This section provides a centralized description of the SAC file format.

The official and up-to-date documentation for the SAC-file format is available from the EarthScope Consortium (formerly IRIS/UNAVCO) here. The following subsections constitute my notes on the format. Below is a quick guide: all credit for the creation of, and documentation for, the SAC file-format belongs to its developers and maintainers (details here).

Floating-point (39)

32-bit (1 word, 4 bytes)

depmin

Pre-data word 001.

Minimum value of the dependent variable (displacement/velocity/acceleration/volts/counts).

depmen

Pre-data word 057.

Mean value of the dependent variable.

depmax

Pre-data word 002.

Maximum value of the dependent variable.

odelta

Pre-data word 004.

Modified (observational) value of delta.

resp(0–9)

Pre-data words 021–030.

Instrument response parameters (poles, zeros, and a constant).

Not used by SAC they're free for other purposes.

stel

Pre-data word 033.

Station elevation in meters above sea level (m.a.s.l).

Not used by SAC free for other purposes.

stdp

Pre-data word 034.

Station depth in meters below surface (borehole/buried vault).

Not used by SAC free for other purposes.

evel

Pre-data word 037.

Event elevation m.a.s.l.

Not used by SAC free for other purposes.

evdp

Pre-data word 038.

Event depth in kilometers (previously meters) below surface.

mag

Pre-data word 039.

Event magnitude.

user(0–9)

Pre-data words 040–049.

Storage for user-defined values.

dist

Pre-data word 050.

Station-Event distance in kilometers.

az

Pre-data word 051.

Azimuth $\color{orange}\mathrm{\left(Event \to Station\right)}$ , decimal degrees from North.

baz

Pre-data word 052.

Back-azimuth $\color{orange}\mathrm{\left(Station \to Event\right)}$ , decimal degrees from North.

gcarc

Pre-data word 053.

Station-Event great circle arc-length, decimal degrees.

cmpaz

Pre-data word 057.

Instrument measurement azimuth, decimal degrees from North.

Value	Direction
0°	North
90°	East
180°	South
270°	West
Other	1/2/3

cmpinc

Pre-data word 058.

Instrument measurement incident angle, decimal degrees from upward vertical (incident 0° = dip -90°).

Value	Direction
0°	Up
90°	Horizontal
180°	Down
270°	Horizontal

NOTE: SEED/MINISEED use dip angle, decimal degrees down from horizontal (dip 0° = incident 90°).

xminimum

Pre-data word 059.

Spectral-only equivalent of depmin ( $\color{orange}f_{0}$ or $\color{orange}\omega_{0}$ ).

xmaximum

Pre-data word 060.

Spectral-only equivalent of depmax ( $\color{orange}f_{max}$ or $\color{orange}\omega_{max}$ ).

yminimum

Pre-data word 061.

Spectral-only equivalent of b.

ymaximum

Pre-data word 062.

Spectral-only equivalent of e.

Double (22)

64-bit (2 words, 8 bytes)

NOTE: in the header section these are floats; they're doubles in the footer section of v7 SAC-files. In memory they're stored as doubles regardless of the SAC-file version.

delta

Pre-data word 000, post-data words 00-01.

Increment between evenly spaced samples ( $\color{orange}\Delta t$ for timeseries, $\color{orange}\Delta f$ or $\color{orange}\Delta\omega$ for spectra).

b

Pre-data word 005, post-data words 02-03.

First value (begin) of independent variable ( $\color{orange}t_{0}$ ).

e

Pre-data word 006, post-data words 04-05.

Final value (end) of independent variable ( $\color{orange}t_{max}$ ).

o

Pre-data word 007, post-data words 06-07.

Event origin time, in seconds relative to the reference time.

a

Pre-data word 008, post-data words 08-09.

Event first arrival time, in seconds relative to the reference time.

t(0–9)

Pre-data words 010–019, post-data words 10–29.

User defined time values, in seconds relative to the reference time.

f

Pre-data word 020, post-data words 30-31.

Event end (fini) time, in seconds relative to the reference time.

stla

Pre-data word 031, post-data words 36-37.

Station latitude in decimal degrees, N/S - positive/negative.

sac-format automatically enforces $\color{orange}\mathrm{stla}\in[-90, 90]$ .

stlo

Pre-data word 032, post-data words 38-39.

Station longitude in decimal degrees, E/W - positive/negative.

sac-format automatically enforces $\color{orange}\mathrm{stlo}\in[-180, 180]$ .

evla

Pre-data word 035, post-data words 32-33.

Event latitude in decimal degrees, N/S - positive/negative.

sac-format automatically enforces $\color{orange}\mathrm{evla}\in[-90, 90]$ .

evlo

Pre-data word 036, post-data words 34-35.

Event longitude in decimal degrees, E/W - positive/negative.

sac-format automatically enforces $\color{orange}\mathrm{evlo}\in[-180, 180]$ .

sb

Pre-data word 054, post-data words 40-41.

Original (saved) b value.

sdelta

Pre-data word 055, post-data words 42-43.

Original (saved) delta value.

Integer (26)

32-bit (1 word, 4 bytes)

nzyear

Pre-data word 070.

Reference time GMT year.

nzjday

Pre-data word 071.

Reference time GMT day-of-year (often called Julian Date) (1–366).

nzhour

Pre-data word 072.

Reference time GMT hour (0–23).

nzmin

Pre-data word 073.

Reference time GMT minute (0–59).

nzsec

Pre-data word 074.

Reference time GMT second (0–59).

nzmsec

Pre-data word 075.

Reference time GMT Millisecond (0–999).

nvhdr

Pre-data word 076.

SAC-file version.

Version	Description
`v7`	Footer (2020+, sac 102.0+)
`v6`	No footer (pre-2020, sac 101.6a-)

norid

Pre-data word 077.

Origin ID.

nevid

Pre-data word 078.

Event ID.

npts

Pre-data word 079.

Number of points in data.

nsnpts

Pre-data word 080.

Original (saved) npts.

nwfid

Pre-data word 081.

Waveform ID.

nxsize

Pre-data word 082.

Spectral-only equivalent of npts (length of spectrum).

nysize

Pre-data word 083.

Spectral-only, width of spectrum.

iftype

Pre-data word 085.

File type.

Value	Type	Description
01	ITIME	Time-series
02	IRLIM	Spectral (real/imaginary)
03	IAMPH	Spectral (amplitude/phase)
04	IXY	General XY file
??	IXYZ∗	General XYZ file

∗Value not listed in the standard.

idep

Pre-data word 086.

Dependent variable type.

Value	Type	Description
05	IUNKN	Unknown
06	IDISP	Displacement (nm)
07	IVEL	Velocity $\color{orange}\mathrm{\left(\frac{nm}{s}\right)}$
08	IACC	Acceleration $\color{orange}\mathrm{\left(\frac{nm}{s^{2}}\right)}$
50	IVOLTS	Velocity (volts)

iztype

Pre-data word 087.

Reference time equivalent.

Value	Type	Description
05	IUNKN	Unknown
09	IB	Recording start time
10	IDAY	Midnight reference GMT day
11	IO	Event origin time
12	IA	First arrival time
13-22	IT(0-9)	User defined time (t) pick

iinst

Pre-data word 089.

Recording instrument type.

Not used by SAC: free for other purposes.

istreg

Pre-data word 090.

Station geographic region.

Not used by SAC: free for other purposes.

ievreg

Pre-data word 091.

Event geographic region.

Not used by SAC: free for other purposes.

ievtyp

Pre-data word 092.

Event type.

Value	Type	Description
05	IUNKN	Unknown
11	IO	Other source of known origin
37	INUCL	Nuclear
38	IPREN	Nuclear pre-shot
39	IPOSTN	Nuclear post-shot
40	IQUAKE	Earthquake
41	IPREQ	Foreshock
42	IPOSTQ	Aftershock
43	ICHEM	Chemical explosion
44	IOTHER	Other
72	IQB	Quarry/mine blast: confirmed by quarry/mine
73	IQB1	Quarry/mine blast: designed shot info-ripple fired
74	IQB2	Quarry/mine blast: observed shot info-ripple fired
75	IQBX	Quarry/mine blast: single shot
76	IQMT	Quarry/mining induced events: tremor and rockbursts
77	IEQ	Earthquake
78	IEQ1	Earthquake in a swarm or in an aftershock sequence
79	IEQ2	Felt earthquake
80	IME	Marine explosion
81	IEX	Other explosion
82	INU	Nuclear explosion
83	INC	Nuclear cavity collapse
85	IL	Local event of unknown origin
86	IR	Region event of unknown origin
87	IT	Teleseismic event of unknown origin
88	IU	Undetermined/conflicting information

iqual

Pre-data word 093.

Quality of data.

Value	Type	Description
44	IOTHER	Other
45	IGOOD	Good
46	IGLCH	Glitches
47	IDROP	Dropouts
48	ILOWSN	Low signal-to-noise ratio

Not used by SAC: free for other purposes.

isynth

Pre-data word 094.

Synthetic data flag.

Value	Type	Description
49	IRLDATA	Real data
XX	∗	Synthetic

∗Values and types not listed in the standard.

imagtyp

Pre-data word 095.

Magnitude type.

Value	Type	Description
52	IMB	Body-wave magnitude ( $\color{orange}M_{b}$ )
53	IMS	Surface-wave magnitude ( $\color{orange}M_{s}$ )
54	IML	Local magnitude ( $\color{orange}M_{l}$ )
55	IMW	Moment magnitude ( $\color{orange}M_{w}$ )
56	IMD	Duration magnitude ( $\color{orange}M_{d}$ )
57	IMX	User-defined magnitude ( $\color{orange}M_{x}$ )

imagsrc

Pre-data word 096.

Source of magnitude information.

Value	Type	Description
58	INEIC	National Earthquake Information Center
61	IPDE	Preliminary Determination of Epicenter
62	IISC	Internation Seismological Centre
63	IREB	Reviewed Event Bulletin
64	IUSGS	U.S. Geological Survey
65	IBRK	UC Berkeley
66	ICALTECH	California Institute of Technology
67	ILLNL	Lawrence Livermore National Laboratory
68	IEVLOC	Event location (computer program)
69	IJSOP	Joint Seismic Observation Program
70	IUSER	The user
71	IUNKNOWN	Unknown

ibody

Pre-data word 097.

Body/spheroid definition used to calculate distances.

Value	Type	Name	Semi-major axis (a [m])	Inverse Flattening (f)
-12345	UNDEF	Earth (Historic)	6378160.0	0.00335293
98	ISUN	Sun	696000000.0	8.189e-6
99	IMERCURY	Mercury	2439700.0	0.0
100	IVENUS	Venus	6051800.0	0.0
101	IEARTH	Earth (WGS84)	6378137.0	0.0033528106647474805
102	IMOON	Moon	1737400.0	0.0
103	IMARS	Mars	3396190.0	0.005886007555525457

Boolean (4)

Pre-data word 105.

32-bit (1 word, 4 bytes) in-file/8-bit (1 byte) in-memory

leven

Pre-data word 106.

REQUIRED Evenly-spaced data flag.

If true, then data is evenly spaced.

lpspol

Pre-data word 107.

Station polarity flag.

If true, then station has positive-polarity; it follows the left-hand convention (for example, North-East-Up [NEZ]).

lovrok

Pre-data word 108.

File overwrite flag.

If true, then it's okay to overwrite the file.

lcalda

Pre-data word 109.

Calculate geometry flag.

If true, then calculate dist, az, baz, and gcarc from stla, stlo, evla, and evlo.

String (23)

32/64-bit (2/4 words, 8/16 bytes, 8/16 characters)

kstnm

Pre-data words 110–111.

Station name.

kevnm

Pre-data words 112–115.

Event name.

∗This is the only four word (16 character) string.

khole

Pre-data words 116–117.

Nuclear: Hole identifier.

Other: Location identifier (LOCID).

ko

Pre-data words 118–119.

Text for o.

ka

Pre-data words 120–121.

Text for a.

kt(0–9)

Pre-data words 112–141.

Text for t(0--9).

kf

Pre-data words 142–143.

Text for f.

kuser(0–2)

Pre-data words 144–149.

Text for the first three of user(0--9).

kcmpnm

Pre-data words 150–151.

Component name.

knetwk

Pre-data words 152–153.

Network name.

kdatrd

Pre-data words 154–155.

Date the data was read onto a computer.

kinst

Pre-data words 156–157.

Text for iinst.

Data (2)

32-bit (2 words, 8 bytes) in-file/64-bit (4 words, 16 bytes) in-memory

Stored as floating-point (32-bit) values in SAC-files; stored as double-precision in memory.

data1

Words 158–(158 + npts)

The first data vector—**always** present in a SAC-file and begins at word 158.

data2

Words (158 + 1 + npts)–(159 + (2 * npts))

The second data vector—**conditionally** present and begins after data1.

Required if leven is false, or if iftype is spectral/XY/XYZ.