Teleseismic Times Research Articles

The Energy Radiated by the 26 December 2004 Sumatra–Andaman Earthquake Estimated from 10-Minute P-Wave Windows

Abstract The rupture process of the MW 9.1 Sumatra–Andaman earthquake lasted for approximately 500 sec, nearly twice as long as the teleseismic time windows between the P and PP arrival times generally used to compute radiated energy. In order to measure the P waves radiated by the entire earthquake, we analyze records that extend from the P-wave to the S-wave arrival times from stations at distances Δ >60°. These 8- to 10-min windows contain the PP, PPP, and ScP arrivals, along with other multiply reflected phases. To gauge the effect of including these additional phases, we form the spectral ratio of the source spectrum estimated from extended windows (between TP and TS) to the source spectrum estimated from normal windows (between TP and TPP). The extended windows are analyzed as though they contained only the P-pP-sP wave group. We analyze four smaller earthquakes that occurred in the vicinity of the MW 9.1 mainshock, with similar depths and focal mechanisms. These smaller events range in magnitude from an MW 6.0 aftershock of 9 January 2005 to the MW 8.6 Nias earthquake that occurred to the south of the Sumatra– Andaman earthquake on 28 March 2005. We average the spectral ratios for these four events to obtain a frequency-dependent operator for the extended windows. We then correct the source spectrum estimated from the extended records of the 26 December 2004 mainshock to obtain a complete or corrected source spectrum for the entire rupture process (∼600 sec) of the great Sumatra–Andaman earthquake. Our estimate of the total seismic energy radiated by this earthquake is 1.4 × 1017 J. When we compare the corrected source spectrum for the entire earthquake to the source spectrum from the first ∼250 sec of the rupture process (obtained from normal teleseismic windows), we find that the mainshock radiated much more seismic energy in the first half of the rupture process than in the second half, especially over the period range from 3 sec to 40 sec.

Global Test of Seismic Event Locations Using Three-Dimensional Earth Models

To prepare to update location procedures at the International Seismological Centre (ISC), we conduct a global test of location accuracy using two 3D Earth models and three 1D models (Jeffreys-Bullens [J-B], Preliminary Reference Earth Model [PREM], ak135) with initial phase arrival times from different distance ranges. The 3D models include one spherical harmonic model and one irregular grid model, representing two important alternative model parameterizations. Our tests include a worldwide distribution of reference events from a previous study by Smith and Ekstrom (1996) and the ground-truth database of the prototype International Data Centre. For each test we use phase arrival times associated with the events in the ISC Bulletin and recompute hypocenters with the same algorithm used in the ISC operations. With teleseismic arrival times alone (Δ ≥ 25°), the overall accuracy of our recomputed hypocenters is similar using either of the 3D models. The teleseismic hypocenters from 3D models are, on an average, closer to the reference locations than the hypocenters computed using any of the 1D models; but the improvement in results using ak135 is small. When we use the irregular grid model with teleseismic, regional, and local initial arrival times together and fix to reference depths, the accuracy of the epicenters is improved over the results with teleseismic times alone. With a 3D model we are able to invert for depth for a larger fraction of earthquakes, and our newly computed depths are more tightly clustered in a subduction zone than depths from the ISC Bulletin, but not as tightly as depths computed by Engdahl et al. (1998) who used depth phases as well as direct phases. In a test of all the locatable events from 1 month of the ISC Bulletin the irregular grid model improved residuals and formal location uncertainties without introducing large systematic changes in the hypocenters.

Effects of earthquake mislocation on estimates of velocity structure

Recently, there have been several attempts at imaging slab structure using teleseismically observed travel times. These times are a function of both event location and velocity structure. Teleseismically determined epicenters of subduction zone earthquakes, which can occasionally be checked by comparison with accurately determined epicenters using local seismographs, are systematically incorrect by as much as 40 km. Observed travel-time residuals are calculated with respect to this incorrect location. Theoretical residuals, on the other hand, are usually calculated by initiating ray tracing at the teleseismically determined hypocenter. In comparing the theoretical and observed times, we make two errors. First, ray tracing should be initiated at the true location of the earthquake, not at the location determined from teleseismic observations. Obviously, this can rarely be done, but fortunately this source of error produces only subtle differences in the theoretical residuals. The second, much more significant, error results from the fact that the observed residuals are calculated with respect to a location that differs from the actual source, whereas, the theoretical residuals are calculated with respect to the same location used to initiate ray tracing. This inconsistent handling of event locations produces profound differences in residuals which, unless accounted for in the velocity determination, could result in velocity estimates that are severely biased. To demonstrate this, we analyze travel-time residuals from a small thrust earthquake in the central Aleutians, which produced impulsive teleseismic arrivals. Times from the local Adak array provide tight constraints on its absolute location. The ISC location, which is constrained primarily by teleseismic times, is 40 km north of the local-array location. We compute residuals from the observed arrival times with respect to the local-array and the teleseismic locations. The residual distribution computed at the local-array location is dominated by large (average of 2 s), negative residuals to the north, as expected for a high-velocity slab dipping to the north. On the other hand, the residual distribution computed at the teleseismic location has negative residuals averaging less than 0.5 s to the north and larger amplitude negative residuals to the south. Theoretical times are calculated by initiating ray tracing within a thermal model of the Aleutian slab at the local-array location. When theoretical residuals are calculated with respect to the location used to initiate ray tracing, these times agree well with observed times at the local array location. The correlation coefficient is 0.84. However, the correlation between these synthetic times and observed times at the teleseismic location drops to 0.04. This latter comparison is often used in the literature, and can lead to biased estimates of velocity structure. The former comparison is only possible when absolute event locations are known. We suggest a technique to correct for this bias. The procedure of calculating travel-time residuals with respect to the hypocenter determined by least-squares minimization of the residuals is equivalent to applying a specific high-pass linear filter to the residuals mapped on the unit focal sphere. In nearly all teleseismic problems this filter has been applied to the data. To make a self-consistent analysis we should also apply this filter to the synthetic times. The correlation between filtered data and filtered synthetic for the Aleutian event is 0.51.

Traveltimes for global earthquake location and phase identification

SUMMARY Over the last three years, a major international effort has been made by the Sub-Commission on Earthquake Algorithms of the International Association of Seismology and the Physics of the Earth's Interior (IASPEI) to generate new global traveltime tables for seismic phases to update the tables of Jeffreys & Bullen (1940). The new tables are specifically designed for convenient computational use, with high-accuracy interpolation in both depth and range. The new iasp91 traveltime tables are derived from a radially stratified velocity model which has been constructed so that the times for the major seismic phases are consistent with the reported times for events in the catalogue of the International Seismological Centre (ISC) for the period 1964–1987. The baseline for the P-wave traveltimes in the iasp91 model has been adjusted to provide only a small bias in origin time for well-constrained events at the main nuclear testing sites around the world. For P-waves at teleseismic distances, the new tables are about 0.7s slower than the 1968 P-tables (Herrin 1968) and on average about 1.8–1.9 s faster than the Jeffreys & Bullen (1940) tables. For S-waves the teleseismic times lie between those of the JB tables and the results of Randall (1971). Because the times for all phases are derived from the same velocity model, there is complete consistency between the traveltimes for different phases at different focal depths. The calculation scheme adopted for the new iasp91 tables is that proposed by Buland & Chapman (1983). Tables of delay time as a function of slowness are stored for each traveltime branch, and interpolated using a specially designed tau spline which takes care of square-root singularities in the derivative of the traveltime curve at certain critical slownesses. With this representation, once the source depth is specified, it is straightforward to find the traveltime explicitly for a given epicentral distance. The computational cost is no higher than a conventional look-up table, but there is increased accuracy in constructing the traveltimes for a source at arbitrary depth. A further advantage over standard tables is that exactly the same procedure can be used for each phase. For a given source depth, it is therefore possible to generate very rapidly a comprehensive list of traveltimes and associated derivatives for the main seismic phases which could be observed at a given epicentral distance.

Seismological evidence for a decoupled lithospheric segment in south Indian shield

Analysis of several geophysical signatures yields a new model for the south Indian continental lithosphere that shows decoupling of the upper 80km from the underlying mantle. The tomographic images map a thick high-velocity upper mantle (1–6 per cent contrast) in the depth range 80–400 km overlain by a low-velocity region with an average contrast of -3 per cent beneath the south Indian shield. Teleseismic time term analysis points the fast anisotropic axis in the NNE direction coinciding well with the major principal stress direction obtained from field observations, and modelling results from vectorial tomography technique over the entire Indian shield. The significant differences in the direction of maximum anisotropy, stress patterns and character of velocity anomalies between the upper 80km and the underlying mantle suggest the presence of a decoupled system beneath the south Indian shield.

Teleseismic time functions for large, shallow subduction zone earthquakes

Broadband vertical P-wave records are analyzed from 63 of the largest shallow subduction zone earthquakes which have occurred in the circum-Pacific in the last 45 yr. Most of the records studied come from a common instrument, the Pasadena, California, Benioff 1-90 seismometer. Propagation and instrument effects are deconvolved from the P-wave records using a damped least-squares inversion to obtain the teleseismic source time function. The inversion has the additional constraint that the time function be positive everywhere. The period band over which the time functions are considered reliable is from 2.5 to 50 sec. Fourier displacement amplitude spectra computed for each of the 1-90 P-wave trains indicate spectral slopes measured between 2 and 50 sec of ω^(−1.0) to ω^(−2.25) with an average value of ω^(−1.5). These values assume an average attenuation of t^* = 1.0. The seismic moments derived from the P-wave time functions compare well with other published values for earthquakes having moments smaller than 2.5 × 10^(28) dyne-cm (M_w = 8.2). Because the 1-90 seismometer has little response at very long periods, this technique underestimates the moments of the very largest events. The time functions are characterized using five parameters: (1) spectral slope between 2 and 50 sec; (2) roughness of the time function; (3) multiplicity of sources; (4) pulse widths of individual sources; and (5) overall signal duration. The 63 earthquakes studied come from 15 subduction zones with a wide range in the ages of subducted lithosphere, convergence rates, and maximum size of earthquakes. Comparing the time function parameters with age, rate, and M_w of the subduction zone does not yield obvious global trends. However, most of the subduction zones do behave characteristically and can be grouped accordingly.

Electrical conductivity under Western North America in relation to heat flow, seismology, and structure.

Western North America between the coastal ranges and the Great Plains, and from the Mexican-United States border to the Trans-Canada Highway has been studied by means of three large two-dimensional arrays of recording magnetometers. The paper discusses the conductive structures found by these arrays in the upper mantle of the region. Highly conductive mantle material at least 100km thick for conductivity 0.2S/m underlies the Basin and Range Province and ridges of still higher conductivity (or greater thickness for a given conductivity) underlie the Wasatch Fault Belt and Southern Rockies. Under the Great Plains the upper mantle is more resistive and is of sub-shield type. North of the Basin and Range the Cordillera have a thin layer (10-20km at 0.2S/m) of conductive material in the upper mantle and the conductivity indicates a lower level of tectonic activity than in the midlatitudes of the U. S. A. Under the Colorado Plateau the conductivity is of Great Plains type or intermediate between this and that beneath the Basin and Range. Alternative conductive models satisfying variation-field anomalies are discussed with special reference to the poor depth and excellent lateral resolution of magnetometer arrays. The distribution of heat-flow is compared with that of electrical conductivity and it is shown that there is generally close correspondence and full support for association of high conductivity with high temperature. Seismological velocity-depth profiles, station terms in P and S teleseismic times and other parameters are considered and are again consistent with the conductivity and geothermal information. The case for partial melting is considered. It is strongly supported by seismological parameters in the low-velocity layer and gives the most plausible hypothesis for the contrasts of order 100 in conductivity required by the magnetometer array results. Three tentative suggestions are made concerning tectonics of the region.