Ide Satoshi, Beroza Gregory C
Department of Earth and Planetary Science, The University of Tokyo, Tokyo 113-0033, Japan.
Department of Geophysics, Stanford University, Stanford, CA 94305.
Proc Natl Acad Sci U S A. 2023 Aug 8;120(32):e2222102120. doi: 10.1073/pnas.2222102120. Epub 2023 Jul 31.
The scaling law for slow earthquakes, which is a linear relationship between seismic moment and duration, was proposed 15 y ago and initiated a debate on the difference in physical processes governing slow vs. fast (ordinary) earthquakes. Based on new observations across a wide period range, we show that linear scaling of slow earthquakes remains valid, but as a well-defined upper bound on moment rate of ~10 Nm/s. The large gap in moment-rate between the scaling of slow and fast earthquakes remains unfilled. Slow earthquakes occur near the detectability threshold, such that we are unable to detect deformation events with lower moment rates. Observed trends within slow earthquake categories support the idea that this unobservable field is populated with events of lower moment rate. This suggests a change in perspective - that the proposed scaling should be considered as a bound, or speed limit, on slow earthquakes. We propose that slow earthquakes represent diffusional propagation, and that the bound on moment rate reflects an upper limit on the speed of those diffusional processes. Ordinary earthquakes, in contrast, occur as a coupled process between seismic wave propagation and fracture. Thus, even though both phenomena occur as shear slip, the difference of scaling reflects a difference in the physical process governing propagation.
慢地震的标度律是地震矩与持续时间之间的线性关系，它于15年前被提出，引发了一场关于控制慢地震与快（普通）地震的物理过程差异的争论。基于在很宽周期范围内的新观测结果，我们表明慢地震的线性标度仍然有效，但作为矩速率的一个明确上限约为10 Nm/s。慢地震和快地震标度之间矩速率的巨大差距仍然未得到填补。慢地震发生在可探测阈值附近，以至于我们无法探测到矩速率更低的变形事件。在慢地震类别中观测到的趋势支持这样一种观点，即这个不可观测的区域存在矩速率更低的事件。这表明了一种视角的转变——即所提出的标度应被视为慢地震的一个界限或速度限制。我们提出慢地震代表扩散传播，并且矩速率的界限反映了那些扩散过程速度的上限。相比之下，普通地震是作为地震波传播和破裂之间的耦合过程发生的。因此，尽管这两种现象都以剪切滑动的形式出现，但标度的差异反映了控制传播的物理过程的差异。
