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.

2015 wildfires take a toll on seismic monitoring

October 1st, 2015

November 1st, 2015



Figure 1: Map of stations in Northern and Central California, including NC (orange), BK (blue), CI (yellow), NP (white), and BG (pink). Not all stations of the California Integrated Seismic Network are shown. The labelled stations are the USGS that have been affected by the 2015 wildfire season as of Oct 1, 2015; a number of stations were damaged and communications were disrupted in the BG network as well. (Click to view larger.)"

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.



Figure 2. Top is a broadscale view of the stations in the greater Geysers area; bottom is a close up showing the locations of the damaged or unknown status stations in the NC and NP networks. Pink stations are those in the BG network, which are not currently being received in real-time monitoring system. (Click to view larger.)"

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.



Figure 3: Number of events per day in the Geysers polygon. Blue is the count of all earthquakes; red is the count of events of magnitude 1 and higher. While this figure illustrates the variability in the number of events per day, the sudden decrease on Sept 13th is associated with the loss of the NCSN stations GBG and GCR and the BG network. (Click to view larger.)"

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.

 

 

 

 

 

 

Data Center Outage 09/18/2015-09/19/2015

September 18th, 2015

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.

If you encounter problems with data or data services, please contact us at: ncedcinfo@ncedc.org

Timing problem at Mammoth analog stations 01/22/2015 - 01/27/2015

January 30th, 2015

January 30th, 2015

The Northern California Earthquake Data Center (NCEDC) archives seismic waveform data from the Northern California Seismic Network (network code NC). This announcement refers to network NC seismic waveform data (both continuously archived data and event gathers) for dates, stations, and channels listed below. Earthquake catalog locations were not affected (see below).

-------------------------------------------------------------------------------------------------

The NCSN experienced problems with the TrueTime clock responsible for providing the IRIGE time at Mammoth from 01/22/2015 23:35:11 through 01/27/2015 23:59:12. During this interval, timing is uncertain for all the analog stations digitized at Mammoth. However, events recorded during this period were timed and located with stations not affected by the problem, and the origin times of the cataloged events are accurate, with fairly good locations.

Beginning of problem - time tears from lost/regained time lock, presumably due to TrueTime IRIGE changes:


20150121_UTC_23:35:11 overlap -0.99 s
20150122_UTC_01:02:58 gap 1.01 s
20150122_UTC_06:02:31 overlap -0.99 s
20150122_UTC_06:40:09 gap 1.01 s
20150122_UTC_07:30:11 overlap -0.99 s


End of problem - TrueTime swap done, adsend synced:
20150127_UTC_23:59:12 Time-code status: Locked-on

Timing between 01:02:58-06:02:31 and 06:40:09-07:30:11 on 1/22 is probably ok.

This timing problem affects the following channels:

IRGMM ATT NC --
MCD EHZ NC --
MCS EHZ NC --
MCV EHZ NC --
MDC EHZ NC 02
MDR EHZ NC 02
MDR ELE NC --
MDR ELN NC --
MEM EHZ NC --
MEM ELE NC --
MEM ELN NC --
MFB EHZ NC --
MGPB EHZ NC --
MLC EHZ NC --
MLH EHZ NC --
MLM EHZ NC --
MMP EHZ NC --
MMS EHZ NC --
MRC EHZ NC --
MRD EHZ NC 02
MTU EHZ NC 02

EGS Catalog Geysers Magnitude Errors

January 3rd, 2015

January 3rd, 2015

The Northern California Earthquake Data Center hosts several earthquake catalogs, including the the northern California earthquake catalogs of the USGS and BSL, the worldwide earthquake catalog of the ANSS, and the Enhanced Geothermal Systems (EGS) catalog. Below please see an announcement regarding magnitude errors for four Geysers-area earthquakes in the EGS catalog:

In reviewing Geysers data, the 4 events listed below had incorrect amplitudes from one station (DEB) that had a large amount of transient electronic noise. These amplitudes were previously included in the moment magnitude calculations for the events listed below, resulting in greatly exaggerated moment magnitude estimates. The corrected magnitude estimates, computed without the erroneous DEB amplitudes, are:

Date       Time              Lat        Lon   Depth   Mag Magt  Nst Gap  Clo  RMS  SRC   Event ID
----------------------------------------------------------------------------------------------
2014/04/05 19:30:20.53  38.84790 -122.83538   2.543 0.43 (formerly 4.92)  ML    0   0    0 0.05  LBL    3383465
2014/04/05 19:52:57.37  38.84423 -122.82652   2.857 1.12 (formerly 4.87)  ML    0   0    0 0.01  LBL    3383466
2014/04/05 20:04:52.30  38.80133 -122.82338   1.035 0.45 (formerly 5.06)  ML    0   0    0 0.03  LBL    3383470
2014/04/07 01:18:36.17  38.83748 -122.78598   0.851 0.51 (formerly 4.65)  ML    0   0    0 0.04  LBL    3383663
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