Large Epicentral Distances Research Articles

Kinematic and Inertial Effects of Earthquakes on Rock Socketed Single Piles in a Two-Layered Medium

The behaviour of piles in a two-layered soil medium subjected to earthquake ground accelerations are investigated using finite element method. The kinematic and inertial effects on rock socketed piles in a two-layered soil medium are investigated in this study considering the soil properties relevant to Sri Lanka. Hyperbolic nonlinear constitutive model is used for modelling the behaviour of the fully coupled soil medium. The effect of the earthquake was simulated by lateral ground acceleration applied to the bedrock and in this respect two earthquake records measured at a considerable distance away from the epicentre are used in the numerical simulation to take into account large epicentral distance to an anticipated earthquake that might affect Sri Lanka. The validity of the results of the proposed model was established by comparing with the trends observed in similar studies reported in the literature. A detailed parametric study of the kinematic bending moments developed in piles at the layer interface of the two-layered medium is carried out using the developed model. Furthermore, the effects of the inertial forces on the kinematic bending moment developed at the layer interfaces are investigated by varying the pile diameter and the effective masses at the pile head. Moreover, the variation of the bending moments, developed in the pile due to the combined effects of the kinematic and inertial responses of the pile-soil system, with the effective mass at the pile head for different pile diameters is also investigated.

A California Statewide Three-Dimensional Seismic Velocity Model from Both Absolute and Differential Times

Abstract We obtain a seismic velocity model of the California crust and uppermost mantle using a regional-scale double-difference tomography algorithm. We begin by using absolute arrival-time picks to solve for a coarse three-dimensional (3D) P velocity ( V P ) model with a uniform 30 km horizontal node spacing, which we then use as the starting model for a finer-scale inversion using double-difference tomography applied to absolute and differential pick times. For computational reasons, we split the state into 5 subregions with a grid spacing of 10 to 20 km and assemble our final statewide V P model by stitching together these local models. We also solve for a statewide S -wave model using S picks from both the Southern California Seismic Network and USArray, assuming a starting model based on the V P results and a V P / V S ratio of 1.732. Our new model has improved areal coverage compared with previous models, extending 570 km in the SW–NE direction and 1320 km in the NW–SE direction. It also extends to greater depth due to the inclusion of substantial data at large epicentral distances. Our V P model generally agrees with previous separate regional models for northern and southern California, but we also observe some new features, such as high-velocity anomalies at shallow depths in the Klamath Mountains and Mount Shasta area, somewhat slow velocities in the northern Coast Ranges, and slow anomalies beneath the Sierra Nevada at midcrustal and greater depths. This model can be applied to a variety of regional-scale studies in California, such as developing a unified statewide earthquake location catalog and performing regional waveform modeling.

Southeast Indian Ocean-Ridge earthquake sequences from cross-correlation analysis of hydroacoustic data

SUMMARY Parameters of earthquake sequences, for instance location and timing of foreshocks and aftershocks, are critical for understanding dynamics of mid-ocean ridge and transform faults. Whole sequences including small earthquakes in the ocean cannot be well recorded by land-based seismometers due to large epicentral distances. Recent hydroacoustic studies have demonstrated that T waves are very effective in detecting small submarine earthquakes because of little energy loss during propagation in Sound Fixing and Ranging channel. For example, an MW 6.2 (2006 March 6, 40.11 ◦ S/78.49 ◦ E) transform-fault earthquake occurred at the Southeastern Indian Ocean Ridge, but National Earthquake Information Center only reported three aftershocks in the first following week. We applied cross-correlation method to hydroacoustic data from the International Monitoring System arrays in the Indian Ocean to examine the whole earthquake sequence. We detected 14 aftershocks and none foreshock for the earthquake and locations of these aftershocks show an irregular pattern. From the observation, we suggest that the feature could be caused by complicated transcurrent plate-boundary dynamics between two overlapped spreading ridges that is possibly explained by the bookshelf faulting model.

Amplitude and phase fluctuations of seismic waves and characterization of small-scale heterogeneities in the earth.

Due to the limitation of sparse seismic observations on the earth’s surface, the deterministic picture of small‐scale heterogeneities in the earth is difficult to obtain in general. Statistical characterization is a viable alternative to study the distribution and properties of the small‐scale heterogeneities. The pioneering work of Aki in 1973 modeled the heterogeneities with a single‐layer uniform random medium having only three parameters: scale, perturbation strength, and layer thickness. This simple model is consistent with the observations which are limited to only the transversal coherence functions of seismic wave (phase and amplitude) across an array and it has no depth resolution. The following work in 1970s and early 1980s were based on similar models. Flatte introduced the key idea of angular coherence functions using different earthquakes at large epicentral distances in the late 1980s (Flatte and Wu, 1988), which opened a new area of earth heterogeneity characterization by extending the single, uniform layer model to a depth‐dependence model. Later, the angular coherence function was extended to the joint transverse‐angular coherence function, which significantly increases the depth resolution of the heterogeneity spectra. In this talk, we summarize the research results along this direction conducted by a group of scientists in the earth science at University of California, Santa Cruz, collaborated with Flatte in different stages.

Geodetic constraint on the slip distribution of the 2006 Central Kuril earthquake

AbstractWe have investigated the slip distribution of the 15 November 2006 Central Kuril earthquake off Simushir Island using continuous GPS data. A dense GPS network on Hokkaido Island, northern Japan, and Sakhalin Island detected coseismic horizontal displacements of ~9 mm despite large epicentral distances of about 900–1200 km. Eastward displacements at the sites decrease to the south, and this spatial distribution feature constrains the slip on the three subfaults. Our data suggest that little slip has occurred in the southern and northern parts, but that a 6-m slip has occurred in the center of the focal region. This interpretation fits well with the detailed slip distribution inferred from teleseismic waveform inversions. The total seismic moment, 2.8 × 1021 N m, is approximately the value indicated using by the Global CMT solution. Our result implies that there remains a seismic gap between this event and the 1952 great Kamchatkan earthquake that is large enough for anM> 8 earthquake.

Seismological constraints on the ultralow velocity zones in the lowermost mantle from core-reflected waves

Analyses of reflected seismic phases PcP and ScP from the core–mantle boundary (CMB) enable us to constrain the physical properties of the ultralow velocity zones (ULVZs) at the base of the mantle. The CMB region beneath the western and southern Pacific is investigated in detail by using a large amount of waveform data recorded by the short-period array stations. Clear arrivals of postcursor to ScP (ScsP) are detected from several event-receiver pairs, which sample the CMB regions beneath Philippine–Kalimantan and East of Australia, indicating the existence of ULVZs under the regions. No ScP precursors (SdP and SPcP) are found in our data set except for a few events with a large epicentral distance over 45 ° . No additional arrivals related to PcP are found in the region where ScsP is observed in the ScP wavelet. Elastic properties of the ULVZ are precisely constrained by waveform modeling of both PcP and ScP which sample the CMB region under Philippine–Kalimantan. Different ULVZ structures are found between the northeastern and southwestern parts of the Philippine–Kalimantan: the ratio of shear-to-compressional velocity reductions is 2.0–2.5 and the ratio of density-to-shear velocity changes is − 0.5 to − 0.33 in the northeastern part, while minimal changes in compressional velocity and large shear velocity decrease in the southwestern part are found. A certain degree of density increases and shear velocity decreases are also observed in the ULVZ beneath East of Australia. Increase in density and decrease in shear velocity throughout our observation imply that the iron incorporation into the lowermost mantle minerals may generate the ULVZ anomaly. However, other mechanisms such as partial melting can explain the structural difference between the northeastern and southwestern parts of the ULVZ beneath the Philippine–Kalimantan region.

Earthquake-induced Groundwater and Gas Changes

Active faults are commonly associated with spatially anomalously high concentrations of soil gases such as carbon dioxide and Rn, suggesting that they are crustal discontinuities with a relatively high vertical permeability through which crustal and subcrustal gases may preferably escape towards the earth's surface. Many earthquake-related hydrologic and geochemical temporal changes have been recorded, mostly along active faults especially at fault intersections, since the 1960s. The reality of such changes is gradually ascertained and their features well delineated and fairly understood. Some coseismic changes recorded in ``near field'' are rather consistent with poroelastic dislocation models of earthquake sources, whereas others are attributable to near-surface permeability enhancement. In addition, coseismic (and postseismic) changes were recorded for many moderate to large earthquakes at certain relatively few ``sensitive sites'' at epicentral distances too large (larger for larger earthquakes, up to 1000 km or more for magnitude 8) to be explained by the poroelastic models. They are probably triggered by seismic shaking. The sensitivity of different sites can be greatly different, even when separated only by meters. The sensitive sites are usually located on or near active faults, especially their intersections and bends, and characterized by some near-critical hydrologic or geochemical condition (e.g., permeability that can be greatly increased by a relatively small seismic shaking or stress increase). Coseismic changes recorded for different earthquakes at a sensitive site are usually similar, regardless of the earthquakes' location and focal mechanism. The sensitivity of a sensitive site may change with time. Also pre-earthquake changes were observed hours to years before some destructive earthquakes at certain sensitive sites, some at large epicentral distances, although these changes are relatively few and less certain. Both long-distance coseismic and preseismic changes call for more realistic models than simple elastic dislocation for explanation. Such models should take into consideration the heterogeneity of the crust where stress is concentrated at certain weak points (sensitive sites) along active faults such that the stress condition is near a critical level prior to the occurrence of the corresponding earthquakes. To explain the preseismic changes, the models should also assume a broad-scaled episodically increasing strain field.

Remotely Triggered Seismicity in the Yellowstone National Park Region by the 2002 Mw 7.9 Denali Fault Earthquake, Alaska

Coincident with the arrival of low-frequency, large-amplitude surface waves of the Mw 7.9 Denali fault earthquake (DFE), an abrupt increase in seismicity was observed in the Yellowstone National Park region, despite the large epicentral distance of 3100 km. Within the first 24 hr following the DFE mainshock, we located more than 250 earthquakes, which occurred throughout the entire Yellowstone Na- tional Park region. The elevated seismicity rate continued for about 30 days and followed a modified Omori law decay with a P value of 1.02 ! 0.07. For a declus- tered earthquake catalog, the seismicity following the 2002 DFE uniquely stands out with a significance of 30r. The increase in seismicity occurred over all magnitude bands. In general, we observed that seismicity following the DFE outlined the spatial pattern of past seismicity routinely observed in the Yellowstone National Parkregion. However, we found significant differences in triggered seismicity inside and outside the caldera. Earthquakes inside the Yellowstone caldera occurred preferentially as clusters close to major hydrothermal systems, were of larger magnitude, and seis- micity decayed more rapidly. This suggests that either different trigger mechanisms were operating inside and outside the caldera or that the crust responded differently to the same trigger mechanism depending on its different mechanicalstate.Compared with other sites that experienced remote earthquake triggering following the 2002 DFE, Yellowstone showed the most vigorous earthquake activity. We attribute this to strong directivity effects of the DFE, which caused relatively large peak dynamic stresses (0.16-0.22 MPa) in Yellowstone, and to the volcanic nature of Yellowstone.

Characteristics of Small-Scale Heterogeneities in the Hidaka, Japan, Region Estimated by Coda Envelope Level

We estimated the spatial distribution of small-scale heterogeneities as anomalous amplification of coda level in the Hidaka, Japan, region. We analyzed 2768 seismograms in a frequency range of 1-32 Hz for 24 earthquakes recorded at 62 stations. First, we estimated the site effect of each station, using the coda nor- malization method with regional earthquake data of large epicentral distances. All the data processing was done after the correction of this site effect. Next, we deter- mined coda amplitude factor (CAF), that is, the amplitude ratio of coda waves on each source-station pair relative to the averaged coda amplitude over stations, for earthquakes inside the region. We found systematic spatial variations of CAF, im- plying the existence of localized heterogeneities. At frequencies lower than 2 Hz, the CAF is relatively large in the west of the Hidaka Mountains, implying the possibility of strong heterogeneities with a scale of 0.6-1.3 km there. In a high-frequency range (� 16 Hz), CAF values are large on paths crossing the Hidaka Mountains. From the corresponding lapse time of coda (65 sec), there may be a zone of concentrated heterogeneities beneath the Hidaka Mountains at the depth of 100-120 km.

An Efficient Method for Computing Green's Functions for a Layered Half-Space at Large Epicentral Distances

Nowadays, with the dramatic increase in computational ability, the dis- crete wavenumber integration method (DWIM) (see, e.g., Bouchon and Aki, 1977; Bouchon, 1979, 1981) has been one of the most favorable techniques of computing the synthetic seismograms for a layered half-space because of its simplicity, accuracy, and fair efficiency for some cases, particularly for the case of the near field. However, it becomes less efficient for the case of far field, that is, at large epicentral distances, and the larger the epicentral distance is, the less efficient DWIM will be. In this study, we propose an efficient numerical wavenumber integration method, the self-adaptive Filon's integration method (SAFIM), to compute efficiently the dynamic Green's functions for a layered half-space at large epicentral distances. This new integration technique is build upon the particular fifth-order Filon's integration scheme (Apsel, 1979; Apsel and Luco, 1983) and the principle of the self-adaptive Simpson inte- gration technique. By using numerical examples, we demonstrate that SAFIM is not only accurate but also very efficient for large epicentral distances. According to our study, we find that at a relatively short epicentral distance (r � 500 km), the classical DWIM is more efficient than SAFIM; at a medium range of epicentral distance (500 kmr � 1200 km), both methods have similar efficiency; at large epicentral dis- tance (r � 1200 km), however, SAFIM is significantly more efficient than DWIM, and the larger the epicentral distance is, the more efficient SAFIM will be. For in- stance, when r 2000 km, SAFIM only needs about 1/3 of the computation time of DWIM. Therefore, this new integration method is expected to be very useful in com- puting synthetic seismograms at large epicentral distances.

Locating Earthquakes: At What Distance Can the Earth No Longer Be Treated as Flat?

Earthquake location programs for events recorded at regional to teleseismic distances have traditionally used travel-time tables such as the J-B tables (Jeffreys and Bullen, 1958), which were based on ray tracing in a laterally homogeneous spherical earth ( e.g., Boyd et al., 1984). Such travel-time tables may not give accurate locations for events located using stations at local to near-regional distances because of regional variations in the crustal and upper-mantle velocity structure—the regions of the Earth in which the first-arrival P and S waves spend most of their time for such events. Until recently, it was a nontrivial task to create travel-time tables for different spherical-earth velocity structures. Accordingly, for the past 30+ years, most locations for local and near-regionally-recorded events have used programs which used flat-earth ray tracing through constant-velocity layers. Examples of such programs are Hypo71 (Lee and Lahr, 1972), Hypoinverse (Klein, 1985), Fasthypo (Herrmann, 1979), and Hypoellipse (Lahr, 1999). With the advent of digital recording and advances in telemetry, there are an increasing number of data sets which include arrival times for small to medium-sized events at both local and regional distances. Most researchers are aware of the fact that the assumption of lateral homogeneity may be invalid for event-station paths at larger epicentral distances, but when shown results as given in Figure 1 below, seismologists often express surprise that the spherical-earth travel-time corrections became significant ( e.g., > 0.1 s) at what seemed relatively small epicentral distances. The purpose of this paper is to document the limitations of using a flat-earth velocity model to approximate travel times within the earth and to suggest strategies for minimizing errors in the analysis of events which are observed at distances for which the spherical-earth corrections become significant. A review of the math and physics involved in flat-earth and spherical-earth ray …

Evaluation of the seismic response coefficient introduced in the Canadian Highway Bridge Design Code

This paper describes results from an evaluation study of the elastic seismic response coefficient introduced in the Canadian Highway Bridge Design Code. The evaluation is conducted by comparing the seismic response coefficient with (i) uniform hazard spectra for selected cities in eastern and western Canada, (ii) spectra of numerically simulated ground motions for sites in British Columbia for scenario earthquakes on the Cascadia subduction zone, (iii) spectra of the 1988 Saguenay, Quebec, earthquake records and selected ensembles of recorded accelerograms from strong earthquakes around the world, and (iv) bridge design spectra of other countries. The results from this study show that the seismic response coefficient of the Canadian Highway Bridge Design Code is conservative in terms of the Canadian seismic hazard, with the exception of (i) sites close to the Cascadia subduction zone and (ii) sites for which the seismic hazard is governed by the effects of strong earthquakes at large epicentral distances. Key words: seismic, response, coefficient, acceleration, spectra, bridge, design, code.

2-D spectral element simulations of destructive ground shaking in Catania (Italy)

This study wants to estimate the strong ground motion in the municipal area of Catania (Italy) for a catastrophic earthquake scenario. It is part of a larger research program funded by the National Research Council – National Group for the Defence Against Earthquakes (CNR-GNDT), The Catania Project, devoted to evaluating the seismic risk of a highly urbanised area, such as that of Catania, located in a seismically active region. The reference earthquake simulates the catastrophic event (M ≈ 7.2) of 1693. The ground shaking is computed solving the 2-D full-wave equation by the Chebyshev spectral element method (SPEM). Particular emphasis is given to the construction of realistic structural models, also including the finest local detail, obtained from the geophysical, geological and geotechnical data available. Simulations are performed for several sources, to account for both a change in source position and orientation, and the finite extension of the fault along its dip. Synthetic seismograms and peak ground acceleration (PGA) envelopes, calculated at the surface for four transects across the Catania area, constitute the main result of this study which can be used for practical purposes. Simulations show that ground motion is strongly influenced by both source characteristics and crustal structure. We have found that PGA values range between 0.1 g and 0.5 g, although particular site conditions strongly affect these values locally. For example, the frequencies of maximum interest in civil engineering (1.5–4 Hz) are enhanced selectively by a thick portion of surface sediments (i.e., 30–100 m for an average shear wave velocity of 500–600 m/s). An unexpected feature is the appreciable increase of PGA at large epicentral distances, which contradicts classical attenuation relations. All the results are examined through an analysis of the propagating wavefield.

Duration of deep earthquakes determined by stacking of Global Seismograph Network seismograms

The duration of each subevent of 48 earthquakes with magnitude larger than 5.5 and depth greater than 100 km was determined from stacked traces of broadband records of Global Seismograph Network stations. We fitted the source time function by one or more triangles convolved with attenuation. We found that global stacks of displacement seismograms yield reliable estimates of the rupture duration. The durations, scaled to a moment of 1019 N m, of both the subevents and the entire earthquake show a slight decrease with depth from 9 s for events at 100 km depth to about 7 s for events at 600 km depth. Assuming that the rupture velocity is a constant fraction of the shear wave speed, this decrease can be completely explained by the increase in shear velocity of 20%. In this sense, deep earthquakes are comparable to intermediate ones. For some intermediate‐depth events, Vidale and Houston [1993] found durations up to twice as long. We find that almost all of their slow events have been recorded at large epicentral distances. At these distances, we conjecture that the end of the P wave train may be extended by the arrival of reflections from the D″ layer.

Building pounding damage during the 1989 Loma Prieta earthquake

This paper presents a survey of building pounding caused by the 1989 Loma Prieta earthquake. Pounding was present over a wide geographical area including the cities of San Francisco, Oakland, Santa Cruz and Watsonville. The survey database contains more than 200 pounding occurrences involving more than 500 building structures. The paper discusses the distribution of pounding damage in the specific areas, types of pounding damage, and examples of pounding damage involving major miltistorey buildings. Some significant pounding occurred at sites with large epicentral distances indicating the possible catastrophic damage that may occur during future earthquakes having closer epicentres.

On the probability of mistaking an earthquake for an explosion using the simplicity of P

Abstract The observation that P waves from an earthquake are generally more complex than those from an underground explosion was first noted in the 1960s. This difference in P-wave character may still be the only way of identifying small earthquakes close to the detection threshold of a given station network. However, there are certain earthquakes that produce simple seismograms. The frequency at which such earthquakes are mistaken for explosions, using the complexity criterion, is known as the false alarm rate. A simple seismogram can be defined as having only one large-amplitude phase that can be caused by several mechanisms. We examine the proportion of simple seismograms produced by a randomly oriented double-couple source buried in three different crustal structures, observed at a single station at a range of epicentral distances. The model predicts that if the hypocenter of a small earthquake (near the detection threshold) is in a region with thick sediments, then the false alarm rate at a single station can be as high as 50% in the teleseismic distance range (30° to 90°). The results suggest that the false alarm rate is minimized if (1) array stations are used, (2) the station is at a large epicentral distance, (3) the source region structure has few layers, or (4) the disturbance is recorded at more than one station.

Ray theoretical analysis of Lg

Abstract Ray theory is used to investigate the effect that irregular crustal interfaces have on the different phases that comprise Lg. We concentrate on the effects of a variable Moho in this initial study, while keeping the surface flat. The Moho models considered are a cosinusoid, a model of the Moho below the German part of the European Geo-Traverse (EGT) and a model representing an ocean-continent transition. It is found that even slight undulations of the Moho give rise to extensive multipathing. This affects the amplitudes of the supercritically reflected crustal multiples comprising Lg in a dramatic way. Two different types of ray behavior are distinguished. The first type, corresponding to regular or weakly chaotic ray behavior, occurs at smaller values of the ray parameter and gives rise to large amplitudes at distinct epicentral distances due to focusing, accompanied by “gaps” containing very little energy due to defocusing in between. The second type, corresponding to strongly chaotic ray behavior, occurs at larger values of the ray parameter. Small changes in the take-off angle in this chaotic regime produce large fluctuations in the epicentral distance of arrivals. The Moho below Germany, for example, produces 140 supercritically reflected arrivals at an epicentral distance of 400 km. Since the amplitudes of arrivals in the chaotic region are smeared out over a large epicentral distance range, they are much less prominent than the arrivals from the regular region, even though they are supercritically reflected. Travel-time fluctuations are less sensitive to the undulations in the Moho than amplitude variations. They are more affected by the magnitude than by the wavelength of the undulations. Amplitude undulations of more than 3 km produce multiple arrivals that have differences in travel time of up to 10 sec.

A hybrid method for the estimation of ground motion in sedimentary basins: Quantitative modeling for Mexico city

To estimate the ground motion in two-dimensional (2D), laterally heterogeneous, anelastic media, a hybrid technique has been developed that combines modal summation and the finite-difference method. In the calculation of the local wave field owing to a seismic event, both for small and large epicentral distances, it is possible to take into account the source, path, and local soil effects. As a practical application, we have simulated the ground motion in Mexico City caused by the Michoacan earthquake of September 19, 1985. By studying the one-dimensional (1D) response of the two sedimentary layers present in Mexico City, it is possible to explain the difference in amplitudes observed between records for receivers inside and outside the lake-bed zone. These simple models show that the sedimentary cover produces the concentration of high-frequency waves (0.2 to 0.5 Hz) on the horizontal components of motion. The large amplitude coda of ground motion observed inside the lake-bed zone and the spectral ratios between signals observed inside and outside the lake-bed zone can only be explained by 2D models of the sedimentary basin. In such models, the ground motion is mainly controlled by the response of the uppermost clay layer. The synthetic signals explain the major characteristics (relative amplitudes, spectral ratios, and frequency content) of the observed ground motion. The large amplitude coda of the ground motion observed in the lake-bed zone can be explained as resonance effects and the excitation of local surface waves in the laterally heterogeneous clay layer. Also, for the 1985 Michoacan event, the energy contributions of the three subevents are important to explain the observed durations.

High-Frequency Seismic Wave Propagation in Northeastern North America

Abstract A key element in the assessment of seismic hazard is the estimation of how energy propagation from a given earthquake is affected by crustal structure near the receiver and along the more distant propagation path. In this paper, we present data from a variety of sources in eastern North America recorded at epicentral distances of a few to 800 km, and characterize and interpret systematic features. Site effects have been classically considered in terms of amplification either within a sediment-filled valley or from a single topographic feature (Geli et al., 1988). We present evidence of high frequency (5–30 Hz) resonances observed in hard-rock recordings of both body waves and Lg waves, and suggest that site effect should be expanded regionally to include structural and topographic information over sufficiently large areas to include several wavelengths of any features that may interact with seismic waves in the frequency range of interest. A growing body of evidence suggests that ground motions at high frequencies recorded at large epicentral distances in eastern North America are controlled by resonance effects. We hypothesize that a fundamental difference between eastern and western North America spectra stems from a combination of differences in the character of topography and near-surface structure. Active tectonics of western North America gives rise to a complex crust that scatters seismic energy in a random manner and results in very effective attenuation of high frequencies. The older eastern North American crust contains scatterers that are more ordered, with characteristic length scales that give rise to resonance phenomena in the frequency band critical for earthquake hazard. We present preliminary analysis of topographic data from the Adirondack Mountains in New York that demonstrates the existence of characteristic length scales on the order of up to 1–3 kilometers. Features with these length scales will effectively scatter energy at frequencies in the 1 to 10 Hz range.

Interpretation of high-frequency coda at large distances: stochastic modelling and method of inversion

Based on previous work, a stochastic convolution model is proposed for the Lg coda observed at large epicentral distances (Δ > 200 km). Coda at these distances differ from local coda in several respects. Effects of various physical processes, such as dispersion, scattering, and mode conversion on coda at large distances are discussed in detail. A spectral ratio method is developed for inversion of Lg coda Q, in which the large variance associated with inversion is greatly reduced. The method is also extended to the inversion of seismic source parameters by jointly using Lg and its coda. We have tested the model and method exhaustively using data from two GDSN stations and found the results to be superior to other methods when those methods used only a small number of stations.