從歷史紀錄中發現台灣1906年的地震新機制

https://www.seismosoc.org/journals/historical-records-uncover-new-mechanism-in-1906-taiwan-quake/

從歷史紀錄中發現台灣1906年的地震新機制

1906年的梅山地震是襲擊台灣的地震中，造成死傷人數最多的其中一次。研究人員重新檢視過往的地震儀紀錄，而找到新的機制來解釋梅山地震的發生原因。

根據發表在期刊《地震研究通訊》(Seismological Research Letters)的論文，作者提出的新機制更符合規模7.1的梅山地震預期中應留下的斷層破裂，以及它的損害和餘震分布模式。

這項發現有助於地震學家更加瞭解該區域錯綜複雜的斷層系統。「瞭解它們的重要之處，在於這提供了更為完整的資訊來讓我們評估地震造成的災害。」任職於國立中央大學與台灣地震災害暨風險評估管理中心(E-DREaM)的馬國鳳表示。

她接著表示這項成果或許也能激勵其他構造活動頻繁國家中的科學家，運用歷史紀錄「來探討整個斷層系統以評估地震造成的災害，而非只用斷層的某個單一區段。」

1906年的梅山地震在與其同名的村落附近造成1258人死亡、2358人受傷並摧毀了6769棟房屋。研究人員預估它的地表晃動程度在麥卡利震度階級中達到9，也就是「猛烈」，代表大型建築物會嚴重受損，像是從地基脫離。

在地震過後進行的野外調查認為這場地震跟梅山斷層上一條東西向的走滑破裂有關。然而，梅山地震是否真的由這道破裂引起，一直以來都有所爭議。舉例來說，相較於推論中要產生如此強烈地震所需的破裂長度，這道地表破裂似乎顯得太短；此外，報告中的損害以及餘震也只有表現出南北向的分布模式，而不是東西向分布。

考慮到這些不一致的結果，馬國鳳和她的同事決定重新探討梅山地震的歷史紀錄。研究人員從東京大學地震研究中心的文獻裡，取得了當初在台灣架設的三座地震站的地震儀原始紀錄。另外，在台灣中央氣象局的協助之下，他們也蒐集了相關歷史文獻。

研究人員運用這些原始紀錄來產生梅山地震的模擬波形，藉此評估幾種關於斷層可能如何破裂的模型。馬國鳳解釋他們之所以從原始的地震儀紀錄中製造模擬波形，是因為要把未知的儀器反應(instrument response)考慮進來，並且把歷史紀錄數位化都相當有難度。

模擬出來的波形顯示P波和S波(分別為一場地震中由地震儀偵測到的第一道和第二道震波)的初動方向有所差異，研究人員表示這代表梅山地震的破裂機制可能不單只有走向滑移。

科學家接著搜尋該地區和數據更為貼切的其他種斷層運動方向。他們的結論中所偏好的機制解，認為梅山斷層是由一條東北―西南向，帶有些微右移分量的逆衝斷層引起。

馬國鳳和她的同事表示，沿著此類逆衝斷層引起的破裂更符合1906年梅山地震的震度大小，以及之後觀察到的餘震模式。他們的研究強調出若要更加瞭解世上其他地方的地震行為，就需要取得歷史上的重大地震事件所留下的歷史紀錄。

Historical records uncover new mechanism in 1906 Taiwan quake

Researchers reexamining historical seismograms from the 1906 Meishan earthquake have uncovered a new mechanism for the quake, one of the deadliest to ever strike Taiwan.

The new mechanism provides a better fit for the fault rupture expected for the magnitude 7.1. earthquake, as well as for the distribution of damage and aftershocks, according to the report in the journal Seismological Research Letters.

The findings will help seismologists better understand the complex fault systems in this area, “which is important to provide better information for seismic hazard assessment,” said Kuo-Fong Ma of National Central University and the Earthquake Disaster & Risk Evaluation and Management (E-DREaM) Center in Taiwan.

The result might also encourage others to use historical records in tectonically active countries to “explore the fault system rather than a single fault segment for seismic hazard evaluation,” she added.

The 1906 Meishan earthquake caused 1258 deaths, 2385 injuries and leveled 6769 houses near its namesake village. Ground shaking has been estimated at a Mercalli intensity of IX or “violent,” meaning that there would have been significant damage to large buildings, including building shifted off their foundations.

Field studies conducted after the earthquake suggested that the earthquake was related an east-west strike-slip rupture on the Meishan Fault. However, there has been a longstanding debate about whether this rupture represents the earthquake’s true origins. The surface rupture was short compared to what might be expected for such an intense quake, for instance, and reports of damage and aftershocks were confined in a north-south pattern, instead of an east-west pattern.

With these inconsistencies in mind, Ma and her colleagues decided to reexamine the historical records of the earthquake. The researchers used the original seismogram recordings from three seismic stations in Taiwan, which were archived at the Earthquake Research Institute, University of Tokyo, along with historical literature collected with the help of Taiwan’s Central Weather Bureau.

The researchers used these original records to generate waveform simulations for the earthquake, to evaluate several models of how the fault might have ruptured. They created simulated waveforms from the original seismograms because of the difficulties in accounting for unknown instrument responses and in digitizing the historical records, Ma explained.

The simulated waveform data uncovered a discrepancy between the first motions of P-waves and S-waves (the first and second waves detected by a seismograph in the event of an earthquake) that suggested the earthquake’s mechanism might not have been a pure strike-slip rupture, the researchers said.

The scientists then hunted for an alternative fault motion in the region that could better explain the data. They concluded that the preferred mechanism would be a thrust fault oriented in a northeast-southwest direction, with a small right-lateral component.

Rupture along this type of thrust fault is more consistent with the level of shaking intensity and the pattern of aftershocks seen after the 1906 earthquake, Ma and colleagues said. Their results highlight the need to achieve important historical seismic recording for better understanding of earthquake behavior around the world.

原始論文：Yi‐Wun Liao, Kuo‐Fong Ma, Ming‐Che Hsieh, Shi‐Nan Cheng, Hao Kuo‐Chen, Chung‐Pai Chang. Resolving the 1906 M_w 7.1 Meishan, Taiwan, Earthquake from Historical Seismic Records. Seismological Research Letters, 2018; DOI: https://doi.org/10.1785/0220170285

引用自：Seismological Society of America. “Historical records uncover new mechanism in 1906 Taiwan quake.”