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 (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 belonged 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 update the magnitude
information and no real association into buffers is needed).
Association stage: After P arrival passes the checking
stage, the 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 one of the 15 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, 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.
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.
