Citation: Lkhagva, D., T. Hara (2020), sPg waves observed for the 2012 Bayanbulag earthquake, Synopsis
of IISEE-GRIPS master report.

sPg WAVES OBSERVED FOR THE 2012
BAYANBULAG EARTHQUAKE
Dagzinmaa Lkhagva1,2Supervisor: Tatsuhiko HARA3

ABSTRACTThe sPg wave is one of the regional depth phases useful for focal depth determination in the Regional
Depth Phase Modeling Technique. We investigated whether sPg phases were observed for events in
Mongolia. We analyzed data from the Central Mongolia Seismic Experiment for the 2012 Bayanbulag
earthquake to investigate whether sPg were observed. There are phases that arrived after Pg on theobserved records. The arrival time differences in the distance range between these phases and Pg do notchange much in the epicentral distance range from 55 km to 70 km, which suggests that the observed
phases are sPg waves.
We calculated synthetic seismograms using the code of the reflectivity method using a model
recently obtained for South-Central Mongolia and measured arrival time differences between sPg and
Pg waves from them. A comparison of the arrival times from the synthetic and observed seismogramsin the epicentral distance range from 55 km to 70 km suggests that the depth of the Bayanbulag
earthquake is around 10 km. It indicates that this epicentral distance range (55 km to 70 km) is good to
be used to determine focal depths using the sPg-Pg pair based on the synthetic waveforms.
Keywords: Focal depth, Regional depth phase, sPg, Bayanbulag earthquake, Synthetic waveform.
1. INTRODUCTION
Focal depth is one of the important parameters in earthquake monitoring and understanding seismicity
in an area of interest. Well constrained focal depths can provide information for researches such as
studies of seismogenic layers, seismic hazard assessments, etc. There are some previous studies about
earthquake focal depths in Mongolia, especially for some significant events in active regions (Huang
and Chen, 1986; Schlupp and Cisternas, 2007).
In routine analyses of the Mongolian National Data Center, the epicenters, origin times, anddepths are determined for events that are recorded by seismic stations in Mongolia. Most of the eventsoccur with magnitudes of less than 4.0 in and around the whole territory of Mongolia. In the routine
processing, focal depths are determined to 2 km or 15 km for most of the events automatically, whichsuggests that they are not well constrained. For further detailed studies, it is desirable to improve theaccuracy of focal depths. In this study, we investigate whether sPg phase, one of the regional depth
phases which can be used to determine focal depths, is observed in Mongolia and the applicability of
the Regional Depth Phase Modeling technique (RDPM; Ma et al., 2003; Ma and Atkinson, 2006; Ma,2010). More else, the RDPM is applicable to small events. Ma and Atkinson (2006) and Ma (2010)
analyzed events whose magnitude (mN) is equal to or greater than 2.8.

1Institute Astronomy and Geophysics of Mongolian Academy of Science (IAG-MAS), Mongolia.
IISEE-GRIPS master course student.
3International Institute of Seismology and Earthquake Engineering, Building Research Institute.
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2. DATA
We used data from the Hangay dome subarray of the Central Mongolia seismic experiment (Meltzer etal.,2012). The Hangay dome subarray was deployed in the period from June 2012 to April 2014, and it
consists of 72 stations (the blue triangles in Figure 1). It extended to approximately 500 x 600 km. The
station code starts from "HD," which stands for "Hangay Dome." The observed seismograms werearchived in the Incorporated Research Institutions for Seismology (IRIS) data management center(DMC). The network code is XL-2012-2016 (Meltzer et al.,2012). In this study, we mainly analyzed the2012 Bayanbulag earthquake that occurred on October 3.

Figure 1. The station distribution of the HD (Hangay dome) subarray of the Central Mongolia Seismic
Experiment (Meltzer et al., 2012).
3. THEORY AND METHODOLOGY
3.1.

Regional depth phase modeling

The Regional Depth Phase Modeling (RDPM) method (Ma et al., 2003; Ma and Atkinson 2006, Ma
2010) is a technique to determine focal depths using arrival time differences between regional depth
phases and their corresponding reference phases. Ma (2010) showed that their time differences are
sensitive to focal depths, and their epicentral distance dependence is small. In our study, we investigate
the observability of sPg and try to model it by calculation of synthetic seismograms.
The sPg phase is a seismic wave that propagates to the surface as the S wave, and then it travels
and arrives at a station as a converted P wave. It arrives just after the arrival of Pg in the short epicentral
distance range.
In this study, we investigate whether there are phases that arrive following the arrivals of Pg.
As we show later, there exist such phases for some events. Then, we compute the theoreticalseismograms with the reflectivity method to examine whether the measured time differences can bemodeled for the selected event.
3.2.

Reflectivity method

The reflectivity method is a technique to compute synthetic seismograms (Fuchs, 1968; Fuchs andMuller, 1971). Synthetic seismograms in the frequency domain are integrated over slowness, and thenthey are converted to synthetic seismograms in the time domain using the Fast Fourier Transform.

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We used the program package ERSZOL3, which was developed by B.L.N. Kennett (1993). It
is possible to compute the seismic response of a horizontally layered structure for a point moment tensor
source.
We set the minimum slowness to 0.0001, maximum slowness to 0.3001, and the number ofslowness to 3600, respectively. In this program package, a cosine taper in frequency is applied between
two specific frequencies. We chose 0.25 and 0.5 Hz for the low-frequency, 2, and 4 Hz for the highfrequency, respectively. For the surface condition, we chose the full calculation for half-space withelastic-free surface conditions.
4. OBSERVATION OF THE sPg WAVE
We show an example of observation of sPg waves for the event named the Bayanbulag earthquake on
October 3, 2012. This event occurred along the South Hangay fault, with body wave magnitude mb 4.6.
The epicenter is 46.8105°N and 98.0587°E by MNDC (Figure 2), and the focal depth is 10 km according
to the USGS information.
To measure the time differences between these phases and Pg waves, we conduct deconvolution
to obtain displacement records (the bandpass is set to that between 0.5 and 2 Hz). The displacement
records are shown in Figures 3, 4, and 5. Following Ma (2010), we took times for the maximum
amplitudes caused by the peaks to reduce reading errors in picking the phase arrival times in thesemeasurements. They are denoted by "T5" and "T6," respectively, in Figures 3, 4, and 5.
Currently, we found two more events that recorded possible sPg phases from the retrieved
data. The one is the event that occurred on June 28, 2012, with magnitude mb =3.8; the epicenter is48.1211 and 97.2674. The other is the event that occurred on May 03, 2013, with magnitude mb =3.5;
the epicenter is 48.5846 and 99.7209.

Figure 3. The displacement record section of theobserved seismograms in the epicentral distance
range from 50 km to 64 km. T5 denotes the time
Figure 2. The epicenter (the red star) of theof the maximum amplitude of Pg. T6 denotes theselected event with mb =4.6 occurred near thetime of the maximum amplitude of sPg. "o"village of Bayanbulag in Bayankhongor province
denotes the origin time.
in Southern Mongolia on October 3, 2012
(latitude:46.812 longitude:98.058).

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Figure 5. The displacement records section of theobserved seismograms in the epicentral distance
range from 76 km to 100 km. T5 denotes the timeof the maximum amplitude of Pg. T6 denotes thetime of the maximum amplitude of sPg. "o"denotes the origin time.

Figure 4. The displacement records section of
the observed seismograms in the epicentral
distance range from 64 km to 74 km. T5 denotes
the time of the maximum amplitude of Pg. T6
denotes the time of the maximum amplitude ofsPg. "o" denotes the origin time.

5. SYNTHETIC SEISMOGRAM CALCULATIONS AND COMPARISON OF ARRIVAL
TIME DIFFERENCES
5.1.

Crust model

We use a model based on that named "1D velocity model for South-Central Mongolia" (Batkhuu 2015,
Progress Report of the 3D tomography research) for our calculation of synthetic seismograms, using thecode of the reflectivity method and measured arrival time differences between sPg and Pg waves fromthem. This model can be used for the whole central and southern parts of Mongolian territory. In order
to use this model for the ERZSOL program package, we have to calculate the value of the S velocities
and densities for each layer, respectively. Figure 6 shows the P wave velocity model used for the
calculation of synthetic seismograms based on his work. We computed densities using the empiricalequation of Kurita (1973). For S wave velocities, we set the velocity ratio Vp/Vs=1.70, based on theanalyses of Batkhuu (2015).

Figure 6. The red line denotes the P wave velocity (km/s) model used, which is based on the study doneby Batkhuu.B. The yellow line indicates the calculated density (g/cm3). The blue line represents thecalculated S velocity by km/s.
5.2.

Synthetic seismograms computed by the reflectivity method

We used a double couple source based on the moment tensor solution of the 2012 Bayanbulag event in
the Global Centroid Moment Tensor catalog. We set the source duration to 0.8 sec, referring to Singh et4al. (2000). The time differences between the Pg wave and the phases which arrive following Pg in thesynthetic seismograms do not change much with respect to the epicentral distance. However, they dochange for a larger epicentral distance, as is shown later. This suggests they are sPg. We calculatedsynthetic seismograms in the epicentral distance range from 50 km to 100 km for focal depths of 5 km,
10 km, 15 km, and 20 km, respectively. Figures 7 and 8 show the examples of the generated synthetic
seismograms obtained from the calculation, as mentioned earlier.
From the generated seismograms, we can see some important characteristics. For example, thesynthetic traces show that the time differences between sPg and Pg become progressively larger, while
they do not change much with respect to epicentral distance.

(km)

(km)
(sec)
(sec)
Figure 7. Synthetic seismograms are generated for Figure 8. Synthetic seismograms generated for aa focal depth of 5 km. T5 and T6 denote the times focal depth of 15 km. T5 and T6 denote the times
for the maximum amplitudes for Pg and sPg, for the maximum amplitudes for Pg and sPg,
respectively.
respectively.

5.3.

Preliminary focal depth determinationsPg-Pg time difference, sec

The observed time differences between sPg and Pg are almost constant in the epicentral distance range
between 55 and 70 km. We used the data in the epicentral distance range between 55 and 70 km forpreliminary focal depth determination in this study. Figure 9 shows the comparison of the arrival timedifferences between sPg and Pg based on the measurements from the synthetic and observed waveforms.
The observed time differences are consistent with those for a focal depth of 10 km.
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
50

60

70

80

90

100

Epicentral distance, km
Depth 5 km

Depth 10 km

Depth 15 km

Depth 20 km

Observation

Figure 9. Comparison of the arrival time differences from the observed seismograms to those from thesynthetic seismograms computed for depths of 5 km, 10 km, 15 km, and 20 km, respectively.

5This indicates that the focal depth is around 10 km. This depth is close to 11 km determined by
Meltzer et al. (2019). The focal depth from the ISC catalog is 15.9 km, which includes teleseismic depth
phase pP; it is necessary to conduct further analyses for teleseismic data for this event.
6. CONCLUSION
In this study, we investigated whether sPg phases were observed in Mongolia and the applicability of
the regional depth phase modeling technique. We have found that there are some events for which there
are possible sPg phases on their observed seismograms. We analyzed one of the events, the 2012
Bayanbulag earthquake mb=4.6, which occurred along the South Hangay fault. The arrival timedifferences between possible sPg waves and Pg waves are almost constant in the distance range from 55
km to 75 km and they are around 3 seconds. We calculated synthetic seismograms using the reflectivity
method using a crust and upper mantle model, which is recently obtained for the Central and Southernparts of Mongolia. We measured time differences between Pg and sPg waves from the synthetic
seismograms. Based on a comparison between the time differences from the synthetic seismograms andthose from the observed records, it is suggested that the depth of the Bayanbulag earthquake is around
10 km. This result indicates that the epicentral distance range from 55 km to 70 km can be used todetermine focal depths using the sPg-Pg pair.
ACKNOWLEDGEMENTS
This research was conducted during the individual study period of the training course “Seismology,
Earthquake Engineering and Tsunami Disaster Mitigation” by the Building Research Institute, JICA,
and GRIPS. I would like to express my sincere gratitude to Dr. Tatsuhiko Hara for his patience,motivation, enthusiasm, and immense knowledge. His guidance helped me all through the research andwriting of this thesis. I am grateful to other BRI professors, including Dr. T. Yokoi, Dr .B. Shibazaki,
and Dr. Kita, for their valuable comments on the improvement of my thesis. Moreover, I would like toexpress my thanks to the director of IAG Sc. D Sodnomsambuu Demberel and all truly dedicated seniorresearchers in the Seismology Department of the IAG for their precious science work and constantly
charging young researchers for their bright future.
REFERENCES
Batkhuu,B., 2015., Progress Report of the 3D tomography research.
Dziewonski, A. M., Chou T.-A. and J. H. Woodhouse., 1981, J. Geophys. Res., 86, 2825-2852.
Fuchs, K., 1968., Z. Geophys., 34, 389-411.
Fuchs, K. and Muller, G., 1971, Geophys. J. R. Astr. Soc., 23, 417–433.
Kennett, B.L.N., 1993, Cambridge University press.
Ma, S., V. Peci, J. Adams, and D. McCormack., 2003, EGS- AGU-EUG Joint Assembly.
Ma and Atkinson 2006, Bull. Seism. Soc. Am., 96, 2, 609.
Ma, S., 2010, Focal Depth Bull. Seism. Soc. Am., 100, 1073–1088.
Meltzer, A., 2012, International Federation of Digital Seismograph Networks.
Meltzer, A., Stachnik, J., Sodnomsambuu, D., Munkhuu, U., Tsagaan, B., Dashdondog, M., Russo, R.,
2019, Seismol. Res. Lett., 90, 1364–1376.

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