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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.
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 Â¢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