Hi All. On June 17, which was Saturday my partner and I were down in South County getting rid a ranchers ground squirrels. We were sitting on a hillside near some springs. It was very quiet with just a gentle breeze blowing. I’m not sure of the time, but it was slightly after 1:00PM when we heard a noise that sounded a little like a large flat board striking water about 2 seconds apart and lasted maybe 10 seconds.

Both of us having the better part of 60 years in the outdoors knew no animal made that sound.

Later that day I learned there was an earthquake in Iceland.

The June 17 earthquake had the following, preliminary, seismological parameters: According to the Icelandic Meteorological Office (IMO) the origin time was 15:40:40.94 GMT, the hypocenter 63.97°N and 20.37°W, and the depth 6.3 km. Preliminary modeling using volumetric strainmeters in the area gave the moment 6.1×1018 Nm, corresponding to moment magnitude of 6.4, noting however that the single fault model used did not comply with all the data, and thus indicating a more complicated model than a single strike-slip fault.

I later contacted Lowell Whiteside and told him what I heard and described the surrounding area to him. He told me that maybe; just maybe we heard the Rayleigh wave. What we heard just might have been the Rayleigh wave disturbing the springs.

Then early this morning while doing some research for Petra I came across this. It is an abstract done by D.J. Andrews and Y. Ben-Zion.

I don’t understand all of the implications this may have in regards to triggering quakes at great distances, but the possibilities are there. Take Care…Don in creepy town.

“The studies done under this proposal attempt to clarify the properties and implications of dynamic rupture and radiated fields in laterally heterogeneous fault zones. Weertman [1980] solved analytically a 2D problem of steady-state rupture along a material interface, and showed that a pulse of slip velocity associated with dynamic reduction of normal stress can propagate along the material interface in a self-sustaining manner. Andrews and Ben-Zion [1995, 1997] used 2D plain-strain finite difference calculations to simulate dynamic ruptures along an interface separating different elastic media. They found that a pulse of the type predicted by Weertman can propagate along a material interface in a wrinkle-like mode, which generates little frictional heat. The wrinkle-like pulse exhibits other interesting dynamic phenomena, including unidirectional propagation and narrowing of the pulse with propagation distance along the material interface. The latter leads to spontaneous break up of the pulse during propagation to a number of smaller pulses. Anooshehpoor and Brune [1996] performed laboratory sliding experiments with two different foam rubber blocks and observed rupture properties compatible to first order with the calculations of Andrews and Ben-Zion.

To simplify the analysis and focus on first order effects, the calculations of Andrews and Ben-Zion employed 20% velocity contrast across the fault, constant coefficient of friction f = 0.75, and uniform initial stress. The research discussed here concentrates on clarifying the range of values of elastic contrast, friction coefficient, and level of strength/stress heterogeneities allowing for the existence of the wrinkle-like pulse. Rupture is initiated in the simulations by imposed slip in a limited space-time domain. Outside the region of the imposed slip, the pulse becomes narrower and higher with propagation distance along the interface. The strength of the wrinkle-like pulse increases with S-wave velocity contrast up to a maximum at about 35% contrast. Beyond such a velocity contrast there is no solution for a generalized Rayleigh wave along a material interface and the strength of the pulse decreases. However, the wrinkle-like pulse can still propagate in a self-sustaining manner for larger velocity contrasts. For a fixed S-wave velocity contrast, the strength has little dependence on density contrast or Poisson's ratio, but the pulse strength increases rapidly with increasing coefficient of friction. Stress/strength heterogeneities with small correlation length have little effect on the pulse, while long wavelength heterogeneities reduce the strength of the pulse. The high mechanical efficiency of the wrinkle-like pulse suggests that earthquake ruptures may favor such mode of failure when possible. We also modified the finite difference code used by Andrews and Ben-Zion to incorporate a number of fault zone layers, multiple faults with possible stepovers, and laboratory-based friction laws. The modified code will be used in future studies.

In addition to the above, we investigated seismic fields radiated from laterally heterogeneous fault zone structures, using a generalized version of the 2D analytical solution of Ben-Zion and Aki [ 1990]. The results indicate that there are significant trade-offs between propagation distance along the structure, FZ width, velocity contrast, source location within the FZ, and Q. The calculations demonstrate that the wavefield depends strongly not only on receiver distance from the FZ, but also on receiver depth below the free surface. The calculations show that the zone connecting sources generating FZ waves and observation points with appreciable wave amplitude can be over an order of magnitude larger than the FZ width.”