S-wave speed anisotropy
S-wave speed anisotropy refers to the phenomenon where S waves have different propagation speeds according to the orientation of oscillation (perpendicular to the orientation of propagation). Analysis of S-wave speed anisotropy using seismograms of crustal earthquakes sometimes helps to reveal crustal stress orientations. From this viewpoint, our system allows for simultaneous plots of S-wave speed anisotropy analysis results and crustal stress measurement results.
Crustal stress / Crustal stress measurement / Stress orientation
Crustal stress refers to the stress that acts on the crust. Stress measurement refers to the act of measuring crustal stresses by in-situ measurement using drilling or indoor experiments using specimens. Stress is represented by the orientations and magnitudes of three orthogonal principal axes, so crustal stress orientation usually refers to the orientation of the maximum or minimum principal axis. Quite often, stress orientation can also refer, more specifically, to the azimuth (like north, south, east or west) of the maximum or minimum horizontal principal (compressional stress) axis, which are horizontal projections of the maximum or minimum principal axis. Our system adopts the latter definition.
Active fault
An active fault refers to a fault that was active recently in geologic times (basically in the Quaternary) and continues to be active now.
Conrad / Moho
The Conrad refers to the discontinuity that separates the upper and lower parts of the crust. The Moho refers to the discontinuity that separates the lower crust and the mantle. Beneath land areas of the Japanese Islands, they are distributed at depths of approximately 15 km and 30 km, respectively.
Seismogenic layer / D10, D90
The depths of earthquakes that occur in the crust are generally restricted within a certain depth range. This depth range is called the seismogenic layer. Quantities called D10 and D90 are measures of the upper and lower limits of the seismogenic layer. They are defined as the depths above which 10% and 90%, respectively, of all crustal earthquakes occur (in terms of the accumulated number of earthquakes, counted in the direction of increasing depth from the ground surface).
Gravity anomaly
A gravity anomaly refers to the difference between the theoretical and observed values of gravity. More specifically, the theoretical value refers to the standard gravity, calculated by using the Earth ellipsoid and corrected for the influence of topographical undulations. The gravity anomaly, where the corrections are based on the assumption that nothing (= just free air) exists between the observation point and the Earth ellipsoid is called the free-air anomaly. When the corrections are based on the assumption that rocks of a homogeneous density exists between the observation point and the Earth ellipsoid, the resulting anomaly is called the Bouguer anomaly. Our system plots Bouguer anomalies for an assumed density of 2.67 g/cm3.
Pacific plate / Philippine Sea plate
According to the idea called plate tectonics, the Earth is covered by more than ten bodies of rock (lithospere), tens of kilometers to about 100 km thick, which move around and clash against one another. Every one of those rock bodies is called a plate, where a plate that covers an oceanic area and one that covers a continental area are called an oceanic plate and a continental plate, respectively. The Pacific plate is a huge oceanic plate that spreads beneath the Pacific Ocean, whereas the Philippine Sea plate is an oceanic plate, noticeably smaller than the Pacific plate, that abuts on its western end. These two oceanic plates are thought to be subducting beneath the Japanese Islands.
Quaternary volcano
A Quaternary volcano refers to a volcano that was active during the Quaternary.
Crust / Mantle
These terms refer to internal structures of the Earth. The outermost part of the Earth is called the crust, and the part underlying it is called the mantle (further inside, there are structures called the outer core and the inner core). Beneath land areas of Japan, the crust is about 30 km thick, and is divided into the upper and lower parts at a depth of about 15 km (called the upper crust and the lower crust, respectively). The mantle usually has a thickness of about 2900 km, and is divided into the upper and lower parts at a depth of about 660 km (called the upper mantle and the lower mantle, respectively).
Tomography / Grid points / conflim / dws
A technique to analyze internal structures of a target body by using signals that propagate through those structures. Broadly used in geophysical exploration and in medical practice. Seismic tomography refers to the analysis, or the output thereof, of three-dimensional distributions of subsurface physical properties using seismic waves (see "About This System"). When analyzing physical property distributions with a computer, the target domain has to be divided into a multitude of sub-domains. In our system, we refer to the points, representing the individual sub-domains, as the grid points. The conflim is an indicator of the degree of confidence of tomography data that we have defined for the purpose of plotting them in our system (see "Data"). The dws is an indicator representing how many seismic rays travel through each sub-domain division. Seismic ray paths depend on the subsurface velocity structures and the geometrical relations between the seismic source and the receiver, so the number of seismic rays traveling through a sub-domain differs depending on its location. The analysis output has relatively higher confidence in sub-domains traveled through by a large number of seismic rays than in sub-domains where seismic rays rarely travel.
Tectonic zone (Strain concentration zone)
A zone where strains due to long-term crustal movements are particularly concentrated. One defined in terms of geologic time scales (tens of thousands to tens of millions of years, or more) is called a geologic strain concentration zone, whereas one defined in terms of geodetic time scales (several years to about 100 years) is called a geodetic strain concentration zone.
Vp (Vs) / dVp (dVs)
Vp and Vs are elastic constants that represent the P-wave (and S-wave) velocities, respectively. The dVp (dVs) is a measure of the extent of deviation of the P-wave (S-wave) velocity from a given reference value, and is calculated by {(velocity)-(reference velocity)} / (reference velocity). Elastic wave speeds tend to increase with increasing depth, and so it is often more convenient to look at their deviations from reference values at corresponding depths when you examine vertical sections of tomography. Our system provides a choice between plotting Vp (Vs) and plotting dVp and dVs.
Poisson's ratio
An elastic constant defined as the ratio between the axial and transverse strains. When Vp and Vs are given, it is calculated by the formula (Poisson's ratio) = {(Vp/Vs)**2-2} / [2{(Vp/Vs)**2-1}], and our system relies on this formula to calculate and plot Poisson's ratios.