The above map shows the epicenter. The black dots and open circles near the coast were quakes where the Cocos Tectonic plate begins to bend downward as it pushes to the northeast and below the American plate. Further inland, the Cocos plate plows deeper beneath the American plate, and the earthquakes are deeper, as shown by the gray dots, and the 70 kilometer deep 1999 JUN15 earthquake.
These seismograms are broadband vertical (bhz), north-south (bhn), and east-west (bhe) earth velocity detected by a Guralp CMG3-TD seismometer in the OGS wakl-in underground vault near Leonard. The signals were digitized 20 times per second. The seismograms show the expected P, S, LQ, and LR phases. However the P is complex, with the largest P motion 27 seconds after first P arrival. This indicates the earthquake had at least one small slip before the main slip. The complex P-wave is shown in detail below.
