1. Earthquakes
• 1.1 Numbers of earthquakes by district and magnitude
• 1.2 Earthquake parameters
• 1.3 Earthquake data
• 1.4 Epicenter distribution
• 1.5 Nodal plane solutions for major earthquakes as determined
  from P-wave initial motion polarity
• 1.6 Centroid moment tensor solutions of major earthquakes

2. Crustal Strain
• 2.1 Hourly cumulative data from borehole volume strainmeters
• 2.2 Volume strain change
• 2.3 Hourly cumulative strain data from multi-component strainmeters
• 2.4 Linear strain change

  The table shown here lists numbers of high-precision hypocenters (table, Section 1.2.3) in and around Japan along with selected low-precision hypocenters with a seismic intensity of 1 or more in Japan by region and magnitude. The Unknown column shows the number of earthquakes whose magnitude could not be determined, and the total number of quakes felt (measuring 1 or more on JMA's seismic intensity scale) is displayed at the bottom. District names for epicenters are based on the specifications of Appendix 1.A.3.. Tremors with a "Near Japan" district designation occurred in regions from No. 311 to No. 341, including Sakhalin, the Kurile Islands, the Korean Peninsula, the Ogasawara Islands, Primorskii and Taiwan.

The earthquake parameters in this bulletin are determined by JMA and agencies in other countries.

1.2.1 Hypocenter calculation

  Hypocenters are calculated using P-wave and S-wave arrival times, and magnitudes are determined using maximum seismic wave amplitudes. Lists of seismic stations and standard seismograph response curves are shown in Appendices 1.A.1. and 1.A.2.,respectively.

Hypocenter determination

The iterative method [Hamada et al. (1983)⁽²⁾;an extension of Geiger's method [Geiger (1910)⁽¹⁾]]is used to calculate hypocenter locations. The data weighting is based on the following formula, where R denotes hypocentral distance [Ueno et al. (2002)⁽³⁾]:

For P-waves ( Wp )	Wp = Rmin 2/R 2
For S-waves ( Ws )	Ws = Wp/3
Rmin : Hypocentral distance from the station nearest the hypocenter (km)
( if Rmin ≤ 50, Rmin = 50; if Wp > 1,Wp = 1)

  Data from stations displaying large travel time residuals are not used for calculation. The depth of focus is initially calculated with no restrictions. If the solution for earthquake parameters is unstable, the best solution is sought with the epicenter fixed and the depth changed in increments of 1 km. For earthquakes located in the shallow region around the Kurile Islands (see Figure 1), the focal depth is fixed at 30 km.

   In hypocenter calculation, JMA employs pre-computed tables to determine theoretical travel times between sources and stations. These are created with the assumption of a spherically layered earth model with constant velocities (application since October 1997 outlined below).

   From October 1997 to September 2020, the JMA2001 travel time table [Ueno et al. (2002)⁽³⁾] was mainly used for theoretical travel time calculation. For earthquakes in the shallow region around the Kurile Islands, the table provided by Ichikawa (1978)⁽⁵⁾ was also used, and the Jeffreys-Bullen table (1958)⁽⁴⁾] was used for earthquakes with an epicentral distance of 2,000 km or farther from JMA's seismic network. These tables were calculated assuming a station elevation of 0 m.

   On September 1 2020, JMA began using the new ocean bottom seismometers (Seafloor observation network for earthquakes and tsunamis along the Japan Trench (referred to here as the S-net) and Dence Oceanfloor Network system for Earthquakes and Tsunamis (referred to here as the DONET2), where seismic velocity structures differ significantly from those of inland stations, in its hypocenter calculation. Four new travel time tables are now used in line with sub-station seismic velocity structures. JMA2001A is used for inland stations, JMA2020A for those within landward slopes, JMA2020B for those in the outer-rise region of the Japan Trench, and JMA2020C for those in the Nankai Trough area.
   The use of these tables in hypocenter calculation enables determination of theoretical travel times between sources and stations at elevations from -4,000 to 3,900 m on land, and from -8,000 to 0 m on the sea bed. For stations with epicentral distances greater than 2,000 km, the time table used until 31 August 2020 continues to be applied(see travel time tables and seismic velocity structure for data).

   At OBS stations, observed travel times are behind those based on velocity structures due to an unconsolidated sedimentary layer (P-wave velocity=1.8 km/s, P-wave velocity/S-wave velocity=3.0). JMA uses the procedure reported by Shinohara et al.(2012)⁽¹⁹⁾ to determine station correction for travel times at each station, and applies these corrections in hypocenter calculation (see station correction list file for data).

Magnitude determination

The maximum amplitude is defined as half of the maximum peak-to-peak amplitude about i～iii.:

1. Local Meteorological Office magnitude M_J
   This is calculated for large shallow (H ≤ 60km)earthquakes using acceleration data from Local Meteorological Office seismic intensity meters.
   M_J is given as the average of
   
      M_J^OBS=1/2log(A_N²+A_E ²)+1.73logΔ−0.83
   
   using the maximum displacement amplitudes recorded at these offices [Tsuboi (1954)⁽⁷⁾]. Here, acceleration data are integrated twice to determine displacement, to which a high-pass (6 s) filter is applied for mechanical strong-motion seismogram simulation.
   This method is provisionally used until the independence of C_D in Eq.(ii) regarding the magnitude of large earthquakes (especially for M7 class or above) is confirmed.


2. Displacement Magnitude M_D
   This is calculated as the average of
   
      M_D^ST=1/2log(A_N²+A_E ²)+β_D(Δ, H)+C_D
   
   over stations in the ranges of R ≥ 30km and Δ≤ 700km that record maximum amplitudes of horizontal displacement [Katsumata (2004)⁽⁸⁾]. For earthquakes located in the region of Figure 2, however, stations in the ranges of R ≥ 30km and Δ≤ 2000km are always used. If the average involves fewer than three stations, Δ is extended up to 2,000km. If only two stations are used to determine M_D, the notation is M_d.


3. Velocity Magnitude M_V
   This is calculated as the average of
   
      M_V^ST=α logA_Z+β_V (Δ, H)+C_V
   
   over stations in the ranges of R >= 5km and R <= 400km (with S-net, R >= 50km and R <= 150km) that record maximum amplitudes of velocity in the vertical component [Funasaki et al. (2004)⁽⁹⁾]. With S-net, the maximum amplitudes of velocity is a composite of the maximum amplitude of the X, Y and Z components. If the average involves fewer than four stations, Δ is extended to 1,000km. If only two or three stations are used to determine M_V, the notation is M_v.


4. Moment magnitude M_w
   M_w is given by the results of Centroid Moment Tensor Solutions

  In the averaging procedure for ⅰ, ⅱ and ⅲ, the initial mean of magnitudes at all stations is first calculated. The mean and standard deviation of magnitudes for the stations are then calculated, with values deviating from the initial mean by more than 0.5 discarded. The mean is adopted as the magnitude only if the standard deviation is less than 0.35.

  The meanings of the symbols used in the above formulas are as follows:

H	: Focal depth (km)
Δ	: Epicentral distance (km)
R	: Hypocentral distance(km)
α	: Constant 1/0.85=1.176
βD, βV	: Terms showing dependence on Δ and H (see Figure 3 and 4)
CD	: Correction value (= 0.2) used for D93-type seismometers
CV	: Correction value depending on seismometer types (see Table 1)
AN, AE	: Maximum displacement amplitude in the horizontal component of D93-type seismometers. The unit is micrometers = 10−6 m.
ASX, ASY, AZY	: Maximum velocity amplitude of the X, Y, and Z observed at the S-net stations The unit is 10−5 m/s.
AZ	: Maximum velocity amplitude in the vertical component of EMT-, EMT76- or E93-type seismometers. The unit is 10−5 m/s.

Table 1: Correction values estimated for individual seismometer types (C_V)

Seismometer type	Value	Seismometer type	Value
JMA-93 type, velocity sensor, remote, surface installation	0.00	JMA velocity sensor, installed in volcanic region	1.13
JMA OBS, velocity sensor	0.47	Other JMA seismometer, velocity sensor	−0.18
JMA-67 type, velocity sensor with logarithmic-amplifier, remote	−0.02	High-sensitivity seismometer, velocity sensor, bore-hole installation	0.43
JMA velocity sensor, local, surface installation	−0.02	Other organization, velocity sensor, ground installation	0.12
JMA velocity sensor, local, buried	0.50	Other organization, velocity sensor, horizontal-tunnel installation	0.30
JMA velocity sensor with logarithmic-amplifier, local, buried	−0.11	Other organization, velocity sensor, bore-hole installation	0.48
JMA velocity sensor, remote, surface installation (except 93 type)	0.03	Other organization OBS, velocity sensor(except S-net)	0.11
S-net, velocity sensor below the sea bed	0.51	S-net, velocity sensor on the sea bed	0.27
JMA velocity sensor, remote, buried	0.29

The Matched Filter (MF) method [Moriwaki (2017)⁽¹⁸⁾] was introduced in March 22 2018 to enable the detection of deep low-frequency earthquakes occurring on the plate boundary along the Nankai Trough. Automatic hypocenter determination using this approach is based on application of the four-dimensional grid search method near the detected event. If the hypocenter is determined to be at the edge of a grid space, the template event hypocenter is adopted.
Magnitude is calculated as the average of
M_v=M^template+αlog(A_z/A_z^template)

Here:

α	:Constant 1/0.85=1.176
Mtemplate	:Magnitudes of the template for low-frequency earthquakes
Az,Aztemplate	:Maximum velocity amplitude in the vertical component of detected and template low-frequency earthquakes

The notation of the magnitude determined is M_v regardless of the number of stations used for determination.

1.2.2 Hypocenter determination

  A new auto hypocenter detection approach (the PF method; [Tamaribuchi et al. (2016)⁽¹⁷⁾]) was adopted in April 2016, and three new hypocenter categories were introduced.

1. Fully checked hypocenter
   In this category, full checking of earthquake locations and automatic phases, along with manual picking are performed by staff for earthquakes with a magnitude exceeding the threshold (Mth). This is the same as the method used until March 2016.
2. Simply checked hypocenter
   In this category, simple checking of earthquake locations and automatic phases, along with some manual picking are performed by staff for earthquakes with a magnitude not exceeding the threshold (Mth).
3. Automatic hypocenter
   In this category, simple checking of earthquake locations and automatic phases is performed, without manual picking by staff for earthquakes with a magnitude not exceeding the threshold (Mth).

  The distribution of Mth values is shown Figure 5. The number is 1.7 on and near land, increasing to 3.5 with distance from land. For earthquakes occurring deeper than 150 km, Mth increases as shown in Table 2. For the prediction of the Tokai Earthquake, Mth for the Tokai area is smaller than conventionally set in other areas for all depths, as shown Figure 6. However, based on examination to determine the precision of results from automatic processing using the PF method, Mth values were unified on March 22, 2018.
The hypocenter determined using the MF method is always classified as C as shown above regardless of Mth values.

   If more than 40 stations report observation in fully checked hypocenter classification, the 40 nearest the focus are used for calculation (as of October 1997) [Earthquake Prediction Information Division, Seismology and Volcanology Department, Japan Meteorological Agency (1998)⁽⁶⁾]. Stations are selected as outlined below.

1. First, the characteristic distance (Δ_lim) is defined using following empirical equation:
   
   Δ_lim=Δ₃²/100+Depth+100
   
   Here, Δ₃ denotes the epicentral distance of the third-nearest station to the epicenter, and Depth is the focal depth. The unit is kilometers.
2. Second, the 16 stations nearest the epicenter are selected. Next, 24 stations within Δ_lim are selected based on their epicentral distances and station quality( given as an attribute constant involving seven classes including the S/N ratio, data transmission capability and average residual travel time). If the number of selected stations are still insufficient for calculation ( in the case of an offshore or deep earthquake,for example) stations outside Δ_lim may be selected until an appropriate number is reached.

  In simply checked hypocenter classification, all available stations are used for calculation.

  In automatic hypocenter classification, the selection applied in fully checked hypocenter categorization is performed only with the PF method.

1.2.3 Hypocenter classification

  The calculated earthquake parameters are categorized into classes K, S, k, s, A and a based on their accuracy and determination methods.

Hypocenter classification and determination

	High-precision hypocenters		Low-precision hypocenters
Over Mth	K (fully checked)		S (fully checked)
Less than Mth	k (simply cheked)	A (Auto)	s (simply cheked)	a (Auto)

  Full checking is performed for earthquakes with a seismic intensity of 1 or more in Japan, and otherwise where necessary.

  The hypocenter classification standards are outlined below. The inland region is defined as shown in Figure 1, and the general region is defined as all areas except the inland region and the region around the Kurile Islands.The hypocenter determined using the MF method is classified as "a (Auto)" as shown above regardless of accuracy.

The standards for the hypocenter classification

High-precision hypocenters (K, k, A)
Region	Inland	General	Region around the Kurile Islands
Origin time error	Less than 0.5 sec	Less than 1.0 sec	Less than 1.5 sec
Epicenter latitude/longitude estimation error	Less than 3.0 min	Less than 5.0 min	Less than 10.0 min

Low-precision hypocenters (S, s, a)
Region	Inland	General	Region around the Kurile Islands
Origin time error	0.5 – 2.0 sec	1.0 – 2.0 sec	1.5 – 2.0 sec
Epicenter latitude/longitude estimation error	3.0 – 10.0 min	5.0 – 10.0 min	10.0 – 15.0 min

1.2.4 Standards for posting

  This bulletin lists class-K, k and A hypocenters only.

1. Origin times are given in hours, minutes and seconds, and origin time errors are given in seconds. Epicenters are given in degrees and minutes, and epicentral errors are given in minutes. Depth and depth errors are given in kilometers.
2. If the focal depth is determined with no restrictions, the focal depth error is also given. If the hypocenter is calculated with the focal depth fixed, "F" is appended to the focal depth value.
3. Magnitude is adopted as the JMA magnitude based on the priority order M_J>M_D>M_V>M_d>M_v, and is generally noted as "MAGNITUDE1". The moment magnitude is generally noted as "MAGNITUDE2" when centroid moment tensor (CMT) solutions are determined. Magnitude types are indicated with the symbols J, D, V, d, v or W. The magnitude columns are left blank if no values are determined.
4. Maximum seismic intensity
   The maximum seismic intensity on the JMA scale as observed by seismic intensity meters is denoted in column C. A, B, C and D represent five lower, five upper, six lower and six upper, respectively(See 1.A.4 for the JMA seismic intensity scale). Column C is left blank if the maximum seismic intensity is below one.
5. Epicenter region name
   The district number, region number and name for the epicenter based on its latitude and longitude are given in Appendix 1.A.3 Geographical region names 1 – 8. The regions are shown in the geographical region figures (1) – (4).
6. Distant earthquakes
   If seismic waves from a distant earthquake that cannot be located by JMA's network are observed, information from other bodies such as the United States Geological Survey (USGS) is carried. The notation "USGS" is placed at the end of the line in such cases. The m_b (body wave magnitude) value calculated by other organizations such as USGS is generally given as the magnitude, but M_s (surface wave magnitude) or M_w (moment magnitude) values calculated by other organizations or the M_w value based on JMA's centroid moment tensor solution may be given in some cases.
7. Focal mechanism solutions
   The notation "NPS" denotes that a focal mechanism solution has been determined. The solution parameters for the solution are given in 1.3 Earthquake data, and a graphical representation of solutions based on equal-area projection onto the lower hemisphere is shown in the figure 1.5 Nodal plane solutions for major earthquakes as determined from P-wave initial motion polarity and the figure 1.6 Centroid moment tensor solutions for major earthquakes.

  Earthquake^* parameters and data on wave arrival at the seismic stations of JMA and other organizations** are listed.

* Earthquakes included on this list meet at least one of the following conditions:
— JMA calculates the hypocenter and magnitude
— USGS calculates the hypocenter and mb or Ms as about 6 or greater, or the tremor is felt at a Japanese station (a JMA seismic intensity of 1 or larger).

** National Research Institute for Earth Science and Disaster Prevention, Hokkaido Univ., Hirosaki Univ., Tohoku Univ., The Univ. of Tokyo, Nagoya Univ., Kyoto Univ., Kochi Univ., Kyushu Univ., Kagoshima Univ., the National Institute of Advanced Industrial Science and Technology, the Geospatial Information Authority of Japan, the Japan Agency for Marine-Earth Science and Technology, Association for the Development of Earthquake Prediction, Aomori Pref., the Tokyo Metropolitan Government, Shizuoka Pref., the Hot Springs Research Institute of Kanagawa Prefecture, Incorporated Research Institutions for Seismology

  Estimated focal parameters are shown at the top of the field assigned to each event. For hypocenters with a fixed focal depth, the symbol (F) is shown after the focal depth value. Focal data with large estimation errors are reported as "POOR SOLUTION" and are excluded from 1.2 Earthquake parameters. For hypocenters determined using the MF method, the notation "MF" is placed in the first line. Template event information is also shown below the focal parameters. For distant earthquakes, parameters provided by other organizations such as USGS are included in the table. The notation "AFTER USGS" is placed in the first line in such cases. However, the M_w value based on JMA's centroid moment tensor solution may alternatively be given as the magnitude. Arrival parameters observed at each station are listed in order of epicentral distance. Arrival data on earthquakes whose focal parameters cannot be determined are given in order of the observed arrival time.

  For earthquakes whose nodal plane solution or CMT solution is determined, information on the solution is shown below the focal parameters.

  Any special remarks or extra information on matters such as damage and/or tsunami heights are included under "REMARK."

1. Station name (Station column)
   Station codes are shown in Appendix 1.A.1 Seismic station list as JMA codes.
2. Phase name (PHA column)
   Impulsive or emergent onsets of individual phases are represented by I or E at the beginning of the phase name (e.g., IP (impulsive P-wave) or ES (emergent S-wave)). The phase names APP and ASS in the PHA column represent pP and sS, respectively. The phase name M denotes data containing only the maximum displacement amplitude.
3. Residual (RES column)
   The residual is the difference between observed (O) and calculated (C) travel times (O−C).
4. Maximum amplitude (N-S AMP, E-W AMP and U-D AMP columns)
   1. If the maximum velocity amplitude of the vertical component is observed, its value is shown with the period of the wave portion under U-D AMP. The units for the maximum amplitude and the period are 10⁻⁵ m/s and seconds, respectively. O.S. stands for off-scale, which represents a saturated record.
      The maximum velocity amplitude value and the period of the wave portion under N-S and E-W AMP are observed using an automatic processing system, and attention should be paid to these values in actual use.
   2. If maximum displacement amplitudes of horizontal components from D93 seismometers are observed, the maximum amplitude value is shown with the period of the wave portion under N-S and E-W AMP and marked with an asterisk. The units for the maximum amplitude and the period are 10⁻⁶ m and seconds, respectively. The maximum amplitude is defined as a half of the maximum peak-to-peak amplitude on the record divided by the standard magnification.
5. Epicentral distance (Δ column)
   Epicentral distance is calculated using the following formula:
   
   cos th=sin ph_E sin ph_S+cos ph_E cos ph _S cos(l_E −l_S)

   where
   	th:	Epicentral angular distance (in radian)
   	phE lE:	Geocentric latitude and longitude of epicenter
   	phS lS:	Geocentric latitude and longitude of station

   
   Δ =r th

   where
   	Δ:	Epicentral distance (km)
   	r:	Average radius of the earth (6,371.009km)

6. Azimuth (AZM column)
   The azimuth from an epicenter to a station is measured clockwise from the local north.
7. Magnitude (MAG column)
   Magnitudes calculated at each station are listed as station magnitudes. The calculation method is outlined in Section 1.2.3.
8. Residual of magnitude (MRES column)
   The residual of magnitude is the difference between the station magnitude shown in the MAG column and the magnitude of the earthquake.
   

  For data on events detected using the MF method, column details are as follows:

1. Station name (Station column)
2. Phase name (PHA column)
3. Head time of the time window (TIME column)
4. Correration coefficient (N-S COR, E-W COR, U-D COR column)
5. Maximum amplitude (N-S AMP, E-W AMP,U-D AMP column)
6. Epicentral distance (DELTA column)
7. Azimuth (AZM column)

  Monthly and annual epicenter distributions are shown in the figures titled "Earthquake epicenters in Japan and adjacent regions" and "Regional distribution of earthquakes." Annual distributions of epicenters within certain depth layers are also shown.

  Nodal plane solutions for major earthquakes are shown using lower-hemisphere equal-area projection. The values of their solution parameters, the strike (STR), dip (DIP) and slip-direction (SLIP) of nodal planes (NP1, NP2), and the azimuth (AZM) and plunge (PLG) of the P-, T- and N-axes are shown under each figure. The strike of each plane and the azimuth of each axis are measured clockwise from north. The dip of each plane and the plunge of each axis are measured downward from the horizontal plane. Slip signifies the motion of a hanging wall relative to a foot wall, and its direction is measured counterclockwise from the strike direction. Nodal plane solutions are determined using the method of Nakamura and Mochizuki(1998)⁽¹⁰⁾, and take-off angles are calculated based on the velocity structure of Ueno et al.(2002)⁽³⁾ Since September 2020, the take-off angle at OBS stations has been calculated using JMA2020A, JMA2020B and JMA2020C (see take-off angle table and seismic velocity structure).

  CMT solutions for major earthquakes are shown using lower-hemisphere equal-area projection. The CMT solution determination method is based on Kawakatsu (1989)⁽¹¹⁾. For further details, see Nakamura et al. (2003)⁽¹²⁾.

  Information on each CMT solution is shown above and below the figure as follows:

1st line	: Origin time (JST)
2nd line	: Region name
Hypo.	: Location (latitude, longitude, depth) of hypocenter
Cent.	: Location (latitude, longitude, depth) of centroid
Δ t	: Centroid time minus origin time (sec.)
Mo	: Scalar momentt
Mw	: Moment magnitude
Mj	: JMA magnitude
mrr,mtt,mff,mrt,mrf,mtf	: Moment tensor components
STR	: Strike of nodal planes (NP1, NP2)
DIP	: Dip angle of nodal planes (NP1, NP2)
SLIP	: Slip angle of nodal planes (NP1, NP2)
MOM	: Moment tensor component of P-, T- and N-axes
AZM	: Azimuth of P-, T- and N-axes
PLG	: Plunge of P-, T- and N-axes
V.R.	: Variance reduction
ε	: Non-double couple component ratio
N	: Number of stations used
COMP	: Number of components used

  The moment tensor is expressed by the spherical polar coordinates (r, t, f), which represent upward, southward and eastward positive respectively.

  The strike of each nodal plane and the azimuth of each axis are measured clockwise from north. The dip of each nodal plane and the plunge of each axis are measured downward from the horizontal plane. Slip signifies the motion of a hanging wall relative to a foot wall, and its direction is measured counterclockwise from the strike direction.

  The data consist of hourly means of strain values. Positive and negative values correspond to crustal expansion and contraction, respectively, relative to the value at the time when observation was started.
  An asterisk indicates that the value has been modified from the original. Some values are accompanied by one of the following:

LOSS	Loss of data
V.O.	Valve Open
I	Instrumental adjustment
P	Power failure
O	Others

1. Hourly data on crustal strain
   Changes in hourly mean crustal strain data are shown. ATM means hourly atmospheric pressure, and PREC means precipitation per hour.
2. Secular crustal strain variations and annual crustal strain variations
   Daily mean strain data for the last seven years and for one year are shown. ATM means daily mean atmospheric pressure, and PREC means daily precipitation.
3. The effects of atmospheric pressure change are removed from the data [Hikawa et al. (1983)⁽¹⁶⁾].
4. Upward and downward directions represent crustal expansion and contraction, respectively.

  The data consist of hourly means of linear strain for each component. Positive and negative values correspond to crustal expansion and contraction, respectively, relative to the value at the time when observation was started.
  An asterisk indicates that the value has been modified from the original. Some values are accompanied by one of the following:

LOSS	Loss of data
Z.S.	Zero Shift
I	Instrumental adjustment
P	Power failure
O	Others

1. Hourly data on crustal strain
   Changes in hourly mean crustal strain data are shown. ATM means hourly atmospheric pressure, and PREC means precipitation per hour.
2. Annual crustal strain variations
   Daily mean strain data for the last year are shown. ATM means daily mean atmospheric pressure, and PREC means daily precipitation.
3. Upward and downward directions represent crustal expansion and contraction, respectively.

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