Major Projects &
LARSE II Frequently Asked Question
Typical questions about the LARSE project are as follows:
1) Why was the project done?
The Southern California Earthquake Center and the U.S. Geologic Survey conducted the seismic imaging survey of the Los Angeles region in October 1999 as part of the National Earthquake Hazards Reduction Program. Four goals of this Program that our survey was designed to address are as follows:
1) To study the geologic structure beneath the Los Angeles Region, so that we can better understand the processes by which earthquakes are generated. This goal includes identifying active faults and defining their geometry. It also includes determining the type and distribution of various rock layers in the subsurface.
The 1987 M 5.9 Whittier, and the 1994 M 6.7 Northridge earthquakes have awakened all of us to the fact that there are many active "blind" thrust faults in the Los Angeles basin. These faults can only be detected (prior to large earthquakes on them) by seismic imaging of the type we will perform. In order to accurately assess seismic hazards, we must know where these faults are located.
2) To acquire data needed for the prediction of strong ground shaking during future large earthquakes. Two important factors that contribute to strong ground shaking are the thickness and seismic velocity of sedimentary rocks. Shaking is stronger for greater thickness and for lower seismic velocities in sedimentary rocks. ("Seismic velocity" is the speed at which seismic waves travel through a given material.) A third factor contributing to strong ground shaking, that became evident during the 1994 M 6.7 Northridge earthquake is focusing of seismic waves by deep rock reflectors in the earth's crust. In pursuing goal 1, we will identify regions that are underlain by significant thicknesses of low-velocity sedimentary rocks, and we will identify deep rock reflectors. Thus, we will be able to predict areas that will shake strongly during future large earthquakes. Information on ground shaking can be used in designing buildings to make them safer.
3) To better locate earthquakes. Our survey will calibrate the permanent Southern California seismographic network, permitting us to more accurately locate earthquakes.
4) To communicate earthquake hazards information to the public. We hope to take advantage of LARSE II, as we did in previous LARSE surveys, to communicate earthquake hazards information to the public.
The first surveys of LARSE, carried out in 1993 and 1994, imaged structures chiefly along "Line 1", which extends northeastward from Seal Beach across the Los Angeles and San Gabriel Valley basins, the San Gabriel Mountains, and Mojave Desert (Figure 1). Line 1 was designed to investigate Earth structure near the 1987 M 5.9 Whittier Narrows and 1991 M 5.8 Sierra Madre earthquakes. The surveys of LARSE II image structures along "Line 2", which extends northward from the coast at Santa Monica through the Santa Monica Mountains, San Fernando Valley, Santa Susana Mountains, Transverse Ranges, and western Mojave Desert. Of considerable interest on Line 2 is structure near the 1994 M 6.7 Northridge earthquake.
During our 1994 survey, we recorded offshore airgun blasts along the onshore part of Line 2 (Figure 1). In order to complete seismic imaging along Line 2 and make this imaging comparable to that on Line 1, the remaining tasks included detonation of onshore buried explosives (called an "active" survey) and "passive" recording of earthquakes.
1) Vibrating-truck sources, such as used by the oil industry for oil exploration, are inadequate for producing clear images at these depths.
Buried explosives were detonated in 8-inch, partly cased boreholes below a depth of 60 feet. The total depth of each borehole varied with charge size. The explosive was a commercial ammonium-nitrate-based product that was pumped into the boreholes. The explosive was covered, or "tamped", with approximately 60 feet of drill cuttings or gravel for containment. The explosive was inert until it was "primed" just prior to on the night of the shot. Detonations were triggered at night, when wind and cultural noise are at their lowest levels at our seismograph sites.
Approximately 93 buried detonations, ranging in size from 10-4000 lbs., were performed along Line 2 for LARSE II. These detonations were recorded by approximately 1000 seismographs spaced 100 meters (~300 feet) apart.
Drilling took place during the 3-month period prior to the survey. Next, the boreholes were be loaded with explosive, capped, locked, and covered with dirt (for camouflage purposes). After deployment of the seismographs, the detonations were triggered one after another over a 3- to 5-night period.
3) Did the detonations trigger earthquakes?
To our knowledge, the only events that DO trigger earthquakes are major earthquakes. The M 7.3 Landers earthquake of June 1992 triggered a M 5.2 earthquake in southern Nevada and numerous smaller earthquakes at several volcanic areas in the western U.S., including Mammoth Lakes, CA, the Geysers, CA, and Yellowstone National Park. The Landers earthquake represents 10's of millions times the energy in our detonations.
4) Did the detonations damage water supplies?
In our 30 years of experience, we have never damaged a spring or well, although we have detonated within a few hundred feet of springs and wells. Except for cases where explosives are detonated directly in a spring or well, the only events that affect springs and wells are major earthquakes. (Major earthquakes apparently increase upper-crustal porosity, by shaking and opening of cracks, and cause water tables to be lowered.)
In siting our boreholes, we used tables of ground velocity that we have established from years of blasting experience in order to ensure that we are beyond the lowest damage threshold for human structures (2 in/sec). That is not to say that our detonations were felt (see 4 below).
Most detonations could be felt only within a few hundred feet of the borehole. The larger detonations could be felt for a few 1000 feet. We made an effort to keep the boreholes well away from houses in order not to disturb people at night. Unfortunately, a few people may have felt the detonations. Prior to the project we communicated the purposes and effects of our activities with the public by way of city council meetings, radio, newspaper, and TV.
The detonations usually sound like a dull "thud." Occasionally, when steam is vented, a hiss or dull roar will occur for a period of seconds following the detonation.
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