Source parameters and rupture velocity of smallM≤ 2.1 reservoir induced earthquakes

Abstract

S U M M A R YWe calculate stress drop and rupture speed for M L ≤ 2.1 shallow reservoir induced earthquakes and find them to be similar to those of large, natural earthquakes. Previous studies have suggested that hydrofractures, mining and reservoir-induced earthquakes have lower average stress drop than natural tectonic earthquakes. This difference might result from the different tectonic setting or the shallower hypocentral depths of induced earthquakes. Alternatively, difficulties in correcting for attenuation and site effects in earlier studies may lead to underestimation of stress drop. In addition, most studies assume the rupture velocity of small reservoir induced earthquakes to be the same as for the large earthquakes. We analyse a set of 101 M L ≤ 2.1 earthquakes induced by changing water level in the Açu Reservoir, NE Brazil. The earthquakes are shallow, (depth <5 km) and the region has negligible natural seismicity. We use three different approaches to calculate the source parameters of the six largest (1.9 ≤ M L ≤ 2.1) earthquakes. We model the individual spectra to find corner frequency, frequency-independent Q, and long period amplitude. We use collocated small earthquakes as empirical Green's functions to calculate the spectral ratios, and determine the relative source time functions. Estimates of the source duration and corner frequency imply stress drops in the range of 26-179 MPa. These are similar to, or higher than tectonic earthquakes suggesting that the shallow hypocentral depth and the presence of water do not affect stress drop. We observe clear directivity for one of the earthquakes, and use the azimuthal variation in pulse width to estimate a rupture velocity of ≥0.6β.

Self‐similar rupture implied by scaling properties of volcanic earthquakes occurring during the 2004‐2008 eruption of Mount St. Helens, Washington

“…Rupture velocity is limited by the Rayleigh wave velocity for subshear rupture speeds, suggesting that rupture velocity is some fraction of the shear wave speed. Although observations suggest a wide range of values, typical values for shallow earthquakes range from V r ≈ (0.6-0.9) [Freund, 1979;Imanishi et al, 2004;Kanamori and Brodsky, 2004;Yamada et al, 2005aYamada et al, , 2005bTomic et al, 2006;Dreger et al, 2007]. Therefore, if f 0 ∝ 1∕ and = r∕V r , it follows that f 0 is proportional to the shear wave velocity, assuming that rise time is significantly shorter than rupture time [Heaton, 1990].…”

Migration of earthquakes with a small stress drop in the Tanzawa Mountains, Japan

“…Goertz- Allmann et al (2011) analyzed stress drops of earthquakes induced by a waterinjection experiment into a borehole and obtained a clear increase in the stress drop as a function of distance from the injection point, thus suggesting that the stress drop correlates with pore pressure perturbations due to the injection. Tomic et al (2009) estimated stress drops of reservoir-induced earthquakes with M ≤ 2.1 and concluded that the values were similar to those of natural earthquakes. They speculated that the reservoirinduced earthquakes they had analyzed were intraplate earthquakes in the fresh rock and these showed higher stress drops than the values of other fluidinduced earthquakes.…”

Attenuation from microseismic datasets by the peak frequency method benchmarked with the spectral ratio method

“…Seismologists use quality factor (Q) to invert earthquake source properties (e.g., Tomic et al, 2009), characterize medium in a reservoir (Aki et al, 1982;Klimentos, 1995), etc. Attenuation determines detectability of perforations and microseismic events in microseismic monitoring (e.g., Einspigel and Eisner, 2014).…”