Imaging a Crustal Low‐Velocity Layer Using Reflected Seismic Waves From the 2014 Earthquake Swarm at Long Valley Caldera, California: The Magmatic System Roof?

Abstract

The waveforms generated by the 2014 Long Valley Caldera earthquake swarm recorded at station MLH show clear reflected waves that are often stronger than direct P and S waves. With waveform analyses, we discover that these waves are reflected at the top of a low‐velocity body, which may be residual magma from the ∼767 ka caldera‐forming eruption. The polarity of the reflection compared to direct P and S waves suggests that the reflection is SP waves (S from hypocenters to reflector and then convert to P waves to the surface). Because the wavefields are coherent among different earthquakes and hold high signal‐to‐noise ratios, we apply them to a wavefield migration method for imaging reflectors. The depth of the imaged magmatic system roof is around 8.2 km below the surface. This is consistent with previous studies. Even though we use only one station and waveforms from one earthquake swarm, the dense cluster of accurately located earthquakes provides a high‐resolution image of the roof.

Structure and mechanics of the subducted Gorda plate: constrained by afterslip simulations and scattered seismic waves

“…The clear arrivals between the direct P and S phases are suggestive of converted phases and we confirm the sense of the conversion (Ps or SP) using a source side 3D beamforming method (Spudich and Bostwick, 1987;Nakata and Shelly, 2018) and estimate the propagation directions and velocities of theses phases as they leave the earthquake source region.…”

Source‐Independent Passive Seismic Reverse‐Time Structure Imaging With Grouping Imaging Condition: Method and Application to Microseismic Events Induced by Hydraulic Fracturing

“…As illustrated in Figure , the PS image from reverse‐time imaging is obtained by applying the imaging condition to the back‐propagated P and S wavefields. In comparison, PS image from conventional elastic reverse‐time migration methods is obtained by applying the zero‐lag imaging condition to the forward‐propagated P / S wavefield and back‐propagated S / P wavefield (Figure ) (Nakata & Shelly, ; Yan & Sava, ). Both methods take advantage of the coherence between the P and S wavefields to image the conversion points (i.e., the discontinuities), and both methods can be expressed with equation .…”

Surface Imaging Functions for Elastic Reverse Time Migration

“…Sources at depth (drill bit noise) were exploited in this fashion in a similar application for imaging the San Andreas Fault at SAFOD (Vasconcelos & Snieder, 2008). Passive seismic imaging with local microseismicity at SAFOD and elsewhere (e.g., Nakata & Shelly, 2018;Reshetnikov et al, 2010;Schmelzbach et al, 2016;Zhang et al, 2009) have also proven feasible based on favorable source and receiver configurations.…”