Differential Time Data Research Articles

Processing automatic seismic event detections: an iterative sorting algorithm improving earthquake hypocentres using interevent cross-correlation

SUMMARYWe present an iterative classification scheme using interevent cross-correlation to update an existing earthquake catalogue with similar events from a list of automatic seismic event detections. The algorithm automatically produces catalogue quality events, with improved hypocentres and reliable P- and S-arrival time information.Detected events are classified into four event categories with the purpose of using the top category, with the highest assessed event quality and highest true-to-false ratio, directly for local earthquake tomography without additional manual analysis. The remaining categories have varying proportions of lower quality events, quality being defined primarily by the number of observed phase onsets, and can be viewed as different priority groups for manual inspection to reduce the time spent by a seismic analyst.A list of 3348 event detections from the geothermally active volcanic region around Hengill, southwest Iceland, produced by our migration and stack detector (Wagner et al. 2017), was processed using a reference catalogue of 1108 manually picked events from the same area.P- and S-phase onset times were automatically determined for the detected events using correlation time lags with respect to manually picked phase arrivals from different multiple reference events at the same station. A significant improvement of the initial hypocentre estimates was achieved after relocating the detected events using the computed phase onset times. The differential time data set resulting from the correlation was successfully used for a double-difference relocation of the final updated catalogue.The routine can potentially be implemented in real-time seismic monitoring environments in combination with a variety of seismic event/phase detectors.

Three Dimensional Velocity Structure of the Koyna-Warna Region using Local Earthquake Tomography

ABSTRACT A dense seismic network (∼100 stations) was operated in the Koyna-Warna region from January 2010 to May 2010, that allow us to collect 400 high-quality local earthquake data of magnitude less than 4. In this region, the fault structure and tectonic setting that accommodate the induced seismicity is not well understood. To investigate the seismotectonics of the region, we have inverted 7826 P- and 7047 S-P arrival times for 3-D Vp and Vp/Vs tomographic models along with hypocenters parameters in the region. Although, Dixit et al. (2014) have performed 3-D local earthquake tomography with double-difference tomography code using catalog differential time data. In this paper, Simulps14 code on the same data set is applied. For better approach P arrival time and S-P travel times are inverted directly for Vp, Vp/Vs variations and earthquake locations. High Vp ∼5.9 to 6.5 and low to high Vp/Vs ∼1.69-1.74 imaged in the hypocenter region. These features interpreted as a fluid bearing rock mass under high pore pressure. It is also observed that below the trap basement form a local topography depression between the Koyna and Warna Reservoirs. To the South of the Warna reservoir, intense seismic activity defines a major cluster of ∼ 5 km width at 3 to7 km deep, located under the trap, where the basement is deepening. Such regions are inferred to be associated with the seismically active faults zones. The obtain velocity anomalies are reliable down to a depth of 10 km. This is also confirmed by the analysis of three resolution parameters viz. Hit count, Derivative Weight sum (DWS) and Resolution Diagonal Elements (RDE).

Array analyses of SmKS waves and the stratification of Earth’s outermost core

We perform array analyses of SmKS waves in order to investigate the Vp structure of the Earth’s outermost core. For earthquakes recorded by broadband seismometer networks in the world, we measure differential travel times between S3KS and S2KS, between S4KS and S3KS, and between S5KS and S3KS by array techniques. The differential times are well fit by a Vp model of the Earth’s outermost core, KHOMC (Kaneshima and Helffrich, 2013). Differential slownesses of S4KS and S2KS relative to S2KS are also measured for the highest quality data. The measured slownesses, with unique sensitivity to the outer core 200–400km below the CMB, are matched by KHOMC. These observations consolidate the evidence for the presence at the top of the outer core of a layer that has a distinctively steeper Vp gradient than the bulk of the outer core. We invert new SmKS differential time data set by a tau-p method and attempt to refine the Vp profile of KHOMC. The essential features of KHOMC are preserved after the model refinement. However, the newly estimated layer thickness is nearly 450km, which is thicker than that of KHOMC. The Vp anomalies relative to PREM for the depths 400–800km below the CMB are less than 0.03km/s, consistent with the degree of agreement between different Vp models for the depth range.

Multiscale Seismic Tomography and Earthquake Relocation Incorporating Differential Time Data: Application to the Maule Subduction Zone, Chile

Abstract We present a multiscale seismic tomography method incorporating differential time data. This is an extension of the double‐difference tomography method to teleseismic distances using a nested regional–global model parameterization. It allows inclusion of absolute and differential time data observed at any distance, potentially improving resolution of structure in areas where local or regional data are sparse. The algorithm, named teletomoDD , is quite flexible and is applicable to a variety of relocation and tomography problems, including simultaneously solving for global velocity structure. To illustrate the technique, we apply it to the Maule region of Chile, where an M w 8.8 earthquake occurred in 2010.

Traveltime Tables for iasp91 and ak135

The purpose of this short paper is to bring attention to the availability of a package for building and applying the traveltime tables as developed by Kennett and Engdahl (1991) for model iasp91 and Kennett et al. (1995) for model ak135 . This package has been tested successfully on Linux, Sun Solaris, and Mac OS X. It can be downloaded from the Incorporated Research Institutions for Seismology (IRIS) Data Management Center (DMC) Software—Processing Programs at http://www.iris.edu/software/downloads/processing/. Kennett and Engdahl (1991) introduced velocity model iasp91 , a product of three years of “a major international effort 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 and Bullen (1940).” The software package to generate and apply these tables was made available to the seismological community by their collaborator, Ray Buland, through a U.S. Geological Survey (USGS) anonymous FTP Web site. Four years later, Kennett, Engdahl, and Buland (1995) produced model ak135 , “which gives a significantly better fit to a broad range of phases than is provided by the iasp91 and sp6 models.... The differences in velocity between ak135 and these models are generally quite small except at the boundary of the inner core, where reduced velocity gradients are needed to achieve satisfactory performance for PKP differential time data.” In 1996, Buland posted an updated package on the USGS anonymous FTP Web site with the ak135 tables and software to generate and use them. That site has not been accessible for about 10 years.1 In the 1991 Web-accessible software package, the radially stratified velocities and …

Constraints on seismic velocities in the Earth from traveltimes

New empirical traveltime curves for the major seismic phases have been derived from the catalogues of the International Seismological Centre by relocating events by using P readings, depth phases and the iasp91 traveltimes, and then re-associating phase picks. A smoothed set of traveltime tables is extracted by a robust procedure which gives estimates of the variance of the traveltimes for each phase branch. This set of smoothed empirical times is then used to construct a range of radial velocity profiles, which are assessed against a number of different measures of the level of fit between the empirical times and the predictions of the models. These measures are constructed from weighted sums of L2 misfits for individual phases. The weights are chosen to provide a measure of the probable reliability of the picks for the different phases. A preferred model, ak135, is proposed which gives a significantly better fit to a broad range of phases than is provided by the iasp91 and sp6 models. The differences in velocity between ak135 and these models are generally quite small except at the boundary of the inner core, where reduced velocity gradients are needed to achieve satisfactory performance for PKP differential time data. The potential resolution of velocity structure has been assessed with the aid of a non-linear search procedure in which 5000 models have been generated in bounds about ak135. Misfit calculations are performed for each of the phases in the empirical traveltime sets, and the models are then sorted using different overall measures of misfit. The best 100 models for each criterion are displayed in a model density plot which indicates the consistency of the different models. The interaction of information from different phases can be analysed by comparing the different misfit measures. Structure in the mantle is well resolved except at the base, and ak135 provides a good representation of core velocities.

Shear velocities at the base of the mantle from observations ofSandScS

The observed amplitude ratios of transversely polarized shear waves ScSH/SH exhibit a minimum at a distance of about 68°. Synthetic seismograms computed for a Jeffreys-Bullen model and for models with negative linear velocity gradients at the base of the mantle fail to explain this feature. Various positive linear velocity gradients above the core-mantle boundary explain the amplitude ratio minimum as well as an apparent difference in arrival times of the transversely and radially polarized core reflections ScS. Good agreement between the observed amplitude ratios and the computed ones cannot be achieved without assigning low Q values to the lower mantle or a small shear velocity value to the outer core. At epicentral distances greater than 70° a substantial part of the energy recorded as a horizontally polarized ScS wave bottoms at various depths in the high-velocity region rather than at the core-mantle interface. These precursory arrivals are in phase with the core reflections on the transverse component and out of phase with them on the radial component. This difference causes an apparent time differential by which the transversely polarized ScS wave seems to arrive slightly earlier than the radially polarized ScS wave. High-velocity regions between 40 and 70 km thick containing increases from 0.3 to 0.5 km/sec above the velocity of a Jeffreys velocity model yield the best explanation to the combined amplitude and differential time data.