تعيين نوسانات هم لرزه پوسته زمين با بهره گيري از داده هاي آهنگ بالاي GPS، مطالعه موردي: زمين‌لرزه سن سيمون 22 دسامبر 2003 (كاليفرنيا- ايالات متحده)

Quantifying the Ground Co-Seismic Variations by using High Rate GPS Data, Case Study: San-Simeon Earthquake 2003 Dec. 22 (California- United States of America)

شناخت پديده زمين‌لرزه، مستلزم دستيابي هر چه دقيق تر به مدل هاي ديناميكي و سينماتيكي براي توضيح چگونگي گسيختگي در گسل است. تحليل حركات زمين به صورت دقيق و در گستره وسيعي از بسامد ها و دامنه ها يكي از راهكارهاي موجود در اين زمينه به شمار مي‌رود. سامانه تعيين موقعيت جهاني، امروزه به ابزار نيرومندي براي اين منظور تبديل شده است. تعداد روز افزوني از ايستگاه هاي دايمي كه با هدف انجام مطالعات ژيوفيزيكي و لرزه شناسي نصب و راه اندازي شده اند، اكنون در بسامدهاي لرزه اي مانند يك هرتز عمل مي كنند. يك گيرنده GPS مي تواند به گونه‌اي دقيق حركات زمين را در مقياس هاي زماني زمين‌شناسي (مثال: 1mm/yr) و لرزه اي (مثال: 500 mm/yr) اندازه گيري كند. در اين پژوهش شكل موج هاي لرزه اي حاصل از 13 ايستگاه GPS در فاصله كانوني 36 تا 74 كيلومتر از زمين لرزه سن‌سيمون 2003 واقع در ايالت كاليفرنياي جنوبي (ايالات متحده) تعيين شده است. در اين مقاله قابليت هاي روش نسبي تعيين موقعيت با بهره گيري از داده هاي با آهنگ بالا (1 ثانيه) بررسي و جابه‌جايي هاي لرزه اي با جابه‌جايي هاي حاصل از انتگرال گيري ركوردهاي لرزه نگاري مقايسه و درست آزمايي شده است.

Better understanding of earthquakes primarily requires more accurate dynamic and kinematic models for fault rupture. There are several methods for ground motion detection; each of them has its own advantages and limitations. The processes, needed for the estimation of displacements by the seismic data, generally increase the noise. Accelerometers, for example, record the details of strong ground motion close to the earthquake source; however it is difficult to transform the measured accelerations into displacement. Broadband seismometers are more sensitive and more accurate than accelerometers but even those may be saturated or clipped in far distances from a large earthquake. InSAR observations can provide good spatial images of some of the surface displacement components in the rupture area. It has, however, drawbacks in some regions, as the InSAR has no sufficient temporal resolution for the analysis of dynamic short period changes during an earthquake. Most of the GPS monitoring systems process the daily or hourly data in order to achieve the station coordinate with millimeter accuracy. But in warning systems, the temporal delay between the natural event and the act of warning must be the least as much as possible. Increasingly more continuous GPS receivers, established primarily for geophysical studies, are now running in seismic frequencies such as 1-Hz. GPS seismology is the unexpected result of the geodetic networks which at first were established to measure the deformation of plates and tectonic plate boundaries. A GPS receiver can accurately measure the movements in the geological time scales (i.e. 1 mm/yr) and that of seismology (i.e. 500 mm/sec). In this paper, the shape of the seismic waves, obtained from thirteen GPS stations, being in 36 to 74 kms of epicentral distances of San-Simeon Earthquake, 2003 are determined. The efficiency of the relative methods of positioning using high rate data has been analyzed; estimated co-seismic displacements have been validated using similar results obtained from the integration of seismic records.