ALBERT

Beschreibung: We present a 1D finite-difference (FD) scheme that is based on the application of Geller and Takeuchi's (1998) optimally accurate FD operators to the heterogeneous strong-form equation of motion developed by Moczo et al. (2002). We numerically compare the scheme with two other FD schemes that approximate the heterogeneous strong-form equation of motion, one using conventional 2nd-order FD operators, the other using staggered-grid 4th-order FD operators. The numerical comparison is based on the envelope and phase misfits between tested and reference solutions. We discuss the error due to internal interface (primarily controlled by the boundary condition and its numerical approximation) and error due to grid dispersion. We demonstrate the superior accuracy of the scheme based on the application of the optimally accurate operators.

Beschreibung: Most instruments used in seismological practice to record ground motion are pendulum seismographs, velocigraphs, or accelerographs. In most cases it is assumed that seismic instruments are only sensitive to the translational motion of the instrument"s base. In this study the full equation of pendulum motion, including the inputs of rotations and tilts, is considered. It is shown that tilting the accelerograph's base can severely impact its response to the ground motion. The method of tilt evaluation using uncorrected strong-motion accelerograms was first suggested by Graizer (1989), and later tested in several laboratory experiments with different strong-motion instruments. The method is based on the difference in the tilt sensitivity of the horizontal and vertical pendulums. The method was applied to many of the strongest records of the M (sub w) 6.7 Northridge earthquake of 1994. Examples are shown when relatively large tilts of up to a few degrees occurred during strong earthquake ground motion. Residual tilt extracted from the strong-motion record at the Pacoima Dam-Upper Left Abutment reached 3.1 degrees in N45 degrees E direction, and was a result of local earthquake-induced tilting due to high-amplitude shaking. This value is in agreement with the residual tilt measured by using electronic level a few days after the earthquake. The method was applied to the building records from the Northridge earthquake. According to the estimates, residual tilt reached 2.6 degrees on the ground floor of the 12-story Hotel in Ventura. Processing of most of the strongest records of the Northridge earthquake shows that tilts, if happened, were within the error of the method, or less than about 0.5 degrees .

Beschreibung: Motivated by recent studies showing that stress modifications due to small quakes are important in earthquake triggering, we use a quasi-dynamic spatially discrete model of a rate- and state-dependent fault to examine the effect of multiple elastic stress transfers on the spatial extent of aftershock activity. We show that multiple stress transfers may significantly alter the spatial distribution of aftershock sequences, and give rise to stress-seismicity relations that are inconsistent with Dieterich's aftershock model. Specifically, we present an example in which, owing to the effect of multiple elastic stress transfers, the area experiencing seismicity rate change is much larger than that subjected to a stress change. We define a parameter that quantifies the proximity to the failure in the model, and show that differences in the magnitude of aftershock rates are due to differences in the proximity to failure that prevailed prior to the mainshock. Furthermore, we show that accelerating and decelerating cumulative Benioff strains are indicative of a region approaching or moving away from failure, respectively. Finally, we compare the cumulative Benioff strains in remote sites triggered either by the Landers or the Hector Mine earthquake, and infer that differences in the state of stress that prevailed prior to these mainshocks may explain the different spatial distribution of their remote aftershocks.

Beschreibung: The Brawley fault zone (BFZ) and the Brawley Seismic Zone constitute the principal transfer zone accommodating strain between the San Andreas and Imperial faults in southernmost California. The BFZ ruptured along with the Imperial fault in the 1940 M (sub w) 6.9 and the 1979 M (sub w) 6.4 earthquakes, although in each case only minor slip apparently occurred on the BFZ; several other episodes of slip and creep have been documented on the BFZ historically. Until this study, it has been unclear whether the past few decades reflect average behavior of the fault. Two trenches were opened and a series of auger holes were bored across three strands of the BFZ at Harris Road to compare the amount of slip observed historically with the displacements observed in the paleoseismic record. Evidence is presented, across the westernmost strand of the BFZ and across the entire BFZ at Harris Road, to show that both the average vertical slip rate observed in modern times (since 1970) and the vertical creep rate (excluding coseismic slip) observed during the 1970s are significantly higher than the long-term average. Across the westernmost strand, the long- term vertical rate is 1.2 (+1.5/-0.5) mm/yr, and the average rate since about A.D. 1710 is determined to be no greater than 2.0 mm/yr; in contrast, the average vertical rate between 1970 and 2004 across that strand was at least 4.3 mm/yr, and the 1970s vertical aseismic creep rate was 10 mm/yr. Likewise, across the entire BFZ, the long-term vertical rate is 2.8 (+4.1/-1.4) mm/yr, whereas the rate between 1970 and 2004 was at least 7.2 mm/yr, and the 1970s aseismic creep rate was 10 mm/yr. The long-term strike-slip rate cannot be determined across any strands of the BFZ but may be significant. In contrast to the commonly accepted higher sedimentation rates inferred for the entire Imperial Valley, we find that the average sedimentation rate on the downthrown side of the BFZ adjacent to Mesquite Basin, in the millennium preceding the onset of agricultural influences, was at most 3.5 mm/yr. Finally, a creep event occurred on the BFZ during our study in 2002 and is documented herein.

Beschreibung: Earthquakes on a specified fault (or fault segment) with magnitudes greater than a specified value have a statistical distribution of recurrence times. The mean recurrence time can be related to the rate of strain accumulation and the strength of the fault. Very few faults have a recorded history of earthquakes that define a distribution well. For hazard assessment, in general, a statistical distribution of recurrence times is assumed along with parameter values. Assumed distributions include the Weibull (stretched exponential) distribution, the lognormal distribution, and the Brownian passage-time (inverse Gaussian) distribution. The distribution of earthquake waiting times is the conditional probability that an earthquake will occur at a time in the future if it has not occurred for a specified time in the past. The distribution of waiting times is very sensitive to the distribution of recurrence times. An exponential distribution of recurrence times is Poissonian, so there is no memory of the last event. The distribution of recurrence times must be thinner than the exponential if the mean waiting time is to decrease as the time since the last earthquake increases. Neither the lognormal or the Brownian passage time distribution satisfies this requirement. We use the "Virtual California" model for earthquake occurrence on the San Andreas fault system to produce a synthetic distribution of earthquake recurrence times on various faults in the San Andreas system. We find that the synthetic data are well represented by Weibull distributions. We also show that the Weibull distribution follows from both damage mechanics and statistical physics.

Beschreibung: Removing the crustal signature from teleseismic travel times is an important procedure to reduce the trade-off between crustal and mantle velocity heterogeneities in seismic tomography. Because reverberations of long- and short-period body-wave arrivals in the crust affect the waveforms of the direct arrivals differently, the crustal effects on travel times measured by waveform cross correlation are frequency dependent. With synthetic responses of selected crustal models, this short note illustrates the significance of frequency-dependent crustal corrections to finite- frequency body-wave travel-time tomography. The differences in crustal correction between long- and short-period body waves at the same station can be as large as 0.6 sec, depending on the crustal thickness, velocity contrast at the Moho, and layering within the crust.

Beschreibung: Earthquakes in central Nevada (1932-1959) were used to develop a modified Mercalli intensity (MMI) attenuation model for estimating moment magnitude M for earthquakes in the Basin and Range province of interior western North America. M is 7.4-7.5 for the 26 March 1872 Owens Valley, California, earthquake, in agreement with Beanland and Clark's (1994) M 7.6 that was estimated from geologic field observations. M is 7.5 for the 3 May 1887 Sonora, Mexico, earthquake, in agreement with Natali and Sbar's (1982) M 7.4 and Suter's (2006) M 7.5, both estimated from geologic field observations. MMI at sites in California for earthquakes in the Nevada Basin and Range apparently are not much affected by the Sierra Nevada except at sites near the Sierra Nevada where MMI is reduced. This reduction in MMI is consistent with a shadow zone produced by the root of the Sierra Nevada. In contrast, MMI assignments for earthquakes located in the eastern Sierra Nevada near the west margin of the Basin and Range are greater than predicted at sites in California. These higher MMI values may result from critical reflections due to layering near the base of the Sierra Nevada.

Beschreibung: Recently, ring laser technology has provided the first consistent observations of rotational ground motions around a vertical axis induced by earthquakes. "Consistent," in this context, implies that the observed waveforms and amplitudes are compatible with collocated recordings of translational ground motions. In particular, transverse accelerations should be in phase with rotation rate and their ratio proportional to local horizontal phase velocity assuming plane-wave propagation. The ring laser installed at the Fundamentalstation Wettzell in the Bavarian Forest, Southeast Germany, is recording the rotation rate around a vertical axis, theoretically a linear combination of the space derivatives of the horizontal components of motion. This suggests that, in principle, rotation can be derived from seismic-array experiments by "finite differencing". This has been attempted previously in several studies; however, the accuracy of these observations could never be tested in the absence of direct measurements. We installed a double cross-shaped array of nine stations from December 2003 to March 2004 around the ring laser instrument and observed several large earthquakes on both the ring laser and the seismic array. Here we present for the first time a comparison of array-derived rotations with direct measurements of rotations for ground motions induced by the M 6.3 Al Hoceima, Morocco, earthquake of 24 February 2004. With complete 3D synthetic seismograms calculated for this event we show that even low levels of noise may considerably influence the accuracy of the array-derived rotations when the minimum number of required stations (three) is used. Nevertheless, when using all nine stations, the overall fit between direct and array-derived measurements is surprisingly good (maximum correlation coefficient of 0.94).

Beschreibung: The present study deals with rapid, automatic, estimation of some earthquake parameters (location, focal depth, and magnitude) in a region of rather high seismic activity, in quasi-real time, through the analysis of incoming broadband records. The method can be applied, in particular, in poorly instrumented countries with high seismic-risk potential. It can also be applied when the analysis of a very important flow of data requires rapid, sophisticated, preferably automatic, data processing. The method requires, as a minimum, a three-component broadband seismographic station and a sufficiently populated database, that is, an instrument operating for a time long enough to have accumulated an appropriate data set, used to construct the knowledge base. The more extensive the knowledge base, the better the accuracy of the method. We proceed in several steps. First, applying the SPARS algorithm to the only vertical component, available waveforms are classified according to the source location taken from National Earthquake Information Center (NEIC) catalog; it results in the sorting out of a subset of waveforms/events which will not be included in the knowledge base. Second, each element of the knowledge base is validated according to the epicentral distance with respect to the reference station (and eventually the azimuth of the corresponding source). Third, new input waveforms are analyzed and compared with one or more elements of the knowledge base to estimate their source location and size. The method can be used to search for doublets (or multiplets); if multiplets are found, their location and focal depth can be determined by using a fuzzy event relocation method. We have tested the capability of the proposed algorithms, processing (broadband) waveforms collected during four and half years at the GEOSCOPE broadband station PVC, operated by Institut de Recherche pour le Developpement, formerly ORSTOM (IRD) at Port Vila, Vanuatu. Among 650 events recorded at this station, 254 ones, meeting a good criterion of quality, have been sorted. The results show that, in a range of distances up to 1000 km, the method is capable of yielding, in a very short time, the location of the input event, the accuracy depending on the local density of known events in the vicinity. We also obtain a reliable estimation of the energy by measuring the maximum surface wave (or S-wave) amplitude, related to the classical magnitude MSZ.

Beschreibung: The Mississippi embayment, located in the central United States, and its thick deposits of sediments (over 1 km in places) have a large effect on earthquake ground motions. Several previous studies have addressed how these thick sediments might modify probabilistic seismic-hazard maps. The high seismic hazard associated with the New Madrid seismic zone makes it particularly important to quantify the uncertainty in modeling site amplification to better represent earthquake hazard in seismic-hazard maps. The methodology of the Memphis urban seismic-hazard-mapping project (Cramer et al., 2004) is combined with the reference profile approach of Toro and Silva (2001) to better estimate seismic hazard in the Mississippi embayment. Improvements over previous approaches include using the 2002 national seismic-hazard model, fully probabilistic hazard calculations, calibration of site amplification with improved nonlinear soil-response estimates, and estimates of uncertainty. Comparisons are made with the results of several previous studies, and estimates of uncertainty inherent in site-amplification modeling for the upper Mississippi embayment are developed. I present new seismic-hazard maps for the upper Mississippi embayment with the effects of site geology incorporating these uncertainties.