加州板块断裂百年大地震的异常间断、可能原因与科学意义

岳中琦

这几天我拜读了美国地震学会题目为“California’s Current Earthquake Hiatus is an Unlikely Pause”的地震新闻报道https://www.seismosoc.org/news/californias-current-earthquake-hiatus-is-an-unlikely-pause/。

我也拜读了美国科学新闻网题目为“California should have had a major earthquake by now, geologists warn” https://www.upi.com/Science_News/2019/04/03/California-should-have-had-a-major-earthquake-by-now-geologists-warn/5601554306980/。

这两个新闻报道均来源于美国地质调查局的地震科学家Glenn P. Biasi博士和Katherine M. Scharer博士，2019年4月3日，发表在《Seismological Research Letters》的最新研究论文。论文题目为“The Current Unlikely Earthquake Hiatus at California’s Transform Boundary Paleoseismic Sites” https://doi.org/10.1785/0220180244。论文摘要如下：

“Paleoseismic and historical earthquake records used to quantify earthquake recurrence rates can also be used to test the likelihood of seismically quiescent periods. At principal paleoseismic sites in California on the San Andreas, San Jacinto, Elsinore, and Hayward faults, no ground‐rupturing earthquake has occurred in the last 100 yr, yet this interval is about three times the average interearthquake period for the ensemble of sites. We examine long paleoseismic records from these faults, as they carry most of the transform fault slip on the plate boundary, to see if the current hiatus has any precedent in the last 1000 yr. The selection of sites is designed to sample fault sections unlikely to have ruptured together, so their conditional probabilities of a hiatus can be combined as independent events. We find a 100‐yr hiatus is not predicted by common time‐dependent or time‐independent recurrence models. Paleoearthquake dating uncertainties can allow long open intervals at individual sites or subsets of sites, but do not explain the observed gap in the ensemble. After approximately removing redundancies in the full paleoearthquake record, the time‐independent probability of the current 100‐yr gap is of order 0.3%. This raises several questions. Do we live in a statistically exceptional time? Or does some wide‐scale effect modulate earthquake occurrence among sites over longer timescales? Finally, how should we understand seismic hazard estimates in California if the recurrence models on which they rely seem, at minimum, incomplete? Whether due to a statistical anomaly, some longer‐term modulation of earthquake occurrence, or another cause, our results emphasize that the hiatus of the last century has been exceptional.”

他们根据古地震研究发现，加州这三条主要大断裂（San Andreas, San Jacinto, Elsinore, and Hayward faults）在过去1000年以来，每100年就要发生大地震！上一次一百年周期的大地震是在1918年结束的。但是，自1918年以来，这三条主要大断裂一直平静！这三条大断裂是分割北美板块和太平洋板块之间的大断裂，具极其重要的地质和地球科学作用和意义。

这是为什么？

美国地震专家们在努力寻找，百年来造成这个断裂滑动缺乏的原因“there is a reason for the dearth of fault slippage over the last 100 years”。

我早已给出了这些板块大断裂百年来大地震间断或不发生大地震的原因。我的原因就是，自1920年代以来，加州盆地的石油和天然气的大量普遍钻井开采。人们不断地将油气从盆地深处地层中储存孔隙抽出，就导致了断裂带无法聚集形成造成地震的极高压天然气囊，和气囊所具有的极大的弹性体积膨胀能。从而，断裂带瘪气了，断裂活动和滑动也就无内在动力了。具体请参见以下相关博文：

1、“加州6.0级地震可再充填加州天然气田http://blog.sciencenet.cn/blog-240687-822411.html”

2、“加州旧金山大地震110周年纪念与地震研究三板斧http://blog.sciencenet.cn/blog-240687-971243.html”

3、“大地震的周期性及其启示http://blog.sciencenet.cn/blog-240687-1107601.html”

美国地震学家对加州三大板块断裂的千年来的古地震周期研究成果的确重要。这项基于加州千年古地震研究的三大断裂的一百年地震周期成果可能证明了，地球深部和内部在恒定的生产极高压甲烷天然气体物质，极高压甲烷气体且恒定地向地壳深大断裂带渗透和聚集，再挤胀逃出深大断裂带内部的圈闭，以100年为周期地造成大地震。

2019年4月12日9：44写成于港大602办公室