Coda Wave Amplitude Research Articles

Determining Attenuation Q-Factor Values for Selected Seismic Stations in Iraq

The coda wave decay (CWD) method was used to estimate the quality factor from the coda wave amplitude decay rate for the broadband seismic stations (IBDR, MSL, and NSR4) using a single backscattering model. This research aims to calculate the attenuation coefficient and the frequency-dependent coefficient (α) for three stations and compare them to comprehend the details of Earth's crust structure in those stations. Then, to test if they are homogenous or not. Also, investigate how sensitive extracted values to tectonic differences in study stations. The attenuation properties are important for providing information about the medium that seismic waves pass through and can also be applied to areas with seismic activity as initial disaster mitigation. The study used data from the Mesopotamian Network (MP) and the Iraqi Meteorological Organization and Seismology (IMOS) from 2011 to 2021. The selected earthquakes have less than < 100 km epicentral distances, and magnitudes ranging from 3.5 to 5.6. Forty-six earthquakes were recorded by three stations (IBDR, MSL, and NSR4). These earthquakes were analyzed at eight center frequencies (1, 2, 3.5, 6, 8, 10, 12, and 16 Hz) and a 30-sec length window by using the SEISAN Earthquake Analysis Software. The QC values are computed by the frequency-dependent relationship QC= Q0 f α, where Q0 is QC at 1Hz and α denotes frequency dependence degree. The attenuation relationship of the station IBDR is obtained as QC =98 f 1.07, QC = 99 f 1.09 in the MSL station, and QC = 77 f 0.98 in the NSR4 station. The results showed that low Q0 corresponds to high attenuation. The comparison of the estimated QC for these three stations indicates that attenuation is highest at NSR4, followed by other stations.

Estimation of Coda Q Wave Attenuation Factors for Northern Iraq

The coda wave decay method was used to estimate the quality factor from the coda wave amplitude decay rate for the broadband seismic stations in northern Iraq using a single back-scattering model. The study aims to calculate the attenuation factor to comprehend the details of the structure of the earth's crust in the study area. The study was conducted using data from the Mesopotamian Network and the Iraqi Meteorological Organization and Seismology from 2007 to 2021. The selected earthquakes have magnitudes ranging from 4 to 6.1 ML and epicentral distances of less than < 100 km. Seventy seven earthquakes were recorded by three stations (SLY1, DHK1, and IKRK). These earthquakes were analyzed at eight center frequencies (1, 2, 3.5, 6, 8, 10, 12, and 16 Hz) and a 30-sec length window by using the Seisan package. The Qc values are computed by the frequency-dependent relationship QC= Q0 f α, where Q0 is QC at 1 Hz and α denotes the degree of frequency dependence. The attenuation relationship of station SLY1 is obtained as QC =88 f 1.02, QC = 90 f 0.97 in the DHK1 station, and QC = 91 f 1.04 in the IKRK station. The comparison of these three stations indicates that attenuation is highest in Sulaymaniyah, followed by Duhok, and then Kirkuk.

Quantitative evaluation of bolt connection using a single piezoceramic transducer and ultrasonic coda wave energy with the consideration of the piezoceramic aging effect

In this paper, a method to quantitatively evaluate the status of bolted connection by using a single piezoceramic transducer and coda wave is proposed. The so-called coda wave was inherited from the seismological community to name the late part of a seismogram. Coda wave is a slowly decaying wave after being scattered multiple times due to medium heterogeneities or boundaries, and it is extremely sensitivity to small changes and highly repeatable. The bolt connection is good application of coda wave since the presence of the microscopically rough surfaces and the boundary along the bolt interface generate scattering events. Stress wave generated by a Lead Zirconate Titanate (PZT) transducer is often used to evaluate the status of bolted connection. However, with the aging of a PZT transducer, the amplitude of the excitation signal decreases, and the signal energy does not correctly reflect the real status of the bolt connection. In this paper, a single PZT transducer, acting as both an actuator and a sensor, is used to transmit and receive stress wave signal. The stress wave signal received by PZT sensor consists of two parts, one is the direct wave whose amplitude is related to the excitation signal, and the other part is the backscatter wave (coda wave) produced by microscopically rough surfaces and the boundary along the interface of a bolt connection. Coda wave amplitude is affected by both heterogeneity of microscopically rough surfaces and PZT performance. To eliminate the effects of PZT aging, coda wave is normalized through the amplitude of the direct wave, and the magnitude of coda wave energy (CWE) only reflects the change of connection states. The main innovation of this paper is to establish the quantitative relationship between the bolt connection status and the CWE with a single PZT transducer. An experiment on the evaluation of the bolt connection status was carried out in this research. The results indicate that quantitative evaluation on the bolt-connected structure can be achieved through CWE, thus verifying the proposed method.

Quantifying the Effects of Nondiffuse Noise on Ballistic and Coda Wave Amplitude From Variances of Seismic Noise Interferometry in Southern California

AbstractWe develop a new approach based on the variance of noise cross‐correlation to characterize the noise source and wave propagation under the influence of a nondiffuse noise field. Based on the standard errors for the Fourier spectra of stacked noise cross‐correlation and the assumption of a diffuse field, we derive an analytical expression for the variance of every time point in the stacked cross‐correlation and validate the theory using synthetic diffuse noise data. The ambient seismic noise field in Southern California is, however, not fully diffuse. The observed correlated neighboring frequencies in the noise data—a definitive character of the nondiffuse field, map into the noise cross‐correlation, biasing its variances from theoretical predictions under fully diffuse field assumption. For the secondary ocean microseism, we find strong positive correlation between the correlated neighboring frequencies and the deviations from diffuse field theory‐predicted variances. The ballistic arrivals on average contain more significant bias than the coda segments of the noise cross‐correlation, which agrees with previous time‐lapse monitoring studies based on ambient seismic noise. In addition, the station pairs having significant bias between actual and diffuse field theory‐predicted variances are aligned with the strongest source direction and exhibit significant beamforming source fluctuation.

Frequency-Dependent Attenuation of Coda Waves in the Crust in Southwest Anatolia (Turkey)

The attenuation of coda waves in the earth’s crust in southwest (SW) Anatolia is estimated by using the coda wave method, which is based on the decrease of coda wave amplitude in time and distance. A total of 159 earthquakes were recorded between 1997 and 2010 by 11 stations belonging to the KOERI array. The coda quality factor Q c is determined from the properties of scattered coda waves in a heterogeneous medium. Firstly, the quality factor Q 0 (the value of Q c at 1 Hz.) and its frequency dependency η are determined from this method depending on the attenuation properties of scattered coda waves for frequencies of 1.5, 3.0, 6.0, 8.0, 12 and 20 Hz. Secondly, the attenuation coefficients (δ) are estimated. The shape of the curve is controlled by the scattering and attenuation in the crustal volume sampled by the coda waves. The average Q c values vary from 110 ± 15 to 1,436 ± 202 for the frequencies above. The Q 0 and η values vary from 63 ± 7 to 95 ± 10 and from 0.87 ± 0.03 to 1.04 ± 0.09, respectively, for SW Anatolia. In this region, the average coda Q–f relation is described by Q c = (78 ± 9)f 0.98±0.07 and δ = 0.012 km−1. The low Q 0 and high η are consistent with a region characterized by high tectonic activity. The Q c values were correlated with the tectonic pattern in SW Anatolia.

Estimation of Coda Wave Attenuation Quality Factor from Digital Seismogram Using Statistical Approach

The main objective of the paper is to estimate seismic wave attenuation from the decay rate of coda wave amplitudes of digital seismogram of local earthquakes (epicentral distance < 100 km) following statistical approach. Three earthquake events have been selected for estimation of coda wave attenuation quality factor (Q<SUB>c</SUB>). The coda wave of 9 seismograms from 3 local earthquakes recorded digitally by three seismic stations in the region have been analyzed for this purpose at ten central frequencies (1, 1.5, 2, 3, 4, 6, 8, 12, 16 and 18Hz) and three lapse time windows of 20, 30 and 40 seconds duration. Single backscattering model proposed by Aki (1969) and extended by Aki and Chouet (1975) and Sato (1977) is employed to determine Q<SUB>c</SUB> values. The mean values of the estimated Q<SUB>c</SUB> vary from 31+12 (at 1Hz) to 1974+211 (at 18Hz) for 20 seconds coda window length. For 30 seconds coda window length Q<SUB>c</SUB> vary from 46+16 (at 1Hz) to 1977+ 256(at 18Hz). Similarly for 40 seconds coda window length Q<SUB>c</SUB> vary from 97+ 22 to 2552 +312. It is observed that Q<SUB>c</SUB> value for the study area is frequency dependent and increase with increase in frequency. Moreover the observed Q<SUB>c</SUB> values increase with increasing lapse time at all frequency bands. The estimated Q<SUB>c</SUB> values show a frequency dependent relationship of the form Q<SUB>c</SUB>=Q<SUB>o</SUB>f<SUP> n</SUP>, where Q<SUB>0</SUB> is Q<SUB>c</SUB> at 1Hz and n represents degree of frequency dependence. The frequency dependent empirical attenuation relationship for 20, 30 and 40 seconds window lengths are obtained as Q<SUB>c</SUB> = 21.49+1.17 <i>f</i> <SUP>1.48+0.08</SUP>, Q<SUB>c</SUB> = 48.6+1.11 <i>f</i> <SUP>1.29+0.06</SUP><SUP> </SUP>and Q<SUB>c</SUB> = 88.86+1.12 <i>f</i> <SUP>1.19+0.06</SUP> respectively.

Lateral variations of coda Q and attenuation of seismic waves in Southwest Anatolia

The seismic quality factor (Qc) and the attenuation coefficient (δ) in the earth’s crust in southwest (SW) Anatolia are estimated by using the coda wave method based on the decrease of coda wave amplitude by time on the seismogram. The quality factor Qo, the value of Qc at 1 Hz, and its frequency dependency η are determined from this method depending on the attenuation properties of scattered coda waves. δ is determined from the observations of amplitude variations of seismic waves. In applying the coda wave method, firstly, a type curve representing the average pattern of the individual coda decay curves for 0.75, 1.5, 3.0, 6.0, 12.0, and 24.0 Hz values was estimated. Secondly, lateral variation of coda Q and the attenuation coefficients for three main tectonic patterns are estimated. The shape of the type curve is controlled by the scattering and attenuation in the crustal volume sampled by the coda waves. The Qo and η values vary from 30 to 180 and from 0.55 to 1.25, respectively for SW Anatolia. In SW Anatolia, coda Q–f relation is described by \(Q = (102 \pm 8)^{*\;f0.82 \pm 0.07} \) and δ = 0.008 km−1. These results are expected to help in understanding the degree of tectonic complexity of the crust in SW Anatolia.

Intrinsic and Scattering Seismic Attenuation in Southwestern Anatolia

The intrinsic dissipation and scattering attenuation in southwestern (SW) Anatolia, which is a tectonically active region, is studied using the coda waves. First the coda quality factor (Q c ) assuming single scattering is estimated from the slope of the coda-wave amplitude decay. Then the Multiple Lapse Time Window (MLTW) analysis is performed with a uniform earth model. Three non-overlapping temporal data windows are used to calculate the scattered seismic energy densities against the source-receiver distances, which, in turn, are used to calculate separate estimates of the intrinsic and scattering factors. In order to explore the frequency dependency, the observed seismograms are band pass-filtered at the center frequencies of 0.75, 1.5, 3.0, 6.0 and 12.0. The scattering attenuation (Q s −1) is found lower than the intrinsic attenuation (Q i −1) at all frequencies except at 0.75 Hz where the opposite is observed. Overall the intrinsic attenuation dominates over the scattering attenuation in the SW Anatolia region. The integrated energy curves obtained for the first energy window (i.e., 0–15 s) are somewhat irregular with distance while the second (i.e., 15–30 s) and third (i.e., 30–45 s) data windows exhibit more regular change with distance at most frequencies. The seismic albedo B0 is determined as 0.61 at 0.75 Hz and 0.34 at 12.0 Hz while the total attenuation factor denoted by L e −1 changes in the range 0.034–0.017. For the source-station range 20–180 km considered the scattering attenuation is found strongly frequency dependent given by the power law Q s −1 = 0.010*f−1.508. The same relations for Q i −1, Q t −1 (total), Q c −1 and $${\tilde{Q}}^{{- 1}}_{c}$$ (expected) hold as Q i −1 = 0.0090*f−1.17, Q t −1 = 0.019*f−1.31, Q c −1 = 0.008*f−0.84 and $${\tilde{Q}}_{c}^{{- 1}} = 0.014*f^{{- 0.87}},$$ respectively. Compared to the other attenuation factors Q c −1 and $${\tilde{Q}}^{{- 1}}_{c}$$ are less dependent on the frequency.

Regional body‐wave attenuation using a coda source normalization method: Application to MEDNET records of earthquakes in Italy

We develop a new methodology to determine apparent attenuation for the regional seismic phases Pn, Pg, Sn, and Lg using coda‐derived source spectra. The apparent Q, combining path and site attenuation, is determined from the difference between the geometrical spreading‐corrected amplitude and the independently determined source spectra derived from the coda methodology. We apply the technique to 50 earthquakes with magnitudes greater than 4 in central Italy as recorded by MEDNET broadband stations around the Mediterranean at local‐to‐regional distances. A power law attenuation of the form Q(f) = Qofγ fit all the phases quite well over the 0.5 to 8 Hz band. The measured apparent Q values are quite repeatable from event to event. Finding the attenuation function in this manner guarantees a close match between inferred source spectra from direct waves and coda techniques. This is important if coda and direct wave amplitudes are to produce consistent seismic results.

Estimation of frequency-dependent coda wave attenuation structure at the vicinity of Cairo Metropolitan area

Estimation of seismic wave attenuation in the shallow crust in terms of coda wave Q structure previously investigated (Sayed et al. 2002) in the vicinity of Cairo Metropolitan area was improved using seismograms of local earthquakes recorded by the Egyptian National Seismic Network (ENSN). The seismic wave attenuation was measured in the study area from the time decay of coda wave amplitudes on narrow bandpass filtered seismograms on the basis of the single scattering theory. The frequency bands of interest are from 1.5 Hz to 18 Hz. In general, the values obtained for paths and events foci and El-Fayoum (FYM) and Wadi Hagul Sations (HAG) are very similar for all frequency bands. A regional attenuation law Qc = 85.66 f0.79 was obtained.

Estimation of frequency dependent coda wave attenuation structure at the vicinity of Cairo Metropolitan Area

Estimation of seismic wave attenuation in the shallow crust in terms of coda wave Q structure previously investigated in the vicinity of Cairo Metropolitan Area was improved using seismograms of local earthquakes recorded by the Egyptian National Seismic Network. The seismic wave attenuation was measured from the time decay of coda wave amplitudes on narrow bandpass filtered seismograms based on the single scattering theory. The frequency bands of interest are from 1.5 to 18 Hz. In general, the values obtained for various events recorded at El-Fayoum and Wadi Hagul stations are very similar for all frequency bands. A regional attenuation law Qc = 85.66 f0.79 was obtained.

Analyses of unusual long‐period earthquakes with extended coda recorded at Katmai National Park, Alaska, USA

A swarm of six long‐period (LP) events with slowly decaying coda wave amplitudes and durations up to 120 s, was recorded by seismic stations located in the proximity of Mt. Griggs, a fumarolically active volcano in the Katmai National Park, Alaska, during December 8–21, 2004. Spectral analyses reveal the quasi‐monochromatic character of the waveforms, dominated by a 2.5 Hz mode frequently accompanied by a weaker high‐frequency onset (6.0–9.0 Hz). Particle motion azimuths and inclination angles show a dominant WNW‐ESE direction of polarization for all the signals, and suggest that seismic energy is radiated by a stable source at shallow depth. Damping coefficients between 0.0014 and 0.0063 are estimated by fitting an exponential decay model to the signal's coda; corresponding quality factors range from 78 to 351. The source of the waveforms is modelled as a resonant cavity filled with a fluid/gas mixture.

S-wave attenuation in the Sea of Marmara, Turkey

This paper presents the estimates of frequency-dependent attenuation of S-waves for five different regions of the crust in the Sea of Marmara. The estimation method for the S-wave attenuation is based on the rate of decay of the S-wave to coda wave amplitude ratio with distance, Q s −1. The epicentral distances of the selected earthquakes are between 15 and 70 km and the duration magnitudes are between 2.4 and 3.9. The quality factors, Q s, are calculated at frequencies 1.5, 3, 6 and 12 Hz and a lapse time 50 s. The frequency dependency of average Q s values calculated at these frequencies for five regions is 40±5 f 1.03±0.06. The estimated Q s values for five regions ranging from 13±1 f 1.22±0.05 to 94±3 f 0.83±0.04 indicate the regional differences in the rheology and tectonic activity of the crust.

Source Parameters and Rupture Velocities of Microearthquakes in Western Nagano, Japan, Determined Using Stopping Phases

We use an inversion method based on stopping phases (Imanishi and Takeo, 2002) to estimate the source dimension, ellipticity, and rupture velocity of microearthquakes and investigate the scaling relationships between source parameters. We studied 25 earthquakes, ranging in size from M 1.3 to M 2.7, that occurred between May and August 1999 at the western Nagano prefecture, Japan, which is characterized by a high rate of shallow earthquakes. The data consist of seismograms recorded in an 800-m borehole and at 46 surface and 2 shallow borehole seismic stations whose spacing is a few kilometers. These data were recorded with a sampling frequency of 10 kHz. In particular, the 800-m-borehole data provide a wide frequency bandwidth with greatly reduced ground noise and coda wave amplitudes compared with surface recordings. High-frequency stopping phases appear in the body waves in Hilbert transform pairs and are readily detected on seismograms recorded in the 800-m borehole. After correcting both borehole and surface data for attenuation, we also measure the rise time, which is defined as the interval from the arrival time of the direct wave to the timing of the maximum amplitude in the displacement pulse. The differential time of the stopping phases and the rise times were used to obtain source parameters. We found that several microearthquakes propagated unilaterally, suggesting that all microearthquakes cannot be modeled as a simple circular crack model. Static stress drops range from approximately 0.1 to 2 MPa and do not vary with seismic moment. It seems that the breakdown in stress drop scaling seen in previous studies using surface data is simply an artifact of attenuation in the crust. The average value of rupture velocity does not depend on earthquake size and is similar to those reported for moderate and large earthquakes. It is likely that earthquakes are self-similar over a wide range of earthquake size and that the dynamics of small and large earthquakes are similar.

S‐wave attenuation in the Marmara Region, northwestern Turkey

The frequency‐dependent attenuation of shear waves in the crust is estimated by using two different methods based on the rate of time decay of coda‐wave amplitude, Qc−1, and the rate of decay of the S‐wave to coda wave amplitude ratio with distance, Qs−1. The epicentral distances of selected earthquakes range between 20 and 110 km and magnitudes are around 3. The quality factors, Qc, are calculated at frequencies of 1.5, 3., 6., 12., and 24. Hz and lapse times of 40, 50, 60, and 70 s. Estimated Qc values show a dependence on frequency and lapse time. We observed the similarity in the frequency dependency of Qc=(41±1.07)f1.08±0.03 and Qs=(50±1.7)f1.09±0.05 for a lapse time of 50 s indicating that the attenuation mechanism for coda waves is similar to that of the direct S waves.

Spatial variation of coda wave amplitude in media with a localized heterogeneous region

Using an indirect boundary integral method, seismograms are computed for elastic media with localized heterogeneities. Models are two-dimensional homogeneous full spaces (P and SV waves) with many circular cavities as heterogeneities or scatterers. Heterogeneities are localized within a depth range, forming a relatively thin random layer. Seismograms are obtained for receivers at depth 0 km (surface) and for several focal depths, with sources radiating S waves isotropically. Seismograms are composed of the direct S wave and all the possible scattered waves by the heterogeneities, exhibiting late arrivals or coda waves. The coda wave amplitude, or coda energy level, and its duration vary for events with different focal depths. As the focal depth increases and the source gets closer to the layer of localized heterogeneities, the coda level becomes small. When the source is within the heterogeneous layer, however, the coda level becomes larger than for a case of the source either above or below the heterogeneous layer. This local enhancement of coda takes place clearly only in the frequency range for which the scattering is the most effective, that is, when the non-dimensional frequency kd takes values from 2 to 3, where k is the wavenumber and d is the size of each heterogeneity. Such enhancement of coda is not observed when the density of cavities, or strength of the heterogeneities, is reduced. Coda level becomes locally large for a source within the heterogeneous layer only in the case that the heterogeneities are strong enough to excite multiply scattered waves, as compared with the singly scattered ones. Robinson (1987) studied the temporal variation of coda-duration magnitude relative to event magnitude based on P and S wave amplitudes using the seismic network of the Wellington, New Zealand, region. He found that coda-duration magnitude relative to amplitude magnitude decreases with focal depth and becomes large locally for events in the depth range from 65 and 75 km. Our synthetic seismograms explain his results well, implying that there must be a region of localized, strong heterogeneity at depths around 70 km. The effective size of the heterogeneous region may be of about several kilometers because the observation was performed with 1 Hz seismometers. This localized heterogeneous layer is probably associated with the subducting Pacific plate underneath the Wellington region.

TheQ value variations in the preparing process of rock rupture

In this paper, the "spectral amplitude ratio method" (SAR), "energy method" (EN) and "coda wave method" (CW) are used to calculate theQ value variations of gneiss in the preparing rupture process. The obtained results show that the variation state ofQ values by SAR features the shape of relative stability—gradual increment to the maximum—then decrement and final rupture. The variation state ofQ values by EN is just contrary to that by SAR, i. e. with the shape of stability—decrement—increment—and final rupture. The varation state ofQ values by CW is similar to that by EN, its main frequency features the shape of relatively high value—decrement to the minimum—increment—and final rupture. But to the high frequency (higher than the main frequency), the variation state ofQ values features the shape of the stable value-increment to the maximum-decrement-and final rupture. At the same time, the results by coda wave amplitude spectrum show that, when stress reaches 70% of rupture stress, the high frequency component of S wave rapidly reduces (Q c increasing); at the time of impending the main rupture, the main frequency component reduces with a large scale (Q c increasing again), this may be the reason which causes the different variation states of two codaQ values. The result of amplitude spectra of P, S (initial wave) waves also show that with the appearance of microcracks the frequency band of S wave turn to be narrow, the high frequency component is reduced quickly, i. e. the S wave spectra have different variation states with different frequency components. That is why theQ s obtained by different methods have different variation characteristics.

Characteristics of Coda Waves and Inhomogeneity of the Earth.

During the last twenty years, the characteristics of seismic waves scattered by inhomogeneous structure in the earth have been gradually clarified by many studies. The composition of P- and S-coda waves has been investigated using seismograms observed by seismic arrays. S to P and P to S wave conversions play an important role in the excitation of P-coda waves. S-coda waves at times later than twice the S-wave travel time are mainly composed of S-waves incoherently scattered by inhomogeneities. In the early part of S-coda waves, the contributions of forward scattering and of surface waves excited by topographic relief are large. Application techniques using coda waves have been developed by many researchers. Regional and temporal variations of coda Q-1 values are related to tectonic activity, although the physical meaning of coda Q-1values is not always clear. A method for the separation of site amplification and source spectra using coda wave amplitudes has been proposed. Multiple scattering models for coda wave energy distribution have been developed for randomly distributed isotropic scatterers. Based on radiative energy transfer theory, analytical solutions have been obtained. Recently, the contribution of scattering loss to the total attenuation factor has been studied. Theoretical studies for coda waves suggest that coda Q-1 values reflect intrinsic attenuation, since multiple scattering energy largely contributes to coda waves. Observational facts, however, imply that the single scattering assumption is still valid. To escape this dilemma, it is necessary to study in more detail the anisotropic scattering process and obtain more realistic models of the inhomogeneous distribution of scatterers.

Attenuation of coda waves in Western Anatolia

By analyzing the decay of coda wave amplitude, we have determined coda Q, Q c, in Western Anatolia (Turkey). Using the single isotropic scattering model, we analyzed 116 earthquakes which registered at the Gebze station by using five narrow frequency bands centered at 1.5, 3, 6, 8 and 10 Hz. Coda Q values were obtained using different lapse times, between 30 and 190 s, in steps of 10 s. Coda Q c for Western Anatolia depends on frequency and lapse time. For a lapse time of 30 s, the frequency dependence of Q c is Q c (ƒ) = 50.7ƒ 1.01 and for a lapse time of 190 s it is Q c (ƒ) = 183.2ƒ 0.76 . In this region, the exponential value of the Q c frequency dependence is practically constant, between 0.7 and 1.0. The obtained coda Q c values were compared with those estimated in other regions. The coda Q c values for lapse times between 60 and 100 s for Western Anatolia and Southern Spain are practically the same, indicating similar coda wave attenuation patterns in both regions of the Mediterranean Basin.

Dense array recordings in the San Bernardino Valley of landers-big bear aftershocks: Basin surface waves, Moho reflections, and three-dimensional simulations

AbstractFourteen GEOS seismic recorders were deployed in the San Bernardino Valley to study the propagation of short-period (T ≈ 1 to 3 sec) surface waves and Moho reflections. Three dense arrays were used to determine the direction and speed of propagation of arrivals in the seismograms. The seismograms for a shallow (d ≈ 1 km) M 4.9 aftershock of the Big Bear earthquake exhibit a very long duration (60 sec) of sustained shaking at periods of about 2 sec. Array analysis indicates that these late arrivals are dominated by surface waves traveling in various directions across the Valley. Some energy is arriving from a direction 180° from the epicenter and was apparently reflected from the edge of the Valley opposite the source. A close-in aftershock (Δ = 25 km, depth = 7 km) displays substantial short-period surface waves at deep-soil sites. A three-dimensional (3D) finite difference simulation produces synthetic seismograms with durations similar to those of the observed records for this event, indicating the importance of S-wave to surface-wave conversion near the edge of the basin. Flat-layered models severely underpredict the duration and spectral amplification of this deep-soil site. I show an example where the coda wave amplitude ratio at 1 to 2 Hz between a deep-soil and a rock site does not equal the S-wave amplitude ratio, because of the presence of surface waves in the coda of the deep-soil site. For one of the events studied (Δ ≈ 90 km), there are sizable phases that are critically reflected from the Moho (PmP and SmS). At one of the rock sites, the SmS phase has a more peaked spectrum that the direct S wave.