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.
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.
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
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.
November 1st, 2015
In a typical year, the USGS might lose one or two seismic stations in Northern and Central California from wildfire damage. Thus far in 2015, over 30 stations operated by the USGS have been affected and an additional 30 maintained by Lawrence Berkeley National Lab in the Geysers have been affected (Figure 1). This loss of monitoring sites has had an impact on the operations of the Northern California Seismic System (NCSS), particularly in the detection and location of small events in the southern Central Valley and the Geysers area.
On September 11, 2015 at 09:30 UTC, the Butte fire damaged the Sierra Vista microwave communications link. This microwave link is responsible for bringing in 23 NCSN (Northern California Seismic Network) stations in the San Joaquin Valley. As a result, we are no longer receiving data from AAS, ABJ, AOH, ASMB, BMS, BRM, HSL, MBE, MBU, MCUB, MHD, MMI, MMT, MNHB, MPR, MRH, MSV, MYL, PAR, PDR, PHB, PJC, PJU, PKE, and PWM. These stations provide coverage in the Sierra Foothills and in the western foothills of the San Joaquin Valley. USGS Menlo Park staff visited the site with personnel from the Army Corps of Engineers (ACOE) on September 21st. We are working with ACOE to restore the link but currently do not have an estimated time for its return to operation. The loss of these stations has reduced the sensitivity of the NCSS for the detection and location of small events, as well as degrading location accuracy. The change is largest in the valley, west and slightly south of Mammoth, of ~0.7-0.8 magnitude units. This calculation was done by Corinne Bachmann, of LBNL, who studied NCSS catalog completeness as part of her Ph.D. in 2009. The new threshold is magnitude ~2.3.
In parallel, the Valley fire has affected our monitoring in the Geysers area. Beginning on September 13, 2015, we began to lose contact with stations NC.GBG, NC.GCR, NP.COB, NP.ADSP, and NP.ADS2 (Figure 2). We were able to access these sites on September 28th to assess damage. Station NC.GBG was restored on the 28th and NC.GCR was restored on the 29th. Station NP.ADSP has been destroyed. NP.ADS2 is in a building which is still standing but without power or communications. The status of NP.COB is still unknown.
We will provide status updates as more information becomes available.
The loss of data from the BG network, combined with the temporary loss of GBG and GCR has significantly lowered the number of events detected and located in the Geysers, as illustrated in Figure 3, which shows the number of events per day in the NCSS Geysers polygon for the month of September.
In addition to the damage to the NCSN and NSMN (National Strong Motion Network) sites, the NCSS lost contact with the stations in the BG network, operated by Lawrence Berkeley National Lab, on Sept 13th at 0955 UTC. This network of ~30 highly sensitive stations, installed as part of the Enhanced Geothermal Systems project, facilitates the location of small events in the Geysers area below magnitude 1.2. LBNL staff are working to restore stations and revive the communications links. As of Oct 1, 20 stations are being recorded and stored on a local computer.
Finally, two other NCSN stations have been damaged or destroyed in other fires this year - NC.NMT to the Jerusalem Fire and NC.NVA to the Wragg fire. We plan to rebuild NC.NMT in the coming weeks and are assessing our options for NC.NVA.
On Friday Sep 18 2015 at approximately 19:00 PT, a small fire in the UC Berkeley campus data center triggered the building's fire suppression system. This ultimately led to the powering down of the entire data center. The NCEDC and UC Berkeley seismic and geodetic acquisition, storage, and distributions systems are housed in the campus data center, and went offline at that time.
Power and network was restored to the data center at ~ 7 AM PT on Saturday Sep 19 2015. BSL staff restored all systems and data service by 17:00 PT.
No archived data was lost. BSL staff will be retrieving missing data from the data loggers, and will archive the missing data as it is made available.
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