Glossary for Slow-to-Fast Earthquakes
Nearly 20 years have passed since the discovery of various slow earthquakes worldwide, and our understanding of the phenomenon has deepened, accompanied by new terms to describe it. On the other hand, it often happens that the diverse terminology becomes a barrier to understanding, especially for students and researchers in other fields. Therefore, in this project, we have compiled a list of frequently used terms regarding slow earthquakes, together with their brief explanations. We hope that this glossary will serve as guidance to promote mutual understanding.
A phenomenon whereby brief and abrupt deformations, such as underground fault movement, generate seismic waves. These waves travel through the Earth’s interior, ultimately causing the ground surface to shake. While large earthquakes can produce intense seismic activity, damaging buildings and structures and leading to catastrophic events, the seismic waves of smaller earthquakes can only be detected by highly sensitive seismometers.
A transient geophysical phenomenon that emits seismic waves and induces ground deformation, similar to earthquakes, yet with minimal or unobservable shaking. While this term has historically been applied to various phenomena, here it is employed as a collective term for low-frequency earthquakes, tectonic tremors and their migrations, very low-frequency earthquakes, and slow slip events, all of which have been discovered around the world since around the year 2000. The mechanism behind their occurrence remains largely a mystery.
Low-frequency earthquake (LFE)
A small earthquake, generally with a magnitude Mw<2, that emits seismic waves with a predominant frequency of a few Hz (a period of around 0.2 s). Since earthquakes of Mw<2 typically emit seismic waves of predominant frequency of 10 Hz or higher (a period of less than 0.1 s), those that deviate from this norm are categorized as a different type of earthquake and are referred to as low-frequency earthquakes (LFEs). They are often observed near volcanoes and along plate boundaries.
Low-frequency earthquake (LFE) on plate boundary
Low-frequency earthquakes(LFEs) occur near plate boundaries. Their focal mechanisms are generally characterized by shear slip. Since they are frequently observed during episodes of tectonic tremor, it is believed that tectonic tremor consist of a series of LFEs. They were discovered around the year of 2000.
LFE beneath volcanoes (Deep long-period earthquake, DLP)
Low-frequency earthquakes(LFEs) occur in the vicinity of volcanoes, typically localized at depths of about 20-30 km. They are also referred to as deep long-period earthquakes (DLP). The focal mechanism is complex and cannot be approximated by a simple shear slip. They can also occur intermittently and in sequences. They were first discovered in the 1960s and are observed not only beneath active volcanoes but also in regions where no volcanic activity is evident at the surface.
Very low-frequency earthquake (VLFE)
Earthquakes that emit seismic waves with periods of 20-100 s (frequencies of 0.01-0.05 Hz), often having magnitudes of around Mw 3-4. Those observed near plate boundaries in subduction zones exhibit focal mechanisms that reflect deformation associated with subduction, and frequently accompany tectonic tremors and low-frequency earthquakes. These were first identified around the year 2005.
An earthquake that produces a disproportionately large tsunami relative to its seismic shaking. While the seismic energy of a typical earthquake is about 10-5 of the seismic moment, for a tsunami earthquake it is about 10-6. This difference is primarily attributed to its prolonged duration. Although these earthquakes are observed near the trench axes in subduction zones, instances are rare and many aspects of their physical mechanisms remain enigmatic. Notable examples include the 1896 Meiji Sanriku earthquake and the 1992 Nicaragua earthquake.
Underground deformation, characterized by slow, localized shear movement—often resembling slip along a fault plane—with minimal seismic wave emission. Typically, this deformation is detected through geodetic measurements. It is also referred to synonymously as ‘creep’ or ‘aseismic slip.’
Slow slip event (SSE)
Slow slip phenomena occurring over a defined duration, ranging from several days to multiple months, and occasionally extending over a year. Often manifesting along plate boundaries or major faults, these events have been globally observed since their initial detection around the year 2000.
Episodic tremor and slip (ETS)
A phenomenon where slow slip events and tectonic tremors occur in roughly the same location and timeframe. This was first discovered in 2003, and similar events have been observed in various locations around the world.
Slow slip occurring in around the source region after an earthquake. Some of the crustal deformation following the earthquake can be explained by afterslip. The slip is substantial immediately after the earthquake, decreasing time. Typically, the seismic moment of afterslip is smaller than that of the earthquake itself, although it can equal or even exceed this value in rare instances.
Slow slip occurring before the onset of an earthquake, in or around its source region. While it is theoretically presumed to happen, there are few reliable observations. It is also referred to as precursory slip.
A geophysical phenomenon similar to earthquakes. However, the term is often used to describe shaking that continues for a relatively longer duration, as opposed to the brief, abrupt characteristics of earthquakes.
Tremors occurring at plate boundaries and around major faults, believed to be induced by the relative motion of tectonic plates. They often occur simultaneously with low-frequency earthquakes, very low-frequency earthquakes, and slow slip events, and are presumed to involve similar slip motions to these phenomena. Seismic waves, marginally above the observational limit at frequencies mainly in 1-10 Hz, continue for tens of seconds, and can more intermittently continue for several hours or days.
Non-volcanic tremor (NVT)
Tremors that occur in the vicinity of volcanoes, believed to be caused by the movement of fluids such as magma and groundwater, and related deformation associated with volcanic activity. The characteristics of the tremors can significantly vary depending on the volcano and the scale or phase of the volcanic activity.
A phenomenon where tremors persist intermittently for a long period which shifting locations as they proceed. These can continue anywhere from a few minutes to several weeks. Since very low-frequency earthquakes and slow slip events are often observed simultaneously, it is believed to reflect a portion of a larger-scale deformation phenomenon.
During an earthquake, two rotational motions, each with the opposite moments (quantities representing the magnitude of rotational motion), occur around the fault. This moment is referred to as the seismic moment. Its value is determined by the product of the fault area, the average slip, and the rigidity of the materials around the fault. It represents the static size of earthquake fault movement during an earthquake.
Energy radiated from the earthquake source into the surrounding area as seismic waves, also referred to as seismically radiated energy. It represents the dynamic size of an earthquake.
Moment magnitude (Mw)
The moment magnitude Mw is defined by a linear relationship with the logarithm of the seismic moment. When the seismic moment increases by a factor of about 30, Mw increases by 1.
Predominant period (frequency) of earthquake
Seismic shaking contains waves with various period (frequency) components. The period (frequency) at which the amplitude becomes particularly large is called the predominant period (frequency) of that seismic shaking. Depending on the definition, it may be referred to in various ways, such as the corner frequency or characteristic frequency.. Typically, larger earthquakes tend to have a longer predominant period (a lower predominant frequency).
A representation of the earthquake source, simplified through a relatively straightforward mechanical system, utilized to describe the characteristics of seismic waves. Various types are recognized, including a combination of forces equivalent to fault slip motion (double couple), an expanding or contracting spherical source, and opening cracks, among others.
Release date Nov. 11, 2023