Surface Rupture Length Research Articles

Extensional reactivation of thrust faults accompanied by coseismic surface rupture, southwestern Wyoming and north-central Utah

Extensional reactivation of thrust faults north of the Uinta Mountains in Utah and Wyoming is accompanied by Quaternary surface faulting, tilting, and monoclinal folding. The 40-km-long Bear River fault zone consists of down-to- the-west, right-stepping, en echelon scarps, each about 3.0 to 3.5 km in length, striking N20°W to N20°E. Scarp-derived colluvial deposits record two Holocene surface ruptures with net vertical tectonic displacements ranging from 5 m per event (West, 1989, in press). Radiocarbon ages indicate surface faulting events occurred at about 4.6 ka and 2.4 ka. The length of surface rupture and net slip per event in the Bear River fault zone imply a seismic potential comparable to the Lost River fault, Hebgen Lake fault, and the Wasatch fault zone. A late Quaternary normal scarp is also coincident with the leading edge of the Absaroka thrust fault 7.2 km west of the Bear River fault zone. Scarp-derived colluvial deposits record one surface faulting event over a length of at least 5.0 km. The age of surface rupture is coeval with the Bear River fault zone, ∼2.4 ka. Normal faults displacing Pleistocene surfaces/deposits and tectonic tilt indicate that the Hogsback thrust east of the Bear River fault zone was reactivated in a normal sense but now may be inactive due to subsequent development of the Bear River fault zone. Cumulative normal throw of 200± m on the Hogsback fault may be responsible for postulated tectonic separation of the Bear River and Green River drainage basins Neotectonic deformation results from east-west extension superposed on the Hogsback and Absaroka thrust plates. Extensional reactivation of thrust faults caused propagation of new normal faults over stress points, particularly at the Hogsback ramp to flat transition, and at the thrust leading edges. The ages of surface rupture and cumulative throws indicate normal reactivation of the Hogsback and Absaroka thrusts, and propagation of the intervening Bear River fault zone proceeded from east to west opposite initial development of the thrusts in Late Cretaceous to Paleocene time. Normally reactivated thrust faults and propagation of listric normal faults accompanied by surface rupture have significant implications for the tectonic/seismogenic development of the eastern Basin and Range transition zone.

A model for the present stress field along the Xian-shui-he fault belt, northwest Sichuan, China

Along the left-lateral strike-slip fault on the eastern border of the Tibetan plateau, 22 M ≥ 6 earthquakes have occurred since 1725. The fault is divided into three segments with two aseismic gaps between the segments. Using the elasticity theory of dislocations, the spatial and temporal variations in stress are calculated using source parameters from the earthquakes such as epicenter, time, length of surface rupture, mean coseismic displacement and magnitude. Further, a total slip rate is assumed in order to calculate the stress increase due to plate motion. The model is optimized towards the smallest variations in the preseismic stresses. Under this constraint, the present stress field is calculated by varying the spatial distribution of the ratio of seismic slip to total slip and the unknown initial stress field. The greatest seismic hazard is found in the central segment, if a spatially constant slip rate is assumed. However, different seismic slip rates for different fault segments lead to the southern segment as the region where the next large earthquake may occur.

Statistical relations among earthquake magnitude, surface rupture length, and surface fault displacement

Abstract In order to refine correlations of surface-wave magnitude, fault rupture length at the ground surface, and fault displacement at the surface by including the uncertainties in these variables, the existing data were critically reviewed and a new data base was compiled. Earthquake magnitudes were redetermined as necessary to make them as consistent as possible with the Gutenberg methods and results, which make up much of the data base. Measurement errors were estimated for the three variables for 58 moderate to large shallow-focus earthquakes. Regression analyses were then made utilizing the estimated measurement errors. The regression analysis demonstrates that the relations among the variables magnitude, length, and displacement are stochastic in nature. The stochastic variance, introduced in part by incomplete surface expression of seismogenic faulting, variation in shear modulus, and regional factors, dominates the estimated measurement errors. Thus, it is appropriate to use ordinary least squares for the regression models, rather than regression models based upon an underlying deterministic relation in which the variance results primarily from measurement errors. Significant differences exist in correlations of certain combinations of length, displacement, and magnitude when events are grouped by fault type or by region, including attenuation regions delineated by Evernden and others. Estimates of the magnitude and the standard deviation of the magnitude of a prehistoric or future earthquake associated with a fault can be made by correlating M s with the logarithms of rupture length, fault displacement, or the product of length and displacement. Fault rupture area could be reliably estimated for about 20 of the events in the data set. Regression of M s on rupture area did not result in a marked improvement over regressions that did not involve rupture area. Because no subduction-zone earthquakes are included in this study, the reported results do not apply to such zones.

Application of linear statistical models of earthquake magnitude versus fault length in estimating maximum expectable earthquakes

Correlation or linear regression estimates of earthquake magnitude from data on historical magnitude and length of surface rupture should be based upon the correct regression. For example, the regression of magnitude on the logarithm of the length of surface rupture L can be used to estimate magnitude, but the regression of log L on magnitude cannot. Regression estimates are most probable values, and estimates of maximum values require consideration of one-sided confidence limits.

Inversion of the body waves from the Borrego Mountain earthquake to the source mechanism

abstract The generalized linear inverse technique has been adapted to the problem of determining an earthquake source model from body-wave data. The technique has been successfully applied to the Borrego Mountain earthquake of April 9, 1968. Synthetic seismograms computed from the resulting model match in close detail the first 25 sec of long-period seismograms from a wide range of azimuths. The main shock source-time function has been determined by a new simultaneous short period-long period deconvolution technique as well as by the inversion technique. The duration and shape of this time function indicate that most of the body-wave energy was radiated from a surface with effective radius of only 8 km. This is much smaller than the total surface rupture length or the length of the aftershock zone. Along with the moment determination of Mo = 11.2 ×1025 dyne-cm, this radius implies a high stress drop of about 96 bars. Evidence in the amplitude data indicates that the polarization angle of shear waves is very sensitive to lateral structure.