Coda Decay Research Articles

A UK MW catalogue derived from coda envelopes

Summary The United Kingdom (UK) experiences low-to-moderate levels of seismicity; only 12 onshore earthquakes with local magnitude (ML) ≥ 4.0 have been recorded in the past 20 years. It is therefore difficult to estimate moment magnitude (MW) using conventional moment tensor inversion for the majority of UK seismicity, resulting in limited reliable estimates of MW. To address this, we calibrated coda envelopes at 16 broadband seismic stations distributed across the UK, to produce a MW catalogue for 100 events with MW≥2.13 that occurred since 2006. This was achieved using the open-source Coda Calibration Tool, which requires independent source parameter estimates for calibration. For 13 UK events between 2006 and 2022, we used spectral modelling to estimate apparent stress (0.32 to 1.74 MPa), and moment tensor inversion to estimate MW (3.35 to 4.52). These independent source parameters formed a subset of the inputs into the final calibration, which used seismic data from 33 events with coda-derived values of 2.57$\le $Mw$\le $4.49. The resultant coda calibration parameters were applied to 67 further events (MW≥2.13). The coda envelopes exhibit slow seismic coda decay across the UK, with significant energy up to 20 Hz, consistent with other regions of low tectonic activity. This MW catalogue, and the application of the calibration to future UK seismic events, will be useful for both assessing seismic hazard and event characterisation.

Estimation of Seismic Attenuation from Ambient Noise Coda Waves: Application to the Hellenic Subduction Zone

ABSTRACT The decay of surface-wave coda in ambient noise cross correlations can facilitate the estimation of seismic attenuation. The coda quality factor (Qc) can be measured in longer period bands using ambient noise cross correlations, which is the main advantage of using them over earthquakes. The classic model of Aki and Chouet (1975) has been applied previously to estimate coda Q in the Alps using ambient noise cross correlations. The Alps represents an ideal environment for ambient noise study, because it has very high and near-uniform station density, and the region is away from oceanic noise sources. However, many regions around the world do not have uniform station density and may be seismically more active than the Alps. One such region is the Hellenic subduction zone (HSZ), because it has a high rate of seismicity, sparser station coverage, and is surrounded by seas from outside and within. In this study, we estimate ambient seismic noise Qc in 2.5–5, 5–10, and 10–20 s period bands for the HSZ, which did not exist previously. Then, we mitigate the effects of lapse time and window length, distance, azimuth, as well as the number of stacked days on the Qc. Mapping of Qc measurements in such a geographically heterogeneous setting poses additional challenges, which we solve using a novel approach that adaptively selects paths based on their lengths and azimuthal distribution. The major tectonic zones are identified in the resulting Qc maps in the form of low Qc such as the North Anatolian fault, the Kefalonia transform zone, the Gulf of Corinth, the volcanic centers, and so on. The results also show a good correlation with large topographical features such as the Hellenides and the Thessalian plains, which have also been noticed from Qc analysis in other parts of the world.

S1222a—The Largest Marsquake Detected by InSight

AbstractNASA's InSight has detected a large magnitude seismic event, labeled S1222a. The event has a moment magnitude of 4.7, with five times more seismic moment compared to the second largest event. The event is so large that features are clearly observed that were not seen in any previously detected events. In addition to body phases and Rayleigh waves, we also see Love waves, minor arc surface wave overtones, and multi‐orbit surface waves. At long periods, the coda event exceeds 10 hr. The event locates close to the North‐South dichotomy and outside the tectonically active Cerberus Fossae region. S1222a does not show any evident geological or tectonic features. The event is extremely rich in frequency content, extending from below 1/30 Hz up to 35 Hz. The event was classified as a broadband type event; we also observe coda decay and polarization similar to that of very high frequency type events.

Energy Envelope and Attenuation Characteristics of High-Frequency (HF) and Very-High-Frequency (VF) Martian Events

ABSTRACTSince its deployment at the surface of Mars, the Seismic Experiment for Interior Structure (SEIS) instrument of the InSight mission has detected hundreds of small-magnitude seismic events. In this work, we highlight some features of two specific families: high-frequency (HF) and very-high-frequency (VF) events. We characterize the shape of the energy envelopes of HF and VF events with two parameters: (1) the delay time td between the onset and the peak of the dominant arrival; and (2) the quality factor Qc, which quantifies the energy decay rate in the coda. We observe that the envelope of HF and VF events is frequency independent. As a consequence, a single delay time suffices to characterize envelope broadening in the 2.5–7.5 Hz band. The typical coda decay time is also frequency independent, as attested by the close to linear increase of Qc with frequency. Finally, we use elastic radiative transfer theory to perform a series of inversion of seismogram envelopes for the attenuation properties of the Martian lithosphere. The good fit between synthetic and observed envelopes confirms that multiple scattering of elastic waves released by internal sources is a plausible explanation of the events characteristics. We quantify scattering and attenuation properties of Mars and highlight the differences and similarities with the Earth and the Moon. The albedo, that is, the contribution of scattering to the total attenuation, derived from VF events is very high, which we interpret as a signature of a mostly dry medium. Our results also suggest a stratification of the scattering and attenuation properties.

Temporal variations in coda attenuation associated with the 2008 Wenchuan (MW7.9) Earthquake in SW, China

In this study, waveform data obtained from Western Sichuan Seismic Array (WSSA) in China was utilized to reveal the temporal variations in coda attenuation around the eastern Tibet Plateau and Western Sichuan. Based on the single-scattering model, coda attenuation factor Qc−1 is calculated in narrower overlapping frequency bands at 1.0–24 Hz by measuring the coda decay rates for local earthquakes before and after the Wenchuan mainshock. The temporal variations in coda attenuation are investigated within three periods. The period I lasted from January 2007 to the end of 2007. The period II lasted from January 2008 till Wenchuan earthquake. The period III lasted from the mainshock to the end of 2008. The resulted temporal variations demonstrate an increase in average Qc−1 by approximately 35%–45% in the vicinity of Longmenshan and 30%–35% in Sichuan Basin after the Wenchuan earthquake in lower frequency bands within 1.25–8 Hz. On the contrary, the average Qc−1 is shown to decrease by approximately 10%–18% in southern segment of Longmenshan, 15%–38% in the Chuan-Dian block, and 10–12% in the South China block. These results are confirmed by a statistical t-test at 99.9% confidence level. No statistically significant change in Qc−1 (<10%) is found in the Songpan-Ganzi block after the mainshock. Temporal variations of coda attenuation differ significantly in individual blocks after the mainshock. The coda attenuation is proposed to be a beacon to tectonic static stress changes associated with the Wenchuan earthquake.

Seismic Envelopes of Coda Decay for Q-coda Attenuation Studies of the Gargano Promontory (Southern Italy) and Surrounding Regions

Here, we describe the dataset of seismic envelopes used to study the S-wave Q-coda attenuation quality factor Qc of the Gargano Promontory (Southern Italy). With this dataset, we investigated the crustal seismic attenuation by the Qc parameter. We collected this dataset starting from two different earthquake catalogues: the first regarding the period from April 2013 to July 2014; the second regarding the period from July 2015 to August 2018. Visual inspection of the envelopes was carried out on recordings filtered with a Butterworth two-poles filter with central frequency fc = 6 Hz. The obtained seismic envelopes of coda decay can be linearly fitted in a bilogarithmic diagram in order to obtain a series of single source-receiver measures of Qc for each seismogram component at different frequency fc. The analysis of the trend Qc(fc) gives important insights into the heterogeneity and the anelasticity of the sampled Earth medium.

Amplitude and decay of long-period coda of great earthquakes

The earthquake coda is the residual vibration energy long after the main arrivals of an earthquake. Previous studies of coda waves have focused on high frequencies and small earthquakes, which are generally attributed to the scattering from the Earth's internal heterogeneities. Here we examined the early coda of great earthquakes at long periods (50 to 300 s) at time window of 10,000 to 50,000 s after the origin time. We used 10 major earthquakes (magnitude > 8.0) in recent decades that were recorded at global seismic networks. We found that the coda energy can be well represented by a simple exponential decay model for the early coda. The variation of the initial coda energy with distance is dominated by the effect of geometric spreading, suggesting that the scattering of the Earth's internal heterogeneities contributes little to the long-period early coda. The energy decay rate is largely insensitive to earthquake source (location, magnitude, focal mechanism, or rupture process), station location, or 3D variation of the earth's internal structure, making it ideal to constrain the average global attenuation structure. A small modification of the quality factor based on a global reference model can fit the coda energy decay well. The long-period coda energy may potentially provide a new way to determine the seismic moment of a major earthquake.

Constraints on small-scale heterogeneity in the lowermost mantle from observations of near podal PcP precursors

Volumetric heterogeneities on large (∼>1000 km) and intermediate scales (∼>100 km) in the lowermost mantle have been established with seismological approaches. However, there are controversies regarding the level of heterogeneity in the lowermost mantle at small scales (a few kilometers to tens of kilometers), with lower bound estimates ranging from 0.1% to a few percent. We take advantage of the small amplitude PcP waves at near podal distances (0–12°) to constrain the level of small-scale heterogeneity within 250 km above the CMB. First, we compute short period synthetic seismograms with a finite difference code for a series of volumetric heterogeneity models in the lowermost mantle, and find that PcP is not identifiable if the small-scale heterogeneity in the lowermost mantle is above 2.5%. We then use a functional form appropriate for coda decay to suppress P coda contamination. By comparing the corrected envelope of PcP and its precursors with synthetic seismograms, we find that perturbations of small-scale (∼8 km) heterogeneity in the lowermost mantle is ∼0.2–0.5% beneath regions of the China–Myanmar border area, Okhotsk Sea and South America. Whereas strong perturbations (∼1.0%) are found beneath Central America. In the regions studied, we find that this particular type of small-scale heterogeneity in the lowermost mantle is weak, yet there are some regions requiring heterogeneity up to 1.0%. Where scattering is stronger, such as under Central America, more chemically complex mineral assemblages may be present at the core–mantle boundary.

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.

Separation of intrinsic and scattering attenuation in the crust of central and eastern Alborz region, Iran

In this study, more than 380 local earthquakes (2<ML<4.5) have been used to estimate the direct-shear waves (Qd), coda (Qc), intrinsic (Qi) and scattering quality factor (QSc) in the crust of central and eastern Alborz region. The events were recorded by one temporary and two permanent networks. The quality factors of shear and coda waves have been individually estimated at different frequency bands by using coda normalization (CNM) method and single backscattering (SBS) method, respectively. Average frequency-dependent relationships have been estimated for Qd and Qc as 111±4f0.85±0.04 and 112±8f1.02±0.06, respectively. The intrinsic quality factor, Qi, has been separated from the scattering quality factor QSc by using individually estimated Qc and Qd values. The average frequency-dependent relationships of Qi and QSc have been calculated in the form of 108f1.00 and 784f0.56, respectively. The results of this study suggest that S-wave’s attenuation (Qd−1) is dominated by the intrinsic attenuation. The attenuation of coda waves has been observed similar to the intrinsic attenuation, which indicates, the coda decay is mostly caused by the intrinsic attenuation. It has been observed that the scattering mean free path is frequency independent at frequencies greater than 6Hz. The results of this study are similar to the tectonically active regions.

Source and high-frequency decay parameters for the Azores region for stochastic finite-fault ground motion simulations

Strong ground motion prediction based on finite-fault simulation requires the identification of the fault (strike, dip, length and width), source kinematics parameters (stress drop, rupture velocity and slip distribution), regional crustal properties (geometrical spreading, anelastic structure, and upper crustal amplification and attenuation parameters) and the determination of amplification effects due to the local site geology. The general purpose of this study is to understand source and attenuation properties in the Azores, by the determination of stress drop, quality factor and kappa, through records obtained by the Portuguese digital seismic and accelerometer network. Source Spectra were obtained, for each record, after correcting observed spectra from geometrical spreading and anelastic attenuation effect: quality factor was estimated based on coda decay in the time domain and the kappa parameter was estimated by fitting the high-frequency decay of the acceleration spectrum with a straight line in a log-linear scale. Mean stress drop value was obtained considering that ω-squared model for the source spectra prevails. Parameters kappa, k, and quality factor, Q, have been estimated to be k = (0.075 ± 0.02)s and $$Q(f) = (76 \pm 11)f^{0.69 \pm 0.09}$$ , respectively. A mean value of stress drop was estimated to be around 90–130 bars for the earthquakes in analysis. The knowledge of source and path parameters, in association with other ground motion parameters, allows improving ground-motion estimates for the Azores and, consequently, will lead to more accurate seismic hazard assessment for the Azores and better characterization of seismic scenarios.

Crustal structure in and around the Onikobe geothermal area, northeastern Honshu, Japan, inferred from the spatial variation of coda decay

The Onikobe area is an active geothermal area situated in the Ou backbone range of northeastern Honshu, Japan. It is home to calderas from the Tertiary to Quaternary eras and active volcanoes. A systematic spatial variation of Qc has been found in this area: Qc values are lower at stations in and around calderas than at other stations. The amplitude of coda waves with high Qcs decreases more slowly after a lapse time of around 7–10s than that with low Qcs. In the present study, to determine causes for these coda decay variations, coda envelopes were synthesized in a structure model in which high attenuation zones existed beneath the Onikobe and Sanzugawa calderas and where scattering coefficients were higher in the lower crust than in the upper crust. Using hypocenters shallower than 10km, envelopes were calculated for 256 station-hypocenter pairs with epicentral distances of less than 10km. It was assumed that the coda waves were composed of S–S scattered waves, and that the scattering was single and isotropic. The observed features of the Qc distribution were reproduced in the synthesis, and synthesized envelopes were found to mostly coincide with observed decay curves. The top of high attenuation zones was thus estimated as being deeper than 7.5km. The structure assumed for the synthesis was consistent with that of previous studies. We consider that the structure model used was appropriate, and that high attenuation zones beneath calderas and the reflective lower crust caused the spatial variation of the Qc and decay curves in the Onikobe area. We also consider that studies using coda decay would be beneficial in detecting high attenuation zones and the reflective lower crust.

Seismic wave attenuation in Israel region estimated from the multiple lapse time window analysis and S-wave coda decay rate

Seismic wave attenuation in Israel region estimated from the multiple lapse time window analysis and S-wave coda decay rate

Tsunami Coda across the Pacific Ocean Following the 2011 Tohoku-Oki Earthquake

Abstract A tsunami coda lasting more than 5 days was clearly recorded across the Pacific Ocean following the 2011 Tohoku‐Oki earthquake. We found that the coda energy per unit area (elapsed time of 60–96 hr from the earthquake origin time) does not show a systematic variation with respect to the travel distance at the stations located at sea depths deeper than 2000 m, whereas the maximum amplitude near the source is significantly larger than those of the far‐field stations. This feature is similar to that of a seismic coda in the sense that the coda energy is distributed uniformly in space. The average tsunami coda decays with a decay time of ∼24 hr. When we carefully examined the tsunami waveform near Japan, a large‐amplitude wave was identified at an elapsed time of ∼48 hr, which corresponds to the round‐trip travel time for waves reflected from Chile. To investigate the excitation mechanisms of the tsunami coda, we conducted a tsunami simulation using a global model. Although linear long‐wave tsunami equations were found to overestimate the observed coda amplitude, the equations including an intrinsic energy loss could successfully simulate the observed coda decay if a bottom friction coefficient of 10 −5 was assumed. This indicates that not only wave scattering but also intrinsic energy loss plays an important role in the tsunami coda excitation and decay process.

Investigation of scattering in lunar seismic coda

We investigate the intrinsic attenuation and scattering properties of the Moon by parameterizing the coda decay of 369 higher‐quality lunar seismograms from 72 events via their characteristic rise and decay times. We investigate any dependence of the decay times on source type, frequency, and epicentral distance. Intrinsic attenuation, scattering, and possible focusing of energy in a near‐surface, low‐velocity layer all contribute to the coda decay. Although it is not possible to quantify the exact contribution of each of these effects in the seismograms, results suggest that scattering in a near‐surface global layer dominates the records of shallow events (∼0–200 km depth), particularly at frequencies above 2 Hz, and for increasing epicentral distance. We propose that the scattering layer is the megaregolith and that energy from shallow sources encounters more scatterers as it travels longer distances in the layer, increasing the coda decay times. A size distribution of ejecta blocks that has more small‐scale than large‐scale scatterers intensifies this effect for increasing frequencies. Deep moonquakes (700–1100 km depth) exhibit no dependence of the decay time on epicentral distance. We suggest that because of their large depths and small amplitudes, deep moonquakes from any distance sample a similar region near a given receiver. Near‐station structure and geology may also control the decay times of local events, as evidenced by two natural impact records. This study provides constraints and testable hypotheses for waveform modeling of the lunar interior that includes the effects of intense scattering and shallow, low‐velocity layers.

Temporal variations of coda Q: An attenuation-coefficient view

Spatial and temporal variations of coda wave attenuation were identified in many studies, and particularly in relation to major earthquakes and volcanic eruptions. Both the coda quality factor, Q c, and its frequency dependence often change following such events, which is often attributed to variations in the properties of large volumes of the subsurface. However, Q c is also strongly sensitive to the assumed theoretical models, which are usually insufficiently accurate for constraining the actual relationships between the geometrical spreading, anelastic dissipation, and scattering. This inaccuracy often leads to significant exaggeration of attenuation effects and complicates the interpretation of temporal variations. To resolve this problem, this study uses a phenomenological approach based on the temporal attenuation coefficient χ instead of Q c. The attenuation coefficient often linearly depends on frequency f, with intercept γ = χ ( 0 ) related to the geometrical spreading and slope giving the “effective quality” factor Q e as d χ / df = π Q e - 1 . Two published examples of temporal variations of local-earthquake coda are revisited: a non-volcanic (Stone Canyon in central California) and volcanic area (Mount St. Helens, Washington). In both cases, linear χ ( f ) patterns are found, with the effects of γ on coda decay rates being significantly stronger than those of Q e - 1 . At Stone Canyon, γ ranged from 0.035 to 0.06 s −1 and Q e varied from 3000 to 10,000, with γ increasing and Q e decreasing during the winter season. At Mount St. Helens, γ remained constant at ∼0.18 s −1, and Q e changed from 400 before the eruption to 750 after it. The observed temporal variations are explained by the near-surface changes caused by seasonal variations in the non-volcanic case and gas-, magma-, and geothermal-system related in the volcanic case. Scattering attenuation does not appear to be a significant factor in these areas, or otherwise it may be indistinguishable due to its fundamental trade-off with the background structure and anelastic attenuation in the data.

Lateral variations of coda Q and attenuation of seismic waves in the Gulf of Suez, Egypt

Gulf of Suez consists mainly of three tectonic provinces that are separated by two accommodation zones. The southern edge of the gulf is bordered by N–S faults which mark the transition between the shallow water, Suez Basin and the deep northern Red Sea Basin. The sensitivity of coda Q measurements with respect to geological differences in the crust is demonstrated in three regions with a large variety of tectonic and geologic properties. The estimation of coda Q (Qc) is performed for 370 local earthquakes recorded at 12 digital seismic stations during the period from 2000 to 2007. The magnitudes of the earthquakes between 1.5 and ~4.5 have been used at central frequencies 1.5, 3, 6, 9, 12, 15, 18, and 24 Hz through three lapse time windows 10, 20, 30 s starting at once and twice the time of the primary S wave from the origin time. The time domain coda decay method of the single isotropic scattering model is employed to calculate frequency-dependent values of coda Q. The Qc values are frequency dependent in the range 1–25 Hz, and are approximated by a least squares fit to the power law [Qc(f) = Qo(f/fo)η]. The observed coda Q indicates that the area is seismically and tectonically active with high heterogeneities. The variation of the quality factor Qc has been estimated at different regions to observe the effect of different tectonic province. The average frequency-dependent estimated relations of Qc vary from 65f1.1 to 96f0.9 at 10 to 30 s window length, respectively. The decreasing value of the frequency parameter with increasing lapse time shows that the crust acquires homogeneity with depth. The variation of Qc with the variations in the geologic and tectonic properties of the crust was investigated. The frequency exponent η might be larger in active tectonic areas and smaller in more stable regions. In the northern region of the Gulf of Suez, the obtained value of η = 0.8 ± 0.011, which might indicate a low level of tectonic activity compared with η = 1.1 ± 0.005 and 1.3 ± 0.009 for the central and southern regions of the gulf.

Frequency-Dependent Attenuation of Body and Coda Waves in the Andaman Sea Basin

We estimated frequency-dependent attenuation of coda waves (![Graphic][1] ) and body waves (![Graphic][2] and ![Graphic][3] ) in 1.5–24 Hz by applying the single isotropic scattering theory and the extended coda-normalization method, respectively, in the crust beneath the Andaman Sea. We used 43 aftershocks of the 13 September 2002 earthquake ( M w 6.5) in the Andaman Sea recorded by three stations installed in the Andaman Islands. The coda Q factors calculated from the amplitude decay rate of the S -wave coda show a dependence on frequency and lapse time. We found that with the increase in lapse time window from 10 to 40 s, Q ( Q C at 1 Hz) increases from 55 to 153, while the frequency-dependent coefficient n decreases from 1.1 to 0.94. The average frequency-dependent relations of ![Graphic][4] vary from 0.02 f -1.1 to 0.01 f -0.94 with an increase in lapse time window from 10 s to 40 s, respectively. The values of ![Graphic][5] and ![Graphic][6] corresponding to spectral amplitude decays show strong frequency dependence and are expressed as 0.02 f -1.01 and 0.01 f -1.0, respectively. Our results are consistent with those of other seismically active regions. The ratio ![Graphic][7] is found to be larger than unity for the whole frequency range. We separated intrinsic absorption (![Graphic][8] ) and scattering attenuation (![Graphic][9] ) using the independent estimates of ![Graphic][10] and ![Graphic][11] . The results show that ![Graphic][12] is close to ![Graphic][13] and both of them are larger than ![Graphic][14] , suggesting that coda decay is predominantly caused by intrinsic attenuation. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif [4]: /embed/inline-graphic-4.gif [5]: /embed/inline-graphic-5.gif [6]: /embed/inline-graphic-6.gif [7]: /embed/inline-graphic-7.gif [8]: /embed/inline-graphic-8.gif [9]: /embed/inline-graphic-9.gif [10]: /embed/inline-graphic-10.gif [11]: /embed/inline-graphic-11.gif [12]: /embed/inline-graphic-12.gif [13]: /embed/inline-graphic-13.gif [14]: /embed/inline-graphic-14.gif

Estimation of coda Q in the mantle and characteristics of regional S-wave envelope

Coda envelope reflects heterogeneity in the Earth medium. Examining seismogram envelopes of regional earthquakes over the world in 1–20 s periods for a wide lapse time range up to 2000 s, we classified them focusing on the envelope characteristics around ScS arrivals. We found that coda decay gradient decreases after ScS arrival irrespective of focal depths and epicentral distances on a semi-logarithmic scale plot. Amplitude offset behavior observed in seismic envelopes is an evidence that scattered waves of ScS waves sometimes dominate over scattered S waves and scattered surface waves. Such an offset associated with ScS arrival is not clear for shallow events (<50 km). Further, the change of coda decay gradient around ScS arrival decreases as the period becomes shorter and the focus shallower. Inverse of coda Q values, which is estimated by applying the single-isotropic scattering model, are 0.11 × 10−2∼0.45 × 10−2 and 0.33 × 10−3∼3.90 × 10−3 at 4 s, 0.32 × 10−2∼0.99 × 10−2 and 0.13 × 10−3∼2.60 × 10−3 at 10 s, 0.53 × 10−2∼1.89 × 10−2 and 0.69 × 10−3∼2.20 × 10−3 and at 15 s for before ScS and after ScS arrival, respectively. These changes in coda decay gradient and offsets in coda level enable us to measure the medium heterogeneity of the mantle.

Array Analysis of Regional-Distance P-Coda in South Asia

Abstract We present a comprehensive study of regional distance P -coda recorded by a medium-aperture array station located in Chiang Mai, Thailand (CMAR). Using array processing techniques, we analyzed data from 955 events that occurred at distances of 13°–30° with primarily continental paths. Nearly all significant, coherent arrivals in the P -coda traveled along the great circle path with ray parameters within of the direct arrival. Most of this energy was created near the source, probably via ( L g , R g )→ P scattering. This was especially true at the larger distances of 28°–30° in which coda decay rates decreased and large-amplitude significant arrivals occurred throughout the first 30 sec of the coda. The codas from approximately 40 intermediate-depth events in our data set were generally less energetic than shallow-event coda at comparable distances. In some cases, the coherent later arrivals we observed were related to upper mantle triplications. At distances of 14°–17° we observed clear secondary arrivals related to the 410 km discontinuity. They appeared on average 2 sec later than expected relative to the first arrivals, which may be partially explained by a depressed 410 beneath Burma. We isolated the diffuse component of the P -coda by subtracting array beams from individual elements to create residual seismograms. The diffuse, residual energy was highly variable from trace to trace and became equal in amplitude to the coherent component after only 20–30 sec of lapse time. We used a high-resolution maximum-likelihood technique to compute frequency–wavenumber spectra of the residual seismograms but did not conclusively observe any deterministic P → R g scatterer locations. The majority of the diffuse energy is created quite close to CMAR and does not travel as a plane wave across the array.