West Coast/Alaska Tsunami Warning Center

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: 7/2008

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:
StaDataFile - seismometer location, elevation, etc.
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

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. The first task of loc_wcatwc is to sort Ps into buffers which contain just picks from the same earthquake. P-picks are sorted by location and arrival time. If the time difference between the new P and all other Ps within the buffer is less than the maximum which could be expected for the distance between the Ps, the new P is placed in that buffer if it within the X closest stations to the picked station. A second check is also made: if the P time is greater than MaxTimBetweenPicks from the last P-pick in the buffer, it will not be included in that buffer. After a buffer has enough P-picks to locate a quake (MinPs), the solution is computed. If a good solution is made, P's 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, P's 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 of the region.

A common problem with this associator/locator is that the same quake may be located in two different buffers. This can happen due to phases other than the P (such as the pP or sP) being picked instead of the real P. A routine is called to check for similar quakes in different buffers. Only the location from the buffer with the most Ps is sent to the HYPO_RING.

After a good location has been computed, magnitude is output based on the amplitude/periods/integrations 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.

Graphical Interface:
This module has no graphical interface. Locations and some errors are logged to the Windows console, if created.