An experimental study of thermally induced microfracturing and its relation to volcanic seismicity

Abstract

In this study, unconfined samples of basalt, sintered and bonded perlite, insulating firebricks, and paraffin were subjected to thermal gradients ranging from 15øC/cm to 100øC/cm. Microshocks produced by thermoelastic stress relief were detected by means of miniature accelerometers. Histograms of microshock occurrences have similar shapes for all tests. Activity begins and increasesabruptly following onset of heating or cooling of the sample. It reaches a maximum, and then decays hyperbolically, in approximate correlation with the time dependence of the thermal gradient. There are later swarms of activity apparently initiated by small thermal fluctuations in the sample. The log-log plots of cumulative number of microshoCks versus amplitude were constructed and show kinks or knees. The value of the negative slope b for the linear approximations to these plots ranges from 1.2 to 2.7. These values are greater than those for normal earthquakes series and are similar to those observed for volcanic B-type earthquakes. The effects of a cellular crack pattern, nonuniformity of stress due to the thermal gradient, and sample inhomogeneity may explain these results. shocks were observed. Lamont-Doherty Geological Observatory Contribution No. 1528.In the first part of this paper we describe the model study and our observed results. In the second part we compare our results to those obtained in earthquake, microearthquake, and microfracture studies. TEST SAMPLESTest materials included basalt, insulating firebrick, paraffin, and perlite. Specimens were generally brick size. The smallest sample tested was approximately 12 x 4 x 3 cm. Specimens used in the vacuum chamber tests, as described below, were approximately 15 X 15 X 7.5 cm.The perlite specimens were manufactured from commercial perlite (a crushed and heattreated perlitic obsidian). The perlite grains are vesicular, irregularly shaped, and microfractured. The test samples were made by two methods: (1) by compressing and sintering the loose grains into bricks, and (2) by bonding the grains with sodium silicate.The insulating firebricks are commercial refactory bricks principally composed of silica and alumina, in a 56/40 ratio by volume. The paraffin samples are polycrystalline and not amorphous. The sample of basalt is aphanitic and nonvesicular with nodules of olivine. The basalt sample was obtained from Vulcan's Throne in the Grand Canyon. 4455 4456 WARREN AND LATHAM

Focal mechanisms and size distribution of earthquakes beneath the Krafla central volcano, NE Iceland

“…McNutt [2005] recognized that there is often a characteristic temporal b-value sequence associated with volcanic intrusions and eruptions. The first short-term b-value peak is attributed to high geothermal gradients [Warren and Latham, 1970], whereas a following longer-lived b-value peak is caused by an increase of the pore pressure analogous to a reservoir undergoing fluid injections [Wyss, 1973]. Thereafter, b values return to normal crustal levels.…”

“…Some factors affecting the b-value are material heterogeneity [Mogi , 1962], thermal gradient [Warren and Latham, 1970], and applied stress [Scholz , 1968;Schorlemmer et al, 2005]. For tectonic regions, b averages to about 1.0 [Frohlich and Davis, 1993].…”

Evidence of non-extensivity in the seismicity observed during the 2011–2012 unrest at the Santorini volcanic complex, Greece

“…While for tectonic earthquakes b has typical values close to 1, higher b-values (up to 3) have been reported for volcano related earthquakes (McNutt, 1996). High b-values associated with volcanic areas have been attributed to crustal heterogeneities (Mogi, 1963), high thermal gradients (Warren and Latham, 1970) and high pore-pressures at the vicinity of the magma chambers, all of which can cause a decrease of the effective normal stress (Wyss, 1973). The FMD for the given data set has a bimodal character (Fig.…”

The locations of magma chambers at Mt. Etna, Italy, mapped by b‐values

“…Wiemer and McNutt [1997] The b-values higher than the usual (b•l), can be achieved by lowering the effective stress through high pore pressure [Wyss, 1973] and by increased heterogeneity of the medium [Mogi, 1962]. Also, there is some evidence that high temperature gradients may be associated with high b-values [Warren and Latham, 1970 The total number of earthquakes in the catalog of the IIVC with M >_Me is 450. The volume within which this seismicity takes place is about 25 km deep and has a radius of about 10 km.…”