Aaron A. Velasco

Rapid estimation of rupture directivity: Application to the 1992 Landers (MS = 7.4) and Cape Mendocino (MS = 7.2), California earthquakes

Using empirical Green functions with regional and teleseismic surface waves, it is possible to resolve fault finiteness effects, in many cases uniquely defining the fault plane for relatively large earthquakes. The technique requires very little data processing and can be applied in near-real time with the current distribution of seismic stations. The Landers strike-slip earthquake was dominated by two sub-events with predominantly north-northwestward rupture. The second sub-event was 1.5 times larger and rotated in strike by 12° counterclockwise relative to the first. The Cape Mendocino thrust event had a relatively smooth rupture that propagated to the southwest on a shallow dipping fault.

Amplitude corrections for regional seismic discriminants

Abstractu200a—u200aA fundamental problem associated with event identification lies in deriving corrections that remove path and earthquake source effects on regional phase amplitudes used to construct discriminants. Our goal is to derive a set of physically based corrections that are independent of magnitude and distance, and amenable to multivariate discrimination by extending the technique described in Taylor and Hartse (1998). For a given station and source region, a number of well-recorded earthquakes is used to estimate source and path corrections. The source model assumes a simple Brune (1970) earthquake source that has been extended to handle non-constant stress drop. The discrimination power in using corrected amplitudes lies in the assumption that the earthquake model will provide a poor fit to the signals from an explosion. The propagation model consists of a frequency-independent geometrical spreading and frequency-dependent power law Q. A grid search is performed simultaneously at each station for all recorded regional phases over stress-drop, geometrical spreading, and frequency-dependent Q to find a suite of good-fitting models that remove the dependence on mb and distance. Seismic moments can either be set to pre-determined values or estimated through inversion and are tied to mb through two additional coefficients. We also solve for frequency-dependent site/phase excitation terms. Once a set of corrections is derived, effects of source scaling and distance as a function of frequency are applied to amplitudes from new events prior to forming discrimination ratios. Thus, all the corrections are tied to just mb (or M0) and distance and can be applied very rapidly in an operational setting. Moreover, phase amplitude residuals as a function of frequency can be spatially interpolated (e.g., using kriging) and used to construct a correction surface for each phase and frequency. The spatial corrections from the correction surfaces can then be applied to the corrected amplitudes based only on the event location. The correction parameters and correction surfaces can be developed offline and entered into an online database for pipeline processing providing multivariate-normal corrected amplitudes for event identification. Examples are shown using events from western China recorded at the station MAKZ.

Broadband source modeling of the November 8, 1997, Tibet (Mw = 7.5) earthquake and its tectonic implications

We studied the source process of a large (Ms = 7.9) intraplate earthquake that occurred on November 8, 1997, at 1002 UT in a remote region of northern Tibet. We used four distinct methods to investigate the broadband source process and thereby better understand the tectonic implications of this event. We relocated aftershocks using a master event technique and found that the distribution of aftershocks covers a region of 200 km in lateral extent. We also employed a surface wave spectral inversion technique to estimate the mainshock moment, depth, centroid location, and centroid time and utilized an empirical Greens function technique to extract rupture directivity information and a detailed source time function from observed seismograms. We also inverted body waves to estimate the moment release along the fault and the source time function. The 1997 earthquake ruptured a strike-slip fault that appears to be an extension or splay of the Kun Lun fault system. This fault is one of the most seismically active strike-slip faults within the Tibetan plateau and has had events with surface wave magnitudes of 6.1, 7.4, and 7.9 in this region since 1973. The rupture released most of the energy within the first 20 s and propagated bilaterally initially, with the later rupture propagating westward for 20–30 s. The absence of large aftershocks suggests that the earthquake efficiently released the stored strain. Comparing mainshock to the largest aftershock energy ratios for this event and other large strike-slip events shows that faulting within the plateau has the characteristics of weak faults (e.g., fracture zone faulting).

Imaging a slow bilateral rupture with broadband seismic waves: The September 2, 1992 Nicaraguan tsunami earthquake

The spatio-temporal rupture history of the September 2, 1992 Nicaragua tsunami earthquake (Ms=7.2; Mw=7.6) is analyzed using long-period (157–288 s) Rayleigh and Love wave spectral inversions and an empirical Green function analysis of very broadband (10–250 s) body and surface waves. The event has a rupture duration exceeding 110 s with a slow (0.6–2.3 km/s) rupture velocity, and involves shallow dipping (6°–10°) thrust faulting at a shallow depth (≤ 10 km), with the latter being primarily responsible for the strong tsunami excitation. Two subevents dominate the source radiation, resulting from a slow asymmetric bilateral, 100–160 km long rupture. Long-period surface wave directivity reveals a predominant rupture azimuth of 140° ± 30°, consistent with asymmetric extension of the bilateral rupture at an azimuth of 125° ± 30° resolved by the source time function analysis. An inverse Radon transform of the source time functions confirms the asymmetric bilateral rupture characteristics.

Improving Regional Seismic Event Location in China

Abstractu200a—u200aIn an effort to improve our ability to locate seismic events in China using only regional data, we have developed empirical propagation path corrections and applied such corrections using traditional location routines. Thus far, we have concentrated on corrections to observed P arrival times for crustal events using travel-time observations available from the USGS Earthquake Data Reports, the International Seismic Centre Bulletin, the preliminary International Data Center Reviewed Event Bulletin, and our own travel-time picks from regional data. Location ground truth for events used in this study ranges from 25u2009km for well-located teleseimic events, down to 2u2009km for nuclear explosions located using satellite imagery. We also use eight events for which depth is constrained using several waveform methods. We relocate events using the EvLoc algorithm from a region encompassing much of China (latitude 20°–55°N; longitude 65°–115°E). We observe that travel-time residuals exhibit a distance-dependent bias using IASPEI91 as our base model. To remedy this bias, we have developed a new 1-D model for China, which removes a significant portion of the distance bias. For individual stations having sufficient P-wave residual data, we produce a map of the regional travel-time residuals from all well-located teleseismic events. Residuals are used only if they are smaller than 10u2009s in absolute value and if the seismic event is located with accuracy better than 25u2009km. From the residual data, correction surfaces are constructed using modified Bayesian kriging. Modified Bayesian kriging offers us the advantage of providing well-behaved interpolants and their errors, but requires that we have adequate error estimates associated with the travel-time residuals from which they are constructed. For our P-wave residual error estimate, we use the sum of measurement and modeling errors, where measurement error is based on signal-to-noise ratios when available, and on the published catalog estimate otherwise. Our modeling error originates from the variance of travel-time residuals for our 1-D China model. We calculate propagation path correction surfaces for 74 stations in and around China, including six stations from the International Monitoring System. The statistical significance of each correction surface is evaluated using a cross-validation technique. We show relocation results for nuclear tests from the Balapan and Lop Nor test sites, and for earthquakes located using interferometric synthetic aperture radar. These examples show that the use of propagation path correction surfaces in regional relocations eliminates distance bias in the residual curves and significantly improves the accuracy and precision of seismic event locations.

Application of regional phase amplitude tomography to seismic verification

Abstractu200a—u200aWe have applied tomographic techniques to amplitude data to quantify regional phase path effects for use in source discrimination studies. Tomography complements interpolation (kriging) methods by extending our ability to apply path corrections into regions devoid of calibration events, as well as raising levels of confidence in the corrections because of their more physical basis. Our tomography technique solves for resolvable combinations of attenuation, source-generation, site and spreading terms. First difference regularization is used to remove singularities and reduce noise effects.¶In initial tests the technique was applied to a data set of 1488, 1.0u2009Hz, Pg/Lg amplitude ratios from 13 stations for paths inside a 30° by 40° box covering western China and surrounding regions. Tomography reduced variance 60%, relative to the power-law distance correction traditionally applied to amplitude ratios. Relative Pg/Lg attenuation varied with geologic region, with low values in Tibet, intermediate values in basins and high values for platforms and older crust. Spatial patterns were consistent with previous path effect studies in Asia, especially local earthquake coda-Q. Relative spreading was consistent with expected values for Pg and Lg. Relative site terms were similar to one another, yet some tradeoff with attenuation was evident.¶Tomography residuals followed systematic trends with distance, which may result from the evolution from direct to coda phases, focusing, model tradeoff or data windowing effects. Examination of the residuals using a kriging interpolator showed coherent geographical variations, indicating unmodeled path effects. The residual patterns often follow geological boundaries, which could result from attenuating zones or minor blockages that are too thin to be resolved, or that have anisotropic effect on regional phases. These results will guide efforts to reparameterize tomography models to more effectively represent regional wave attenuation and blockage. The interpolated residuals also can be combined with predictions of the tomographic model to account for path effects in discrimination studies on a station by station basis.

Rupture process of the 1990 Luzon, Philippines (Mw = 7.7), earthquake

The rupture process of the destructive July 16, 1990, Luzon, Philippines, earthquake is analyzed using a long-period surface wave spectral inversion technique to estimate the average source properties and a broadband body wave and surface wave empirical Green function technique to investigate the slip distribution. The average source model has a seismic moment of 4.2 ± 0.1 × 1020 N m (Mw = 7.7), a total duration of 30 to 35 s, a left lateral strike-slip focal mechanism with strike, ϕ = 153°, dip, δ = 89°, and rake, λ = 16°, and a north-northwestward directivity. These results are consistent with previous work on the Luzon earthquake. However, the broadband analysis indicates that, for periods longer than about 20 s, the teleseismic source time function is characterized by a single, relatively smooth pulse of energy release with azimuthal variations in duration of 24 to 60 s. This contrasts with previous body wave results that inferred two discrete pulses of energy release. The simpler source process is inferred as a result of using empirical Green functions rather than standard theoretical Green functions for a layered crustal model, and by considering only the seismic energy with periods longer than 20 s. The Luzon earthquake began rupturing bilaterally, but evolved into a predominantly northwest directed rupture that extended for 75 to 100 km. The largest slip (10–15 m) occurred about 25 km northwest of the epicenter, beyond which the slip decreased gradually as the rupture propagated into a restraining bend. The peak slip at depth appears to exceed the surface rupture by a factor of 2 to 3. There is little evidence for significant energy release later than 50 s after the rupture initiated.

Improved resolution of earthquake source parameters from long-period surface wave inversions

Abstract Source centroid location optimization moment tensor inversions of long-period fundamental mode surface wave spectra. Joint inversions of Rayleigh and Love wave spectra are used to find source moment tensors for the 18 October 1989 Loma Prieta, California, earthquake ( M w = 6.9) and the 22 April 1991 Valle de la Estrella, Costa Rica, earthquake ( M w = 7.6). For the Loma Prieta event, the focal mechanism at the optimal centroid location is consistent with that at the epicenter, and both agree with independent information. For the Costa Rica event, the solution at the optimal centroid location is consistent with geodetic and body wave observations, whereas the solution found for the epicenter is not. We conclude that optimizing for centroid location in spectral inversions is possible and improves the determination of long-period surface wave source parameters.

Long-period surface wave inversion for source parameters of the 18 October 1989 Loma Prieta earthquake

Abstract We performed simultaneous seismic moment tensor inversions of long-period (157–288 s) fundamental mode Rayleigh and Love waves from the 1989 Loma Prieta earthquake to further constrain the source process of the event. Utilizing a two-step spectral inversion technique, we explored the model dependence and centroid location sensitivity of the long-period surface wave analysis to assess the confidence bounds on the results. We found that estimates of the source duration and depth are highly dependent on the choice of propagation, attenuation and source velocity structure models. Including centroid location parameters in the inversion stabilizes moment tensor estimates but yields a biased location away from the epicenter due to model inaccuracies. Our source duration estimate is 11 ± 5 s using a recent velocity model, MPA, with the centroid time of 6 s being significantly less than earlier surface wave studies (centroids of 10–22 s) and hence more compatible with both body wave and strong motion duration estimates. An unconstrained moment tensor inversion at the optimum centroid location yields a stable major double-couple solution (strike = 124 ± 6°; dip = 67 ± 6°; rake = 126 ± 7°) and a seismic moment estimate (3.0 ± 0.2 × 1019 Nm; Mw = 6.9) similar to earlier long-period studies and body wave and geodetic results. The surface wave centroid depth estimate is 22 ± 11 km, which overlaps the body wave estimates (13 ± 5 km). Thus, surface wave source parameters for the Loma Prieta event, allowing for plausible model dependence, are fully compatible with body wave determinations, and there is no evidence for any anomalous coseismic long-period source process.