THE APRIL 2015 SANDPOINT, IDAHO, EARTHQUAKE SEQUENCE: A SPATIAL CONSTRAINT ON BASIN-AND-RANGE STYLE EXTENSION ON THE WESTERN PORTION OF THE LEWIS AND CLARK FAULT ZONE

The Lewis and Clark Fault Zone (LCFZ) is a megashear that extends WNW about 800 km across the northern Rockies. In Montana, the LCFZ is associated with recurrent seismicity including multiple M6 events near Helena. This raises the question of whether similar damaging earthquakes might occur along the western part of the LCFZ in northern Idaho and eastern Washington. Background seismicity is low in this region. However, three widely felt earthquakes (~M4) representing the first significant seismicity since 1942 in the Idaho Panhandle occurred near Sandpoint on April 24th 2015. Fault plane solutions of these events, along with recent GPS velocity results and a re-analysis of the 2001 swarm of M<4 events in Spokane, show that the stress field reactivating relict structures in the western LCFZ causes reverse mechanisms and contractional strain fundamentally different from the basin-and-range style extension along the eastern LCFZ in Montana. The M6 Montana events involved dextral strike-slip motion on the steeply-dipping WNW trending faults. Similar faults occur close to population centers in northern Idaho and eastern Washington. However, the stress field revealed by our study does not favor such fault motion in the western LCFZ. Our results constrain the western extent of the basin-and-range style extension along the LCFZ.