2017 年 24 巻 p. 23-29
We have constructed a seismic travel time database as an aid to research on the interior structure of the Earth. We measured various types of travel times from seismic waveform data collected by stations around the world, including broadband ocean bottom seismometers (BBOBSs). We measured absolute arrival times of various seismic phases by manual picking of the phase onset, differential travel times of P waves between two stations, and differential travel times between PP and P waves. The differential travel times were measured using waveform cross-correlation method. So far, we have measured more than 80,000 travel times. These data have been used to improve the spatial resolution of our tomography model, particularly in regions of poor seismic ray path coverage such as the Pacific Ocean. The database is continuing to develop and is publically available at our own web site; we welcome anyone to use the data for further research.
Complete data set is available via site: http://www.godac.jamstec.go.jp/catalog/jamstec-r_repository/metadataDisp/JAMSTEC-R_24DP01?lang=en
Travel time data of various seismic phases are useful in studies of the Earth's interior. Seismic travel time tomography using such data is a key technique to obtain the seismic velocity structure. However, the resolution of seismic tomography models depends on the coverage of seismic ray paths. Efforts toward the improvement of data coverage are thus necessary in order to increase the spatial resolution of seismic tomography. One way is to increase seismic observations in the areas of poor station coverage, e.g., by deploying a temporal land seismic network or/and BBOBSs on deep seafloors. In addition, extracting additional travel time information from seismic records improves the resolution of travel time tomography. Travel time data for both first arrival and later phases are important because different phases interact differently with the Earth's interior.
We collected both broadband and short period seismic waveform data worldwide, including from BBOBSs deployed in the Pacific region, and measured absolute arrival times of various seismic phases by manual picking of the phase onset, differential travel times of P waves between two stations, and differential travel times between PP and P waves. These data have been used in our tomography studies (e.g., Obayashi et al., 2006, 2009, 2013, 2016; Fukao et al., 2009, 2013; Suetsugu et al., 2010). Our database is made available to the public at our web page. Here we introduce our database.
Various seismic observation networks have been deployed globally; at present, it is possible to obtain seismic waveform data from stations around the world. Data from several seismic networks are publically available online; we used data from the following land seismic networks: SKIPPY, LDG, SPANET, JISNET, OHP, Hi-net, F-net, J-array, GEOFON, GSN and other networks served by IRIS DMC.
We also collected waveform data from broadband ocean bottom seismometers (BBOBSs) such as the Stagnant Slab Project network, Polynesia BBOBS array, TIARES network, and Petit Spot network. These BBOBS waveform data are publically available at <http://p21.jamstec.go.jp/top/>. Details of these observation networks are summarized in Table 1.
List of seismic networks we used travel time measurement.
Fig. 1 shows a distribution map of the seismic stations used for travel time measurements.
Distribution of the seismic stations. Triangles, diamonds, dots, and stars are seismic stations used in this work. Colors and shapes represent different network as follows,
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We measured waveform travel times in suitable ways depending on the seismic phase and the measurement method as described below.
2.2.1 Measurement of absolute arrival time by manual pickingWe picked the onset times of various seismic phases such as P- and S-waves on raw seismograms by referring to the theoretical travel times and the filtered seismogram record. We also changed the filter types, such as low-pass, high-pass, and band-pass, and adjusted the filter parameters to make the target phase stand out. Fig. 2(a) shows an example of manual picking where we determined the P-wave onset time at the red line. We selected the arrival times of P, pP, and sP waves on the vertical component of the seismogram, and S waves on the transverse component. We recorded the absolute arrival time with the quality (a, b, or c) determined by visual inspection of the onset clarity and polarity (＋ or －) for each phase (see Fig. 3 for examples).
(a) An example of seismic record used for the arrival time measurement by the manual picking method. The upper trace is the raw seismic record. The second trace is its filtered record. Blue lines are theoretical arrival times expected from the AK135. Red line is the picked arrival time of P-wave by referring to the filtered trace as well as the original raw record.
(b) Example of the seismic records used for the differential travel time measurement of P-wave between two stations. Black traces are seismogram of BBOBSs and red trace are reference waveform recorded on the land. The reference waveform is shifted in time to give the best correlation.
Examples of seismic record with the quality a, b, and c determined by visual inspection of the onset clarity of P-wave.
In previous tomography studies using the database (e.g., Obayashi et al., 2006, 2009, 2013, 2016; Fukao et al., 2009, 2013; Suetsugu et al., 2010), the standard deviations of the observational errors are assumed to be 0.1, 0.3 and 1.0 sec for quality notations a, b and c, respectively.
2.2.2 Measurement of differential P wave travel time between two stationsDifferential travel time information can be extracted even when the onset of the phase is too ambiguous to pick manually due to high background noise. We measured P wave travel times between two different observational stations using a waveform cross-correlation method by selecting a station where the P wave is recorded most clearly as a reference station for each seismic event. We band-pass filtered the seismograms and defined a window in the reference station's record that enclosed the first swing of the P wave. Then we cross-correlated the record in the window with the records of other stations. Fig. 2(b) shows examples of differential travel time measurements between two stations. The reference waveform (shown in red) is shifted in time to give the best correlation with BBOBS waveforms (shown in black) within the time windows indicated by the shaded area. We applied this method to the BBOBS networks because the background noise level at periods less than 5 sec is generally high on the deep seafloor. As a reference, we also used data from land stations in the vicinity of BBOBS networks for the measurement.
2.2.3 Determining differential travel times between P and PP phasesA P wave that has traveled through the mantle, undergone one reflection from the underside of Earth's surface, and traveled again through the mantle to the station is designated a PP wave. We measured the difference between PP and P wave travel times by cross-correlating the observed PP waveform with a synthetic PP waveform calculated from the observed P waveform. We obtained the synthetic PP waveform by:
At step (iv), the crustal response was calculated for a layered crustal (and sea water, if necessary) structure of the CRUST2.0 (Bassin et al., 2000) using the Haskell matrix method (Haskell, 1962). See also Obayashi et al., (2004) and Fukao et al., (2003) for the detail of the measurement.
The travel time dataset is provided in ASCII text format with all necessary supplementary information described below (as summarized in Table 2).
List of contents in our database.
(a) Example of absolute arrival times in our database.
(b) Example of differential travel time of P-wave between two stations.
(c) Example of differential travel time between PP and P (PP-P).
The data contain absolute arrival time measured by manual picking, event and station information, channel code, phase name, filter information, polarity of picked phase, and quality of picking precision. Table 2(a) shows some examples of absolute arrival times in our database.
3.2 Differential P-wave travel time between two stationsThe data contain measured differential travel times between two stations, correlation coefficient, corner frequencies of the applied filter, seismic event information, and station information. Table 2(b) shows some examples of differential P-wave travel times between two stations in our database.
3.3 Differential travel time between P and PP phasesThe data contain measured differential travel times between P and PP phases, seismic event information, station information and quality of measurement precision. Table 2(c) shows some examples of differential travel times between P and PP phases in our database.
The database is expected to be used for research on the seismic structure of the Earth, seismic source and so on.
Our database is made available to the public at our web page <http://d-earth.jamstec.go.jp/Traveltime/index.html>.
Refer to this article for using our travel time data. Note that the database is continuing to develop.
These data belong to the Department of Deep Earth Structure and Dynamics Research, Japan Agency for Marine-Earth Science and Technology.
We would like to thank the data centers of seismic networks SKIPPY, LDG, SPANET, JISNET, OHP, Hi-net, F-net, J-array, GEOFON, IRIS, Stagnant Slab Project, Polynesia BBOBS array, TIARES, and Petit Spot for providing and managing the seismic data.
