Semiautomated Estimates of Directivity and Related Source Properties of Small to Moderate Southern California Earthquakes Using Second Seismic Moments

Abstract

We develop a semiautomated method for estimating with second seismic moments the directivity, rupture area, duration, and centroid velocity of earthquakes. The method is applied to 41 southern California earthquakes with magnitude in the range 3.5-5.2 and provides stable results for 28 events. Apparent source time functions (ASTFs) of P and S phases are derived using deconvolution with three stacked empirical Green's functions (seGf). The use of seGf suppresses nongeneric source effects, improves the focal mechanism correspondence to the analyzed earthquakes, and typically allows inclusion of 5 to 15 more ASTFs compared with analysis using a single eGf. Most analyzed earthquakes in the Trifurcation area of the San Jacinto Fault have directivities toward the northwest, while events around Cajon Pass and San Gabriel Mountain tend to propagate toward the southeast. These results are generally consistent with predictions for dynamic rupture on bimaterial interfaces associated with the imaged velocity contrasts in the area. The second moment inversions also provide constraints on the upper and lower bounds of rupture areas in our data set. Stress drops and uncertainties are estimated for elliptical ruptures using the derived characteristic rupture length and width. The semiautomated second moment method with seGfs can be used for routine application to moderate earthquakes in locations with good station coverage.

A Bimaterial Interface Along the Northern San Jacinto Fault Through Cajon Pass

“…Earthquakes propagating in the opposite direction are expected to occur less often as they require favorable (along‐strike asymmetric) initial stress conditions (e.g., Ampuero & Ben‐Zion, ; Erickson & Day, ) and/or supershear velocity (e.g., Shi & Ben‐Zion, ; Shlomai & Fineberg, ; Weertman, ). Preferred propagation to the southeast agrees well with directivity of small to moderate earthquakes on this fault section (Kurzon et al, ; Meng et al, ), suggesting that large earthquakes on the NSJFZ are not likely to propagate toward CP and continue to rupture the SAF. On the other hand, slip on the NSJFZ triggered by earthquakes on the SAF around CP can evolve relatively easily into a large rupture propagating on the SJFZ to the southeast.…”

Immediate Foreshocks Indicating Cascading Rupture Developments for 527 M 0.9 to 5.4 Ridgecrest Earthquakes

“…These signals share high resemblance with the P waves, and the onsets of both phases can be fit by scaling the records of a 3.7 earthquake that is 1.5 km away from the hypocenter (SCEDC catalog; Hutton et al., 2010). We further implement the records of the 3.7 earthquake as empirical Green's functions (eGfs) to remove the path effects to obtain apparent source time functions (ASTFs) of the 5.4 earthquake for both P and S waves (Fan & McGuire, 2018; McGuire, 2004; Meng et al., 2020). The ASTFs show that there are at least two distinct subevents constituting the 5.4 earthquake: the first subevent (E1) as the immediate foreshock releasing about 4.8% of the total seismic moment (equivalent to a 4.5 earthquake), while the second subevent (E2) occurred about 0.8 s later and released the remaining moment (Figures 1b and ).…”

Rupture Directivity in 3D Inferred From Acoustic Emissions Events in a Mine-Scale Hydraulic Fracturing Experiment

“…Earthquake ruptures can be classified in pure unilateral or pure bilateral (symmetric) ruptures, as well as asymmetric bilateral ruptures, describing an intermediate case between the two previous models. It is well know that a predominance of unilateral ruptures is observed for large earthquakes (McGuire et al, 2002); however, recent studies demonstrated that directivity might be also a common feature of small to moderate events (e.g., Kane et al, 2013;Kurzon et al, 2014;Calderoni et al, 2015;Meng et al, 2020;Ross et al, 2020). So far, rupture directivity has also been identified for a few cases of weak fluid-injection induced earthquakes (Folesky et al, 2016;López-Comino and Cesca, 2018;Király Proag et al, 2019;Wu et al, 2019) and fluid-induced microcracks or acoustic emission (AE) events in mine-scale hydraulic fracturing (HF) experiments (Dahm, 2001), suggesting that such properties could persist at smaller magnitudes.…”