Rupture Segment Research Articles

Evolution of the stress field in southern California and triggering of moderate‐size earthquakes: A 200‐year perspective

Changes in stress in southern California are modeled from 1812 to 2025 using as input (1) stress drops associated with six large (7.0≤M<7.5) to great (M≥7.5) earthquakes through 1995 and (2) stress buildup associated with major faults with slip rates ≥3 mm/yr as constrained by geodetic, paleoseismic, and seismic measurements. Evolution of stress and the triggering of moderate to large earthquakes are treated in a tensorial rather than a scalar manner. We present snapshots of the cumulative Coulomb failure function (ΔCFF) as a function of time for faults of various strike, dip, and rake throughout southern California. We take ΔCFF to be zero everywhere just prior to the great shock of 1812. We find that about 95% of those well‐located M≥6 earthquakes whose mechanisms involve either strike‐slip or reverse faulting are consistent with the Coulomb stress evolutionary model; that is, they occurred in areas of positive ΔCFF. The interaction between slow‐moving faults and stresses generated by faster‐moving faults significantly advanced the occurrence of the 1933 Long Beach and 1992 Landers events in their earthquake cycles. Coulomb stresses near major thrust faults of the western and central Transverse Ranges have been accumulating for a long time. Future great earthquakes along the San Andreas fault, especially if the San Bernardino and Coachella Valley segments rupture together, can trigger moderate to large earthquakes in the Transverse Ranges, as appears to have happened in the Santa Barbara earthquake that occurred 13 days after the great San Andreas shock of 1812. Maps of current ΔCFF provide additional guides to long‐term earthquake prediction.

Late Quaternary stream incision and uplift in the forearc of the Cascadia subduction zone, western Oregon

Documentation of a latest Pleistocene/earliest Holocene episode of strath formation and fluvial aggradation in the Oregon Coast Range provides a datum from which long‐term bedrock stream incision rates are determined. Variations in long‐term incision rates probably reflect cumulative differential uplift in the forearc of the Cascadia subduction zone, although factors such as bedrock and climatic controls and isostatic adjustments to erosion obscure the precise relationship between surface uplift and stream incision. Patterns of differential incision are most striking near the latitude of Newport, where a steep gradient divides a region of higher rates (∼0.6–0.9 mm/yr) in the northern Coast Range from a region of lower rates (∼0.1–0.3 mm/yr) in the central Coast Range. The steep incision gradient is nearly coincident with abrupt changes in marine terrace (∼80–125 kyr) uplift rates, the locations of Quaternary faults, and the southern flank of a saddle of low historic (∼40–70 years) uplift. The exact causes of these variable patterns of incision/uplift are unknown. Analogies with uplift patterns in other subduction zones and comparisons with other neotectonic data in the region indicate that patterns of differential incision probably are caused by variations in permanent strain accumulation along the Cascadia subduction zone. Such variations may be related to differences in seismic moment release during individual earthquakes, to changes in plate geometry or rates of wedge accretion, to segmentation of earthquake ruptures, and/or to deformation on active structures in the North American plate and accretionary wedge.

Modeling of short-period surface-wave propagation in southern California

Abstract For the 1992 Landers mainshock, the lateral heterogeneity of the lithosphere combined with an extended earthquake source may account for the complexity of the strong-motion observations that cannot be easily interpreted by a simplified point-source model in a laterally homogeneous medium. Both the effect of an extended source and that of lateral heterogeneity are examined using a forward modeling technique that combines distributed nonuniform slip on a fault plane in a 3D medium. Surface-wave synthetics are constructed using surface-wave Gaussian beam method, which allows for a mild degree of lateral heterogeneity. We analyze the amplitude and waveform data of the Landers strong-motion recordings in southern California in order to identify the path effects and the effect of an extended source. The overall source parameters of the Landers earthquake are obtained by comparing synthetics with recordings at near-source stations which lie within relatively homogeneous path structures. This procedure yields a unilateral rupture propagating northwesterly over two distinct rupture segments with somewhat different focal mechanisms. The surface-wave phase velocity map is modified to fit the data recorded at larger hypocentral distances. The final improved model generates a very good agreement between observations and synthetics. In the presence of lateral heterogeneity, the focusing, defocusing, and multipath interference are found to contribute significantly to the strong-motion shaking in a 3D basin.

Rupture segmentation and assessment of probabilities of seismic potential on the Xiaojiang fault zone

Spatial distribution of sources of strong and large earthquakes on the Xiaojiang fault zone in eastern Yunnan is studied according to historical earthquake data. 7 segments of relatively independent sources or basic units of rupture along the fault zone have been identified preliminarily. On every segment, time intervals between main historical earthquakes are generally characterized by “time-predictable” recurrence behavior with indetermination. A statistic model for the time intervals between earthquakes of the fault zone has been preliminarily established. And a mathematical method has been introduced into this paper to reckon average recurrence interval between earthquakes under the condition of having known the size of the last event at a specific segment. Based on these, ranges of the average recurrence intervals given confidence have been estimated for events of various sizes on the fault zone. Further, the author puts forward a real-time probabilistic model that is suitable to analyze seismic potential for individual segments along a fault zone on which earthquake recurrence intervals have been characterized by quasi-time-predictable behavior, and applies this model to calculate conditional probabilities and probability gains of earthquake recurring on the individual segments of the Xiaojiang fault zone during the period from 1991 to 2005. As a consequence, it has shown that two parts of this fault zone, from south of Dongchuan to Songming and from Chengjiang to Huaning, have relatively high likelihoods for strong or large earthquake recurring in the future.

Structures in segment boundary zones of the Lost River and Lemhi Faults, east central Idaho

Geologic mapping and structural analysis of the Lost River and Lemhi normal faults in east central Idaho suggest that many rupture segment boundaries, previously inferred from geomorphic data coincide with two classes of structural complexities: (1) zones of distributed faults and steps or jogs in the range front faults, and (2) intersections with preexisting Eocene to Oligocene(?) cross faults at 10–15 km depth. Segment boundaries contain as many as 13 fault splays and are as much as 13 km long and 5–6 km wide (in plan view). In contrast, simple fault traces characterize the range front faults in the interiors of segments. Thus segment boundaries of normal faults are broad zones of intersecting faults, not discrete points. The structural data further imply that segment boundaries contain large volumes of fractured and faulted rock and that zones of distributed normal faults have influenced rupture propagation and arrest along these two normal faults. Intersections of preexisting cross faults and range front faults may also have influenced the locations of rupture boundaries along the Lost River and Lemhi faults. Downdip projections of preexisting Eocene to Oligocene(?) normal faults in the footwall of the Lost River and Lemhi faults may coincide with four or five segment boundary zones at about 15 km depth, the depth at which large earthquakes nucleate in the Basin and Range province. Structural analyses also show that some cross faults, which coincide with segment boundary zones at the surface, are distant from these zones at depth. Conversely, some potentially significant structures present at depth are far removed from the segment boundary zone at the surface. Thus a close spatial relationship between cross faults and segment boundary zones at the surface cannot prove or disprove a genetic relationship between these features because the near‐surface geometry of segment boundary zones may differ dramatically from the subsurface geometry of the zones. Where cross faults define segment boundaries, the enhanced fracture toughness of preexisting fault planes and their damage zones may impede or arrest rupture on younger crosscutting faults.

Fluid inclusions and hydrothermal alteration on the Dixie Valley Fault, Nevada

Footwall rocks of the 1954 rupture segment of the Dixie Valley fault show extensive hydrothermal alteration related to fluids that were present on the fault during tectonic events. Hydrothermal alteration of granitic host rocks consists of temporally and spatially overlapping mineral assemblages. An early, biotite‐feldspar assemblage is followed by later Fe‐chlorite and epidote. Both chlorite and epidote are replaced by hydrothermal sericite and cross‐cut by calcite‐hematite and quartz‐calcite veins. Biotite is partially replaced by prehnite. The latest hydrothermal minerals are stilbite, laumontite, kaolinite, alunite, smectite, illite, and pervasive replacement of rock units with fine grained quartz, chalcedony, and opal. Secondary fluid inclusions trapped in healed microfractures in igneous quartz include type I inclusions that contain a moderate salinity aqueous liquid and vapor, type II inclusions that contain a moderate salinity aqueous liquid and CO2; type III inclusions that show eutectic melting temperatures below the NaCl‐H2O eutectic and contain substantial CaCl2, and type IV inclusions containing halite and other daughter minerals. Microthermometric measurements on these inclusions yield variable compositions and homogenization temperatures. Salinities of type I inclusions vary from 0.1 to 12.9 wt % NaCl with the mode in the interval 0 to 1%. Salinities of type II CO2 bearing inclusions range from 0.62 to 6.81 wt % NaCl relative to H2O, and salinities of type III inclusions with low eutectic melting temperatures are 12.9 to 25.3 NaCl equivalent wt %. Salinities of halite‐bearing inclusions are 30.1 to 39.2 wt % NaCl. Homogenization temperatures span the range 120° to 400°C. The processes of isochemical cooling with upward displacement of the footwall, mixing of cool low‐salinity water with hotter components, and mixing of cool, evaporite brine with hotter components could be responsible for variable fluid inclusion compositions, homogenization temperatures, and densities. The P‐T path of the fault fluids is established by mineral equilibria and fluid inclusion characteristics. The path includes a lithostatic fluid pressure at 305°C and 1570 bars. Along with cooling and escape of CO2 from fluids, the fluid P‐T path probably approaches hydrostatic pressure conditions at lower temperatures. Hydrothermal alteration product minerals, fluid temperatures, pressures, and compositions in the footwall of the Dixie Valley fault constrain minimum fault age to 20 to 25 Ma, displacement to 6 km with about 3 km of pre‐10 to 13 Ma and 3 km of post‐10 Ma uplift. Fluid compositions and P‐T data suggest the following mechanism for rupture initiation and arrest. Ruptures may be initiated as a result of high fluid pressures, then opening of dilatant fractures causes drastic decrease in fluid pressure, separation of steam and CO2. The drastic reduction in fluid bulk modulus that accompanies volatile phase separation permits propagation of the ruptures even though fluid pressure is reduced. In areas where fluid pressure reduction is not accompanied by phase separation, fractures are arrested by dilatant hardening.

Horizontal deformation associated with the Loma Prieta earthquake

AbstractCo-seismic horizontal displacements for the 1989 Loma Prieta earthquake were derived from preseismic triangulation/trilateration observations and post-seismic GPS observations. As part of this process, the empirical model entitled TDP-H91 was applied to “correct” the preseismic measurements for the crustal motion that occurred during the seven decades spanned by these data. These newly derived displacements were combined with previously documented geodetic results to generate a dislocation model for the earthquake. Our preferred model consists of a vertically segmented rupture surface represented by two rectangles that share a common edge at a depth of 9 km. The upper rectangle dips 90° and the lower rectangle dips 70°SW. Via a trial-and-error technique, the following estimates were found for the remaining parameters: strike = 134.4 ± 0.7°, fault length = 32.4 ± 0.7 km, upper depth = 4.8 ± 0.1 km, lower depth = 15.1 ± 0.3 km, right-lateral strike slip = 1.86 ± 0.06 m for the upper rectangle and 1.96 ± 0.13 m for the lower rectangle, and thrusting dip slip = 1.06 ± 0.06 m for the upper rectangle and 2.30 ± 0.18 m for the lower rectangle.

Late Quaternary fault segmentation from analysis of scarp morphology

A thorough knowledge of the lateral extent of individual prehistoric ruptures along a fault is necessary to assess the locations and geologic characteristics of boundaries between independent faultrupture segments. Paleoseismic studies employing only trenching may yield insufficient data to evaluate the lateral extent of individual rupture events. Systematic mapping of fault scarps and degradation-equation modeling of relative scarp ages on the Lemhi fault, a range-bounding normal fault in east-central Idaho, provide a method to assess rupture extent and boundaries between rupture segments. The youngest latest Quaternary surface ruptures occurred at distinctly different times on adjacent segments of the fault. The boundaries between rupture segments differ in characteristics, although most are at intersections of the currently active range-bounding fault and mid-Tertiary normal faults. Several major structural and tectonic discontinuities did not form segment boundaries in the most recent rupture cycle or (probably) in the previous cycle. The Lemhi fault illustrates both the importance of preexisting structures in controlling extent of ruptures and the need for detailed analysis along an entire fault zone.

Historical surface faulting in the Basin and Range province, western North America: implications for fault segmentation

The distribution of surface ruptures caused by 11 historical earthquakes in the Basin and Range province of western North America provides a basis for evaluating earthquake segmentation behavior of faults in extensional tectonic settings. Two of the three moderate magnitude (5.5 < M < 7) events appear to be confined to individual geometric or structural segments. The remaining nine events, eight of which had large magnitudes (M ≥ 7), ruptured multiple geometric or structural segments. Several of these events had widely distributed surface-rupture patterns, ruptured in complex manners, and extended beyond distinct fault-zone discontinuities. Some of the surface ruptures associated with these events may have resulted from sympathetic or secondary surface faulting. Approximately one-half of the surface rupture end points coincided with distinct fault-zone discontinuities. This study indicates that earthquake ruptures in extensional tectonic settings may not be confined to individual geometric or structural segments. Some rupture-controlling discontinuities may be difficult to identify and significant faulting may occur beyond postulated rupture end points. Rupture of multiple geometric or structural segments should be considered in the evaluation of large earthquakes. Several lines of evidence, particularly timing information, are needed to delineate potential earthquake segments in the Basin and Range province.

Variation in fault behaviour in different tectonic provinces of New Zealand

Fault behaviour appears to be different in different tectonic provinces of New Zealand. Uplifted Holocene marine terraces along the east coast of the North Island suggest time-predictable behaviour on reverse faults of the Hikurangi subduction margin. Major dextral faults of the strike-slip fault province in central New Zealand appear to show characteristic slip behaviour. Short normal faults in the Central Volcanic Region show highly variable increments of displacement. Finally, reverse faults of the Central Otago and Northwest Nelson region may have intermittently characteristic patterns of activity. Each of these behaviour patterns has been observed elsewhere, although, where there are good data to constrain the actual dates of palaeoearthquakes, a uniform slip or coupled model best explains most situations. Temporal and spatial clustering of fault rupture and major earthquakes are increasingly being recognized. It is crucial to obtain better dates and to determine the extent of individual rupture segments in order to better characterize fault behaviour. Non-characteristic fault behaviour has serious implications for current methods of seismic hazard evaluation. Seismic hazard studies should utilize specific fault data in preference to assuming a particular fault behaviour model.

Geodetic measurement of deformation in the Loma Prieta, California Earthquake with very long baseline interferometry

Following the Loma Prieta earthquake, two mobile Very Long Baseline Interferometry (VLBI) systems operated by the NASA Crustal Dynamics Project and the NOAA National Geodetic Survey were deployed at three previously established VLBI sites in the earthquake area: Fort Ord (near Monterey), the Presidio (in San Francisco) and Point Reyes. From repeated VLBI occupations of these sites since 1983, the pre‐earthquake rates of deformation have been determined with respect to a North American reference frame with 1σ formal standard errors of ∼1 mm/yr. The VLBI measurements immediately following the earthquake showed that the Fort Ord site was displaced 49 ± 4 mm at an azimuth of 11 ± 4° and that the Presidio site was displaced 12 ± 5 mm at an azimuth of 148 ± 13°. No anomalous change was detected at Point Reyes with 1σ uncertainty of 4 mm. The estimated displacements at Ford Ord and the Presidio are consistent with the static displacements predicted on the basis of a coseismic slip model in which slip on the southern segment is shallower than slip on the more northern segment of the fault rupture. We also give the Cartesian positions at epoch 1990.0 of a set of VLBI fiducial stations and the three mobile sites in the vicinity of the earthquake.

Historic surface slip along the San Andreas Fault near Parkfield, California

The Parkfield Earthquake Prediction Experiment is focusing close attention on the 44‐km‐long section of the San Andreas fault that last ruptured seismically in 1966 (Ms 6.0). The 20‐km‐long central segment of the 1966 Parkfield rupture, extending from the mainshock epicenter at Middle Mountain southeastward to Gold Hill, forms a 1‐ to 2‐km salient northeastward away from the dominant N40°W strike. Following the 1966 earthquake afterslip, aseismic slip has been nearly constant. Moderate Parkfield earthquakes have recurred on average every 21 years since 1857, when a great earthquake (M≈8) ruptured at least as far north as the southern Parkfield segment. Many measurements of slip have been made near Parkfield since 1966. Nevertheless, much of the history of surface slip remained uncertain, especially the total amount associated with the 1966 event. In 1985 we measured accumulated slip on the four oldest cultural features offset by the fault along the 1966 Parkfield rupture segment. We interpret net slip on each feature as a sum of event slip (sum of coseismic and rapid preseismic and postseismic slip) from Parkfield earthquakes and steady interseismic slip as measured over the last 20 years on nearby alinement arrays, creep meters, and trilateration lines. We assumed for each site that event slip was identical for the 1922, 1934, and 1966 Parkfield events and that long‐term average rate of interseismic slip was constant between all events. Two fences on the southern segment, southeast of Gold Hill, indicate event slip of 13 and 15 cm and interseismic slip rate of 0.36 and 0.30 cm/yr since 1959 and 1908, respectively. At these sites, redundant independent data support our assumption that both event and interseismic slip occur uniformly. On the central segment, near Parkfield, both the 1934 and the 1966 ruptures offset a bridge built in 1932. Interseismic slip rate near the bridge has been about 1.1 cm/yr since 1966; thus we deduce an average event slip of 31 cm for the 1934 and 1966 earthquakes. On a parallel fault trace, 1 km to the southwest, slip was about 8 cm in 1966; thus total event slip summed across the entire fault zone near Parkfield was nearly 40 cm. On Middle Mountain, 4 km north of the 1966 mainshock epicenter, an offset fence indicates 17 cm of slip in 1966 and a 2.26‐cm/yr interseismic slip rate since circa 1946. Thus the central segment of the 1966 rupture is characterized by much larger event slip (∼40 cm) than both distal segments (∼15 cm). This amount of surface slip per event is about twice what had been previously assumed. Larger 1966 surface slip in the central part of the rupture is geodetically compatible with a coseismic slip of 65±10 cm slip on a narrow, buried asperity between Middle Mountain and Gold Hill that has been inferred from the depth distribution of early aftershocks. Assuming our characteristic surface slip model, one can further deduce a deficit in slip since the great 1857 earthquake. Taking the long‐term slip rate as 3.3 cm/yr, the surface slip deficit is 3±0.2 m south of Gold Hill but only 0.3±0.3 m northward from Parkfield.

Space-time Clustering of Large Earthquakes: A Western U. S. Perspective with Implications for the East

Abstract Results of ongoing paleoseismicity studies along the Wasatch fault zone, Utah, and Lost River fault zone, Idaho, suggest that spatial and temporal clustering of large magnitude surface-faulting earthquakes occur on each. The timing of individual past events is based primarily on radiocarbon and thermoluminescence dating of faulted deposits and on identification of displaced volcanic ashes, all observed in trenches. Figure 1 shows the faulting history of the Wasatch for the past 6 ka. Even though there is variability, distinct patterns emerge. Five of the seven active segments ruptured between 0.3 and 1 ka; if the Salt Lake segment is included, six segments have ruptured between 0.3 and 1.5 ka. From Salt Lake south, a similar sequence of events occurred between about 4.5 and 6 ka. Figure 2 shows timing of events along the Lost River during the past 12 ka. No events occurred from at least 12 ka until middle Holocene. Between 5.5 and 8 ka the northern three segments ruptured; actual timing of events could have been considerably closer. The Thousand Springs segment produced the 1983, MS 7.3, Borah Peak earthquake. These observations suggest thousands of years of quiescence can occur between clusters of activity during which adjacent segments fail in short periods of time (hours, months, decades, a few hundred years?). Segments adjacent to historical ruptures may have a higher potential to produce the next earthquakes, especially if they have elapsed times similar to the historical rupture segment. Similar clustering with even longer periods of quiescence could characterize seismic sources in the East.

Strike‐slip fault geometry in Turkey and its influence on earthquake activity

The geometry of Turkish strike‐slip faults has been reviewed and described. From this data set it appears that fault geometry (the distribution of discontinuities such as bends and stepovers along the main fault trace) plays an important role in controlling the location of large earthquake rupture segments along the fault zones. Large earthquake ruptures generally do not propagate past individual stepovers that are wider than 5 km or bends that have angles greater than about 30°. More important than the size of each discontinuity, however, is the total “geometric pattern,” i.e., the distribution of adjacent bends and stepovers based not only on distance from one another but also on relative discontinuity size. Certain geometric patterns (restraining single or double bends) are particularly common and can be viewed as responsible for strain accumulation along portions of the fault zone. Fault geometry not only plays a role in the extent of earthquake rupture but also in characteristics of earthquake behavior. For example, large earthquake epicenters often occur near restraining bends or double bends. Furthermore, aftershocks and swarm activity can often be associated with releasing areas (areas along the strike‐slip fault that are undergoing a relatively large amount of extension).

Rupture characteristics of normal faults: an example from the Wasatch fault zone, Utah

Summary The Salt Lake fault segment is one of several independent rupture segments in the Wasatch normal fault zone, Utah. The segment is about 35 km long and consists of several, approximately linear fault sections that intersect in geometrical barriers defined by bends and branching of the fault trace. The dips of these fault sections vary based on a preliminary model of the fault zone, with estimated dips ranging from 45 to 90°. The palaeo-stress tensor was characterized by sub-vertical maximum principal compressive stress, a minimum principal stress axis trending about 050° and φ ≈ 0.2−0.5. The average slip direction across the fault segment was ≈240°. Three large, non-conservative barriers occur within the fault segment—at the northern and southern ends of the segment, and a branch point in the centre of the segment. Bifurcation of the rupture zone into two primary fault systems has presumably increased the fracture toughness of the northern part of the segment due to the overlap of secondary faulting adjacent to the primary fault planes. Two large historic earthquakes in the eastern Basin and Range and Rocky Mountain transition region have ruptured bifurcated fault zones similar in structure to the Salt Lake segment; the Hebgen Lake, Montana earthquake ( M = 7.5) in 1959 and the Borah Peak, Idaho earthquake ( M = 7.3) in 1983. The earthquake foci were located at depths of 12–18 km and the ruptures propagated towards the regions of fault-zone bifurcation in both cases. A characteristic earthquake ( M ≈ 7−7.5) may initiate at either the centre or the southern end of the Salt Lake segment. In the first case, the rupture would propagate bilaterally, but in the latter case the rupture would propagate unilaterally to the N. A unilateral propagation direction is preferred based on inferences drawn from the structural geology of the segment and direct comparison with the rupture traces of the Borah Peak and Hebgen Lake earthquakes.

The Use of Surface Faults for Estimating Design Earthquakes; Implications of the 28 October 1983 Idaho Earthquake

Research Article| August 01, 1986 The Use of Surface Faults for Estimating Design Earthquakes; Implications of the 28 October 1983 Idaho Earthquake KEVIN J. FREEMAN; KEVIN J. FREEMAN Associate The Earth Technology Corporation, 200 W. Mercer St., Seattle, WA 98119 Search for other works by this author on: GSW Google Scholar STEVE FULLER; STEVE FULLER Project Geologist The Earth Technology Corporation, 200 W. Mercer St., Seattle, WA 98119 Search for other works by this author on: GSW Google Scholar BRUCE A. SCHELL BRUCE A. SCHELL Senior Geologist The Earth Technology Corporation, 3777 Long Beach Blvd., Long Beach, CA 90807 Search for other works by this author on: GSW Google Scholar Author and Article Information KEVIN J. FREEMAN Associate The Earth Technology Corporation, 200 W. Mercer St., Seattle, WA 98119 STEVE FULLER Project Geologist The Earth Technology Corporation, 200 W. Mercer St., Seattle, WA 98119 BRUCE A. SCHELL Senior Geologist The Earth Technology Corporation, 3777 Long Beach Blvd., Long Beach, CA 90807 Publisher: Association of Environmental & Engineering Geologists First Online: 02 Mar 2017 Online Issn: 1558-9161 Print Issn: 1078-7275 © 1986 Association of Engineering Geologists Environmental & Engineering Geoscience (1986) xxiii (3): 325–332. https://doi.org/10.2113/gseegeosci.xxiii.3.325 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation KEVIN J. FREEMAN, STEVE FULLER, BRUCE A. SCHELL; The Use of Surface Faults for Estimating Design Earthquakes; Implications of the 28 October 1983 Idaho Earthquake. Environmental & Engineering Geoscience 1986;; xxiii (3): 325–332. doi: https://doi.org/10.2113/gseegeosci.xxiii.3.325 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 SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract Comparison of surface ruptures of the 28 October 1983 Idaho earthquake to other historic surface ruptures such as the 1915 Pleasant Valley and 1954 Fairview Peak, Nevada earthquakes shows that maximum earthquake estimates for seismic design based on surface rupture can be misleading. Surface ruptures associated with these earthquakes are in a variety of geologic media ranging from valley alluvium to colluvium to bedrock, and occur at various positions relative to mountain fronts, all along the same rupture segments. Ruptures occurred both as continuous, interconnected segments and as discrete separated segments. Such characteristics indicate several pitfalls in the use of empirical fault-length data to estimate earthquake magnitudes for seismic-design purposes. The common practice of using well-defined surface fault segments instead of the entire length of the fault zone or fault system for estimating maximum earthquakes can lead to underestimating earthquake potential. 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.

Seismic sequences in Greece interpreted in terms of barrier model

Between 1978 and 1981, five shallow main shocks occurred in Greece and the surrounding region. Here we consider the space–time distribution of the foreshocks and the aftershocks of these earthquakes by interpreting the migration characteristics of the epicentres in terms of the barrier model1. This model suggests that segmented ruptures can occur in a seismic fault but that the rupture front may be stopped by a barrier, or may propagate over the whole fault plane leaving the barrier unbroken. These unbroken barriers result in aftershocks due to the concentration of stress there, and cause migration of major events along a plate boundary. This migration may be an important factor for the prediction of earthquakes.

A seismotectonic analysis of the Anza Seismic Gap, San Jacinto Fault Zone, southern California

Small earthquake epicenters near the Anza seismic gap define a 20‐km quiescent segment of fault bounded to the northwest and southeast by areas of relatively high seismicity. Recent moderate earthquakes on and near the San Jacinto fault in the gap and their relatively depressed aftershock activity indicate that the fault is seismogenic and highly stressed but locked by some mechanism. The locked nature of the fault may be due to relatively high compressive stress normal to the fault resulting from the convergent geometries of the local, active, discontinuous faults and the oblique orientation of the regional maximum compressive stress. Strain is not being relieved by aseismic fault creep. A swarm of small earthquakes in the crustal block 13 km southwest of the Anza gap beneath the Cahuilla Valley recently released stress in an area which was previously highly active before the 1918 (M 6.8) and 1937 (ML 6.0) earthquakes. The occurrence of these periods of increased seismicity near Cahuilla in the years immediately before the nearby (closer than 35 km) large earthquakes and the recent swarm suggest that the ground beneath Cahuilla may be acting as a stress meter signaling the presence of high stresses before large local earthquakes. The length of the quiescent fault segment suggests potential for about an M 6.5 earthquake if the entire segment ruptures at once.

The 23 November 1980 southern Italy earthquake

abstractThe seismic activity associated with the catastrophic southern Italy earthquake was monitored by 11 seismic stations operating before this event, within an epicentral distance of 200 km, and by 32 additional short-period seismometers installed soon after the main shock. The hypocenter of this event was located at 40°46′N and 15°18′E, at 16 km depth. The fault-plane solution reveals normal faulting, with tensile axis dipping 18° and oriented orthogonal to the axis of the Apennines chain. This mechanism is in good agreement with the stress pattern inferred from some previous earthquakes and the local seismotectonics.The hypocenter locations of more than 600 aftershocks, with local magnitudes greater than 2.4, show a pronounced alignment extending for about 70 km, oriented north 120° and scattered laterally less than 15 km. These events are mostly concentrated between 8 and 16 km depth. A cluster of aftershocks occurred close to the hypocenter of the main shock covering a region elongated 25 km which corresponds also to the highly damaged area. No significant spreading of the aftershock area with time is observed, but one of the events with higher magnitude (ML = 4.8, 14 February 1981) is displaced 20 km NW from the tip of the aftershock region. The time evolution of the number of aftershocks fits well Omori's hyperbolic law with a decay coeffcient of 1.07 ± 0.06.The possibility of a future delayed multiple sequence of large events, as already observed in the past along the central and southern Apennines, is discussed. In particular, a relatively high seismic potential seems to exist along the northern boundary of the 1980 rupture segment.

Origin, extent, and effect of molecular stress concentration in fibrillar structures

AbstractElectron spin resonance and infrared spectroscopy investigations of the deformation of oriented polymers have given insight into the loading and rupture of chain segments. Whereas there is agreement on the phenomenology of chain scission, fiber deformation, and stress relaxation, quite controversial interpretations have been given of the observed phenomena. Particularly disputed have been the extent of interfibrillar slip, the effect of chain scission on macroscopic stress‐strain behavior, and the cause of microcrack formation. Dealing with polyamide‐6 fibers it is shown that the microfibrils do not unload through slippage and that the lateral rigidity of the crystal blocks within the sandwich‐structured microfibrils must be large enough so as to permit the build‐up of large stress concentrations. The extent of molecular stress concentration is calculated. The qualitative agreement between the slopes of stress‐strain and free radical‐strain curves is analyzed. Unless this agreement is coincidental, it can be sought in an amplification effect accompanying the rapid release of stored elastic energy. The ensuing local temperature rise will facilitate the extension of kinked chains tinder annihilation of kinks. This is practically equivalent to unloading those chains without breaking them. The formation of submicrocracks is not related to the breakage of chains. The direct influence of submicrocracks as potential stress concentrators is weak and ineffective with regard to accelerating chain scission.