We report two decades of changing behavior of the Earth's inner core boundary (ICB), which provides the simplest explanation for the observed temporal change of the compressional seismic waves that are reflected from the ICB (PKiKP) and refracted in the inner core (PKIKP), from earthquake doublets occurring in South Sandwich Islands between 1993 and 2013. In the early period (before 2003), the ICB is enlarged beneath the western coast of Gabon, Republic of Congo, and southwest Tanzania in the reflected points of the PKiKP observed at seismic stations OBN, AAK, and ARU, while it experiences little change beneath Zimbabwe or/and Kenya, and beneath west Angola or/and north Central African Republic, in the PKIKP entry or/and exit points of AAK and ARU observations, respectively. In the later period (after 1998), the ICB regions beneath the western coast of Gabon, Republic of Congo, and southwest Tanzania either shrink or remain unchanged, and the temporal change migrates to beneath Zimbabwe or/and Kenya, and beneath west Angola or/and north Central African Republic, with a decrease of inner core surface by 5.59 km between 1998 and 2009 beneath Zimbabwe or Kenya and by 1.73 km beneath west Angola or north Central African Republic between 1998 and 2013. These results indicate that ICB temporal change occurs in localized regions and is episodic, rapidly migrating, and alternately enlarged and shrunk.

We discuss the question whether inner core (IC) differential rotation or temporal change of the IC surface provides a consistent interpretation for temporal changes of the IC-related phases and their coda. While temporal change of the IC surface is required and provides a consistent explanation to all the seismic observations, we present three lines of seismic evidence showing that IC differential rotation cannot provide a consistent or reasonable interpretation for the observed temporal change of seismic waves from repeating earthquakes in the South Sandwich Islands (SSI) and the Middle America subduction zone. (1) Changed PKIKP/PKiKP coda between events in a doublet in SSI indicates an IC surface scatterer that simply disappeared, with no associated energy in the later event for any assumed IC differential rotation.(2) Within a cluster in SSI, comparisons between temporal changes of PKIKP wave and its coda of the earlier and later event pairs yield contradictory estimates of differential rotation rate change by a factor of at least 27, using different portions of seismic data. (3) The seismic data from repeating earthquakes in Middle America indicate a PKiKP temporal change of 0.017-0.04 s on a timescale of 8-85 days, requiring an unreasonable rotation rate of at least 8.6°/year. We conclude that the observed temporal changes of IC phases are caused by temporal changes of the IC surface, which occur in some localized regions within a timescale of days or months, a phenomenon that should provide important clues to our understanding of core dynamics.

P‐wave azimuthal anisotropic tomography reveals that the July 6, 2019 Mw 7.1 Ridgecrest earthquake occurred in a region with clockwise crustal rotation. The rotation together with the sinistral slip on the Garlock Fault is a response to the northwest‐trending, dextral shear within the Eastern California Shear Zone due to the relative motion between the Pacific and North America Plates. The hypocentral area of the Ridgecrest mainshock is characterized by a sharp lateral velocity contrast which has a reversal in contrast polarity at about 5 km depth. We find high Vp/Vs ratio structures covering the rupture zones of the Mw 6.4 foreshock and the Mw 7.1 mainshock, which may indicate the existence of fluids in the fault zones. We speculate that fluids and crustal rotation may have played important mechanical roles in causing the 2019 Ridgecrest earthquake sequence.

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