Near-Source Waveform Modeling to Estimate Shallow Crustal Attenuation
and Radiated Energy of Mw 2.0-4.5 Earthquakes
Abstract
Estimating the radiated energy of small-to-moderate (Mw < 5)
events remains challenging because their waveforms are strongly
distorted during wave propagation. Even when near-source records are
available, seismic waves pass through the shallow crust with strong
attenuation; consequently, high-frequency energy may be significantly
dissipated. Here, we evaluated the degree of energy dissipation in the
shallow crust by estimating the depth-dependent attenuation (Q-1) by
modeling near-source (< 12 km) waveform data in northern
Ibaraki Prefecture, Japan. High-quality waveforms recorded by a downhole
sensor confined by granite with high seismic velocity helped to
investigate this issue. We first estimated the moment tensors for M1–4
events and computed their synthetic waveforms, assuming a tentative
one-dimensional -model. We then modified the -model in the 5–20 Hz
range such that the frequency components of the synthetic and observed
waveforms of small events (Mw < 1.7) matched. The results show
that the Q-value is 55 at depths of < 4 km and shows no
obvious frequency dependence. Using the derived -model, we estimated the
moment-scaled energy (eR) of 3,884 events with Mw 2.0–4.5. The median
eR is 3.6×10-5 , similar to the values reported for Mw >6
events, with no obvious Mw dependence. If we use an empirically derived
Q-model (~350), the median eR becomes a one-order
underestimation (3.1×10-6). These results indicate the importance of
accurately assuming the Q-value in the shallow crust for energy
estimation of small events, even when near-source high-quality waveforms
are available.