Complex Faulting in the Yuha Desert: Implications for Fault Interaction

We have determined precise hypocentral locations for over 3,600 aftershocks that occurred in the Yuha Desert region following the 4 April 2010 Mw 7.2 El Mayor-Cucapah (EMC) earthquake until 14 June 2010 through a series of absolute and relative relocation procedures with algorithms including hpoinverse, velest and hypoDD. Location errors were reduced to ~20 m horizontally and ~80 m vertically. The locations reveal a complex pattern of faulting with en echelon fault segments trending in toward the northwest, approximately parallel to the North American-Pacific plate boundary and en echelon, conjugate features trending to the northeast. The relocated seismicity is highly correlated with the mapped faults showing triggered surface slip in response to the EMC mainshock. Aftershocks are located between depths of 2 km and 11 km, consistent with previous studies of seismogenic thickness in the region. Three-dimensional analysis reveals individual and intersecting fault planes between 5 km and 10 km in the along-strike and along-dip directions. These fault planes remain distinct structures at depth, indicative of conjugate faulting, and do not appear to coalesce onto a through-going fault segment. Measurements of surface displacement along the Laguna Salada-West and East (LS-W and LS-E, respectively) branches reveal no triggered surface creep on either branch following the EMC mainshock. Approximately 2 mm of triggered surface creep was observed on the LS-W in response to the 14 June 2010 M5.7 Ocotillo earthquake, however no creep was measured along the LS-E. Relocated seismicity shows that aftershock activity occurs along the LS-E, however not along the LS-W in the time period between these earthquakes. The area east of the LS-W was seismically active following the EMC mainshock, however; no systematic migration patterns were observed. Aftershock activity abruptly shuts off in this area following the Ocotillo earthquake. The majority of seismicity in the two-year period following the Ocotillo earthquake suggesting a complex spatial and temporal distribution of aftershock activity. The conjugate pattern of faulting, combination of seismicity and triggered surface creep and non-uniform aftershock migration patterns suggest that strain in the Yuha Desert is being accommodated in a complex m