Comparison of actual and seismologically inferred stress drops in asperity-type dynamic source models of microseismicity

Yen-Yu Lin, & Nadia Lapusta

Published August 15, 2017, SCEC Contribution #7786, 2017 SCEC Annual Meeting Poster #181

Estimating source parameters for small earthquakes is commonly based on either Brune or Madariaga source models. These models assume circular rupture that starts from the center of a fault and spreads axisymmetrically with a constant rupture speed. The resulting stress drops are moment-independent, with large scatter. However, more complex source behaviors are commonly discovered by finite-fault inversions for both large and small earthquakes, including directivity, heterogeneous slip, and non-circular shapes. Recent studies (Noda, Lapusta, and Kanamori, GJI, 2013; Kaneko and Shearer, GJI, 2014; JGR, 2015) have shown that slip heterogeneity and directivity can result in large discrepancies between the actual and estimated stress drops.

We explore the relation between the actual and seismologically estimated stress drops for microearthquakes numerically produced on an asperity-type fault patch. The asperity-type circular fault patch with increasing normal stress towards the middle of the patch, surrounded by a creeping region, is a potentially common microseismicity source. In our models of the patch, a number of events rupture the portion of the patch near its circumference, producing ring-like ruptures, before a patch-spanning event occurs. We calculate the far-field synthetic waveforms for our simulated sources and estimate their spectral properties. The distribution of corner frequencies over the focal sphere is markedly different for the ring-like sources compared to the Madariaga model. Furthermore, most waveforms for the ring-like sources are better fitted by a high-frequency fall-off rate different from the commonly assumed value of 2 (from the so-called omega-squared model), with the average value over the focal sphere being 1.5. The application of Brune- or Madariaga-type analysis to these sources results in the stress drops estimates different from the actual stress drops by a factor of up to 125 in the models we considered. Our current studies aim to determine whether such sources can be identified from their focal spectral signatures and studied using a more tailored seismological analysis.

Citation
Lin, Y., & Lapusta, N. (2017, 08). Comparison of actual and seismologically inferred stress drops in asperity-type dynamic source models of microseismicity. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)