ALBERT

Description: Stress transfer from the great 1964 Prince William Sound earthquake is modeled on the Denali fault, including the Denali-Totschunda fault segments that ruptured in 2002, and on other regional fault systems where M 7.5 and larger earthquakes have occurred since 1900. The results indicate that analysis of Coulomb stress transfer from the dominant earthquake in a region is a potentially powerful tool in assessing time-varying earthquake hazard. Modeled Coulomb stress increases on the northern Denali and Totschunda faults from the great 1964 earthquake coincide with zones that ruptured in the 2002 Denali fault earthquake, although stress on the Susitna Glacier thrust plane, where the 2002 event initiated, was decreased. A southeasterly-trending Coulomb stress transect along the right-lateral Totschunda-Fairweather-Queen Charlotte trend shows stress transfer from the 1964 event advancing slip on the Totschunda, Fairweather, and Queen Charlotte segments, including the southern Fairweather segment that ruptured in 1972. Stress transfer retarding right-lateral strike slip was observed from the southern part of the Totschunda fault to the northern end of the Fairweather fault (1958 rupture). This region encompasses a gap with shallow thrust faulting but with little evidence of strike-slip faulting connecting the segments to the northwest and southeast. Stress transfer toward failure was computed on the north-south trending right-lateral strike-slip faults in the Gulf of Alaska that ruptured in 1987 and 1988, with inhibitory stress changes at the northern end of the northernmost (1987) rupture. The northern Denali and Totschunda faults, including the zones that ruptured in the 2002 earthquakes, follow very closely (within 3%), for about 90 degrees , an arc of a circle of radius 375 km. The center of this circle is within a few kilometers of the intersection at depth of the Patton Bay fault with the Alaskan megathrust. This inferred asperity edge may be the pole of counterclockwise rotation of the block south of the Denali fault. These observations suggest that the asperity and its recurrent rupture in great earthquakes as in 1964 may have influenced the tectonics of the region during the later stages of evolution of the Denali strike-slip fault system.

Description: In this article we analyze the spatial and temporal variations in the seismicity and stress state within the central Denali fault system, Alaska, before and during the 2002 Denali fault earthquake sequence. Seismicity for 30 years prior to the 2002 earthquake sequence along the Denali fault was very light, with an average of four events with magnitude M (sub L) 〉 or =3 per year. We observe a significant increase in the seismicity rate prior to the M (sub w) 7.9 event of 3 November 2002 within its epicentral region, starting about 8 months before its occurrence. The majority of the aftershocks of the M (sub w) 7.9 event are located within the upper 11 km of the crust and form several persistent clusters with a few aseismic patches along the ruptured fault. The most active aftershock source is associated with the epicentral region of the earthquake. The overall b-value of the aftershock sequence is 0.96 with the highest b-values within the epicentral region. We estimate that it will take 14 years for the seismicity rate to drop back to the background level. The stress regime across the region varies in space and time. The inferred stress regime prior to the 2002 sequence is predominately strike slip. Along the central part of the rupture zone, the orientations of the least- and intermediate-stress axes are reversed after the 2002 earthquake sequence. The maximum compressive stresses along the Denali fault rotate clockwise by up to 35 degrees ; the greatest rotations occur in the area of the rupture step-over from the Denali to the Totschunda fault. The inferred stress regime after the 2002 sequence reflects an interchanging thrusting and strike-slip faulting along the ruptured fault. The thrust faulting is concentrated in the epicentral region of the M (sub w) 7.9 event and along the rupture segments showing the largest surface offsets.

Description: Geophysical information, including deep-crustal seismic reflection, magnetotelluric (MT), gravity, and magnetic data, cross the aftershock zone of the 3 November 2002 M (sub w) 7.9 Denali fault earthquake. These data and aftershock seismicity, jointly interpreted, reveal the crustal structure of the right-lateral-slip Denali fault and the eastern Alaska Range orogen, as well as the relationship between this structure and seismicity. North of the Denali fault, strong seismic reflections from within the Alaska Range orogen show features that dip as steeply as 25 degrees north and extend downward to depths between 20 and 25 km. These reflections reveal crustal structures, probably ductile shear zones, that most likely formed during the Late Cretaceous, but these structures appear to be inactive, having produced little seismicity during the past 20 years. Furthermore, seismic reflections mainly dip north, whereas alignments in aftershock hypocenters dip south. The Denali fault is nonreflective, but modeling of MT, gravity, and magnetic data suggests that the Denali fault dips steeply to vertically. However, in an alternative structural model, the Denali fault is defined by one of the reflection bands that dips to the north and flattens into the middle crust of the Alaska Range orogen. Modeling of MT data indicates a rock body, having low electrical resistivity (〉10 Omega .m), that lies mainly at depths greater than 10 km, directly beneath aftershocks of the Denali fault earthquake. The maximum depth of aftershocks along the Denali fault is 10 km. This shallow depth may arise from a higher-than-normal geothermal gradient. Alternatively, the low electrical resistivity of deep rocks along the Denali fault may be associated with fluids that have weakened the lower crust and helped determine the depth extent of the aftershock zone.

Description: The M (sub w) 7.9, Denali fault earthquake (DFE) is the largest continental strike-slip earthquake to occur since the development of Interferometric Synthetic Aperture Radar (InSAR). We use five interferograms, constructed using radar images from the Canadian Radarsat-1 satellite, to map the surface deformation at the western end of the fault rupture. Additional geodetic data are provided by displacements observed at 40 campaign and continuous Global Positioning System (GPS) sites. We use the data to determine the geometry of the Susitna Glacier fault, thrusting on which initiated the DFE, and to determine a slip model for the entire event that is consistent with both the InSAR and GPS data. We find there was an average of 7.3+ or -0.4 m slip on the Susitna Glacier fault, between 1 and 9.5 km depth on a 29 km long fault that dips north at 41+ or -0.7 degrees and has a surface projection close to the mapped rupture. On the Denali fault, a simple model with large slip patches finds a maximum of 8.7+ or -0.7 m of slip between the surface and 14.3+ or -0.2 km depth. A more complex distributed slip model finds a peak of 12.5+ or -0.8 m in the upper 4 km, significantly higher than the observed surface slip. We estimate a geodetic moment of 670+ or -10X10 (super 18) N m (M (sub w) 7.9), consistent with seismic estimates. Lack of preseismic data resulted in an absence of InSAR coverage for the eastern half of the DFE rupture. A dedicated geodetic InSAR mission could obviate coverage problems in the future.

Description: Seismic-event location within the context of monitoring the Comprehensive Nuclear-Test-Ban Treaty entails a priori knowledge of the travel time of seismic phases for a given source to stations of the International Monitoring System (IMS). Such travel-time information (or ground truth, GT) is provided empirically by seismic reference events, events that have well-determined hypocenter locations (epicenters typically known to + or -5 km with high confidence) and origin times. In this study we present new reference events for the calibration of six seismic stations of the IMS in China, a region with high seismic activity. We use the Annual Bulletin of Chinese Earthquakes, which lists about 1000 earthquakes in and near China each year with consistent phase picks at regional stations, to determine precise relative earthquake locations from double-difference cluster analysis. The resulting high-resolution image of active faulting at seismogenic depths in areas of dense seismicity is correlated with the tectonic structure derived from mapped fault information at the surface to validate the absolute locations. We generated 59 reference events with M〉 or =3.5, distributed in six clusters in central and eastern China, and recorded by at least one of the six IMS stations. The scatter in relative travel-time residuals is reduced from 1.28 sec before to 0.61 sec after relocation, consistent with the relocated positions of the events. The degree of correlation between seismicity structure and well-characterized fault data indicates that, in four clusters, the locations of the new reference events are accurate to within 5 km (GT5), and in two clusters within 10 km (GT10).

Description: This article compares the variation in observed intensity of the 2001 M (sub w) 6.8 Nisqually, Washington, earthquake in Victoria, British Columbia (150 km from the epicenter), with amplification hazard predictions based on the average shearwave velocity of geologic units. Modified Mercalli intensities were assigned from 750 felt reports collected by online Web submission augmented by door-to-door canvassing in regions of particular interest. An intensity map was created based on high-resolution (sub-city block) georeferencing with the Canadian postal code system. Site-specific comparisons of earthquake intensity and geology indicate significant differences in observed felt effects between high and low shear-wave velocity substrates (bedrock and glacial till versus soft clay and peat). Overall, the observed intensity map for weak levels of shaking supports the assignment of amplification hazard based on shear-wave velocities across greater Victoria.

Description: An extension of probabilistic seismic hazard analysis is proposed to introduce a priori information about seismic source parameters. In particular, faulting style is taken into account with a theoretical corrective coefficient applied to the attenuation law. The validity of this correction is assessed through a comparison with observed data, attenuation law predictions corrected and not corrected, and the results of attenuation laws containing faulting style parameters. The probabilistic nature of the analysis is maintained, introducing into the classical hazard formulation a 2D probability density function describing the most probable focal mechanisms associated with each seismic source zone. This new expression may also be used in the framework of deaggregation analysis. Thus, the design earthquake resulting from the deaggregation is characterized by a focal mechanism. An application to a site located in the Southern Apennines, Italy, is shown. The result of the analysis emphasizes the importance of strike-slip events in the seismic hazard context, compared with normal faulting seismic activity in this region.

Description: We analyze shear-wave splitting (SWS) in a high-quality waveform data set recorded at surface and downhole (0.2 km) seismometers in a region around the 20 September 1999 M (sub w) 7.6 Chi-Chi, Taiwan, earthquake sequence. The data set was generated by events in a 5-year period before, during, and after the mainshock. The purpose is to investigate the depth extent of the crustal anisotropy and its possible temporal evolution in relation to the occurrence of large earthquakes. Results from downhole records show a stable polarization direction of the fast shear wave that matches well the local Global Positioning System (GPS) velocity field. A slightly different polarization direction of the fast shear wave is obtained from surface data. This suggests a possible anisotropy change between the top 0.2 km structure and the deeper section of the crust. Measured time delays below the downhole station have an average value of 0.16 sec without systematic changes for sources from about 8 km to 20 km in depth. Estimates of time delays in the top 0.2 km of the crust based on shear waves reflected from the free surface give a constant 0.04 sec. A likely depth distribution inferred from these two types of measurements and an S-velocity model indicates that the crustal anisotropy in the region is dominated by the top 2 to 3 km. The measured polarization directions and time delays give essentially constant values over the 2.7-year premainshock and 2.3-year postmainshock periods in the region adjacent to the Chi-Chi rupture and within 10 km from the epicentral region of its two large M〉 or =6.0 aftershocks. Analysis of SWS in waveforms produced by earthquake multiplets confirms further the lack of precursory temporal variations of crustal anisotropy in the immediate neighborhood of the Chi-Chi earthquake sequence. The results raise doubts on the general usefulness of SWS measurements for earthquake forecasting. An apparent coseismic increase in anisotropy time delay of approximately 10% is observed for depths 〉0.2 km; however, this value is clearly affected by spatial changes associated with different event locations before and after the Chi-Chi mainshock.

Description: The use of ground-motion-prediction equations to estimate ground shaking has become a very popular approach for seismic-hazard assessment, especially in the framework of a logic-tree approach. Owing to the large number of existing published ground-motion models, however, the selection and ranking of appropriate models for a particular target area often pose serious practical problems. Here we show how observed ground-motion records can help to guide this process in a systematic and comprehensible way. A key element in this context is a new, likelihood based, goodness-of-fit measure that has the property not only to quantify the model fit but also to measure in some degree how well the underlying statistical model assumptions are met. By design, this measure naturally scales between 0 and 1, with a value of 0.5 for a situation in which the model perfectly matches the sample distribution both in terms of mean and standard deviation. We have used it in combination with other goodness-of-fit measures to derive a simple classification scheme to quantify how well a candidate ground-motion-prediction equation models a particular set of observed-response spectra. This scheme is demonstrated to perform well in recognizing a number of popular ground-motion models from their rock-site-recording subsets. This indicates its potential for aiding the assignment of logic-tree weights in a consistent and reproducible way. We have applied our scheme to the border region of France, Germany, and Switzerland where the M (sub w) 4.8 St. Die earthquake of 22 February 2003 in eastern France recently provided a small set of observed-response spectra. These records are best modeled by the ground-motion-prediction equation of Berge-Thierry et al. (2003), which is based on the analysis of predominantly European data. The fact that the Swiss model of Bay et al. (2003) is not able to model the observed records in an acceptable way may indicate general problems arising from the use of weak-motion data for strong-motion prediction.