EES Focus AreasEES Group Pages |
Shake, Rattle, and Roll: The Los Alamos Seismic NetworkLA-UR-06-7624 "Mommy, do we have earthquakes here in New Mexico?" Do you know the answer to that? The answer is Yes, we do! In fact, several dozen earthquakes occur in northern New Mexico each year. A quick Google search on "Los Alamos" and "seismic network" or "seismic hazard" and you'll soon discover that not only does Los Alamos have earthquakes but LANL has its very own seismic network too. In fact, it's the only one in northern New Mexico and has located more than 2500 earthquakes in north-central New Mexico since its inception. The Los Alamos Seismic Network (LASN) has been run continuously by members of the Geophysics group since the Fall of 1973. The network was originally established as part of LANL's nuclear test ban treaty verification capability but is now used by engineers and earth scientists for LANL's seismic hazards program. Back in the 1970s we ran more than 20 stations (http://neic.usgs.gov/neis/station_book/LOA_NETWORK.html); now we're down to 7. Before you ask, no, we did not detect the recent North Korea event on our network and yes, we did see the much bigger recent Hawaii earthquakes. In fact, the seismogram in the title shows the ground motion recorded during the first and second big Hawaii earthquakes at one of our stations. Do we really need a seismic network at LANL? Yes, we do. The same Google search mentioned above will also turn up an interesting document about the seismic hazards at the Laboratory and nicely lays out the reasons why we need to monitor local seismicity. Interestingly, it's published by the Los Alamos Study Group, an independent watchdog organization often critical of laboratory's policies---see http://www.lasg.org/archive/1997/seismic.htm. In that document, although it is a bit dated and distorted, is a nice description of the seismic potential and general kinds of seismic hazards concerns for the area around Los Alamos. We're at the western margin of the Rio Grand Rift, only about 5 miles away from the Jemez volcanic field, and also only about 5 miles away from the Valles caldera, one of the three largest, youngest and potentially active calderas in the United States. More importantly, to Los Alamos residents and LANL facilities, an active tectonic feature known as the Pajarito Fault System passes just west of town and has major sub-faults dissecting LANL property. LASN stations detect roughly 2 to 5 earthquakes each year from the Pajarito Fault alone. Another reason for recording and studying earthquakes is to gather historical data to predict future effects of seismic activity on people and structures, especially LANL facilities. As a nuclear facility, Los Alamos National Laboratory is required to monitor local seismic activity to comply with DOE Order 420.1B. Moreover, we also are participating members of the Advanced National Seismic System, (http://earthquake.usgs.gov/research/monitoring/anss) filling a valuable role to the nation by being able to provide earthquake information for northern New Mexico where there would otherwise be nothing. We see earthquakes that no one else sees. For those of you who are curious, one of our stations sends its signals to the rotating drum recorder at the Bradbury Science Museum in town.
Click on map for larger view
As you can see on the map figure, Los Alamos has 7 permanent stations continuously monitoring seismic activity: 4 located around the Lab, and 3 in the Jemez Mountains. Many of these stations can measure not only vertical (up/down) motion, but also record side-to-side motion (North/South and East/West). These stations are generally used to monitor the smaller seismic activity. In addition to the Pajarito fault earthquakes, we can hear whether or not our local volcano, the Valles caldera, has indigestion—it's actually very quiet—or possibly feel the effects of pumping in the Buckman well field or aquifer recharge events. We can also listen to seismic activity on the Nacimiento uplift west of the Jemez Pueblo. Naturally, we also pick up our share of explosions, mine blasts, sonic booms, thunderstorms and, of course, distant earthquakes. Every event needs to be examined as a potential source of trouble. This year alone we've had dozens of small local earthquakes, the smallest just below magnitude 1, the largest about magnitude 2.4. Recently, there has been a swarm of local events in the Caja del Rio Plateau area (see map above and waveforms below), which is interesting because of that area's past volcanic history and because of its proximity to the Buckman wells field, a source of water for the city of Santa Fe. To get a better handle on where any future earthquakes in this area are located, we recently installed a new temporary station just west of Santa Fe (labeled LKS on the map). Eventually, we hope to permanently reoccupy one of the old abandoned stations located in the middle of the Caja del Rio Plateau.
We also field several strong motion sensors placed at critical laboratory facilities. These "forensic data" instruments record data only in the event of a strong earthquake. With such information, engineers can help design buildings that are survivable during future large earthquakes. Over the last 11,000 years, there is geological evidence of at least 3 large events around magnitude 7 associated with the Pajarito Fault System, and the fault is definit*ely still active. "Engineers are the consumers of the information that geoscientists come up with," to quote Jamie Gardner (EES-9). "It's their [engineers] job to predict how structures will be affected by seismic activity. Our job is really public safety, and predicting the probable effects from a local quake on facilities at the Laboratory and in Northern New Mexico." |
|
