Hi Kate. Your welcome. Here is some more from the Carnegie Institution. Take Care…Don in creepy town.

Alan T. Linde

Crustal Deformation and the DTM Borehole Strainmeter Program

Earthquakes and volcanic activity result from changes within the Earth. Those internal changes also cause deformation of the Earth's surface. For a number of years, working with Selwyn Sacks, I have carried out a program of both measuring such deformations in tectonically active areas and interpreting the data in terms of processes at depth. The primary source of data for this program comes from Sacks-Evertson borehole strainmeters installed in a number of different active areas, particularly California, Japan, and Iceland. These high-resolution data have allowed identification of new processes, such as slow earthquakes in seismogenic zones and changes in magma reservoirs during episodes of volcanic activity, which are not detectable with other techniques.

Work on this program in the last year has resulted in the completion, or near completion, of several studies. With Icelandic colleagues Kristján Ágústsson and Ragnar Stefánsson, we analyzed strain changes associated with a moderate (magnitude 5.8) earthquake in southern Iceland. Coseismic changes were consistent with the earthquake's parameters determined from seismic waves and allowed resolution of spatial variation in the slip distribution. Preceding and following the earthquake were slow strain changes, which were interpreted as being due to magma movement. It appears that this earthquake may have been triggered by an episode of dike growth.

Our work on slow earthquakes, based on recordings from the strainmeters, led us to reexamine reports of slow changes before the great Tonankai (1944) and Nankaido (1946) earthquakes along the Nankai trough south of Honshu, Japan. The evidence for those slow changes came from leveling surveys, tide gauge records, and reports of the drying up of water wells. Because such changes have had significance in earthquake prediction efforts in Japan, we were interested to see if the slow changes were consistent with slow earthquakes before the main shocks. Our results show that such is the case and that the slow rupture takes place on the subduction interface down-dip from the seismic rupture zone. This situation, similar to what happened before the 1960 great Chile earthquake, is such that the slow events increased the shear stress in the seismogenic zone, presumably triggering the earthquakes.

New insight into volcanic activity has come from analyzing borehole strain data from sites close to Hekla, Iceland, and Long Valley, California. Sacks and I, together with colleagues Osamu Kamigaichi, Kenji Kanjo, and Masaaki Churei of the Japan Meteorological Agency (JMA), are analyzing the deformations caused by the 1986 eruption of Miharayama on the island Izu-Oshima in Japan. The strainmeters, which recorded that activity, are part of a network installed by the JMA as part of their earthquake prediction research program. The data show that, during the first stage of the eruption, the relatively shallow reservoir (which was the ultimate source of magma for the eruption) was continuously being replenished from a much deeper (about 30 km) source. Additionally, the rate of that replenishment was changed at the time of small volcanic earthquakes. Strain changes preceding the second stage of the eruption were clear at distances up to 50 km. Those strain changes were due to the formation and propagation of dikes from a depth of several kilometers up to the surface. Somewhat surprising was the fact that most of the magma movement from the reservoir was not to the surface but rather into a large dike that did not break the surface.

The strainmeter observational program continues to be enhanced. Over the next several years, in collaboration with the U.S. Geological Survey and the University of California at Berkeley and San Diego, about 10 new sites will be instrumented in California in the San Francisco Bay area. In the summer of 1999, borehole strainmeters together with tiltmeters and seismometers were installed for the first time in deep drill holes (about 1,100 m) below the seafloor, as described at greater length below. Data from these sites should yield new insight into the processes of plate motion and earthquake generation.

• Alan Linde