Resolving Complicated Faulting Process Using Multi‐Point‐Source Representation: Iterative Inversion Algorithm Improvement and Application to Recent Complex Earthquakes

Abstract

Complicated faulting processes of large earthquakes may involve simultaneous or sequential rupture of multiple fault planes, often with different focal mechanisms. Determination of these subevent focal mechanisms is essential for multifault parameterization and complex rupture process analysis. Iterative inversion for Multi‐Point‐Source (MPS) faulting representations using broadband teleseismic body waves is an established strategy to estimate the focal mechanism variation but tends to be an unstable procedure. We enhance an iterative inversion algorithm developed by M. Kikuchi and H. Kanamori by specifying extra a priori constraints in the inversion. These constraints include the specific search time window, location, and source time function of each point source subevent to enable controls over the temporal and spatial properties of each point source. We apply the improved algorithm to seven recent earthquakes with complex faulting: the 2008 Wenchuan, 2009 Samoa, 2010 El‐Mayor Cucapah, 2010 Darfield, 2012 Indian Ocean, 2016 Kaikoura, and 2018 Gulf of Alaska earthquakes. The faulting complexity of these events has been independently constrained using geodetic, seismic, tsunami, and/or field observations, enabling tests of the MPS inversion algorithm performance. The improved iterative inversion resolves varying focal mechanisms of multiple point sources with specific timing consistent with detailed solutions, but spatial resolution tends to be relatively low (>20 km). Summation of subevent point‐source solutions recovers 30% to 95% of the long‐period point‐source seismic moments, varying with different levels of parameterization. The improved iterative MPS inversion algorithm is a promising approach for prompt (within a few hours) analysis of primary focal mechanism variation with reasonable overall space‐time distribution for complicated events.

Rupture Process of the 7 January 2020, MW 6.4 Puerto Rico Earthquake

“…The majority of strong‐motion stations are located on coastal areas of Puerto Rico, which are covered with carbonate and other sedimentary rocks (Jolly et al, 1998), so misfit of minor features at some stations may be due to uncertainties in the velocity model. The observed teleseismic P waves (Figure S5) have some variable coda signals between 15 and 30 s after the P arrival that are not accounted for by our model, and we also observed some relatively large later arrivals after 40 s in the teleseismic P signals but only very weak energy in the strong‐motion recordings after 30 s. The teleseismic P coda is not readily modeled simply by allowing for prolonged rupture on a single plane, so we applied a multiple point‐source inversion code (Yue & Lay, 2020), without finding robust secondary sources compatible with the P and SH observations. This was confirmed by performing joint finite‐fault inversions with normal‐fault and strike‐slip fault geometries, which did not place significant slip on the strike‐slip faults assigned orientations similar to those of some of the foreshocks and aftershocks.…”

Seismological Characterization of the 2021 Yangbi Foreshock‐Mainshock Sequence, Yunnan, China: More than a Triggered Cascade

“…We adopt the multipoint‐source (MPS) inversion technique (Yue & Lay, 2020) to resolve the moment tensor of the largest foreshock and the mainshock. The MPS method utilizes different subevents to model three‐component broadband records in the near field.…”

“…The MPS method utilizes different subevents to model three‐component broadband records in the near field. It is primarily developed by Kikuchi and Kanamori (1982, 1986, 1991) and is improved by Yue and Lay (2020) with an iterative inversion algorithm. In this method, a priori constraints are set on the search time window of subevents, their potential location (mesh grids), and the shape of STF.…”

Supershear triggering and cascading fault ruptures of the 2023 Kahramanmaraş, Türkiye, earthquake doublet

“…To determine the geometry of fault orientation at depth, we performed a multi-point-source (MPS) inversion ( 19 ) using teleseismic body waves [section S1.6 in ( 18 )]. This method decomposes the kinematic rupture process into sequential ruptures of a cluster of point sources whose locations and focal mechanisms reflect the fault planes and slips of different segments.…”