WC/ATWC Communications and Networking Architecture

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Operations Manual

NOAA/NWS/WCATWC

910 South Felton Street

Palmer, Alaska 99645

http://wcatwc.arh.noaa.gov

Section 4.1.8 – loc_wcatwc

Last Updated: 9/2010

Date Written: 2/2001

Development Language: MicroSoft C/C++ v7.0

Necessary Rings:
Input Ring – PICK_RING; reads TYPE_PICKTWC format messages
Output Ring - HYPO_RING; writes TYPE_HYPOTWC and TYPE_H71SUM2K format messages
Output Ring - ALARM_RING; writes TYPE_ALARM format messages

Necessary Data Files:
StaFile - picker configuration file
StaDataFile - seismometer location, elevation, etc.
ResponseFile - Broadband instrument response in poles/zeroes format

BValFile - Richter B-value data fo Mb
OldQuakes - List of previously located quakes
AutoLoc - Trigger file for ANALYZE program
DummyFile - Hypo-parameters of last located quake
MapFile - Trigger file for EarthVu

RTPFile - P-data used in last location

QLogFile – Log of all quakes located

MwFile – Mw determined from Mm module

DepthFile – File with average and max depths for entire world (1 deg. Spacing) based on historical data

CityFiles – City names and locations that epicenters are referenced

Flinn-EngdahlFiles – Files controlling Flinn-Engdahl regions

Initialization File:
loc_wcatwc.d - read on module start-up

Module Description:
This program locates earthquakes given TYPE_PICKTWC format P-picks placed in the InputRing (Pick_ring). The work done by the loc_wcatwc can be divided into 3 stages; checking, association, and location.

Checking stage: Check to see if this is a new P arrival or previous P arrival.

Step 1. Checks to see if the P arrival is more than 30 minutes old, if it is then it is discarded.

Step 2. Checks to see if the P arrival has a tag that indicates that it belongs to a specific buffer and if so, put it into that specific buffer. (If an analyst updated the P-pick in the hypo_display widow, then the P is tagged with that buffer. It came from an already associated and located event, so the P arrival belongs to that buffer).

Step 3. Checks to see if this is a repeat P arrival (i.e. the P just had updated mb/mL information, if so then just update the magnitude information for that P arrival).

Association stage: After the P arrival passes the checking stage, loc_wcatwc needs to find a buffer (bucket) for the P arrival.

Step 1: Scan all buffers to see if there is a solution and if this P arrival fits the solution, then put P in that buffer and update the location.

Step 2: Check to see if this is a known phase of an existing solution in a buffer. If it is then the arrival is ignored.

Step 3: Order the buffers without assigned solutions by number of p arrivals in the buffer. Use following rules to place the p arrival in a buffer (added 2007).

Rule 1: Is P-station one of the X closest stations (configurable) from this P arrival station in a buffer? If the answer is yes, check rule 2. Otherwise, put it in a new buffer*.

Rule 2: Is the P arrival within theoretical limit of other P arrivals in the buffer. If the answer is yes, put P arrival into that buffer. If the answer is no, then put the P arrival into a new buffer*.

*If all the buffers are full, then empty the buffer with the oldest P time and place this P arrival into that buffer.

After a buffer has enough P-picks to locate a quake (MinPs), the solution is computed. If a good solution is made, Ps from other buffers are compared to this solution and are added back into the buffer if they fit (unless they are in a buffer which has produced a good location). Also, Ps which were eliminated by the Bad P discriminator in the locate routine are placed in a different P buffer. This scavenging and removal of Ps after locations are made is the second sorting of Ps throughout the different buffers. As new P-data enters a buffer which has more than MinPs picks, the hypocenter is updated for that buffer. See the link for a flow diagram of this process.

Quake locations are computed using Geiger's method given an initial location. The initial location estimate is first assigned to the location of the first P-time in the buffer. If a solution can not be computed from this initial location, a routine is called to compute the initial location from azimuth and distance determined from a quadrapartite of stations. If a location can still not be determined, a bad P-pick discriminator is called. This simply throws out stations one-at-a-time (up to three stations at once) and re-computes the location. Good solutions are verified by total residual, azimuthal coverage, nearest station distance, and frequency/S:N ratios.

The IASPEI91 travel times are used as the basis for quake locations in this program. A time/distance/depth table has been created from software provided by the National Earthquake Information Center. Locations with this set of P times have been compared to those made with the Jefferey's-Bullen set of times and were found to be superior in regards to depth discrimination and epicentral location with poor azimuthal control. The P-table is arranged on 10km depth increments and 0.5 degree distance increments.

A routine was added in 2006 which provides better depth control for solutions. The quake depth is fixed to the average depth for the region (based on USGS historical data on a one degree by one degree grid). When enough P control is attained in the buffer, the depth will float, but will be limited by the maximum depth plus 50km of the region.

After a good location has been computed, magnitude is output based on the amplitude/period/integration reported by the P-picker, develo, hypo_display, and/or lpproc and mm. mb, Ml, MS, Mw, and Mwp magnitudes are computed depending on epicentral distance.

The locations/magnitudes are sent to the HYPO_RING. Alarms based on location and magnitude can also be issued to the AlarmRing if desired.