A new algorithm for three‐dimensional joint inversion of body wave and surface wave data and its application to the Southern California plate boundary region

Abstract

We introduce a new algorithm for joint inversion of body wave and surface wave data to get better 3‐D P wave (Vp) and S wave (Vs) velocity models by taking advantage of the complementary strengths of each data set. Our joint inversion algorithm uses a one‐step inversion of surface wave traveltime measurements at different periods for 3‐D Vs and Vp models without constructing the intermediate phase or group velocity maps. This allows a more straightforward modeling of surface wave traveltime data with the body wave arrival times. We take into consideration the sensitivity of surface wave data with respect to Vp in addition to its large sensitivity to Vs, which means both models are constrained by two different data types. The method is applied to determine 3‐D crustal Vp and Vs models using body wave and Rayleigh wave data in the Southern California plate boundary region, which has previously been studied with both double‐difference tomography method using body wave arrival times and ambient noise tomography method with Rayleigh and Love wave group velocity dispersion measurements. Our approach creates self‐consistent and unique models with no prominent gaps, with Rayleigh wave data resolving shallow and large‐scale features and body wave data constraining relatively deeper structures where their ray coverage is good. The velocity model from the joint inversion is consistent with local geological structures and produces better fits to observed seismic waveforms than the current Southern California Earthquake Center (SCEC) model.

Internal structure of the San Jacinto fault zone at Blackburn Saddle from seismic data of a linear array

“…This is consistent with tomographic results for the SJFZ region based on local earthquakes and ambient seismic noise (Allam & Ben-Zion 2012;Zigone et al 2015;Fang et al 2016). The inferred contrast is also consistent with the surface geology, showing pre mid-Cretaceous banded gneisses on the southwest side of the fault juxtaposed against mid-Cretaceous tonalitic rocks on the northeast (Sharp 1967).…”

Ground Motion Residuals, Path Effects, and Crustal Properties: A Pilot Study in Southern California

“…None of these other metrics show relationships with the path residual, either with magnitude or distance dependence. This suggests that bulk velocity is not related to path‐specific ground‐motions for the Fang et al () model and that heterogeneity in seismic attenuation is also not related.…”

“…FM3D is based on the fast‐marching method, which uses a finite‐difference eikonal solver. Thus, we find the fastest‐arrival raypaths for a given velocity model and compute the P wave and S wave raypaths for the Fang et al () V P and V S models, respectively (though in this paper we focus on the rays for the V S models only as they are the only values that show robust relationships). The vast majority of rays travel through the crust and are upgoing from the event location.…”

“…In a physical sense, this is representative of the degree of heterogeneity in velocity along a raypath (i.e., larger dVI represents greater heterogeneity). We apply this metric to the velocity model of Fang et al (), which features a grid spacing of 0.03° in both latitude and longitude (~3‐km horizontal spacing). Another metric we devise and investigate is the normalized path integral metric for attenuation, QI norm : where Q max is the maximum value of Q found in the crustal model; the numerator is the path integral of seismic quality factor (here denoted Q S ), and the denominator is the path integral of seismic quality, but assuming that the value of Q along the entire ray is Q max .…”

“…For every recording ij in the data set, we first interpolate the raypath obtained when raytracing through the Fang et al () crustal property model, to obtain the property value at every discretized point along the ray. Then, along each ray, we compute these metrics for the values of the gradient of velocity for the model of Fang et al () and seismic quality of Hauksson and Shearer (). These results are then compared to the path residuals for recording ij .…”