About Monitoring Stations

North Texas Earthquakes

Range of equipment, permanent and temporary, is recording data from North Texas earthquakes

SMU seismologists are frequently asked about the equipment they use to monitor the increased seismic activity occurring in the Fort Worth Basin since 2008.

How many permanent seismic stations do we have in North Texas? How many are operated/owned by USGS and how many belong to SMU or other parties?

Since SMU has been operating a temporary research network, the number of SMU station depends upon when the question is asked.  In April of 2015, SMU was completing studies in the Azle/Reno area and beginning a study in the Irving/Dallas area.  For that time, an interesting look at this question is provided by the table below showing partial information from the stations contributing to the National Earthquake Information Center (NEIC) locations for the April 2, 2015 magnitude 3.3 earthquake in Dallas/Irving. On each line, the channel names start with a two-letter network code that in general indicates the owner of the station and something about the planned duration for the station operation. The phase arrival table lists the times that the seismic waves arrive at each station sorted in order of distance from the earthquake.  Distance between the station and the estimated earthquake epicenter is given in degrees. One degree is about 60 miles. The table lists the contributing networks:

Phase Arrival Times

Channel

Distance

Azimuth

Phase

Arrival Time

Status

NQ UDFB HNZ 01

0.01°

114.41°

Pg

22:36:22.30

manual

ZW IFS3 EHZ 00

0.03°

149.48°

Pg

22:36:22.50

manual

NQ NLKCP HNZ 01

0.03°

303.79°

Pg

22:36:22.58

manual

ZW IFBF EHZ 00

0.07°

15.49°

Pg

22:36:22.69

manual

ZW ITL1 EHZ 00

0.07°

15.49°

Pg

22:36:22.30

manual

ZW ITSC EHZ 00

0.08°

63.24°

Pg

22:36:22.67

automatic

ZW AZDA EHZ 01

0.53°

283.06°

Pg

22:36:31.39

manual

N4 Z35B BHZ --

0.55°

330.94°

Pg

22:36:31.07

manual

TA WHTX BHZ --

0.97°

207.21°

Pg

22:36:39.20

manual

N4 237B BHZ --

1.28°

131.38°

Pn

22:36:44.59

manual

N4 Z38B BHZ --

1.69°

75.57°

Pn

22:36:49.58

manual

TA 435B BHZ --

2.14°

195.14°

Pn

22:36:59.37

automatic

OK X37A BHZ --

2.17°

36.57°

Pn

22:36:58.00

automatic

US NATX BHZ 00

2.21°

118.89°

Pn

22:36:57.50

automatic

TA ABTX BHZ --

2.29°

265.00°

Pn

22:36:59.57

automatic

US WMOK BHZ 10

2.43°

321.26°

Pn

22:37:01.90

automatic

OK FNO HHZ 01

2.43°

350.99°

Pn

22:37:03.23

automatic

OK OKCSW EHZ --

2.58°

350.86°

Pn

22:37:05.82

automatic

OK OKCFA HHZ --

2.59°

350.66°

Pn

22:37:06.07

automatic

GS OK025 HHZ 00

2.74°

353.14°

Pn

22:37:08.02

automatic

OK BCOK HHZ --

2.85°

348.92°

Pn

22:37:10.06

automatic

GS OK029 HHZ 00

2.97°

351.83°

Pn

22:37:11.64

automatic

US MIAR BHZ 00

3.27°

58.01°

Pn

22:37:11.67

automatic

AG WLAR HHZ 00

3.31°

74.39°

Pn

22:37:12.75

automatic

AG WLAR HHZ 00

3.31°

74.39°

Pg

22:37:20.35

automatic

US JCT BHZ 00

3.40°

226.67°

Pn

22:37:14.90

automatic

NQ - Netquakes stations.  These are portable stations deployed to catch aftershocks or new events when an earthquake sequence starts.  They are temporary and not as sensitive as the more useful permanent stations, but because they are close to the events, they help tremendously with locations.  They are typically deployed by local volunteers at sites where the site owner can contribute a network connection.  SMU has installed and maintains seven of these in the North Texas area. They are on loan from the USGS.

ZW - SMU's temporary network. These are instruments on loan to SMU, and the bulk of the 18 currently operating stations come from Incorporated Research Institutes for Seismology (IRIS).  SMU is currently operating these.

N4 - Central and Eastern US network (CEUSN). These are high quality redeployed stations from the temporary TA network.  They are funded through 2017.

TA - Transportable Array stations.  This is the National Science Foundation-funded IRIS operated transportable array, The stations listed here are the U.S. Reference stations and are expected to operate through the end of the EarthScope experiment. 

OK - Oklahoma Geological Survey Network.

US - USGS Network

GS – USGS-funded stations, usually temporary.

AG - Arkansas Network

SS – Single, miscellaneous stations.  SMU's DAL station falls into this category.  SMU operates this station on the SMU campus.

IM - International Monitoring System (this is only the site in far West Texas).  SMU operates and maintains this array (a set of 9 stations)

In the April 2, 2015 M3.3 earthquake, the NEIC used stations out to about 240 miles away in solving for the location.  This included stations in Texas, Oklahoma, and Arkansas.  The closest station was one of the temporary SMU stations, about six miles from the epicenter.  The closest long-term station was N4.Z35B, at about 40 miles.  The closest miscellaneous station was SMU's basement seismometer in Heroy Hall, an instrument graciously on loan to SMU from Geotech Instruments here in Dallas.  The closest permanent government-funded station was OK.X37A at about 150 miles.  The closest “permanent” station owned by the USGS was US.NATX just beyond 150 miles.  N4 and TA stations are National Science Foundation research stations currently operated as part of an experiment, but will likely evolve into permanent stations once the experiment is complete.

Following the M4.0 earthquake reported by the USGS near Venus, Tx on May 7, 2015, SMU decided to expand their North Texas Research program into the Mansfield/Venus area and to collaborate with the University of Texas at Austin in this area.  As part of the research, SMU installed an additional 2 USGS NetQuakes stations, an additional 8 stations using equipment borrowed from IRIS, and is exchanging data with the 4 stations installed by UT in the area.  In June 16, 2015 SMU was operating 40 stations as part of the research network.  T

At the end of this text you will find an image of the regional stations in Texas operating after 12/15/2013.

How many permanent stations are needed in North Texas?

The role of permanent stations in an integrated network is to first detect earthquakes where no portable network exists, provide accurate enough locations and magnitudes to make an initial assessment of the hazard that the earthquakes might represent, and to provide a long-term monitoring network for any active faults.  Networks with more stations generally mean better detection of small magnitude earthquakes, better estimates of magnitudes and locations, and better estimates of time constants for developing earthquake sequences.

To answer the question with some scientific basis, seismologists typically look at network simulations or calculations that take into account the detection capability of stations in various locations.  The network is designed to solve a particular problem, for example to have a 90% probability of detection of any earthquake larger than magnitude 2.0, and to be able to locate the hypocenter to an accuracy of +/- 1 mile for earthquakes within 150 miles of DFW.  In addition it might be designed to reliably detect earthquakes down to magnitude 1 for any fault showing recent activity. (These are arbitrary figures - so far no one has published a set of criteria for Texas).  Alternatively, in seismically active areas seismologists evolve a network, starting with roughly evenly spaced seismic stations and then infilling where the network does not meet the defined capability criteria.

As a rough illustration, the magnitude 1.1 Irving/Dallas earthquake occurring at 16:01 UTC Jan. 23, 2014, was barely visible on only one existing regional station about 40 miles from the station.  To reliably detect an earthquake, seismologists like to see the earthquake at three or more stations.  In urban areas, where traffic can often “blind” a station for a moment, additional stations are used. 

The magnitude 2.0 Venus, Texas earthquake occurring at 5:45 UTC Nov. 2, 2013 in an area with recent activity not monitored by portable stations, was recorded by nine regional stations BUT only one of the stations was within 100 miles of the earthquake, producing a location uncertainty so large that it would be impossible to attribute the earthquake to any specific fault in the area.  In these situations, locations may be in error by 10 miles or more.  Initial deployment of portable stations requires a large number of stations to be certain of getting enough stations near the true epicenter.

In 2008 to 2012 the EarthScope experiment covered Texas with stations spaced roughly every 45 miles (except in the urbanized DFW areas where there were none).  This network, with each station operating for two years, provided significantly better seismic coverage than the permanent stations of today, but was insufficient to reliably and accurately locate small earthquakes.  As an evolutionary improvement in the 45-mile spacing, and without developing a network criterion, SMU suggested reducing the spacing used during the experiment in North Texas to roughly half, 22.5 miles.  Combined with the few existing permanent stations, this would cover the currently active part of the North Texas area with 16-20 permanent stations.

How many portable stations do we have in North Texas? Who owns/operates them? 

Currently, SMU is the only station operator that has portable stations deployed and is producing publicly available data that may be used by the USGS or other interested parties for earthquake analysis.  SMU currently has deployed a total of 40 stations, with most borrowed from IRIS and the USGS.  Only two of the stations use SMU owned equipment, the rest have all been borrowed.  Currently 9 of the stations are scheduled to be returned at the end of 2015 with most others on indefinite, but on-call loans.  Most portable networks operate only for a year or two.

In June, 2015, researchers from the University of Texas at Austin deployed 4 seismic stations in Johnson County.  SMU is collaborating and exchanging data with these researchers.

How many portable stations do we need in addition to those?

Because some of the earthquake sequences seem to continue over a number of years, some of the portable stations might need to stay in a location for 5 or more years.  In our current situation, none of the portable stations is likely to be available for that period of time.  For each earthquake sequence, SMU has found that having a set of 10 instruments provides enough initial coverage.  While it is possible to cover an area with as little as five instruments, provided there is a good permanent network with the personnel to operate and do routine analysis, the coverage is not as reliable and it takes substantially more time to establish the optimum placement for the stations in the network.  We experienced this in the cases of Azle and Cleburne where the initial work was done with just 5 instruments.

To keep the current temporary network running, we'd like to see 15-20 portable instruments in the North Texas area.  The number of additional instruments beyond the initial 20 that could be useful is probably more limited by the funding for investigators and technicians than places to put seismometers.  An investigation in Johnson County where there has been recent activity is using about 10 additional instruments.  There are areas to the north and west that are potential areas of study as well.  To some extent this also depends on the type of installation.

Is the number of stations proposed under TexNet enough?

The network design for TexNet (station locations, equipment capability, emplacement depth, etc.) has not been published and we are therefore unable to comment on the capabilities of the network for earthquakes in North Texas.  However, if the total number of stations is spread evenly over the state, the network capability in heavily urbanized regions may not be adequate for induced earthquake hazard assessments.  It may be that an uneven network could improve the capability. 

However, it isn't just the number of stations that is needed, but also an analysis and alert capability.  As additional stations are added to the network, additional manpower is needed to conduct data analysis in a timely enough manner to support the more intensive portable deployments.

Additional stations, coupled with a local analysis center, provide two advantages:  First, the overall detection threshold (or catalog completeness) magnitude is reduced.  Thus, if we currently reliably detect all M3.0 earthquakes in Texas, the additional stations may allow us to capture data from more earthquakes - such as those at M2.5 or lower.  While smaller earthquake are not generally considered damaging, they do provide insight into the probability and location of possible larger earthquakes, and if induced, may give an indication of areas where some form of mediation or changes in practices are needed.  It is also an open research questions as to whether the characteristics of small earthquakes may give an indication as to whether a particular sequence is likely induced or natural.  Knowing this may refine hazard estimates.

The map below shows the network of permanent stations currently operating in Texas:

About Monitoring Stations Networks