TCDP井下地震儀之微地震紀錄的特性

1999年9月21日凌晨台灣中部車籠埔斷層大規模破裂，發生芮氏規模7.3的強震，由於在井中觀測可大大降低雜訊的干擾，因此我們便對國科會贊助，在台中大坑鑽探通過車籠埔斷層的井(TCDP井)中，所設置的地震儀陣列獲得的地震目錄加以研究，此地震目錄能夠完整紀錄到最小規模為0.6的微地震 (micro-earthquake)，期望對台灣地區地震活動的特性有進一步的了解。研究方式主要可分為：(1)計算TCDP微地震目錄的b值，並和氣象局地震目錄的結果做比較，討論地震在不同尺度下的活動特性是否一致，結果由TCDP微地震目錄所得到的b值顯著大於氣象局地震目錄所得到的b值，說明在不同尺度下，地震的發震率並不同。 (2)藉由「區段傅立葉轉換 (short time Fourier transform) 」可以將大於完整規模的微地震次數時序資料轉換到頻率域中，發現一天一個周期 (1 cycle per day, 1 cpd) 的頻率有較高的振幅，將理論計算的潮汐高度做相同的轉換並做比較，發現兩者在頻率1 cpd的振幅上都有月周期，不論是比較振幅或是相位，在1 cpd頻率上地震和地潮高度都有密切的關係，而對照0.6 cpd的結果可知這並不是一個隨機的現象。 (3)在時間域中我們也可以藉由計算地震活動度了解地震活動和潮汐應力之間的關聯性，經由低通濾波除去高頻訊號後，計算地震和潮汐應力的相關係數，由不同長度的時間窗去計算相關係數隨時間的變化，可以看到不同周期的相關係數在正負之間變化是有規律的，這暗示在時間域中地震和地潮應力有相關性存在。研究TCDP微地震，讓我們發現在台灣中部不同尺度的地震有不同的發震率，且不論在時間域或頻率域中的分析，微地震和地潮應力都有高相關性。

關鍵字

b值； TCDP ；微地震；潮汐應力

並列摘要

Taiwan Chelungpu-fault Drilling Project (TCDP) deployed a set of comprehensive 7-level borehole seismometers (BHS) after a successful 2-km-deep vertical hole drilling across the slip zone generated by the 1999 Mw 7.6 Chi-Chi earthquake. The TCDP BHS have capability to record micro-events down to magnitude 0.6, which are complete for recording and are all located at a seismogenic layer around 10 km deep. We calculate the b value of frequency-magnitude distribution of the TCDP micro-earthquake and compare with the Central Weather Bureau (CWB) catalog. The result shows the difference of seismicity in different magnitude scale. In additional, we apply the power spectral analysis to time series of hourly counts of the TCDP micro-earthquake sequence. We find from the power spectrum of those micro-events a strong signal with the frequency of 1 cycle per day (cpd), which possibly indicates tidal triggering on those micro-events. We also find the power of that 1-cpd signal shows a strong seasonal variation associated with the large gradient in the power of 1-cpd Earth tide. We also calculate correlation coefficient between observational and theoretic seismicity rate in time domain. Our result elucidates that the tidal triggering effect can be shown clearly in the both of power spectrum and time analysis of the TCDP micro-earthquake sequence.

並列關鍵字

TCDP ； microearthquake ； tidal stress ； b value

參考文獻

Agnew, D. C., “NLOADF: A program for computing ocean-tide loading” , J. Geophys. Res, 102, 5109-5110, 1997.

Chen, C.-C., Telesca, L., Ma, K.-F. and Lin, Y.-Y., “Scaling in spectral behavior of regional to single-fault seismicity”, EPL, 90, 48004, 2010.

Deng, J., and Sykes, L. R., “Evolution of the stress field in southern California and triggering of moderate-size earthquakes: A 200-year perspective”, J. Geophys. Res., 102, 9859–9886, 1997.

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Fischer, T., Kalenda, P. and Skalasky, L., “Weak tidal correlation of NW- Bohemia/Vogtland earthquake swarms”, Tectonophysics, 424, 259-269, 2006.

國際替代計量

TCDP井下地震儀之微地震紀錄的特性

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