Geology topics

J.C. Savage

Publications and source records attributed to J.C. Savage.

At least 37 records · Page 2Linked to original sources

USGS2007

Postseismic relaxation and aftershocks

[1] Perfettini et al. (2005) suggested that the temporal dependence of surface displacements u ( t ) measured in the epicentral area following an earthquake is related to N ( t ), the cumulative number of aftershocks, by the equation u ( t ) = a + bt + cN ( t ) + d (1 − e − αt ), where a , b , c , d , and α are constants chosen to fit the data and t is the postearthquake time. N ( t ) appears in the expression for u ( t ) because both the aftershocks and a portion of u ( t ) are thought to be driven by the same source, postseismic fault creep at subseismogenic depths on the downdip extension of the coseismic rupture. We show that this equation with the actually observed N ( t ) fits the postseismic displacements recorded on several baselines following each of five earthquakes: 1999 M7.6 Chi-Chi (Taiwan), 1999 M7.1 Hector Mine (southern California), 2002 M7.9 Denali (central Alaska), 2003 M6.5 San Simeon (central California), and 2004 M6.0 Parkfield (central California) earthquakes. Although there are plausible physical interpretations for each of the terms in the expression for u ( t ), the large number of adjustable constants ( a , b , c , d , and α ) involved in fitting the rather simple postseismic displacements diminishes the significance of the fit. Because the observed N ( t ) is well fit by the modified Omori's law, fault creep at depth presumably exhibits the same temporal dependence. That dependence could be explained if the rheology of the fault downdip from the coseismic rupture is consistent with ordinary transient creep. Montesi (2004) demonstrated that power law creep across a shear zone at depth would also produce that temporal signal.

Journal of Geophysical Research B: Solid Earth↗

USGS2007

Postearthquake relaxation and aftershock accumulation linearly related after the 2003 M 6.5 Chengkung, Taiwan, and the 2004 M 6.0 Parkfield, California, earthquakes

We treat both the number of earthquakes and the deformation following a mainshock as the superposition of a steady background accumulation and the postearthquake process. The preseismic displacement and seismicity rates r u and r E are used as estimates of the background rates. Let t be the time after the mainshock, u ( t ) + u 0 the postseismic displacement less the background accumulation r u t , and Δ N ( t ) the observed cumulative number of postseismic earthquakes less the background accumulation r E t . For the first 160 days (duration limited by the occurrence of another nearby earthquake) following the Chengkung ( M 6.5, 10 December 2003, eastern Taiwan) and the first 560 days following the Parkfield ( M 6.0, 28 September 2004, central California) earthquakes u ( t ) + u 0 is a linear function of Δ N ( t ). The aftershock accumulation Δ N ( t ) for both earthquakes is described by the modified Omori Law d Δ N / dt ∝ (1 + t / τ ) − p with p = 0.96 and τ = 0.03 days. Although the Chengkung earthquake involved sinistral, reverse slip on a moderately dipping fault and the Parkfield earthquake right-lateral slip on a near-vertical fault, the earthquakes share an unusual feature: both occurred on faults exhibiting interseismic fault creep at the surface. The source of the observed postseismic deformation appears to be afterslip on the coseismic rupture. The linear relation between u ( t ) + u 0 and N ( t ) suggests that this afterslip also generates the aftershocks. The linear relation between u ( t ) + u 0 and Δ N ( t ) obtains after neither the 1999 M 7.1 Hector Mine (southern California) nor the 1999 M 7.6 Chi-Chi (central Taiwan) earthquakes, neither of which occurred on fault segments exhibiting fault creep.

Bulletin of the Seismological Society of America↗

USGS2006

Dislocation pileup as a representation of strain accumulation on a strike-slip fault

The conventional model of strain accumulation on a vertical transform fault is a discrete screw dislocation in an elastic half-space with the Burgers vector of the dislocation increasing at the rate of relative plate motion. It would be more realistic to replace that discrete dislocation by a dislocation distribution, presumably a pileup in which the individual dislocations are in equilibrium. The length of the pileup depends upon the applied stress and the amount of slip that has occurred at depth. I argue here that the dislocation pileup (the transition on the fault from no slip to slip at the full plate rate) occupies a substantial portion of the lithosphere thickness. A discrete dislocation at an adjustable depth can reproduce the surface deformation profile predicted by a pileup so closely that it will be difficult to distinguish between the two models. The locking depth (dislocation depth) of that discrete dislocation approximation is substantially (???30%) larger than that (depth to top of the pileup) in the pileup model. Thus, in inverting surface deformation data using the discrete dislocation model, the locking depth in the model should not be interpreted as the true locking depth. Although dislocation pileup models should provide a good explanation of the surface deformation near the fault trace, that explanation may not be adequate at greater distances from the fault trace because approximating the expected horizontally distributed deformation at subcrustal depths by uniform slip concentrated on the fault is not justified.

Journal of Geophysical Research B: Solid Earth↗

USGS2005

Postseismic relaxation and transient creep

Postseismic deformation has been observed in the epicentral area following the 1992 Landers (M = 7.3), 1999 Chi-Chi (M = 7.6), 1999 Hector Mine (M = 7.1), 2002 Denali (M = 7.9), 2003 San Simeon (M = 6.5), and 2004 Parkfield (M = 6.0) earthquakes. The observations consist of repeated GPS measurements of the position of one monument relative to another (separation ???100 km). The early observations (t < 0.1 year) are well fit by the function a' + c'log(t), where t is the time after the earthquake and a' and c' are constants chosen to fit the data. Because a log(t) time dependence is characteristic of transient (primary) creep, the early postseismic response may be governed by transient creep as Benioff proposed in 1951. That inference is provisional as the stress conditions prevailing in postseismic relaxation are not identical to the constant stress condition in creep experiments. The observed logarithmic time dependence includes no characteristic time that might aid in identifying the micromechanical cause.

Journal of Geophysical Research B: Solid Earth↗

USGS2004

Strain accumulation across the Coast Ranges at the latitude of San Francisco, 1994-2000

A 66-monument geodetic array spanning the Coast Ranges near San Francisco has been surveyed more than eight times by GIPS between late 1993 and early 2001. The measured horizontal velocities of the monuments are well represented by uniform, right-lateral, simple shear parallel to N29°W. (The local strike of the San Andreas Fault is ∼N34°W.) The observed areal dilatation rate of 6.9 ± 10.0 nstrain yr −1 (quoted uncertainty is one standard deviation and extension is reckoned positive) is not significantly different from zero, which implies that the observed strain accumulation could be released by strike-slip faulting alone. Our results are consistent with the slip rates assigned by the Working Group on California Earthquake Probabilities [2003] to the principal faults (San Gregorio, San Andreas, Hayward-Rodgers Creek, Calaveras-Concord-Green Valley, and Greenville Faults) cutting across the GPS array. The vector sum of those slip rates is is 39.8 ± 2.6 mm yr −1 N29.8°W ± 2.8°, whereas the motion across the GPS array (breadth 120 km) inferred from the uniform strain rate approximation is 38.7 ± 1.2 mm yr −1 N29.0°W ± 0.9° right-lateral shear and 0.4 ± 0.9 mm yr −1 N61°E ± 0.9° extension. We interpret the near coincidence of these rates and the absence of significant accumulation of areal dilatation to imply that right-lateral slip on the principal faults can release the accumulating strain; major strain release on reverse faults subparallel to the San Andreas Fault within the Coast Ranges is not required.

California↗

USGS2004

Interseismic strain and rotation rates in the northeast Mojave domain, eastern California

The northeast Mojave domain, a type locality for bookshelf faulting, is a region of east striking, left-lateral faults in the northeast corner of the Mojave block, a block otherwise dominated by ∼N40°W striking, right-lateral faults. Paleomagnetic evidence suggests that blocks within the domain have rotated clockwise about a vertical axis as much as 60° since 12.8 Ma [Schermer et al., 1996] . In 1994, and again in 2002, the U.S. Geological Survey surveyed an array of 14 geodetic monuments distributed across the northeast Mojave domain. The 2002 survey results were adjusted to remove the coseismic offsets imposed by the nearby Hector Mine earthquake (16 October 1999, M w = 7.1). The adjusted deformation across the array appears to be uniform and can be approximated by the principal strain rates ε 1 = 28.9 ± 9.1 N77.2°W ± 4.8° and ε 2 = −48.2 ± 8.9 N12.8°E ± 4.8° nstrain yr −1 ; extension reckoned positive, and quoted uncertainties are standard deviations. That strain accumulation could be released by slip on faults striking N32°W but not by bookshelf faulting on the east striking faults alone. The vertical axis rotation rate of the northeast Mojave domain as a whole relative to fixed North America is 71.0 ± 6.4 nrad yr −1 (4.07° ± 0.37° Myr −1 ) clockwise, about twice the maximum tensor shear strain rate. The observed rotation rate acting over 12.8 Myr would produce a clockwise rotation of 52.1° ± 4.7°, exclusive of possible coseismic rotations. That rotation is in rough agreement with the paleomagnetic rotation accumulated in the individual fault blocks within the northeast Mojave domain since 12.8 Ma.

California↗

USGS2003

Near-field postseismic deformation associated with the 1992 Landers and 1999 Hector Mine, California, earthquakes

After the Landers earthquake (Mw = 7.3, 1992.489) a linear array of 10 monuments extending about 30 km N50??E on either side of the earthquake rupture plus a nearby offtrend reference monument were surveyed frequently by GPS until 2003.2. The array also spans the rupture of the subsequent Hector Mine earthquake (Mw = 7.1, 1999.792 . The pre-Landers velocities of monuments in the array relative to interior North America were estimated from earlier trilateration and very long baseline interferometry measurements. Except at the reference monument, the post-Landers velocities of the individual monuments in the array relaxed to their preseismic values within 4 years. Following the Hector Mine earthquake the velocities of the monuments relaxed to steady rates within 1 year. Those steady rates for the east components are about equal to the pre-Landers rates as is the steady rate for the north component of the one monument east of the Hector Mine rupture. However, the steady rates for the north components of the 10 monuments west of the rupture are systematically ???10 mm yr1 larger than the pre-Landers rates. The relaxation to a steady rate is approximately exponential with decay times of 0.50 ?? 0.10 year following the Landers earthquake and 0.32 ?? 0.18 year following the Hector Mine earthquake. The postearthquake motions of the Landers array following the Landers earthquake are not well approximated by the viscoelastic-coupling model of Pollitz et al. [2000]. A similar viscoelastic-coupling model [Pollitz et al., 2001] is more successful in representing the deformation after the Hector Mine earthquake.

Journal of Geophysical Research B: Solid Earth↗

USGS2002

Strain accumulation and rotation in western Nevada, 1993-2000

The positions of 44 GPS monuments in an array extending from the Sierra Nevada at the latitude of Reno to near Austin, Nevada, have been measured several times in the 1993–2000 interval. The western half of the array spans the Walker Lane belt, whereas the eastern half spans the central Nevada seismic zone (CNSZ). The principal strain rates in the Walker Lane belt are 29.6 ± 5.3 nstrain yr −1 N88.4°E ± 5.4° and −12.8 ± 6.0 nanostrain yr −1 N01.6°W ± 5.4°, extension reckoned positive, and the clockwise (as seen from above the Earth) rotation rate about a vertical axis is 13.6 ± 4.0 nrad yr −1 . The quoted uncertainties are standard deviations. The motion in the Walker Lane belt can then be represented by a zone striking N35°W subject to 16.8 ± 4.9 nstrain yr −1 extension perpendicular to it and 19.5 ± 4.0 nstrain yr −1 right-lateral, simple shear across it. The N35°W strike of the zone is the same as the direction of the local tangent to the small circle drawn about the Pacific-North America pole of rotation. The principal strain rates for the CNSZ are 46.2 ± 11.0 nstrain yr −1 N49.9°W ± 6.0° and −13.6 ± 6.1 nstrain yr −1 N40.1°E ± 6.0°, and the clockwise rotation rate about a vertical axis is 20.3 ± 6.3 nrad yr −1 . The motion across the CNSZ can then be represented by a zone striking N12°E subject to 32.6 ± 11.0 nstrain yr −1 extension perpendicular to it and 25.1 ± 6.3 nstrain yr −1 right-lateral, simple shear across it. The N12°E strike of the zone is similar to the strikes of the faults (Rainbow Mountain, Fairview Peak, and Dixie Valley) within it.

California, Nevada↗

USGS2002

Strain accumulation and rotation in western Oregon and southwestern Washington

Velocities of 75 geodetic monuments in western Oregon and southwestern Washington extending from the coast to more than 300 km inland have been determined from GPS surveys over the interval 1992–2000. The average standard deviation in each of the horizontal velocity components is ∼1 mm yr −1 . The observed velocity field is approximated by a combination of rigid rotation (Euler vector relative to interior North America: 43.40°N ± 0.14°, 119.33°W ± 0.28°, and 0.822 ± 0.057° Myr −1 clockwise; quoted uncertainties are standard deviations), uniform regional strain rate (ε EE = −7.4 ± 1.8, ε EN = −3.4 ± 1.0, and ε NN = −5.0 ± 0.8 nstrain yr −1 , extension reckoned positive), and a dislocation model representing subduction of the Juan de Fuca plate beneath North America. Subduction south of 44.5°N was represented by a 40‐km‐wide locked thrust and subduction north of 44.5°N by a 75‐km‐wide locked thrust.

Oregon, Washington↗

USGS2001

Strain accumulation and rotation in the Eastern California Shear Zone

Although the Eastern California Shear Zone (ECSZ) (strike ∼N25°W) does not quite coincide with a small circle drawn about the Pacific-North America pole of rotation, trilateration and GPS measurements demonstrate that the motion within the zone corresponds to right-lateral simple shear across a vertical plane (strike N33°W±5°) roughly parallel to the tangent to that local small circle (strike ∼N40°W). If the simple shear is released by slip on faults subparallel to the shear zone, the accumulated rotation is also released, leaving no secular rotation. South of the Garlock fault the principal faults (e.g., Calico-Blackwater fault) strike ∼N40°W, close enough to the strike of the vertical plane across which maximum right-lateral shear accumulates to almost wholly accommodate that accumulation of both strain and rotation by right-lateral slip. North of the Garlock fault dip slip as well as strike slip on the principal faults (strike ∼N20°W) is required to accommodate the simple shear accumulation. In both cases the accumulated rotation is released with the shear strain. The Garlock fault, which transects the ECSZ, is not offset by north-northwest striking faults nor, despite geological evidence for long-term left-lateral slip, does it appear at the present time to be accumulating left-lateral simple shear strain across the fault due to slip at depth. Rather the motion is explained by right-lateral simple shear across the orthogonal ECSZ. Left-lateral slip on the Garlock fault will release the shear strain accumulating there but would augment the accumulating rotation, resulting in a secular clockwise rotation rate ∼80 nrad yr −1 (4.6° Myr −1 ).

California↗

USGS2001

Deformation across the Pacific-North America plate boundary near San Francisco, California

We have detected a narrow zone of compression between the Coast Ranges and the Great Valley, and we have estimated slip rates for the San Andreas, Rodgers Creek, and Green Valley faults just north of San Francisco. These results are based on an analysis of campaign and continuous Global Positioning System (GPS) data collected between 1992 and 2000 in central California. The zone of compression between the Coast Ranges and the Great Valley is 25 km wide. The observations clearly show 3.8±1.5 mm yr −1 of shortening over this narrow zone. The strike slip components are best fit by a model with 20.8±1.9 mm yr −1 slip on the San Andreas fault, 10.3±2.6 mm yr −1 on the Rodgers Creek fault, and 8.1±2.1 mm yr −1 on the Green Valley fault. The Pacific-Sierra Nevada-Great Valley motion totals 39.2±3.8 mm yr −1 across a zone that is 120 km wide (at the latitude of San Francisco). Standard deviations are one σ. The geodetic results suggest a higher than geologic rate for the Green Valley fault. The geodetic results also suggest an inconsistency between geologic estimates of the San Andreas rate and seismologic estimates of the depth of locking on the San Andreas fault. The only convergence observed is in the narrow zone along the border between the Great Valley and the Coast Ranges.

California↗

USGS2000

Viscoelastic-coupling model for the earthquake cycle driven from below

In a linear system the earthquake cycle can be represented as the sum of a solution which reproduces the earthquake cycle itself (viscoelastic-coupling model) and a solution that provides the driving force. We consider two cases, one in which the earthquake cycle is driven by stresses transmitted along the schizosphere and a second in which the cycle is driven from below by stresses transmitted along the upper mantle (i.e., the schizosphere and upper mantle, respectively, act as stress guides in the lithosphere). In both cases the driving stress is attributed to steady motion of the stress guide, and the upper crust is assumed to be elastic. The surface deformation that accumulates during the interseismic interval depends solely upon the earthquake-cycle solution (viscoelastic-coupling model) not upon the driving source solution. Thus geodetic observations of interseismic deformation are insensitive to the source of the driving forces in a linear system. In particular, the suggestion of Bourne et al . [1998] that the deformation that accumulates across a transform fault system in the interseismic interval is a replica of the deformation that accumulates in the upper mantle during the same interval does not appear to be correct for linear systems.

Journal of Geophysical Research B: Solid Earth↗

USGS2000

Strain accumulation across the Eastern California Shear Zone at latitude 36°30'N

The motion of a linear array of monuments extending across the Eastern California Shear Zone (ECSZ) has been measured from 1994 to 1999 with the Global Positioning System. The linear array is oriented N54°E, perpendicular to the tangent to the local small circle drawn about the Pacific-North America pole of rotation, and the observed motion across the ECSZ is approximated by differential rotation about that pole. The observations suggest uniform deformation within the ECSZ (strike N23°W) (26 nstrain yr −1 extension normal to the zone and 39 nstrain yr −1 simple right-lateral shear across it) with no significant deformation in the two blocks (the Sierra Nevada mountains and southern Nevada) on either side. The deformation may be imposed by right-lateral slip at depth on the individual major fault systems within the zone if the slip rates are: Death Valley-Furnace Creek fault 3.2±0.9 mm yr −1 , Hunter Mountain-Panamint Valley fault 3.3±1.6 mm yr −1 , and Owens Valley fault 6.9±1.6 mm yr −1 . However, this estimate of the slip rate on the Owens Valley fault is 3 times greater than the geologic estimate.

Journal of Geophysical Research B: Solid Earth↗

USGS2000

Deformation across the forearc of the Cascadia subduction zone at Cape Blanco, Oregon

Over the interval 1992-1999 the U.S. Geological Survey measured the deformation of a geodetic array extending N880°E (approximate direction of plate convergence) from Cape Blanco on the Oregon coast to the volcanic arc near Newberry Crater (55 and 350 km, respectively, from the deformation front). Within about 150 km from the deformation front, the forearc is being compressed arcward (N80°E) by coupling to the subducting Juan de Fuca plate. Dislocation modeling of the observed N80°E compression suggests that the main thrust zone (the locked portion of the Juan de Fuca-forearc interface) is about 40 km wide in the downdip direction. The transverse (N10°W) velocity component of the forearc measured with respect to the fixed interior of North America decreases with distance from the deformation front at a rate of about 0.03 mm yr -1 km -1 . That gradient appears to be a consequence of rigid rotation of the forearc block relative to fixed interior North America (Euler vector of 43.4°±0.1° N, 120.0°±0.4° W, and -1.67±0.17° (m.y.) -1 ; quoted uncertainties are standard deviations). The rotation rate is similar to the paleomagnetically measured rotation rate (-1.0±0.2° (m.y.) -1 ) of the 15 Ma lava flows along the Columbia River 250 km farther north. The back arc does not appear to participate in this rotation but rather is migrating at a rate of about 3.6 mm yr -1 northward with respect to fixed North America. That migration could be partly an artifact of an imperfect tie of our reference coordinate system to the interior of North America.

Journal of Geophysical Research B: Solid Earth↗

USGS1999

Deformation across the Alaska-Aleutian Subduction Zone near Kodiak

The Kodiak-Katmai geodetic array, nine monuments distributed along a profile trending north-northwestward across Kodiak Island and the Alaska Peninsula, was surveyed in 1993, 1995 and 1997 to determine the deformation at the Alaska-Aleutian subduction zone. Velocities on Kodiak island measured relative to the stable North American plate decrease with distance from the Alaska-Aleutian trench (distance range 106 to 250 km), whereas no appreciable deformation was measured on the Alaska Peninsula (distances 250 to 370 km from the trench). The measured deformation is reasonably well predicted by the conventional dislocation representation of subduction with the model parameters determined independently (i.e., not simply by fitting the observations). The deformation of Kodiak Island is in striking contrast to the very minor deformation measured in the similarly situated Shumagin Islands, 450 km southwest of Kodiak along the Alaska-Aleutian trench.

Geophysical Research Letters↗

USGS1999

Geodetic estimates of fault slip rates in the San Francisco Bay area

Bourne et al. [1998] have suggested that the interseismic velocity profile at the surface across a transform plate boundary is a replica of the secular velocity profile at depth in the plastosphere. On the other hand, in the viscoelastic coupling model the shape of the interseismic surface velocity profile is a consequence of plastosphere relaxation following the previous rupture of the faults that make up the plate boundary and is not directly related to the secular flow in the plastosphere. The two models appear to be incompatible. If the plate boundary is composed of several subparallel faults and the interseismic surface velocity profile across the boundary known, each model predicts the secular slip rates on the faults which make up the boundary. As suggested by Bourne et al., the models can then be tested by comparing the predicted secular slip rates to those estimated from long-term offsets inferred from geology. Here we apply that test to the secular slip rates predicted for the principal faults (San Andreas, San Gregorio, Hayward, Calaveras, Rodgers Creek, Green Valley and Greenville faults) in the San Andreas fault system in the San Francisco Bay area. The estimates from the two models generally agree with one another and to a lesser extent with the geologic estimate. Because the viscoelastic coupling model has been equally successful in estimating secular slip rates on the various fault strands at a diffuse plate boundary, the success of the model of Bourne et at. [1998] in doing the same thing should not be taken as proof that the interseismic velocity profile across the plate boundary at the surface is a replica of the velocity profile at depth in the plastosphere.

Journal of Geophysical Research B: Solid Earth↗

USGS1998

Strain accumulation rates in the San Francisco Bay area, 1972–1989

Maps of the strain accumulation rate in the San Francisco Bay area have been constructed from trilateration observations extending from about 1972 until the Loma Prieta earthquake in late 1989. The observations were corrected to remove offsets imposed by shallow fault creep and by four M ∼6 earthquakes that occurred in the Bay area during that time interval. The Bay area was divided into 32 contiguous polygons, and the uniform (in both space and time) strain rates that best explain the changes in the corrected (earthquake and shallow fault creep offsets removed) distances within each polygon were calculated. In a coordinate system with the 1 axis directed N58°E and the 2 axis N32°W (perpendicular and parallel to the local tangent to the small circle drawn about the Pacific-Sierra Nevada pole of rotation) the averages of these 32 strain rates (each weighted by the area of the polygon) are ε11=9.2±7.4, ε12=−160.7±4.6, and ε22=8.2±6.2 nanostrain/yr, where extension is reckoned positive and quoted uncertainties are standard deviations. As expected from the Pacific-Sierra Nevada relative plate motion, the overall strain rate is predominantly right-lateral shear across a vertical plane striking N32°W. The net increase in the 12,225 km2 area of the trilateration network is only 212±110 m2/yr, which arises from almost equal extensions in the N32°W and N58°E directions. Within the network the strain rates vary from polygon to polygon. Significant areal dilatation rates are observed in almost ⅓ of the individual polygons and the N32°W extension rates tend to be negative to the west of the Hayward-Rodgers Creek fault trend and positive east of it. The N58E extension rate is positive in 22 out of 32 polygons, a proportion that is significantly larger than would be expected by chance if the N58°E extension rate were zero or negative. The pre-1989 strain accumulation across the eventual site of the Loma Prieta rupture involves fault normal contraction as well as right-lateral shear, consistent with the rupture mechanism.

California↗

USGS1998

Viscoelastic coupling model of the San Andreas fault along the Big Bend, southern California

The big bend segment of the San Andreas fault is the 300-km-long segment in southern California that strikes about N65°W, roughly 25° counterclockwise from the local tangent to the small circle about the Pacific-North America pole of rotation. The broad distribution of deformation of trilateration networks along this segment implies a locking depth of at least 25 km as interpreted by the conventional model of strain accumulation (continuous slip on the fault below the locking depth at the rate of relative plate motion), whereas the observed seismicity and laboratory data on fault strength suggest that the locking depth should be no greater than 10 to 15 km. The discrepancy is explained by the viscoelastic coupling model which accounts for the viscoelastic response of the lower crust. Thus the broad distribution of deformation observed across the big bend segment can be largely associated with the San Andreas fault itself, not subsidiary faults distributed throughout the region. The Working Group on California Earthquake Probabilities [1995] in using geodetic data to estimate the seismic risk in southern California has assumed that strain accumulated off the San Andreas fault is released by earthquakes located off the San Andreas fault. Thus they count the San Andreas contribution to total seismic moment accumulation more than once, leading to an overestimate of the seismicity for magnitude 6 and greater earthquakes in their Type C zones.

Journal of Geophysical Research B: Solid Earth↗