mutil_thread append string cause core

i use multi thread to update each item(string) of global vector each thread update item(string) with different index i think is a good way to avoid updating same data but i still get core, i do not know why

extern vector<string> gTestVec;
#define NUM 10

void * worker(void * args) {
  thread_data * p = (thread_data *)args;
  int i = p->thread_id;

  for (int j=0; j<100; j++) {
    gTestVec[i] += "a";
  }

  return NULL;
}


void do_complete_stage_test::excute() {
  int i = 0;
  pthread_t thd[NUM];
  thread_data data[NUM];

  for (i=0; i<NUM; i++) {
    gTestVec.push_back(format("%d", i));
    data[i].thread_id = i;
    
    if (0 != pthread_create(&(thd[i]), NULL, &worker, (void *)&data[i])) {
      printf("pthread_create failed");
    }
  }

  for (int i=0; i<NUM; i++) {
    if (0 != pthread_join(thd[i], NULL)) {
      printf("pthread_join failed");
    }
  }
}

when i run the code,sometimes get coredump

Starting program: /data/settle_script/isp_tran_collect/bin/isp_tran_collect -p 2134234
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
[New Thread 0x7ffff2623700 (LWP 6316)]
[New Thread 0x7fffefb0e700 (LWP 6317)]
[New Thread 0x7fffef30d700 (LWP 6318)]

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffef30d700 (LWP 6318)]
0x00007ffff6787d67 in ?? () from /lib64/libstdc++.so.6
(gdb) bt
#0  0x00007ffff6787d67 in ?? () from /lib64/libstdc++.so.6
#1  0x00007ffff678899b in std::string::reserve(unsigned long) () from /lib64/libstdc++.so.6
#2  0x00007ffff6788bbf in std::string::append(char const*, unsigned long) () from /lib64/libstdc++.so.6
#3  0x000000000044babe in append (__s=0x880492 "a", this=<optimized out>) at /usr/include/c++/4.8.2/bits/basic_string.h:1009
#4  operator+= (__s=0x880492 "a", this=<optimized out>) at /usr/include/c++/4.8.2/bits/basic_string.h:942
#5  worker (args=<optimized out>) at ../src/do_complete_stage_test.cpp:21
#6  0x00007ffff7bc6e25 in start_thread () from /lib64/libpthread.so.0
#7  0x00007ffff5ee635d in clone () from /lib64/libc.so.6

thanks for your help!!!