Source parameters of the 23 April 1992 M 6.1 Joshua Tree, California, earthquake and its aftershocks: Empirical Green's function analysis of GEOS and TERRAscope data

Source parameters of the M 6.1 23 April 1992 Joshua Tree mainshock and 86 M 1.8 to 4.9 aftershocks are determined using an empirical Green's function methodology. For the aftershocks, deconvolved P- and S-wave spectra are calculated for 126 pairs of closely spaced events recorded on portable GEOS stations; S-wave spectra from the two horizontal components are averaged. The deconvolved spectra are fit by a ratio of omega-square source models, yielding an optimal (least-squares) corner frequency for both the large and the small event in each pair. We find no resolved difference between the inferred P- and S-wave corner frequencies. Using the standard Brune model for stress drop, we also find no resolved nonconstant scaling of stress drop with moment, although we also conclude that detailed scaling systematics would be difficult to resolve. In particular, a weak increase of stress drop with moment over a limited moment/magnitude cannot be ruled out. For magnitudes smaller than M 3 to 3.5, the inferred stress-drop values will be limited by the maximum observable corner frequency value of 60 Hz. For the mainshock, source-time functions are obtained from mainshock recordings at three TERRAscope stations (PFO, PAS, and GSC) using an M 4.3 foreshock as an empirical Green's function. The results indicate a fairly simple, single-pulse source-time function, with clear south-to-north directivity and an inferred rupture radius of 5 to 6 km. The deconvolved source-time functions are inverted to obtain a finite-rupture model that gives a robust estimate of rupture dimension. Early aftershocks are found to lie along the perimeters of regions with high mainshock slip. The inferred mainshock stress-drop value, 56 bars, is within the range determined for the aftershocks. Our derived mainshock source spectra do not show resolvable deviation from the omega-square model

Complex faulting deduced from broadband modeling of the 28 February 1990 Upland earthquake (M_L = 5.2)

The 1990 Upland earthquake was one of the first sizable local events to be recorded broadband at Pasadena, where the Green's functions appropriate for the path are known from a previous study. The synthetics developed in modeling the 1988 Upland sequence were available for use in rapid assessment of the activity. First-motion studies from the Caltech-USGS array data gave two solutions for the 1990 main shock based on the choice of regional velocity models. Although these focal mechanisms differ by less than 5° in strike and 20° rake, it proved possible to further constrain the solution using these derived Green's functions and a three-component waveform inversion scheme. We obtain from long-period waves a fault-plane solution of θ = 216°, δ = 77°, λ = 5.0°, M_0 = 2.5 × 10^(24) dyne-cm, depth = 6 km, and a source duration of 1.2 sec, for which the orientation and source depth are in good agreement with the first-motion results of Hauksson and Jones (1991). Comparisons of the broadband displacement records with the high-pass Wood-Anderson simulations suggests the 1990 earthquake was a complicated event with a strong asperity at depth. Double point-source models indicate that about 30 per cent of the moment was released from a 9-km deep asperity following the initial source by 0.0 to 0.5 sec. Our best-fitting distributed fault model indicates that the timing of our point-source results is feasible assuming a reasonable rupture velocity. The rupture initiated at a depth of about 6 km and propagated downward on a 3.5 by 3.5 km (length by width) fault. Both the inversion of long-period waves and the distributed fault modeling indicate that the main shock did not rupture the entire depth extent of the fault defined by the aftershock zone. A relatively small asperity (about 1.0 km^2) with a greater than 1 kbar stress drop controls the short-period Wood-Anderson waveforms. This asperity appears to be located in a region where seismicity shows a bend in the fault plane

Study on the feasibility of a tool to measure the macroeconomic impact of structural reforms

The main aim of this study is to assess the feasibility of empirical tools to study the impact of structural reforms on the macroeconomic performance in the member countries of the European Union (EU). This report presents the results of the project "Study on the feasibility of a tool to measure the macroeconomic impact of structural reforms" (ECFIN-E/2005/001) and amalgamates the findings of the two previous interim reports and the main conclusions of the workshop held in Brussels on May 11th. The main goal of the project is to determine the most reliable and robust methods to investigate the impacts of economy-wide structural reforms as well as reforms in individual markets or sectors, and to make suggestions as to how they best to implement them and possible improvements of the institutional dataset. In addition, a roadmap has been created which includes the main steps in the model-developing process, and solutions feasible even in the short term are discussed.The most relevant conclusion to be drawn from the study is that the most appropriate tool that can be developed in the short term is the integration of a DSGE model (preferably QUEST due to its in-house availability) with different satellite models, to be developed.structural reforms, product markets, labour markets, financial markets, Dreger, Art�s, Moreno, Ramos, Suri�ach

Broadband modeling of local earthquakes

Three-component broadband waveforms of two small earthquakes near Upland, California, recorded on the Pasadena broadband, high dynamic range instrument, were modeled to obtain useful Green's functions for this path and to examine the sensitivity of the synthetic seismograms to perturbations of the crustal model. We assumed that the source of each event was both simple and known, as determined from the Caltech-USGS array first motions. A trapezoidal time function was chosen to fit the width of the direct S wave. Generalized rays, reflectivity, and finite-difference techniques were used to compute the synthetic seismograms. We found that a simple layer over a half-space model is an adequate approximation of the upper crust along this profile. In particular, the waveforms are controlled by a relatively slow, 4-km-thick surficial layer (α = 4.5km-s^(−1), β = 2.6 km-s^(−1)) over a faster layer (α = 5.9 km-s^(−1), β = 3.5 km-s^(−1)). The relative amplitudes of direct and multiple S indicate that the main shock occurred at a depth of 6 km, while the aftershock occurred at a depth of 8 to 9 km. Sensitivity analyses indicate that for distances less than 50 km and for periods longer than 1 sec, the synthetic seismograms are not very sensitive to perturbations of the deep crustal structure. Analysis of upper crustal model perturbations revealed that the surficial layer is between 3 to 5 km thick. In addition, the contact between this layer and the underlying material can be smoothed with a 2-km-wide velocity gradient without adversely affecting the fit to the data. Two-dimensional finite-difference calculations show that a ridge structure beneath the recorder acts as a lowpass filter (the lower frequency phases are largely unaffected). Other two-dimensional models with ridges between the source and receiver clearly did not fit the data. Synthetic seismograms computed for the best fitting model were used to estimate a long-period moment of (6 ± 2) × 10^(22) dyne-cm (M_L = 4.6) and 1 × 10^(22) dyne-cm (M_L = 3.7) with identical triangular source-time durations of 0.3 sec. Assuming the same fault dimension of 0.4 km from standard scaling laws, stress drop estimates of 410 and 70 bars are obtained for the two events, respectively. Generally, we found that it is possible to reproduce local waveforms at frequencies up to 1 Hz without a complete knowledge of fine structural detail. Resulting Green's functions can be useful in studying historic events, and in simulations of large events from a given source region

Moment tensor inversions of icequakes on Gornergletscher, Switzerland

We have determined seismic source mechanisms for shallow and intermediate-depth icequake clusters recorded on the glacier Gornergletscher, Switzerland, during the summers of 2004 and 2006. The selected seismic events are part of a large data set of over 80,000 seismic events acquired with a dense seismic network deployed in order to study the yearly rapid drainage of Gornersee lake, a nearby ice-marginal lake. Using simple frequency and distance scaling and Green’s functions for a homogeneous half-space, we calculated moment tensor solutions for icequakes with M_w-1.5 using a full-waveform inversion method usually applied to moderate seismic events (M_w>4) recorded at local to regional distances (≈50–700 km). Inversions from typical shallow events are shown to represent tensile crack openings. This explains well the dominating Rayleigh waves and compressive first motions observed at all recording seismograms. As these characteristics can be observed in most icequake signals, we believe that the vast majority of icequakes recorded in the 2 yr is due to tensile faulting, most likely caused by surface crevasse openings. We also identified a shallow cluster with somewhat atypical waveforms in that they show less dominant Rayleigh waves and quadrantal radiation patterns of first motions. Their moment tensors are dominated by a large double-couple component, which is strong evidence for shear faulting. Although less than a dozen such icequakes have been identified, this is a substantial result as it shows that shear faulting in glacier ice is generally possible even in the absence of extreme flow changes such as during glacier surges. A third source of icequakes was located at 100 m depth. These sources can be represented by tensile crack openings. Because of the high-hydrostatic pressure within the ice at these depths, these events are most likely related to the presence of water lenses that reduce the effective stress to allow for tensile faulting

The impact of institutions on the employment performance in European labour markets

This paper investigates the role of institutions for labour market performance across European countries. As participation rates have been rather stable over the past, the unemployment problem is mainly caused by shortages in labour demand. Labour demand is expressed by its structural parameters, such as the elasticities of employment to output and factor prices. Institutional variables include employment protection legislation, the structure of wage bargaining, measures describing the tax and transfer system and active labour market policies. As cointegration between employment, output and factor prices is detected, labour demand equations are fitted in levels by efficient estimation techniques. To account for possible structural change, time varying parameter models and aysmmetries due to the business cycle situation are considered. Then, labour demand elasticities are explained by institutions using panel fixed effects regressions. The results suggest that higher flexibility and incentives of households to work appear to be appropriate strategies to improve the employment record. The employment response to economic conditions is stronger in a more deregulated environment, and the absorption of shocks can be relieved. However, the institutional database should be improved in order to arrive at more definite policy conclusions

Moment tensors for rapid characterization of megathrust earthquakes: the example of the 2011 M9 Tohoku-oki, Japan earthquake

The rapid detection and characterization of megathrust earthquakes is a difficult task given their large rupture zone and duration. These events produce very strong ground vibrations in the near field that can cause weak motion instruments to clip, and they are also capable of generating large-scale tsunamis. The 2011 M9 Tohoku-oki earthquake that occurred offshore Japan is one member of a series of great earthquakes for which extended geophysical observations are available. Here, we test an automated scanning algorithm for great earthquakes using continuous very long-period (100-200 s) seismic records from K-NET strong-motion seismograms of the earthquake. By continuously performing the cross-correlation of data and Green's functions (GFs) in a moment tensor analysis, we show that the algorithm automatically detects, locates and determines source parameters including the moment magnitude and mechanism of the great Tohoku-oki earthquake within 8 min of its origin time. The method does not saturate. We also show that quasi-finite-source GFs, which take into account the effects of a finite-source, in a single-point source moment tensor algorithm better fit the data, especially in the near-field. We show that this technique allows the correct characterization of the earthquake using a limited number of stations. This can yield information usable for tsunami early warnin

Modeling of Energy Amplification Recorded within Greater Los Angeles Using Irregular Structure

We have investigated energy amplification observed within Greater Los Angeles basin by analyzing regional waveforms recorded from several Nevada Test Site (NTS) nuclear explosions. Although the stations are located nearly at the same azimuth (distances ranging from 350 to 400 km), the seismograms recorded in Compton (the central part of the basin), Long Beach (the southern edge of the basin), and downtown Los Angeles are remarkably different, even for a common explosion. Following the onset of L_g waves, the Long Beach sites have recorded surface waves for more than 100 sec. From one explosion, the sites within downtown Los Angeles have recorded seismograms with strong 3-sec surface waves. These waves are not observed on the seismograms recorded in the neighboring hard-rock site California Institute of Technology (CIT) station. Thus, they must have been generated by local wave guides. Numerically, we modeled these 3-sec waves by convolving the CIT seismogram with the response of a sedimentary strata dipping gently (about 6°) from CIT toward downtown. We also examined the irregular basin effect by analyzing the variation of cumulative temporal energy across the basin relative to the energy recorded at CIT from the same explosion. Variation up to a factor of 30 was observed. To model the energy variation that is caused by extended surface waves in the Long Beach area, we used numerically simulated site transfer functions (STF) from a NNE-SSW oriented two-dimensional basin structure extending from Montebello to Palos Verdes that included low-velocity sedimentary material in the uppermost layers. These STFs were convolved with the CIT seismogram recorded from the MAST explosion. To simulate elongated duration of surface waves, we introduced in the upper sedimentary structure some discontinuous microbasin structures of varying size, each microbasin delaying the seismic waves propagating through them. Consequently, the surface-reflected phases through these structures are delayed and reflected into the upper medium by the underlying interfaces. This mechanism helps delayed energy to appear at a later time and result in a longer time duration at sites located at southern edge of the basin