We present a new method to estimate stable seismic source parameters, such as energy, moment, and Orowan stress drop, using regional coda envelopes from as few as one broadband station. We use the method to compute path‐ and site‐corrected seismic moment‐rate spectra for 117 recent western United States earthquakes. Empirical Green's function corrections were applied to our surface‐ and body‐wave coda envelope measurements to generate S‐wave source spectra. These source spectra provide stable, single‐station estimates of radiated seismic energy Es and seismic moment Mo that for common events are in excellent agreement with network‐averaged estimates obtained using local and regional data. Teleseismic moment estimates are compatible with our regional results, but teleseismic energy estimates appear to be nearly an order of magnitude low. We estimated the seismic moment of events ranging between Mw 2.2 and 7.3, and energy estimates for which we had measured at least 70% of the total energy, generally events above Mw 3.3. We use these estimates to examine the behavior of derived parameters such as the Orowan stress drop (Δσ = 2μEs/Mo). While the earthquakes we studied have a small range in Orowan stress drop, generally between 0.1 and 20 MPa, they show a strong tendency for Orowan stress drop to increase with moment, approximately as Mo0.25. We believe this is a source effect and is not due to inadequate bandwidth or attenuation correction, and note that this trend appears to continue for microearthquakes as described in a recent deep borehole study in southern California. Many of the large high stress drop earthquakes show complexity in their moment‐rate spectra near the corner frequency and cannot be fit by a simple ω‐square model. Instead, above the first corner frequency, the spectral decay ranges between f−1.0 and f−1.5. This leads to larger estimates of radiated energy than predicted with a simple ω‐square model and has implications for seismic hazard estimation. Coda envelopes have three main advantages over direct arrivals for estimating seismic moment and energy: (1) Coda amplitudes vary little with geology and source‐radiation anisotropy and allow accurate single‐station applications; (2) path‐corrected coda amplitude measurements can be applied to very large regions, allowing a comparison of source parameters throughout the western United States using a common methodology and stations; (3) because long‐period coda can last for hours for large local and regional events, it allows the analysis of seismograms with clipped early arrivals.
Abstract. We inverted 1510 P arrival times from regional distances (333-1600 km), in and around the Tibetan Plateau to map the lateral velocity variation within the uppermost mantle. Previous studies have placed first-order constraints on upper mantle velocities but relied on data recorded almost exclusively at stations outside of the plateau. We improve resolution by using 40 events recorded at stations within the Tibetan Plateau. We combine these data with observations obtained from the International Seismological Centre (ISC) to extend our coverage by including Pn arrivals from 85 additional plateau events, relocated in previous studies, and recorded at stations in and around the Tibetan Plateau. We use synthetic travel time data to evaluate the resolution of our data set. The observations provide good resolution to about 1 ø over most of the plateau and surrounding regions. Our results show average Pn velocities that are about 3% lower in the northern plateau relative to the southern plateau. These variations correlate well with major tectonic features and previous geophysical observations. In the Qiangtang terrane of the northern plateau, an area known to be inefficient for Sn propagation, Pn is slow relative to both the plateau south of the Banggong-Nujiang suture and the tectonically stable Tarim basin north of the plateau. This is strong evidence for the existence of partial melt within the uppermost mantle beneath the northern Tibetan Plateau. However, when laboratory estimates of relationships between temperature, velocity, and attenuation are applied, a relatively small temperature variation (2400 to 3700C) is required to explain our Pn velocity observations. When combined with geochemical constraints from volcanics in the northern plateau, our results strongly suggest that the mantle lid is intact beneath the northern plateau. This result would preclude tectonic models involving wholesale delamination of the mantle lithosphere in the northern Tibetan Plateau.
[1] The use of local and regional S-wave coda is shown to provide stable amplitude ratios that better constrains source differences between event pairs. We first compared amplitude ratio performance between local and nearregional S and coda waves in the San Francisco Bay region for moderate-sized events, then applied the coda spectral ratio method to the 1999 Hector Mine mainshock and its larger aftershocks. We find (1) average amplitude ratio standard deviations using coda are $0.05 to 0.12, roughly a factor of 3 smaller than direct S-waves for 0.2 < f < 15.0 Hz; (2) coda spectral ratios for the M w 7.0 Hector Mine earthquake and its aftershocks show a clear departure from self-similarity, consistent with other studies using the same datasets; and (3) event-pairs (Green's function and target events) can be separated by $25 km for coda amplitudes without any appreciable degradation, in sharp contrast to direct waves. Citation: Mayeda, K., L. Malagnini, and W. R. Walter (2007), A new spectral ratio method using narrow band coda envelopes: Evidence for non-self-similarity in the Hector Mine sequence, Geophys.
We present observations of regional phase velocity and propagation characteristics using data recorded during a 1‐year deployment of broadband digital seismic stations across the central Tibetan Plateau along the Qinghai‐Tibet highway from Golmud to Lhasa. Previous seismological studies within this region have had to rely on earthquakes recorded almost exclusively at stations outside of the plateau. We have the opportunity to study numerous source‐receiver paths confined entirely within the Tibetan Plateau. Our analysis concentrates on travel time, amplitude, and frequency content measurements of the Pg, Pn and Sn phases. Pn can be clearly picked for all observed paths and propagates at an average velocity of 8.16±0.07 km/s within the Tibetan Plateau. Sn, however, shows dramatic variations in propagation efficiency across the Tibetan Plateau that is strongly dependent on frequency. We observe that Sn rapidly decreases in frequency and amplitude as it passes through the northern portion of the plateau. We show that in general, Sn propagation efficiency decreases with increasing frequency content. We use 122 events from outside of the plateau and 61 from within to refine the boundaries of a region of inefficient high‐frequency Sn propagation. Specifically, we show that a larger portion of the northern Tibetan Plateau attenuates Sn energy than was previously suggested. In the southern plateau, where high‐frequency Sn is observed, we computed an average velocity of 4.59±0.18 km/s. We also observed that the Pn velocity within this region of inefficient high‐frequency Sn propagation is nearly 4% slower than the Pn velocity computed for paths restricted to the southern plateau and that the crust is about 10 km thinner than in the south. The coincident locations of inefficient Sn propagation and slow Pn velocity is commonly observed in regions of active tectonics. Our results add constraints to the velocity structure of the lithosphere beneath the Tibetan Plateau and require first‐order lateral variations in the uppermost mantle structure, despite the relatively uniform topography of the plateau.
[1] We calculate the deviatoric and isotropic source components for 17 explosions at the Nevada Test Site, as well as 12 earthquakes and 3 collapses in the surrounding region of the western United States, using a regional time domain full waveform inversion for the complete moment tensor. The events separate into specific populations according to their deviation from a pure double-couple and ratio of isotropic to deviatoric energy. The separation allows for anomalous event identification and discrimination between explosions, earthquakes, and collapses. Confidence regions of the model parameters are estimated from the data misfit by assuming normally distributed parameter values. We investigate the sensitivity of the resolved parameters of an explosion to imperfect Earth models, inaccurate event depths, and data with low signal-to-noise ratio (SNR) assuming a reasonable azimuthal distribution of stations. In the band of interest (0.02-0.10 Hz) the source-type calculated from complete moment tensor inversion is insensitive to velocity model perturbations that cause less than a half-cycle shift (<5 s) in arrival time error if shifting of the waveforms is allowed. The explosion source-type is insensitive to an incorrect depth assumption (for a true depth of 1 km), and the goodness of fit of the inversion result cannot be used to resolve the true depth of the explosion. Noise degrades the explosive character of the result, and a good fit and accurate result are obtained when the signal-to-noise ratio is greater than 5. We assess the depth and frequency dependence upon the resolved explosive moment. As the depth decreases from 1 km to 200 m, the isotropic moment is no longer accurately resolved and is in error between 50 and 200%. However, even at the most shallow depth the resultant moment tensor is dominated by the explosive component when the data have a good SNR.
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