NCEDC Catalogs snapshot available on Github

June 6th, 2017

The NCEDC is pleased to announce the availability of daily snapshots for the earthquake parameters catalog of the Northern California Seismic System through GitHub. This means that at any time the catalog can be retrieved as it was on a given date. The catalog repository is available at:

https://github.com/NCEDC/NCSS-catalogs

The files contain information from 1966 to present and are available in the EHP CSV format. For documentation on the format, please go to:

https://earthquake.usgs.gov/earthquakes/feed/v1.0/csv.php

A current snapshot of these files can also be found at:

http://ncedc.org/ftp/pub/catalogs/NCSS-catalogs/

In the GitHub repository, a file 'index.txt' is provided that contains the mapping between the commit times and the hash keys. It can be used to retrieve a version of the catalog at a given time.

How to use the index file

This file contains entries similar to the following:

...
NCSS catalogs as of 2017-03-13 01:00:02 --> 4f3c8891402c14c260350afcde15f55d46f36974
NCSS catalogs as of 2017-03-12 01:00:01 --> 84e6697c9f7f482cc47751fada331dc1b360d4b3
NCSS catalogs as of 2017-03-11 01:00:01 --> 5a14aebedc36c05339d511ae9ae6190c37ff73b4
NCSS catalogs as of 2017-03-10 01:00:02 --> d1a3f36bd71b04dc175c7711409c5b9c2fc20454
...

For example, a snapshot of the catalog on 2017/03/11 13:00:00 would be available at:

https://github.com/NCEDC/NCSS-catalogs/tree/5a14aebedc36c05339d511ae9ae6190c37ff73b4

Specific snapshots are available at:

https://github.com/NCEDC/NCSS-catalogs/tree/<HASH_KEY>

The NCEDC is a joint project of the University of California, Berkeley and the USGS. Please send us email at ncedcinfo@ncedc.org or fill out our comments form if you have questions or comments about these changes.

FISSURES DHI services to be retired (Jan 2017)

January 6th, 2017

The NCEDC provides a number of web services that provide improved functionality formerly provided by the FISSURES Data Handling Interface (DHI) system. FISSURES DHI, initially developed by IRIS, defined a client/server system for querying and delivering station and channel metadata as well as time series data. IRIS and all other data centers have already retired their FISSURES DHI services.

The FDSN-compatible and NCEDC web services are available at
http://service.ncedc.org

The NCEDC will shut down its FISSURES DHI servers and primary DHI name server for DHI server lookups on Fri Jan 20 2017. Please contact the NCEDC at ncedcinfo@ncedc.org if you have any questions.

NCSS monitoring returned to normal after Butte and Valley fires

November 19th, 2015

November 19th, 2015

The dataflow into the Northern California Seismic Network (NCSN, netowrk code NC) has nearly returned to normal, following significant infrastructure damage from the Valley and Butte fires.



This figure shows the distribution of magnitudes as a function of time in the NCSS Geysers polygon. Note the apparent decrease in earthquakes with magnitudes between 0 and 0.5 during the interval that no BG data was being received. (Click to view larger.)

The Valley fire affected stations in and around the Geysers. Data from the 32 stations of the Lawrence Berkeley Lab (LBL) Geysers network (network code BG) are collected at the Geysers Administration Center (GAC). The link from the GAC to Menlo Park was re-established Nov 3rd at 1900 UTC. The NCSN is once again receiving data from 24 BG stations. The 8 remaining LBL stations are operating, but communications from the stations on Socrates Rd have not been re-established to the GAC. Telemetry links from NCSN stations NC.GBG and NC.GBR were restored in late Sept, while sites NP.COB, NP.ADSP, and NP.ADS2 are remain down. Discussions are underway with Calpine and LBL about restorating strong motion monitoring.

Stations in the Sierra Foothills were affected by the Butte fire. EMP staff worked with the Army Core of Engineers (ACOE) and other agencies to restore communications at the Sierra Vista microwave site on Oct 29th, 2304 UTC. Since then, data are flow in from stations ABJ, AOH, BMS, BRM, HSL, MBE, MBU, MCUB, MHD, MMI, MMT, MPR, MRH, MYL, PAR, PDR, PHB, PJC, PJU, PKE, and PWM.

The NCSS now reports earthquake depth relative to the geoid (sea level)

October 7th, 2015

Starting 2015/10/07, the Northern California Seismic System (NCSS) is reporting earthquake depths relative to the earth geoid surface (which is approximately mean sea level) instead of the previously used surface of the velocity model used to locate the events. Geoid depths have been determined for all events in the NCSS database from 1966 to the present. (Learn more about Geoid FAQ) Requests for event information from the NCSS catalog will now return depths with respect to the geoid surface. This change is to comply with recently established USGS Advanced National Seismic Network (ANSS) convention regarding event depth reporting. Additional concurrent changes made to the NCSS earthquake information database are also described below.



The offset between the geoid and model depths is the "depth datum." The depth datum is normally the average elevation (above the geoid) of the nearest five stations used to locate the earthquake. Only stations closer than 100 km are used in the average, even if there are fewer than five. Earthquakes for which the nearest station is more than 100 km distant have very poorly determined depths, and the elevation of the nearest station is used as the estimate of the depth datum. Details of the algorithm for determining depth datum, and the handling of model and geoid depths can be found in the hypoinverse manual. For example, deep borehole stations where the sensor is far below the surface are not used in the nearest-five average. Also, the few catalog earthquakes with no station phases cannot be adjusted, and the model and geoid depths are the same. (Click to view larger.)

To accomplish this transition, an additional step has been added to the location process. The NCSS locates earthquakes with the program hypoinverse using a flat-topped crustal model with linear velocity gradients, referred to as "CRT" layer crustal models. The earthquake depth below this model surface is called the model depth. To determine the geoid depth, the elevation of the flat model surface is inferred from the nearby seismic stations used to locate the event and is subtracted from the model depth. The relationship between model depth and geoid depth is illustrated in Figure 1, taken from the hypoinverse v.1.41 manual (ftp://ehzftp.wr.usgs.gov/klein/hyp1.41/doc/hyp1.41.pdf, p. 25). Model depth must continue to be used internally in hypoinverse, for purposes such as setting trial or fixed depths. The fundamental location algorithm and multiple crustal models used in the NCSS are unchanged, and the geoid depth adjustment is applied after the earthquake is located. The geoid depth is stored in the database field "depth" in the origin table; the model depth is stored in the field "mdepth."

The model depth is always positive or zero, but the geoid depth can be negative for events above sea level. Catalog searches using depth as a criterion must now use a new minimum negative depth (such as -4 km) to be sure of retrieving all events, including shallow earthquakes in the high Sierras or the Geysers areas. The geoid used for earthquake depths and for station elevations are the same, WGS84.

Networks using hypoellipse "CRE" layer crustal models already calculate geoid depths because both stations and earthquakes are embedded within a layer model that is referred to the geoid. The NCSS will continue to use crustal models with linear gradients ("CRT" models) as there are many benefits. It is very difficult to modify hypoinverse to embed stations in gradient models that allow for shadow zones and interpolating in travel time tables. The NCSS decided it was better to adjust model depths to the geoid rather than attempt calculation of geoid depths directly.

Double-difference depths will also now (eventually) be geoid depths, but DD depths can't be simply adjusted from the model depth. The DD procedure begins with single-event depths from hypoinverse and adjusts locations based on differences between common stations for nearby events. DD locations must be computed from the new geoid depths because the mix of associated events will change once their depths are adjusted to the geoid. We are developing a plan to implement this transition.

New crust model fields were added to the event table in the catalog database that are computed by hypoinverse. These fields are output to hypoinverse format, but are not written to simpler output formats. The crust model fields are not a property of the physical earthquake and are not implemented as search criteria. This involved a schema change to the database.

Additional catalog output for multiple hypoinverse crustal models

The field "crust type " is one letter for the type of hypoinverse crust model, corresponds to the 3rd letter of the hypoinverse command and thus the model type, and is in the database origin table. Currently the NC and BK nets use gradient type T, and CI uses layer type H.

 H - Homogeneous layers, all stations at top (CRH model)
T - Travel time table with linear gradient, stations at top (CRT model)
E - Hypoellipse layer model, using station elevations(CRE model)
L - Hypoellipse single gradient model, using station elevations (CRL)
V - Hypoellipse single gradient over halfspace, using station elevations (CRV) 

The field "crust model" is the 3-letter code for the regional crust model. The code is for the "dominant" model because a blend of up to 3 different models can be used for smooth transitions between adjacent models, and the one with the highest weight is saved in this field. In the NCSS, model zones were picked to enclose zones of high seismicity and faults, thus most earthquakes use a single model. Each crust model has its own set of station delays, now in their 4th revision but soon to be in the 5th revision. Many of the crust models are those originally used when individual regions were processed by different research groups, but other models have been added as necessary. The models denoted by east and west are the paired "alternate" models, where one model is used for stations on the east side of the San Andreas fault and the other model for those on the west side. The alternate models are assigned to earthquakes in the same geographic region. The crust model code is in the db origin table. The table is from the shadow2000.doc file used in the NCSS.

Code Name            Source of original model              How derived
AUB  Auburn          Eaton & Simirenko (OFR 80-604, 1980)  test&modify
BAE  Bartlett Spr E  Castillo pers. comm. (1991)             VELEST
BAR  Bartlett Spr Ft Castillo pers. comm. (1991)             VELEST
BAS  Basin & Range   averaged Prodehl (Prof Pap 1034, 1979)  refract
COA  Coalinga        Eaton OFR 85-44 (1985)                test&modify
CON  Concord-Calaveras Klein pers. comm. (1991)              VELEST
COY  Coyote Lake     Reasenberg & Ellsworth (JGR 1982)       VELEST
CST  Central Coast   Poley & Eaton pers. comm.             test&modify
DIA  Diablo-Bear Valley Dietz pers. comm. &                  VELEST       
     (west)        Walter & Mooney (BSSA 1982)             refract
GAB  Gabilan-Bear Valley Dietz pers. comm. &                 VELEST       
     (east)        Walter & Mooney (BSSA 1982)             refract
GEY  Geysers         Eberhart-Phillips & Oppenheimer JGR ¢84 VELEST 
HAY  Hayward Fault   Klein pers. comm. (1990)                VELEST
LAS  Lassen          S. Walter pers. comm.                   VELEST
LEW  Mt. Lewis       Klein pers. comm. (1990)                VELEST
LIV  Livermore-Antioch Klein pers. comm. (1991)              VELEST
LOM  Loma Prieta (west) J. Olsen and M. Zoback (1995)        VELEST
LON  Loma Prieta (east) J. Olsen and M. Zoback (1995)        VELEST
MAA  Maacama Fault   Castillo pers. comm. (1991)             VELEST
MAM  Mammoth Lakes   Kissling (PhD thesis 1986?) &         refract &                     
                     Cockerham & Kissling pers. comm.      inversion
MAN  Maacama North   Castillo pers. comm. (1991)             VELEST
MEN  Mendocino       Magee pers. comm. (1991)                VELEST
MOR  Morgan Hill     Cockerham & Eaton                     
                     (USGS Bulletin 1639, 1987)              VELEST
NBY  North SF Bay Eberhart-Phillips & Oppenheimer (JGR 1984) VELEST
NCG  N. Calif gradient default model derived from                     
                     Eaton pers. comm.                    test&modify
PEN  SF Peninsula north Olson (Proc. Rec. Crus. Mvmts. 1987?) VELEST
PES  SF Peninsula south Olson (Proc. Rec. Crus. Mvmts. 1987?) VELEST
PGH  Park.-Gold Hill Poley & Eaton pers. comm.            test&modify
PMM  Park.-Middle Mtn. same as PGH.                       test&modify
PSM  Park.-Simmler   same as PGH                          test&modify
PTA  Pt Arena, Ft Bragg Castillo pers. comm. (1991)           VELEST
SCA  Southern Calif. Jones pers. comm.                       refract?
SHA  Shasta & Oregon average from Zucca et al. (JGR 1986)    refract
SIM  San Simeon & central coast Klein pers. Comm.             VELEST
TRA  Transverse Ranges Prodehl PP-1034 (1979)                refract
TRE  Tres Pinos      Dietz pers. comm. &                      VELEST                     
                     Walter & Mooney (BSSA 1982)             refract
WAL  Walker Pass     Jones & Dollar (BSSA 1986)           test&modify

Geoid Depth FAQ

October 7th, 2015

What is Geoid depth?

The depth of an earthquake location can be reported relative to the mathematical
geoid surface within the earth, which is very close to sea level. In simplified terms, the geoid is an
imaginary surface within the earth which is close to sea level over the oceans and approximates what the
ocean height would be over continents if the ocean could extend inland (see http://en.wikipedia.org/wiki/Geoid for a more complete explanation of the geoid).

What are the advantages of geoid depth?

We have many seismic networks around the country that independently locate
earthquakes within their networks. If all networks report geoid depths, then there
are no systematic shifts between earthquake depths located by different networks due
to differences in datum. Earthquake geoid depths also eliminate systematic bias caused
by the topography of mountain ranges.

How can an earthquake have a negative geoid depth?

Earthquakes are always within the solid earth. If a computed earthquake hypocenter
is above sea level, it will have a negative geoid depth, and still be below the earthquake's
surface. For example, if the earth's surface near the earthquake is 2 kilometers (1.24
miles or about 6,300 ft) above sea level, and the earthquake focus is 1 kilometer below the
surface, it has a geoid depth of -1 kilometer. If it is 4 kilometers below the surface, it
has a geoid depth of 2 kilometers. Areas where the earthquakes are very shallow and the
ground's surface is well above sea level, such as the Geysers geothermal area in northern
California, can thus have events with negative geoid depths.

What depths did we calculate before we started reporting geoid depths?

To simplify the calculation of locating earthquakes, both before and with computers, we
assumed the earth has a smooth surface and no topography. The earth model has a seismic velocity
structure below its top surface where rays propagate from the earthquake source and travel times
are calculated to the seismic stations that record it. If all stations are at the earthquake's surface
with no topography, calculations are simplified. Depths calculated within this simplified smooth-surface
earth model are called model depths. Model depths are essentially depths below the earth surface near the
earthquake, and model depths were reported before we changed to reporting geoid depths. Model depths are
still recoverable from some versions of the earthquake catalog.


How do we get geoid depths now?

Model depths are still calculated for every earthquake. That is because the calculation of travel
times in an earth model with a smooth surface is much more practical, even with complications like
velocity layers with linear velocity gradients. We get the geoid depth by correcting the model depths that
we calculate (which are always positive) by subtracting the elevation of the nearby ground surface. Thus
for an earthquake with a model depth of 3 kilometers (below the surface) and a nearby surface that is one
kilometer above sea level, the geoid depth is 3-1 = 2 kilometers.

We use the average elevation of the closest 5 stations to estimate the elevation of the nearby ground
surface. The geoid datum used to measure the station elevations thus becomes the reference datum of the
earthquake depths, currently WGS84. Geoid models continue to evolve, but the differences between sea level
and between various geoid datums are typically a few meters, which is much smaller than the accuracy of
locating earthquakes. Some seismic networks using simpler crustal velocity models consisting of constant-velocity
layers use a calculation procedure that gives geoid depths directly. This approach has the disadvantages of
giving up models with velocity gradients and would have introduced artifacts into the catalog. The earth
velocity models used normally for routine and fast earthquake locations are one-dimenstional (vary only
with depth), and are an approximation to a three dimensional earth.


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