Earthquake physics and real-time seismology

Abstract

Simply stated, an earthquake is caused by slip on a fault. However, the slip motion is complex, reflecting the variation in basic physics that governs fault motion in different tectonic environments. Seismologists can learn a great deal about earthquakes from studying the details of slip motion. The size of great earthquakesSeismic slip motion involves a broad 'period' (or frequency) range, at least from 0.1 s to 1 hour, and a wide range of amplitudes, roughly from 1 µm to 30 m. Most seismographs available before the 1960s could record ground motions over only short periods -less than 30 s -which prevented seismologists from studying important details of earthquake processes. In the 1960s, longer-period analogue seismographs became available, allowing seismologists to study great earthquakes (in general, magnitude ≥ 8) over an extended period range; this has resulted in substantial progress in our understanding of earthquakes. For example, being able to measure long-period waves of up to 1 hour has made it possible for seismologists to establish the overall size of great earthquakes accurately. With the old instruments, wave amplitudes were measured over only a short period range, leading to underestimates of the magnitude of great earthquakes (Fig. 1). The monitoring of longperiod waves, combined with rigorous use of wave theory, rectified this problem, and, as a result, our perception of global seismicity changed drastically during the twentieth century. With the old estimates, global seismic activity seemed to have been relatively constant over the century, but according to the new estimates, a burst of activity occurred between 1952 and 1965; about 40% of the total seismic energy released during the century was released during this period (see ref. 1 for a review).Another important finding is that most earthquakes involve a relatively low stress change, 1-10 MPa. This contrasts with the much higher stress -100 MPa or greater -involved in fracture of rocks at high confining pressure. This indicates that the fracture process in Earth's crust involves special physics, and this is currently the subject of extensive research.A better understanding of great earthquakes has also contributed to an improved understanding of the relationship between earthquakes and global plate motion.

Brittle Failure Mechanism of Multiple Locked Patches in a Seismogenic Fault and Exploration of a New Approach to Earthquake Prediction

“…Thus, at a subduction zone, slow or silent quakes provide a precursor clue to the future great event. This clue is particularly significant at the ancient continental subduction zone of Cascadia where detailed records of historical quakes are available and recurrence of similar major events seems inevitable [36] .…”

Potential of 3‐D vertical seismic profiles to characterize seismogenic fault zones

“…Thus a patchwork of frictional behaviors appears to develop along the plate interface fault. This patchwork in space [ Sage et al , 2006] changes character in time during the seismogenic cycle [ Bilek and Lay , 2002; Wang and Hu , 2006; Kanamori , 2008]. The frictional patchwork from material and rupture character is speculative because seismic resolution at the scale of the roughness and sampling of eroded upper plate debris on the lower plate requires riser drilling that has yet to be realized.…”

IODP New Ventures in Exploring Scientific Targets (INVEST): Defining the New Goals of an International Drilling Program

“…The new paradigm in earthquakes includes not only classically understood locked zones that break in fast slip producing earthquakes and adjacent creeping zones that nearly continuously deform without generating earthquakes, but also faults with episodic slow-slip zones that produce tremors. Additionally some subduction zones generate tsunami earthquakes that produce inordinately large tsunamis compared to the size of the earthquake (from Kanamori, 2008). …”