin, M., L. Perg, D. Blumentritt, S. Mukhopadhyay, and A. Iriondo (2007), Slip rate of the Calico fault: Implications for geologic versus geodetic rate discrepancy in the Eastern California Shear Zone,
Journal of Geophysical Research: Solid Earth, 2015
Campaign GPS data collected from 2002 to 2014 result in 41 new site velocities from the San Bernardino Mountains and vicinity. We combined these velocities with 93 continuous GPS velocities and 216 published velocities to obtain a velocity profile across the Pacific-North America plate boundary through the San Bernardino Mountains. We modeled the plate boundary-parallel, horizontal deformation with 5-14 parallel and one obliquely oriented screw dislocations within an elastic half-space. Our rate for the San Bernardino strand of the San Andreas Fault (6.5 ± 3.6 mm/yr) is consistent with recently published latest Quaternary rates at the 95% confidence level and is slower than our rate for the San Jacinto Fault (14.1 ± 2.9 mm/yr). Our modeled rate for all faults of the Eastern California Shear Zone (ECSZ) combined (15.7 ± 2.9 mm/yr) is faster than the summed latest Quaternary rates for these faults, even when an estimate of permanent, off-fault deformation is included. The rate discrepancy is concentrated on faults near the 1992 Landers and 1999 Hector Mine earthquakes; the geodetic and geologic rates agree within uncertainties for other faults within the ECSZ. Coupled with the observation that postearthquake deformation is faster than the pre-1992 deformation, this suggests that the ECSZ geodetic-geologic rate discrepancy is directly related to the timing and location of these earthquakes and is likely the result of viscoelastic deformation in the mantle that varies over the timescale of an earthquake cycle, rather than a redistribution of plate boundary slip at a timescale of multiple earthquake cycles or longer. The relative role of the San Andreas and San Jacinto Faults also remains a topic of ongoing investigation (see summary in McGill et al. [2013]). Bedrock offsets along the San Andreas Fault [e.g., Powell, 1993] are an order of magnitude larger than for the (younger) San Jacinto Fault [Sharp, 1967; Matti and Morton, 1993], but the relative significance of the two faults in terms of present-day activity and seismic hazard may be more equal. South of San Gorgonio Pass (Figure 1), most geodetic studies suggest an equal or slightly larger role for the San Andreas Fault (16-26 mm/yr) than for the San Jacinto Fault (9-21 mm/yr) [Bennett et al., 1996; MCGILL ET AL.
International Geology Review, 2018
Accurate estimation of fault slip rate is fundamental to seismic hazard assessment. Previous work suggested a discrepancy between short-term geodetic and long-term geologic slip rates in the Mojave Desert section of the Eastern California Shear Zone (ECSZ). Understanding the origin of this discrepancy can improve understanding of earthquake hazard and fault evolution. We measured offsets in alluvial fans along the Calico Fault near Newberry Springs, California, and used several techniques to date the offset landforms and determine a slip rate. Our preferred slip rate estimate is 3.2 ± 0.4 mm/yr, representing an average over the last few hundred thousand years, faster than previous estimates. Seismic hazard associated with this fault may therefore be higher than previously assumed. We discuss possible biases in the various slip rate estimates and discuss possible reasons for the rate discrepancy. We suggest that the ECSZ discrepancy is an artefact of limited data, and represents a combination of faster slip on the Calico Fault, off-fault deformation, unmapped fault strands, and uncertainties in the geologic rates that have been underestimated. Assuming our new rate estimate is correct and a fair amount (40%) of off-fault deformation occurs on major ECSZ faults, the summed geologic rate estimate across the Mojave section of the ECSZ is 10.5 ± 3.1 mm/yr, which is equivalent within uncertainties to the geodetic rate estimate.
The Structure and Composition of Exhumed Faults, and Their Implications for Seismic Processes
2000
Field studies of faults exhumed from seismogenic depths provide useful data to constrain seismologic models of fault zone processes and properties. Data collected on the San Andreas Fault in the San Gabriel Mountains has shown that large-displacement faults consist of one to several very narrow slip zones embedded in a cataclastically deformed sheared region several meters thick. However these faults have not been buried to depths greater than 5 km. Fault zones in the Sierra Nevada, California allow us to study the microstructures resulting from the deformation mechanisms active at seismogenic depths. Syn-fault mineralization shows that these left-lateral strike-slip faults formed at 5-12 km depth. Detailed microstructural analyses of the small faults reveal that they evolved from cooling joints filled by chlorite, epidote and quartz. These joints were then reactivated to form shear faults with accompanying brittle fracture and cataclastic deformation, ultimately developing very fined-grained cataclasites and ultracataclasites. The shear-induced microstructures are developed on faults with as little as several mm of slip showing that narrow slip-surfaces develop early in the lifetime of these faults. Subsequent slip has little effect on the microstructures. The inferred similarity of deformation mechanisms in faults 10 m to 10 km long indicates that basic slip processes on the faults are scale invariant, and may be a cause for the inferred constant b-value for small earthquakes. Analysis of map-scale fault linkages and terminations indicate that linkage zones are up to 400 m wide and 1 km long, and consist of altered and fractured rocks with numerous through-going slip surfaces. Terminations are regions of numerous splay faults that have cumulative offsets approaching those of the main faults. The slip distribution and structure of the terminations and linkage zones suggest that seismic slip may propagate into these zones of enhanced toughness, and that through-going slip can occur when a sufficient linkage of faults in the zone allow slip to be transmitted.
Journal of Geophysical Research, 2006
1] It is poorly known if fault slip repeats regularly through many earthquake cycles. Well-documented measurements of successive slips rarely span more than three earthquake cycles. In this paper, we present evidence of six sequential offsets across the San Andreas fault at a site in the Carrizo Plain, using stream channels as piercing lines. We opened a latticework of trenches across the offset channels on both sides of the fault to expose their subsurface stratigraphy. We can correlate the channels across the fault on the basis of their elevations, shapes, stratigraphy, and ages. The three-dimensional excavations allow us to locate accurately the offset channel pairs and to determine the amounts of motion for each pair. We find that the dextral slips associated with the six events in the last millennium are, from oldest to youngest, !5.4 ± 0.6, 8.0 ± 0.5, 1.4 ± 0.5, 5.2 ± 0.6, 7.6 ± 0.4 and 7.9 ± 0.1 m. In this series, three and possibly four of the six offset values are between 7 and 8 m. The common occurrence of 7-8 m offsets suggests remarkably regular, but not strictly uniform, slip behavior. Age constraints for these events at our site, combined with previous paleoseismic investigations within a few kilometers, allow a construction of offset history and a preliminary evaluation of slip-and timepredictable models. The average slip rate over the span of the past five events (between A.D. 1210 and A.D. 1857.) has been 34 mm/yr, not resolvably different from the previously determined late Holocene slip rate and the modern geodetic strain accumulation rate. We find that the slip-predictable model is a better fit than the timepredictable model. In general, earthquake slip is positively correlated with the time interval preceding the event. Smaller offsets coincide with shorter prior intervals and larger offset with longer prior intervals.
Earth and Planetary Science Letters, 2009
Repeating earthquakes (REs) are sequences of events that have virtually identical waveforms and are interpreted to represent fault asperities driven to failure by loading from aseismic creep on the surrounding fault surface at depth. To investigate the postseismic deformation after the 1984 M6.2 Morgan Hill earthquake, we identify RE sequences occurring on the central Calaveras fault between 1984 and 2005 using a combination of cross-correlation and spectral coherence techniques. Both the accelerated slip transients due to the earthquake as well as the return to interseismic background creep rates can be imaged from our dataset. A comparison between the regions of the fault that ruptured coseismically and the locations of the REs show that REs preferentially occur in areas adjacent to the coseismic rupture. Using calculated RE-derived subsurface slip distributions at 6 months and 18 months after the mainshock, we predict surface electronic distance meter (EDM) line length changes between stations near the Morgan Hill rupture area. The RE-derived slip model underpredicts a subset of the observed line-length changes. Inclusion of transient aseismic slip below the seismogenic zone is needed to better match the measured surface deformation.
Spatial and temporal distribution of slip for the 1992 Landers, California, earthquake
Bulletin of the Seismological Society of America, 1994
We have determined a source rupture model for the 1992 Landers earthquake (Mw 7.2) compatible with multiple data sets, spanning a frequency range from zero to 0.5 Hz. Geodetic survey displacements, near-field and regional strong motions, broadband teleseismic waveforms, and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface. Our fault parameterization involves a variable-slip, multiple-segment, finite-fault model which treats the diverse data sets in a self-consistent manner, allowing them to be inverted both independently and in unison. The high-quality data available for the Landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern. In all models, consistent features include the following: (1) similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 x 1026 dyne-cm (seismic potency of 2.3 to 2.7 km3);