天搖地動：美國中部由人類引發的地震跟自然產生的地震「本質上是相似的」

原文網址：http://www.ns.umich.edu/new/releases/24990-shake-it-up-human-induced-and-natural-earthquakes-in-central-u-s-are-inherently-similar

天搖地動：美國中部由人類引發的地震跟自然產生的地震「本質上是相似的」

Jim Erickson

美國中部人類引發的地震跟自然發生的地震兩者釋放出來的應力在許多案例中是無法分辨的，代表現有用來預估晃動造成損害的工具可以適用於這兩種類型的地震。

這是8月2日刊登在期刊《科學進展》(Science Advances)線上版一篇研究的主要結論，作者為一名美國密西根大學的地震學家和兩位美國史丹佛大學的研究人員。

此篇刊於《科學進展》論文的第一作者，密西根大學地球和環境科學系的助理教授Yihe Huang表示：「我們的研究顯示美國中部的誘發地震和自然地震本質上是相同的，而且我們可以運用跟自然地震相同的理論基礎，來預測誘發地震造成的損害。」

共同作者，史丹佛大學誘發與觸發地震研究中心的共同主持人Gregory Beroza表示：「我們的發現簡化了災害評估作業，因為我們不需要分別處理這兩種類型的地震所造成的震動。」

在美國中部和東部，過去幾年來地震數目有激增的現象。根據美國地質調查局表示，經常跟石油和天然氣開採有關的深井廢水處置是美國中部地震最近增加的主因。

美國地質調查局於三月總結在美國中部及東部居住工作的大約350萬人，有很大的可能性會在2017年因為人為地震的晃動而受到損害。其中大部分是住在奧克拉荷馬州和堪薩斯州南部。

一項關鍵問題是所謂的誘發地震造成的地動是否跟自然發生的地震有重大差異。若是如此，那人類誘發的地震可能會對建築物和公共設施造成不同層級和類型的損害。

但如果誘發地震和自然地震產生的地動大抵相同，則用來預測自然地震產生損害的算式也可以運用在人類誘發的地震。

為了解答這項問題，Huang和她的同事利用可以取得的儀器紀錄來估計地震的應力降(stress drop)，也就是斷層承受的應力在地震前後的差異。對象為美國中部和北美東部總共39起中等規模的誘發地震與自然地震。

他們發現一旦把地震的斷層機制和深度考量進來，則美國中部誘發和自然地震的應力降是「無法分辨的」。這項發現認為用來預估自然發生地震產生的地動，並進一步估算損害的算式――強地動預估式(GMPEs)也可以運用在誘發地震。

此結果也跟一則說法相符，其認為自然發生的地震和誘發地震都是因為地質斷層上的應力導致。注入深層處置井的廢水讓誘發地震的發生時間提前而讓它們形成。

研究人員分析了美國中部和北美東部規模3.8到5.8的自然和誘發地震，其中包含三種類群。

他們觀察美國中部新馬德里和沃八什谷地震帶內部和周圍的自然產生地震；以及在其西方但洛磯山脈以東，主要是位於奧克拉荷馬州和堪薩斯州南部的誘發地震。

在美國中部一半以上的誘發地震深度都小於5公里，但所有的自然產生地震都在5公里之下。

在北美東部，他們探討了發生在美國和加拿大，阿帕拉契山附近和東側的自然地震。

共同作者，史丹佛大學誘發與觸發地震研究中心的共同主持人William Ellsworth表示：「我們發現東邊的自然發生地震造成的晃動程度可能比美國中部的自然地震還要強烈，此差異的成因可能是斷層類型。」

東部的地震大部分是發生在逆斷層，而美國中部大部分的自然和誘發地震是發生在走向滑移斷層。一般來說逆斷層產生的地震晃動幅度會比走向滑移斷層的地震更加強烈。

根據美國地質調查局統計，在1980年至2000年奧克拉荷馬州每年大於等於規模2.7的地震平均大概只有兩起。此數目在2014年躍升至將近2500起，2015年更有將近4000起，2016年又下跌至2500起。在2016年9月3日，一場規模5.8的地震襲擊了奧克拉荷馬州，是目前為止該州發生的最大一次地震。

據美國地質調查局表示，奧克拉荷馬州和美國中部其他地方的許多地震是由跟石油和天然氣開採有關的廢水注入導致。在某些案例中，注入深井的廢水處置還會結合水力壓裂法使用。然而，許多研究提出僅靠破裂作用本身，很少成為這些地震的直接成因。

Shake it up: Human-induced and natural earthquakes in central U.S. are ‘inherently similar’

The stresses released by human-induced and naturally occurring earthquakes in the central United States are in many cases indistinguishable, meaning that existing tools to predict shaking damage can be applied to both types.

That's the main conclusion of a study by a University of Michigan seismologist and two Stanford University colleagues, published online Aug. 2 in the journal Science Advances.

"Our study shows that induced earthquakes and natural earthquakes in the central U.S. are inherently similar, and we can predict the damaging effects of induced earthquakes using the same framework as natural earthquakes," said Yihe Huang, first author of the Science Advances paper and an assistant professor in the U-M Department of Earth and Environmental Sciences.

"Our finding simplifies the task of hazard assessment because we don't have to treat the shaking from these two kinds of earthquakes differently," said co-author Gregory Beroza, co-director of Stanford's Center for Induced and Triggered Seismicity.

Within the central and eastern U.S., the number of earthquakes has increased dramatically over the past few years. Wastewater disposal in deep wells, often associated with oil and natural gas extraction, is the primary cause of the recent increase in the central U.S., according to the U.S. Geological Survey.

In March, USGS concluded that about 3.5 million people live and work in areas of the central and eastern U.S. with significant potential for damaging shaking from human-caused earthquakes in 2017. Most of them live in Oklahoma and southern Kansas.

A key question is whether these so-called induced earthquakes excite ground motions that are substantially different than those of naturally occurring earthquakes. If that's the case, then human-induced earthquakes could result in different levels and types of damage to buildings and infrastructure.

But if the ground motions in induced and natural earthquakes are largely the same, then equations used to predict damage from natural earthquakes can also be applied to human-induced quakes.

To answer this question, Huang and her colleagues used available instrumental recordings to estimate the stress drop—the difference between the stress across a fault before and after an earthquake—of 39 moderate-magnitude induced and natural earthquakes in the central U.S and in eastern North America.

They found that the stress drops of induced and natural earthquakes in the central U.S. are "indistinguishable" once the faulting mechanism and the depth of the quakes are accounted for. The finding suggests that ground motion prediction equations, known as GMPEs, used to predict damage from naturally occurring earthquakes can also be applied to induced quakes.

The results are also consistent with the idea that both naturally occurring and induced earthquakes are driven by stresses along geologic faults and that the injection of fluids in deep disposal wells advances the timing of induced earthquakes, triggering them.

The researchers analyzed natural and induced earthquakes with magnitudes between 3.8 and 5.8 in the central U.S. and eastern North America, comprising three populations.

They looked at naturally occurring earthquakes in and around the New Madrid and Wabash Valley seismic zones in the central U.S., as well as induced earthquakes occurring further to the west but east of the Rocky Mountains, mainly in Oklahoma and southern Kansas.

In the central U.S., more than half of the induced earthquakes were shallower than 5 kilometers (3.1 miles), while all of the naturally occurring earthquakes were deeper than 5 kilometers.

In eastern North America, they studied naturally occurring U.S. and Canadian earthquakes around and to the east of the Appalachian Mountains.

"We found that naturally occurring earthquakes in the east may lead to stronger shaking than natural earthquakes in the central U.S., differences that may be due in part to fault type," said co-author William Ellsworth, co-director of Stanford's Center for Induced and Triggered Seismicity.

Most eastern earthquakes occur on reverse faults, while most central U.S. induced and natural earthquakes occur on strike-slip faults. Reverse-faulting earthquakes typically have stronger shaking than strike-slip earthquakes.

Between 1980 and 2000, Oklahoma averaged about two earthquakes greater than or equal to magnitude 2.7 per year, according to the U.S. Geological Survey. That number jumped to about 2,500 in 2014 and 4,000 in 2015, then dropped to 2,500 in 2016, according to USGS. On Sept. 3, 2016, a magnitude-5.8 earthquake struck Oklahoma, the state's largest earthquake to date.

According to USGS, many earthquakes in Oklahoma and other parts of the central U.S. have been triggered by wastewater fluid injection associated with oil and gas operations. In some cases, wastewater disposal in deep wells is associated with hydraulic fracturing sites. However, studies suggest that the fracking process itself is rarely the direct cause of these earthquakes.

原始論文：Yihe Huang, William L. Ellsworth, Gregory C. Beroza. Stress drops of induced and tectonic earthquakes in the central United States are indistinguishable. Science Advances, 2017; 3 (8): e1700772 DOI: 10.1126/sciadv.1700772

引用自：Michigan University. “Shake it up: Human-induced and natural earthquakes in central U.S. are ‘inherently similar.’”