|
Abstract
|
Global Positioning System (GPS) measurements suggest hours-long precursors to many large earthquakes.
A meaningful earthquake prediction must clearly define the expected time, location, and magnitude of a future event (1). Short-term earthquake prediction--that is, the capability to issue a warning from minutes to a few months before a mainshock--is impossible without the existence of an observable and actionable precursor. Thus, a key goal is the discovery of a common preparatory faulting process that can tell us where, when, and how big an impending earthquake is going to be (2). On page 297 of this issue, Bletery and Nocquet (3) present a systematic analysis of changes in horizontal position of approximately 3000 geodetic stations, which were measured by using the Global Positioning System (GPS), near 90 global earthquakes with magnitudes greater than 7. They found that on average, horizontal movements of the stations exponentially accelerated in a direction consistent with slow fault slip near the eventual earthquake nucleation point in the last 2 hours before the earthquake ruptures.
There is a long history of retrospective studies, carried out after a large earthquake has already happened, that suggests a wide variety of possible precursors that could potentially have been used to predict the earthquake (2). In the 1970s, there was some optimism that observations of reliable precursors should be possible with improved geophysical observations, but that optimism waned in subsequent decades. There is a single case of an officially issued prospective prediction of a large earthquake, based largely on the observation of hundreds of small earthquakes, in the days preceding the 1975 Haicheng earthquake in China. Even then, luck apparently played a big role in the identification of these events as foreshocks (4). Nonetheless, many earthquakes are preceded by foreshock sequences (5), and a few of the largest earthquakes of the 21st century, including the 2011 magnitude 9 Tohoku-oki earthquake near the Japan Trench, followed slow faultslip episodes of varying sizes and durations (6). That is, the fault that eventually ruptured and produced earthquake shaking sometimes started moving much more slowly before the mainshock...
Language: en |