人為地震的關鍵因素

 原文網址：https://www.gfz-potsdam.de/en/press/news/details/schluesselfaktoren-bei-der-entstehung-von-menschgemachten-erdbeben

人為地震，或稱為誘發地震(induced seismicity)，引起了越來越多人的關注。在油氣儲集層、廢水處置場址、地熱儲集層，有流體注入或抽出來的時候可能會造成這些事件。少數規模較大而被稱為「失控性誘發地震」(runaway induced earthquakes)的案例，發生的強烈震動會讓公眾關注而造成計畫暫停(像是2006瑞士巴賽爾的事件)，甚至是造成嚴重損害(2017年韓國浦項)。不過詳細的研究可以成功地避免這類失控事件發生，像是2018年赫爾辛基的地熱計畫。若要有系統地避免大型誘發地震，關鍵是要更加深入了解這些事件背後的物理機制。

位在地質力學高壓實驗室的這座儀器，可以對內部的圓柱型岩石樣品施加通常在地殼內部數公里深才有的壓力，甚至還能繼續施加更大的壓力來引發斷層滑移。圖片來源：GFZ。

發表在《美國國家研究院院刊》(Proceedings of the National Academy of Sciences)的新研究中，德國地球科學研究中心(GFZ)「地質力學與科學鑽探」部門的Lei Wang博士與同僚，聯合挪威奧斯陸大學的研究人員指出在造成這類失控事件的因素當中，關鍵是在地質儲集層當中已經存在的斷層的粗糙度，以及連帶造成的應力分布不均。研究結合了GFZ地質力學實驗室以聲學技術監測的新型流體注入實驗還有數值模擬的結果。「我們的實驗室試驗發現岩石裡粗糙與光滑的斷層展現的行為完全不同。這項觀察結果令我們十分興奮，因為我們證實了在流體灌注的過程中，大型誘發事件發生之前微地震的逐漸局部化可以指示應力如何轉移，」研究第一作者，負責設計與進行這些實驗的Lei Wang博士表示。

在實驗室中，灌注誘發的地震強調出斷層粗糙度的重要性

在地質儲集層當中可能有活斷層、構造斷層沿線的破裂以及早已存在但停止活動的斷層，不過要確立它們的粗糙度卻相當困難。為了克服在自然情況下，監測或是繪製這類斷層時解析度不足的問題，研究團隊把它們的尺度「縮小」到數十公分，在實驗室中做出表面粗糙度已知的斷層，接著利用MTS的三軸壓縮儀把斷層加壓到將近臨界應力的狀態。這些岩石樣品也裝有許多感應器，像是運用實驗室壓電式地震儀來監測數以千計稱為「聲射」(Acoustic Emissions)的微小地震，其指示了受壓岩石在破裂之前的變形過程。最後他們把液體注入岩石來模擬液體注入地質儲集層的過程。「在實驗室我們可以控制邊界條件並且裝設密集的監測網，使我們能呈現實驗做出來的誘發地震以及緩慢的非地震變形是如何演變，並且推導斷層滑移量及滑移速率等關鍵參數，這讓我們可以更加全面地瞭解灌注誘發地震的物理機制，」GFZ地質力學與科學鑽探部門的教授Georg Dresen表示。他是這項研究的發起人與指導者。

相比於平滑的斷層，灌注誘發的滑移在粗糙的斷層上會製造空間局部化的聲射叢集，其發生在高度受壓的突起附近。此處的誘發滑移速率比較高，也會伴隨著較多的大型事件。這道機制對於測量應力的方法「古登堡-芮克特b值」來說是典型的結果。流體灌注首先透過緩慢、不會造成地震的滑移來重新啟動斷層的部分區段，並只形成少量的小型地震事件，隨著地震的逐漸局部化，最後大型的誘發事件才會發生。「這項研究對於誘發地震而言具有重大意義。它意味著在流體灌注到地質儲集層的過程中，即時監測或許可以辨識出這類局部化過程，而在大型誘發事件成核之前避免它們發生，」GFZ地質力學與科學鑽探部門的主任Marco Bohnhoff教授表示。

實驗室與現地尺度的誘發地震相似性

為了進一步探討實驗室試驗對地質儲集層發生的地震來說有多實用，作者匯整了許多誘發地震的資料庫，關於流體能量與發散出來的能量之間關係式的研究，來源包括了在實驗室尺度下與現地進行的流體灌注試驗、儲集層尺度下的水力壓裂試驗，以及世界各地的地熱開採和廢水處理計畫。地震灌注功率(seismic injection efficiency，即地震散發的能量與流體灌注到系統的水力能量之間的比例)區分了壓力受控的破裂以及失控的破裂。從誘發地震看出長期處於壓力受控下的破裂，其地震灌注功率相較於失控事件所擁有的低了許多。Wang博士強調「我們實驗裡的誘發斷層滑移會在停止灌注流體之後不久跟著停止，反映出我們在實驗室的觀察結果跟現地尺度的誘發地震中，屬於壓力受控的破裂有類似之處。」

最終目標：控制並避免大型誘發地震

這項成果隸屬於最近剛發起的一項研究提案，其目標是更加準確地預測地質儲集層中的誘發地震，最終推及至會造成重大災害的自然地震。這項提案的一部份是把現地尺度的作用帶入實驗室，使科學家可以控制邊界條件並且重現引發地震事件的過程。Marco Bohnhoff總結：「若要大幅了解岩石變形的過程細節，唯有奠基於新的資料處理方法，並且調整地震分析的古典地震學方法，以及現在也能在實驗室進行的研究方法。在能源轉型的過程中，很重要的一部分是利用地下岩層作為儲能或產能的地點。若要讓公眾接受，前提條件之一便是要減輕人為造成的地震災害，而Wang博士和共同作者最近發表的這類研究，就有潛力可以達成此目的。」

 

Key factors in man-made earthquakes

Man-made earthquakes, so called induced seismicity, have become an increasing concern. These events can occur during fluid injection or extraction such as in oil or gas reservoirs, wastewater disposal, or geothermal reservoirs. In few cases larger co-called ‘runaway induced earthquakes’ were strong enough to cause public concern and stopping projects (e.g. 2006 Basel/Switzerland) or even substantial damage (2017 Pohang/South Korea). Intense research, however, has resulted in successful attempts to avoid such runaway events such as in the Helsinki geothermal project in 2018. The key to systematically avoid large induced earthquakes is to better understand the underlying physical processes.

In a new study published in the Proceedings of the National Academy of Sciences, Dr. Lei Wang and his colleagues from the GFZ Section ‘Geomechanics and Scientific Drilling’, together with researchers from the University of Oslo, Norway, report that the roughness of pre-existing faults and associated stress heterogeneity in geological reservoirs play a key role for causing such runaway events. The study combines novel fluid injection experiments under acoustic monitoring performed in GFZ’s geomechanical laboratory with numerical modelling results. ‘’We found that rough and smooth faults in the rocks behaved entirely different during our laboratory experiments. This is an exciting observation as we evidenced the progressive localization of microseismic activity indicating stress transfer before large induced events during fluid injection’’, says the first author Dr. Wang who designed and performed the experiments and the modelling.

Injection-induced seismicity in the lab highlights the important role of fault roughness

Roughness for active faults and fractures along tectonic faults as well as pre-existing but inactive faults in geological reservoirs are difficult to characterize. To overcome the insufficient resolution when imaging or monitoring such faults in nature, the research group ‘down-sized’ to the decimeter-scale preparing laboratory faults with defined surface roughness. Those were then pressurized to near critical stress states using a triaxial MTS compression apparatus. The rock samples were also equipped with multiple sensors including piezo-based lab seismometers to monitor thousands of tiny earthquakes, so-called Acoustic Emissions, indicating deformation inside the pressurized rocks before they break. Fluid injection was then performed into the samples simulating fluid injection in geological reservoirs.  ‘’Controlling the boundary conditions and using a dense monitoring network in the lab enabled us to image the evolution of induced laboratory earthquakes as well as slow aseismic deformation and derive key-parameters such as fault slip and slip rate, providing a comprehensive image to better understand the physics of injection-induced seismicity’’, says Georg Dresen, professor in GFZ’s Section Geomechanics and Scientific Drilling, who supervised and initiated the study.

Compared to smooth faults, injection-induced slip on rough faults produces spatially localized clusters of Acoustic Emissions occurring around highly stressed asperities. It is there that induced local slip rates are higher, accompanied by a relatively higher number of large events. This mechanism is typically measured in the ‘Gutenberg–Richter b-value’ as a measure for stress. Fluid injection first reactivates the fault patches through slow, aseismic slip and causing only few and small seismic events, followed by a progressive localization ultimately leading to large induced events. ‘’This study has important implications for induced earthquakes: It means that when monitoring fluid injections in geological reservoirs in real-time, this may allow identifying such localization processes before the nucleation of larger induced events allowing to avoid them’’, says Prof. Marco Bohnhoff, head of GFZ’s section Geomechanics and Scientific Drilling.

The similarities between laboratory-scale and field-scale induced seismicity

To further investigate the relevance of the lab experiments for earthquakes in geological reservoirs, the authors compiled a wide range of datasets of induced seismicity studying the emitted energy as a function of hydraulic energy from laboratory-scale and in-situ fluid injection experiments, as well as from reservoir-scale hydraulic fracturing, from geothermal and waste-water disposal projects around the world. The value of seismic injection efficiency (i.e. the ratio of energy emitted in earthquakes to the hydraulic energy that is put in the system by fluid injection) separates pressure-controlled ruptures from runaway ruptures. In contrast to runaway events with high seismic injection efficiencies, induced seismicity showing an extended pressure-controlled rupture typically shows a much lower seismic injection efficiency. Dr. Wang emphasizes that ‘’our laboratory observations bear similarities with those field-scale induced earthquakes corresponding to pressure-controlled ruptures, as reflected by the fact that in our experiments the induced fault slip terminates shortly after we stop fluid injection’’.

The goal is to ultimately control and avoid large induced earthquakes

The study is part of a recently started research initiative aiming to better forecast induced earthquakes in geological reservoirs and ultimately also large catastrophic natural earthquakes. Part of this initiative is to bring the processes from the field scale into the laboratory where boundary parameters can be controlled and the processes leading to seismic events can be reproduced. Marco Bohnhoff concludes: “Only novel data processing approaches and tuning classical seismological methods to analyze earthquakes, now also in the lab, form the basis for understanding the rock deformation processes in greater detail. Studies such as the one now published by Wang and his co-authors host the potential to mitigate man-made seismic hazards, which is a pre-condition to reach public acceptance when using the geological underground for energy storage and extraction as a key element of the energy transition.”

原始論文：Lei Wang, Grzegorz Kwiatek, François Renard, Simon Guérin-Marthe, Erik Rybacki, Marco Bohnhoff, Michael Naumann, Georg Dresen. Fault roughness controls injection-induced seismicity. Proceedings of the National Academy of Sciences, 2024; 121 (3) DOI: 10.1073/pnas.2310039121

引用自：GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre. "Key factors in human-made earthquakes."