Intermediate-period Surface Waves Research Articles

Determination of the Earth’s structure based on intermediate-period surface wave recordings of tidal gravimeters: a case study

Tidal gravimeters can detect intermediate-period surface waves with high accuracy. Three gravimetric stations with estimated transfer functions and co-located with seismic stations were selected: two in Belgium (Membach and Rochefort, in Western Europe) and one in Germany (Black Forest, in Central Europe). The compatibility of gravimetric and seismic recordings of earthquakes in the period range of 10-180 s has been presented. The series of monochromatic signals separated from surface waves for selected events have been calculated using the multiple-filtering procedure, and averaged fundamental-mode Rayleigh wave group-velocity curves have been estimated on a regional scale. Next, averaged dispersion curves for three regions (Italy, Greece, and Western Turkey) were inverted by weighted linear inversion methods. Additionally, a quantitative analysis of resolution tests of inverted models was presented to show the capabilities of the gravimetric data to retrieve a shear-wave velocity distribution with a depth. In particular, a method for determining the depth range of the inverted model has been proposed. Results obtained based on gravimetric data were verified by results from seismic data obtained by applying the same procedure. This study shows the novel application of tidal gravimetric data in the determination of the regional Earth’s structure based on intermediate-period surface waves recordings, as well as a comprehensive approach to the quantitative estimation of a final model resolution.Graphical

The global CMT project 2004–2010: Centroid-moment tensors for 13,017 earthquakes

Earthquake moment tensors reflecting seven years of global seismic activity (2004–2010) are presented. The results are the product of the global centroid-moment-tensor (GCMT) project, which maintains and extends a catalog of global seismic moment tensors beginning with earthquakes in 1976. Starting with earthquakes in 2004, the GCMT analysis takes advantage of advances in the mapping of propagation characteristics of intermediate-period surface waves, and includes these waves in the moment-tensor inversions. This modification of the CMT algorithm makes possible the globally uniform determination of moment tensors for earthquakes as small as MW=5.0. For the period 2004–2010, 13,017 new centroid-moment tensors are reported.

The Italian CMT dataset from 1977 to the present

The INGV-Harvard European-Mediterranean Regional Centroid Moment Tensor (RCMT) Catalog collects solutions routinely computed since 1997 for earthquakes with moderate magnitude ( 4.5 ≤ M ≤ 5.5 ) in the Mediterranean region. The database represents an extension to smaller magnitudes of the Harvard global CMT catalog, based on analysis of seismograms recorded at regional distance, and modeling of intermediate period surface waves. The catalog includes about 600 events, 200 of which in the Italian region. This study extends the catalog back in time, for the Italian region, as long as made possible by available digital data – i.e. since 1977 – with the same analysis and inversion method used for current seismicity. As a result, we present here 65 new moment tensors, for years between 1977 and 1997. These solutions represent 45% of the total number of events analyzed, the existing seismograms being often too scarce to allow a stable solution. The new dataset includes events in many seismic zones where moderate seismicity had previously been scarcely documented, e.g., the Po Plain, the Central to Southern Apennines and the Adriatic Sea. The complete dataset, including previously determined RCMTs and CMTs, represents the seismic deformation in the Italian area during the last 25 years.

Rayleigh Waves Generated by Mining Explosions and Upper Crustal Structure around the Powder River Basin, Wyoming

Fundamental-mode, intermediate period Rayleigh waves generated by mining explosions are utilized to constrain the crustal structure of Wyoming. Broadband seismic stations recorded data during two regional seismic experiments conducted in Wyoming in 1996 and 1997. The stations were deployed in a ring surrounding the Powder River Basin (PRB) at ranges 100-360 km from the Black Thunder Coal Mine located in the northeast corner of Wyoming. Signals generated by four explosions with total explosive weight from 1.1 kiloton to 2.7 kiloton are analyzed with the goal of resolving the crustal structure in northeastern Wyoming. Data from the experiments show evidence that kiloton-sized millisecond-delay-fired cast blasts produce strong 1- to 15-sec period surface waves, some extending to 20-sec period. Group velocities of fundamental-mode Rayleigh waves were estimated using the Multiple Filter Analysis (MFA) technique and refined with Phase Matched Filtering (PMF). The surface waves are dependent on propagation direction and source orientation. The northwest-southeast trend of the PRB means that some propagation paths are parallel to the basin orientation and others perpendicular. Group velocities exhibit normal dispersion and range from 0.97 (1 sec) to 3.1 (20 sec) km/sec. A least square inversion technique was used to invert group velocity dispersion curves for the shallow shear-wave velocity structure. The average model consists of nine layers in the upper 20 km of the crust. The two top layers are thin (0.35 and 0.45 km) with slow velocities (1.0 to 1.6 km/sec) as a result of the basin sediments. At depths below 15 km, the shear velocity is nearly the same for paths parallel and perpendicular to the PRB axis. A possible low-velocity zone is found along the PRB axis with a velocity contrast of 0.7 km/sec at depth of 9.8 km to 14.8 km. These detailed models give a much better fit to dispersion curves than the global CRUST2.0, particularly for periods below 5 sec, which indicates the need for path-specific models when analyzing mid-period surface waves. Comparison of theoretical seismograms using the empirically derived structure with the observed seismograms provides a basis for constraining the source processes of these mining explosions. The generation of the intermediate-period surface waves is directly related to the total source time duration that is on the order of several seconds for these cast blasts. Observational data suggest that source orientation is important as well, resulting from some combination of mine free face orientation, blasting direction, and material casting. Manuscript received 24 July 2003.

Measuring surface wave phase velocities beneath small broad-band arrays: tests of an improved algorithm and application to the French Alps

The local measurement of dispersion curves of intermediate-period surface waves is particularly difficult because of the long wavelengths involved. We suggest an improved procedure for measuring dispersion curves using small-aperture broad-band arrays. The method is based on the hypotheses of plane incoming waves and that averaging over a set of events with a good backazimuth distribution will suppress the effects of diffraction outside the array. None of the elements of the processing are new in themselves, but each step is optimized so we can obtain a reliable dispersion curve with a well-defined uncertainty. The method is based on the inversion for the slowness vector at each event and frequency using time delays between pairs of stations, where the time delays Δt are obtained by frequency-domain Wiener filtering. The interstation distance projected on to the slowness vector (D) is then calculated. The final dispersion curve is found by, at each frequency, calculating the inverse of the slope of the best-fitting line of all (D, Δt) points. To test the algorithm, it is applied to synthetic seismograms of fundamental mode Rayleigh waves in different configurations: (1) the sum of several incident waves; (2) an array located next to or above a crustal thickening; and (3) added white noise, using regular and irregular backazimuth distributions. In each case, a circular array of 23 km diameter and composed of six stations is used. The algorithm is stable over a large range of wavelengths (between half and a tenth of the array size), depending on the configuration. The situations of several, simultaneously incoming waves or neighbouring heterogeneities are well handled and the inferred dispersion curve corresponds to that of the underlying medium. Above a strong lateral heterogeneity, the inferred dispersion curve corresponds to that of the underlying medium up to wavelengths of eight times the array size in the configuration considered, but further work is needed to better understand the limits under which the obtained dispersion curve is not biased. In the case of 5 per cent spectral amplitude white noise, the dispersion curve is also stable for wavelengths up to approximately eight times the array size, but this limit depends of course strongly on the noise level. The method is finally applied to data from two arrays in the French Alps located 50 km apart. It is possible to measure the dispersion curves up to wavelengths approximately ten times bigger than the array diameter. The difference in the dispersion curves is compatible with a crustal and lithospheric thickening under the Alps. However, the observed errors are large, which result in severe limits on the interpretation in terms of lithospheric structure. Longer recording periods may help to reduce the errors. Otherwise the algorithm is likely to be of use mainly in areas where the lateral variations outside the array are smaller than those of the French Alps.

European–Mediterranean regional centroid-moment tensors: 1997–2000

The Mediterranean region is seismically very active and undergoing rapid deformation. Earthquakes as strong as M≥6 are present, but relatively rare. On the other hand, moderate-magnitude seismicity (4.5< M<5.5) is widespread and, often, damaging. An improved knowedge of moderate-magnitude earthquakes can contribute significantly to a better understanding of active tectonics in the region. Harvard centroid-moment tensors (CMT) today represent the most widely used reference for reliable source mechanisms, but modelling seismic sources with the CMT technique for earthquakes with magnitude less than M≈5–5.5 is quite difficult. Since 1997, we are using an extension of the standard CMT algorithm to compute regional centroid-moment tensors (RCMT) for moderate-magnitude earthquakes ( M W as low as 4.0), analyzing intermediate-period surface waves from regional and teleseismic seismograms. We routinely apply the algorithm to European–Mediterranean seismicity to produce a catalog of seismic moment tensors for events with magnitude between 4.5 and 5.5, which generally are not included in the Harvard CMT catalog. In this paper, we show the results for earthquakes that occurred between 1997 and 2000, consisting of 252 solutions, and we discuss the characteristics of the catalog and the areas where these new data are most relevant.

Seismotectonic re-evaluation of the 1976 Friuli, Italy, seismic sequence

The 1976 Friuli seismic sequence is known for itscomplexity, with several unusually large aftershocksoccurring in the epicentral area. The source region, where the northeastern part of the Alpine chain meetsthe northern Dinaric chain, is characterized by a complexcompressional tectonic regime. Previous studies have not clearly identified which of the two main tectonic systems was activated, in part due to the limited precision of derived earthquake parameters, such as hypocentral coordinates and focal mechanisms. We review the locations for the 10 largest events of the sequence, including theSeptember 16, 1977 and April 18, 1979 earthquakes andwe compute their centroid moment tensors. Source parameters are calculated using intermediate period surfacewaves and the modification of the Harvard centroid-moment tensoralgorithm proposed by Arvidsson and Ekstrom (1998). A summary of all available geological, geodetic and seismological data show that most of the earthquakes may be associated with the Periadriatic overthrust and other related thrust faults. Based on their locations and focal mechanisms only two ofthe early aftershocks (May 7, 1976 and May 9, 1976) appear instead to have occurred on Dinaric structures.

Off-great-circle propagation of intermediate-period surface waves observed on a dense array in the French Alps

Array analysis is performed on surface waves recorded in the French Alps using a small-aperture (25 km) temporary array of six broad-band stations. The analysis shows that both Rayleigh and Love waves deviate relative to the great-circle path. The deviations are particularly strong, up to 30°, between 20 and 40 s period. To interpret these observations, we first study the effect of large-scale structures using ray tracing in a smooth, laterally heterogeneous model of the Earth. Second, we evaluate the local effect by considering a model for the French Alps including strong lateral heterogeneities around the array that were not taken into account in the ray tracing. By combining these two possible causes of the observed deviations, we propose an explanation for the general trend in the observed deviations. Finally, we show that by taking into account azimuthal deviations, phase velocities measured at a regional scale can be significantly improved.

Global CMT analysis of moderate earthquakes,Mw≧ 4.5, using intermediate-period surface waves

AbstractA new method for calculating centroid moment tensor (CMT) solutions for moderate-sized earthquakes is presented and tested. In the new algorithm, which is a modification to the standard Harvard CMT method, intermediate-period surface waves are included in the inversion for source parameters. The dispersion of teleseismic surface waves in the period range 40 to 150 sec is predicted by interpolation of recent global phase velocity maps. The method is tested on a set of 20 moderate and large (5.2 ≦ Mw ≦ 7.2) earthquakes with known mechanisms. The retrieved mechanisms are similar to the previous results, even when noise is added to the seismograms to simulate earthquakes of a smaller magnitude (Mw ∼ 4.5). The inversion algorithm is applied to the 1993 Klamath Falls mainshock and aftershock sequence, and the results compare well with earlier results. A catalog of Mw ≧ 4.5 focal mechanisms for Greece and surrounding areas for the first six months of 1995 is derived using the new method of analysis.

Rupture process of the 19 August 1992 Susamyr, Kyrgyzstan, earthquake

The Susamyr earthquake of August 19, 1992 in Kyrgyzstan is one of the largest events (Ms = 7.4, Mb = 6.8) of this century in this region of Central Asia. We used broadband and long period digital data from IRIS and GEOSCOPE networks to investigate the source parameters, and their space-time distribution by modeling both body and surface waves. The seismic moment (M0 = 6.8 × 1019 N m) and the focal mechanism were determined from frequency-time analysis (FTAN) of the fundamental mode of long period surface waves (100–250 s). Then, the second order integral moments of the moment-rate release were estimated from the amplitude spectra of intermediate period surface waves(40–70 s). From these moments we determined a source duration of 11–13 s, major and minor axes of the source of 30 km and 10–22 km, respectively; and an instant centroid velocity of 1.2 km/s. Finally, we performed a waveform inversion of P and SH waves at periods from 5–60 s. We found a source duration of 18–20 s, longer than the integral estimate from surface wave amplitudes. All the other focal parameters inverted from body waves are similar to those obtained by surface waves (θ = 87° ± 6°, δ = 49° ± 6°, λ = 105° ± 3°, h = 14 ± 2 km, and M0 = 5.8 ± 0.7 × 1019 N m). The initial rupture of this shallow earthquake was located at the south-west border of Susamyr depression in the western part of northern Tien Shan. A finite source analysis along the strike suggests a westward propagation of the rupture. The main shock of this event was preceded 2 s earlier by small foreshock. The main event was almost immediately followed by a very strong series of aftershocks. Our surface and body wave inversion results agree with the general seismotectonic features of the region.

Analysis of Rayleigh wave refraction from three-component seismic spectra

SUMMARY Lateral refraction of intermediate period surface waves is caused by horizontal gradients in local phase velocity in the vicinity of the source-receiver minor arc. For smooth structural perturbations, this is manifested by an off-azimuth arrival (azimuth deviation) of the Airy phase. We present a method for estimating azimuth deviation as a function of frequency for Rayleigh waves observed on a three-component seismogram. By taking advantage of the different dispersion characteristics of the Rayleigh and Love wave group arrivals, the method essentially constructs an optimal narrow-band taper for each frequency component, in order to minimize the effect of Love wave contamination in the Rayleigh wave azimuth estimation. We have measured the frequency-dependent azimuth deviation of fundamental mode Rayleigh waves of period 20-100s under a variety of conditions. The most coherent results are obtained for (1) seismic records with a relatively high Rayleigh wave excitation, (2) ray paths which are confined to purely oceanic or purely continental lithosphere, and (3) ray paths of intermediate offset (30°-50°).

Propagation Characteristics of Intermediate-Period (1-10 Seconds) Surface Waves in the Kanto Plain, Japan

Propagation characteristics of the intermediate-period (1-10 s) surface waves predominated in strong motion records are analyzed on the basis of an array observation deployed in metropolitan Tokyo area, Japan. Seismic energy source is the main shock of the 1984 western Nagano earthquake (M 6.8) with a focal depth of 2 km; epicentral distances (Δ) for the stations are about 157-205 km. Phase analyses using the sub-array at KOG (Δ_??_7175 km) indicate that dominant phases in the period range 1-10 s are Love and Rayleigh waves with clear dispersion. Apparent of their phases are lying in the range of 800-2, 100 m/s. They are matched with the fundamental mode of Love waves for transverse motion but are stretching over the two modes of Rayleigh waves (M11 and M21) for radial component; the crustal model, on which the normal mode solution is based, includes thick (-52, 000 m) sedimentary layers lying on the Pre-Tertiary bedrock. The above results in conjunction with direction of wave propagation and particle orbit of the ground motion suggest that the intermediate-period surface waves are perturbed by other phases such as diffracted surface waves which are owed to lateral heterogeneities in the uppermost crust. Correspondence of major phases in the strong motion records of the long-span array gives rise to inter-station group velocities of the surface waves of the intermediate-period. Thus derived group enable us to interpret that the well-dispersed later phases are on a branch near the Airy phase of which propagation velocity is 300-800 m/s.

Inversion of regional Pn1 and surface‐wave data for source parameters of a Borah Peak aftershock

We present results of a comparative source study on one of the largest aftershocks of the Borah Peak, Idaho, earthquake series. The mechanism of this event had been previously studied using first motions and teleseismic p‐wave data, and we report here our results from analysis of regional Pn1 and surface waves. The Pn1 waves gave an estimate of the source duration near 5 sec, in agreement with the teleseismic p‐waves. Both the source duration and the effect of surface‐wave dispersion through laterally heterogeneous crustal structures must be accounted for in the moment‐tensor inversions of intermediate period surface waves. All data sources, both long period and short, gave consistent source mechanisms, and the long periods also yield three independent and consistent estimates of the seismic moment. The results demonstrate the promise of formal source‐parameter retrieval techniques applied to regional data for the study of earthquakes with magnitudes below the magnitude threshold of teleseismic source determinations.