A “strange phase” in PP precursor

Abstract:

Teleseismic PP waves and their precursors have been routinely used to study the upper mantle discontinuities as well as lower mantle scatterers (or discontinuities) in recent studies. By extracting the arrival time and slowness of the weak precursors, we are able to locate these heterogeneities in the mantle. For most precursors, the slowness is close to that of PP. However, some precursors signals are still not well understood. In this study, in many records with the epicenter distance larger than 100º, we often observe a “strange phase” visible as a PP precursor, which arrives 20-50s before PP but with a slowness between PP and P_diff, which is not present for reference Earth models. In order to determine the origin of this strange phase, we collect the vertical-component records of China National Seismic Network of large earthquakes (Mw>5.8) that occurred between 2010 and 2020 within the epicentral distance range from 80° to 130°. The whole network is divided into 5°-15° square subarrays according to their geographical distribution and 4th-root vespagrams are generated for each subarray and tests are carried out to confirm that the wave travel on the great circle path. We observe strong strange phases from earthquakes that occurred in the region of Central America, Tonga-Fiji, and Africa. We find that the strange phases can be divided in two forms: i) a strong arrival with a unique slowness and ii) a wave train with gradually increasing slowness. Those two forms may reflect different types of lower-mantle structures. To determine the location and physical properties of heterogeneities generating the strange phase we performed 1D and 2D waveform simulations. Our results indicate that the “strange phase” may originate from a low-velocity layer in the lower mantle, which may represent a chemically distinct region related to the accumulation of subducted oceanic crust or lithosphere. But the arrival cannot always be explained by a topside or underside reflection, waves diffracted along the surface can explain some observations. Deciphering the origin of the strange phase will provide a new way to search for deep mantle structures.