Analysis Of Body Waves Research Articles

Fast and semi-automatic S-wave and P-wave velocity estimations from landstreamer data: a field case from the Middle East

Seismic surface and body wave analyses are powerful tools for the geotechnical characterization of sites. The use of landstreamers facilitates the acquisition of dense data sets over large areas. However, efficient processing workflows are needed to estimate 3D velocity models from these massive data sets. For surface wave analysis, the manual picking of dispersion curves (DCs) of large data sets is very time-consuming, whereas the accuracy can be biased by operator choices. We apply a semi-automatic workflow for the analysis, processing, and interpretation of a large-scale landstreamer data set acquired for engineering purposes in the Middle East. The workflow involves the application of a validated automatic DC picking algorithm, and the transformation of the DCs into S- and P-wave velocity models through the wavelength-depth technique. The method has a high level of automation, is data driven and does not require extensive data inversion. Another remarkable benefit is that the auto-picking is more than 1,000 times more efficient than standard manual picking and the estimated velocities are in good agreement with available geotechnical and geophysical information. We conclude that the semi-automatic approach may represent a fast and straightforward method suitable for both research and industrial projects, thus enhancing further collaborations and developments.

Modeling subduction earthquake sources in the central-western region of Colombia using waveform inversion of body waves

Source parameters of four earthquakes located within the Wadati-Benioff zone of the Nazca plate subducting beneath the South American plate in Colombia were determined. The seismic moments for these events were recalculated and their approximate equivalent rupture area, slip distribution and stress drop were estimated. The source parameters for these earthquakes were obtained by deconvolving multiple events through teleseismic analysis of body waves recorded in long period stations and with simultaneous inversion of P and SH waves. The calculated source time functions for these events showed different stages that suggest that these earthquakes can reasonably be thought of being composed of two subevents. Even though two of the overall focal mechanisms obtained yielded similar results to those reported by the CMT catalogue, the two other mechanisms showed a clear difference compared to those officially reported. Despite this, it appropriate to mention that the mechanisms inverted in this work agree well with the expected orientation of faulting at that depth as well as with the wave forms they are expected to produce. In some of the solutions achieved, one of the two subevents exhibited a focal mechanism considerably different from the total earthquake mechanism; this could be interpreted as the result of a slight deviation from the overall motion due the complex stress field as well as the possibility of a combination of different sources of energy release analogous to the ones that may occur in deeper earthquakes. In those cases, the subevents with very different focal mechanism compared to the total earthquake mechanism had little contribution to the final solution and thus little contribution to the total amount of energy released.

Attenuation Characteristics of High Frequency Seismic Waves in Southern India

We present a systematic study of seismic attenuation and its related Q structure derived from the spectral analysis of P-, S-waves in the southern India. The study region is separated into parts of EDC (Eastern Dharwar Craton), Western Dharwar Craton (WDC) and Southern Granulite Terrain (SGT). The study is carried out in the frequency range 1–20 Hz, using a single-station spectral ratio technique. We make use of about 45 earthquakes, recorded in a network of about 32 broadband 3-component seismograph-stations, having magnitudes (M L) varying from 1.6 to 4.5, to estimate the average seismic body wave attenuation quality factors; Q P and Q S. Their estimated average values are observed to be fitting to the power law form of Q = Q 0 f n . The averaged power law relations for Southern Indian region (as a whole) are obtained as Q P = (95 ± 1.12)f (1.32±0.01); Q S = (128 ± 1.84)f (1.49±0.01). Based on the stations and recorded local earthquakes, for parts of EDC, WDC and SGT, the average power law estimates are obtained as: Q P = (97 ± 5)f (1.40±0.03), Q S = (116 ± 1.5)f (1.48±0.01) for EDC region; Q P = (130 ± 7)f (1.20±0.03), Q S = (103 ± 3)f (1.49±0.02) for WDC region; Q P = (68 ± 2)f (1.4±0.02), Q S = (152 ± 6)f (1.48±0.02) for SGT region. These estimates are weighed against coda Q (Q C) estimates, using the coda decay technique, which is based on a weak backscattering of S-waves. A major observation in the study of body wave analysis is the low body wave Q (Q 0 0.5) and Q S/Q P ≫ 1, suggesting lateral stretches of dominant scattering mode of seismic wave propagation. This primarily could be attributed to possible thermal anomalies and spread of partially fluid-saturated rock-masses in the crust and upper mantle of the southern Indian region, which, however, needs further laboratory studies. Such physical conditions might partly be correlated to the active seismicity and intraplate tectonism, especially in SGT and EDC regions, as per the observed low-Q P and Q S values. Additionally, the enrichment of coda waves and significance of scattering mechanisms is evidenced in our observation of Q C > Q S estimates. Lapse time study shows Q C values increasing with lapse time. High Q C values at 40 s lapse times in WDC indicate that it may be a relatively stable region. In the absence of detailed body wave attenuation studies in this region, the frequency dependent Q relationships developed here are useful for the estimation of earthquake source parameters of the region. Also, these relations may be used for the simulation of earthquake strong ground motions which are required for the estimation of seismic hazard, geotechnical and retrofitting analysis of critical structures in the region.

Deriving Sensitivity Kernels of Coda-Wave Travel Times to Velocity Changes Based on the Three-Dimensional Single Isotropic Scattering Model

Recently, coda-wave interferometry has been used to monitor temporal changes in subsurface structures. Seismic velocity changes have been detected by coda-wave interferometry in association with large earthquakes and volcanic eruptions. To constrain the spatial extent of the velocity changes, spatial homogeneity is often assumed. However, it is important to locate the region of the velocity changes correctly to understand physical mechanisms causing them. In this paper, we are concerned with the sensitivity kernels relating travel times of coda waves to velocity changes. In previous studies, sensitivity kernels have been formulated for two-dimensional single scattering and multiple scattering, three-dimensional multiple scattering, and diffusion. In this paper, we formulate and derive analytical expressions of the sensitivity kernels for three-dimensional single-scattering case. These sensitivity kernels show two peaks at both source and receiver locations, which is similar to the previous studies using different scattering models. The two peaks are more pronounced for later lapse time. We validate our formulation by comparing it with finite-difference simulations of acoustic wave propagation. Our formulation enables us to evaluate the sensitivity kernels analytically, which is particularly useful for the analysis of body waves from deeper earthquakes.

The lack of equipartitioning in global body wave coda

AbstractCorrelation of the coda of large earthquakes can be used to retrieve body waves at teleseismic distances. This retrieval depends on waves propagating with equal energy in all directions (equipartitioning). We carry out a beamforming analysis of body waves at periods of 10 and 40 s recorded at USArray and show that the late coda, up to 10 h after the Okhotsk earthquake, is dominated by waves propagating in the great circle direction; late coda waves are not equipartitioned. This implies for seismic interferometry that teleseismic body waves can only be extracted from the coda when the earthquake and the receivers are located on a great circle. This happens automatically when the employed stations are antipodal or when the autocorrelation of waves is used. The use of long‐period ambient seismic noise relies on the distribution of sources because scattering only marginally randomizes directions. This provides insight into recent observations of core phases extracted from seismic interferometry.

A deep outer‐rise reverse‐fault earthquake immediately triggered a shallow normal‐fault earthquake: The 7 December 2012 off‐Sanriku earthquake (MW 7.3)

AbstractTeleseismic body wave analysis revealed that the 7 December 2012 off‐Sanriku earthquake (MW 7.3) at the outer rise of Japan Trench consisted of two successive subevents. The first subevent with reverse‐fault mechanism (Event 1, MW 7.1) at 56 km depth was followed by, approximately 20 s later, the second subevent with normal‐fault mechanism (Event 2, MW 7.2) at 6 km depth. Finite‐fault slip models show that the slip of Event 1 was concentrated around the initial rupture point with the maximum of 2.7 m and that Event 2 had two asperities with the maximum of 4.5 m at both sides of the initial rupture point. The static Coulomb Failure Function analyses suggested that Event 1 triggered Event 2 and that both subevents were promoted by the 2011 Tohoku earthquake (MW 9.1).

Seismic anisotropy of the mantle lithosphere beneath the Swedish National Seismological Network (SNSN)

Body-wave analysis — shear-wave splitting and P travel time residuals — detect anisotropic structure of the upper mantle beneath the Swedish part of Fennoscandia. Geographic variations of both the splitting measurements and the P-residual spheres map regions of different fabrics of the mantle lithosphere. The fabric of individual mantle domains is internally consistent, usually with sudden changes at their boundaries. Distinct backazimuth dependence of SKS splitting excludes single-layer anisotropy models with horizontal symmetry axes for the whole region. Based upon joint inversion of body-wave anisotropic parameters, we instead propose 3D self-consistent anisotropic models of well-defined mantle lithosphere domains with differently oriented fabrics approximated by hexagonal aggregates with plunging symmetry axes. The domain-like structure of the Precambrian mantle lithosphere, most probably retaining fossil fabric since the domains' origin, supports the idea of the existence of an early form of plate tectonics during the formation of continental cratons already in the Archean. Similarly to different geochemical and geological constraints, the 3D anisotropy modelling and mapping of fabrics of the lithosphere domains contribute to tracking plate tectonics regimes back in time.

Resolution potential of surface wave phase velocity measurements at small arrays

The deployment of temporary arrays of broadband seismological stations over dedicated targets is common practice. Measurement of surface wave phase velocity across a small array and its depth-inversion gives us information about the structure below the array which is complementary to the information obtained from body-wave analysis. The question is however: what do we actually measure when the array is much smaller than the wave length, and how does the measured phase velocity relates to the real structure below the array? We quantify this relationship by performing a series of numerical simulations of surface wave propagation in 3-D structures and by measuring the apparent phase velocity across the array on the synthetics. A principal conclusion is that heterogeneities located outside the array can map in a complex way onto the phase velocities measured by the array. In order to minimize this effect, it is necessary to have a large number of events and to average measurements from events well-distributed in backazimuth. A second observation is that the period of the wave has a remarkably small influence on the lateral resolution of the measurement, which is dominantly controlled by the size of the array. We analyse if the artefacts created by heterogeneities can be mistaken for azimuthal variations caused by anisotropy. We also show that if the amplitude of the surface waves can be measured precisely enough, phase velocities can be corrected and the artefacts which occur due to reflections and diffractions in 3-D structures greatly reduced.

Anomalous Seismic Amplitudes Measured in the Los Angeles Basin Interpreted as a Basin-Edge Diffraction Catastrophe

The Los Angeles Basin Passive Seismic Experiment (LABPSE) involved the installation of an array of 18 seismic stations along a line crossing the Los Angeles basin from the foothills of the San Gabriel Mountains through the Puente Hills to the coast. At 3-5 km spacing between stations the array has much higher resolution than the permanent network of stations in southern California. This resolution was found to be important for analyzing the factors that govern the amplitude variation across the basin. We inverted spectra of P- and S-body-wave seismograms from local earthquakes (ML 2.1-4.8) for site effects, attenuation, and corner frequency factor using a standard model that assumes geometric spreading varying as inverse distance, exponential attenuation, and an x 2 source model. The S-wave attenuation was sep- arable into basin and bedrock contributions. In addition to the body-wave analysis, S-wave coda were analyzed for coda Q and coda-determined site effects. We find S- wave Q (QS) in bedrock is higher than in the basin. High-frequency QS is higher than low-frequency QS. Coda Q (Qc) is higher than QS. P-wave Q (QP) was not separable into basement and bedrock values, so we determined an average value only. The corner frequencies for P and S waves were found to be nearly the same. The standard model fit over 97% of the S-wave data, but data from six clustered events incident along the basin edge within a restricted range of incidence and azimuth angles gen- erated anomalous amplitudes of up to a factor of 5 higher than predicted. We test whether such basin-edge focusing might be modeled by catastrophe theory. After ruling out site, attenuation, and radiation effects, we conclude a caustic modeled as a diffraction catastrophe could explain both the frequency and spatial dependence of the anomalous variation.

Energy radiation from the Sumatra earthquake

We determined the duration of high-frequency energy radiation from Indonesia's great Sumatra-Andaman earthquake (26 December 2004) to be about 500 seconds. This duration can be translated into a rupture length of about 1,200 km, which is more than twice as long as that inferred from body-wave analyses performed soon after the event. Our analysis was able rapidly to define the extent of rupture, thereby aiding the assessment of seismic hazard in the immediate future.

Surface expression of thrust faulting in eastern Iran: source parameters and surface deformation of the 1978 Tabas and 1968 Ferdows earthquake sequences

SUMMARY Previously unrecognised thrust faults in eastern Iran were responsible for a destructive earthquake at Tabas (1978, September 16), which produced over 80 km of distributed and discontinuous surface ruptures above a series of low anticlinal hills to the west of a major range-front. Analysis of long-period body-wave seismograms shows a simple rupture on a gently dipping (∼16 ◦ ) thrust, with a slight right-lateral component. This is compatible with the locally recorded aftershock distribution. Body wave analysis of two later, smaller events show similar source orientations. Several indicators of long-term active folding at Tabas can be recognised in the geomorphology, and surface ruptures from 1978 are consistent with co-seismic fold growth. Drainage incision also indicates uplift at depth on thrust faults dipping eastwards beneath the folds. Body-wave seismograms for two earthquakes near Ferdows, 150 km east of Tabas, in September 1968 also show thrust faulting at depths of ∼10 km. Again, the surface geomorphology indicates a region of folding above an eastward-dipping thrust fault, which is ∼10 km west of a fault bounded range-front. In both Tabas and Ferdows, the active faulting appears to show Quaternary migration away from the range-front, possibly in response to stresses produced by the elevated topography. The identification of zones of active fault-related folding is important for earthquake hazard assessment and also for an understanding of the local tectonics. We conclude that the structures which gave rise to the 1968 and 1978 earthquakes could have been recognised beforehand from the clear signals in the geomorphology, had we known what to look for at that time.

Seismic anisotropy - a method for studying the fabric of deep continental lithosphere

Independent seismological results, obtained from shear-wave splitting, azimuthal variation of Pn velocities, P-residual spheres and dispersion of surface waves, have demonstrated to observed anisotropies originates in preferred orientation of olivine crystals in the mantle lithosphere. In comparison with oceanic regions, the large-scale fabric of the continental lithosphere is more complex. While the surface-wave data provide an integral property of a number of tectonic units, results on splitting of sub-vertically incident teleseismic shear waves show consistent orientations of the fast S waves within individual lithospheric blocks. In many provinces, the mean azimuth of the fast S polarization parallels the regional geological fabric at the surface (e.g. the strike of a mountain belt). The azimuth-incidence angle dependent terms of teleseismic P-wave residuals, cleared of the effects of source regions and deep mantle paths, show a systematic directional dependence of the relative residuals over distances of several hundred kilometres. Abrupt changes in orientations of the high- and low-velocity directions are related to important tectonic boundaries (deep-seated suture zones). The continental lithosphere related anisotropy scanned by P waves requires a tectonic block structure with dipping high- and low-velocity directions within the lithosphere. Present-day models and interpretations of individual observations of seismic anisotropy depend on the wavelength of different types of seismic waves, as well as on directions of the propagation (both as to azimuths and dips from the horizontal). A joint analysis of body waves - both anisotropic P-residual spheres and shear-wave splitting - allows detecting large-scale anisotropic structures approximated either by hexagonal or orthorhombic symmetries with inclined symmetry axes.

Teleseismic body‐wave analysis of the 9 October, 1995 (Mw = 8.0), Colima‐Jalisco, Mexico Earthquake, and its largest foreshock and aftershock

The source process of the October 9, 1995, Colima‐Jalisco, Mexico, earthquake (Mw=8.0), and its largest foreshock and aftershock were determined from teleseismic body waves, using a least‐squares inversion scheme. The three events are shallow‐dipping, thrust‐fault earthquakes, in agreement with the relative plate motions for Rivera‐North America and Cocos‐North America plate boundaries. Neither the foreshock nor the largest aftershock show significant differences in focal mechanism nor centroidal depth with respect to the average mechanism of the mainshock. The rupture of the mainshock began near the foreshock hypocenter and propagated, with a variable rupture velocity, towards the NW. The source‐time function for this event shows four distinct episodes of energy release. About 50% of the moment was released at the northern end of the rupture, between 90 and 110 km from the epicenter. This rupture geometry is in agreement with crustal deformation observed inland using GPS measurements. However, the scalar seismic moment of the main shock obtained in this study (Mo=1.84×1020) is smaller with respect to the lower frequency CMT results reported by Harvard (Mo=1.15×1021)

Examination of slow and late moment release of the 1988 Spitak and 1991 Racha, Caucasus, earthquakes

SUMMARY Two large shallow earthquakes occurred in the Caucasus, part of the collisional belt caused by the northward movement of the Arabian plate with respect to the Eurasian plate: the 1988 December 7 Spitak (Armenian) (M,6.7) and 1991 April 29 Racha (M,6.9) earthquakes. In the present study, slow and late moment releases of the earthquakes, which were previously proposed based on the body-wave analyses, are examined using long-period surface waves. Results from spectral inversions using fundamental-mode surface waves show that the source durations of both earthquakes are about 20 s. Although additional investigations utilizing the long-period surface waves, which are optimized to detect slow and late moment release, were employed, no evidence of slow and late moment release was found for either earthquake. For the Racha earthquake, the obtained source duration is consistent with the result from body-wave analyses. Within the resolution of the body-wave data, the simple P and SH waveforms are well modelled by a source time history model with a duration of 20 s. However, for the Spitak earthquake, the source duration obtained from the surface waves is about 30 s shorter than the source model suggested in the body-wave analysis. This discrepancy is attributed to a slow and late thrust-fault slip in the body-wave source model, which was proposed in order to interpret the late arrivals of P waves as the result of source complexity. Numerical experiments predict that Rayleigh-wave radiation is sensitive to the radiation from the proposed slow and late slip. A comparison between the observed and predicted Rayleigh-wave radiation patterns, however, provides no indication of the surface-wave radiation from the slow and late slip. It is unlikely that the discrepancy in the source model is only due to errors in modelling the propagation of long-period surface waves, because for the Racha earthquake the source models obtained from body and surface waves are consistent. The present results thus indicate that the late arrivals of P waves are probably caused by something other than source complexity, for example complex local structure near the earthquake source in the continental collisional belt. For the Spitak earthquake, a large non-double-couple component is observed in the moment tensor solution obtained at long periods. If the non-double-couple component is due to source complexity, it should be produced within the 20 s rupture process.

The 1987–1992 Gulf of Alaska earthquakes

We present a study of the 1987–1992 Gulf of Alaska earthquake sequence using relocated seismicity data together with body wave analysis of selected larger events. Most of the sequence is located on a N-S-trending fault, directly south of the Yakataga seismic gap and consists of four main events of M w = 7.2, 7.8, 7.7 and 6.8 and associated aftershocks. The first earthquake is of left-lateral strike-slip type on an ENE-WSW-trending fault. The second event is a right-lateral strike-slip earthquake on the N-S-trending fault. The fault plane of the third strike-slip event was identified as an ENE-WSW-trending fault, located to the south of that for the first event, in a previous study using body wave analysis. We show that though the body wave study cannot unambiguously identify the fault plane, the temporal development of the seismicity together with the pattern of aftershock distribution on the conjugate fault suggests that this event also occurred on the N-S-trending fault and is of right-lateral type. The seismicity on the conjugate fault is interpreted as being triggered by the increase of the shear stress in the direction of the normal to the fault plane due to the main shock. The occurrence of the fourth main shock, a right-lateral strike-slip event in 1992, which itself can be considered an aftershock of the 1987–1988 sequence with epicentres distributed along a N-S-trending fault, favours this conclusion. The first three events of the sequence have been described in earlier studies as having too short a rupture length for their seismic moment. If the rupture lengths inferred from the aftershock zones in this study are used we find that this is not the case. The events in the 1987–1992 sequence lie along magnetic anomaly 13 and related conjugate fracture zones, indicating that the oceanic crust is rupturing along pre-existing zones of weakness in response to plate boundary stresses. At the southern end, the seismic activity terminates near a seamount, which rises 1 km above an otherwise relatively low relief ocean floor, and close to a fracture zone perpendicular to the N-S trend of the earthquake sequence, suggesting that the N-S-striking rupture along the N-S-trending fault is terminated here by a barrier. At the northern end, the sequence terminates at the Alaska trench near the Yakataga seismic gap, where there is a right-lateral offset in the bathymetry along the Transition fault, this offset also being visible on the gravity map of the region. Thus, this offset has the same right-lateral sense as the N-S-trending strike-slip fault planes of three of the main shocks and suggests that though there was no significant seismic activity in this region of the Gulf of Alaska since 1964, faulting here has been active in the past.

Very broadband seismic analysis of the 1992 Flores, Indonesia, earthquake (Mw = 7.9)

The December 12, 1992, Flores Island earthquake (Mw = 7.9), which caused a destructive tsunami, is located in the back arc of the transition zone between the Sunda and Banda arcs, a region in active collision with the Australian continental margin. We study the earthquake using four seismological techniques spanning a period range of 1–1000 s and infer that the Flores event may be characterized as a shallow back arc thrust with fault plane strike 70°±30°, dip 28°±10° and rake 80° to 140° (linear trade‐off with strike), a rupture consisting of three discrete pulses with a total duration of 70 s (centroid time 26.3 s) and an estimated moment release between 7.5×1020 and 8.0×1020 N m. From the slip vector we infer that compressional forces play an important role in the deformation mechanism of the back arc north of Flores Island. Body waves constrain the hypocentral depth to be 16 km with rupture mainly propagating updip to shallow depth and bilaterally along strike with most subevents located toward the northeast. Evidence for rupture directivity from surface wave deconvolutions is inconclusive, but the overall source function is consistent with body wave results. Our body wave finite fault model is in rough agreement with two models proposed to explain tsunami run‐up observations. Normal mode modeling of W phase observations at periods longer than 70 s shows that slow slip is negligible. From this and the slip distribution determined from the body wave analysis we infer that the seismic source is an unlikely candidate to explain anomalously high tsunami run‐ups measured on northeastern Flores Island.

The June 9 Bolivia and March 9 Fiji deep earthquakes of 1994: I. Source processes

We examined the source characteristics of the 1994 deep earthquakes of Bolivia (June 9) and Fiji (March 9) over a period range of 1–1000 s using moment tensor and body wave analyses. Moment tensor inversions from 100–1000 s reveal that these two earthquakes were essentially point sources at very long period with insignificant isotropic and non‐double couple components with scalar moments Mo = 2.56 × 1021 Nm for the Bolivia and Mo = 0.26 × 1021 Nm for the Fiji earthquakes. Body wave analysis for the Bolivia event reveals a 45 s duration, a nearly linear E‐NE 1 km/s rupture on the subhorizontal focal mechanism plane giving a stress drop Δσ = 114 MPa. For the Fiji earthquake we find a change in focal mechanism and a linear 3 km/s rupture plunging shallowly to the north on the steep dipping nodal plane with a duration of at least 20 s and Δσ = 14 MPa. Both ruptures propagated parallel their null (σ2) axis and suggests that their width is controlled by the thickness of the seismogenic volume in deep slabs.

Rupture directivity and slip distribution for the Ms 6.8 earthquake of 6 April 1992, Offshore British Columbia: An application of the empirical Green's function method using surface waves

Abstract On 6 April 1992 at 13:55 UT, an earthquake of Ms = 6.8 occurred along the Revere-Dellwood-Wilson (RDW) fault, in the triple-junction region at the northern end of the Cascadia subduction zone. This was the first significant event in this region recorded by modern digital broadband seismic networks, thus providing the first opportunity to examine the rupture process of a major earthquake along this young oceanic transform fault. In this article, an empirical Green's Function technique is applied to regional and teleseismic surface waves to estimate the rupture directivity, the extent of rupture, and the slip distribution along the Revere-Dellwood-Wilson fault associated with this earthquake. The 20-sec low-pass-filtered relative source time functions (RSTF's) are single pulses with an azimuthal variation in the pulse width. This suggests that the rupture propagated to the NW (315° ± 20°), along the Revere-Dellwood-Wilson fault (striking 326°). Higher frequency RSTF's reveal two discrete subevents. The clear azimuthal variation in the time separation of these subevents requires that relative to the first subevent, the second is located 13 to 20 km in the direction 345° ± 20°. Using the RSTF at HRV (perpendicular to the rupture direction), a total rupture length of 35 km is estimated, with the bulk of the slip concentrated in a 20-km-long segment of the RDW fault to the northwest of the epicenter. Two peaks are observed in the estimated slip distribution, with maximum values of 1.8 and 1.1 m, respectively. The rupture model derived from this analysis is similar to that obtained from the analysis of body waves and is consistent with the results of aftershock studies. The latter indicate a paucity of aftershock activity (and low moment release) in the 20-km-long segment of the RDW fault to the NW of the epicenter. A distinct peak in aftershock activity 30 to 40 km to the NW of the epicenter likely represents the termination of rupture. The good agreement between the results of this study, the rupture model estimated from body-wave analysis, and the aftershock distribution bode well for the application of the empirical Green's function method using surface waves. It suggests that this method could be applied to large, historic earthquakes in this region, for which regional and teleseismic surface waves are often the most reliable data set.

A broadband seismological investigation of the 1989 Loma Prieta, California, earthquake: Evidence for deep slow slip?

Analysis of regional and teleseismic broadband body waves and long-period surface waves from the 1989 Loma Prieta earthquake demonstrates that a fairly simple average fault model explains most characteristics of seismic wave radiation over a broad period range (1 to 300 sec), but there are some systematic period-dependent characteristics that are not yet understood. Determinations of the preferred point source focal mechanism are consistent for different seismic waves, as long as optimal source-time functions and depths are chosen for each wavetype. The major double couple orientation compatible with the entire period range has a strike = 128 +_ 3 °, dip = 67 _+ 5 ° , and rake = 133 _+ 8% While this source mechanism consistency is encouraging and indicates that the average source process is quite simple, there are some systematic discrepancies in the source parameter estimates from different period waves. The source duration estimated from the principal body-wave ground motions is 8 + 2 sec, compatible with the duration of near-field strong-motion vibrations, but the surface waves indicate total durations of 18 to 30 sec, depending on the earth model selected for computing propagation corrections. The model dependence of the surface-wave results suggests that it may be possible to reconcile the duration estimates with improved earth models, but the discrepancy is quite large. There is some complexity in the source-time function, comprised of three subevents at different depths, but this complexity is only manifested in short-period signals and does not explain the duration discrepancy. Changes of fault geometry or slip direction during rupture, as suggested by recent finite-fault inversions, are unlikely to cause a bias in source duration estimates. The centroid depth determined from our body-wave analysis is 10 to 12 km, consistent with the finite-fault models, which indicate rupture extending from 5 to 18 km. Surface-wave centroid depth estimates (15 to 30 km) vary substantially for different choices of global Q model and lithospheric structure in the source region, but they do tend to be deeper than the body-wave results. It is again not clear whether this depth discrepancy is due to the source process or inadequate earth models. The body-wave and surface-wave seismic moment estimates are also not fully consistent; the moment from our body-wave inversions is 2.4 _+ 0.3 x 1019 N-m, while our surface-wave analysis gives 3.2 _+ 0.5 x 1019 N-m. It is not known whether reasonable changes of the earth models can bring the moment estimates into full agreement. The systematic discrepancies in source duration, centroid depth, and seismic moment, despite the consistency in source orientation, raise the possibility of a deep, relatively slow co-seismic slip component, but trade-offs with modeling assumptions preclude us from confirming this hypothesis at present.

Inversion of regional surface-wave spectra for source parameters of aftershocks from the Loma Prieta earthquake

Abstract Surface waves recorded at regional distances are used to study the source parameters for three of the larger aftershocks of the 18 October 1989, Loma Prieta, California, earthquake. The short-period P-wave first-motion focal mechanisms indicate a complex aftershock sequence with a wide variety of mechanisms. Many of these events are too small for teleseismic body-wave analysis; therefore, the regional surface-waves provide important long-period information on the source parameters. Intermediate-period Rayleigh- and Love-wave spectra are inverted for the seismic moment tensor elements at a fixed depth and repeated for different depths to find the source depth that gives the best fit to the observed spectra. For the aftershock on 19 October at 10:14:35 (md = 4.2), we find a strike-slip focal mechanism with right lateral motion on a NW-trending vertical fault consistent with the mapped trace of the local faults. For the aftershock on 18 October at 10:22:04 (md = 4.4), the surface waves indicate a pure reverse fault with the nodal planes striking WNW. For the aftershock on 19 October at 09:53:50 (md = 4.4), the surface waves indicate a strike-slip focal mechanism with a NW-trending vertical nodal plane consistent with the local strike of the San Andreas fault. Differences between the surface-wave focal mechanisms and the short-period P-wave first-motion mechanisms are observed for the aftershocks analyzed. This discrepancy may reflect the real variations due to differences in the band width of the two observations. However, the differences may also be due to (1) errors in the first-motion mechanism due to incorrect near-source velocity structure and (2) errors in the surface-wave mechanisms due to inadequate propagation path corrections.