Inversion Of Body Waves Research Articles

Source mechanism of the September 19, 1985 Michoacan Earthquake and its implications

Moment tensor inversion of low frequency teleseismic data for the September 19, 1985 Michoacan, Mexico earthquake gave an average strike 298°, slip 268°, and seismic moment 10.3×1027 dyne‐cm. Low frequency body waves clearly show the complex nature of this event. Cross‐correlation of the September 19 P‐wave with that of the smaller, and colocated October 25, 1981 Playa Azul earthquake, show that the Michoacan earthquake consisted of two subevents separated in time by 25 s, and located approximately 30 km northwest and 40 km southeast respectively, of the Playa Azul epicenter. Body wave inversion of the first subevent gave strike 298°, slip 266°, and seismic moment 3.0×1027 dyne‐cm. The second subevent had a comparable seismic moment. The cumulative moment for large earthquakes occurring along the Michoacan‐northwest Guerrero coast indicate that seismic displacement along this segment of the Mexican subduction zone is similar to that occurring on other portions, indicating that the "former" Michoacan gap was not anomalously aseismic.

Geometry and mechanism of faulting of the 1980 El Asnam, Algeria, earthquake from inversion of teleseismic body waves and comparison with field observations

The El Asnam earthquake of October 10, 1980 (Ms = 7.3), provided a wealth of geological and seismological data and is an ideal event for comparing geologically and seismologically derived models. The event produced extensive surface faulting. In addition to the main tectonic deformation, which is clearly a thrust, widespread secondary normal faulting was observed at the surface. In the southwest region the surface break of a thrust fault could be traced for 24 km. In the northeast part of the fault zone, normal faults with throws of several meters were observed. Although no clear thrust type surface breaks were observed in this region, geodetic measurements and aftershocks indicate that thrusting was nonetheless the main tectonic mode of deformation. In this paper the rupture process of the El Asnam earthquake is investigated by inversion of teleseismic P and SH waves. The effort is concentrated on the late part of the waveforms which carries information about the faulting in the least understood, north‐eastern segment of the fault zone. During the earthquake, rupture initiated at the southern terminus of the southwestern fault segment and propagated northward as indicated by the epicentral location and the observable azimuthal directivity of the body wave shapes and amplitudes. The seismogenic faulting in the southwestern segment has a thrust mechanism with the following average parameters: seismic moment 3.9×1019 N m, depth 6 km, strike 220°, dip 46°, and rake 72°. The duration of rupture on this fault segment was approximately 10 s which, together with the observed fault length of 24 km, indicates a rupture velocity of 2.4 km/s. Unconstrained inversions suggest normal faulting in the northeast part of the source region that began immediately following the arrival of the rupture to the northeast region. The normal faulting mechanism is in agreement with that observed for the large surface fault scarps in this region. Guided by the evidence from aftershocks and geodetic measurements, a solution with a thrust‐type mechanism was also sought. Using a priori constraints based on the field observations (the observed strike and slip angle), we obtain the following thrust source mechanism for the northeast segment: seismic moment 3.6×1019 N m, depth 6 km, strike 230°, dip 20°, and rake 91°. This solution requires that the thrusting in the northeast be initiated 10 s after the arrival of the rupture from the southwest, suggesting that some time is required to overcome the geometric barrier marked by the abrupt change in the dip and azimuth. Inversions are nonunique with respect to the seismic moment associated with normal faulting; the ambiguity being caused by the limited bandwidth of the observed body waves. The moment estimated from the surface waves at periods about 300 s, however, favors thrust faulting to be dominant in the northeast region.

Iterative deconvolution of complex body waves from great earthquakes—the Tokachi-Oki earthquake of 1968

A method of body-wave inversion is developed in an attempt to extract the information about asperities or barriers in a fault zone. A sequence of point sources, each being characterized with the seismic moment, the onset time and the location, are iteratively derived from observed records at multi-stations, where the two-dimensional extent of the source location is taken into account. A modification is made of the iterative method of Kikuchi and Kanamori on the formulation of inversion procedure to facilitate the computation. Using this method, we analyse long period P waves of the Tokachi-Oki earthquake of 1968 ( M w = 8.2) and obtain several significant subevents with time durations of ∼ 10 s. Their spatio-temporal distribution shows that the rupture process consists of three characteristic stages: (A) a stage of introductory rupture, (B) a stage of main rupture and (C) a stage of aftershocks. The main rupture takes place in the form of clustering around a few sites of the fault plane. The largest subevent occurs in the northwestern corner. The stress drop associated with this event is estimated to be ∼ 200 bars, one order of magnitude higher than the stress drop averaged over the entire fault plane. The sum of the seismic moments of the individual subevents amounts to 2.3 × 10 28 dyn. cm which approximately coincides with the one estimated from the analysis of long-period surface waves. This implies that the source of the Tokachi-Oki earthquake consists of several major subevents with time durations of ∼ 10 s in addition to other minor subevents.

Inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 Imperial Valley, California, earthquake

AbstractA least-squares point-by-point inversion of strong ground motion and teleseismic body waves is used to infer the fault rupture history of the 1979 Imperial Valley, California, earthquake. The Imperial fault is represented by a plane embedded in a half-space where the elastic properties vary with depth. The inversion yields both the spatial and temporal variations in dislocation on the fault plane for both right-lateral strike-slip and normal dip-slip components of motion. Inversions are run for different fault dips and for both constant and variable rupture velocity models. Effects of different data sets are also investigated. Inversions are compared which use the strong ground motions alone, the teleseismic body waves alone, and simultaneously the strong ground motion and teleseismic records. The inversions are stabilized by adding both smoothing and positivity constraints.The moment is estimated to be 5.0 × 1025 dyne-cm and the fault dip 90° ± 5°. Dislocation in the hypocentral region south of the United States-Mexican border is relatively small and almost dies out near the border. Dislocation then increases sharply north of the border to a maximum of about 2 m under Interstate 8. Dipslip motion is minor compared to strike-slip motion and is concentrated in the sediments. The best-fitting constant rupture velocity is 80 per cent of the local shear-wave velocity. However, there is a suggestion that the rupture front accelerated from the hypocenter northward. The 1979 Imperial Valley earthquake can be characterized as a magnitude 5 earthquake at the hypocenter which then grew into or triggered a magnitude 6 earthquake north of the border.

Body wave inversion: Moment tensors and depths of oceanic intraplate bending earthquakes

This paper develops a moment tensor inversion technique suited for analysis of long‐period body waves from distant, shallow earthquakes. In addition to the moment tensor the procedure resolves precise source depths (±5 km) and estimates the duration of faulting. The technique is applied to digital SRO‐DWWSSN recordings of outer rise, intraplate earthquakes. These earthquakes result from bending of oceanic plates prior to subduction. Bending induces horizontal tension near the top of the lithosphere and horizontal compression near its base. A neutral surface separates compressional from tensional regimes. An examination of 11 bending earthquakes in the period January 1981 to July 1982 indicates that the depth to the neutral surface is not geographically uniform. Although the seismicity sample is very small, the depth of the neutral surface appears to correlate with the position of the trench‐arc in the evolutionary sequence of Uyeda and Kanamori (1979). In particular, neutral surfaces at strongly coupled Chilean‐type plate boundaries are elevated as much as 15 km above those at weakly coupled Mariana‐type boundaries. This correlation implies that nonuniformities in neutral surface height are created by variations in regional compressive stress resulting from contrasts in plate‐plate coupling. To account for the observed heights, maximum interplate compressive stresses must be comparable with stresses derived from bending.

Inversion of teleseismic body waves for the moment tensor of the 1978 Thessaloniki, Greece, earthquake

abstractSeismograms from WWSSN and Canadian network stations were modeled to determine the source parameters of the 20 June 1978 Thessaloniki, Greece, earthquake (Ms = 6.4). The depth of the initial rupture was constrained to 11 ± 1 km by comparison of the arrival times of surface reflections with synthetic short-period seismograms. A focal sphere plot of first motion polarities provided little constraint on other focal parameters, except to indicate that predominantly normal faulting was involved. A generalized inverse technique utilizing the moment tensor formalism was applied to teleseismic P and SH waves for six increments of depth. The moment tensor obtained indicated a nearly horizontal, N-trending tension axis and a nearly vertical compression axis, and yielded the following double-couple source parameters: strike 280° ± 7°; dip 55° ± 3°; rake −65° ± 5°; seismic moment 5.7 × 1025 dyne-cm; and a skewed triangular source time function with a rise time of about 1 sec and duration of 6 to 8 sec. Due to indications of multiple or finite source effects for this event, and the assumption in the moment tensor formalism of a point source, a low-pass filter was applied to the data and the inversions were repeated. The results were nearly identical with those of the original inversion, suggesting that any individual sources had similar mechanisms, or that the point source model is sufficient for this earthquake.

Equation of state fits to the lower mantle and outer core

The lower mantle and outer core are subjected to tests for homogeneity and adiabaticity. An earth model is used which is based on the inversion of body waves and Q-corrected normal-mode data. Homogeneous regions are found at radii between 5125 and 4825 km, 4600 and 3850 km, and 3200 and 2200 km. The lower mantle and outer core are inhomogeneous on the whole and are only homogeneous in the above local regions. Finite-strain and atomistic equations of state are fit to the homogeneous regions. The apparent convergence of the finite-strain relations is examined to judge their applicability to a given region. In some cases the observed pressure derivatives of the elastic moduli are used as additional constraints. The effect of minor deviations from adiabaticity on the extrapolations is also considered. An ensemble of zero-pressure values of the density and seismic velocities are found for these regions. The range of extrapolated values from these several approaches provides a measure of uncertainties involved.

A body wave inversion of the Koyna, India, earthquake of December 10, 1967, and some implications for body wave focal mechanisms

With a generalized inverse technique, WWSSN (World-Wide Standard Seismograph Network) long-period P and SH wave forms are analyzed from the Koyna earthquake. The effects of local plane-layered earth structure near an imbedded point dislocation source are put in by using a modified plane-wave ray theory which includes the standard reflection and transmission coefficients plus source corrections for radiation pattern and geometrical spreading. The generalized inverse compares synthetic seismograms to the observed ones in the time domain through the use of a correlation function. By using published crustal models of the Koyna region and primarily by modelling the crustal phases P, pP, and sP, the first 25 s of the long-period wave forms is synthesized for 17 stations, and a focal mechanism is obtained for the Koyna earthquake which is significantly different from previous mechanisms. The fault orientation is 67° dip to the east, −29° rake plunging to the northeast, and N16°E strike, all angles being ±6°. This is an eastward dipping, left lateral oblique slip fault which agrees favorably with the trend of fissures in the meizoseismal area. The source time duration is estimated to be 6.5±1.5 s from a triangular time pulse which has a rise time of 2.5 s, a tail-off of 3.9 s, source depth of 4.5±1.5 km, and seismic moment of 3.2±1.4×10^(25) dyn cm. Some short-period complexity in the time function is indicated by modelling shortperiod WWSSN records but is complicated by crustal phases. The long-period P wave forms exhibit complicated behavior due to intense crustal phase interference caused by the shallow source depth and radiation pattern effects. These structure effects can explain much of the apparent multiplicity of the Koyna source. An interpretation of the Koyna dam accelerograms has yielded an S-P time which can be used along with the IMD (Indian Meteorological Department) epicenter and present depth determination to place the epicenter directly on the meizoseismal area.

Amplitude constraints and the inversion of Canadian Shield early rise explosion data

The inclusion of amplitude constraints in body-wave inversions allows the separation of earth models which seem indistinguishable on the basis of traveltime observations. Amplitude constraints can be included through the calculation of synthetic seismograms by the quantized ray theory formulation. As an example, a preliminary model for the velocity structure of the upper mantle beneath part of the Canadian Shield is derived from P -wave amplitude and arrival-time data from Project Early Rise. The amplitude behavior observed in this data set is consistent with the existence of positive velocity gradients near 150- and 200-km depths and prominent discontinuities near 400- and 650-km depths.

P-Wave Train Synthetic Seismograms Calculated by Quantized Ray Theory

The quantized ray theory formulation is used to construct synthetic body wave seismograms from a ray parameter-distance curve. The synthetic seismograms calculated by this new technique compare favourably with the exact Cagniard-de Hoop solutions, but require far less time to produce. The ease and rapidity with which the calculations can be done allow the routine use of synthetic seismograms in body wave inversions. Inclusion of amplitude constraints greatly reduces the range of valid Earth models. As an example, a P-wave velocity-depth model is found for the upper mantle beneath the Canadian Arctic. The model is constrained by ray parameter and travel time data as well as the observed amplitude behaviour in the first 10-15 s of real vertical component seismograms recorded between 10 and 40 degrees. As each arrival observed on each record provides a number of constraints for the ray parameter curve, a relatively few wave trains provide a great amount of information on the velocity structure. Even when individual arrivals cannot be separated on a record, because of interference with each other, synthetic seismograms can be used. The interference pattern itself can be reproduced, and by tracing back through the calculations, one can define the number, amplitudes and travel times of the interfering arrivals.