This is determined by fault dip (See figure below).

When a fault ruptures, either side of it has a name, the hanging wall, or the footwall. In addition to being an interesting fact, this shape also has a profound effect on seismic hazard. This variable dip was determined by magnetic imaging and has a profound effect on seismic hazard.Д«ecause of these variable dips, at depth, the southern portion of the San Andreas has a shape described as a crude propeller.

This figure from Fuis et al., 2012 shows the propeller shape the San Andreas Fault has at depth. This is a striking difference compared to the dip of the San Andreas within the western Traverse Range, where it dips approximately 55В° to the southwest. Subsurface magnetic data shows that in areas such as San Bernardino and Indio, the fault dips between 37В°-65В° to the northeast. While in the straight portions, like along the Mojave Desert, fault dips often exceed 80В°, in the bends, the story is quite different. As the San Andreas weaves its way through Southern California, it has many bends separated by straight sections. While most people only focus on where faults are at the surface, the subsurface geometry of the southern San Andreas is quite intricate. TodayвЂ™s earthquake is marked by the blue pin. It also indicates approximate fault dips at various locations, which help create the crude propeller shape the San Andreas has at depth. This Temblor map shows the southern portion of the San Andreas Fault. This event is almost purely strike-slip, which is perhaps most consistent with San Andreas motion. Shearer (2012) showed that both strike-slip and thrust slip events occur at this location. Wenzheng Yang, Egill Hauksson, and Peter M. This is an extremely shallow dip compared to other locations, where it becomes nearly vertical. According to Fuis et al., 2012, this portion of the main strand of the San Andreas Fault is inclined (вЂdipsвЂ™) about 40В° to the northeast. The Mill Creek Fault is a northern strand of the San Andreas Fault Zone as it slices through the San Bernardino mountains. On the basis of the M=3.4 earthquake location, depth, and вЂfocal mechanismвЂ™ (its orientation and sense of slip), and the likely northeast dip of the San Andreas, the event probably lies on the Mill Creek Fault, very close to the NE-вЂdippingвЂ™ (inclined) main strand of the San Andreas Fault. This earthquake occurred in the San Andreas Fault Zone (SAFZ), along the San Bernardino Mountains section. This location has a Temblor Hazard Rank of 76. This Temblor map shows the location of todayвЂ™s M=3.4 earthquake within the San Andreas Fault Zone, along the Mill Creek Fault. Around the epicenter, there was moderate shaking, with weak shaking felt in and around Riverside. Having said that, based on the shaking, it is highly likely that many more felt it. this morning, there were 436 felt reports on the USGS website. local time, a M=3.4 earthquake within the San Andreas Fault Zone just east of Los Angeles woke residents from their nightly slumber. By David Jacobson and Ross Stein, TemblorД¬heck your seismic hazard rank TodayвЂ™s M=3.4 earthquake within the San Bernardino Mountains occurred along the San Andreas Fault.

San andreas fault earthquake prediction