The M 6.7 Northridge earthquake of January 17,1994, produced concentrated ground deformation and associated damage in a 5-km-long linear belt within suburban Granada Hills and Mission Hills in the northern San Fernando Valley. Small-displacement cracks comprise discrete zones with characteristics that reflect control by local physiographic and near-surface conditions. Several semi-arcuate zones of tension cracks and a zone of compression (expressed by crumpled pipelines and crushed pavement) formed in Holocene alluvium on the gently sloping piedmont west of Bull Canyon. These observations, along with evidence from trenches and surveys of road deformation, point to shallow mass movement as the primary cause of deformation in the area. A combination of tension cracks and compressional buckles developed along the pre-development course of Bull Creek in lower Bull Canyon and above filled tributary drainages farther east. The relation of this deformation to stream channels and, locally, to small sand boils implicates liquefaction, sediment compaction, and/or lurching of surficial deposits as the likely cause(s) of deformation. Tension cracks also developed on the moderately steep flank of the Mission Hills anticline at the east end of the deformation belt. Locally, these features are clearly related to slope failure, but cracks in the largest set also follow downslope-dipping bedding and thus could be an expression of slip accommodation related to possible coselsmic folding in the anticline. The various characteristics of deformation observed in the Granada Hills-Mission Hills belt are consistent with one or more types of shallow-seated phenomena and are not consistent with a tectonic faulting origin. The nature of these phenomena implicates strong ground shaking as a primary driving mechanism for deformation. The distribution of ground deformation strongly reflects the distribution of damage (to roads, utility lines, and structures) in the Granada Hills-Mission Hills-area. This correlation points to the value of understanding factors that control the location of shaking-induced ground failures (such as location of concealed thrust faults), as well as how frequently these failures occur. Understanding the risk posed by this hazard requires more work, however; both locally, to better establish recurrence times, and regionally, to identify the extent of the hazard.

Afterslip data from the Superstition Hills fault in southern California, a creep event on the same fault, the modified Omori law, and cumulative moments from aftershocks of the 1957 Aleutian Islands earthquake all indicate that the original formulation by Dieterich (1981) [Constitutive properties of faults with simulated gouge. AGU, Geophys. Monogr. 24, 103–120] for friction evolution is more appropriate for systems far from instability than the commonly used approximation developed by Ruina (1983) [Slip instability and state variable friction laws. J. Geophys. Res. 88, 10359–10370] to study instability. The mathematical framework we use to test the friction models is a one-dimensional, massless spring-slider under the simplifying assumption, proposed by Scholz (1990) [The Mechanics of Earthquakes and Faulting. Cambridge University Press] and used by Marone et al. (1991) [On the mechanics of earthquake afterslip. J. Geophys. Res., 96: 8441–8452], that the state variable takes on its velocity-dependent steady-state value throughout motion in response to a step in stress. This assumption removes explicit state-variable dependence from the model, obviating the need to consider state-variable evolution equations. Anti-derivatives of the modified Omori law fit our data very well and are very good approximate solutions to our model equations. A plausible friction model with Omori-law solutions used by Wesson (1988) [Dynamics of fault creep. J. Geophys. Res. 93, 8929–8951] to model fault creep and generalized by Rice (1983) [Constitutive relations for fault slip and earthquake instabilities. Pure Appl. Geophys. 121, 443–475] to a rate-and-state variable friction model yields exactly Omori's law with exponents greater than 1, but yields unstable solutions for Omori exponents less than 1. We estimate from the Dieterich formulation the dimensionless parameter a∗ which is equal to the product of the nominal coefficient of friction and the more commonly reported friction parameter a. We find that a∗ is typically positive, qualitatively consistent with laboratory observations, although our observations are considerably larger than laboratory values. However, we also find good model fits for a∗ < 0 when data correspond to Omori exponents less than 1. A modification of the stability analysis by Rice and Ruina (1983) [Stability of steady frictional slipping. J. Appl. Mech. 50, 343–349] indicates that a∗ < 0 is not a consequence of our assumption regarding state-variable evolution. A consistent interpretation of a∗ < 0 in terms of laboratory models appears to be that the data are from later portions of processes better characterized by two-state-variable friction models. a∗ < 0 is explained by assuming that our data cannot resolve the co-seismic evolution of a short-length-scale state variable to a velocity-weakening state; our parameterization leads to an apparent negative instantaneous viscosity. We estimate the largest critical slip distance associated with afterslip to be ∼1–10 cm, consistent with other estimates for near-surface materials. We assume that our observed large values for a∗ reflect the fact that our model ignores the geometrical complexities of three-dimensional stresses in fractured crustal materials around a fault zone with frictional stresses that vary on a fault surface. Our one-dimensional model parameters reflect spatially averaged, bulk, stress and frictional properties of a fault zone, where we clearly cannot specify the details of the averaging process. Our analysis of Omori's law suggests that bulk-frictional properties of a fault zone are well described by our simple laboratory-based models, but they would need to change during the seismic cycle for a mainshock instability to recur, unless a mainshock-aftershock sequence were characterized by a process similar to the arrested instabilities possible in two-state-variable systems.

The M 6.7 Northridge earthquake of January 17,1994, produced concentrated ground deformation and associated damage in a 5-km-long linear belt within suburban Granada Hills and Mission Hills in the northern San Fernando Valley.Small-displacement cracks comprise discrete zones with characteristics that reflect control by local physiographic and near-surface conditions.Several semi-arcuate zones of tension cracks and a zone of compression (expressed by crumpled pipelines and crushed pavement) formed in Holocene alluvium on the gently sloping piedmont west of Bull Canyon.These observations, along with evidence from trenches and surveys of road deformation, point to shallow mass movement as the primary cause of deformation in the area.A combination of tension cracks and compressional buckles developed along the pre-development course of Bull Creek in lower Bull Canyon and above filled tributary drainages farther east.The relation of this deformation to stream channels and, locally, to small sand boils implicates liquefaction, sediment compaction, and/or lurching of surficial deposits as the likely cause(s) of deformation.Tension cracks also developed on the moderately steep flank of the Mission Hills anticline at the east end of the deformation belt.Locally, these features are clearly related to slope failure, but cracks in the largest set also follow downslope-dipping bedding and thus could be an expression of slip accommodation related to possible coselsmic folding in the anticline.The various characteristics of deformation observed in the Granada Hills-Mission Hills belt are consistent with one or more types of shallow-seated phenomena and are not consistent with a tectonic faulting origin.The nature of these phenomena implicates strong ground shaking as a primary driving mechanism for deformation.The distribution of ground deformation strongly reflects the distribution of damage (to roads, utility lines, and structures) in the Granada Hills-Mission Hills-area.This correlation points to the value of understanding factors that control the location of shaking-induced ground failures (such as location of concealed thrust faults), as well as how frequently these failures occur.Understanding the risk posed by this hazard requires more work, however; both locally, to better establish recurrence times, and regionally, to identify the extent of the hazard.

The 12 October 1992 earthquake (MB 5.9; Ms 5.2) that occurred near the village of Dahshur, Egypt, was the latest in a long history of earthquakes that occasionally cause damage to buildings in Cairo and surrounding areas in northeastern Egypt.The epicenter of the earthquake was about 18 km south of the center of Cairo and located at a depth of about 25 km.Damage to engineered buildings in Cairo appeared to be mainly due to poor construction materials, poor construction detailing, overloading of building columns beyond their design values, inferior workmanship, or deficient foundation systems.Damage to non-engineered structures, such as dwellings, was substantial in the earthquake region, particularly for adobe construction.Adobe is known to be one of the most vulnerable building materials to earthquake shaking; widespread use of this material in villages along the Nile valley contributed significantly to the high number of deaths and injuries from this moderate-sized earthquake.Inspections of bridges across the Nile and overpasses in Cairo revealed no earthquake damage.Geologic effects of the earthquake were minimal, restricted to a small area of minor liquefaction near the village of Manshyat Fadil on the west side of the Nile, approximately 20 km south of the epicenter.The area of liquefaction coincided with the severest shaking damage from the earthquake in Manshyat Fadil.Field investigations by motor vehicles and by airplane revealed no surface fault rupture from the earthquake.That more widespread damage did not occur in the Cairo metropolitan area was due to the moderate size of this earthquake.However, considerable vulnerability to earthquake shaking exists in the Cairo building stock and far more severe effects would be expected from a somewhat larger earthquake in the vicinity of Cairo, or from an earthquake of comparable magnitude directly under the city.The important lesson learned from this earthquake was not of a technical or scientific nature.Rather, it was the tragic demonstration of the potential for catastrophe in metropolitan Cairo if the warning of this moderate earthquake is not heeded in future development, planning, and earthquake preparedness.With this lesson in mind, we offer the subsequent recommendations on an earthquake preparedness strategy for the region affected by the 12 October 1992 Dahshur, Egypt, earthquake.They are divided into three categories: A) Engineering design and building code development, B) Scientific and engineering research needs, and C) Civil preparedness and governmental disaster response. A. Engineering Design and Building-Code Development:An earthquake itself does not pose a significant threat to people.Rather, it is the damage to, and collapse of buildings and other structures that result in casualties and fatalities.If construction practices are improved so that buildings withstand the expected earthquake ground motions with less damage and without collapse, lives will be saved and the number of injuries will be reduced.Much engineering knowledge has been gained from the study of earthquake effects on all types of construction around the world and the task of improving building construction and design begins with communicating this knowledge to local authorities and the local engineering community.Significant improvement to the earthquake resistance of common structures, such as dwellings, can be made with little additional cost if proper construction techniques are used.A lasting contribution to improved earthquake-resistant design for all types of construction is to formalize appropriate earthquake-resistant construction techniques into a building code.Equitable application of building-code requirements throughout a region follows a scheme of zoning based on the prevalent earthquake ground-motion hazard.Thus, the ground-motion hazard needs to be quantified in hazard maps for rational decisions to be made on the degree of earthquake resistance that is required of construction in any particular area.Seismic-hazard maps of this type have also found wide-spread use in land-use planning and insurance analyses.Recommendations in this category are aimed at near-term objectives (approximately three years or less to realize results) to deal with the immediate earthquake information needs of the practicing engineers and government agencies charged with overseeing construction practice.The recommendations, in large part, implement common knowledge that has been gained from earthquake experiences around the world.1. Establish an influential task force consisting of knowledgeable engineers, seismologists, and geologists from various sectors ~ academic, private, government ~ to motivate professional and governmental concern and understanding of earthquake mitigation measures.Such a body could form the core group of a local professional organization promoting earthquake education and governmental earthquake-mitigation policy.2. Quantify the earthquake ground-motion hazard throughout Egypt, with particular attention to northeastern Egypt and the vicinity of Cairo, in terms useful to modern engineering design and code development, land-use and economic planning.3. Qualitatively establish the vulnerability of various types of common, high-occupancy construction in Cairo to earthquake shaking for estimates of earthquake risk (monetary loss) from past or potential earthquakes.4. Establish earthquake-engineering-related curricula in the university system and encourage faculty and engineering students to become involved in all aspects of earthquake mitigation engineering including earthquake reconnaissance, recovery and reconstruction.5 Conduct fora for exchanging information.Most observed damage to engineered buildings in the Dahshur earthquake was due to poor detailing and/or quality control.Commonly known errors made during design and construction of earthquake resistant structures could be avoided with simple communication/education vehicles, such as workshops and professional meetings, in which experts are brought together with practicing engineers and local authorities to exchange information on "lessons learned" from past earthquakes.6. Provide guidance and education for non-engineered construction to the general public through information campaigns (see C below).7. The General Authority for Roads and Bridges should adopt established standards for the retrofit of bridges and overpasses.

AbstractThe M 6.2 Elmore Desert Ranch earthquake of 24 November 1987 was associated spatially and probably temporally with left-lateral surface rupture on many northeast-trending faults in and near the Superstition Hills in western Imperial Valley. Three curving discontinuous principal zones of rupture among these breaks extended northeastward from near the Superstition Hills fault zone as far as 9 km; the maximum observed surface slip, 12.5 cm, was on the northern of the three, the Elmore Ranch fault, at a point near the epicenter. Twelve hours after the Elmore Ranch earthquake, the M 6.6 Superstition Hills earthquake occurred near the northwest end of the right-lateral Superstition Hills fault zone. Surface rupture associated with the second event occurred along three strands of the zone, here named North and South strands of the Superstition Hills fault and the Wienert fault, for 27 km southeastward from the epicenter. In contrast to the left-lateral faulting, which remained unchanged throughout the period of investigation, the right-lateral movement on the Superstition hills fault zone continued to increase with time, a behavior that was similar to other recent historical surface ruptures on northwest-trending faults in the Imperial Valley region.We measured displacements over 339 days at as many as 296 sites along the Superstition Hills fault zone, and repeated measurements at 49 sites provided sufficient data to fit with a simple power law. Data for each of the 49 sites were used to compute longitudinal displacement profiles for 1 day and to estimate the final displacement that measured slips will approach asymptotically several years after the earthquakes. The maximum right-lateral slip at 1 day was about 50 cm near the south-central part of the North strand of Superstition Hills fault, and the predicted maximum final displacement is probably about 112 cm at Imler Road near the center of the South strand of the Superstition Hills fault. The overall distributions of right-lateral displacement at 1 day and the estimated final slip are nearly symmetrical about the midpoint of the surface rupture. The average estimated final right-lateral slip for the Superstition Hills fault zone is about 54 cm. The average left-lateral slip for the conjugate faults trending northeastward is about 23 cm.The southernmost ruptured member of the Superstition Hills fault zone, newly named the Wienert fault, extends the known length of the zone by about 4 km. The southern half of this fault, south of New River, expressed only vertical displacement on a sinuous trace. The maximum vertical slip by the end of the observation period there was about 25 cm, but its growth had not ceased. Photolineaments southeast of the end of new surface rupture suggest continuation of the Superstition Hills fault zone in farmland toward Mexico.

Abstract Seven quadrilaterals, constructed at broadly distributed points on surface breaks within the Superstition Hills fault zone, were repeatedly remeasured after the pair of 24 November 1987 earthquakes to monitor the growing surface displacement. Changes in the dimensions of the quadrilaterals are recalculated to right-lateral and extensional components at millimeter resolution, and vertical components of change are resolved at 0.2 mm precision. The displacement component data for four of the seven quadrilaterals record the complete fault movement with respect to an October 1986 base. These data fit with remarkable agreement the power law U ( t ) = U f ( B t 1 + B t ) c , where U(t) is a displacement component at time t after the second main shock and Uf, B, and c are constants. This power law permits estimation of the final displacement, Uf, from the data obtained within the period of observation. Data from one quadrilateral, located near the epicenter of the second main shock and northeast-trending conjugate faults, allow that about 5 cm of right-lateral slip may have been associated with the first main shock there. Data from the other quadrilaterals confirm that the surface faulting on most of the Superstition Hills fault zone did initiate at the time of the second main shock of the 1987 earthquakes. The three-dimensional motion vectors all describe nearly linear trajectories throughout the observation period, and they indicate smooth shearing on their respective fault surfaces. The inclination of the shear surfaces is generally nearly vertical, except near the south end of the Superstition Hills fault zone where two strands dip northeastward at about 70°. Surface displacement on these strands is right reverse. Another kind of deformation, superimposed on the fault displacements, has been recorded at all quadrilateral sites. It consists of a northwest-southeast contraction or component of contraction that ranged from 0 to 0.1 per cent of the quadrilateral lengths between November 1987 and April 1988.

AbstractTwo right-lateral slip events, about 3 weeks apart in November 1987, broke the surface discontinuously along probably similar, nearly 20 km lengths of the northern Imperial fault. The first displacement, at about the beginning of November, was accompanied by a surface tilt representing deep vertical motion or distributed strain. This movement may have been part of a more regional event that also involved the southern San Andreas fault, although the evidence there is questionable. The later surface offset was triggered, probably by the second main shock of the 24 November earthquakes located in the Superstition Hills, about 37 km northwest of the Imperial fault. The maximum observed displacement was less than 4 cm on both occasions; for the triggered movement the maximum slip occurred on a branch strand near the northern extremity of the fault.Pre-earthquake resurveys of short-length leveling lines indicated combined surface displacement and an eastward tilt at two locations between surveys 712 months apart and 13 months apart; the tilt at Harris Road during this pre-earthquake interval is modeled as a 1.4 cm vertical component of slip deeper than 100 m on a 70° northeastward-dipping fault. Later resurveys showed a marked reduction of deep movement in the time period including the triggered slip, and between December 1987 and January 1988, it had ceased.No definite evidence of surface fracturing was found in the Brawley fault zone during either period of time when the Imperial fault moved. Remeasurement of leveling lines there indicated small, very near-surface vertical components of movement in the northern half of the zone. Elevation changes in the southern part indicated distributed strain and/or movement only deeper than about 50 m. The Brawley zone changes are not fixed in time well enough to clearly relate them to the surface movements on the Imperial fault.

AbstractParts of the Imperial and the Superstition Hills faults moved right laterally at the ground surface at the time of or shortly following the ML 5.6 Westmorland earthquake of 26 April 1981. The displacements probably occurred before any significant aftershocks on either fault and thus are classed as triggered slips. Although the main shock was located in an exceptionally seismogenic part of Imperial Valley, about 20 km distant from either fault, no clear evidence of past surface faulting is known in the epicentral area. Horizontal displacement on the Imperial and Superstition Hills faults, southeast and southwest of the epicenter, respectively, reached maxima of 8 and 14 mm, and the discontinuous surface ruptures formed along approximately equal lengths of northern segments of the two structures (16.8 and 15.7 km, respectively). The maximum vertical component of slip measured on surface cracks on the Imperial fault (6 mm) was located on the west side of Mesquite basin near Harris Road. Leveling data at Harris Road suggest slightly larger vertical movement at the surface there. Fault dislocation modeling of this leveling data further suggests that the maximum shallow subsurface dip-slip component of the triggered movement was as much a factor of 4 larger than that measured at the fault trace. The map pattern of the rupture may reveal the principal strand among the several near the north end of the Imperial fault.No surface displacement was found along traces of the Brawley fault zone, the San Andreas fault, or the part of the Coyote Creek fault that slipped during the 1968 Borrego Mountain earthquake. Ground search in the epicentral area of the main shock and in the widely dispersed aftershock region in northern Imperial Valley failed to locate any definite evidence of new surface faulting.

Three strike slip displacements of strata with known approximate ages have been measured at two locations on the San Jacinto fault zone. Minimum horizontal offset between 5.7 and 8.6 km in no more than 0.73 m.y. northeast of Anza indicates 8–12‐mm/yr average slip rate since late Pleistocene time. Two measures of more recent displacement are based on trenching studies of stratigraphic offsets on the Coyote Creek fault in western Imperial Valley. Horizontal slip of 1.7 m has been calculated for the youngest sediment of Lake Cahuilla since its deposition 275–510 yr B.P. The corresponding slip rate is 2.8–5.0 mm/yr. Right lateral offset of 10.9 m measured on a buried stream channel older than 5060 yr B.P. but younger than 6820 yr B.P. yields average slip rates for the intermediate time periods, 400± to 6000± yr B.P., of 1–2 mm/yr. The average rates of slip for these three time intervals suggest a relatively quiescent period for the San Jacinto fault zone from about 4000 B.C. to about 1600 A.D. To the extent that long‐term variations in seismic activity of major strike slip faults elsewhere are known, fluctuations in slip rate for the San Jacinto fault would not appear to be abnormal. If the San Jacinto and adjacent segments of the San Andreas fault alternately assume dominant roles in absorbing motion between the Pacific and American plates, perhaps even more recently than 400 yr ago, the San Andreas fault south of the Transverse Ranges expressed most of the motion but has since become relatively inactive.

Research Article| March 01, 1978 Fault movement (afterslip) following the Guatemala earthquake of February 4, 1976 R. C. Bucknam; R. C. Bucknam 1U.S. Geological Survey, Stop 966, Denver Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar George Plafker; George Plafker 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar R. V. Sharp R. V. Sharp 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information R. C. Bucknam 1U.S. Geological Survey, Stop 966, Denver Federal Center, Denver, Colorado 80225 George Plafker 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 R. V. Sharp 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1978) 6 (3): 170–173. https://doi.org/10.1130/0091-7613(1978)6<170:FMAFTG>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. C. Bucknam, George Plafker, R. V. Sharp; Fault movement (afterslip) following the Guatemala earthquake of February 4, 1976. Geology 1978;; 6 (3): 170–173. doi: https://doi.org/10.1130/0091-7613(1978)6<170:FMAFTG>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Field studies of surface faulting associated with the Guatemala earthquake of February 4, 1976, have documented the occurrence of afterslip at seven locations along the 230 km of surface rupture. The total displacement across the fault as measured in April 1976 averaged 110 cm. Displacement at one location increased from 60 cm on February 8, 1976, to 91 cm on October 6, 1977. Afterslip time histories determined at three sites show the afterslip to be proportional to the logarithm of time since the earthquake, and that the rate of afterslip is inversely related to the amount of displacement at a site. The regular variation in total slip and afterslip along about 50 km of the fault trace suggests that the afterslip is not controlled by local, near-surface geologic factors such as alluvial cover. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The m b 5.9 earthquake of August 1, 1975, near Lake Oroville, California, was accompanied and followed by normal faulting at the surface. The distribution of aftershocks led directly to our discovery of the 3.8 km-long N- to NNW-trending zone of new fractures. In position, orientation, and sense of slip, the surface faulting agrees with the fault zone defined by mapped aftershocks and focal mechanisms, and is compatible with deformation shown by comparisons between pre- and postearthquake level surveys. The block to the east of the fault moved upward relative to that to the west, as shown by at least 55 mm of slip across the surface ruptures and 180 mm of vertical movement of benchmarks near the rupture zone. The faulting follows a zone of earlier displacement that may have been active in Quaternary time.

Research Article| December 01, 1973 Reinterpretation of the Boundary between the Cosumnes and Logtown Ridge Formations, Amador County, California ROBERT V. SHARP; ROBERT V. SHARP 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar WENDELL A. DUFFIELD WENDELL A. DUFFIELD 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information ROBERT V. SHARP 1U.S. Geological Survey, Menlo Park, California 94025 WENDELL A. DUFFIELD 1U.S. Geological Survey, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (12): 3969–3976. https://doi.org/10.1130/0016-7606(1973)84<3969:ROTBBT>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ROBERT V. SHARP, WENDELL A. DUFFIELD; Reinterpretation of the Boundary between the Cosumnes and Logtown Ridge Formations, Amador County, California. GSA Bulletin 1973;; 84 (12): 3969–3976. doi: https://doi.org/10.1130/0016-7606(1973)84<3969:ROTBBT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Recent detailed geologic mapping in the Sierran foothills reveals that rocks previously included in the Jurassic Amador Group must be redefined. The term “Amador Group” was applied by Taliaferro and Clark to a section of epiclastic metasedimentary rocks (the Cosumnes Formation) and the seemingly conformable overlying metavolcanic rocks (the Logtown Ridge Formation).New structural and stratigraphic evidence indicates that at their type localities on the banks of the Cosumnes River the boundary between the two formations should be relocated about 610 m downsection from the position shown by Clark. This change removes all known paleontological control on the age of the Cosumnes Formation.Structural relations show that the type Cosumnes and Logtown Ridge Formations are in fault contact at the Cosumnes River. Rocks of the Cosumnes Formation are now grouped with a complex unit of megabreccia that includes other strata previously termed the “western belt” of the Calaveras Formation. The megabreccia formed, at least partly, sometime between late Paleozoic and Late Jurassic times, but rocks in the megabreccia, including the Cosumnes Formation, could be older than late Paleozoic.The term “Amador Group” is herein abandoned. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.