S U M M A R YThree-component broad-band displacement seismograms with paths sampling the Basin and Range province are studied to constrain the crustal structure. To find an average model that fits the data in both absolute time and waveform, we generate broad-band reflectivity synthetics and conduct sensitivity tests on different parts of a layered crustal model, where only a few layers are involved. Generalized rays are used to help identify the various phases. It proves useful to decompose a regional seismogram into segments so that the impact of model parameters on each segment can be clearly identified. Thus, for mid-crustal earthquakes, it is established that the top crustal layer controls the Rayleigh wave, the Airy phase, in shape over the range from 300 to 600 km, and the crustal layer just above the source depth controls its timing. The P,,, waves, the P , and P,, portion, are controlled in broad-band character by the mid-crust while the top layer contributes to their long-period motion. These crustal parameters control the tangential motion similarly. The SV wave, the segment between the P,, wave and the Rayleigh wave, is mostly controlled by the shear velocity of the lower crust. In judging the goodness of fit between the array observations and synthetic waveforms, we allow individual data segments to shift relative to the I-D synthetics by a few seconds to account for some lateral variation. The amount of time shift is found by the cross-correlation in displacement between the data segment and the synthetics. Applying these tests in a forward modelling approach, we find that a simple two-layer crustal model is effective in explaining this data set. In this model, the main crustal layer has P and S velocities of 6.1 km s-' and 3.6 km sC1, similar to those found by Langston & Helmberger (1974). A surface layer of thickness 2.5 to 3.5 km is required to fit the Rayleigh waves. The refined model can be used as a reference model for further studies in this region.

Abstract. We present a study of regional earthquakes in the western Mediterranean geared toward the development of methodologies and path calibrations for source characterization using regional broadband stations. The results of this study are useful for the monitoring and discrimination of seismic events under a comprehensive test ban treaty, as well as the routine analysis of seismicity and seismic hazard using a sparse array of stations. The area consists of several contrasting geological provinces with distinct seismic properties, which complicates the modeling of seismic wave propagation. We started by analyzing surface wave group velocities throughout the region and developed a preliminary model for each of the major geological provinces. We found variations of crustal thickness ranging from 45 km under the Atlas and Betic mountains and 37 km under the Saharan shield, to 20 km for the oceanic crust of the western Mediterranean Sea, which is consistent with earlier works. Throughout most of the region, the upper mantle velocities are low which is typical for tectonically active regions. The most complex areas in terms of wave propagation are the Betic Cordillera in southern Spain and its north African counterparts, the Rif and Tell Atlas mountains, as well as the Alboran Sea, between Spain and Morocco. The complexity of the wave propagation in these regions is probably due to the sharp velocity contrasts between the oceanic and continental regions as well as the the existence of deep sedimentary basins that have a very strong influence on the surface wave dispersion. We used this preliminary regionalized velocity model to correct the surface wave source spectra for propagation effects which we then inverted for source mechanism. We found that this method, which is in use in many parts of the word, works very well, provided that data from several stations are available. In order to study the events in the region using very few broadband stations or even a single station, we developed a hybrid inversion method which combines Phi waveforms synthesized with the traditional body wave methods, with surface waves that are computed using normal modes. This procedure facilitates the inclusion of laterally varying structure in the Green's functions for the surface waves and allows us to determine source mechanisms for many of the larger earthquakes (M > 4) throughout the region with just one station. We compared our results with those available from other methods and found that they agree quite well. The epicentral depths that we have obtained from regional waveforms are consistent with observed teleseismic depth phases, as far as they are available. We also show that the particular upper mantle structure under the region causes the various Pn and Sn phases to be impulsive, which makes them a useful tool for depth determination as well. Thus we conclude that with proper calibration of the seismic structure in the region and high-quality broadband data, it is now possible to characterize and study events in this regio...

No abstract

Broadband and long-period displacement waveforms from a selection of Northridge aftershocks recorded by the TERRAscope array are modeled to study source characteristics. Source mechanisms and moments are determined with long-period data using an algorithm developed by Zhao and Helmberger (1994). These results are compared with those by Hauksson et al. (1995) and Thio and Kanamori (1996). The width of the direct pulses at the nearest stations PAS and CALB are measured as indications of the source duration. Another measurement of the source-time functions of these earthquakes is obtained by comparing the short-period to long-period energy ratio in the data to that in the synthetics. These measurements are used to estimate the relative stress drop using a formula given by Cohn et al. (1982). The depth distribution of the relative stress drops indicates that the largest stress drops are in the depth range of 5 to 15 km for an aftershock population of 24 events. A correlation of extended surface wave train with source depth is demonstrated for paths crossing the San Fernando basin.

Broadband regional records are modeled to determine source mechanism, seismic moment, fault dimension, and rupture directivity for the 17 January 1994 Northridge earthquake. Modeling is done using both theoretical Green's functions (tGf) and empirical Green's functions (eGf). From the theoretical modeling, we obtain a source mechanism with strike 128°, dip 33°, and rake 106° for the mainshock, using a source estimation algorithm by Zhao and Helmberger (1994). While the fault orientation seems resolvable from regional data, the moment estimation is less reliable due to inadequate synthetic waveform fits to the observed surface waves. This appears to be caused by the combination of propagational effects and fault complexities. Further investigation of the source characteristics is carried out with a new method of using eGf's. As an eGf, we select the 17 January 1994 17:56 GMT aftershock, which occurred near the onset of the mainshock and had a similar source mechanism. The source duration of the mainshock, as seen from the regional surface waves observed at various stations, is obtained by searching for the trapezodial far-field source-time function for each station that, when convolved with the aftershock data, best simulates the mainshock data. Stations to the north record shorter source durations than stations to the south. Modeling these with theoretical predictions of rupture on a square fault, we constrain the effective fault dimension to be 14 km with rupture along the direction of the average rake vector. A moment of (1.4 ± 0.9) × 1026 dyne-cm with a stress drop of ∼120 bars is obtained for the mainshock from our eGf study.