Fake P Phases
In early 1998, the ISC learned that "fake P" readings from the US National Seismic Network had been finding their way into the published ISC Bulletin. In response to an enquiry from the ISC, NEIC's Bruce Presgrave described the purpose and properties of these spurious phase readings. Body wave arrivals are required to anchor surface wave amplitudes in the NEIC data management system. Because of high-frequency noise at some USNSN stations, however, it is sometimes possible to measure a surface wave amplitude where no body wave is detectable. To avoid excluding useful amplitudes from the data, P arrivals are inserted into the database with properties that are intended to prevent their use by any careful seismologist, including a large positive travel time residual.
The ISC accepts phase readings from NEIC, and the fake P's were included. The outcome has almost always been harmless: the association and identification are unchanged, the residual remains large, and the arrival time is automatically assigned an effectively null weight in computing the event location. Occasionally, however, the phase is reassociated or re-identified by an ISC Bulletin editor as pP or sP, potentially leading to an inaccurate event depth.
An algorithm has been developed to identify the fake P's in NEIC data files with very high confidence. In arrivals from the first month of 1996, the algorithm identified 621 fake P's and a comprehensive manual review of the flagged arrivals showed one apparent mis-identification. From the start of 1996, fake P's will routinely have an operator identification code 111 and a null ISC identification in ISC products, and be neglected in processing.
Fake P's are likely to be have been included in the ISC Bulletin at least back to 1992, and we will be evaluating means for properly identifying these arrivals in the processed data. Further details are given below.
Fake P times are created at NEIC in order to enable measurements of surface wave amplitude and period to be used in the determination of event surface wave magnitudes. There are occasions when a surface wave amplitudes and periods can be measured but the first onset is non determinable. This often applies at oceanic stations such as HON. NEIC phase association rules will not associate the data unless a primary phase time is reported.
At the ISC, fake P's are identified when all of these conditions hold:
- The data comes from NEIC
- The time is exactly on a ten second boundary
- The NEIC priority code is '10*' or '20*' (The priority code discriminant reported by Presgrave was somewhat more restricted than this, but the ISC does not appear to retain the third character of the priority code.)
- There are exactly two 'phases' reported. The first being the onset time and the second the surface wave amplitude and period
- The phase identification is P.
- No amplitude, log a/t, period etc. for the first phase
- No onset time, phase id, etc. for second phase
- Amplitude and period are from vertical component
These rules were established based on discussions with Bruce Presgrave that following notification that NEIC had routinely included fake P times in the data stream reported to the ISC. A study of the NEIC phase readings data stream as supplied to ISC was undertaken.
Using the data for 1996 January as a sample, the following facts emerged.
The total number of RAGs (Reading Associated Group) for 199601 was 98524.
Primary times are given to 1/100th of a second.
|Times falling exactly on an hour boundary||2||0|
|Times falling exactly on a ten minute boundary||54||2|
|Times falling exactly on a one minute boundary||465||16|
|Times falling exactly on a ten second boundary||2678||98|
|Times falling exactly on a one second boundary||18537||985|
|Times falling exactly on one tenth second boundary||72621||9852|
|Times with 1/100th non zero||25903||88672|
There are 1153 RAGs with times exactly on a ten second mark which also report surface wave data. From these there are 620 RAGs which report the phase id as 'P', leave the emergent/impulsive indicator and identification quality fields as blank and only report the vertical component in the surface wave field. All these records report Priority path code as 106 or 206. Where
character 1 = 1 quick and dirty 2 preliminary character 2 = 0 local (interpretation generated at NEIS) or phoned to NEIS character 3 = 6 treat amplitudes as ground amplitudes
The following table shows the stations found and their frequency of occurrence
11 ALQ Albuquerque New Mexico W 34:56:33 N 106:27:27 W 1849 4 ARU Arti Sverdlovskaya B 56:25:48.7N 58:33:45.0E 250 1 BDFB Brasilia Distrito Federal 15:38:28.3S 48:00:50.9W 1095 1 BGCA Bogoin Central African Republic/ 5:10:34 N 18:25:27 E 576 16 BINY Binghamton New York 42:11:57.5N 75:59:10.0W 498 19 BMN Battle Mountain/Nevada 40:25:53.3N 117:13:18.4W 1594 3 BOSA Boshof South Africa 28:36:50.7S 25:15:19 E 1202 15 CBKS Cedar Bluff Kansas 38:48:50.4N 99:44:14.6W 667 11 CCM Cathedral Cave Missouri 38:03:20.4N 91:14:40.5W 223 19 CEH Chapel Hill North Carolina 35:53:27 N 79:05:34 W 152 19 CMB Columbia College/California 38:02:06 N 120:23:06 W 719 12 COR Corvallis Oregon BW 44:35:08.6N 123:18:11.5W 121 15 DUG Dugway Utah BW 40:11:42 N 112:48:48 W 1477 4 ELK Elko Nevada 40:44:41.4N 115:14:19.6W 2210 2 EYMN Ely Minnesota 47:56:46.3N 91:29:42.0W 475 9 FVM French Village Missouri 37:59:02.4N 90:25:33.6W 310 14 GLD Golden Colorado 39:45:02 N 105:13:17 W 1762 1 GOGA Godfrey Georgia 33:24:40.3N 83:27:59.8W 150 18 GOL Golden Colorado BCW 39:42:01 N 105:22:16 W 2359 10 HKT Hockley Texas 29:57 N 95:50 W -415 36 HON Honolulu Hawaii W 21:19:27 N 158:00:02 W 2 20 HRV Harvard Massachusetts 42:30:23 N 71:33:30 W 180 20 ISA Isabella California B 35:39:48 N 118:28:24 W 835 7 JFWS Jewell Farm Wisconsin 42:54:51.3N 90:14:53.1W 335 18 LBNH Lisbon New Hampshire 44:14:24.4N 71:55:33.2W 367 1 LBTB Lobatse Botswana 25:00:52.2S 25:35:49.2E 1028 1 LKWY Lake Wyoming 44:33:54.7N 110:24:00.0W 2424 19 LSCT Lakeside Connecticut 41:40:42.2N 73:13:27.8W 318 9 LTX Lajitas Texas 29:20:02 N 103:40:01 W 1013 19 MCWV Mont Chateau West Virginia 39:39:29.2N 79:50:44.2W 280 19 MIAR Mount Ida Arkansas 34:32:44.5N 93:34:22.8W 207 21 MNV Mina Nevada B 38:25:55.9N 118:09:15.8W 1507 17 MYNC Murphy North Carolina 35:04:26.0N 84:07:40.4W 550 19 NEW Newport Washington 48:15:48 N 117:07:12 W 760 12 OBN Obninsk Kaluzhskaya B 55:10 N 36:36 E 12 PFO Pinyon Flat Obs./California 33:36:33 N 116:27:19 W 1280 19 SAO SanAndreas Obs./California B 36:45:54 N 121:26:42 W 350 1 SBA Scott Base Victoria Land BW 77:51:01 S 166:45:22 E 38 1 SDN Sand Point Alaska and Aleutians 55:20:28.8N 160:29:49.8W 23 4 SIT Sitka Alaska and Aleutians 57:03:25 N 135:19:28 W 19 12 SLM Saint Louis Missouri B 38:38:10 N 90:14:10 W 161 15 SMTC Superstition Mt./California 32:56:56.4N 115:43:12.0W -50 1 SMY Shemya Alaska and Aleutians 52:43:51 N 174:06:11 E 58 18 SSPA Standing Stone Pennsylvania 40:38:08.9N 77:53:16.8W 158 20 TPNV Topopah Spring Nevada 36:56:55.8N 116:14:58.2W 1600 16 TUC Tucson Arizona BW 32:18:35 N 110:47:03 W 906 1 VNDA Vanda Victoria Land 77:30:50.2S 161:50:44.2E 98 13 WDC Whiskeytown California 40:34:48 N 122:32:23 W 300 18 WMOK Wichita Mts Oklahoma 34:44:16.4N 98:46:51.6W 486 8 WVOR Wild Horse Valley/Oregon 42:26:02.2N 118:38:12.2W 1344 19 YSNY Yorkshire New York 42:28:32.9N 78:32:15.0W 628
A program to count the frequency of records of different types produced:
|3418||records with surface wave values|
|2057||records with only vertical component surface wave data|
|2678||records with time exactly on ten second boundary|
|80998||records with primary phase code given as P|
|20378||records with onset code blank|
|954||records with path code 106 or 206|
|1090||records with only first arrival and surface wave|
|903||only first arrival and Z component surface wave|
|621||satisfy all above criteria|
|1||satisfy all criteria except time is not on 10 sec. marker|
|0||satisfy all criteria except path not 106 or 206|
|0||satisfy all criteria except onset not blank|
|0||satisfy all criteria except ident not P|
|0||satisfy all criteria except no surface wave|
|0||satisfy all criteria except no Z surface wave|
|0||satisfy all criteria except not primary and sw only|
The records from the NEIS station list seem to show that all the stations belong to appropriate networks to be treated in this fashion.