The Effect of Secondary Arrivals on Regional Earthquake Locations
Michael Begnaud, Lee K. Steck and Aaron A. Velasco
Geophysics Group (EES-11), Los Alamos National Laboratory
Secondary arrivals can provide added constraints on earthquake locations, particularly for events detected by few stations and/or with poor azimuthal coverage. In an attempt to improve earthquake locations for China as a first step for velocity inversion, we will assess the quality of secondary arrivals and analyze the effect of including them in location calculations. Utilizing data from the ISC, the USGS EDR, and the pIDC REB catalogs, as well as regional picks from the Los Alamos National Laboratory database, we will identify secondary arrivals and weight them accordingly in the event location process. While shear arrivals are commonly identified in regional waveforms, they are not systematically defined for use in location determination. Preliminary results using the ISC catalog show that including secondary arrivals in relocations (using the IASPEI model) results in lateral shifts of up to ~60 km, with depths generally increasing by ~5 km. Instead of strictly incorporating secondary arrivals in the IASPEI model, we will relocate events using regional models for China. As a means of assessing the relocations, we will also show the effect of the secondary arrivals and regional velocity models on the 1997 November 8 Tibet (Mw=7.5) mainshock (which produced a significant surface rupture) and surrounding historical events. By analyzing the effect of secondary arrivals on earthquake locations and whether such arrivals improve regional locations, we will be able to produce an accurate data set for later use in velocity inversion for China.
Study Area and Velocity Models
Left - Western China study area. Stations with 2-D propagation path corrections (PPCs) are shown (red triangles) [Steck et al., 2000] as well as nuclear test sites (blue squares). Red box is major study region. Blue box is InSAR region of detailed study using actual and synthetic data shown in Figures 3-6.
Right - P and S Velocity models used in study.
CLICK ON IMAGE TO ENLARGE.
Catalog Locations with Secondary Phases |
Ground Truth Constraints on Relocations Near a Large-Magnitude Event
Synthetic Tests Pt. 1 |
Synthetic Tests Pt. 2 |
Relocations of Actual Data |
Relocations Using S Phases for the Asia Study Area |
Please direct comments and questions to the principal author.