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
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. This can be a very difficult task
for some station geometries. The first sort is just an initial guess and is
based on some simple logic. 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. 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.
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.
