Epi-Center

震源決定作図用の波形および地図作成の勘所． Yoshio Okamoto 27Dec.2025

Tips for Seismograms and Maps for Epicenter Finding
Exercise Resources are comming soon-----.

１．動機 Motivation
　タイ訪問授業の際のミャンマー地震の話で，震源決定実習を行うことにした．この作業のための地震波形と地図を作成する．他の地震にも応用が可能．
During a lecture visit to Thailand, we discussed the 2025Myanmar earthquake and decided to conduct an epicenter determination exercise. We will make seismograms and maps for this task. This method can be applied to other earthquakes as well.

２．データ準備 Data Preparation

Data: https://ds.iris.edu/wilber3/

より，日時とイベントを検索して当該の地震のページを開く．
Search for the date and event to open the page for that specific earthquake.
NetworksをAllに変更．これで観測点数が増える．
Change Networks to All. This will increase the number of observation sites.

観測点一覧を距離順に並べて，距離の近いものから順に波形データの確認を行う．リストの青くリンクの貼っている観測点記号を押す．
Sort the observation point list by epi-central distance and review the seismograms starting from the closest points. Click the blue linked OBS codes in the list.

波形をダウンロードしたい観測点がまとまったら，次に画面右にある紫色のDataRequestボタンを押す．
Once you have selected all the observation sites for which you wish to download seismograms, press the purple DataRequest button on the right side of the screen.

ダウンロードが成功すれば，別のページから波形をダウンロードする．その際データ形式（SAC形式）や切り取りの時間窓は自動で処理されるが，自分で変更も可能．
If the download is successful, you will download the data from a separate page. The data format (SAC format) and the time window for the selected segment will be processed automatically, but you can also modify these settings yourself.

３．波形処理ソフトSeisGram2k70

SAC形式の波形データ処理を行うのはSeisGram2k70が便利（これはChatGPTに教わった）．

これは下記からダウンロードできる．あらかじめJAVA環境が必要なので，そちらも設定する．そして実行ファイルのある場所で，次のコマンドラインを打つと，ソフトが立ち上がる．/home/seagull/は私の環境なので調整が必要．画面がさきほどダウンロードしたSACファイルをopenしたところ．観測点を換えてopenしていくと，このように波形の比較表示ができる．波形の振幅や時間軸の変更もできる（私は振幅の変更しか使っていない．時間軸の変更はクセがあり，なかなか元に戻らない）．振幅を大きくしてとくに小さなP波の初動を読むことができる．あとはP，Sなどの位置のセットも可能．セットすると自動でS-P時間を読み取って，下のコンソールに書き出してくれる．この画面をスクショしてパワポに貼り付ける．

This can be downloaded from the link below. A Java environment is required beforehand, so configure that as well. Then, in the directory containing the executable file, enter the following command line to launch the software. /home/seagull/ is my environment path, so adjust accordingly. The screen shows the SAC file downloaded earlier being opened. By opening files for different observation sites, you can compare seismograms displayed like this. You can also modify the amplitude and time axis. (I only use the amplitude adjustment. Changing the time axis is tricky and often doesn't revert back easily). Increasing the amplitude allows you to read the initial arrival of particularly small P-waves. You can also set the arrivals for P, S, etc. Once set, it automatically reads the S-P time and outputs it to the console below. Take a screenshot of these seismograms and paste it into PowerPoint.

http://alomax.free.fr/seisgram/SeisGram2K.html

On your terminal, execute the following command,
java -jar /home/seagull/SeisGram2K/SeisGram2K70.jar

４．観測点地図の作成．Observation sitet map.

最後に観測点地図をGMTを用いて作成する．Finally, create the observation site map using GMT6.5.

GMTの地図作成スクリプトはChatGPTに依頼した．この種の作業にはLLM AIは極めて有能である．
I requested the GMT script to ChatGPT. LLM AI is exceptionally capable for this type of task.
ただし，スケールバーはGMTのバージョンによってはうまく操作しない．その場合は下記サイトでスケールバーのみを作成し，貼り付ける．
However, the scale bar may not function properly depending on the GMT version. In that case, create only the scale bar using the site below and paste it in.
Scale bar generator
https://scale.twir.re/

#!/bin/bash
# Map-GMT6-A3h-Scale2000km-QIZ-majorrivers.sh
# For GMT6.5, A3 Landscape, Thailand Map, Stations/User sites/IC.QIZ, 5 deg grid, 2000km scale bar, major rivers only
# GMT Symbols https://thoth.blog/gmt/symbols/ 
# Yoshio Okamoto 26 Nov.2025 with ChatGPT5 

OUTFILE=SE_Asia_station_map_A3h_3000km_majorrivers

# --- Observation Stations (Upper Triangle) ---
cat << EOF > u_stations.txt
98.94447 18.81401 CHTO  
91.12757 29.70317 LSA 
109.84398 19.02928 QIZ 
97.034073 20.768545 TGI
99.589798 21.2857 KTN
108.34161 9.75406 VIVO
EOF

# --- Observation Stations (Lower Triangle) ---
cat << EOF > d_stations.txt
80.70220  7.27280 PALK
114.14150 22.27760 HKPS
96.1376 19.7785 NPW
94.916618 21.205721 NGU
EOF

# --- Observation Stations (Circle) ---
cat << EOF > n_stations.txt
102.74748 25.14825 KMI
96.153397 16.864799 YGN
103.953796 1.4179 UBIN
EOF

# --- Epicenter (Red Star) ---
cat << EOF > epi_center.txt
95.922 22.013
EOF

# --- User Installed Sites (Blue Triangle) ---
cat << EOF > user_stations.txt
104.7231 16.5420 Mukdahan
100.0440 13.8190 Nakhon_Pathom
EOF

gmt begin $OUTFILE png A3h
    gmt set MAP_FRAME_TYPE plain
    gmt set MAP_GRID_PEN_PRIMARY 0.5p,gray30
    gmt set MAP_GRID_PEN_SECONDARY 0.2p,gray50

    # Projection and Region
    gmt basemap -R70/130/0/40 -JM60c -Bxa5f1 -Bya5f1 -B+t"SE Asia Seismic Stations Map"

    # Coastlines, Borders, and Major Rivers
    gmt coast -Glightgray -Slightblue -W0.5p -N1/1p,red -N2/0.8p,gray50 \
              -I1/1p,blue -I2/0.5p,blue   # Grade 1 rivers, pen width 0.5p

    # Lat/Lon Grids every 5 degrees
    gmt basemap -Bg5
    # Observation Stations (Upper Triangle)
  #  gmt plot epi_center.txt -SA1.0c -Gred -W0.5p
    #gmt text u_stations.txt -F+f14p,Helvetica-Bold+jTL -Dj0.3c/0.3c
    # Observation Stations (Upper Triangle)
    gmt plot u_stations.txt -ST0.7c -Gorange -W0.5p
    gmt text u_stations.txt -F+f14p,Helvetica-Bold+jTL -Dj0.3c/0.3c
    # Observation Stations (Lower Triangle)
    gmt plot d_stations.txt -SI0.7c -Gblue -W0.5p
    gmt text d_stations.txt -F+f14p,Helvetica-Bold+jTL -Dj0.3c/0.3c
    # Observation Stations (Circle)
    gmt plot n_stations.txt -SC0.7c -W0.5p
    gmt text n_stations.txt -F+f14p,Helvetica-Bold+jTL -Dj0.3c/0.3c
    # User Installed Sites (Blue Triangle)
    gmt plot user_stations.txt -SC0.7c -Gwhite -W0.5p
    gmt text user_stations.txt -F+f14p,Helvetica-Bold+jTL -Dj0.3c/0.3c

    # Scale Bar: Bottom center, total 2000 km, 50 km intervals
    gmt basemap -Lx14/-2.0+c14+w3000k+f+l50k  # x0 for center, y=-1.2c for below frame

gmt end show